Views: 100 Author: Site Editor Publish Time: 2026-06-15 Origin: Site
H13 (1.2344/SKD61/4Cr5MoSiV1) is a widely used hot work die steel worldwide. It features excellent high-temperature strength, wear resistance and thermal fatigue resistance, so it is extensively applied to manufacture die cast dies, hot forging dies and other tooling.
As a critical pre-process, forging directly determines the final mechanical properties of materials. A well-designed forging process can refine grains and homogenize carbide distribution, which greatly improves the thermal fatigue resistance and high-temperature strength of the steel.
When a high-speed forging press is used for upsetting and drawing of H13 hot work die steel, its powerful pressure as well as axial and radial deformation capacity can achieve thorough forging of the workpieces. After upsetting and drawing on the high-speed forging press, internal defects of steel ingots such as segregation, porosity, blowholes and inclusions are compacted and welded. The microstructure becomes denser, and the plasticity and mechanical properties of the metal are enhanced, thus extending the service life of the H13 finished parts.
After conventional forging, H13 steel presents a mixed microstructure of martensite and bainite with relatively high hardness, which makes machining difficult. Therefore, spheroidizing annealing is mandatory. This process effectively relieves residual stress, reduces hardness and uniformly distributes alloy carbides, and also prepares favorable microstructure for the subsequent quenching and tempering treatments.
Due to the slow cooling rate after forging of H13 steel, coarse microstructure and network carbides tend to form. After conventional low-temperature spheroidizing annealing, coarse eutectic carbides and compositional segregation remain in the core region. Carbides accumulate at grain boundaries and even form continuous chain-like structures. The aggregation of eutectic carbides and secondary carbides at grain boundaries severely deteriorates the impact toughness of die blocks. Hence, it is necessary to investigate the spheroidizing heat treatment process for forged H13 steel to optimize the annealed microstructure and improve its service performance.
Three processing schemes were formulated for spheroidizing treatment of forged H13 steel:
Scheme 1:Low-temperature spheroidizing annealing at 830 ℃ for 5 hours;
Scheme 2:Ultra-refining treatment at 960 ℃ for 2 hours + low-temperature spheroidizing annealing at 830 ℃ for 5 hours;
Scheme 3:Ultra-refining treatment at 1050 ℃ + low-temperature spheroidizing annealing at 830 ℃.
Low-temperature spheroidizing annealing is a heat treatment process: heat the steel to approximately 20 ℃ below the Ac₁ critical temperature, hold at this temperature for a long period (duration depends on steel grade and required spheroidization degree), then cool slowly or air-cool to room temperature to obtain spheroidal pearlite.
After this process, although intragranular carbides are spheroidized, a large number of chain-like network carbides still exist at grain boundaries, accompanied by banded segregation. This phenomenon results from high final forging temperature and slow post-forging cooling. As a hypereutectoid steel, H13 tends to form network secondary carbides during slow cooling. Besides, its high alloy content inevitably leads to inherent segregation in raw materials.
The core function of this combined process is to fully dissolve carbides into austenite and enable long-term diffusion of alloying elements. After heat preservation, the steel is rapidly cooled to a temperature above the martensite transformation point, which prevents the formation of network carbides and refines the microstructure. After water quenching, conventional spheroidizing annealing is carried out.
The spheroidized microstructures obtained by Scheme 2 and Scheme 3 are significantly superior to those from the single low-temperature spheroidizing annealing (Scheme 1). The carbides are fine and uniformly distributed, and network carbide structures are completely eliminated.
In particular, Scheme 3 achieves a remarkable improvement in alloy element segregation.The reasons are that the high-temperature holding followed by water quenching realizes rapid cooling and inhibits the precipitation of network secondary carbides and the subsequent spheroidizing annealing allows the uniformly dispersed carbides precipitated from the matrix to grow into spherical particles, optimizing the annealed microstructure.
Furthermore, after ultra-refining treatment at 1050 ℃(Scheme 3), chromium and molybdenum carbides in H13 steel are fully dissolved and uniformly distributed in the matrix, which greatly mitigates alloy element segregation after spheroidizing annealing.
1.Optimizing the forging process of H13 steel can effectively improve the mechanical properties of forgings and prolong their service life.
2.Forged H13 steel treated by single low-temperature spheroidizing annealing at 830 ℃ features coarse microstructure, severe micro-segregation and incomplete spheroidization. Most carbides form network structures, which seriously impair the service performance.
3.The combined process of ultra-refining at 1050 ℃ plus low-temperature spheroidizing annealing at 830 ℃ refines the microstructure, alleviates alloy element segregation and banded segregation, and produces uniform and fine spheroidized microstructure.
4.High final forging temperature and slow cooling rate of forged H13 steel easily induce coarse network carbides, which cannot be eliminated by conventional annealing. Adding a high-temperature ultra-refining process prior to spheroidizing annealing can suppress the precipitation of network secondary carbides and form fine lamellar pearlite. Meanwhile, alloying elements are fully dissolved and segregation is substantially improved.
FAQ
Q1:What advantages does forging with a high-speed forging press have over conventional forging when processing H13 steel?
A high-speed forging press delivers powerful forging force and enables bidirectional deformation in both axial and radial directions. When performing upsetting and drawing on H13 steel, it achieves superior full forging penetration. It thoroughly compacts internal defects in all directions, optimizes metal flow lines, and greatly improves the internal microstructure of forgings. The enhancement in mechanical properties is far more significant than that of conventional forging processes.
Q2:What is the working principle of high-temperature ultra-refinement treatment?
At high temperatures, carbides inside the matrix fully dissolve into austenite and alloy elements diffuse sufficiently. After heat preservation, the material is rapidly water-cooled to a temperature above the martensite transformation point. This fundamentally prevents the formation of network carbides and refines the original coarse microstructure.
Q3:Why is ultra-refinement treatment used in combination with low-temperature spheroidizing annealing?
High-temperature ultra-refinement alone cannot realize carbide spheroidization. Combined with low-temperature spheroidizing annealing, the finely dispersed precipitated carbides in the matrix transform into a spherical structure, yielding uniform and high-quality annealed microstructure.
