The "four fires" in heat treatment
1. Annealing steel is heated to a certain temperature and held for a period of time, and then it is slowly cooled, known as annealing. Annealing of steel is the steel heated to the temperature at which the phase change or partial phase change occurs, after holding the heat treatment method of slow cooling. The purpose of annealing is to eliminate organizational defects, improve the organization so that the composition of uniformity and refinement of grain, improve the mechanical properties of steel, reduce residual stress; at the same time can reduce hardness, improve plasticity and toughness, improve cutting performance. So annealing is not only to eliminate and improve the organization defects and internal stresses left by the previous process, but also to prepare for the subsequent process, so annealing is a semi-finished product heat treatment, also known as pre-heat treatment. Iron-carbon balance of steel
2. Normalizing Normalizing is a heat treatment method to heat the steel above the critical temperature, so that all the steel is transformed into uniform austenite, and then cooled naturally in the air. It can eliminate over-eutectoid steel mesh carburite, for sub-eutectoid steel normalizing can refine the lattice, improve the overall mechanical properties, the requirements of the parts are not high with normalizing instead of annealing process is more economical.
Quenching Quenching is the steel is heated to a critical temperature above, hold for a period of time, and then quickly into the quenching agent, so that its temperature suddenly lowered to a rate greater than the critical cooling rate of rapid cooling, and to obtain the martensite unbalanced organization of the main heat treatment method. Quenching can increase the strength and hardness of steel, but to reduce its plasticity. The quenching agent commonly used in quenching are: water, oil, alkaline water and salt solutions. The quenching agent for high-speed steel can be "wind", so high-speed steel is also known as "wind steel".
4. Temper the steel has been quenched and reheated to a certain temperature, and then cooled by a certain method called tempering. Its purpose is to eliminate the internal stresses generated by quenching, reduce hardness and brittleness, in order to obtain the desired mechanical properties. Tempering is divided into high-temperature tempering, tempering and low-temperature tempering three types. Tempering is mostly used in conjunction with quenching and normalizing. Tempering treatment: quenching and tempering after high-temperature heat treatment method is called tempering treatment. High-temperature tempering refers to tempering between 500-650 ℃. Tempering can make the performance of the steel, the material is largely adjusted, its strength, plasticity and toughness are better, with good overall mechanical properties. Ageing treatment: In order to eliminate precision gauges or molds, parts in long-term use size, shape changes, often in the low-temperature tempering (low-temperature tempering temperature 150-250 ℃) before finishing, the workpiece is reheated to 100-150 ℃, hold 5-20 hours, this treatment for the quality of stable precision parts, called ageing. It is especially important to age steel components under low temperature or dynamic load conditions to eliminate residual stresses and stabilize the steel organization and dimensions.