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.
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.
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.
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.
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 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.
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.
Typical applications of grade 431 stainless steels include the following:
Pump and propeller shafts
Nuts and bolts