Stainless Steel Grades 440A, 440B and 440C

Grade 440C is capable of attaining, after heat treatment, the highest strength, hardness (Rockwell C 60) and wear resistance of all the stainless alloys. Its very high carbon content of 1.0% is responsible for these characteristics, which make 440C particularly suited to such applications as ball bearings and valve parts.

Stainless steel Rolling element bearing

Grades 440A and 440B are identical except for slightly lower carbon contents (0.60 - 0.75% and 0.75 - 0.95% respectively); these have lower attainable hardnesses but slightly higher corrosion resistances. Although all three versions of this grade are standard, in practice 440C is more available than the A or B variants; none of these are regularly stocked in Australia however.

A free-machining variant 440F (UNS S44020) also exists, with the same high carbon content as 440C. Again this grade is not readily available in Australia.

Martensitic stainless steels are optimised for high hardness, and other properties are to some degree compromised. Fabrication must be by methods that allow for poor weldability and also allow for a final harden and temper heat treatment. Corrosion resistance is generally lower than the common austenitic grades, and their useful operating temperature range is limited by their loss of ductility at sub-zero temperatures and loss of strength by over-tempering at elevated temperatures.

Corrosion Resistance

Good resistance to the atmosphere, fresh water, foods, alkalies and mild acids. Best resistance in the hardened and tempered and passivated condition. A smooth polished surface also assists.

The corrosion resistance of Grade 440C is close to that of grade 304 in many environments.

Consult Atlas Technical Assistance for specific environmental recommendations.

Heat Resistance

Not recommended for use in temperatures above the relevant tempering temperature, because of reduction in mechanical properties by over-tempering.

Heat Treatment Annealing

Full anneal - 840-870°C, slow furnace cool to about 600°C and then air cool.

Sub-critical Annealing - 735-760°C and slow furnace cool.

Hardening

Heat to 1010-1065°C, followed by quenching in warm oil or air. Oil quenching is necessary for heavy sections. Immediately temper at 150-370°C to obtain the hardness values and mechanical properties as indicated in the accompanying table.

Best corrosion resistance is when tempered below 425°C. Tempering in the range 425-565°C is to be avoided because of reduced impact resistance and corrosion resistance. Tempering in the range 600-675°C results in lower hardness – the product becomes machinable.

Maximum achievable hardnesses are approximately HRC56 for Grade 440A, HRC58 for 440B and HRC60 for 440C.

Welding

Welding is seldom carried out because of the grades’ high hardenability. If welding is necessary pre-heat at 250°C and follow welding with a full anneal. Grade 420 filler will give a high hardness weld (although not as high as the 440C), but 309 or 310 will produce soft welds with higher ductility.

Machining

In the annealed condition this grade is relatively easily machined; approximately the same as for high speed steel. Chips are tough and stringy so chip breakers are important. If these grades are hardened machining becomes more difficult and probably impossible.

Typical Applications

Rolling element bearings, valve seats, high quality knife blades, surgical instruments and chisels.

These properties are specified for bar product in ASTM A276. Similar but not necessarily identical properties are specified for other products such as wire and forgings in their respective specifications. These grades are not normally available in flat rolled or fluids products.

COLLECTION OF COMMON STAINLESS STEEL GRADE

Source:

http://www.atlassteels.co.nz/