440C Steel

440 stainless steel, a high martensitic stainless steel, has high strength, good hardness, wear resistance and moderate corrosion resistance. After the correct heat treatment, 440C has the capability to reach the highest strength, hardness and wear resistance among the corrosion and heat resistant stainless steels. Corrosion resistance is lower than for the usual austenitic grades, and their useful operating temperature range is limited by loss of ductility at temperatures below zero and loss of strength due to over-tempering at higher temperatures as well as the reduction in corrosion resistance. In both its pre-hardened and post-hardened condition 440C is magnetic.

Applications

Applications for this stainless steel include: ball bearings, bushings, valve parts, cutlery, knives and blades, nozzles, pump parts.

Corrosion resistance

440C stainless steel has good resistance to atmosphere, fresh water, fruit and vegetable juices, alkalis and mild acids and its corrosion resistance is similar in many environments to 304 stainless steel. Achieve maximum corrosion resistance by hardening from 2000°F (1093°C), to provide a better carbide solution, meaning the better the finish of the hardened and tempered part, the better its corrosion resistance. A smooth polished surface will support this as well. Care should be taken, however, to minimize the time at 2000°F (1093°C) to avoid excessive coarsening of the grain. For optimum corrosion resistance, the tempering temperature should be below 800°F (427°C).

Heat resistance

The use at temperatures above the respective tempering temperature is not recommended, as mechanical properties will deteriorate due to over-hardening.

Heat treatment

440C is a martensitic stainless steel, thus it is hardenable by heat treatment. To achieve the creation of martensite, it is necessary to reduce the amount of austenite by heat treatment.

Annealing

Full anneal at 1562-1652°F (850-900°C), then slowly cool in the furnace to about 1112°F (600°C) and then cool further in air to room temperature.

Sub-critical anneal at 1355-1445°F (735-785°C), then slowly cool to room temperature in the furnace.

Hardening

Heat to 1850-1950°F (1010-1066°C), followed by quenching in warm oil or cooling in air. Oil quenching is necessary for large sections or complex parts. Be aware not to overheat, as full hardness cannot be reached and temper immediately to achieve a wide range of hardness values.

Tempering

440C is tempered at a temperature range of 302-698°F (150-370°C) where several levels of hardness and mechanical properties can be obtained. Tempering in the range of 797-1049°F (425-565°C) reduces impact strength and corrosion resistance and should be avoided.

For our tempering diagram, please click here

Sub-zero Treatment

Quenching 440C down to cryogenic temperatures reduces the austenite content of the steel. Before the tempering process, 440C can be treated either by deep or shallow cryogenic treatment.

To increase hardness up to 7% with the deep cryogenic treatment cool the steel to -321°F (-196°C) and hold at this temperature for 24 hours, then bring back to room temperature.

To increase hardness up to 4% with the shallow cryogenic treatment cool the steel to -112°F (-80°C) and hold for 5 hours and bring the temperature back to room temperature.

The steel then can be tempered at 392°F (200°C) after the cryogenic treatment.

Machinability

Machining, such as turning or drilling, is mostly done in the annealed condition. As a result to its high carbon content it machines similar to high speed steel. Chip curlers and breakers should be used as chips are tough and stringy.

Heat treated 440C is more difficult and probably impossible to machine due to its high hardness.

Welding

As 440C has high-hardness capabilities, and it can air harden easily, this alloy is not welded often. Though if welding is necessary, preheat and maintain at 500°F (260°C). Immediately after the parts need a 6-8 hour anneal at 1350-1400°F (732-760°C) with a slow furnace cool. Do not allow to cool below 500°F (260°C) between welding and annealing. Use high welding inputs and consider a similar filler metal to achieve complementary mechanical properties.

Resistance to scaling (oxidation)

At about 1400°F (760°C) 440C stainless steel scales. With the type of atmosphere, construction and cycle of operation, this temperature will vary.

Grinding and Polishing

For cutlery applications, grinding and polishing is very important. Loss of hardness and corrosion resistance can occur when overheating.