1、 Longitudinal crack
The reason for the longitudinal crack is that when the die is completely quenched, the core is transformed into the quenched martensite with the largest specific capacity, thus forming the tangential tensile stress. Moreover, the more carbon content in the die steel is, the greater the tangential tensile stress is. When the tensile stress exceeds the limit of the steel strength, the longitudinal crack occurs.
The crack direction is axial, and the shape is fine and long.
2、 Transverse crack
The transverse crack is caused by the large tensile stress in the hardened area and the transition part of the hardened area, which is not hardened yet. When the large mold is cooled rapidly, it is easy to produce a large tensile stress peak value. Because the axial stress is greater than the tangential stress, the transverse crack is formed.
The transverse crack is perpendicular to the axial direction.
3、 Arc crack
The reason for the arc crack is that the stress produced at the corner during quenching is 10 times of the average stress on the smooth surface
The arc cracks mainly appear in the sharp corner, boss, knife pattern, sharp angle, right angle, notch, hole, and die wiring flash.
4、 Peeling crack
Because the specific volume of surface structure and core structure of die is different, the surface layer forms axial and tangential quenching stress, radial tensile stress and internal mutation, and produces peeling crack in the narrow range of stress sharp change, which often occurs in the cooling process of die after surface chemical heat treatment, because the surface chemical modification and steel matrix phase transformation are different, the inner and outer layers are caused The quenching martensite expansion does not occur at the same time, resulting in large transformation stress, resulting in chemical treatment of the layer from the matrix structure. Such as flame surface hardening layer, high frequency surface hardening layer, carburizing layer, carbonitriding layer, nitriding layer, boronizing layer, etc. After quenching, the chemical layer should not be tempered rapidly, especially when the temperature is below 300 ℃, the tensile stress will be formed in the surface layer, while the compressive stress will be formed in the core and transition layer of steel matrix. When the tensile stress is greater than the compressive stress, the chemical layer will be cracked and peeled.
The peeling crack is that when the die is in service, under the action of stress, the hardened layer is stripped from the steel matrix one by one.
5、 Reticular crack
There are several reasons for the formation of network cracks: 1. The raw material itself has a relatively deep decarburized layer, which is not removed during cooling processing; 2. The metal structure of decarburized surface of die and the amount of carbon contained in martensite are different, and the specific capacity is different. During decarburization surface quenching, the decarburized surface metal produces large tensile stress, and the surface metal is often pulled into a network along the grain boundary; 3. Coarse grain coarse grain steel of raw material The original structure is coarse and there are massive ferrite, which can not be eliminated by ordinary quenching. If it is left in the quenched structure, it may be that the temperature control is not accurate, or the instrument is out of order, and network cracks may occur.
The reticular crack is also known as craze. Its depth is relatively shallow, generally about 0.01-1.5mm in depth, showing a radial shape.
6、 Cold treatment crack
More die steel is medium and high carbon alloy steel. After quenching, some undercooled austenite is not transformed into martensite, and remains in service state to become retained austenite, which affects service performance. If it is cooled below zero, retained austenite can undergo martensitic transformation. Therefore, the essence of cold treatment is to continue quenching. When the superposition of quenching stress at room temperature and quenching stress at zero, cold treatment crack will be formed when the superimposed stress exceeds the strength limit of the material.
7、 Grinding crack
The causes of grinding cracks are as follows: improper pretreatment of raw materials, failure to eliminate massive, reticular and banded carbides of raw materials and serious decarburization; over heating temperature of quenching leads to overheating, coarse grains and more retained austenite;
The micro cracks are perpendicular to the grinding direction and the depth is about 0.05-1.0 mm.
8、 WEDM crack
The reason for the formation of WEDM crack is that the raw material has serious carbide segregation or the instrument is broken, the quenching heating temperature is too high, the grain is coarse, the strength and toughness of the material is reduced, the brittleness is increased, the quenched workpiece is not tempered in time and the tempering is not sufficient, there is excessive residual internal stress and the new internal stress formed in the process of WEDM.
The WEDM cracks are arc tail shaped rigid metamorphic layer cracks
9、 Fatigue crack
The micro fatigue crack of die under the repeated action of alternating stress propagates slowly, resulting in sudden fatigue fracture.
10、 Stress corrosion cracking
Stress corrosion cracking is the process of chemical reaction or electrochemical reaction, which leads to the damage of the structure from the surface to the inside.