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

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.

Martensitic stainless steel types

References:

• https://www.azom.com/
• https://www.makeitfrom.com/
• htps://www.ugitech.com
• http://www.interlloy.com.au

We know the various names of stainless steel that are included in the austenitic grade category, namely: AISI / SUS 201, 202, 301, 302, 303, 304, 305, 309, 310, 316, 317, 327, 347. In this article we will discuss SS 316 and SS 316L and 316H.

Austenitic Stainless Steel

Austenitic stainless steel is a specific type of stainless steel alloy. Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic and duplex stainless steel.

[1] Austenitic stainless steels possess austenite as their primary crystalline structure (face centered cubic).

This austenite crystalline structure is achieved by sufficient additions of the austenite stabilizing elements nickel, manganese and nitrogen. Due to their crystalline structure, austenitic steels are not hardenable by heat treatment and are essentially non-magnetic.

There are two subgroups of austenitic stainless steel. 300 series stainless steels achieve their austenitic structure primarily by a nickel addition while 200 series stainless steels substitute manganese and nitrogen for nickel, though there is still a small nickel content.

300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type SS 304, also known as 18/8 or A2.

Type SS 304 is extensively used in such items as, cookware, cutlery, and kitchen equipment.

Type SS 316 is the next most common austenitic stainless steel. Some 300 series, such as Type SS 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attack (e.g. pitting and crevice corrosion). The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series

Other notable austenitic stainless steels are Type 309 and 310, which are utilized in high temperature applications greater than 800°C.

Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless.

This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid. Alloy 20 has excellent mechanical properties and the presence of niobium in the alloy minimizes the precipitation of carbides during welding.

Austenitic stainless steel can be tested by nondestructive testing using the dye penetrant inspection method but not the magnetic particle inspection method. Eddy-current testing may also be used.

Stainless Steel – Grade SS 316 (UNS S31600)

Chemical Formula :
Fe, <0.03% C, 16-18.5% Cr, 10-14% Ni, 2-3% Mo, <2% Mn, <1% Si, <0.045% P, <0.03% S

Grade 316 is the standard molybdenum-bearing grade, second in importance to 304 amongst the austenitic stainless steels.

The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments.

It has excellent forming and welding characteristics. It is readily brake or roll formed into a variety of parts for applications in the industrial, architectural, and transportation fields.

Grade 316 also has outstanding welding characteristics. Post-weld annealing is not required when welding thin sections.

What is the difference between 316, 316L and 316H ?

Grade 316L, the low carbon version of 316 ( L=Low Carbon. ) and is immune from sensitisation (grain boundary carbide precipitation).

Thus it is extensively used in heavy gauge welded components (over about 6mm).

Grade 316H, with its higher carbon content has application at elevated temperatures, as does stabilised grade 316Ti.

The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Composition ranges for 316 grade of stainless steels.

The following is the chemical composition between SS 316, 316L and 316H

Mechanical properties of 316 grade stainless steels.

The following are mechanical properties (tensile strength, yield strength, elongation and hardness) of stainless steel SS 316, 316 L and 316 H

Note: 316H also has a requirement for a grain size of ASTM no. 7 or coarser.

 Typical physical properties for 316 grade stainless steels.

Grade specifications for 316 grade stainless steels.

Note: These comparisons are approximate only. The list is intended as a comparison of functionally similar materials not as a schedule of contractual equivalents. If exact equivalents are needed original specifications must be consulted.

Possible alternative grades to 316 stainless steel.

Corrosion Resistance

Excellent in a range of atmospheric environments and many corrosive media – generally more resistant than 304. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60 °C. Considered resistant to potable water with up to about 1000 mg/L chlorides at ambient temperatures, reducing to about 500 mg/L at 60 °C.

316 is usually regarded as the standard “marine grade stainless steel”, but it is not resistant to warm sea water. In many marine environments 316 does exhibit surface corrosion, usually visible as brown staining. This is particularly associated with crevices and rough surface finish.

Heat Resistance

Good oxidation resistance in intermittent service to 870 °C and in continuous service to 925 °C. Continuous use of 316 in the 425-860 °C range is not recommended if subsequent aqueous corrosion resistance is important. Grade 316L is more resistant to carbide precipitation and can be used in the above temperature range. Grade 316H has higher strength at elevated temperatures and is sometimes used for structural and pressure-containing applications at temperatures above about 500 °C.

Heat Treatment

Solution Treatment (Annealing) – Heat to 1010-1120 °C and cool rapidly. These grades cannot be hardened by thermal treatment.

Welding

Excellent weldability by all standard fusion methods, both with and without filler metals. AS 1554.6 pre-qualifies welding of 316 with Grade 316 and 316L with Grade 316L rods or electrodes (or their high silicon equivalents). Heavy welded sections in Grade 316 require post-weld annealing for maximum corrosion resistance. This is not required for 316L. Grade 316Ti may also be used as an alternative to 316 for heavy section welding.

Machining

A “Ugima” improved machinability version of grade 316 is available in round and hollow bar products. This machines significantly better than standard 316 or 316L, giving higher machining rates and lower tool wear in many operations.

Dual Certification

It is common for 316 and 316L to be stocked in “Dual Certified” form – mainly in plate and pipe. These items have chemical and mechanical properties complying with both 316 and 316L specifications. Such dual certified product does not meet 316H specification and may be unacceptable for high temperature applications.

Applications

• Food preparation equipment particularly in chloride environments.
• Laboratory benches & equipment.
• Coastal architectural panelling, railings & trim.
• Boat fittings.
• Chemical containers, including for transport.
• Heat Exchangers.
• Woven or welded screens for mining, quarrying & water filtration.
• Threaded fasteners.
• Springs.

Typical applications include:

SS 316 Equivalent Grade

References:

1.  The International Nickel Company (1974). “Standard Wrought Austenitic Stainless Steels”. Nickel Institute.
2. ^ “Stainless Steel”. Encyclopaedia Britannica.
3. ^ American Iron and Steel Institute. “Design Guidelines for the Selection and Use of Stainless Steels”. Nickel Institute.
4. Atlas Steels Australia