Basic factors affecting the service life of dies

The basic factors affecting the service life of dies are as follows
Die structure design, die material, cold and hot processing technology, heat treatment, grinding, adjustment and operation of machine tool, properties and state of processed materials, lubrication conditions and service environment of dies are the eight basic factors affecting the service life of dies

1. Structural design
Unreasonable structural design is often an important factor for the early failure and heat treatment deformation and cracking of the die. In the structural design of the die, sharp fillet and excessive cross-section change should be avoided as far as possible. The stress concentration caused by sharp fillet can be as high as 10 times of the average calculated stress, For example, the inner square head bolt was upset by cold upsetting die in the original design, and the service life was 500 pieces, which broke at the stress concentration part of punch fillet transition; later, the design was improved to increase the radius of fillet transition part from R = 0.127mm to 0.381-0.5mm, and the service life was increased to 12000-27000 pieces, In order to prevent heat treatment deformation and cracking, the cross-section size should be uniform, the shape should be symmetrical and simple, the blind hole should be opened as far as possible, the process hole could be opened if necessary, and the mold with complex shape and easy deformation and cracking could be changed into combined type

2. Die material and heat treatment
The influence of die material on die life is reflected in three aspects: whether the die material is selected correctly, whether the material is good and whether it is used reasonably. When selecting material, the requirements of die performance must be taken into consideration. For cold stamping die, the strength, toughness and wear resistance of steel should be mainly considered. The strength and toughness, as well as toughness and wear resistance, often go up and down When cracking, it is necessary to select materials with lower strength but better toughness, or formulate reasonable heat treatment process to improve the toughness of steel, or select high-grade alloy steel with both high strength and high toughness according to the actual situation, Reasonable surface treatment is used to improve the wear resistance of the die. The strength, wear resistance and corrosion resistance of the plastic die steel at the plastic forming temperature should be taken into account, and the processability and specularity should also be considered

Improper heat treatment is an important factor leading to early failure of dies and moulds. The influence of heat treatment on die life is mainly reflected in two aspects: unreasonable technical requirements for heat treatment and poor quality of heat treatment. Statistical data show that about 70% of dies fail early due to improper material selection and heat treatment. This problem is discussed separately in sections 4 and 5

3. Hot and cold processing technology
The influence of forging and machining on die life is often ignored. Incorrect forging and machining often become the key to early failure of dies. Taking Cr12MoV steel as an example, this steel is one of the most commonly used cold working die steels in China. It belongs to high carbon and high chromium ledeburite steel, which contains a lot of primary and secondary carbides and has a large segregation, It is important to improve the distribution of carbide in Cr12MoV steel. Table 1 shows the effect of carbide grade on mechanical properties of Cr12MoV steel

It is difficult to eliminate the eutectic network carbide by heat treatment, so it must be refined and homogenized by forging. In the national standard, the grade of network carbide is wide. In actual use, it needs to be re forged to meet the requirement of no more than grade 2 carbide. Therefore, it is necessary to upsetting and drawing steel billet from different directions, It should be forged according to “two light and one heavy” method. That is, when the billet is heated to 1100 ~ 1150 ℃, it should be struck gently to prevent forging crack; when the temperature is 1000 ~ 1100 ℃, it should be hammered again to ensure that the carbide is broken; when the plasticity of steel is lower than 1000 ℃, it is necessary to strike again to prevent internal crack, and to ensure that the direction of the carbide formed is perpendicular to the working face of the die, The final forging temperature is 850 ~ 900 ℃ and the forging ratio is generally controlled at 2 ~ 2.5. The crypto needle martensite, fine dispersed carbides and a small amount of retained austenite can be obtained by quenching with residual heat after forging and tempering at low temperature. The service life of the die can be greatly improved

Incorrect machining may lead to early failure of dies in the following three aspects:
① Improper cutting, forming sharp fillet or too small fillet radius, often causes stress concentration and makes die early failure;
② The surface finish is not enough and there are not allowed tool marks, which often lead to the failure of die due to early fatigue failure;
③ The decarburization layer formed by rolling and forging was not removed completely and evenly by machining, which resulted in the formation of soft spots and excessive residual stress after heat treatment, which led to the early failure of the die