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.

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