Stainless Steel, Martensitic
431 Stainless Steel (S43100) Bar
Martensitic stainless steel that is the aerospace standard similar to 431 stainless steel.
This alloy is a high chromium-nickel corrosion-resistant martensitic stainless steel. It is extremely corrosion resistant and offers very good strength and toughness. It is typically used in aerospace and defence applications or where higher mechanical properties than 410 are required and where corrosive conditions are not too severe.
The grade is generally supplied in the hardened and tempered condition. This grade is one of the most corrosion resistant grades of martensitic stainless steels, displaying excellent strength and toughness, resistance to stress corrosion and to oxidation up to 800°C.
316H stainless steel is a high-carbon, high-temperature variant of 316, combining enhanced tensile strength at elevated temperatures with excellent corrosion resistance. It is widely used in power generation, chemical processing, aerospace, and industrial applications where both high strength and corrosion resistance are essential.
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431 Stainless Steel Related Specifications
| System / Standard | Country / Region | Grade / Designation |
| AISI | USA | 431 |
| UNS | International | S43100 |
| EN / W.Nr. | Europe | 1.4057 |
| EN Name | Europe | X17CrNi16-2 |
| JIS | Japan | SUS431 |
| GB | China | 1Cr17Ni2 |
| BS (旧标准) | UK | 431S29 |
| ASTM A276 | USA | Type 431 (bars, shapes) |
| ASTM A182 | USA | F431 (forgings, flanges) |
Properties
Chemical Composition
AMS 5628
| Chemical Element | % Present |
| Carbon (C) | 0.12 - 0.17 |
| Chromium (Cr) | 15.50 - 16.50 |
| Manganese (Mn) | 0.30 - 0.80 |
| Silicon (Si) | 0.20 - 0.60 |
| Phosphorous (P) | 0.04 max |
| Sulphur (S) | 0.03 max |
| Nickel (Ni) | 2.00 - 3.00 |
| Molybdenum (Mo) | 0.25 max |
| Copper (Cu) | 0.50 max |
| Nitrogen (N) | 0.10 max |
Mechanical Properties
Minimum Tensile Properties
AMS 5628
| Mechanical Property | Value |
| Yield Strength at 0.2% Offset | 150 ksi (1034 MPa) |
| Tensile Strength | 200 ksi (1379 MPa) |
| Elongation in 2 inches or 4D | 10% |
| Reduction of Area | 40% |
General Physical Properties
General Physical Properties of 431 Stainless Steel Bar
The general physical properties of 431 stainless steel (AISI 431 / EN 1.4057 / X17CrNi16-2, martensitic stainless steel) are as follows (typical values at room temperature):
Density: approx. 7.7–7.8 g/cm³
Melting range: roughly 1425–1510 °C
Modulus of elasticity (Young’s modulus): about 200–215 GPa in the quenched and tempered condition
Poisson’s ratio: approx. 0.27–0.30
Thermal conductivity (at 20 °C): about 24–26 W/m·K, increasing with temperature
Specific heat capacity (at 20 °C): approx. 460–500 J/kg·K
Linear coefficient of thermal expansion: around 10.5–11.5 × 10⁻⁶ /K (between 20–100 °C), similar to other Cr-rich martensitic stainless steels
Electrical resistivity: typically 0.6–0.8 μΩ·m at 20 °C
Magnetic properties: 431 is a fully martensitic, strongly magnetic stainless steel in all hardened and tempered conditions.
These physical properties, combined with its high tensile strength and good corrosion resistance, make 431 stainless steel bar suitable for shafts, fasteners and high-strength mechanical components where dimensional stability and predictable behaviour under load and temperature are important.
Applications of 431 Stainless Steel
431 stainless steel is a high-strength martensitic stainless steel that combines good corrosion resistance with high hardness and tensile strength. It is widely used for shafts, fasteners and mechanical components operating in mildly to moderately corrosive environments where higher strength is required than standard austenitic grades.
1. Pump Shafts and Rotating Equipment
Pump and mixer shafts in water and mildly corrosive process media
Impellers, rotors and agitator components
Drive shafts and transmission parts in industrial machinery
Components requiring high strength, wear resistance and dimensional stability under rotation
2. Valve and Fluid-Control Components
Valve stems, spindles, seats and internal trim
Hydraulic and pneumatic components operating under pressure
Plugs, shafts and sealing surfaces in water and mildly corrosive fluids
Parts requiring a combination of corrosion resistance, hardness and sealing integrity
3. High-Strength Fasteners and Mechanical Fixings
High-strength bolts, screws and studs for structural and machinery use
Pins, axles, clevis pins and dowels in outdoor or industrial environments
Fasteners used in marine, construction and general engineering applications
Components where higher strength than 304 / 316 fasteners is needed
4. Marine and Offshore Hardware
Propeller shafts and stern gear for small craft and light marine equipment
Couplings, flanges and drive parts on deck machinery
Hardware and fittings exposed to seawater spray, splash zones and salt-laden air
Applications needing better corrosion resistance than carbon steel with higher strength than austenitic grades
5. Aerospace, Automotive and Power-Transmission Parts
Non-critical aerospace hardware such as pins, bushes and linkages
Steering, suspension and driveline components in vehicles
Gears, rings, couplings and other power-transmission elements
Parts subjected to cyclic or shock loading where strength, toughness and moderate corrosion resistance are important
6. General Engineering, Tooling and Wear Parts
High-strength machine components and shafts in industrial equipment
Plastic mould inserts and tooling where corrosion and wear resistance are required
Guides, jigs, fixtures and wear-resistant parts operating under sliding or rolling contact
Applications needing a hardened, polishable stainless steel with good mechanical performance
Summary
431 stainless steel is mainly used for high-strength shafts, fasteners, valves, pumps, marine hardware, power-transmission components and general engineering parts that must combine high hardness and load-bearing capacity with better corrosion resistance than standard martensitic grades in mildly to moderately corrosive environments.
Characteristics of 431 Stainless Steel
1. Martensitic Stainless Steel with High Cr + Ni
431 stainless steel (AISI 431 / EN 1.4057 / X17CrNi16-2, SUS431) is a martensitic stainless steel with about 15–17% Cr and 1.25–2.5% Ni. It is designed to combine high mechanical strength and good corrosion resistance in bar and forged products.
2. High Strength and Hardness After Heat Treatment
After hardening and tempering, 431 offers:
High tensile strength and high yield strength compared with 410 / 420
High hardness suitable for wear- and load-bearing components
Strength levels adjustable via tempering temperature, allowing a balance between hardness and toughness
This makes 431 suitable for shafts, fasteners and high-load mechanical parts.
3. Better Corrosion Resistance than Standard Martensitic Grades
Thanks to its higher chromium and nickel content, 431 provides:
Better corrosion resistance than 410, 416, 420 in many environments
Reliable performance in mild marine, industrial and atmospheric conditions
Adequate resistance for pump, valve and shaft components in water and mildly corrosive media
However, its corrosion resistance is lower than austenitic grades such as 304 / 316 and not intended for the most aggressive chloride or acid environments.
4. Good Toughness for a High-Hardness Steel
Compared with low-alloy tool steels or standard martensitic stainless steels at similar hardness, 431 offers:
Improved toughness and impact resistance
Reduced risk of brittle fracture when correctly tempered
More reliable performance under dynamic or shock loading
This toughness–hardness balance is a key reason why 431 is widely used for rotating and power-transmission components.
5. Wear and Galling Resistance
In the hardened and tempered condition, 431 exhibits:
Good abrasion and wear resistance
Improved anti-galling performance versus austenitic grades (e.g., 304)
Stable surface behaviour under sliding and rolling contact
These features make it suitable for valve parts, pump shafts, pins, couplings, and drive components that see repeated mechanical contact.
6. Machinability and Surface Finish
431 is usually supplied as bar, forged bar or machined parts:
Machinability is moderate—easier than many tool steels, but generally more difficult than 304 due to higher hardness
Best machined in the annealed or tempered condition, with rigid tooling and appropriate cutting parameters
Can achieve high-quality surface finish after turning, grinding and polishing, which is important for shaft and sealing surfaces
7. Fully Magnetic Stainless Steel
As a martensitic alloy, 431 is:
Strongly magnetic in all hardened and tempered conditions
Suitable for applications involving magnetic sensing, couplings or magnetic clamping
Different from austenitic stainless steels (304 / 316), which are essentially non-magnetic in the annealed state
8. Heat Treatment Flexibility
431 stainless steel can be heat treated to obtain different property levels:
Quenching from high temperature to form martensite
Tempering over a wide temperature range to tune hardness, strength and toughness
Possibility to tailor the final condition for high strength, better toughness, or a compromise depending on the application
This flexibility allows designers to use the same grade for different components with different property requirements.
One-Sentence Summary
431 stainless steel is a high-strength, martensitic stainless steel with better corrosion resistance than standard 410/420, good toughness, wear and galling resistance, full magnetism and flexible heat treatment, making it ideal for shafts, fasteners, valve and pump components and other high-load mechanical parts in mildly to moderately corrosive environments.
Additional Information
Weldability
Weldability of 431 Stainless Steel
431 stainless steel is a high-strength martensitic stainless steel; it is weldable, but not “easy to weld”. Successful welding requires strict control of heat input, preheat, interpass temperature, hydrogen level and post-weld heat treatment to avoid cracking and loss of toughness.
1. General Weldability Overview
431 has high carbon + martensitic structure, which makes it prone to cold cracking and heat-affected zone (HAZ) hardening.
It can be welded using common arc processes, but must be treated as a sensitive, hardenable steel, not like 304/316.
Best practice: preheat + low-hydrogen welding + PWHT whenever possible.
2. Welding Processes and Filler Metals
Suitable welding processes include:
GTAW / TIG – preferred for controlled heat input and high-quality, small welds.
GMAW / MIG – used for production work with suitable shielding gas and parameters.
SMAW / MMA – possible with low-hydrogen electrodes and careful technique.
Typical filler choices:
Matching filler (e.g., 431-type or 410 Ni-modified) for strength and similar composition.
Austenitic fillers (e.g., 309/309L or 312) to improve weld toughness and crack resistance, often used for dissimilar joints or highly restrained welds.
For critical parts, filler selection should consider required strength, toughness, and service environment.
3. Preheat and Interpass Temperature Control
Preheat is strongly recommended to reduce hardness and cracking risk:
Typical preheat: 150–300°C, depending on section thickness and restraint.
Maintain interpass temperature in the same range, avoiding large temperature swings.
Slow, uniform heating and cooling help reduce residual stresses and HAZ hardening.
4. Post-Weld Heat Treatment (PWHT)
For highly stressed or thick-section components, PWHT is highly desirable:
Usually a tempering or stress-relieving treatment to reduce hardness and restore toughness.
PWHT helps:
Reduce residual stresses
Improve HAZ toughness
Stabilise mechanical properties and reduce the risk of delayed cracking
In non-critical or thin parts, PWHT may be omitted, but with some loss of toughness and increased risk of cracking.
5. Control of Hydrogen and Cracking Risk
Use low-hydrogen processes and consumables (basic-coated electrodes, dried wire/flux).
Keep joint surfaces clean and dry (no oil, grease, moisture, rust or scale).
Avoid high restraint joints and sharp geometries that concentrate stress.
Where possible, butter with an austenitic filler (e.g., 309) then weld onto the buttered layer to further reduce cracking risk.
6. Effect of Welding on Mechanical Properties
The HAZ can become very hard and brittle if not properly preheated and tempered.
Without PWHT, tensile strength may remain high but impact toughness and fatigue resistance can be significantly reduced.
Matching-filler welds can achieve comparable strength to the base metal after appropriate tempering.
Overheating or too-high heat input may cause grain growth and local softening, reducing fatigue performance.
7. Effect of Welding on Corrosion Resistance
Improper welding (no preheat, no PWHT, excessive heat input) can lead to:
Sensitisation and reduced corrosion resistance in the HAZ
Local areas of high hardness and residual stress, more susceptible to stress corrosion cracking in chloride-containing environments
Using austenitic filler metals can improve weld metal corrosion resistance and toughness, especially in wet or mildly marine environments.
Good practice: ensure full gas shielding, avoid undercut and slag inclusions, and grind and clean welds exposed to corrosive media.
8. Practical Design and Fabrication Guidelines
Prefer lower-carbon heats if frequent welding is expected.
Design joints to minimise restraint and allow for some movement during cooling.
Use multi-pass, controlled heat input rather than a single high-heat pass.
For highly critical components (shafts, pressure parts, safety-related items):
Qualify welding procedures (WPS/PQR)
Use NDE (UT, RT, MT, PT) as appropriate
Specify PWHT and mechanical testing of representative welds.
Summary
431 stainless steel is weldable but should be treated as a crack-sensitive, hardenable martensitic stainless steel: reliable welds require low-hydrogen procedures, proper preheat and interpass control, and often post-weld tempering to achieve the desired combination of strength, toughness and corrosion performance.
Fabrication
Fabrication of 431 Stainless Steel
431 stainless steel is a hardenable martensitic stainless steel, so its fabrication requires more control than austenitic grades such as 304 or 316. Successful forming, machining and heat treatment depend on understanding its tendency to harden, its magnetic nature and its sensitivity to heat input.
1. Forming and Cold Working
431 has limited cold formability compared with austenitic stainless steels.
Simple operations such as straightening, light bending or swaging can be carried out in the annealed or tempered condition.
Heavy cold forming (tight-radius bending, deep drawing, complex shaping) is generally not recommended, as high work hardening and reduced ductility increase cracking risk.
If significant cold work is applied, a subsequent stress-relief or tempering treatment is often advisable to restore toughness and dimensional stability.
2. Hot Working and Forging
431 is usually hot worked in the range ~950–1200°C (1740–2190°F), followed by air cooling or oil quenching depending on the desired condition.
Parts should be uniformly heated and worked aggressively while the temperature remains within the recommended range; avoid working below about 900°C, where ductility drops.
After hot working, a full anneal or harden-and-temper cycle is commonly applied to refine the microstructure and obtain consistent mechanical properties.
Slow cooling from hot-working temperatures through the martensite formation range should be avoided if high strength and toughness are required.
3. Machining and Cutting
In the annealed or tempered condition, 431 has moderate machinability—better than many tool steels but generally more difficult to machine than 304.
For best results:
Machine in the softest practical condition (annealed or lower-strength tempered state) before final hardening, if design allows.
Use rigid tooling, sharp carbide tools and adequate machine power.
Apply lower cutting speeds with higher feed compared with austenitic stainless steels to reduce work hardening.
Ensure effective coolant to control heat and extend tool life.
Thermal cutting (plasma, oxy-fuel) should be followed by grinding or machining of the heat-affected zone if high mechanical performance is required.
4. Heat Treatment and Hardening
Typical heat-treatment steps for 431 include:
Annealing to soften and homogenise the structure.
Quenching from the austenitizing temperature to form martensite and achieve high hardness.
Tempering over an appropriate temperature range to adjust hardness, strength and toughness.
The final property balance (hardness vs. toughness) can be tuned via tempering temperature and time, allowing the same grade to be used for different component types.
Careful control of furnace atmosphere and cooling rate helps minimise scaling, distortion and residual stress.
5. Surface Finishing and Grinding
431 can be finished to high-quality machined, ground or polished surfaces, which is important for:
Shafts and bearing seats
Seal surfaces and valve components
Precision mechanical parts
After grinding or heavy machining:
Avoid overheating the surface, which can cause burning, micro-cracking or soft spots.
Use proper cooling and conservative parameters, especially on hardened material.
Pickling and chemical cleaning are more difficult than for austenitic grades; mechanical finishing (grinding, blasting, polishing) is often preferred.
6. Welding Considerations in Fabrication
431 is weldable with precautions, but more crack-sensitive than 304/316.
Good fabrication practice includes:
Preheat and controlled interpass temperature for thicker sections.
Use of low-hydrogen processes and consumables.
Post-weld tempering or stress relief for highly stressed parts to restore toughness and reduce residual stresses.
For complex fabrications, design should allow for access to PWHT, and joints should be configured to minimise restraint and distortion.
7. Dimensional Stability and Distortion Control
Because 431 is hardened by martensitic transformation, distortion can occur during quenching and heavy machining.
To improve dimensional control:
Use symmetrical designs and uniform section thickness where possible.
Perform rough machining → heat treatment → finish machining for tight-tolerance components.
Apply stress-relief treatments after heavy machining or welding.
Proper fixturing, controlled heating/cooling and balanced machining operations all help maintain final dimensional accuracy.
Summary
431 stainless steel can be fabricated into high-strength shafts, fasteners, valves and other mechanical components when its martensitic, hardenable nature is respected—using limited cold forming, controlled hot working, carefully planned machining, appropriate heat treatment and well-managed welding and stress relief to deliver stable dimensions, good toughness and reliable service performance.
Hot Working
Hot Working of 431 Stainless Steel
431 stainless steel is a hardenable martensitic stainless steel that can be hot worked successfully when temperature and cooling are carefully controlled. Proper hot working practice helps achieve a refined microstructure, good toughness and consistent mechanical properties.
1. Recommended Temperature Range
Typical heating / forging range: about 950–1200°C (1740–2190°F)
Start hot working towards the upper end of the range for best plasticity.
Avoid working below ~900°C (1650°F), where ductility falls and cracking risk increases.
Use uniform, thorough preheating to minimise thermal gradients and internal stresses.
2. Forging and Hot Forming Practice
Begin deformation once the entire section is at the target temperature.
Use firm, continuous reductions rather than light hammering to promote good grain refinement.
For large forgings, work the piece progressively, returning to the furnace as soon as temperature falls near the lower limit.
Avoid excessive holding times at high temperature to reduce grain growth and surface scaling.
3. Cooling After Hot Working
After forging, cool the material in still air for general purposes, unless a specific hardening or annealing cycle is planned.
For components that will be quenched and tempered to high strength, hot working is usually followed by:
Normalising or annealing (if required), then
Austenitising, quenching and tempering to final properties.
Avoid very slow furnace cooling through the martensitic transformation range if high strength and toughness are required, as this can lead to non-uniform structures.
4. Surface Protection and Scale Control
High-temperature exposure can cause oxide scale and surface decarburisation.
Where surface quality is important (e.g. shafts, sealing surfaces), plan for:
Sufficient machining allowance to remove scale and decarburised layers.
Use of controlled atmosphere or protective coatings where practical.
After hot working, shot blasting, grinding or machining is typically used to restore clean, sound surfaces.
5. Distortion and Defect Prevention
Design preforms and forgings with even section thickness to reduce distortion and internal stresses.
Avoid sharp transitions and abrupt changes in cross-section that concentrate stress during cooling.
Inspect forgings (e.g. visually and, for critical parts, with NDE) for cracks, laps, folds or overfills before further processing.
If heavy reductions or complex shapes are involved, consider an intermediate stress-relief or normalising step before final hardening and tempering.
Summary
Hot working of 431 stainless steel should be carried out in the approximate range of 950–1200°C with uniform heating, firm deformation and controlled air cooling, followed by appropriate heat treatment; careful control of temperature, section design and post-forging cleanup is essential to avoid cracking, excessive scale and distortion and to achieve a refined, tough martensitic structure in the finished component.
Heat Resistance
Heat Resistance of 431 Stainless Steel
431 stainless steel is a martensitic stainless steel that offers moderate heat resistance compared with carbon steels and standard martensitic grades, but it is not a dedicated high-temperature alloy like heat-resistant austenitic or nickel-based grades. Its high strength depends on a tempered martensitic structure, which can be degraded by prolonged exposure to elevated temperatures.
1. Maximum Service Temperature Range
In general engineering service, 431 is typically used at temperatures up to about 300–400°C for long-term operation.
Short-term or intermittent exposure to somewhat higher temperatures may be acceptable, but continuous high-temperature service is not recommended.
Above its tempering temperature range, 431 will gradually lose hardness and tensile strength and may suffer reduced fatigue performance.
2. Behaviour During Tempering and Elevated Exposure
The strength and hardness of 431 are developed by quenching and tempering; the final tempering temperature determines the usable upper service temperature.
Prolonged exposure near or above the tempering temperature can:
Reduce strength and hardness (over-tempering).
Alter the carbide structure and reduce wear resistance.
As a rule of thumb, for critical parts, the continuous service temperature should stay well below the final tempering temperature chosen during heat treatment.
3. Oxidation and Scaling Resistance
With ~15–17% Cr, 431 has better oxidation resistance than carbon steels and low-alloy steels at moderate temperatures.
It can tolerate intermittent exposure to elevated temperatures in air without rapid scaling, but its oxidation resistance is inferior to austenitic grades such as 304 and 316 at high temperatures.
For sustained temperatures above the low-hundreds °C, austenitic stainless steels or special heat-resisting alloys are generally preferred.
4. Effect of Heat on Mechanical Properties
At elevated temperatures, 431 will exhibit:
Reduced yield and tensile strength compared with room temperature.
Lower fatigue strength under cyclic loading.
Possible temper embrittlement or loss of toughness if exposed for long periods in unfavourable temperature ranges.
Designs using 431 at elevated temperature should de-rate allowable stresses and consider the effect of temperature on impact toughness and fatigue life.
5. Comparison with Austenitic Stainless Steels
Compared with 304 / 316, 431 offers:
Higher room-temperature strength and hardness,
But poorer high-temperature strength and creep resistance for long-term service.
Austenitic grades maintain better ductility, toughness and corrosion/oxidation resistance at elevated temperatures and are generally preferred for continuous high-temperature or heat-resisting applications.
431 is best viewed as a high-strength mechanical steel with stainless capability, not as a primary heat-resistant alloy.
Summary
431 stainless steel provides useful heat resistance for short-term or moderate-temperature service (typically up to about 300–400°C), but prolonged exposure to higher temperatures will reduce its hardness, strength and fatigue performance; it should therefore be treated as a high-strength martensitic stainless steel with limited high-temperature capability rather than a dedicated heat-resistant material, with austenitic grades like 304/316 being preferred for continuous high-temperature applications.
Machinability
Machinability of 431 Stainless Steel
431 stainless steel is a hardenable martensitic stainless steel with higher strength and hardness than 304/316, so its machinability is moderate: generally more difficult to machine than austenitic grades, but easier than many tool steels when processed in a suitable condition.
1. General Machining Behaviour
Machinability is best in the annealed or low–medium tempered condition.
As hardness and strength increase, tool wear, cutting forces and heat generation all rise.
The alloy tends to work harden less than austenitic grades, but its higher base hardness makes cutting more demanding.
Stable fixturing and rigid machines are important to prevent vibration and poor surface finish.
2. Recommended Condition for Machining
Whenever possible, rough machining should be done in the softest practical condition (annealed or lower-strength tempered).
Final hardening and tempering can then be applied, followed by light finishing passes or grinding.
If machining must be performed in a high-strength condition, expect reduced tool life and lower achievable cutting speeds.
3. Cutting Tools and Parameters
Carbide tools are generally preferred for turning, milling and drilling, especially at higher hardness levels.
Use:
Lower cutting speeds than for 304/316
Moderate to high feed rates to keep the tool cutting below the work-hardened layer
Positive rake geometries to reduce cutting forces and heat
Insert grades designed for stainless or hardened steels usually give the best balance of wear resistance and toughness.
4. Coolant and Chip Control
Apply copious cutting fluid or coolant to:
Remove heat from the cutting zone
Improve tool life and surface finish
Help chip flow and reduce built-up edge
431 can produce tough, continuous chips in some operations; use:
Chip-breaker geometries
Proper feed and depth of cut to avoid long stringy chips that hinder automatic operation and may damage surfaces.
5. Drilling, Tapping and Threading
For drilling:
Use high-quality HSS-Co or carbide drills
Moderate speed, steady feed, and frequent chip break/pecking for deeper holes
For tapping and thread milling:
Prefer strong, high-quality taps with good lubrication
Consider thread milling for critical or hard components to reduce risk of tap breakage
Use thread forms and tolerances appropriate for high-strength materials.
6. Surface Finish and Dimensional Control
431 can achieve excellent surface finishes after turning, grinding and polishing, which is important for:
Shafts and bearing seats
Seal and valve surfaces
To maintain tolerance and finish:
Avoid excessive heat input that may cause surface tempering or distortion
Use light finishing cuts with sharp tools on hardened parts
Allow for spring-back in high-strength conditions when setting final dimensions.
Summary
431 stainless steel has moderate machinability: it is more difficult to machine than 304/316 but easier than many tool steels, and it responds well to machining when worked in the annealed or tempered condition using rigid setups, sharp carbide tooling, controlled cutting speeds, generous coolant and appropriate drilling/tapping strategies to achieve accurate, high-quality surfaces on shafts, fasteners and other precision components.
Corrosion Resistance
Corrosion Resistance of 431 Stainless Steel
431 stainless steel is a martensitic stainless steel with higher chromium and nickel than 410/420, giving better overall corrosion resistance than standard martensitic grades, but still below austenitic alloys such as 304 and 316, especially in aggressive chloride or acidic environments.
1. General Corrosion Behaviour
Good resistance to uniform corrosion in many mildly corrosive media.
Clearly better than 410 / 416 / 420 due to higher Cr + Ni content.
Does not match the pitting and crevice corrosion resistance of 304 / 316, especially in chlorides.
Best performance is obtained in the hardened and tempered condition with a clean, smooth surface.
2. Atmospheric and Fresh-Water Environments
Performs well in rural, urban and light industrial atmospheres, with good resistance to rust staining.
Suitable for fresh water, drinking water and many cooling water systems when chloride levels are low to moderate.
Commonly used for shafts, fasteners and valve parts exposed to weather, splash and condensation.
Regular cleaning and good drainage further improve service life.
3. Marine and Chloride-Containing Environments
In marine atmospheres and seawater splash zones, 431 offers:
Better performance than carbon steel and low-alloy steels.
Better resistance than 410 / 420, but
Lower resistance than 316 / duplex grades, especially for pitting and crevice attack.
Continuous immersion in seawater or stagnant chloride solutions is not ideal; risk of pitting and crevice corrosion increases, particularly at higher temperatures.
For critical submerged or very high-chloride service, 316, super-austenitic or duplex stainless steels are usually preferred.
4. Resistance to Chemical Media
Suitable for many mild acids, alkaline solutions and industrial process fluids at low to moderate concentrations.
Performs well in oils, fuels and many organic media.
Not recommended for strong mineral acids, strong reducing acids or hot concentrated chlorides, where rapid attack or localised corrosion can occur.
For chemical service, actual medium composition, temperature and concentration must be checked against corrosion data or testing.
5. Stress Corrosion Cracking and Hydrogen Effects
Like other martensitic stainless steels, 431 is more susceptible to:
Stress corrosion cracking (SCC) in chloride-bearing environments under tensile stress.
Hydrogen embrittlement or delayed cracking when exposed to hydrogen-charging conditions (e.g. cathodic protection, pickling, plating).
Good practice includes:
Avoiding high residual stresses (proper heat treatment and stress relief).
Minimising sustained tensile stress in corrosive environments.
Carefully controlling pickling, plating and cathodic protection conditions.
6. Surface Condition, Heat Treatment and Design Factors
Corrosion resistance is strongly influenced by:
Heat treatment: proper tempering improves both toughness and corrosion behaviour.
Surface finish: smooth, polished surfaces resist pitting and crevice corrosion better than rough ones.
Cleanliness: removal of scale, weld spatter and contamination is essential.
Good design practice:
Avoid stagnant crevices, tight gaps and water traps.
Provide drainage and access for cleaning.
Choose appropriate weld procedures and fillers, as weld metal and HAZ can be more corrosion-sensitive if improperly produced.
Summary
431 stainless steel offers good corrosion resistance in atmospheric, fresh-water and many mildly corrosive environments, outperforming standard martensitic grades such as 410/420 but not reaching the chloride and chemical resistance of 304/316; with correct heat treatment, clean smooth surfaces and sensible design, it provides reliable service for high-strength mechanical components in mildly to moderately corrosive conditions, while more aggressive chloride or chemical environments should use higher-alloy stainless steels.
Heat Treatment
Heat Treatment of 431 Stainless Steel
431 stainless steel is a hardenable martensitic stainless steel. Its final properties depend strongly on the chosen heat treatment route—typically involving hardening (austenitizing and quenching) followed by tempering to balance hardness, strength and toughness.
1. Objectives of Heat Treatment
Develop a martensitic structure for high strength and hardness.
Use tempering to adjust hardness and improve toughness and fatigue resistance.
Reduce residual stresses and distortion from machining, welding or quenching.
Optimise performance for shafts, fasteners, valve parts and other high-load components.
2. Annealing / Softening Treatment
Used to soften the material, improve machinability and homogenise the structure.
Typically involves heating to a temperature high enough to dissolve carbides and then slow cooling to form a softer microstructure.
Applied to:
Bar and forging stock before heavy machining.
Components that have been heavily cold worked or require dimensional correction.
3. Hardening (Austenitizing and Quenching)
Hardening is achieved by:
Heating into the austenitizing range (high temperature where the structure becomes austenitic).
Holding long enough for uniform temperature and solution of carbides.
Quenching (usually in air or oil, depending on section size and specification) to form hard martensite.
Correct hardening practice aims to:
Maximise strength and hardness.
Avoid excessive grain growth.
Minimise distortion and cracking by controlled heating and quenching.
4. Tempering to Balance Hardness and Toughness
After quenching, 431 is very hard and brittle; tempering is essential.
Tempering involves reheating to an intermediate temperature and holding for a defined time, then cooling in air.
Effects of tempering:
Reduces internal stresses and brittleness.
Adjusts hardness and tensile strength to the required level.
Improves toughness, ductility and fatigue resistance.
Lower tempering temperatures → higher hardness and strength but lower toughness.
Higher tempering temperatures → lower hardness but better toughness and impact strength.
5. Stress Relief Heat Treatment
Used when full hardening is not required, but residual stresses from machining or welding must be reduced.
Typically performed at a sub-critical temperature (below the austenitizing range) to relax stresses without significantly changing strength or microstructure.
Recommended for:
Heavily machined shafts and precision components.
Welded fabrications where distortion and cracking risk must be minimised.
6. Typical Heat Treatment Sequences in Practice
Route for high-strength shafts / fasteners:
Rough machining in softened/annealed condition.
Austenitize and quench to form martensite.
Temper to required hardness and strength.
Finish machining and grinding to final size and surface finish.
Route for welded or highly stressed parts:
Weld using controlled procedure.
Post-weld stress relief or tempering to restore toughness and reduce cracking risk.
Final machining or grinding as required.
7. Precautions During Heat Treatment
Avoid overheating during austenitizing, which can cause grain coarsening and reduced toughness.
Use controlled heating and cooling rates to minimise distortion and cracking, especially for long, slender shafts or complex shapes.
Protect surfaces from scaling and decarburisation (or allow sufficient machining allowance to remove affected layers).
Ensure that tempering temperatures are chosen above any previous tempering temperature to maintain predictable properties.
Summary
Heat treatment of 431 stainless steel is based on hardening (austenitizing and quenching) followed by tempering, with optional annealing or stress relief as needed; careful control of temperatures, quenching and tempering allows engineers to tailor hardness, strength and toughness for high-strength shafts, fasteners, valve components and other demanding mechanical parts.
Cold Working
Cold Working of 431 Stainless Steel
431 stainless steel is a hardenable martensitic stainless steel with relatively high strength and limited ductility, so its cold working capability is restricted compared with austenitic grades such as 304 and 316. Light to moderate cold deformation is possible, but heavy forming operations are generally not recommended.
1. General Cold Workability
431 can be cold worked to a limited extent, mainly for small shape adjustments or sizing.
Ductility is significantly lower than that of austenitic stainless steels, especially in hardened or highly tempered conditions.
Excessive cold strain can lead to cracking, edge tearing or loss of toughness, particularly in thick or complex sections.
2. Preferred Condition for Cold Working
Cold working should be carried out in the annealed or lower-strength tempered condition, where ductility is highest.
In fully hardened/high-strength conditions, only very light straightening or minor bending should be attempted.
If substantial cold work is required, components are often:
Softened by annealing,
Cold formed, then
Hardened and tempered to final properties.
3. Typical Cold Working Operations
Suitable cold working operations include:
Straightening, light bending and swaging of bars and shafts.
Sizing, light drawing or reducing of cross-sections within modest strain limits.
Operations generally not recommended (except on very thin sections) include:
Deep drawing and complex press forming.
Tight-radius bending of thick sections.
Severe cold heading where high upset is required.
4. Work Hardening and Mechanical Effects
431 will increase in strength and hardness with cold work, but its work hardening rate is not as high as austenitic grades.
Cold work can:
Raise yield strength and hardness locally.
Reduce toughness and ductility, especially in highly strained regions.
Introduce residual stresses that may contribute to distortion, fatigue or stress-corrosion sensitivity.
5. Stress Relief After Cold Working
After significant cold deformation, a stress-relief heat treatment is often beneficial:
Reduces internal stresses from bending, straightening or swaging.
Helps restore toughness and dimensional stability before final machining or service.
For components requiring high integrity (e.g. shafts, fasteners, critical fittings), stress relief or a full harden-and-temper cycle is recommended after major cold working steps.
6. Design and Process Recommendations
Keep cold deformation moderate and avoid sharp bends or severe section changes.
Use larger bend radii and gradual transitions to reduce local strain.
For tight tolerances, use a sequence such as:
Rough machining → limited cold working (if needed) → stress relief / heat treatment → finish machining and grinding.
When extensive shaping is required, consider hot working plus finish machining instead of heavy cold forming.
Summary
Cold working of 431 stainless steel should be limited to light to moderate operations such as straightening, sizing and gentle bending in the annealed or low-strength tempered condition; heavier deformation risks cracking and loss of toughness, so significant cold work should be followed by stress relief or full heat treatment to restore dimensional stability and mechanical performance for critical components.
