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Influence of Forging Ratio on Microstructure and Mechanical Properties of H13

H13 steel is a widely used hot work tool steel in the world. It has good heat strength, toughness and hardenability, which is widely used in the production fields of hot forging mold and copper-aluminum alloy mold, it is one of the most widely used hot work tool steels in the world. As we all know, ESR process plays an important role in improving the quality of H13 steel. However, for various reasons, the most common production process of domestic H13 steel is still the traditional electric furnace smelting. Compared with electroslag remelting smelting, Compared with ESR, the advantage of electric furnace smelting process is that it can save production cost by 20% to 30%. Therefore, through the reasonable forging process and heat treatment process can improve the quality, performance and service life of the electric furnace steel, this will bring higher economic benefits.

Some people systematically studied the effects of different forging ratios on the microstructure and mechanical properties of H13 steel produced by electric furnace smelting, and summarized and analyzed its laws, which has great reference value for the production and development of traditional electric furnace steel.

The test material is H13 electric furnace ingot produced by Fushun mill, the upper diameter is 410 mm and the lower diameter is 330 mm. Drawing by one-way hot forging, the forging ratio is 2, 4, 6 and 10, Spheroidizing annealing at 830℃, comes out for air cooling until slowly cooling down to 500℃ at a rate less than or equal to 30-40℃/h, The chemical composition of ingots is shown in Table1.

Table1 Chemical composition (mass fraction) of test steel%
chemical com. C Mn Si S P Ni Cr V Mo Cu
content 0.415 0.42 1.13 0.0025 0.021 0.16 4.98 0.99 1.32 0.12

Cut the sample at the center, 1/4 in diameter and edge of the ingot, observation of structure in different parts of ingot transversely; After annealing, cut annealed sample and observe different section of horizontal and longitudinal in it, take six standard 10 x 10 x 55mm Charpy V notched impact specimens respectively. Measurement of impact properties of quenched and tempered samples by JB30B impact tester, the fracture morphology was observed and the hardness and structure differences of different parts in quenching and tempering state were measured. The heat treatment processes for Quenched and tempered impact specimens are oil-cooled quenching after holding at 1030 C for 30 minutes and tempering twice at 600 C x 2 h. Microstructure specimens were corroded by 4% nitric acid and alcohol, microstructure observation and analysis by LEICA MEF4M optical microscope and S-4300 cold field emission scanning electron microscopy (SEM); The grading standard of non-metallic inclusions in steel adopts GB/T10561-2005. In order to ensure the accuracy of the experimental data and its changing rules, all the materials used in this experiment are taken from the same ingot furnace, and the annealing and quenching tempering processes are also completely consistent.

Heat Treatment Process of H13 Steel

Preheat Treatment

The H13 steel and die blank supplied on the market have been annealed in the steel plant, which ensures good metallographic structure, proper hardness, good workability and no further annealing. However, the original structure and properties are destroyed and the forging stress is increased after forging modification by the manufacturer, so it must be annealed again.

Isothermal spheroidizing annealing process is as follows: heating at 860-890℃ for 2 hours, cooling to 740-760℃ for 4 hours, and discharging from furnace at 500℃ or so.

Quenching and Tempering

Requirements for good toughness of the die quenching process specifications: heating temperature 1020-1050℃, oil-cooled or air-cooled, hardness 54-58HRC; Requirements for hot-hardened die quenching process specifications, heating temperature 1050-1080℃, oil-cooled, hardness 56-58HRC.

Recommended tempering temperature: 530-560℃, hardness 48-52HR; Tempering temperature is 560-580 C, hardness is 47-49HRC.

Tempering should be done twice. Secondary hardening peak appears at tempering temperature of 500℃. The hardness of tempering is the highest, and the peak value is about 55 HRC, but the toughness is the worst. Therefore, it is advisable to avoid the tempering process at about 500℃. According to the needs of the die, tempering is better in the range of 540-620℃.

Quenching heating should be preheated twice(600~650℃,800~850℃), Reducing thermal stress during heating.

Chemical Teat Treatment

H13 steel can be further strengthened by gas nitriding or nitrocarburizing, but its nitriding temperature should not be higher than tempering temperature, so as to ensure that the strength of the core does not decrease, thereby improving the service life of the mold.

The results show that the transverse impact energy of H13 EAF steel at the same forging ratio is obviously less than that of the longitudinal one, and the impact energy is getting worse and worse from the transverse and longitudinal edges to the center of the ingot; the different forging ratios have little effect on the transverse impact energy, but the longitudinal impact energy increases with the increase of forging ratio.

The results show that unidirectional hot forging elongation can not effectively improve the transverse crack arrest performance of steel, and the forging ratio of EAF H13 hot work tool steel should be controlled in the range of 4-6.