AISI 431 Martensitic Stainless Steel

AISI 431 – Martensitic stainless steel – A combination of corrosion resistance, mechanical properties and machinability. 431 is martensitic stainless steel with chemical composition as C 0,12-0,22 • Cr 15-17 • Ni 1,5-2,5 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. 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 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.

Complies with standards
EN 10088-3 : 1.4057 X17CrNi 16-2 – AISI 431 – JIS SUS 431 – EN10272 : 1.4057

Related Specifications

AustraliaAS 2837-1986 431
GermanyW.Nr 1.4057 X20CrNi17 2
Great BritainBS970 Part3 1991 431S29
BS970 – 1955 EN57
JapanJIS G4303 SuS 431
USAASTM A276-98b 431
SAE 51431 AISI 431
UNS S43100

Grade 431 stainless steels are martensitic, heat-treatable grades with excellent corrosion resistance, torque strength, high toughness and tensile properties. All these properties make them ideal for bolt and shaft applications. These steels, however, cannot be cold-worked owing to their high yield strength, hence they are suitable for operations such as spinning, deep drawing, bending or cold heading.

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.

Corrosion Resistance

Grade 431 stainless steels have considerable resistance to salt water, but they are less resistant to tropical water when compared to that of grade 316 steels. Grade 431 steels have overall corrosion resistance similar to, or slightly lower than, that of grade 304 steels.

Grade 431 steels with a smooth surface finish perform well in tempered and hardened conditions.

Heat Resistance

Grade 431 steels are resistant to scaling at temperatures of 925°C in intermittent conditions, and 870°C during continuous operations. In general, these steels are not to be used at temperatures above standard tempering temperatures, owing to loss of mechanical properties.

Heat Treatment

Full anneal — Full annealing cannot be performed on grade 431 steels. This grade gets hardened even during slow cooling.

Process anneal — Grade 431 steels are heated to 620 to 660°C and then air-cooled.

Grade 431 steels are generally hardened by heating at temperatures from 980 to 1065°C, holding for nearly ½ h, followed by oil or air quenching. Complex or hardened parts of grade 431 steels can be pre-heated to temperatures from 760 to 790°C and tempered, to improve their mechanical properties. Tempering of these steels at 425 to 600°C should be avoided, owing to the loss of impact toughness at this temperature range.

Welding

Welding of grade 431 stainless steels is difficult due to the chances of cracking. It is recommended to pre-heat the materials to 200 – 300°C before welding, and carry out post-weld heat treatment at 650°C. Welding can be performed using grade 410 filler rods, but ductile welds can be achieved using grades 308L, 309 or 310 steels.

Machining

Grade 431 steels can be easily machined in their annealed state. However, it is extremely difficult to machine these steels if they are hardened above 30HRC.

Applications

Typical applications of grade 431 stainless steels include the following:

  • Laboratory equipment
  • Marine systems
  • Beater bars
  • Pump and propeller shafts
  • Nuts and bolts

Fabrication of AISI 431

Fabrication of martensitic steels is generally carried out using techniques that allow hardening and tempering treatments and poor weldability. The corrosion resistance properties of grade 431 steels are lower than that of austenitic grades. The operations of grade 431 are limited by their loss of strength at high temperatures, due to over-tempering, and loss of ductility at negative temperatures.

Mill’s certificate sample of AISI 431 – Martensitic Stainless Steel

Chrome content on AISI 431 vs. 420/410

AISI 431, 420 and 410 are the same type of martensitic stainless steel. A striking difference from AISI 431 compared to AISI 410 and 420 is the chrome (Cr) content of AISI 431 which is higher than AISI 410 and 420. Where, Cr content for AISI 431 is 15-18%, while the content of Cr content in AISI 410 is 11.5- 13.5%. Meanwhile, the content of Cr in AISI 420 is 12-14%.

Here we show the chemical compositions of grades 431, 420 and 410 where all of these grades are classified as martensitic stainless steel. You can compare the elements of Fe, C, Cr, and others.

Grade

Fe

C

Mn

Si

P

S

Cr

Ni

Mo

431

min.

max.

78.2

83.8

0.12

0.20

1

1

0.04

0.03

15

17

1.25

2.50

 

420

min.

max.

83.5

88.4

0.15

0.4

1

1

0.04

0.03

12

14

0.75

 

410

min.

max.

82.3

87.9

0.08

0.15

1

1

0.04

0.03

11.5

13.5

0.75

0

0.5

Martensitic stainless steel types

Type

Description

410

Basic martensitic grade, containing the lowest alloy content of the three basic stainless steels (304, 430, and 410). Low cost, general purpose, heat treatable stainless steel. Used widely where corrosion is not severe (air, water, some chemicals, and food acids. Typical applications include highly stressed parts needing the combination of strength and corrosion resistance such as fasteners.

410S

Contains lower carbon than Type 410, offers improved weldability but lower hardenability. Type 410S is a general purpose corrosion and heat resisting chromium steel recommended for corrosion resisting applications.

414

Has nickel added (2%) for improved corrosion resistance. Typical applications include springs and cutlery.

416

Contains added phosphorus and sulfur for improved machinability. Typical applications include screw machine parts.

420

Contains increased carbon to improve mechanical properties. Typical applications include surgical instruments.

431

Contains increased chromium for greater corrosion resistance and good mechanical properties. Typical applications include high strength parts such as valves and pumps.

440

Further increases chromium and carbon to improve toughness and corrosion resistance. Typical applications include instruments.

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