SS440

ss-440

Description

Grade 440C stainless steels are high carbon steels, which attain the highest hardness, wear resistance and strength of all stainless steel grades after heat treatment. These properties make this grade suitable for applications such as valve components and ball bearings. Grade 440A and 440B stainless steels, on the other hand, have similar properties – except for a slightly lower percentage of carbon in grade 440A.

All three forms of grade 440 steels are commonly used. However, grade 440C is more readily available than the other standard grades. Grade 440F, a free-machining type of grade 440 series, is also available with a high carbon content similar to that of grade 440C. Martensitic steel grades are high-hardness steels, usually fabricated using techniques that require hardening and tempering treatments.

These grades have a corrosion resistance lower than that of other austenitic grades. The applications of martensitic steels are limited by the loss of strength caused by over-tempering at high temperatures, and loss of ductility at temperatures below zero.

Key Properties

The following section discusses the properties of grade 440 bar products covered under ASTM A276. These values may not be similar to that of other forms such as forgings and plates.

Corrosion Resistance

Grade 440 stainless steels exhibit excellent resistance to mild acids, alkalis, foods, fresh water and air. A smooth polished surface also helps grade 440 steels resist corrosion in tempered, passivated and hardened conditions.

Grade 440C steels exhibit corrosion resistance similar to that of grade 304 steels.

Heat Resistance

Grade 440 stainless steels should not be used at temperatures below the relevant tempering temperature, due to loss of mechanical properties caused by over-tempering.

Heat Treatment

Annealing — Grade 440 stainless steels are full annealed at 850 to 900°C, followed by slow furnace-cooling at about 600°C and air-cooling. Sub-critical annealing is carried out at 735 to 785°C, followed by slow furnace-cooling.

Hardening — Grade 440 stainless steels are heated at 1010 to 1065°C, then quenched in air or oil. Oil quenching is usually carried out for heavy sections. Following this process, these steels are tempered at temperatures from 150 to 370°C, to achieve a high hardness and improved mechanical properties.

Tempering at temperatures between 425 and 565°C should be avoided as the corrosion and impact resistance properties of grade 440 tend to reduce in this range. Also, tempering of these grades at 590 to 675°C will result in high impact resistance and loss of hardness.