440C STAINLESS STEEL | 1.4125 | SUS440C

4. Heat treatment

The heat treating of 440C stainless steel, like other martensitic stainless steels, typically involves austenitizing, quenching, and tempering. The process aims to transform the steel into a martensitic microstructure, which is then tempered to optimize its balance of hardness, strength, ductility, and corrosion resistance. 

4.1 Austenitizing(hardening)

The austenitizing temperature is 925 to 1065 °C (1700 to 1950 °F). We recommend a temperature of 1038°C (1900°F), as we believe this temperature will provide the best hardness and corrosion resistance. Additionally, we suggest preheating the material to 650°C (1200°F) before this step to minimize distortion, warping, or cracking from high thermal gradients during rapid heating or quenching. Soaking time should be sufficient to ensure carbides dissolve into the austenite, which can be slower than in plain carbon steels due to the presence of chromium.

4.2 Quenching

Following austenitizing, 440C steel is typically quenched in air or oil. Oil quenching from temperatures like 1038°C (1900°F) or 1040°C (1900°F) is a common choice. After quenching, the steel transforms into martensite, a hard yet brittle structure.

4.3 Tempering

The step following quenching is tempering, which aims to reduce hardness, enhance toughness and ductility, and relieve internal stress.

The tempering temperature is 392°F to 662°F (200°C to 350°C). Different tempering temperatures result in varying hardness levels, as follows:

It should be noted that tempering 440C stainless steel and similar high-carbon martensitic stainless steels in the range of 427 °C to 566 °C (800 °F to 1050 °F) will result in temper brittleness and a decrease in impact strength and corrosion resistance.
If tempering outside the aforementioned temperature range is necessary to enhance toughness, we recommend a tempering temperature of 593°C (1100°F) or higher. Tempering at this temperature improves impact strength and corrosion resistance but sacrifices some hardness.

4.4 Special Considerations in 440C Steel Heat Treatment

• Preheating: Before the main hardening(austenitizing) process, we suggest preheating 440C stainless steel to approximately 650°C (1200°F). This helps to ensure a more uniform temperature distribution and minimize thermal shock during the subsequent austenitizing stage.
• Slow Cooling After Forging: For high-carbon steels such as 440C, we recommend slow cooling after forging, or intermittent cooling, which involves cooling to 150-250°C, reheating to approximately 650°C, and then final cooling. This helps prevent cracking and the formation of grain boundary carbides.
• Managing Retained Austenite: As-quenched 440C steel can sometimes contain a significant amount of retained austenite (potentially 20-30% by volume). If not addressed, this retained austenite can transform spontaneously over time, leading to dimensional changes and an increased risk of cracking. Performing a subzero cold treatment after quenching can effectively transform this retained austenite into untempered martensite. This cold treatment must be followed by at least one standard tempering cycle to temper the newly formed martensite and relieve internal stresses.
• Post-Weld Heat Treatment (PWHT): For welded 440C material components, PWHT is almost always a necessary step. This treatment serves to temper the martensite formed in the weld and heat-affected zone, as well as to relieve residual stresses. The PWHT temperature range for 440C is 480°C to 750°C (895°F to 1380°F).