North
American
Version
Imperial Units
Type 254 SMO® Comparative
to Other 6Mo Stainless Grades
This brochure briefly describes the many similarities and few
differences between Outokumpu 254 SMO®1 alloy and other
6Mo stainless steel “alloys”, such as the ATI Allegheny Ludlum
AL-6XN®2 alloy. It is based on Outokumpu’s 30+ years of
experience with developing, producing, and marketing of 6%
molybdenum-containing stainless steels. While extensive, this
brochure does not provide all detailed information that has been
published in applicable literature or observed by Outokumpu
over the years with respect to 6Mo stainless steels. For additional
details regarding such information as references, testing
standards, and results, please contact Outokumpu.
254 SMO and Most Other 6Mo Grades
are Essentially Equivalent
The physical properties of 6Mo grades, including
both the 254 SMO®1 and AL-6XN®2 alloys
are essentially equivalent as described herein.
Because of the similarities among the 6Mo grades,
they may be used interchangeably, may be used
together and the selection of a specific grade
should be based on availability, price, and service.
254 SMO: The Leanest* 6Mo Stainless
on the Market
The 6Mo stainless steels, whether 254 SMO
(S31254), AL-6XN (N08367), or 1925 hMo
and 25-6Mo (both covered by N08926), are
substantially similar with respect to significant
performance characteristics.3 The chemical
composition of the 254 SMO stainless steel
typically has a slightly higher copper content than
the AL-6XN alloy (Table 1). The original Avesta
Jernverk AB (predecessor to Outokumpu) patent4
on 254 SMO was based, to some extent, on
data that demonstrated that 0.5-1.0% copper
in a 6Mo stainless steel produced an optimal
combination of resistance to reducing acids and
resistance to chlorides.4 The AL-6XN alloy
contains copper only as a residual element, as
do most austenitic stainless steels. Based on
Outokumpu’s experience, most austenitic
stainless steels have a residual copper content of
about 0.20%.
*“Leanest” meaning lowest Ni content of all the 6Mo grades
listed in reference #3.
In practice, there will be only a small difference
between the AL-6XN and 254 SMO alloys with
respect to copper. The N08926 alloy requires a
copper content of 0.50-1.50%.5 The effect of the
different concentrations of copper on corrosion
resistance would be difficult to detect in laboratory
corrosion tests and are not considered to be
statistically significant in any application, other
than reducing acid environments, such as the
intermediate concentration range of pure sulfuric
acid, depending on the exact exposure and
evaluation criteria.4
Composition, wt. pct
Element
Carbon
Chromium
Nickel
Molybdenum
Nitrogen
Copper
Sulfur
Phosphorus
Silicon
Manganese
Iron
Outokumpu
254 SMO®
(wrought products)
0.020 max
19.5 - 20.5
17.5 - 18.5
6.0 - 6.5
.018 - 0.22
0.50 - 1.00
0.010 max
0.030 max
0.80 max
1.00 max
Balance
Table 1
ATI Allegheny Ludlum
Typical AL-6XN®
0.02
20.5
24.0
6.2
0.22
0.2
0.001
0.020
0.40
0.40
Balance
** See producers’ websites for published alloy surcharges.
The most notable difference in composition
between the 254 SMO and AL-6XN alloys is
the nickel content (Table 1). The 254 SMO
alloy contains about 6% less nickel than the
AL-6XN, which results in a negligible difference
in performance6, yet often achieves a significant
savings in cost due to the volatility in nickel
prices**.
The 254 SMO alloy was the first nitrogen-alloyed
6Mo grade stainless steel.6,7 Avesta Jernverks
AB was granted a patent for such composition
that required various other elements.4 When
other producers decided to produce a nitrogencontaining 6Mo steel, the use of a Ni content higher
than that in the 254 SMO alloy avoided conflict
with the patent without substantial detrimental
effects.8
As shown in Figure 1, the most likely performance
characteristic that is potentially affected by the
lower 254 SMO alloy nickel content is the resistance
to chloride stress-corrosion cracking.9 However,
laboratory testing has failed to demonstrate
any detectable influence on the chloride stress
corrosion cracking resistance due to the 6%
nickel content difference. Both the 254 SMO
and AL-6XN alloys will pass the wick test10 and
the boiling 25% NaCl test for SCC resistance,
yet neither will pass the boiling 42% magnesium
chloride test (Table 2).
In the sophisticated drop-evaporation test, it is
possible to make either the 254 SMO alloy or the
AL-6XN alloy look superior, depending on precise
conditions. However, any differences in such
drop-evaporation tests are not believed to be
statistically significant.11,12,13
Chloride Stress Corrosion
Cracking Resistance
Grade
Boiling
Wick
42% MgCL2 Test
Table 2
Boiling
25% NaCl
Outokumpu
254 SMO®
Fail
Pass
Pass
ATI Allegheny
Ludlum AL-6XN®
Fail
Pass
Pass
Outokumpu
Fail
2205 Code Plus Two®
Pass
Pass
Alloy 20
Fail
Pass
Pass
Alloy 904L
Fail
Pass or
Fail
Pass
or Fail
2 - Type 254 SMO® Comparison
The time to failure in boiling
magnesium chloride solution of
stainless steel wire as a function of Ni
content
Figure 1
Isocorrosion Curves 0.1 mm/year for given steels
in pure sulfuric acid
Figure 2
Note: While this data seems to indicate a small advantage for the AL-6XN alloy over the 254 SMO alloy in this specific
environment, the data has not been produced under exactly the same conditions. Thus, small differences in test procedures
and sampling may account for the minor differences in test results.
Outokumpu has observed that when one 6Mo
alloy is not satisfactory for use in a particular
application due to susceptibility to SCC, then
selection of the other 6Mo alloys for that
application would be an exceedingly risky
proposition. The relative level of SCC resistance
of the 6Mo alloys, which corresponds to resistance
in boiling NaCl solutions, but susceptiblein boiling
MgCl2, is the same level of relative resistance for
the 20Cb-3®14 alloy, 904L, the duplex grades
2205 and 2507, and any ferritic stainless steel with
nickel content in the range of 1% or higher, such
as the SEA-CURE®15 alloy.10
Based on the Copson curve (see Figure 1), a
minimum of 34% nickel content is required to
pass the magnesium chloride test.
Pitting and Crevice Corrosion
Based on published results on pitting and crevice
corrosion resistance16,17 “lab” data give little
concrete evidence to choose between these alloys.
A study by NASA18 to evaluate the resistance of
the 254 SMO and AL-6XN alloys to chloride-
bearing launch environments found both alloys
performed substantially better than 304 stainless
steel and the 254 SMO alloy performed slightly
better than the AL-6XN alloy. Support for the
similar performance comes from the very similar
Pitting Resistance Equivalent number (PREn)
for these alloys. The PREn employs statistical
regression to relate pitting resistance to the
chemical composition of a stainless steel.19
The PREn calculations from various investigators
have demonstrated that the pitting resistance
depends primarily on the level of Cr, Mo, and N
content. It has also been shown that nickel has
very little statistically detectable effect on pitting
corrosion resistance over the full range of austenitic
stainless steels.20 The sensitivity of the PREn data
is such that within grade variations of Cr, Mo, and
N near the nominal values for these stainless steels,
the apparent PREn variation is of minor import
compared to effects from surface finish, normal
variations in practical annealing conditions, and
variations in the corrosiveness of the environment.
These effects will overshadow any apparent
differences in PREn values for the nominal
compositions that are available in the various
6Mo alloys. In view of the above, Outokumpu
recommends a conservative approach to use the
standardized minimums for Cr, Mo, and N for the
6Mo alloys in the PREn calculation, and concludes
that there are no statistically significant differences.
In oil & gas production environments, the 254
SMO, AL-X6N, and other 6Mo alloys have been
extensively researched and compared and are all
typically specified interchangeably.21
Welding
High amounts of nickel in lower alloyed
austenitic materials have been shown to increase
the tendency for hot cracking due to the mode
of solidification of the weld metal.22 Low nickel
levels favor either complete solidification as
primary ferrite (termed Type F), or solidification
of primary ferrite followed by some austenite
formation (termed Type FA).22 Higher levels
of nickel favor solidification either as primary
austenite followed by the formation of some ferrite
(termed Type AF) solidification, or complete
Type 254 SMO® Comparison - 3
Isocorrosion Diagrams,
Corrosion rate 0.1 mm/yr,
in hydrochloric acid
Figure 3
austenitic solidification (termed Type A).22 Ferrite
formation during solidification has been shown to
improve hot workability and cracking resistance.22
Requirements for some 6Mo stainless steels
according to ASTM A240-09a
Table 3
254 AL 16 UNS
SMO 6XN N08926
45
45
45
45
43
43
Tensile Strength, Sheet and
min (kpi)
Strip Plate
100
95
100
95
94
94
Elongation in
2”, %
35
35
30
30
35
35
Yield Strength,
min (kpi)
4 - Type 254 SMO® Comparison
Sheet and
Strip Plate
Sheet and
Strip Plate
At Room Temperature
The lower nickel content in the 254 SMO alloy, as
compared to other 6Mo alloys, was designed to
simultaneously precipitate ferrite and austenite
from the melt, resulting in better hot workability.
Other 6Mo grades that contain a higher nickel
content than the 254 SMO alloy generally solidify
entirely as primary austenite and as a result do not
have the same improved hot workability.4 This
solidification mode also has the potential to increase
the resistance to hot cracking during welding.4
Property
However, this potential advantage will only be
realized with autogenous welds, which typically
with 6Mo steels are only used in conjunction with
a post weld solution anneal.16 Welding the 254
SMO alloy with a nickel alloy filler would not
result in any advantage over other 6Mo steels due
to lower nickel.
Tensile Strength
Minimum Tensile Test
Physical Properties
There is different strength data published for the
various 6Mo grades (Table 3).5 However, the
apparent differences in strength data result from an
apples-to-oranges comparison. In other words, the
strength data specifically relates to product form
and the original data developed for the grades. The
strength data for the 254 SMO alloy was originally
developed for thick plate because that was the
initial product of interest. The strength data for
the AL-6XN alloy was originally developed for
light gauge sheet and strip, because tubing was the
initial product form.7 As a result, the basis for the
strength data for the AL-6XN alloy is different
than the basis for the strength data of the 254
SMO alloy, and therefore not directly comparable.
For many years, Outokumpu followed the
traditional and more conservative approach that
quotes a single minimum value for yield and
tensile strengths at all thicknesses. However,
in order to address market concerns about
performance, Outokumpu introduced a higher
strength quote for sheet gauges of the 254 SMO alloy.
Table 4
Outokumpu
254 SMO®
ATI
Allegheny
AL-6XN®
Modulus of elasticity
psi x 106
29
29
Coefficient of thermal
expansion
(68˚F to 212˚F) x 10-6/˚F
8.9
7.9
Thermal Conductivity
Btu/h ft˚F
7.5
7.5
Heat Capacity Btu/lb˚F
0.120
0.11
Density lb/in3
0.287
0.291
Magnetic Permeability
1.003
1.0028
The N08926 alloy has a composition that most
closely resembles the AL-6XN alloy (N08367) and
continues to use the conservative single strength
value for all thicknesses which is the same strength
quote originally quoted for the 254 SMO alloy
(S31254).5
Designations
The AL-6XN alloy was originally listed in the
ASTM and ASME B-specs, rather than A-specs
that listed nickel-base alloys, because ASTM
formerly defined “stainless steel” in a way that
excluded the 6Mo grades other than the 254 SMO
alloy.23 The old rule stated that in a stainless
steel, iron had to be at least 50% by weight of
the alloying additions.24 In the last ten years,
the ASTM has harmonized its steel definitions
with the rest of the world.23 As a result, the AL6XN alloy is now listed as a stainless steel in the
ASTM A-specs. The new rule states that iron is
the element with the largest weight percentage.24
Basically, the ASTM “grandfathered” the
specifications for nickel-base alloys (such as
the AL-6XN alloy, the 904L alloy, and many
other 6Mo stainless steels with original UNS
N-numbering), stating its intent to maintain
these specifications for a period of about ten
years for the convenience of previous users and
to withdraw these specifications—or at least the
grades that are now considered as stainless steels
and covered in the A-specs.23 As a result, the
AL-6XN alloy has been introduced into most of
the same specifications that the 254 SMO alloy
has been in for over twenty years. Outokumpu
believes that there is no significance to the original
specifications or to the changes, except that it is
now convenient to specify both grades using the
same standard specifications, thereby facilitating
the best service to the user. The knowledgeable
user will specify both grades as acceptable
alternatives.
The physical properties of 6Mo grades, including
both the 254 SMO and AL-6XN alloys, are
essentially equivalent (Table 4). Because of the
similarities among the 6Mo grades, they may be
used interchangeably3, may be used together and
the selection of a specific grade should be based on
availability, price, and service.
Technical Support
Outokumpu assists users and fabricators in the
selection, qualification, installation, operation,
and maintenance of the 254 SMO stainless steel.
Technical personnel, supported by the research
laboratory of Outokumpu, can draw on years of
field experience with the 254 SMO alloy to help
you make the technically and economically correct
materials decision. Outokumpu is prepared to discuss
individual applications and to provide data and
experience as a basis for selection and application
of the 254 SMO alloy.
Outokumpu works closely with its distributors to
ensure timely availability of the 254 SMO alloy
in the forms, sizes, and quantities required by the
user. For assistance with technical questions and
to obtain top quality 254 SMO products, call
Outokumpu at 1-800-833-8703.
Type 254 SMO® Comparison - 5
References
1
254 SMO is a trademark of Outokumpu OYJ, registered in the United States and other countries.
2
AL-6XN is a trademark of ATI Properties, Inc., registered in the United States.
3
Ralph M. Davison and James D. Redmond, Practical Guide to Using 6Mo Austenitic Stainless Steel, Material
Performance, vol. 27, Number 12, December 1988, pp 39 – 43.
4
United States Patent Number 4,078,920, Austenitic Stainless Steel with High Molybdenum Content, Liljas et al,
March 14, 1978.
5
ASTM A240/240M, Standard Specification for Chromium, and Chromium-Nickel Stainless Steel Plate, Sheet, and
Strip for Pressure Vessels and for General Applications, ASTM International, West Conshohocken, PA.
6
Mats Liljas, Development of Superaustenitic Stainless Steels, ACOM 1-1995, Avesta Sheffield AB, Avesta, Sweden.
7
CASTI Handbook of Stainless Steels & Nickel Alloys, Stephen Lamb Technical Editor, CASTI Publishing Inc.
Edmonton, Alberta, 1999.
8
United States Patent Number 4,545,826, Method For Producing A Weldable Austenitic Stainless Steel in Heavy Sections,
Thomas H. McCunn, John P. Ziemianski, and Ivan Franson, October 1985.
9
H. R. Copson, Effect of Composition on Stress Corrosion Cracking of Some Alloys Containing Nickel, Physical Metallurgy
of Stress Corrosion Fracture, T.N. Rhodin, Editor, Interscience, 1959, pp 247 – 272.
10
Corrosion of Stainless Steels, Second Edition, A. John Sedriks, John Wiley & Sons, Inc., 1996, pp 293.
11
Poul-Erik Arnvig and Wioletta Wasielewska, Stress Corrosion Behaviour of Highly Alloyed Stainless Steels Under Severe
Evaporative Conditions, ACOM 3-1993, Avesta Sheffield, Avesta, Sweden.
12
Helle Anderson, Poul-Erik Arnvig, Wioletta Wasielewska, Lena Wegrelius, and Christian Wolfe, SCC of Stainless Steel
Under Evaporative Conditions, ACOM 3-1998, Avesta Sheffield, Avesta, Sweden.
13
Unpublished work by Poul-Erik Arnvig.
14
20Cb-3 is a trademark of CRS Holdings, Inc., registered in the United States.
15
SEA-CURE is a trademark of Plymouth Tube Company, registered in the United States.
16
ATI AL-6XN® Alloy (UNS N08367) Sourcebook, Ed. 4, ©2010 ATI Allegheny Ludlum.
17
Corrosion Handbook, Outokumpu Oyj, Tenth Edition, 2009.
18
L.M. Calle, M.R. Kolody. R.D. Vinje, M.C. Whitten, and W. Li, Electrochemical Impedance Study of Alloys in a Simulated
Space Shuttle Launch Environment, NASA Government Publication 153, (http://corrosion.ksc.nasa.gov/pubs/153.pdf )
19
ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing, and Protection, Stephen D. Cramer and Bernard S.
Covino, jr., Volume Editors, ASM International, Materials Park, Ohio 2003, pp 266 – 274.
20
Elisabet Alfonsson and Rolf Qvarfort, Investigation of the Applicability of Some PRE Expressions for Austenitic Stainless
Steels, ACOM 1-1992, Avesta AB, Avesta Sweden, 1991.
21
International Standard NACE MRO175/ISO 15156-1:2001
22
Welding Metallurgy and Weldability of Stainless Steels, John C. Lippold and Mamian J. Kotecki, Wily Interscience
2005, pp 173 – 189.
23
Discussions with Ralph Davison former Chairman of the ASTM A1.17 Subcommittee.
24
ASTM A941-06, Standard Terminology Relating to Steel, Stainless Steel, Related Alloy, and Ferroalloys, ASTM
International, West Conshohocken, PA.
6 - Type 254 SMO® Comparison
Type 254 SMO® Comparison - 7
We work with our customers and partners
to create long lasting solutions for the tools
of modern life and the world’s most critical problems:
clean energy, clean water and efficient infrastructure.
Because we believe in a world that lasts forever.
Information given in this brochure may be subject to alterations without notice. Care has been taken to ensure that the contents
of this publication are accurate but Outokumpu and its affiliated companies do not accept responsibility for errors or for
information which is found to be misleading. Suggestions for or descriptions of the end use or application of products or methods
of working are for information only and Outokumpu and its affiliated companies accept no liability in respect thereof. Before using
products supplied or manufactured by the company the customer should satisfy himself of their suitability.
254 SMO® is a registered trademark of Outokumpu Stainless.
2205 Code Plus Two® is a registered trademark of Outokumpu Stainless, Inc.
Outokumpu High Performance Stainless
2275 E. Half Day Road, Suite 300, Bannockburn, IL 60015 USA
Tel. 1-847-317-1400 Fax 1-847-317-1404
outokumpu.com
1245EN, Bannockburn, USA. October 2014. Edition 3 (US)
Working towards forever.