H13 hot working die steel has excellent thermal resistance, cold and hot fatigue resistance and liquid metal erosion resistance. 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, as the die needs to withstand substantial impact force.
Pressure casting technology is predominantly used in the production of automotive parts, such as brackets, clutches and crankcases, 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. Compared with extrusion mold, die casting mold has to bear more impact energy during the production process, especially when manufacturing large parts. This requires the use of high-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: cast iron pretreatment → casting in a 20t electric furnace → refining in an LF furnace (ladle refining furnace) → vacuum treatment in VD furnace (vacuum refining furnace) → casting in 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 → non-destructive testing → sampling inspection.
During impact energy testing of a steel plate, it was discovered that the impact performance did not meet the expected standard.
To identify the cause of low-impact performance, researchers including Li Yongdeng and Yang E from Daye Special Steel Co., Ltd., Hubei Provincial Key Laboratory of High-Quality Special Steel and Hubei Huangshi Institute of Product Quality Supervision and Inspection analyzed the materials. They identified the reason for unsatisfactory impact energy and provided a basis for subsequent production improvement.
1. Physical and chemical inspection
1.1 Analysis of chemical composition
The chemical composition of H13 steel module with unqualified impact energy was detected and the results comply with the requirements of GB/T 1299-2014 for steel tools and dies.
1.2 Impact performance test
The impact performance test must be carried out using impact test specimens without transverse notches.
Samples must be taken from the central part of the module and then subjected to quenching and tempering treatment after making the blank, followed by machining to the final sample size.
Three samples were tested and the impact sample size was 55 mm x 10 mm x 7 mm.
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 sample 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 area of fracture origin.
Samples with higher impact energy showed smaller areas of intergranular fracture, while those with lower impact energy showed larger areas of intergranular fracture.
Samples that reached the expected impact energy exhibited a Bremsstrahlung fracture morphology, with no intergranular cracks observed. Furthermore, no defects such as large inclusions were found on the fracture surface.
The fracture morphologies of samples with low and high impact energy are shown in Figure 1 and Figure 2, respectively.
In general, intergranular fracture is a form of grain boundary.
Fig. 1 Fracture Micromorphology of Low Impact Energy Specimen
Fig. 2 Fracture Micromorphology of High Impact Energy Specimen
1.4 Metallographic inspection
After sanding and polishing the fracture surface of the impact sample, it was attacked 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 carbides were found.
Annealed samples were taken from the same batch with low impact energy. After sanding, polishing and etching with alcohol and nitric acid, the microstructure showed spherical pearlite. Spherical carbides were distributed in chains locally and no apparent aggregation of carbides was found. This indicates that segregation in the casting process is at a normal level.
Figure 3 shows the microstructure of the samples with low impact energy.
Fig. 3 Fracture Microstructure of Low-Impact Energy Specimen
For the sample with the highest 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 (see Fig. 4).
Fig. 4 Fracture Microstructure of High Impact Energy Specimen
2. Comprehensive analysis
The chemical composition of H13 steel, melted by electroslag remelting, meets the requirements of the GB/T 1299-2014 standard.
Observations of the microstructure indicate that there is no apparent carbide accumulation or band segregation, and there are no significant non-metallic inclusions on the fracture surface. This indicates that the casting process is under normal control.
Based on the analysis of the micromorphology 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 has a dimple morphology and its structure is uniform.
Intergranular fracture occurs when the grain boundary of the steel supports 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 mainly V8C7, Cr23C6 and Cr3C2 (Cr2VC2).
Insufficient heating during forging and inadequate cooling after the process contribute to the accumulation of these carbides along the grain boundary. This build-up 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 lattice carbides can be effectively reduced.
Refining and dispersing carbides in steel can be achieved through high-temperature homogenization, increasing deformation during forging and decreasing the 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 segregation of components that occurs during casting 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 cross-linked carbides in the steel, as well as prevent secondary carbides from precipitating along the grain boundary to form carbide chains.
Rapid cooling followed by annealing after forging can produce a uniform spheroidal pearlite structure in steel.
Increasing deformation during the forging process can improve the internal structure of the steel. Large cast structures and unstable eutectic carbides can be broken by applying large stresses.
If feasible, the upsetting and stretch forging process can be employed to further improve the H13 steel structure and its properties.
3. Conclusion and suggestions
(1) The main reason why the transverse impact performance of electroslag remelting cast H13 steel cannot reach the expected target is due to the lack of adequate control over the forging process.
After heat treatment, secondary carbides precipitate along grain boundaries, weakening them. To effectively improve the transverse impact toughness of H13 steel, it is essential to prevent secondary precipitation of carbide in a network along the grain boundary.
(2) The impact resistance of H13 steel can be significantly improved by implementing high-temperature homogenization treatment, increasing forging deformation, improving the cooling rate after forging, minimizing segregation and preventing carbide precipitation when along grain boundaries.
