1.What are the main reasons for the decrease in hardness?
Eliminating work hardening: During cold rolling, metal grains are elongated and broken, generating numerous dislocations and distortions, leading to increased hardness (work hardening). High-temperature tempering (usually recrystallization annealing) enhances atomic diffusion, reduces dislocation rearrangement and distortion, thereby softening the material.
Recrystallization process: When heated above the recrystallization temperature (typically above 450℃ for steel), the cold-deformed structure is gradually replaced by new, undistorted equiaxed grains, significantly reducing hardness and strength while restoring plasticity and toughness.
Carbide precipitation and spheroidization: For steels with high carbon content (such as medium-high carbon steel), high-temperature tempering may promote cementite spheroidization, further reducing hardness.
2.What are the effects of process parameters on hardness?
Temperature: The higher the tempering temperature, the more complete the recrystallization, and the more significant the decrease in hardness. For example:
Low-temperature tempering (150-250℃): Primarily relieves stress, with minimal change in hardness.
High-temperature tempering (500-700℃): Primarily driven by recrystallization, resulting in a significant decrease in hardness.
Time: Extending the holding time will lead to more complete softening, but the effect tends to plateau after a certain time.
Cooling method: Generally air cooling or furnace cooling; the cooling rate has a relatively small impact (unlike quenching).
3.What are the variations in hardness of typical materials?
Low-carbon steel has a hardness of 180-220 after cold rolling and 90-120 after high-temperature tempering.
Medium carbon steel has a hardness of 250-300 after cold rolling and 150-200 after high-temperature tempering.
Stainless steel has a hardness of 400-500 after cold rolling and 200-250 after high-temperature tempering.
4.What other performance changes are there?
Reduced strength: Yield strength and tensile strength decrease with decreasing hardness.
Increased plasticity: Elongation and stamping formability are significantly improved.
Stress relief: Reduces deformation and cracking tendency, and improves dimensional stability.
5.What are the precautions for application?
Target matching: High-temperature tempering is often used for subsequent processing requiring good formability (such as deep drawing and bending).
Oxidation control: A protective atmosphere (such as nitrogen) is required at high temperatures to prevent surface oxidation and decarburization.
Material specificity: Materials with high alloying element content (such as high-strength steel) have higher recrystallization temperatures, requiring process adjustments.