Parts subjected to high loads and shock loads require high strength, but at the same time sufficient toughness.This performance requirement is met by suitable tempered steels.
The heat treatment method of tempered steel is: after quenching, and then between 500-700 tempering. This heat treatment method is called quenching and tempering.
Quenching and tempering is mainly used for transmission shafts, crankshafts, bolts, rods, screws, connecting rods and other parts.
The parts have high strength and good toughness through quenching and tempering.
Both unalloyed and alloyed steels can be quenched and tempered. Non-alloy quenched and tempered steel contains 0.2%-0.6% carbon, and alloy quenched and tempered steel adds a small amount of chromium, molybdenum, nickel or manganese. Frequently used quenched and tempered steels are: C45E, 28Mn6, 42CrMo4. Strength achievable by tempering: unalloyed steel up to 1000N/mm², alloy steel up to 1400N/mm².
Quenched steel is very hard and has high strength, but is brittle and prone to fracture. The subsequent tempering process reduces the hardness, tensile strength and yield strength, but increases the toughness and elongation at break of the material.
■The internal change process of the material during quenching and tempering
After quenching, acicular martensite is formed, a brittle and hard structure.
When tempered to 400°C, part of the martensite decomposes into finely distributed ferrite and acicular cementite precipitated in the remaining martensite. As the tempering temperature increases, the decomposition of martensite accelerates.
When the tempering temperature reaches 550 °C, the martensite has been completely decomposed into ferrite and acicular cementite.
When the tempering temperature reaches 700°C, the needle-like cementite finally aggregates into cementite grains.
■Heat treatment of quenched and tempered steel
The heat treatment methods of quenched and tempered steel include annealing treatment and quenching and tempering. During the annealing treatment, softening annealing and normalizing can be used respectively as required. The purpose of the former is to transform the strip cementite into fine-grained cementite, and the purpose of the latter is to obtain a uniform and fine structure. Quenching and tempering is the standard heat treatment method for quenched and tempered steel. The purpose of tempering is to obtain high strength and high yield strength and good toughness (high elongation at break) of the material. Depending on the tempering temperature, the material can be tempered to achieve higher strength or better toughness. Therefore, we divide quenched and tempered steel into hardness quenched and tempered steel and toughness quenched and tempered steel.The tempering temperature of the quenched and tempered steel required to achieve the required ratio of hardness to toughness can be read from the quenched and tempered curve chart, which is formulated by the steel manufacturer for each standardized quenched and tempered steel