Heat treatment and characteristics of steel
Heat treatment refers to a metal hot working process in which materials are heated, insulated and cooled in the solid state to obtain the expected microstructure and properties.
There are four basic processes for overall heat treatment of iron and steel: annealing, normalizing, quenching and tempering.
Annealing: the process of heating the workpiece to a temperature below or above AC1, holding it for a certain time, and then cooling with the furnace to obtain a balanced structure. Classification: complete annealing, incomplete annealing, spheroidizing annealing, diffusion annealing, recrystallization annealing and stress relief annealing.
Second normalizing: the process of heating steel above AC3 or ACCM and cooling it in air to obtain fine pearlite. Differences from annealing: 1. The cooling speed is faster than annealing. 2. The normalized and annealed structures are p-type structures. The normalized state is pseudo P and the annealed state is equilibrium structure. 3 Normalized granular pearlite is finer than annealed pearlite.
The bainite is usually obtained by heating the steel to a certain heat treatment speed (AC) or above after quenching at a certain heat treatment speed (VC). Necessary conditions for quenching: 1. The steel parts must be heated above AC1 or AC3 for a certain time to obtain a structure 2. The cooling rate must be greater than the critical cooling rate VC to avoid a high-temperature decomposition 3 and obtain martensite (or lower bainite) structure, which is the essence of quenching.
- tempering: reheat the quenched parts to a certain temperature lower than the critical point A1 for heating and insulation, which is a heat treatment process in which the quenched Yawen structure is transformed into a stable tempering structure and cooled to room temperature at an appropriate cooling rate. It is divided into low temperature tempering, medium temperature tempering and high temperature tempering.
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