Medium carbon steel is a kind of carbon steel with a carbon content of 0.25% to 0.60%. It includes most of high-quality carbon structural steel and part of ordinary carbon structural steel. Medium carbon steel is mostly used to make various mechanical parts, and some are used to make engineering structural parts.
The final heat treatment methods of medium carbon steel include quenching and tempering, low temperature tempering after quenching, low temperature tempering after high frequency quenching, austempering and medium temperature tempering after quenching.
(1) Tempering. The structure is tempered sorbite. This kind of structure has good comprehensive mechanical properties, high strength, good plasticity and toughness. The hardened and tempered steel must have good hardenability to ensure uniform structure and performance on the entire section of the hardened and tempered part. Compared with alloy steel, carbon steel has poor hardenability, so it is only suitable for medium carbon steel parts with small cross-sectional dimensions for hardening and tempering.
(2) Low temperature tempering after quenching. The structure is tempered martensite, which has high strength and appropriate plasticity and toughness.
(3) Low temperature tempering after high frequency quenching. The structure of the high-frequency quenching layer is extremely fine cryptic martensite, which is tempered at low temperature to obtain tempered martensite. This treatment can obtain a similar effect to the carburizing treatment. Quenching or normalizing is generally carried out before induction hardening. Therefore, after high-frequency quenching and tempering, the core of the part has high strength, good plasticity and toughness. And its surface layer has high hardness and good wear resistance. In addition, the surface layer of high-frequency hardened parts produces pressure stress, which has a high fatigue limit and a long service life.
(4) Austempering. The structure is bainite, which has high strength and good plasticity and toughness.
(5) Middle temperature tempering after quenching. The structure is tempered sorbite.
The medium carbon steel uses:
1. High-strength medium carbon tempered steel has certain plasticity, toughness and strength, thus it has good machinability, good comprehensive mechanical properties after hardening and tempering treatment. But it is in poor hardenability and low welding performance and easy to crack, so it needs to be preheated well before welding, and heat treatment is required after welding.
2. Medium carbon steel is mainly used to manufacture high-strength moving parts, such as air compressors, pump pistons, steam turbine impellers, heavy machinery shafts, worms, gears, parts with wear-resistant surfaces, crankshafts , machine tool spindles, drums, and fitter tools.