Monday, 27 December 2010

High Alloy Valve Supplier Spearheads Use of Corrosion Resistant High Alloys in Petrochemical Process



The Alloy Valve Stockist, the Barcelona-based stockist of alloy valves, is spearheading an initiative that studies the use of forged super duplex valves in A182 F53, A182 F55 and cast super duplex valves in A890 6A and A890 5A for corrosive processes

The Alloy Valve Stockist, the Barcelona-based stockist of alloy valves, is spearheading an initiative that studies the use of forged super duplex valves in A182 F53, A182 F55 and cast super duplex valves in A890 6A and A890 5A for corrosive processes.

Alloy SAF 2507 (UNS S32750) is a super duplex stainless steel with 25% chromium, 4% molybdenum, and 7% nickel designed for demanding applications which require exceptional strength and corrosion resistance, such as chemical process, petrochemical, and seawater equipment. The steel has excellent resistance to chloride stress corrosion cracking, high thermal conductivity, and a low coefficient of thermal expansion. The high chromium, molybdenum, and nitrogen levels provide excellent resistance to pitting, crevice, and general corrosion. A detailed overview of the corrosion resistance properties of super duplex can be found on

Nuria González, who heads the initiative team, commented that "the use of super duplex valves as opposed to duplex valves, for corrosive applications is still relatively rare, despite the higher corrosion resistance of super duplex valve and the often negligible price difference." The Alloy Valve Stockists' super duplex valve specialist site, , offers further details of the initiative.

They generally stock the following material range: A182 F347H, A182 F44, A182 F51, A182 F53, A182 F55, A182 F91, 254 SMO, 6moly, inconel 600, 625, UNS N06600, UNS N06625, A890 Gr. 4A, duplex 2205, A890 Gr. 5A, A890 Gr. 6A, titanium A182 316Ti, titanium Gr. 2, B367, B381, Alloy 20, A351 CN7M, Carpenter 20, UNS N08020, Monel 400, K500, A494 M35-1, UNS N04400, 904L, Avesta, inconel 600, inconel 625, UNS N06600, UNS N06625, incoloy 800, 800H, 800HT, 825, UNS N08800, UNS N08825, hastelloy C276, B3, A494 CW12MW, A182 F11, A182 F22, A351 CG8M, and A494-CY40. Their website, forms an integral part of the initiative.

They supply valves in ANSI and DIN standards, in a wide range of sizes and pressure ratings, and stock premium brand valves, approved leading oil, gas and chemical companies. Their customers are end users and engineering firms, from the chemical and petrochemical sectors in the Middle East, Europe and Asia.

# # #

Grupo Compás is a premier alloy valve stockist of duplex, super duplex, alloy 20, monel alloy 400, hastelloy, inconel alloy 600 and incoloy alloy 800 valves in stock, and can also offer them on short manufactuing lead times.

Saturday, 15 May 2010

6 Mo Valves and their properties

6 Moly valves are rare.; the material is also known as A182 F44 valves, 254 SMO valves, 904L valves or Avesta valves. The following article "describes the general metallurgical characteristics, corrosion resistance, mechanical properties, economics and the specific investigations conducted on the various components relating to the Soga Snorre seawater piping system, where over 1OOO tons of alloy 1925hMo were supplied in various product forms". It is entitled: "6% MOLYBDENUM SUPER AUSTENITIC STAINLESS STEELS IN OFFSHORE APPLICATIONS", and reproduced from the original site http://www.isope.org/publications/proceedings/ISOPE/ISOPE%201992/Abstract%20Pages/I92v4p022.pdf


ABSTRACT
With greater emphasis on offshore exploration to find new sources of energy,
the need to find cost effective materials of construction for handling seawater
and hydrocarbonbrine mixtures with and without hydrogen sulfide has become
very critical. This paper describes the metallurgical characteristics, corrosion
resistance and applications of a new class of 6 moly nitrogen strengthened
stainless steels, known as CroniferR alloy 1925hM0, (UNS # N08926).

INTRODUCTION
Materials used in the offshore industry encounter numerous corrosion
and mechanical problems. They consist of submerged and atmospheric marine
corrosion, mechanical and mechanical wave action and marine fouling.
Unfortunately, many other factors other than the mere considerations of
corrosion resistance in so-called "normal seawater" have to be taken into
consideration when making material selection. This applies in particular for
piping systems.

A seawater piping system consists not only of pipe (seamless and
welded) but other accessories such as pipe spools, pipe bends, reducers,
flanges, return bends, tees, elbows, pumps, valves and metering devices. The
important properties governing proper material selection are:

o resistance to various forms of corrosion, especially localized corrosion
and stress corrosion cracking.
o physical and mechanical properties (coefficient of thermal expansion,
thermal conductivity, tensile properties, erosion resistance, low temp.
toughness)
o galvanic compatibility
o good fabricability and weldability
o resistance to marine growth
o availability of various product forms
o successful case histories
o costs

Even though the precise determination of all corrosion variables as
relating to site specific marine corrosion is not fully categorized, there is ample
laboratory, field and case history experience available to make cost effective and
functionally reliable maintenance-free selection. Table 1 lists the various classes
of materials, usually specified and used in seawater service. Table 2 lists the
nominal chemistry of some of the alloys used iuncluding the 6 Mo stainless
steels. Carbon steel, along with most of the materials listed in Table 1 & Table
2, have been successfully used in marine applications although in certain very
specific conditions the performance of some has not been totally satisfactory.

METALLURGICAL CHARACTERISTICS OF 6 MO SS
The standard austenitic grades of stainless steet, although acceptable
from uniform corrosion and erosion considerations, are not suitable due to their
poor localized corrosion resistance and susceptibility to chloride stress corrosion
cracking. It has been shown (Loren?, & Medawar, 1969) that the pitting index
(P.I.) or Pitting Resistance Equivalent (P.R.E.) as measured by %Cr + 3.3%
Mo %Cr + 3.3% Mo + 16 ... 30 N, if nitrogen is piesent, must be greater
than 38 as a rule of thumb for having adequate localized corrosion resistance
to marine corrosion. A research & development effort at VDM resulted in two
new alloys of the 6 Moly family. These are

1) Cronifer'"* 1925hMo - alloy 926 - UNS# NO8926
2) NicroferiR' 3127hMo - alloy 31 - UNS# NO8031

Within the 6 Mo stainless steels there are 2 alloys containing different
levels of nickel and one alloy containing higher levels of both chromium and
nickel. The 25% Nickel version of the 6 Mo SS has shown some advantages
over the 18% nickel version of 6 Mo SS. Some of these advantages are: . improved stability of austenite . improved resistance to stress corrosion cracking . improved passivation characteristics . slower formation of precipitates, even in the temperature range of 700 - 1000°C (1290 - 1830°F) . slower sensitization kinetics.

The increased Mo of alloy 926 (UNS# N08926) and alloy 31 (UNS#
N08031) had to be metallurgically balanced by addition of nitrogen. Nitrogen,
being an austenite stabilizer, made these new alloys thermally stable by slowing
down the kinetics of precipitation of detrimental phases such as carbides, Chi,
and others during hot working and welding operations.

Other benefits of this nitrogen addition include increased resistance to
localized corrosion, enhanced mechanical properties, increased resistance to
SCC and a much lower cost substitute for nickel.

CORROSION RESISTANCE 1 MECHANICAL PROPERTIES
As mentioned earlier and documented in open literature, the main
problem with standard austenitic stainless steels has been their poor resistance
to pitting and crevice corrosion in chloride bearing media. To provide greater
resistance, judicious increase in molybdenum, chromium & nitrogen contents
of the "Fe-Ni-Cr-Mo" alloys was necessary. This had to be accomplished
without increasing the cost significantly, as is with nickel base alloys of Ni-
Cr-Mo familty such as alloy 625, alloy C-276 & alloy 59 and without
sacrificing thermal stability. Table 3 compares for various alloys including the
6 Moly SS, Pitting Resistance Equivalent, the critical pitting & crevice
corrosion temperature as measured in 10% Ferric-chloride solution (ASTM G-
48 test) and a cost ratio comparison to alloy 316L.

Saturday, 8 May 2010

Super duplex valve categorization

Super duplex valves can be categorized as follows:

* Super Duplex 2507,
* Super Duplex SAF 2507,
* Super Duplex UNS S32750
Duplex 2205, Duplex UNS S31803 Duplex SAF2205, Duplex SAF 2205 Super Duplex 2507, Super Duplex UNS S32750, Super Duplex SAF2507, Super Duplex SAF 2507

A category of stainless steel with high amounts of chromium and moderate nickel content. The duplex class is so named because it is a mixture of austenitic (chromium-nickel stainless class) and ferritic (plain chromium stainless category) structures. This combination was originated to offer more strength than either of those stainless steels. Duplex stainless steels provide high resistance to stress corrosion cracking (formation of cracks caused by a combination of corrosion and stress) and are suitable for heat exchangers, desalination plants, and marine applications
Categories:

* Super Duplex 2507,
* Super Duplex SAF 2507,
* Super Duplex UNS S32750

Thursday, 6 May 2010

Duplex valves and alloys for Offshore Applications – Duplex and Super Duplex Stainless Steels, Cupronickels and Corrosion Mechanisms

This is a great summary on Duplex Stainless steel which can be used in duplex valves and super duplex valves, for example, in corrosive applications.

Primary author By K.C. Bendall

Background

Materials selections must be given detailed attention at every stage of the design, construction and operation of systems and equipment for application in offshore oil and gas production. Full attention must be given to general corrosion resistance, selective corrosion resistance (by pitting and crevice attack) and stress corrosion cracking susceptibility in sour hydrogen sulphide environments if failures, loss of production and costly maintenance are to be avoided. Even more important than these considerations is the need to maintain offshore safety. Thus the specification and use of materials which combine corrosion resistance with high mechanical strength is a fundamental requirement.

A greater understanding of the offshore environment and more detailed knowledge of the conditions under which offshore structures and systems have to operate will obviously contribute to the selection of the correct materials.

Corrosion in Sea Water and Offshore Environments

Sea water is highly corrosive and offshore installations are often exposed to temperature extremes. The corrosion resistance of a material is therefore equally as important as mechanical strength. The introduction of chlorine by adding hypochlorite solution to sea water to give biofouling resistance can reduce the corrosion resistance of certain stainless steels, particularly under crevice conditions. Hydrocarbon process systems often have to withstand the potentially corrosive effects of hydrogen sulphide and acid conditions associated with the dissolved carbon dioxide which is often present. Corrosion can weaken elements of an otherwise well designed ,structure or affect individual equipment components to such an extent that they cease to be serviceable. Unfortunately, the fight against corrosion itself can lead to equally damaging side effects such as the release of nascent hydrogen. This can be generated as a result of cathodic protection measures adopted to protect a structure or by dissimilar metal coupling. The presence of such hydrogen can given rise to hydrogen-induced cracking of steels and nickel base alloys.

Alloys for Offshore Applications

Metals manufacturers have spent much time and effort in developing alloys specifically to meet offshore needs. The alloys developed have had to be suitable for shafts and bolting as wellas many other applications. These have included sea water and process pipework, water injection and booster pumps, line shaft pumps, emergency shutdown valves, anchorages and tensioners for riser protection systems, multiphase pumps and remotely operated vehicle components.

The Development of Marinel

One particularly significant corrosion-resistant alloy (CRA) development led to the introduction of an ultra high strength cupronickel alloy (Marinel), approximately five years ago. This alloy was added to the range of alloys available for selection with reference to particular equipment where corrosion and hydrogen embrittlement could occur offshore. Most high strength iron and nickel based alloys and titanium alloys are prone to hydrogen embrittlement, the effect usually becoming more severe as the strength increases. Thus these alloys when operating in a high-stress condition will be more susceptible to hydrogen embrittlement than the same alloys operating under lower stress. Hydrogen embrittlement is of particular concern where high strength (usually B7 carbon steel, 720 N.mm-2 yield point) bolting is used on subsea structures. The operating stress level usually taken to represent a critical situation with respect to hydrogen embrittlement is that given by the yield stress of B7 carbon steel which has the value of 720 N.mm-2.

Use of Cathodic Protection

Cathodic protection by sacrificial anodes or impressed current is extensively used to protect subsea structures from corrosion. This technique can generate hydrogen which, if absorbed, may lead to embrittlement of metallic components with the resultant danger of premature failure. The time-dependent nature of the ingress of hydrogen may mean that an apparently unaffected subsea critical component, for example a bolt, fails in an instant after it has performed satisfactorily for several years in service. Failure occurs when the residual ductile core is reduced in area by an encroaching hydrogen embrittlement front to a cross-section which cannot carry the load placed upon it. As an example, the failure of alloy K-500 riser clamp bolts has been reported in the April 1985 issue of Materials Performance (p37). Charging of UNS N 05500 (high strength 70Ni-3OCu alloy) with hydrogen has been shown to result in the hydrogen embrittlement of nonmagnetic drill collars. This has been thought to be due to galvanic coupling of the collars with carbon steel (see the October 1986 issue of Materials Performance, p28). It has also been suggested that a documented example of cracking in high strength steel legs of jack-up rigs was associated with hydrogen-induced stress corrosion cracking, the hydrogen being generated by the cathodic protection system operating in hydrogen sulphide contaminated seawater (February 1989 issue of Veritec Offshore Technology Journal).

Transport of Hydrogen into a Metal


The entry of hydrogen into a metal can be purely diffusion-controlled, or can be assisted by dislocation transport and the latter effect has been experimentally demonstrated by the measurement of hydrogen permeation rates through nickel whilst it is undergoing plastic deformation (see volume 13, 1979 of Scripta Metallurgica, pp 927-932). Dislocation sweep-in of hydrogen from the surface in the case of several different metals has been found to be consistent with the calculated energy of activation of hydrogen-induced cracking (see pp 233-239 of the proceedings of the 1976 TMSAIME international conference on the effects of hydrogen on the behaviour of metals). During hydrogen transport, the hydrogen can be deposited at various ‘trap-sites’ or internal discontinuities such as grain boundaries or precipitates.

Susceptibility to Hydrogen Embrittlement

These can take the form of ‘reversible’ traps which the hydrogen can subsequently leave, or ‘irreversible’ traps, which the hydrogen cannot leave and which tend to encourage local fracture through a lowering of the surface energy of the material. The effectiveness of the traps in promoting hydrogen embrittlement is related to the degree of strengthening present in the material matrix, as it is well established that materials in a higher strength state (i.e. cold worked or age hardened) are more susceptible to hydrogen embrittlement than the same materials in a lower strength condition. Thus, measurement of both the hydrogen entry kinetics of a metal (or alloy) and the ability of the metal to trap hydrogen would give an indication of its hydrogen embrittlement susceptibility. Overall solubility of hydrogen does have an influence on hydrogen embrittlement characteristics, as iron, nickel and titanium have relatively high hydrogen solubilities (>1cc/cc) and these materials are more susceptible to hydrogen embrittlement than aluminium and copper alloys, whose solubilities are generally less than 0.1 cc/cc. The hydrogen diffusion coefficients of steel and titanium are greater than 10-6 cm2.s-1, whereas the hydrogen diffusion coefficients of nickel, aluminium and copper alloys are approximately 10-10 cm2.s-1, although this does not take into account dislocation transport or grain boundary diffusion.

Nickel-Copper Alloys and Hydrogen Embrittlement

Two alloys which are interesting to compare are the age hardening nickel-copper alloy K-500 and age hardening cupronickel Marinel, which have similar mechanical properties and hydrogen diffusion characteristics. In comparing the chemical composition of these two alloys, see Table 1, it is apparent that they contain almost the same basic elements, the major difference between them being the Cu:Ni ratio. In the case of Marinel the high Cu:Ni ratio renders the alloy immune to hydrogen embrittlement and this has been found to be largely due to the reduced ability of this alloy to trap the hydrogen irreversibly.

Table 1. Typical composition of bolting.

MaterialTiCrMnNbCuNiFeAl
K-5000.6-1.0-30Bal.1.02.8
Marinel-0.45.00.7Bal.181.01.8

Marinel in Offshore Applications

In offshore situations many developments have widely employed Marinel bolting for splash zone and subsea. Bolting subsea has been used with 13Cr steel, 22Cr duplex and 25Cr duplex steel manifold, valve and choke flanges. Subsea developments using the alloy include Lyell, Strathspey, Nelson, Heidrun, Johnston and Nelson.

Good galling resistance obviates the need for a lubricant during assembly and nuts can be readily removed after a period of service if required.

For the Conoco Lyell subsea manifold Marinel bolting was chosen for its greater mechanical strength and corrosion resistance compared with grade 660 steel. The bolts were bolt tensioned and assembled without lubricant. Stud bolts have been subjected to a laboratory examination after 18 months service (nearly 12 months with the manifold in operation) and apart from the expected calcareous deposit, appeared completely unaffected by service.

Duplex Stainless Steels in Offshore Applications

A most significant contribution to the fight against corrosion offshore has been made by duplex stainless steels. These have often been adopted on offshore structures in preference to carbon steel or other stainless steels. The value of the duplex stainless steel is that it combines the basic toughness of the more common austenitic stainless steels with the higher strength and improved corrosion resistance of ferritic steels. The optimum chemical composition of these steels provides a high level of corrosion resistance in chloride media together with high mechanical strength and ductility. Other benefits include the ability of some duplex stainless steels to be used at quite low sub-zero temperatures and be able to resist stress corrosion cracking.

A significant feature of duplex stainless steel is that its pitting and crevice corrosion resistance is greatly superior to that of standard austenitic alloys. Pitting resistance equivalent numbers (PREN), a standard industry measure, are often in the high 30s while the latest duplex alloys exceed a PREN of 40. This is an increasingly common specification for certain offshore duties. However, PREN numbers only provide an approximate grading of alloys and do not account for the microstructure of the material. An acceptance corrosion test on material in the supply condition is so much more meaningful.

The Evolution of Duplex Stainless Steels

Ferralium alloy 255 was the world’s first commercial 25% chromium duplex stainless steel when it was introduced over 20 years ago. It pioneered the use of a deliberate nitrogen addition in order to improve ductility and corrosion resistance. Further research has demonstrated the importance of using duplex stainless steels containing both nitrogen and copper.

Super Duplex Stainless Steels for Offshore Applications

For offshore and indeed, onshore applications, the availability of a super duplex (25% chromium) stainless steel alloy in a variety of forms is important. For example, bar, forgings, castings, sheet, plate, pipe/tube, welding consumables, flanges, fittings, dished ends and fasteners are available. In terms of other benefits, the high allowable design stress of this alloy type in comparison with other duplex stainless steels and austenitic stainless steels, including 6% Mo type, is significant. It also offers excellent castability, weldability and machinability. These features are complemented by excellent fatigue resistance and galvanic compatibility with other high alloy stainless steels.

Twenty-two percent chromium stainless steels provide better pitting resistance and resistance to crevice corrosion than type 316 stainless steel by virtue of a more stable passive film and also have greater mechanical strength. However, for optimum corrosion resistance, a 25% chromium high alloy duplex stainless steel is required and these alloys are often referred to as super duplex stainless. Even within this category, it is important to select the correct grade of material to get versatility in handling a wide range of corrosive media and for confidence that the alloy will cope with any excursions or transient operating conditions which make the environment more aggressive.

Materials Selection for Offshore Applications

Offshore structures themselves present different requirements of materials depending upon whether their application is topside, splash zone or subsea. Topside, duplex materials are suitable for a wide range of bolting applications and material such as Ferralium alloy 255 provide up to B7 steel strength, excellent corrosion resistance and a service life equal to the life of the system, thereby contributing to reduced maintenance costs. In the splash zone, the alloy has already demonstrated its suitability for sea water resistance with over 15 years service on North Sea installations and has been widely employed for riser bolting and components on riser protection system on TLPs.

Emergence of New Super Duplex Stainless Steels


Improved materials in the super duplex stainless steel category continue to be developed by manufacturers offering better or differently combined characteristics, features and benefits. These alloys, generally with a PREN > 40, are produced to conform to a number of UNS designations which appear in ASTM product form specifications. Castings and wrought forms are available. Typical of recent developments is Ferralium alloy SD40 (conforming to UNS S 32550) with a PREN > 40.0 and providing a minimum 0.2% proof stress of 550N.mm-2 and a UTS of 760 N.mm-2. This 25% chromium super duplex material results from a carefully controlled composition and balanced austenitic/ferritic structure with a substantial content of molybdenum and nitrogen.

Applications for Super Duplex Stainless Steels

Applications which can benefit from the use of these high alloy super duplex steels involve piping systems, pumps (where the good erosion and abrasion resistance is employed), valves, heat exchangers and diverse other equipment.

Recently, the excellent corrosion resistance of the new super duplex Ferralium alloy SD40 has been exploited for subsea electrical connectors on the Saga Snorre and Total South Ellon developments. In one case the super duplex material was chosen to replace standard austenitic stainless steel which had suffered from corrosion attack.


Figure 1. Super duplex stainless steel alloy is available in a variety of forms for both on and offshore applications.

Conclusions

Several types of alloys have been developed in recent years to combat the degradation of existing alloys by corrosion attack and in some cases hydrogen embrittlement in the harsh offshore environment. Super (25 Cr) duplex stainless steels and an ultra high strength cupronickel have provided the solution to many material selection dilemmas.

Wednesday, 5 May 2010

Duplex valves: mechanical and chemical properties

Duplex is an austenitic ferritic Iron Chromium-Nickel alloy with Molybdenim addition. It has good resitance to pitting, a high tensile strength and higher resistance to stress corrosion cracking at moderate temperatures to that of conventional austenitic stainless steels.  Valves made of duplex are highly suitable for marine and other saline environments.


Duplex valve for corrosive, seawater application


Duplex is a material having an approximate equal amount of austenite and ferrite. These combine excellent corrosion resistance with high strength. Mechanical properties are approximately double those of singular austenitic steel and resistance to stress corrosion cracking is superior to type 316 stainless steel in chloride solutions. Duplex material has a ductile / brittle transition at approximately -50°C. High temperature use is usually restricted to a maximum temperature of 300°C for indefinite use due to embrittlement.

Sunday, 25 April 2010

Duplex: a material overview

Duplex stainless steels are extremely corrosion resistant, work hardenable alloys. Their microstructures consist of a mixture of austenite and ferrite phases. As a result, duplex stainless steels display properties characteristic of both austenitic and ferritic stainless steels. This combination of properties can mean some compromise when compared with pure austenitic and pure ferritic grades.

Duplex stainless steels are in most cases, tougher than ferritic stainless steel. Strengths of duplex stainless steels can in some cases be double that for austenitic stainless steels.

Whilst duplex stainless steels are considered resistant to stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless steel. However, the corrosion resistance of the least resistant duplex stainless steel is greater than that for the most commonly used grades of stainless steel, i.e. 304 and 316.

Duplex stainless steels are also magnetic, a property that can be used to easily differentiate them from common austenitic grades of stainless. Due to excellent corrosion resistance and high strength, the most widely used duplex stainless steel is 2205. The name 2205 comes from a typical composition of 22% Chromium and 5% Nickel. Other designations for 2205 are given below. There are many other duplex stainless steels, most of which are proprietary grades developed and produced by individual manufacturers.

Corrosion Resistance of Duplex stainless steel

Duplex stainless steels are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, duplex stainless steel exhibit very high resistance to stress corrosion cracking. The super duplex grades are even more resistant to corrosion.

Heat Resistance of Duplex stainless steel

The high chromium content of duplex stainless steel that protects against corrosion, causes embrittlement at temperatures over about 300°C . At low temperatures duplex stainless steels have better ductility than the ferritic and martensitic grades. Duplex grades can readily be used down to at least -50°C .

Fabrication of Duplex stainless steel

Fabrication of all stainless steel should be done only with tools dedicated to stainless steel materials. Tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolour the surface of the fabricated product.

Heat Treatment of Duplex stainless steel

Duplex stainless steel cannot be hardened by heat treatment. They can however be work hardened. Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C .

Machinability

Although machinable, the high strengths of duplex stainless steel makes machining difficult. As an example, machining of 2205 is around 20% slower than for 304.

Machining can be enhanced by using the following rules:

Welding of Duplex stainless steel

Duplex stainless steel have good weldability. All standard welding processes can be used. They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding. The recommended filler material for 2205 stainless steel is 2209.

Applications of Duplex stainless steel

• Chemical processing, transport and storage
• Oil and gas exploration and offshore rigs, as well as refining
• Marine environments
• Pulp & paper manufacturing
• Chemical process plants

Corresponding Standards:

• EN/DIN 1.4462 X2CrNiMoN22.5.3
• AFNOR Z2CND22.05Az
• AISI A182 F51
• UNS ASTM A276 S31803 / S32205
• BRANDS SAF 2205 - SANMAC
• URANUS 45N
• EN (10283) 1.4470

Duplex stainless steel is among various metals used in corrosion-resistant applications. Duplex's machining rate of travel is similar to that of 316 stainless steel with high-speed steel tooling, and duplex requires fewer machining labor hours than high-nickel alloys. Duplex alloys are in a class with several other materials for various types of corrosion resistance. Some of the corrosive environments listed below depict these differences. There are many more types of corrosive conditions but these are some for which stainless and nickel alloys are suitable.

Duplex is better than the 300 series in nearly every category. The consideration in the case of chlorides and halides must include costs. Duplex alloys are nearly equal in strength to the C276 and AL6XN alloys. The pitting and crevice corrosion resistance of C276 and AL6XN are only slightly superior to the duplex alloys'. The corrosion rate for duplex is 23 mil/year; AL6XN is 17 mil/year. The elements that provide the strength and corrosion resistance in duplex are much more common than those found in some high-nickel alloys. For instance C276 contains tungsten. Carpenter 20Cb contains columbium and tantalum. The 625 alloy contains columbium, tantalum, cobalt, and titanium. Chromium, nickel, molybdenum, and nitrogen are common elements that provide the strength and corrosion resistance in the duplex alloys.

Duplex alloy -- patent comment

A stainless duplex ferritic-austenitic steel with improved corrosion resistance. The steel consists essentially of about: 0.03 w/o Max. carbon, 3.0 w/o Max. manganese, 1.0 w/o Max. silicon, 26.0 to 29.0 w/o chromium, 3.5 to 5.2 w/o nickel, 3.5 w/o Max. molybdenum, 0.15 w/o Min. nitrogen and the balance essentially iron. The steel preferably contains about 0.17 to 0.35 w/o nitrogen for improved pitting resistance and for increased austenite content. Welds of the steel preferably contain at least about 17% austenite in the as-welded condition for improved pitting and intergranular corrosion resistance.


Inventors:
Debold, Terry A. (Wyomissing, PA)
Englehart, David A. (Shillington, PA)
Martin, James W. (Spring Township, Berks County, PA)

Application Number:
07/015584

Publication Date:
05/23/1989

Filing Date:
02/17/1987
















DE2457089June, 1975
148/38
JP0000716January, 1977
751/28E
JP0052716January, 1977


JP0046117April, 1979
751/28E
JP5446117April, 1979


JP0044528March, 1980
148/37
JP5544528March, 1980


JP0044757April, 1981
751/28E
JP5644757April, 1981


JP0142855November, 1981
148/37
JP56142855November, 1981

TWO-PHASE STAINLESS STEEL EXCELLENT IN HOT PROCESSABILITY AND LOCAL CORROSION RESISTANCE
JP0047852March, 1982
148/37
GB1248980October, 1971



Other References:
Sandvik 3RE60, Stainless Steel with High Resistance to Stress Corrosion Cracking, Sanvik Steel Catalogue, 1.25E, Mar. 1976, pp. 2, 8 & 9.
Avesta Jernverks AB, Information Sheet No. 7416, re: Avesta 453S Welding Electrodes, (1980).
ASTM Specification A268-81, "Seamless and Welded Ferritic Stainless Steel Tubing for General Service", pp. 200-206, (1981).
Bungardt et al., "Untersuchungen uber das Superplastische Verhalten Ferritisch-Austenitischer Chrom-Nickel (Molybdan)-Stahle", DEW-Technische Berichte, vol. 10, issue No. 2, pp. 85-96, (1970).
Carpenter Technology Corporation, Tube Division, Technical Data Sheet: "Carpenter Stainless 7-Mo, (Type 329)", (1974).
Firth Vickers Foundry Limited, Brochure: "Firth Vickers FMN Casting Alloy", (no date).
Firth Vickers Foundry Limited, Reprint of: Matthews, "Properties of Duplex Austenitic Ferritic Stainless Steels", The Metallurgist and Materials Technologist, (May 1982).
Firth Vickers Foundry Limited, NACE Uniform Material Testing Report Form, (Dec. 10, 1981).
Irving, "Duplex Stainless Keeps Corrosion in Check", Iron Age, pp. 93-94, (May 4, 1981).
Lawson et al., "Evaluating Material Performance in a 3000-GPD Stainless Steel Desalination Test Plant", Materials Performance, pp. 11-16, (Mar. 1974).
Nordin (Uddeholm Steel Research), "Condensate of Research Report No. FM76-865-3", (1977).
Nordin, "Properties of a Modified Type 329 Weldable and SCC Resistant Stainless Steel", Metaux Corrosion-Industries, No. 659-660, pp. 229-240, (Jul.-Aug. 1980).
SKF Industries, Inc., Material Specification No. 471,878, re: Type 329 Steel, (1966).
Stahlschlussel Key to Steel, 11th Edition, C. W. Wegst, Ed., Verlag Stahlschlussel Wegst, KG., Marbach, pp. 255 and 307, (1977).
Wessling et al., "Properties and Applications of a Recently Developed Ferritic/Austenitic Steel Containing 0.02% C, 22% Cr, 3% Mo and 0.12% N in Comparison with Molybdenum-Alloyed Austenitic Steels", Stainless Steel 1977, published by Climax Molybdenum, Inc., pp. 217-225, (1977).

Primary Examiner:
Yee, Deborah

Attorney, Agent or Firm:
Jay, Edgar N.
Pace, Vincent T.

Parent Case Data:
This application is a continuation of our application Ser. No. 773,857, filed Sept. 9, 1985, now abandoned, which in turn was a continuation of our application Ser. No. 455,870, filed Jan. 5, 1983, now abandoned.

Claims:
We claim:

1. A stainless duplex ferritic-austenitic steel consisting essentially in weight percent of about:
______________________________________
Elements w/o
______________________________________


C .03 Max.

Mn 3.0 Max.

Si 1.0 Max.

Cr 26.0-29.0

Ni 3.5-5.2

Mo 1.0-3.5

Cu 2.0 Max.

B .005 Max.

N 0.15-0.4

______________________________________
the balance of the steel being essentially iron; the total of chromium w/o plus nickel w/o plus molybdenum w/o being no more than about 34.0; and the total of nickel w/o plus molybdenum w/o being no more than about 7.0.
.

2. The steel of claim 1 wherein nitrogen is about 0.17 to 0.35 w/o.

3. The steel of claim 2 wherein the elements are balanced to provide a weld of the steel with at least about 17% austenite and enhanced intergranular corrosion resistance and pitting resistance in the as-welded condition.

4. The steel of claim 3 wherein the chromium equivalent minus the nickel equivalent is no more than about 16.4.

5. The steel of claim 3 wherein the chromium equivalent minus the nickel equivalent is no more than about 15.3.

6. The steel of claim 3 wherein nickel is about 4.0 to 5.0 w/o.

7. The steel of claim 5 wherein molybdenum is about 1.0 to 2.5 w/o.

8. The steel of claim 6 wherein chromium is about 26.0 to 28.0 w/o.

9. The steel of claim 8 wherein manganese is about 1.0 w/o Max.

10. The steel of claim 1 which contains about:
______________________________________
Elements w/o
______________________________________


C .01-.028

Mn 1.0 Max.

Si 0.75 Max.

Cr 26.0-28.0

Ni 4.0-5.0

Mo 1.0-2.5

Cu 1.0 Max.

N 0.17-0.35

______________________________________
.


11. The steel of claim 10 wherein the chromium equivalent minus the nickel equivalent is no more than about 16.4.

12. The steel of claim 11 wherein the chromium equivalent minus the nickel equivalent is no more than about 15.3.

13. The steel of claim 1 which contains about:
______________________________________
Elements w/o
______________________________________


C .015-.025

Mn 0.5 Max.

Si 0.5 Max.

Cr 26.5-27.5

Ni 4.5-5.0

Mo 1.25-2.25

Cu 1.0 Max.

N 0.18-0.28

______________________________________
.


14. The steel of claim 13 wherein the chromium equivalent minus the nickel equivalent is no more than about 16.4.

15. The steel of claim 14 wherein the chromium equivalent minus the nickel equivalent is no more than about 15.3.

16. A hot rolled, annealed plus welded plate made from the stainless duplex ferritic-austenitic steel of claim 4.

17. The plate of claim 16 wherein the chromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.

18. A cold rolled, annealed plus welded sheet or strip made from the stainless duplex ferritic-austenitic steel of claim 3.

19. The sheet or strip of claim 18 wherein the chromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.

20. A welded article made from the stainless duplex ferritic-austenitic steel of claim 4.

21. The article of claim 20 wherein the chromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.

22. A stainless duplex ferritic-austenitic steel consisting essentially of about:
______________________________________
Elements w/o
______________________________________


C .03 Max.

Mn 3.0 Max.

Si 1.0 Max.

Cr 26.0-29.0

Ni 3.5-5.2

Mo 1.0-2.5

Cu 2.0 Max.

B .005 Max.

N 0.15-0.40

______________________________________
the balance of the steel being essentially iron, the total of chromium w/o plus nickel w/o plus molybdenum w/o being no more than about 34, and the total of nickel w/o plus molybdenum w/o being no more than about 7.
.

23. The steel of claim 22 wherein nitrogen is about 0.17 to 0.35 w/o.

24. The steel of claim 22 wherein the elements are balanced to provide a weld of the steel with at least about 17% austenite and enhanced intergranular corrosion resistance and pitting resistance in the as-welded condition.

25. The steel of claim 24 wherein the chromium equivalent minus the nickel equivalent is no more than about 16.4.

26. A hot rolled, annealed plus welded plate made from the stainless duplex ferritic-austenitic steel of claim 25.

27. The plate of claim 26 wherein the chromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.

28. A cold rolled, annealed plus welded sheet or strip made from the stainless duplex ferritic-austenitic steel of claim 25.

29. The sheet or strip of claim 28 wherein the chromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.

30. A welded article made from the stainless duplex ferritic-austenitic steel of claim 25.

31. The article of claim 30 wherein the chromium equivalent minus the nickel equivalent of the steel is no more than about 15.3.


Description:
BACKGROUND OF THE INVENTION
This invention relates to a stainless duplex ferritic-austenitic steel having a unique combination of good mechanical properties and good corrosion resistance properties.
Heretofore, a stainless duplex ferritic-austenitic steel, designated AISI Type 329, has been commercially available with: a) good mechanical properties such as high annealed yield strength; and b) good corrosion resistance such as resistance to general corrosion in the presence of strong oxidizing agents (e.g., boiling nitric acid). Typical uses for Type 329 steel have included tube or pipe for heat exchange applications involving severely corrosive, oxidizing environments such as are found in the petroleum refining, petrochemical, chemical, and pulp and paper industries (e.g., in nitric acid cooler-condensers). Type 329 steel has typically had a composition of about 0.08 weight percent (w/o) Max. carbon, 1.0 w/o Max. manganese, 0.75 w/o Max. silicon, 23.0 to 28.0 w/o chromium, 2.5 to 5.0 w/o nickel, 1.0 to 2.0 w/o molybdenum, with the balance essentially iron. Compositions similar to Type 329 steel have been sold, containing down to about .02 w/o carbon, up to about 2 w/o manganese, up to about 6 w/o nickel, up to about 30 w/o chromium, up to about 3.5 w/o molybdenum and/or up to about 0.25 w/o nitrogen.
However, the resistance to intergranular corrosion in the presence of strong oxidizing agents and the resistance to pitting in the presence of halides, particularly chlorides, of Type 329 steel has left something to be desired in areas of the steel which have been welded, particularly in areas which have been welded but not subsequently annealed (e.g., in areas of a tube, formed from the steel, which have been welded into a tube sheet of a heat exchanger). Hence, a steel has been sought with mechanical properties and corrosion resistance properties at least as good as Type 329 steel and with intergranular corrosion resistance and pitting resistance, as welded or as welded plus annealed, that are superior to Type 329 steel.
SUMMARY OF THE INVENTION
In accordance with this invention, a stainless duplex ferritic-austenitic steel is provided, the broad, preferred and particularly preferred forms of which are conveniently summarized as consisting essentially of about:
______________________________________
Broad Preferred Particularly Ranges Ranges Preferred Elements (w/o) (w/o) Ranges (w/o)
______________________________________


C .03 Max. .01-.028 .015-.025

Mn 3.0 Max. 1.0 Max. 0.5 Max.

Si 1.0 Max. 0.75 Max. 0.5 Max.

Cr 26.0-29.0 26.0-28.0 26.5-27.5

Ni 3.5-5.2 4.0-5.0 4.5-5.0

Mo 3.5 Max. 1.0-2.5 1.25-2.25

Cu 2.0 Max. 1.0 Max. 1.0 Max.

B .005 Max.

N 0.15 Min. 0.17-0.35 0.18-0.28

______________________________________
The remainder of the steel is iron except for incidental impurities which can comprise: up to about 0.04 w/o, preferably up to about 0.025 w/o, phosphorous; up to about 0.03 w/o, preferably up to about 0.005 w/o, sulphur; up to about 0.2 w/o tungsten; up to about 0.25 w/o vanadium; up to about 0.2 w/o cobalt; and up to about 0.1 w/o of elements such as aluminum, calcium, magnesium and titanium and up to about 0.1 w/o of misch metal which can be used in refining the steel.
In the foregoing tabulation, it is not intended to restrict the preferred ranges of the elements of the steel of this invention for use solely in combination with each other or to restrict the particularly preferred ranges of the elements of the steel for use solely in combination with each other. Thus, one or more of the preferred ranges can be used with one or more of the broad ranges for the remaining elements and/or with one or more of the particularly preferred ranges for the remaining elements. In addition, a preferred range limit for an element can be used with a broad range limit or with a particularly preferred range limit for that element.
The steel of this invention has: a) the good mechanical properties of Type 329 steel; and b) corrosion resistance properties, particularly resistance to intergranular corrosion and pitting in weld areas, that are superior to Type 329 steel.
DETAILED DESCRIPTION OF THE INVENTION
In the stainless duplex ferritic-austenitic steel of this invention, carbon, which is a strong austenite former, is kept to a minimum to minimize the formation of chromium-rich carbonitrides or carbides (e.g., M 23 C 6 ) at grain boundaries when the steel is heated. In this regard, no more than about 0.03 w/o carbon, preferably no more than about 0.025 w/o carbon (e.g., down to about 0.001 to 0.005 w/o carbon), is utilized. Thereby, the susceptibility of the steel to intergranular corrosion is reduced. About .01 w/o carbon is considered a practical and hence preferred, but not an essential, minimum because of the cost of reducing the carbon below about 0.01 w/o. A particularly preferred range for carbon is about 0.015 to 0.025 w/o.
Manganese is an austenite former and also increases the solubility of nitrogen in the steel of this invention. In addition, manganese is a scavenger for unwanted elements (e.g., sulfur). Hence, at least about 0.2 w/o manganese is preferably present in the steel. However, manganese can promote the formation of sigma phase which, if present: (a) makes the steel hard and brittle and thereby makes it difficult to handle and work the steel; and (b) makes the steel prone to corrosion. Also, most of the benefit from having manganese present can be attained with up to about 3.0 w/o manganese, and more than about 1.0 w/o manganese may adversely affect the pitting resistance of the steel. Hence, only up to about 3.0 w/o manganese is utilized in the steel. Preferably, no more than about 1.0 w/o, better yet no more than about 0.5 w/o, manganese is present in the steel.
Silicon acts as a deoxidizing agent and a strong ferrite former. Silicon also improves the weldability of the steel by increasing the fluidity of the steel when it is molten. Hence, at least about 0.2 w/o silicon is preferably present in the steel. However, silicon promotes the formation of sigma phase. Hence, only up to about 1.0 w/o silicon is utilized in the steel. Preferably, no more than about 0.75 w/o, better yet no more than about 0.5 w/o, silicon is present in the steel.
Chromium is a ferrite former and provides significant corrosion resistance to the steel of this invention. In this regard, chromium provides significant resistance to: (a) general and intergranular corrosion in the presence of strong oxidizing agents such as nitric acid heated above its atmospheric boiling point; and (b) pitting in the presence of chlorides. Chromium also increases the solubility of nitrogen in the steel. Hence, at least about 26.0 w/o chromium is present in the steel. However, chromium promotes the formation of sigma phase. Hence, no more than about 29.0 w/o chromium is utilized in the steel, and preferably no more than about 28.0 w/o chromium is utilized. The use of about 26.5 to 27.5 w/o chromium is particularly preferred in the steel, bu the use of about 28.0 to 29.0 w/o chromium may be preferred for providing corrosion resistance if little or no (e.g., about 0.2 w/o Max.) molybdenum is used in the steel.
Nickel is a strong austenite former, and for this reason, at least about 3.5 w/o nickel is present in the steel of this invention. Nickel also provides general corrosion resistance in acid environments, particularly in sulfuric acid. However, nickel is relatively expensive. Nickel also decreases the solubility of nitrogen in the steel and promotes the formation of sigma phase. Moreover, most of the corrosion resistance benefits, obtained by adding nickel, can be attained with up to about 5.2 w/o nickel. Hence, not more than about 5.2 w/o nickel is present in the steel. Preferably, about 4.0 to 5.0 w/o, better yet about 4.5 to 5.0 w/o, nickel is used in the steel.
Molybdenum is a strong ferrite former and, if added to the steel of this invention, provides significant corrosion resistance, particularly pitting resistance. Molybdenum also increases the solubility of nitrogen in the steel. However, molybdenum promotes the formation of sigma phase, and hence, not more than about 3.5 w/o molybdenum, preferably not more than about 2.5 w/o molybdenum, is used. Preferably, at least about 1.0 w/o molybdenum is present in the steel for pitting resistance. It is particularly preferred that the steel contain about 1.25 to 2.25 w/o molybdenum for use in a wide variety of corrosive environments, particularly those containing chlorides. However, for corrosive environments containing little or no chlorides, it is contemplated that the steel can contain little or no (e.g., about 0.2 2/o Max.) molybdenum.
In a preferred steel of this invention, the total of chromium w/o plus nickel w/o plus molybdenum w/o in the steel does not exceed about 34.0 and the total of nickel w/o plus molybdenum w/o does not exceed about 7.0. This inhibits sigma phase formation during the processing of this preferred steel which could adversely affect the workability and the corrosion resistance of the steel. By so limiting the total of chromium, nickel and molybdenum, the workability of this preferred steel is made comparable to Type 329 steel, and this steel can be processed in the same general manner as Type 329 steel, as will be described below, to remove any minor amounts of sigma phase that might form. In this regard, by controlling the total of chromium, nickel and molybdenum in this preferred steel, the hardness of the steel is kept from exceeding about 30 on the Rockwell C (Rc) scale when the steel is sensitized by heating at 1400 F (760 C) for two hours and then air cooling. By so limiting the total of chromium, nickel and molybdenum, the risk of forming sigma phase in weld areas of this preferred steel, as a result of the welding process, is also substantially reduced. Of course, the total of chromium, nickel and molybdenum in the steel of this invention need not be so limited, provided sigma phase formation is not a problem in the processing or welding of the steel. For example, the total of chromium, nickel and molybdenum need not be so limited: (a) if the dimensions of the articles (including intermediate and final shaped articles), formed from the steel, allow the articles to be rapidly cooled through the sigma phase sensitization range of about 1250 to 1650 F (about 675 to 900 C); or (b) if any sigma phase can subsequently be removed in a conventional manner from the articles.
Copper, if added to the steel of this invention, can provide significant corrosion resistance, particularly resistance to general corrosion in acids such as sulfuric acid. Copper is also an austenite former. However, most of the benefit from adding copper can be attained with up to about 2.0 w/o copper, and more than about 1.0 w/o copper can adversely affect pitting resistance. For these reasons and to minimize the cost of the steel, copper is limited to 2.0 w/o maximum, preferably 1.0 w/o maximum.
Nitrogen is a strong austenite former and contributes to the tensile strength and pitting resistance of the steel of this invention. Nitrogen also seems to inhibit the formation of sigma phase. Hence, nitrogen can be present in the steel up to its limit of solubility, which may be up to about 0.4 w/o, provided the steel is not to be welded or heated for a prolonged period at a temperature at which nitrides or carbonitrides could form, i.e., at about 1050 to 1750 F (about 565 to 955 C). In accordance with this invention, the steel contains a minimum of about 0.15 w/o nitrogen. When the steel is to be welded, it is preferred that the steel contain at least about 0.17 w/o, better yet at least about 0.18 w/o, nitrogen to provide enhanced pitting resistance and high levels, i.e., at least about 17%, of austenite in weld areas of the steel, even without subsequent annealing. When the steel is to be welded, it is also preferred that the nitrogen content not exceed about 0.35 w/o, better yet about 0.28 w/o, to avoid porosity in the weld. In this regard, when nitrogen exceeds the stated preferred limits, some of the nitrogen in solid solution can come out of solution during welding and can be trapped during subsequent solidification of the steel. This can produce pores in the weld area, thereby making the weld area prone to corrosion and mechanical failure. Thus, to assure good weldability of the steel by conventional welding techniques, nitrogen in the steel is preferably limited, for example, to: about 0.28 w/o Max. when using autogenous gas tungsten arc (GTA) welding techniques; and about 0.35 w/o Max when using electron beam welding or laser welding techniques.
In the steel of this invention, it is preferred that the austenite formers, nickel, manganese, copper and carbon, not be present in minimum amounts in the steel when the ferrite formers, chromium, silicon and molybdenum, are present in maximum amounts. In this regard, one should not rely on using nitrogen to form austenite in the steel when the remainder of the alloy balance would produce a totally ferritic structure such as would be obtained with a significant excess of ferrite formers, beyond the levels required to produce 100% ferrite. This is because, when the steel is heated (e.g., welded), nitrogen may form chromium nitrides, thereby reducing the amount of nitrogen that is present interstitially in the austenite and that stabilizes the austenite.
Up to about 0.005 w/o boron can be present in the steel of this invention. In this regard, a small but effective amount (e.g., 0.0005 w/o or more) of boron can be used, because it is believed to have a beneficial effect on corrosion resistance, as well as hot workability.
Small amounts of one or more other elements may also be present in the steel because of their beneficial effect in refining (e.g., deoxidizing and/or desulfurizing) the melt. For example, elements such as calcium, magnesium, aluminum and/or titanium, in addition to silicon, can be added to the melt to aid in deoxidizing and also to benefit hot workability as measured by high temperature ductility. When added, the amounts of such elements should be adjusted so that the amounts retained in the steel do not undesirably affect corrosion resistance or other desired properties. Misch metal (a mixture of rare earths primarily comprising cerium and lanthanum) can also be added to the melt for, inter alia, removing sulfur, and its use is believed to have a beneficial effect upon hot workability. However, for that effect, no definite amount of misch metal need be retained in the steel because its beneficial effect is provided during the melting process when, if used, up to about 0.4 w/o, preferably no more than about 0.3 w/o, is added.
In a preferred steel of this invention containing about 0.17 to 0.35 w/o nitrogen, the elements are preferably balanced so that the value of the chromium equivalent ("Cr Eq.") minus the nickel equivalent ("Ni Eq."), calculated by the following equations, is no more than about 16.4, preferably no more than about 15.3: Cr Eq.=Cr w/o+Mo w/o+1.5×Si w/o Ni Eq.=40 (C w/o+N w/o)+Ni w/o+0.5×(cu w/o+Mn w/o).
It is believed that such a value of chromium equivalent minus nickel equivalent can be used to provide a weld of this preferred steel (containing about 0.17 to 0.35 w/o nitrogen) with an austenite content of at least about 17%, as welded. Of course, reasonable care should be taken in welding and then cooling this preferred steel in order to be sure of obtaining at least about 17% austenite in the weld. Nevertheless, a weld can be provided with at least about 17% austenite simply by: (a) welding this preferred steel using techniques conventionally employed in commercial welding of stainless duplex austenitic-ferritic steel tubing or vessels (e.g., by GTA); and (b) then allowing the weld area to cool in any manner that is (i) conventionally used in commercial welding of such steel tubing or vessels and (ii) slow enough so that at least about 17% austenite forms in the weld as the weld cools. However, the cooling of the weld should not be so slow as to cause excessive carbonitride precipation in the weld which could reduce its pitting and/or intergranular corrosion resistance.
It is believed that an austenite content of at least about 17% in a weld of a preferred steel of this invention, containing about 0.17 to 0.35 w/o nitrogen, provides the weld and the high-temperature heat affected zone of the steel, in the as-welded condition, with improved pitting and intergranular corrosion resistance, even without subsequent annealing of the weld area. In this regard, a weld in a preferred steel of the invention can contain up to about 50%, but typically no more than about 25%, austenite in the as-welded condition. Austenite reduces the continuity and the amount of ferrite-to-ferrite grain boundaries in welds of the steel. As a result, austenite reduces the amount and continuity of chromium-rich carbides and carbonitrides which can form at ferrite-to-ferrite grain boundaries in the welds. This prevents the chromium from being depleted from the adjacent ferrite matrix.
However, the advantages of providing at least about 17% austenite in a weld are not confined to the preferred steel of this invention containing about 0.17 to 0.35 w/o nitrogen. The intergranular corrosion resistance of a weld, in the as-welded condition, can be improved by providing at least about 17% austenite in the weld for any stainless duplex ferritic-austenitic steel consisting essentially of about:
______________________________________
Elements w/o
______________________________________


C .01-.03

Mn 3 Max.

Si 1 Max.

Cr 11-30

Ni 3.5-20

Mo 3.5 Max.

Cu 2 Max.

B .005 Max.

N 0.10-0.35

______________________________________
where the balance of the steel is essentially iron. In addition, the pitting resistance of a weld, in the as-welded condition, can be improved by providing at least about 17% austenite in the weld for any of the aforementioned duplex steels wherein nickel is limited to about 3.5 to 5.2 w/o.
Where large ferrite grains with extensive and continuous ferrite-to-ferrite grain boundaries, containing carbides and carbonitrides, are more likely to be formed in the steel of this invention during processing, as in large section-size pieces, it is also preferred that the parent or base metal of the steel have an austenite content of at least about 30%, better yet at least about 40%, up to about 60%. The austenite present in the base metal reduces the tendency to form larger ferritic grains and thereby improves the impact strength and tensile ductility of the steel. As in the case of the weld area, the austenite present also reduces the continuity and amount of the carbides and carbonitrides which can form at ferrite-to-ferrite grain boundaries and thereby improves the pitting and intergranular corrosion resistance of the steel. However, the base metal of the steel of this invention can, if desired, contain somewhat less than the preferred amount of austenite, i.e., down to about 25% austenite.
No special techniques are required in melting, casting and working the steel of this invention. In general, arc melting with argon-oxygen decarburization, is preferred, but other practices can be used. In some instances, an initial ingot, cast as an electrode, can be remelted, or powder metallurgy techniques can be used to provide better control of unwanted constituents or phases. Good hot workability is attained by hot working from a furnace temperature of about 2050 F (about 1120 C), preferably from about 1950 F (about 1065 C), and reheating as necessary. Process annealing is carried out above about 1750 F (about 955 C), preferably at about 1850 to 1950 F (about 1010 to 1065 C), for a time depending upon the dimensions of the article which is then preferably quenched in water.
The steel of this invention is suitable for forming to a great variety of shapes and products for a wide variety of uses, for which Type 329 steel has heretofore been used. The steel of this invention lends itself to the formation of billets, bars, rod, wire, strip, plate or sheet using conventional practices. The steel of this invention is particularly suited to be used in cold rolled, annealed sheet or strip and hot rolled, annealed plate that are to be welded. As compared to Type 329 steel, the steel of this invention has, inter alia: superior resistance to embrittlement when heated at about 700 to 1000 F (about 370 to 540 C) for prolonged periods; higher tensile strength in the base metal; and higher tensile strength in weld areas. As compared to Type 329 steel, the steel of this invention also has superior corrosion resistance, particularly intergranular corrosion and pitting resistance. The steel of this invention has especially superior intergranular corrosion and pitting resistance in weld areas, particularly in the as-welded condition. Moreover, like Type 329 steel, the corrosion resistance in weld areas of the steel of this invention can be improved by annealing to increase the austenite in the weld areas and to dissolve carbides, particularly intergranular carbides. In this regard, the steel of this invention can be annealed at about 1750 to 2050 F (about 950 to 1120 C), preferably about 1825 to 1950 F (about 995 to 1065 C), for as short as a few seconds or up to about 30 minutes, followed by air cooling.
The steel of this invention is advantageously used in the manufacture of tubing for use in heat exchangers or condensers. Because of its good weldability by conventional welding techniques, this steel is suitable for the manufacture of welded tubing, preferably by GTA welding. For some purposes, it is useful to provide this steel in the form of a weld filler wire.
Any minor amounts of sigma phase which may form in a steel of this invention, such as a preferred steel in which the total of chromium w/o plus nickel w/o plus molybdenum w/o is no more than about 34.0 and the total of nickel w/o plus molybdenum w/o is no more than about 7.0, can be removed in a conventional manner such as would be satisfactory for Type 329 steel. In this regard, sigma phase can be removed by heating or heating plus working of the steel followed by rapid cooling (e.g., air cooling of small section-sizes or water quenching of large section-sizes).
The heats A to V used in the examples, which follow, were prepared as small experimental heats, induction melted under argon. Heats A, B, J, N, R, T, U and V were each a steel of this invention ("invent."), and none of the other heats was a steel of this invention. The heats were analyzed as set forth in Table I, below. The tolerances for the anlayses did not exceed: ±0.003 w/o for carbon; ±0.02 w/o for manganese and for silicon; ±0.08 w/o for nickel; ±0.05 w/o for molybdenum; ±0.18 w/o for chromium; ±0.01 w/o for 0.10 to 0.19 w/o nitrogen; and ±0.02 w/o for 0.20 to 0.49 w/o nitrogen.
Each heat was hot worked to form a strip, annealed as required, cold rolled to .125 inch (.3 cm) thickness, annealed in neutral salt at 1850 F (1010 C) for three minutes and then air cooled. The austenite content of the base metal of each strip was determined by x-ray diffraction to ±2% of the reported value. The austenite content of the base metal of each strip is set forth in Table I, below.
Welding of a strip from each heat, when carried out in the examples, was carried out with a GTA apparatus, and after welding, the strip was cooled at a rate which approximated conventional commercial weld-cooling rates. The austenite content of the weld area of each strip was determined by point counting of one typical field using 300 intersections at 500X magnification. The austenite content of the weld of each strip is set forth in Table I, below.
TABLE I
________________________________________________________ __________________
Elements* (w/o) Austenite Heats C Mn Si P S Cr Ni Mo Cu** N Base Metal (%) Weld (%)
________________________________________________________ __________________


A (invent.)

.024

.37

.31

.020

.008

26.19

4.79

1.44

N.A.

.21

45 N.A.

B (invent.)

.024

.38

.32

.021

008

26.47

4.83

1.44

N.A.

.20

40 24

C .056

.40

.32

.020

.008

26.36

4.81

1.44

N.A.

.22

54 30

D .052

.39

.32

.021

.008

27.00

4.86

1.44

N.A.

.20

42 21

E .023

.39

.32

.020

.007

26.55

5.55

1.44

N.A.

.19

48 21

F .026

.38

.32

.021

.007

26.76

5.56

1.44

N.A.

.18

44 20

G .025

.38

.33

.023

.007

26.95

6.13

1.43

N.A.

.16

46 18

H .027

.40

.33

.022

.007

27.11

6.21

1.45

N.A.

.15

44 11

I .025

.42

.32

.021

.008

25.73

4.84

1.43

N.A.

.17

48 N.A.

J (invent.)

.026

.42

.32

.021

.008

26.98

4.82

1.43

N.A.

.15

40 15

K .026

.42

.34

.023

.007

26.64

4.76

1.44

N.A.

.13

36 5

L .025

.42

.32

.021

.007

26.88

4.78

1.42

N.A.

.13

40 6

M .021

.41

.34

.021

.008

27.19

5.47

1.43

N.A.

.15

39 11

N (invent.)

.030

.38

.34

.022

.007

26.54

4.94

1.46

N.A.

.19

40 19

O .023

.40

.35

.020

.008

26.68

6.32

1.47

N.A.

.20

47 22

P .027

.38

.32

.019

.008

27.21

6.20

1.41

N.A.

.20

43 23

Q .026

.39

.32

.017

.007

25.48

4.92

1.43

N.A.

.20

43 N.A.

R (invent.)

.028

.38

.34

.022

.007

27.15

4.76

1.48

N.A.

.20

41 22

S .028

.40

.34

.021

.007

27.06

5.45

1.46

N.A.

.19

43 N.A.

T (invent.)

.021

.42

.36

.019

.008

26.69

4.80

2.36

.02

.21

N.A. N.A.

U (invent.)

.021

.44

.40

.023

.008

26.29

4.70

2.35

.02

.19

N.A. N.A.

V (invent.)

.021

.44

.39

.025

.008

26.25

4.82

2.36

.84

.18

N.A. N.A.

________________________________________________________ __________________
*Oxygen was no more than about .02 w/o. **Copper, when not analyzed ("N.A."), did not exceed about .05 w/o.
EXAMPLE 1
The hardness of strips from certain heats was determined after: a) annealing each strip in salt at 1850 F (1010 C) for three minutes and then air cooling; and b) annealing each strip as in a), followed by heat treating each strip at 1400 F (760 C) for two hours and then air cooling. The results are set forth in Table II, below.
TABLE II
______________________________________
Heat Elements Annealed Treated Cr Ni Mo Hardness Hardness Heats (w/o) (w/o) (w/o) (Rc) (Rc)
______________________________________


R (invent.)

27.15 4.76 1.48 21.8 24.5

J (invent.)

26.98 4.82 1.43 20.5 24.7

B (invent.)

26.47 4.83 1.44 21.9 23.1

N (invent.)

26.54 4.94 1.46 21.1 23.1

S 27.06 5.45 1.46 21.7 31.8

M 27.19 5.47 1.43 21.2 31.5

E 26.55 5.55 1.44 22.7 32.2

F 26.76 5.56 1.44 22.4 32.5

G 26.95 6.l3 l.43 22.0 37.9

P 27.2l 6.20 l.4l 22.6 35.8

H 27.11 6.2l l.45 2l.0 40.5

0 26.68 6.32 1.47 22.3 37.5

T (invent.)

26.69 4.80 2.36 23.3 36.0

U (invent.)

26.29 4.70 2.35 22.2 36.9

V (invent.)

26.25 4.82 2.36 22.8 36.7

______________________________________
Table II shows that, in a preferred steel of this invention in heats B, J, N and R, the use of no more than about 5.2 w/o nickel, a total of chromium w/o plus nickel w/o plus molybdenum w/o of no more than about 34.0, and a total of nickel w/o plus molybdenum w/o of no more than about 7.0 prevents the hardness of the preferred steel from exceeding about Rc 30 when the steel is heated at about 1400 F (760 C) for two hours and then air cooled. This indicates that any sigma phase, which may form in the preferred steel, will not significantly impair the hot workability or the corrosion resistance of the steel and can be removed by conventional heating or heating plus working techniques in making a finished product.
EXAMPLE 2
The intergranular corrosion resistance, as welded, of strips from certain heats was determined in ferric sulfate plus sulfuric acid (ASTM A262-B). The test was significantly more severe than ASTM A262-B, because three periods of 120 hours each were used. Each strip had been welded and machine ground to a 1.25×1×0.125 inch (3.2×2.5×0.3 cm) sample with a 120 grit finish before being tested.
The results are set forth in Tables IIIA and IIIB, below. Corrosion rates were determined in mils per year (MPY) and converted to millimeters per year (MMPY). The depth of attack in the weld and the high-temperature heat affected zone (HAZ), immediately adjacent the weld, was measured in inches, using cross-sections of the weld areas, and converted to centimeters.
TABLE III
________________________________________________________ __________________
Austenite Corrosion Rates In 120 Hour Periods Elements in Weld 1st Period 2nd Period 3rd Period Heats C w/o N w/o (%) (MPY) (MMPY) (MPY) (MMPY) (MPY) (MMPY)
________________________________________________________ __________________


C .056

.22 30 19.4

.49 32.2 .82 56.6 1.44

D .052

.20 21 21.0

.53 44.1 1.12 56.2 1.43

B (invent.)

.024

.20 24 16.4

.42 19.1 .49 22.0 .56

J (invent.)

.026

.15 15 19.9

.51 35.8 .91 46.8 1.19

K .026

.13 5 22.7

.58 38.8 .99 48.9 1.24

L .025

.13 6 22.9

.58 57.0 1.45 81.1 2.06

N (invent.)

.030

.19 19 20.1

.51 29.7 .75 31.2 .79

R (invent.)

.028

.20 22 21.5

.55 26.1 .66 25.0 .64

________________________________________________________ __________________


Depth of Attack After 1st Depth of Attack After 3rd

120 Hr. Period 120 Hr. Period

Weld Weld HAZ HAZ Weld Weld HAZ HAZ

Heats (inches)

(cm) (inches)

(cm) (inches)

(cm) (inches)

(cm)

________________________________________________________ __________________


C .0042

.011 .0065

.017 .0076 .019 .0118 .030

D .0057

.014 .0053

.014 .0118 .030 .0193 .049

B (invent.)

.0025

.006 .0021

.005 .0047 .012 .0069 .018

J (invent.)

.0082

.021 .0086

.022 .0114 .029 .0224 .057

K .0037

.009 .0065

.017 .0155 .039 .0264 .067

L .0074

.019 .0043

.011 .0215 .055 .0396 .101

N (invent.)

.0036

.009 .0024

.006 .0076 .019 .0068 .017

R (invent.)

.0031

.008 .0029

.007 .0060 .015 .0066 .017

________________________________________________________ __________________
Tables IIIA and IIIB show that, in a preferred steel of this invention in heats B, N and R as welded, the use of more than about 0.15 w/o (i.e., at least about 0.17 w/o) nitrogen and no more than about 0.03 w/o carbon and the presence of more than about 15% (i.e., at least about 17%) austenite in the weld provides improved intergranular corrosion resistance in the weld and the heat affected zone of the steel, particularly after the third period of exposure to ferric sulfate plus sulfuric acid.
EXAMPLE 3
The general corrosion resistance of strips from certain heats was determined in boiling 65 w/o nitric acid for five 48 hour periods (ASTM A262-C). The test was significantly more severe than ASTM A-262C, because the nitric acid contained 0.5 g/l potassium dichromate so that it provided a severe oxidizing environment such as is found in nitric acid heated above its atmospheric boiling point (e.g., in a nitric acid cooler-condenser). Each strip had been hand ground to an approximately 1.5×0.5×0.125 inch (3.8×1.3×0.3 cm) sample with a 120 grit finish before being tested.
The results are set forth in Tables IVA and IVB, below, for duplicate test strips. Corrosion rates were determined in mils per year (MPY) and converted to millimeters per year (MMPY).
TABLE IV
________________________________________________________ __________________
Corrosion Rates In 48 Hour Periods Elements 1st Period 2nd Period 3rd Period Heats Cr w/o N w/o (MPY) (MMPY) MPY (MMPY) (MPY) (MMPY)
________________________________________________________ __________________


Q 25.48

.20 581/554

14.8/14.1

284/353

7.2/9.0

474/523

12.0/13.3

I 25.73

.17 297/281

7.5/7.1

407/365

10.3/9.3

246/242

6.2/6.1

A (invent.)

26.19

.21 257/243

6.5/6.2

392/356

10.0/9.0

456/437

11.6/11.1

J (invent.)

26.98

.15 75/77

1.9/2.0

283/272

7.2/6.9

208/199

5.3/5.1

________________________________________________________ __________________


Corrosion Rates In 48 Hour Periods

Average of

4th Period

5th Period

Periods Tested

Heats (MPY)

(MMPY)

(MPY)

(MMPY)

(MPY)

(MMPY)

________________________________________________________ __________________


Q 337/469

8.6/11.9

198/192

5.0/4.9

375/418

9.5/10.6

I 419/355

10.6/9.0

435/532

11.0/13.5

361/355

9.2/9.0

A (invent.)

300/308

7.6/7.8

146/154

3.7/3.9

311/300

7.9/7.6

J (invent.)

164/134

4.2/3.4

318/319

8.1/8.1

210/200

5.3/5.1

________________________________________________________ __________________
Tables IVA and IVB show that, in the steel of this invention in heats A and J, the use of at least about 26.0 w/o chromium provides improved general corrosion resistance.
EXAMPLE 4
The pitting resistance of strips from certain heats was determined in 6 w/o ferric chloride (solution from ASTM-G48). The tests were carried out at 40 C, and each strip was immersed in 150 ml of ferric chloride solution for 72 hours. Each strip had been welded, annealed at 1850 F (1010 C) for 10 minutes, air cooled and then machine ground to a 1.25×1×0.125 inch (3×2.5×0.3 cm) sample with a 120 grit finish before being tested.
The results are set forth in Table V, below, for duplicate test strips. Corrosion rates were determined in milligrams per square centimeter (Mg/cm 2 ). No strip was observed to have suffered preferential attack in its weld area.
TABLE V
______________________________________
Elements Corrosion C Cr Ni N Rates Heats w/o w/o w/o w/o (Mg/cm 2 )
______________________________________


B (invent.)

.024 26.47 4.83 .20 0/0

K .026 26.64 4.76 .13 1.4/2.0

L .025 26.88 4.78 .13 2.3/3.0

N (invent.)

.030 26.54 4.94 .19 .1/1.3

R (invent.)

.028 27.15 4.76 .21 1.2/0

______________________________________
Table V shows that, in a preferred steel of this invention in heats B, N and R as welded plus annealed, the use of more than about 0.13 w/o (i.e., at least about 0.17 w/o) nitrogen provides improved pitting resistance.
EXAMPLE 5
The pitting resistance of strips from certain heats was determined in 6 w/o ferric chloride at 22 C for three days (ASTM-G48). Unlike ASTM-G48, each strip was immersed in 150 ml of ferric chloride solution in the tests. Each strip had been welded and then machine ground as in Example 4 before being tested.
The results are set forth in Table VI, below, for duplicate test strips. Corrosion rates were determined in milligrams per square centimeter (Mg/cm 2 ). The test strips also were visually compared at the end of the tests to determine the relative extent of pitting which had been suffered. The pitting resistance of the strips was rated either good (G), moderate (M) or bad (B) from this visual comparison.
TABLE VI
______________________________________
Auste- Elements nite in Corrosion C Cr Ni N Weld Rates Visual Heats w/o w/o w/o w/o (%) (Mg/cm 2 ) Ratings
______________________________________


B .024 26.47 4.83 .20 24 1.7/2.6 G

(invent.)

J .026 26.98 4.82 .15 15 13.4/14.0

B

(invent.)

E .023 26.55 5.55 .19 21 .8/.9 G

F .026 26.76 5.56 .18 20 1.1/2.9 G

M .021 27.19 5.47 .15 11 7.7/8.7 B

G .025 26.95 6.13 .16 18 1.3/2.3 G/M

H .027 27.11 6.21 .15 11 6.9/8.1 B

______________________________________
Table VI shows that, in a stainless duplex ferritic-austenitic steel such as the steel of this invention, as welded, the presence of at least about 17% austenite in the weld provides improved pitting resistance in the weld areas of the steel. Table VI also shows that, in a steel of this invention in heats B and J as welded, the use of more than about 0.15 w/o (i.e., at least about 0.17 w/o) nitrogen and the presence of more than about 15% (i.e., at least about 17%) austenite in the weld is preferred to provide improved pitting resistance in the weld areas of the steel.
The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
_________________________

Duplex and super duplex usage increases

During the last twenty years there has been a large increase in the use of duplex and super duplex stainless steels. These are the "second generation" of duplex stainless steels. These grades are distinguished from the first generation by their ability to retain a good balance of austenite and ferrite, and thereby toughness and corrosion resistance, in the welded condition. This improvement results from the use of nitrogen as an intentional and precisely controlled alloying addition.

Second generation duplex stainless steels have enjoyed wide acceptance because they offer excellent combinations of strength, pitting and crevice corrosion resistance as well as chloride stress corrosion cracking resistance, for a very economical cost. As fabrication experience with these duplex alloys increases, it has become recognized that these technically complex duplex grades require increased qualification and care in production and fabrication to assure safe and economical results.

Use of duplex and super duplex increases

During the last twenty years there has been a large increase in the use of duplex and super duplex stainless steels. These are the "second generation" of duplex stainless steels. These grades are distinguished from the first generation by their ability to retain a good balance of austenite and ferrite, and thereby toughness and corrosion resistance, in the welded condition. This improvement results from the use of nitrogen as an intentional and precisely controlled alloying addition. Second generation duplex stainless steels have enjoyed wide acceptance because they offer excellent combinations of strength, pitting and crevice corrosion resistance as well as chloride stress corrosion cracking resistance, for a very economical cost. As fabrication experience with these duplex alloys increases, it has become recognized that these technically complex duplex grades require increased qualification and care in production and fabrication to assure safe and economical results.

Tuesday, 20 April 2010

Super duplex valves: the stars at desalination plants

From Intoco f55superduplex.co.uk:

The Super Duplex material is NACE and NORSOK approved and has a PREN (pitting resistance equivalent number - an empirical relationship to predict the pitting resistance of austenitic and duplex stainless steels) of 40. Super Duplex grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due to the further additions of chromium, molybdenum, and nitrogen to these grades. It also conforms, or is similar to, EN / DIN (Werkstoff) 4501 / 1.4501 / X2CrNiMoCuWN25.7.4 / UNS 32760 / S32760, UNS - ASTM A276 S32760 / AISI F55 / GOST 12Kh13 / Afnor Z3CND25.06Az and branded steel Zeron 100. Also 1.4410 / UNS S32550 / UNS S32750 / Z3CNDU25.07Az / SS2328 / X2CrNiMoCuN25.6.3 / SAF 2507 SANMAC / Uranus 52N.

Super Duplex 1.4501 / 32760 / F55 Stainless steel Acier, inoxydable, Rostfreier, Stahl, Acciaio, Inossidabile, Aco, Inoxidável, Acero, Inox, Inoxidables, Rostfrei, Roestvrij materials get their name, Super Duplex , because they contain both ferritic and austenitic microstructure. They have a high chrome / chromium content (25%) and a moderate nickel content (7%). They also have 3 to 4 % Moly./ Molybdenum. At this level, F55 / UNS 32760 / 1.4501 materials, are too low in nickel to produce a fully austenitic structure, thus producing the Super Duplex microstructure (ferrite and austenite). The main advantage of Super Duplex F55 / UNS 32760 / 1.4501 stainless steel is the combination of properties given by both the austenitic and ferritic structure (austeno-ferritic).

Super Duplex stainless steels Acier, inoxydable, Rostfreier, Stahl, Acciaio, Inossidabile, Aco, Inoxidável, Acero, Inox, Inoxidables, Rostfrei, Roestvrij to F55 / UNS S32760 / EN / DIN (Werkstoff) 4501 / 1.4501 / X2CrNiMoCuWN25.7.4 / UNS 32760 / UNS - ASTM A276 S32760 / AISI F55 / GOST 12Kh13 / Afnor Z3CND25.06Az and branded steel Zeron 100 have excellent corrosion resistance, increased resistance to chloride attack, good resistance to stress corrosion cracking, tensile and yield strength higher then conventional austenitic or ferritic grades of stainless steel, good weldability and good formability.

Stainless steel Super Duplex grade F55 / UNS S32760 / EN / DIN (Werkstoff) 4501 / 1.4501 / X2CrNiMoCuWN25.7.4 / UNS 32760 / UNS - ASTM A276 S32760 / AISI F55 / GOST 12Kh13 / Afnor Z3CND25.06Az and branded steel Zeron 100, can be used for heat exchangers, chemical tanks, refineries, shafts (marine, etc) pressure vessel parts, flanges, fittings & pipes for the oil and gas industries and offshore technology, paper industry, compressor parts and seawater desalination plants amongst many other applications. Intoco offer Super Duplex materials in round bar (rolled / forged) and sheet / plate, complete with the stainless mill certs conforming to EN10204/3.1 & 3.2 if required with certification supplied by Lloyd's Register, etc. ASTM/AISI F51 / UNS 31803 / 1.4462 Super Duplex stainless steel Acier, inoxydable, Rostfreier, Stahl, Acciaio, Inossidabile, Aco, Inoxidável, Acero, Inox, Inoxidables, Rostfrei, Roestvrij.

The above text is given as an overview of Super Duplex Stainless Steel and is not to be relied upon for a specification.