Seven Ways to Prevent Mold Cracks

01 Methods to prevent quenching cracks

(1) Reasonably design part shape

The design of equal wall thickness shall be adopted as far as possible to avoid abrupt changes in cross section shape:

① The parts with large difference in wall thickness shall not be made into a whole, and the mosaic structure shall be adopted so that each block can be as strong as possible;

② Large parts shall be designed as hollow structure as far as possible;

③ If necessary, process holes shall be opened;

④ Sharp corners shall be avoided at the corners of the contour, and larger radius R shall be added as much as possible. The fillet radius of tool steel heat treated parts shall not be less than 2mm.

(2) Strictly control the heat treatment process

① The correct heat treatment process specification shall be formulated according to the size, shape characteristics and material of the parts, and the heating speed, temperature and time shall be reasonably selected;

② Perfect temperature control measures shall be taken to master the real temperature of the parts in real time, and the temperature control device shall be calibrated regularly to prevent overheating caused by aging or inaccurate placement of thermocouples.

02 Methods to prevent tempering cracks

Tempering crack refers to that the structure of quenched steel martensite is in expansion state. When tempering, it first shrinks at about 100 ℃. At this time, if the quenched part is heated rapidly, the surface of the part shrinks, while the interior is in expansion state, leading to cracking. The measure to prevent such cracking is not to conduct rapid heating before the parts are heated to 300 ℃.

Tempering and quenching also produce tempering cracks. This is because the residual austenite in the quenching structure becomes the same state as quenching when it is quenched. The method to prevent such cracks is air cooling from the tempering temperature.

03 Methods to prevent spontaneous cracks

As the surface structure of quenched parts is residual austenite, when placed at room temperature, the transformation from austenite to martensite will produce phase transformation expansion, which will cause parts cracking. The measures to prevent such cracks are: tempering the parts immediately after quenching. The time interval between quenching and tempering shall not exceed 3h. If the tempering cannot be carried out on the same day, the parts shall be placed in a 100 ℃ furnace or water for thermal insulation, so as to wait for the normal tempering treatment on the next day.

04 Measures to prevent grinding cracks

Grinding cracks are mainly caused by grinding heat. The same reason as tempering cracks is that grinding heat shrinks the surface quenched structure by the same fire. The direction of cracking is generally at right angles to the grinding direction.

Small preventive measures are:

① Before grinding the quenched parts, the parts shall be tempered at a low temperature of 150 ℃ or a high temperature of about 300 ℃ in advance (when the grinding amount is large);

② The grinding wheel shall be trimmed strictly to keep the grinding tool sharp and prevent the grinding wheel from blocking to reduce grinding heat;

③ Reasonable selection of grinding wheel size, machining practice has proved that the selection of coarser grinding wheel can reduce the occurrence of cracks;

④ It is proved by practice that the grinding burn can be avoided and the probability of workpiece cracking can be reduced by selecting appropriate grinding parameters, using higher grinding wheel speed and increasing workpiece speed.

05 Methods to prevent decarburization cracking

The reason for decarburization is that the heating temperature is too high or proper protection is not taken for heating in the air. To prevent decarburization, vacuum furnace can be used to heat the workpiece or the workpiece can be heated in the protective atmosphere to strictly control the temperature to avoid over burning.

06 Measures to prevent low-temperature cracks

As the residual stress of quenched parts will increase during cold treatment, resulting in low-temperature cracking, low-temperature tempering process shall be added before cold treatment to reduce the occurrence of cracks.

07 Measures to prevent EDM cracks

The instantaneous high temperature and rapid cooling of EDM are very easy to form microcracks on the surface of quenched parts. The method to prevent microcracks is to use a smaller electrical gauge for processing (the processing speed will be affected), or to conduct necessary polishing treatment on the modified layer after processing.