Two methods to prevent quenching defects

In the production of heat treatment, the improper heat treatment process often brings defects to the workpiece, such as oxidation, decarburization, overheating, overburning, insufficient hardness, deformation and cracking, etc.

（1） . oxidation and decarbonization

1. Oxidation is because when steel is heated in oxidizing gas, it will oxidize and form a layer of oxide skin on the surface. At high temperature, even the grain boundary will oxidize again.

2. When the steel is heated in some media, the carbon content on the steel surface will decrease. We call this phenomenon "decarburization". The methods to reduce or prevent the oxidation and decarburization of steel in quenching are as follows:

1） . heating in a salt dissolving furnace with good deoxidation;

2) . heating in a controlled atmosphere furnace;

3) . heating in a vacuum furnace;

4) . reserve enough machining allowance.

（2） . deformation and cracking

As long as the deformation and cracking in quenching are caused by the internal stress formed during quenching, they can be divided into two types: thermal stress and organizational stress, which cause the deformation characteristics of workpiece: making the plane edge convex, the right angle edge obtuse angle, the long direction shorter, the short direction longer, in a word, making the workpiece tend to be spherical. The characteristics of the deformation of the workpiece caused by the tissue stress are the opposite: the plane becomes concave, the right angle becomes obtuse, the long direction becomes longer; the short direction shortens, in a word, the sharp angle tends to be prominent. The deformation and cracking of workpieces are the result of the combination of thermal stress and tissue stress, but the direction of thermal stress and tissue stress is exactly opposite. If the heat treatment is proper, they can partially offset each other and reduce the residual stress. However, when the residual stress exceeds the yield strength of steel, the workpieces will deform, and when the residual stress exceeds the tensile strength of steel, the workpieces will crack. In order to reduce the deformation or cracking, the following measures can be taken in the process besides the correct selection of steel and the reasonable design of workpiece structure:

1. Adopt reasonable forging and pre heat treatment;

2. Adopt reasonable heat treatment process;

3. Adopt correct operation methods;

4. The parts that are easy to crack after quenching, such as keyways and holes, shall be blocked with asbestos.