H13 Steel Module: Reasons for Unqualified Transverse Impact Energy

H13 hot work die steel has good thermal strength, cold and hot fatigue resistance and liquid metal erosion resistance, and is widely used in hot extrusion dies, aluminum alloy die-casting dies and other types of dies.

The service life of the die is determined by the impact performance because the die needs to bear a large impact force in the process of use.

With the development of the automobile industry, die casting technology is mainly used in the production of auto bracket, clutch, oil pan and other parts.

High pressure and high speed filling of the die casting mold cavity are the two major characteristics of die casting. Compared with the extrusion mold, the die casting mold needs to bear more impact energy in the production process, especially when manufacturing large parts, the quality of the mold steel is required to be higher.

The life of the extrusion die made of H13 steel bars produced by conventional process and relatively small modules can reach the expected effect.

The production process flow of a batch of H13 steel modules in a factory is as follows: pretreatment of molten iron → smelting in 20t electric furnace → refining in LF furnace (ladle refining furnace) → vacuum treatment in VD furnace (vacuum refining furnace) → casting into 16t ingots → remelting 16t ingots in 16t gas shielded electroslag furnace → ingot annealing → heating (1180 ℃, 20h) → 45MN rapid forging billet/finished product (section specification: 400mm × 500mm) → annealing → nondestructive testing → sampling inspection.

During the impact energy test of steel plate, it was found that the impact performance did not meet the expected goal.

In order to find out the reason for the low impact performance, researchers such as Li Yongdeng and Yang E from Daye Special Steel Co., Ltd., Hubei Provincial Key Laboratory of High Quality Special Steel and Hubei Huangshi Product Quality Supervision and Inspection Institute analyzed the materials, found out the reason for the unqualified impact energy, and provided a basis for improvement for subsequent production.

1. Physical and chemical inspection

1.1 Chemical composition analysis

Detect the chemical composition of H13 steel module with unqualified impact energy, and the results meet the requirements of GB/T 1299-2014 Tool and Die Steel.

1.2 Impact performance test

The impact test specimen without transverse notch shall be selected for impact performance test.

Take samples from the central part of the module, and then conduct quenching and tempering treatment after making the blank, and then machine to the final sample size.

Three samples were tested, and the impact sample size was 55mm×10mm×7mm.

The impact energy of the sample with good impact performance can reach more than 300J, while the impact energy of the sample with poor impact performance is less than 100J.

1.3 SEM analysis of impact specimen fracture

After ultrasonic cleaning, the fracture surface of the impact sample was analyzed by scanning electron microscope.

For the sample whose impact energy does not reach the expected goal, the fracture surface is relatively flat as a whole.

After magnifying observation, it is found that the fracture source area has different degrees of intergranular fracture characteristics, and the sample with higher impact energy has smaller intergranular fracture area;

On the contrary, the intergranular fracture area of the sample with lower impact energy is larger.

The fracture morphology of the sample with impact energy reaching the expected target is bremsstrahlung, and no intergranular cracking is found.

No defects such as large inclusions are found on the fracture surface.

The fracture morphology of samples with low and high impact energy is shown in Fig. 1 and Fig. 2.

Generally speaking, intergranular fracture is a form of grain boundary.

H13 Steel Module: Reasons for Unqualified Transverse Impact Energy 1

Fig. 1 Fracture Micromorphology of Specimen with Low Impact Energy

H13 Steel Module: Reasons for Unqualified Transverse Impact Energy 2

Fig. 2 Fracture Micromorphology of Specimen with High Impact Energy

1.4 Metallographic inspection

After directly grinding and polishing the fracture surface of the impact sample, it was etched by nitric acid and alcohol, and observed by metallographic microscope.

It was found that the local grain boundary of the sample with low impact energy was obvious.

It was visible that the carbide was clustered and banded at the grain boundary, and no large primary carbide was found.

Take the annealed samples of the same batch with low impact energy.

After grinding and polishing and nitric acid alcohol etching, observe with metallographic microscope, the microstructure is spherical pearlite.

Spherical carbides are distributed in chains locally, and no obvious carbide aggregation is found. This indicates that the segregation in the smelting process is at a normal level.

The microstructure of the samples with low impact energy is shown in Fig. 3.

H13 Steel Module: Reasons for Unqualified Transverse Impact Energy 3

Fig. 3 Microstructure of Fracture of Specimen with Low Impact Energy

For the sample with higher impact energy, the quenched and tempered structure is homogeneous tempered martensite, and no obvious grain boundary carbide is found;

The corresponding annealed structure is uniform spheroidal pearlite, and no carbide aggregation network phenomenon is found (see Fig. 4).

H13 Steel Module: Reasons for Unqualified Transverse Impact Energy 4

Fig. 4 Microstructure of Fracture of Specimen with High Impact Energy

2. Comprehensive analysis

The chemical composition of H13 steel smelted by electroslag remelting meets the requirements of the standard GB/T 1299-2014.

According to the microstructure observation, it is found that there is no obvious carbide accumulation and band segregation, and no obvious non-metallic inclusions are found on the fracture surface, indicating that the smelting process is under normal control.

According to the analysis of the micro morphology and metallographic structure of the impact fracture, the fracture of the sample with low impact energy presents intergranular characteristics, and there are obviously network carbides in the structure.

The fracture of the sample with high impact energy is dimple morphology, and the structure is uniform.

Because the grain boundary of steel is relatively weak, intergranular fracture will be formed when bearing impact load.

Secondary carbide precipitation along grain boundary is the main reason for low impact toughness.

The research shows that the carbides in H13 steel are mainly V8C7, Cr23C6 and Cr3C2 (Cr2VC2).

Influenced by insufficient forging heating and improper control of cooling after forging, these carbides are easy to accumulate on the grain boundary, weaken the grain boundary, and thus reduce the impact toughness of steel.

The key factor to improve the impact properties of H13 steel is to avoid secondary carbide precipitation along the grain boundary.

As long as the heating temperature before forging and the cooling rate after forging are strictly controlled, the precipitation of network carbide of the steel can be effectively improved.

Homogenization at high temperature, increasing the deformation during forging, and decreasing the final forging temperature can make the carbides in the steel fully refined and dispersed, which is conducive to inhibiting the precipitation of secondary carbides along the grain boundary.

After high temperature homogenization treatment of H13 steel, the component segregation formed during smelting and solidification can be effectively improved, and the tendency of carbide and impurities to segregate at grain boundaries is weakened.

The rapid cooling process after forging can effectively prevent the precipitation of coarse or reticulated carbides in steel, and prevent the secondary carbides in the structure from precipitating along the grain boundary to form carbide chains.

The rapid cooling and re annealing process after forging can make the steel form uniform spheroidal pearlite structure.

The internal structure of steel can be improved as long as the deformation in forging process is increased, and large as cast structure and unstable eutectic carbide are broken by large stress.

If conditions permit, upsetting and drawing forging process can be used to further improve the structure of H13 steel to improve its properties.

3. Conclusion and Suggestions

(1) The main reason why the transverse impact performance of H13 steel smelted by electroslag remelting can not reach the expected goal is that the forging link is not properly controlled.

After heat treatment, secondary carbide precipitates along the grain boundary, weakening the grain boundary.

The transverse impact toughness of H13 steel module can be effectively improved as long as the secondary carbide is prevented from precipitating into a network along the grain boundary.

(2) The impact toughness of H13 steel can be effectively enhanced by adopting high temperature homogenization treatment, increasing forging deformation, increasing cooling rate after forging, and minimizing segregation and avoiding carbide precipitation along grain boundaries.

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