Technological change in the extraction of large hardness chromium-containing bolts

In order to ensure the production progress of the bolt and reduce the production cost, it was decided to manufacture the bolt by using 40Cr material instead of 20MnTiB material. However, the results of the conventional quenching process for the bolts made of 40Cr materials show that the matching of strength, plasticity and impact toughness is not good. After the improved quenching process, the performance indexes of bolts have met the design requirements and obtained Good overall mechanical properties.

In the literature [1], the critical diameter of the 40Cr material is 25 to 30 mm, and the usual quenching temperature is 850 to 870 °C. Accordingly, the bolt is subjected to a conventional quenching process for heat treatment, heated by a salt bath furnace, cooled by L-AN32 mechanical oil, and tempered by a box type electric resistance furnace.

After quenching and tempering, a D-R4 tensile specimen and a 10×10×55 Charpy U-notch impact specimen were taken at the center of the material for mechanical performance tests. The test results of the conventional quenching process and mechanical properties of bolts are shown in Table 1.

The test results show that the 40Cr high-strength bolt adopts the conventional L-AN32 mechanical oil quenching process, it is difficult to ensure the design requirements of strength, plasticity and impact toughness at the same time. Under the condition that the strength and plasticity index are barely qualified, the impact toughness index is unqualified. And the design requirements are quite different; in the case of the impact toughness and plasticity indicators are barely qualified, the strength index is not qualified, and the design requirements are also different. Figure 2 shows the mechanical properties of Ф25mm40Cr material after tempering at different temperatures. The comparison curve analysis shows that the experimental results are very similar to the data. Therefore, it is difficult to meet the bolt design requirements under the conventional heat treatment process conditions. To this end, the conventional quenching process has been improved.

With the development of the process, the new sub-temperature quenching process of the sub-eutectoid steel is increasingly used in production. The original microstructure quenched by sub-temperature should be fine-grained structure, martensite, bainite, and quenched and tempered, but there should be no reticulated or massive ferrite. The hypoeutectoid steel is heated in the optimum sub-temperature quenching two-phase heating temperature zone of Ac3-(5~10) °C close to Ac3, and austenite and a small amount of fine residual ferrite are uniformly distributed, and obtained after sub-temperature quenching. Martensite and a small amount of evenly distributed fine ferrite, and then obtained by tempering at medium temperature to obtain a small amount of fine ferrite which is uniformly distributed in the form of troostite. This fine ferrite can absorb the energy of crack propagation and hinder cracking. Expansion, at the same time, can enrich P, Si, Sb and other impurities causing reversible temper brittleness in the ferrite, reduce their chance of segregation on the austenite grain boundary, reduce the brittleness of the grain boundary, and ensure the material The strength does not decrease. This can significantly improve the toughness, reduce the cold and brittle transition temperature, and reduce the high temperature temper brittleness under the premise of ensuring the strength of the part.

According to the theory of sub-temperature quenching and the Ac3 point of 40Cr (782 °C), and the bolts are made of 40Cr round steel with small original hot-rolled annealing state, the sub-temperature quenching process of bolts is established: bolt quenching at 780±10°C The salt bath furnace is heated, and after being kept for 12 minutes, it is first cooled with brine for 7 s, and then cooled with L-AN32 mechanical oil. The tempering was heated in a box-type resistance furnace at (470±20) °C, and air-cooled after being kept for 90 minutes. After the sub-temperature quenching and tempering bolts, a D-R4 tensile specimen and a 10×10×55 Charpy U-notch impact specimen were taken at the physical center for mechanical performance test. The test results are shown in Table 2. .

The test results show that after the sub-temperature quenching treatment, the 40Cr high-strength bolt greatly improves the impact toughness and the plasticity index under the premise of ensuring the strength requirement, thus obtaining good comprehensive mechanical properties and achieving the product design. Claim.

Since the 40Cr material is quenched and water-cooled, it is easy to form quenching cracks. For this reason, the blasting treatment of the fine sand is added to the bolts, and the surface magnetic powder inspection is performed after the bolt blasting, and no quenching crack is found.

Conclusion (1) Using 40Cr material instead of 20MnTiB to manufacture M24 high-strength bolts, the sub-temperature quenching process not only ensures product quality and production progress, but also reduces production costs. (2) The original microstructure quenched by sub-temperature should be fine-grained structure, martensite, bainite or quenched and tempered, but there should be no reticulated or massive ferrite.