The Failure Analysis of H13 Steel Die Casting Mould

Publish Time: 05/10 2021 Author: Site Editor Visit: 17828

Using optical microscope, scanning electron microscope, hardness tester, impact testing machine, etc., the early failure causes of H13 steel die-casting die for aluminum alloy forming were analyzed. The results show that the failure mode of the mold is overall brittle fracture. The main reason is that there are more serious structural defects such as band segregation, non-metallic inclusions, and liquid carbide in the mold steel. At the same time, the heat treatment process is unreasonable; non-metallic Cracks are formed around inclusions and liquefied carbides under the action of thermal stress and mechanical force. Band segregation and unreasonable heat treatment process reduce the impact toughness of the mold, make the cracks propagate rapidly, and ultimately lead to the early failure of the mold.

$Using optical microscope, scanning electron microscope, hardness tester, impact testing machine, etc., the early failure causes of H13 steel die-casting die for aluminum alloy forming were analyzed. The results show that the failure mode of the mold is overall brittle fracture. The main reason is that there are more serious structural defects such as band segregation, non-metallic inclusions, and liquid carbide in the mold steel.$

H13 steel is currently the most widely used hot work die steel. Due to its high high temperature strength and hardness, it has good toughness, thermal fatigue performance and certain wear resistance under medium temperature conditions, and it can resist corrosion of molten metal. , Often used to make die-casting molds.

During use, the die-casting mold has to withstand the impact and compression stress of high-temperature molten metal, and also withstand the tensile stress generated by the compression of the die-casting metal during demolding. The stress situation is more complicated, and the process of use is often due to thermal cracks and overall Failure due to brittle fracture, corrosion or erosion.

There are many factors that cause die-casting die failure. It is difficult to correctly determine the cause of the failure. In addition, the quality of H13 steel produced by domestic manufacturers is uneven and the heat treatment process is not reasonable. This brings a great deal to the failure analysis of die-casting die. difficult.

A metallurgical plant used aluminum alloy die-casting molds made of H13 steel, and only trial-produced more than 100 products. The mold was broken in whole after the use time was less than one day, which caused certain economic losses to the plant. In order to find the reason for the failure of the H13 steel die-casting die, the author carried out
Failure analysis.

Organizational Defects

There are obvious band segregation defects in the annealed structure of die blank steel. Band segregation is a kind of chemical composition segregation. When the steel ingot is forged and rolled, the dendritic segregation formed during the solidification process is rolled and elongated to form a segregation zone. During annealing, the carbide precipitates along the segregation zone to form a band with different degrees of density. Segregation. Band segregation is the simplest and most important indicator to measure the degree of segregation of H13 steel. It can reflect the segregation of alloying elements and dendrites in the steel ingot structure and whether the hot working process is appropriate. It has a significant impact on the transverse impact toughness of the steel. Therefore, the NADCA#2007-2003 standard clearly stipulates the acceptable level of the annealed structure and band segregation of H13 steel. Band segregation has a great influence on the structure and properties after quenching. After quenching, the low-carbon martensite structure is formed in the carbon-poor zone, and the high-carbon cryptone martensite structure is formed in the carbon-rich zone, which is finally inherited. Tempered state. The band segregation of the failed die steel is serious and the structure is very uneven, which seriously affects the transverse toughness of the die.

Non-metallic inclusions and liquefied carbides in the segregation zone. Studies have pointed out that the reheating and diffusion of the ingot can reduce the element segregation, but for H13 steel, the segregation is difficult to completely eliminate, and once it appears in the segregation zone A large number of non-metallic inclusions and liquefied carbides will further reduce the transverse impact toughness of the steel. This is also an important basis for distinguishing whether the band segregation level is qualified or not in NADCA#2007-2003. According to the test results, the purity of the die steel is low, and the segregation zone contains a large number of non-metallic inclusions. Among them, the DS Al 2 O 3 large particle inclusions have reached the level of 2.0, which seriously damages the continuity of the matrix. , Under the action of external force, cracks are easily formed. The strength of steel decreases with the increase in the number of inclusions, and the larger the size of the inclusions, the greater the impact on toughness. The liquefied carbides are coarse and continuous blocks in the H13 steel ingot, which are broken after forging and distributed in chains along the forging direction. The conventional heat treatment process has basically no effect on the distribution and morphology of the liquefied carbides. Therefore, the chain-like distribution of the liquefied carbides can still be seen in the belt-shaped area of the tempered structure. Similar to inclusions, liquefied carbides can increase the brittleness of steel due to their own fracture or separation from the interface of the matrix. In addition, local sharp-angled chain-like carbides can easily cause stress concentration and microcracks. The concentrated distribution of non-metallic inclusions and liquefied carbides, on the one hand, seriously affects the transverse toughness of steel, and on the other hand, it is easy to form crack sources during use.

Mold hardness is too high

It can be seen from the hardness test results that the hardness of the failed mold is higher than the recommended hardness range of NADCA#2007-2003, and the distribution is uneven. According to the quenching and tempering curve of H13 steel, it can be seen that excessively high quenching temperature or low tempering temperature may cause the hardness of H13 steel to be higher, and insufficient tempering may cause uneven hardness distribution of the mold. The mold may have high hardness after quenching and tempering due to improper operation or furnace temperature control during the heat treatment process, which further affects the impact toughness of the mold, and finally makes the microstructure in an unstable state and excessive residual internal stress. Large, easy to crack when external force acts, causing early failure of the mold.

Failure process

During use, the die-casting mold has to withstand the impact and compressive stress of high-temperature molten metal, as well as the tensile stress generated by the compression of the die-cast metal during demolding, and the service environment is relatively harsh. It can be seen from the test results that a large number of inclusions and liquefied carbides are concentrated near the crack source on the surface. There are differences in the elasticity, plasticity, and thermal expansion coefficient of inclusions and liquefied carbides from the matrix. When thermal stress and mechanical force are repeatedly applied, stress concentration is easily formed around the inclusions and liquefied carbides, and microcracks eventually occur. Due to the low toughness of the die steel, when the microcracks are formed, the die does not have enough toughness to prevent crack propagation. When the stress exceeds its fracture strength, it is easy to cause cracks to penetrate the die, causing the die to crack and be scrapped. From this, it can be judged that the non-metallic inclusions and liquid-deposited carbides in the die steel caused early micro-cracks on the die surface, and the extremely low toughness of the die steel caused the cracks to propagate rapidly, which is an important cause of die cracking.

Improvement measures

According to the above analysis, for H13 steel and its heat treatment process,
The following improvements have been made:

H13 steel adopts electroslag remelting process to improve the purity of steel and reduce the content of non-metallic inclusions; control the remelting speed or use other smelting processes to control the size and quantity of liquid carbide.
Through high temperature diffusion annealing and repeated multi-directional forging with large forging ratio, the band segregation is improved and the liquid carbide is reduced.
The heat treatment process parameters of the mold should be strictly controlled to ensure that the overall hardness of the mold is within the specified range.

Knot Discussion

The fracture of the mold is brittle fracture. The reason is that there is a relatively serious band segregation in the microstructure of the die steel, and there are more non-metallic inclusions and liquid carbides in the segregation zone, plus no Reasonable heat treatment process causes the overall hardness of the mold to be higher. The combined effect of these factors results in the extremely low impact toughness of the mold.
The non-metallic inclusions in the die steel and the vicinity of the liquid carbide are easy to form early micro-cracks, and the extremely low toughness of the die steel causes the cracks to propagate rapidly, and finally the overall die is broken.
In the future production, the factory selected high-quality H13 die steel and strictly controlled the heat treatment process parameters. The service life of the die was significantly improved. No large through cracks were seen after die-casting 10　000 pieces.

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