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.

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.

Do not overheat as this will cause a loss of toughness and ductility.

Do not forge below 900 ^oC

Finished forgings should be cooled as slowly as possible in dry lime or ashes to room temperature and sub-critical annealed immediately.

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.

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.

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.

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.

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.

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.