H13 hot work die steel possesses excellent thermal strength, cold and hot fatigue resistance, and resistance to liquid metal erosion. As a result, it finds wide applications in hot extrusion dies, aluminum alloy die-casting dies, and other types of dies.
In the process of use, the impact performance of the die determines its service life since the die needs to withstand a substantial impact force.
Die casting technology is predominantly used in the production of auto parts, such as auto brackets, clutches, and oil pans, due to the growth of the automobile industry.
The two significant features of die casting are high pressure and high-speed filling of the mold cavity. In comparison to the extrusion mold, the die casting mold has to endure more impact energy during the production process, particularly when fabricating large parts. This necessitates the use of higher-quality mold steel.
The extrusion die made of H13 steel bars produced by conventional processes and relatively small modules can achieve the desired effect in terms of longevity.
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 a steel plate, it was discovered that the impact performance did not meet the expected standard.
To identify the cause of 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. They identified the reason for the unsatisfactory impact energy and provided a foundation for subsequent production improvement.
1. Physical and chemical inspection
1.1 Chemical composition analysis
The chemical composition of the H13 steel module with unqualified impact energy has been detected, and the results comply with the requirements of GB/T 1299-2014 for Tool and Die Steel.
1.2 Impact performance test
The impact performance test should be conducted using impact test specimens without transverse notches.
Samples should be taken from the central part of the module, and then subjected to quenching and tempering treatment after the blank is made, followed by machining to the final sample size.
Three samples were tested, and the impact sample size was 55mm x 10mm x 7mm.
A sample with good impact performance should have an impact energy of more than 300J, while a sample with poor impact performance should have an impact energy of less than 100J.
1.3 SEM analysis of impact specimen fracture
After undergoing ultrasonic cleaning, the fracture surface of the impact sample was analyzed using a scanning electron microscope.
For samples that did not reach the expected impact energy, the fracture surface appears relatively flat overall. Upon closer examination, varying degrees of intergranular fracture characteristics were observed in the fracture source area.
Samples with higher impact energy displayed smaller intergranular fracture areas, while those with lower impact energy showed larger intergranular fracture areas.
Samples that reached the expected impact energy displayed a bremsstrahlung fracture morphology, with no intergranular cracking observed. Additionally, no defects such as large inclusions were found on the fracture surface.
The fracture morphologies of samples with low and high impact energy are depicted in Fig. 1 and Fig. 2, respectively.
In general, intergranular fracture is a form of grain boundary.
Fig. 1 Fracture Micromorphology of Specimen with Low Impact Energy
Fig. 2 Fracture Micromorphology of Specimen with High Impact Energy
1.4 Metallographic inspection
After grinding and polishing the fracture surface of the impact sample, it was etched with nitric acid and alcohol, and observed under a metallographic microscope.
It was observed that the local grain boundary of the sample with low impact energy was evident. The carbide was clustered and banded at the grain boundary, and no significant primary carbide was found.
The annealed samples from the same batch with low impact energy were taken. After grinding, polishing, and nitric acid alcohol etching, the microstructure showed spherical pearlite. Spherical carbides were distributed in chains locally, and no apparent carbide aggregation was found. This indicates that the segregation in the smelting process is at a normal level.
Figure 3 shows the microstructure of the samples with low impact energy.
Fig. 3 Microstructure of Fracture of Specimen with Low Impact Energy
For the sample with higher impact energy, the quenched and tempered structure shows a homogeneous tempered martensite, and no obvious grain boundary carbides were found.
On the other hand, the corresponding annealed structure shows a uniform spheroidal pearlite, and no carbide aggregation network phenomenon was observed (refer to Fig. 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 GB/T 1299-2014 standard.
Microstructure observations indicate that there is no apparent carbide accumulation or band segregation, and no significant non-metallic inclusions on the fracture surface. This indicates that the smelting process is under normal control.
Based on the analysis of micro morphology and metallographic structure of the impact fracture, the sample with low impact energy presents intergranular characteristics, and has evident network carbides in its structure.
The sample with high impact energy shows a dimple morphology, and its structure is uniform.
The intergranular fracture occurs when steel’s grain boundary bears the impact load because it is relatively weak.
The main reason for the low impact toughness of H13 steel is the precipitation of secondary carbides along the grain boundary. Research indicates that the carbides found in H13 steel are primarily V8C7, Cr23C6, and Cr3C2 (Cr2VC2).
Insufficient heating during forging and improper cooling after the process contribute to the accumulation of these carbides along the grain boundary. This accumulation weakens the grain boundary and, as a result, reduces the impact toughness of the steel.
To improve the impact properties of H13 steel, it is crucial to prevent the precipitation of secondary carbides along the grain boundary. This can be achieved by strictly controlling the heating temperature before forging and the cooling rate after the process. By doing so, the precipitation of network carbides can be effectively reduced.
Refining and dispersing carbides in steel can be achieved through homogenization at high temperature, increasing deformation during forging, and decreasing final forging temperature. This process is beneficial in inhibiting the precipitation of secondary carbides along grain boundaries.
By subjecting H13 steel to high temperature homogenization treatment, the component segregation that occurs during smelting and solidification can be effectively improved, and the tendency of carbides and impurities to segregate at grain boundaries is weakened.
Rapid cooling after forging can prevent the precipitation of coarse or reticulated carbides in steel, as well as prevent secondary carbides from precipitating along the grain boundary to form carbide chains.
Rapid cooling followed by re-annealing after forging can produce a uniform spheroidal pearlite structure in the steel.
Increasing deformation during the forging process can improve the internal structure of the steel. Large as-cast structures and unstable eutectic carbides can be broken down by applying large stress.
If feasible, the upsetting and drawing forging process can be employed to further enhance the structure of H13 steel and its properties.
3. Conclusion and Suggestions
(1) The main reason why the transverse impact performance of H13 steel smelted by electroslag remelting cannot reach the expected goal is due to the lack of proper control over the forging process.
After heat treatment, secondary carbides precipitate along the grain boundaries, weakening them. To effectively improve the transverse impact toughness of the H13 steel, it is essential to prevent secondary carbide precipitation into a network along the grain boundary.
(2) The impact toughness of H13 steel can be significantly enhanced by implementing high-temperature homogenization treatment, increasing forging deformation, improving the cooling rate after forging, minimizing segregation, and avoiding carbide precipitation along grain boundaries.
