431 Stainless Steel

431 stainless steel is a high chromium-low nickel high hardenability Martensitic stainless steel with high strength and good corrosion resistance, as generally supplied hardened and tempered in the tensile range 850 – 1000 Mpa (condition T) Brinell range 248 – 302. Characterised by very good corrosion resistance in general atmospheric corrosive environments, good resistance to mild marine and industrial atmospheres, resistant to many organic materials, nitric acid and petroleum products coupled with high tensile and high yield strength plus excellent toughness in the hardened and tempered condition.

431 due to its excellent hardenability is capable of being through hardened up to Rc44, depending upon carbon content and section size. Small sections can be air cooled and larger sections oil quenched for maximum through hardness. Pre-hardened and tempered 431 will also respond readily to nitriding achieving a typical surface hardness of over Rc65. The nitriding process however reduces the corrosion resistance and is therefore not generally recommended except for critical applications where the benefit outweighs all other considerations.

Used extensively for parts requiring a combination of high tensile strength, good toughness and good corrosion resistant properties. Typical applications are: Aircraft Parts and Components, Bolts and Nuts, Fasteners, Pump Shafts, Propellor Shafts, Studs, Valve Parts etc.. Material magnetic in all conditions.

TABLE

431 Martensitic Stainless Steel Bar / Interlloy / Engineering Steels & Alloys

Min % Max %
Carbon 0.12 0.20
Silicon 0 1.00
Manganese 0 1.00
Nickel 1.25 3.00
Chromium 15.00 18.00
Phosphorous 0 0.04
Sulphur 0 0.03

 

  • Carbon range can vary considerably
  • Nickel addition optional

Mechanical Property Requirements For Material in the Annealed and Heat Treated – Condition T To AS2837 – 1986 431 and BS970 Part 3 1991 431S29

Condition Annealed *T
Tensile Strength (MPa) Min 850
Max 1000
0.2% Yield Strength (MPa) Min 635
Elongation on 5.65√S0 % Min 11
Izod Impact Valua J mm Min 63 34
63 20
Hardness HB Min 248
Max 277 302

 

  • Material stocked generally in condition T.
  • NB Check the mill certificate if critical for end use.

Typical Mechanical Properties At Room Temperature - *Hardened and Tempered to Condition T

Tensile Strength (MPa) 940
0.2 Yield Strength (MPa) 750
Elongation in 50mm % 19
Impact Izod J 65
Hardness (HB) 280
Hardness (Rc) 30
*Typical Hardening Temperatures 980°C - 1020°C
*Typical Tempering Temperatures 640°C - 660°C
590°C - 610°C

 

Typical Mechanical Properties At Room Temperature – Hardened By Oil Quench at 980ºC and Tempered as Indicated

Tempering Temperature °C 250 370 480 590 650
Tensile Strength (MPa) 1370 1390 1410 980 920
0.2% Yield Strength (MPa) 1030 1130 1200 790 690
Elongation in 50mm % 16 16 16 19 20
Impact Charpy J 54 *34 *16 65 70
Hardness (HB) 410 420 425 295 270
Hardness (Rc) 44 45 46 32 29

 

High tensile strength and high yield strength with slightly lower impact properties when tempered below 370ºC. Section size 30mm

*Note drop in impact properties Tempering within the range 370ºC – 565ºC should be avoided.

Elevated Temperature Properties

431 displays good resistance to scaling in continuous service up to 700ºC. Its use however at these higher working temperatures results in a substantial drop in tensile strength and hardness, with subsequent increase in ductility.

Typical Mechanical Properties at Elevated Temperatures, Hardened at 1010ºC and Tempered at 30ºC Above Working Temperature

Tempering Temperatue °C 510 570 620
Working Temperature °C 480 540 590
Tensile Strength (MPa) 1350 720 435
Elongation in 50mm % 15 20 26
Room Temperature Hardness after Test (HB) 440 330 280
Room Temperature Hardness after Test (Rc) 47 37 30

 

NB: Creep and stress rupture strength is also substantially reduced at these higher working temperatures.

Low Temperature Properties

431 stainless steel is not recommended for use at sub-zero temperatures due to a substantial drop in impact properties consistent with most steels other than the austenitic steel types.

Cold Bending

In the hardened and tempered as supplied condition will be extremely difficult due to the high yield strength and is not generally recommended.

Hot Bending

In the hardened and tempered as supplied condition it is not recommended due to its affect on the mechanical properties within the heat affected zone.

Corrosion Resistance

431 has the highest corrosion resistance of all the Martensitic stainless steels, and while not as high as the austenitic stainless steels it is in certain corrosive environments similar to that of 301 and 302 grades. NB it has optimum corrosion resistance in all environments in the hardened and tempered condition and is not therefore recommended for use in the annealed condition. It is most important that oxygen is always allowed to circulate freely on all stainless steel surfaces to ensure that a chrome oxide film is always present to protect it. If this is not the case, rusting will occur as with other types of non stainless steels. For optimum corrosion resistance surfaces must be free of scale and foreign particles.

Finished parts should be passivated.

Forging

Heat uniformly to 1150ºC – 1200ºC, hold until temperature is uniform throughout the section. Do not soak but commence forging immediately. Do not overheat as this will cause a loss of toughness and ductility. Do not forge below 900ºC Finished forgings should be cooled as slowly as possible in dry lime or ashes to room temperature and sub-critical annealed immediately.

Heat Treatment

Sub-Critical Annealing

Heat uniformerly to 620ºC-660ºC hold until temperature is uniform throughout the section.

  • Soak as required – suggested time 6 to 12 hours but can be longer, in cool air.

Annealing

Heat to 950ºC – 1020ºC, hold until temperature is uniform throughout the section *soak as required.

Quench in oil or air cool. Temper immediately while still hand warm. Note: Best impact properties achieved by hardening from above 1020ºC. Best corrosion resistance plus mechanical properties achieved by hardening from about 980ºC and tempering above 590ºC.

Hardening

Heat to 950ºC – 1020ºC, hold until temperature is uniform throughout the section. *soak as required. Quench in oil or air cool. Temper immediately while still hand warm. Note: Hardening from 1020ºC – 1060ºC will give optimum corrosion resistance, but hardening from about 980ºC will give the best combination of corrosion resistance and mechanical properties.

Nitriding

Prior to nitriding, the chrome oxide film which protects the surface must be broken down by pickling or fine sand blasting. Nitriding is carried out at 500ºC – 550ºC followed by slow cooling (no quench) reducing the problem of distortion. Parts can therefore be machined to near final size, leaving a grinding tolerance only. Always ensure that the tempering temperature employed during the initial heat treatment was higher than the nitriding temperature otherwise the core strength will be affected.

Tempering (Condition T)

Heat to 590ºC – 680ºC as required hold until temperature is uniform throughout the section, soak as required, cool in air.

A Double Tempering Treatment, as follows is recommended for optimum toughness

Heat to 640ºC – 680ºC. *Soak as required, cool in air. Followed by:

Re heat to 590ºC – 610ºC. *Soak as required, cool in air. 431 stainless steel can of course be tempered at much lower temperatures producing much higher tensile strengths with subsequent lower impact properties. NB Tempering however within the range 370ºC – 565ºC should be avoided due to temper brittleness, resulting in a considerable reduction in impact properties and loss of corrosion resistance. *Heating temperatures, rate of heating, cooling and soaking times will vary due to factors such as work piece size/shape, also furnace type employed, quenching medium and work piece transfer facilities etc.. Please consult your heat treater for best results.

Machining

431 machines best in the hardened and tempered as supplied condition and is regarded as being readily machineable with all operations such as turning and drilling etc.. capable of being carried out satisfactorily. It does not work harden to the same extent as the 300 series austenitic stainless steels, but is more similar in this respect to the low allow high tensile steels such as 4140 etc.. Allowing therefore for its high tensile properties, all machining should be carried out as per machine manufacturers recommendations for suitable tool type, feeds and speeds.

Welding

431 is not generally recommended for welding in either the annealed or hardened and tempered condition, due to its air hardening capability which can lead to the formation of brittle martensite, resulting in cold cracking due to contraction stresses within the weld and heat affected zone. The higher the carbon content the higher the hardening capability and the greater the risk of cracking. Pre heating and interpass temperature control during welding, plus very slow cooling and post-weld annealing is the best method to prevent cracking. The following welding procedure and post-weld heat treatment may be taken as a guide only if welding is necessary.

Welding Procedure

Welding electrodes or rods should be low hydrogen types 410 or * similar when good strength is required otherwise an austenitic stainless electrode or rod such as 308 or * similar may be used resulting in a more ductile weld when strength is not so critical and post-weld annealing is not possible or intended. Pre-heat at 200ºC – 300ºC and maintain interpass temperature at 200ºC minimum. On completion of welding cool slowly as possible until hand warm if possible: Post-weld sub-critical anneal at 620ºC-660ºC and cool in air *Please consult your welding consumables supplier.