What Is 6Mo Stainless Steel?

Quick Answer

6Mo stainless steel refers to super austenitic grades containing approximately 6% molybdenum, achieving a PREN above 40; comparable to super duplex stainless steel. The two most common 6Mo grades are 254 SMO (UNS S31254) and AL-6XN (UNS N08367). These alloys bridge the gap between standard austenitic 316L (PREN ~26) and nickel alloys like Inconel 625 (PREN >50) for seawater and chloride service.

Common 6Mo Grades

Property	254 SMO (S31254)	AL-6XN (N08367)
Chromium	19.5-20.5%	20.0-22.0%
Nickel	17.5-18.5%	23.5-25.5%
Molybdenum	6.0-6.5%	6.0-7.0%
Nitrogen	0.18-0.22%	0.18-0.25%
Copper	0.50-1.00%	0.75% max
PREN	42-44	44-46
Tensile (min)	650 MPa (94 ksi)	655 MPa (95 ksi)
Yield (min)	300 MPa (44 ksi)	310 MPa (45 ksi)
Elongation (min)	35%	30%
Max service temp	300°C (572°F)	300°C (572°F)
Cryogenic rating	-196°C	-196°C
Microstructure	Fully austenitic	Fully austenitic

6Mo vs Duplex vs 316: Positioning

Property	316L	Duplex 2205	6Mo (254 SMO)	Super Duplex 2507
PREN	~26	~35	~43	~42
Yield (min)	205 MPa	450 MPa	300 MPa	550 MPa
SCC resistance	Moderate	Excellent	Excellent	Excellent
Cryogenic service	Yes (-254°C)	Limited (-40°C)	Yes (-196°C)	Limited (-40°C)
Impact toughness	Excellent	Good	Excellent	Good
Sigma phase risk	None	Yes (>315°C)	None	Yes (>315°C)
Relative cost	1x	1.3x	2.5x	2x

6Mo grades are preferred over duplex when cryogenic service, superior impact toughness at low temperatures, or immunity to sigma phase embrittlement is required. Duplex grades offer higher strength at lower cost for ambient-temperature chloride service.

Applications

• Seawater cooling systems: heat exchanger tubes, piping, and valve bodies on offshore platforms
• Desalination: reverse osmosis HP piping and evaporator circuits
• Pulp and paper: chlorine dioxide bleaching systems
• Flue gas desulfurization: scrubber piping and outlet ducting
• Chemical processing: chloride-bearing acids, bleach production
• LNG and cryogenic: piping components where chloride resistance is needed below -40°C

ASTM Specifications

Product Form	254 SMO Spec	AL-6XN Spec
Seamless pipe	A312 (S31254)	B690 (N08367)
Welded pipe	A358 (S31254)	B690 (N08367)
Forgings	A182 (F44)	B564 (N08367)
Plate	A240 (S31254)	B688 (N08367)
Castings	A351 (CK3MCuN)	A494 (CK3MCuN)

Testing

6Mo grades require solution annealing (1150-1200°C, water quench). Mill test certificates per EN 10204 3.1 must report chemistry (Mo and N are critical), mechanical properties, and corrosion test results per ASTM G48 (ferric chloride pitting at 50°C minimum). NDT and hydrostatic testing follow the applicable piping standard.

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