Stainless Steel 431

General Characteristics of Stainless 431

Stainless 431 is a machinable and workable steel with added nickel that toughens it while eliminating unwelcome delta ferrite. This alloy displays improved corrosion resistance and toughness as a quench-hardenable stainless steel. Its corrosion resistance makes it ideal for marine and aircraft applications.

Chemical Analysis

C% Mn% P% S% Si% Cr%
0.20 max 1.00 max 0.040 max 0.030 max 1.00 max 15.0/17.0




This alloy is used for aircraft fasteners and fittings and should be considered for structural parts in marine atmospheres.

Type 431 is used in highly-stressed aircraft components, fasteners, bomb racks, bolting, pump shafts and valve stems. It is suitable for applications requiring a combination of corrosion resistance, hardness and toughness from –100/1200ºF (-70/650ºC.)


This alloy takes well to hot heading and drop forging. Heat to 2100/2200ºF (1150/1205ºC), then forge; cool small forgings slowly and large forgings in dry lime or ashes. Cool to room temperature before post-forge annealing. Do not forge below 1650ºF (900ºC.)

Heat Treatment

Annealing: Heat to 1200/1250ºF (650/675ºC), soak and air cool. This treatment is best for most machining operations.

Harden: Heat to 1800/1950ºF (980/1065ºC) – soak and oil quench or air cool. If tempering below 700ºF (370ºC) use the high side of the austenitizing temperature for the best toughness, and similarly the low side of the austenitizing temperature when tempering above 1100ºF (540ºC) for best toughness.

Temper: at a temperature to give desired mechanical properties. Soak at least one hour – longer for large parts – and air cool. Temper between 700/1050ºF (370/565ºC): lower toughness and corrosion resistance may obtain outside this temperature range, but depending upon other required properties it may still be necessary to forsake some toughness and corrosion resistance. Experience will be the guide here.


The alloy shows fairly good machinability, but there will be some gall and build up on the cutting edge of the tools.


The base metal should be preheated to 400/600ºF (205/315ºC) and the temperature maintained at 400ºF (205ºC) during welding. A post-weld treatment at 1200ºF (650ºC) should be carried out on the weldment as soon as possible after cooling to room temperature.


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