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Heat Treatment of Steel

Heat treatment is a metal hot working process. The solid-state material will get better properties by heating, cooling and other ways of heat treatment. Steel heat treatment would not change the shape and chemical composition of steel products. It will change the microscopic constitution of steel core or change the chemical composition of the steel surface. The characteristic is to change the steel quality, but those things could not be seen by the eyes. In order to make it get the proper mechanical property, physical property and mechanical property, heat treatment can make a great impact on that. The annealing, normalizing, quenching, tempering are several types of basic steel heat treatment.

Annealing

Heat the steel to make its temperature up to a point then take it to heat preservation. Finally, make it slow cooling to indoor temperature.  The purposes of annealing for steel as below.

1. Decrease the hardness of steel and raise the plastic property, that will make the process of cutting and cold-forming.
2. Make the organization of steel distribute more uniform. Raise the properties of steel and be ready for other heat treatment steps.
3. Remove the internal stress of steel to prevent the steel from being out of shape and cracking.

Normalizing

Heat the steel until the temperature on the range for 40℃-60℃. Then take time to do heat preservation. The last thing is air cooling. The goals of steel normalizing as below.

1. Make the weave construction of steel get better and improve machinability.
2. The normalizing always be used for the last step of heat treatment for the fittings which with lower requirements of mechanical property.

Quenching

Heat the steel products up to quenching temperature, then stay the temperature. Finally, take rapid cooling in the water or oil. It makes steel get higher hardness and wear resistance.

Tempering

Heat the steel which has been quenching up to close critical temperature then stay temperature for a certain time. Then take it to fast cooling in the air or oil. After the quenching, then must take the steel products to temper. It is the last step for steel heat treatment. There are several goals for steel tempering as below.

1. It will make the steel get better mechanical property. The strength and hardness of fittings will be raised after the quenching. But the plastic property and toughness will be lower. But during the use, it has high requirements for those two factors. So choose the right temperature to temper can change this problem.
2. Make the organization structure become more stable and maintain size and shape.
3. Eliminate internal stress.

Steel heat treatment is a big subject. Everyone who works for the steel company needs to understand fully. It has great impact during steel production and use, but the ultimate purpose is to get proper properties. The best way for that is by taking a different heat treatment according to the requirements of use.