Analysis and measures of 7 kinds of cracks in die steel quenching

4 peeling crack

When the mold is in service, under stress, the quenched hardened layer is peeled off from the steel matrix piece by piece. Due to the difference in specific volume between the surface layer structure and the core structure of the mold, the surface layer forms axial and tangential quenching stresses during quenching, tensile stress is generated in the radial direction, and sudden changes in the interior, and peeling cracks occur at a narrow range of sharp stress changes, which often occur in During the cooling process of the surface chemical heat treatment mold, the inner and outer layers of quenched martensite will not expand at the same time due to the different time between the chemical modification of the surface and the phase transformation of the steel matrix, resulting in a large phase transformation stress, resulting in the chemical treatment of the permeated layer from the matrix structure Peel off. Such as flame surface hardened layer, high frequency surface hardened layer, carburized layer, carbonitrided layer, nitrided layer, boronized layer, metalized layer, etc. After quenching, the chemically infiltrated layer is not suitable for rapid tempering, especially the rapid heating of low temperature tempering below 300~C, which will promote the formation of tensile stress on the surface layer, while the core of the steel matrix and the transition layer will form compressive stress. The chemically permeable layer was peeled off by tear.

Precaution:

(1) The concentration and hardness of the chemical infiltration layer of mold steel should be gradually reduced from the surface to the inside, and the bonding force between the infiltration layer and the matrix should be enhanced. After infiltration, the diffusion treatment can make the transition between the chemical infiltration layer and the substrate uniform;

(2) Diffusion annealing, spheroidizing annealing, and quenching and tempering treatment are carried out before the chemical treatment of die steel to fully refine the original structure, which can effectively prevent and avoid peeling cracks and ensure product quality.

5 mesh cracks

The depth of the crack is relatively shallow, generally about 0.01-1.5mm deep, radial, alias crack.

The main reasons are:

(1) The raw material has a deep decarburization layer, which is not removed by cold cutting, or the finished mold is heated in an oxidizing atmosphere furnace to cause oxidative decarburization;

(2) The metal structure of the decarburized surface layer of the mold is different from the martensite of the steel matrix. The carbon content and specific volume are different. The steel decarburized surface layer produces large tensile stress during quenching. Therefore, the surface metal is often pulled into a network along the grain boundary. ;

(3) The raw material is coarse-grained steel, the original structure is coarse, there is large ferrite, which can not be eliminated by conventional quenching, and it remains in the quenched structure, or the temperature is not accurately controlled, the instrument fails, the structure is overheated, or even burned, The grains are coarsened and the grain boundary bonding force is lost. When the mold is quenched and cooled, the carbides of the steel precipitate along the austenite grain boundaries. The grain boundary strength is greatly reduced, the toughness is poor, and the brittleness is large. It is networked along the grain boundaries under tensile stress. split.

Precaution:

(1) Strict chemical composition of raw materials. Metallographic structure and flaw detection inspection, unqualified raw materials and coarse grain steel are not suitable for mold materials;

(2) Choose fine-grain steel and vacuum electric furnace steel, review the depth of the decarburization layer of the raw materials before putting it into production, and the cold cutting machining allowance must be greater than the depth of the decarburization layer;

(3) Develop an advanced and reasonable heat treatment process, select a microcomputer temperature control instrument, and the control accuracy can reach ±1.5℃, and the instrument will be checked on site regularly;

(4) The final treatment of mold products selects vacuum electric furnaces, protective atmosphere furnaces and fully deoxidized salt bath furnaces to heat the mold products to effectively prevent and avoid the formation of network cracks.