The quenching and tempering of steel

The quenching and tempering of steel has a hardness range, generally between 225-297HB. If it is 45# steel quenching and tempering, that is, quenching + high temperature tempering, the highest hardness should be 297HB, that is, 31 ~ 32HRC, which is the most ideal state.

In the process of heat treatment, whether the steel cracks doesn’t have anything to do with its hardness, but has a relation with the shape of the workpiece and the way of heat treatment. 

Normalizing is generally used for medium and low carbon steels. After normalizing, the pearlite structure is obtained, which is convenient for cutting and processing. High carbon steel and alloy steel use annealing (phase transformation recrystallization, spheroidization, and isothermal.)

Quenching and tempering is a double heat treatment of quenching and high temperature tempering, and its purpose is to make the workpiece have good comprehensive mechanical properties.

Quenched and tempered steel has two categories: carbon quenched and tempered steel and alloy quenched and tempered steel. Regardless of whether it is carbon steel or alloy steel, its carbon content is controlled strictly. If the carbon content is too high, the strength of the workpiece after quenching and tempering is high, but the toughness is not enough. For example, if the carbon content is too low, the toughness will increase, but the strength will be insufficient. In order to obtain a good comprehensive performance of the quenched and tempered workpiece, the carbon content is generally controlled at 0.30~0.50%.

During quenching and tempering, the entire section of the workpiece is required to be quenched, so that the workpiece can obtain a microstructure dominated by fine needle-shaped quenched martensite. By tempering high temperature, a microstructure dominated by uniform tempered sorbite is obtained. Some small factories may not perform metallographic analysis for each furnace of steel; generally only perform the steel hardness testing, that is, whether the hardness meets requirements of the material after quenching, just check the hardness of the workpiece after tempering according to the requirements of the drawing 

The operation of the workpiece quenching and tempering treatment must be carried out strictly in accordance with the process documents, here only some suggestions are provided on how to implement the process during the operation.

1. Quenching and tempering of 45 steel

45 steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide sources, so it is widely used. Its biggest weakness is that it has low hardenability, for workpieces which with large cross-sectional dimensions and have relatively high-demand should not be used.

The quenching temperature of 45 steel is A3+(30~50) ℃, if it is quenched by using water, it can reach 40----60HRC (depending on the water temperature and the size of the material), and the quenching using oil is generally between 30----55HRC ( It depends on the thickness of the material and the type of quenching oil).

 In the actual operation of 45 steel quenching, the upper limit temperature is generally taken. A higher quenching temperature can speed up the heating of the workpiece, reduce surface oxidation, and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual amount of furnace installed is large, the holding time needs to be extended appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the holding time is too long, coarse grains and serious oxidative decarburization will also occur, which will affect the quenching quality. Therefore, it is recommended that if the furnace load of the workpiece is greater than the provisions of the process documents, the heating and holding time should be extended by 1/5.

Because 45 steel has low hardenability, a 10% saline solution with a high cooling rate should be used. After the workpiece enters the water, it should be quenched, but not cold through. If the workpiece is thoroughly cooled in salt water, it is possible to crack the workpiece. This is because that the workpiece is cooled to about 180°C, the rapid transformation of austenite into marten site caused by excessive tissue stress. Therefore, the slow cooling method should be adopted when the quenched workpiece is rapidly cooled to this temperature region. Since the temperature of the outlet water is difficult to control, it must be operated by experience. When the workpiece stops shaking in the water, it can be out of water for air-cooled (oil cooling is better if possible). In addition, the workpiece into the water should be dynamic rather than static it should be moved regularly in accordance with the geometric shape of the workpiece. The static cooling medium and the static workpiece will lead to uneven hardness, uneven stress and large deformation of even cracking of the workpiece.