431 Stainless Steel Sheet

Stainless Steel – Grade 431 (UNS S43100)

Grade Specification Comparison

Grade specifications of grade 431 stainless steels

Possible Alternative Grades

Introduction

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.

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.

Specifications

The following specifications cover Stainless Steel 431

• AISI 431
• AMS 5628
• ASTM A276
• ASTM A473
• ASTM A493
• ASTM A511
• ASTM A579
• ASTM A580
• DIN 1.4057
• MIL S-18732
• MIL S-8967
• UNS S43100

Key Properties

The specified properties are for bar products in ASTM A276. The properties may not be necessarily similar to other forms such as forgings and plates.

Principal Design Features

431 is a martensitic stainless combining excellent impact strength at high hardness levels with the best corrosion resistance of any martensitic stainless steels.

Composition

Table 1. Composition ranges of grade 431 stainless steels

Mechanical Properties

Table 2. Mechanical properties of grade 431 stainless steels

Grade 431 is frequently stocked and supplied in “Condition T”, with specified tensile strength of 850 – 1000MPa.

* Annealed tensile properties are typical for Condition A of ASTM A276; annealed hardness is the specified maximum. Grade 431 is only rarely stocked in annealed Condition A.

# Grade 431 should not be tempered between 425 and 600°C, due to the low impact resistance associated with this temperature range.

Physical Properties

Table 3. Typical physical properties of grade 431 stainless steels

Hot Working

Heat to 2100-2200 F (1149-1204 C) for best results. Do not work material below 1650 F (900 C).

Cold Working

This alloy is easily drawn, spun, headed, sheared and bent compared with other stainless steels.

Forging

Heat uniformly to 1150^oC – 1200^oC, 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 900^oCFinished 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 620^oC – 660^oC hold until temperature is uniform throughout the section.
*Soak as required – suggested time 6 to 12 hours but can be longer, cool in air.
Annealing
Heat to 950^oC – 1020^oC, 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 1020^oC. Best corrosion resistance plus mechanical properties achieved by hardening from about 980^oC and tempering above 590^oC.

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.
Hardening

Heat to 950^oC – 1020^oC, 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 1020^oC – 1060^oC will give optimum corrosion resistance, but hardening from about 980^oC will give the best combination of corrosion resistance and mechanical properties.

1800-1950 F (982-1066 C), oil quench or ail cool for maximum 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 500^oC – 550^oC 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 590^oC – 680^oC as required hold until temperature is uniform throughout the section, soak as required, cool in air. Temper for desired properties. Temperatures between 700-1050 F (371-565 C) will adversely affect impact strength and corrosion properties.
A Double Tempering Treatment,as follows is recommended for optimum toughness.
Heat to 640^oC – 680^oC. *Soak as required, cool in air.
Followed by:
Re heat to 590^oC – 610^oC. *Soak as required, cool in air.431 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 370^oC – 565^oC 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 alloy 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.

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.

In the fully annealed condition, 431 will gall and build up on the tools. Good surface finishes are not easily obtained.

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 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.Most electric welding procedures have proven successful with 431 stainless.

Filler metal should be AWS E/ER410.To avoid cracking, pre-heat the workpiece to 400-600 F (204-316 C). After air cooling, treat at 1200 F (649 C) to reattain maximum properties.

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 200^oC – 300^oC and maintain interpass temperature at 200^oC minimum. On completion of welding cool slowly as possible until hand warm if possible: Post-weld sub-critical anneal at 620^oC – 660^oC, and cool in air.*Please consult your welding consumables supplier.

Applications

Typical applications of grade 431 stainless steels include the following:

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