In the original process, the forged gear blank is directly air cooled to room temperature after forging, and then rough turning and tempering. This process has the following defects: coarse grain, uneven hardness and insufficient depth of surface hardened layer after finishing. If the normalizing process is added after forging, it can refine the grain, eliminate the network carbide and improve the machinability. The increase of Mn and Cr content can improve the hardenability of the material, but also increase the possibility of quenching cracking. Therefore, the hierarchical quenching process can effectively reduce the structural stress and thermal stress during cooling and prevent the cracking and deformation of the workpiece.

1 Improved heat treatment process

When the forged gear blank is air cooled to 350 ~ 400 ℃ after forging, the furnace normalizes. The normalizing process is shown in Figure 1. Because the previous improved forging process reduces the deformation stress and temperature stress, the holding time can be appropriately reduced (from the original 1 ∙ 6min · mm-1 to 1 ∙ 2min · mm-1), which saves energy and improves the production efficiency. The quenching and tempering process after normalizing is shown in Figure 2, and the quenching medium is AQ251 medium water.

2 Mechanical properties and metallographic analysis results

Requirements for mechanical properties of forged gear blanks after heat treatment, normalizing and quenching and tempering. After finishing turning, the hardness of the forged gear blank is uniform, and the hardness deviation of each surface is not greater than 25hb, as shown in Figure 3.

Non metallic inclusion content. The grain size is 6 ∙ 5 ~ 7, and the microstructure is tempered sorbite + a small amount of ferrite distributed along the crystal, as shown in Fig. 4.