Introduction of Commonly Used Alloy Quenched and Tempered Steel and Heat Treatment

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Alloy quenched and tempered steel is often divided into three categories according to hardenability.

1. Low hardenability quenched and tempered steel. The critical hardening diameter of oil quenching is 20~40mm, and the strength after quenching and tempering is higher than that of carbon steel, generally σb=800~1000MPa, σs=600~800MPa, αK=60~90J/cm2. The total amount of alloying elements WMe<2.5%, it is often used as a medium cross-section, requiring higher mechanical properties than carbon steel quenched and tempered parts. This type of steel includes manganese, silicon-manganese, and chromium quenched and tempered steels. The most used machine tool is 40Cr (substitute material is 40MnB or 35SiMn).

2. Medium-hardenable quenched and tempered steel. The critical hardening diameter of oil quenching is 40~60mm, and the strength after quenching and tempering is very high. Generally, it can reach σb=900~1000MPa, σs=700~900MPa, αK=50~80J/cm2, which can be used for large cross-section and heavy bearing. Loaded machine parts. This type of steel includes chromium-molybdenum series, chromium-manganese series, and chromium-nickel alloy quenched and tempered steels, such as 30CrMnSi, 35CrMo, 38CrMoAlA, 40CrMn, 40CrNi steel and so on. Among them, 30CrMnSi is the most widely used for the manufacture of important aircraft and machine parts.

3. High hardenability quenched and tempered steel. The critical hardening diameter of oil quenching is not less than 60~100mm. This kind of quenched and tempered steel has the highest strength and good toughness after quenching and tempering. Generally, σb=1000~1200, σs=800~1000MPa, αK=60~120J/cm2, which can be used as an important adjustment for large cross-section and greater load. Quality parts. This type of steel includes chromium-manganese series, chromium-nickel series, chromium-nickel-tungsten series quenched and tempered steels, such as 40CrNiMoA, 37CrNi3, 25CrNi4WA steel and so on.

Quenched and tempered steel must be heat treated after hot processing (such as forging) to reduce the hardness and facilitate cutting. Quenched and tempered steel with low content of alloying elements and low hardenability can be annealed; quenched and tempered steel with high hardenability should be normalized and tempered at high temperature. For example, the hardness of 40CrNiMoA steel after normalizing is above 400HBW, and after high temperature tempering, the hardness drops to 207~240HBW, which meets the requirements of cutting. The final heat treatment of quenched and tempered steel is high temperature tempering after quenching, and the tempering temperature is generally 500~650℃.

If in addition to good comprehensive mechanical properties, good wear resistance is required on the surface, surface quenching or nitriding treatment can be carried out after quenching and tempering.

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