What is the cause of the network crack

What is the cause of the network crack
The crack depth is relatively shallow, generally about 0.01~1.5mm deep, radiating, and the alias is crack. The main reasons are:
(1) The raw material has a deep decarburization layer, which is not removed by cooling and cutting, or the finished mold is heated in an oxidizing atmosphere furnace to cause oxidative decarburization;
(2) The structure of the decarburized surface metal of the die is different from the carbon content and specific volume of the martensite in the steel matrix. The decarburized surface of the steel produces large tensile stress during quenching. Therefore, the surface metal is often cracked into a network along the grain boundary;
(3) The raw material is coarse grain steel. The original structure is coarse, and there is massive ferrite, which cannot be eliminated by conventional quenching. It is retained in the quenching structure, or the temperature control is inaccurate, the instrument fails, the structure overheats, or even burns, the grain coarsens, and the grain boundary bonding force is lost. When the die is quenched and cooled, the steel carbide precipitates along the austenite grain boundary, the grain boundary strength is greatly reduced, the toughness is poor, and the brittleness is large, Under the action of tensile stress, there is a network crack along the grain boundary.

Preventive measures:
(1) The chemical composition, metallographic structure and flaw detection of raw materials shall be strictly checked, and unqualified raw materials and coarse grain steel shall not be used as die materials;
(2) Fine grain steel and vacuum electric furnace steel shall be selected, and the depth of decarburization layer of raw materials shall be rechecked before production. The cold cutting allowance shall be greater than the depth of decarburization layer;
(3) Formulate advanced and reasonable heat treatment process, select microcomputer temperature control instrument with control accuracy of ± 1.5 ℃, and calibrate the instrument on site regularly;
(4) Vacuum electric furnace, protective atmosphere furnace and fully deoxidized salt bath furnace are selected for mold product treatment to effectively prevent and avoid the formation of network cracks.