Heat treatment is not only a heating and cooling process of workpiece, but also a process of stress generation and release. The theory of heat treatment proves that the shape of the workpiece surface has a great influence on the defects caused by heat treatment. Before heat treatment, the workpiece surface should be smooth as far as possible without edges and corners and more depressions and protrusions. Any sharp corner may have stress concentration and crack during heating.
Comparing the appearance shape of the tooth surface of the cast steel big gear before gear milling with that after gear milling, it is obvious that after the preliminary semi milling, the shape of the surface is much more complex, and there are many more edges, corners and convex concave, which creates conditions for the stress release to produce cracks, and more crack defects will appear in the heat treatment after gear milling. As shown in the table, the random three groups of quarter cast steel big gears are compared with the defects of the old and new processes. It is not difficult to see from the table that although the new process solves the thermal deformation caused by flaw detection and repair welding, it also increases the number of defects and significantly increases the length of defects. The reason for the defect is that the surface shape after milling is complex, which increases the difficulty of heat treatment. Heat treatment after milling is not impossible, and most of the heat treatment after milling is surface quenching treatment of hard tooth surface.
| New process | New process | Traditional craft | Traditional craft | |
| Random group | Number of defects | Maximum defect length mm | Number of defects | Maximum defect length mm |
| 1 | 247 | 451 | 98 | 245 |
| 2 | 327 | 547 | 82 | 157 |
| 3 | 199 | 354 | 127 | 204 |
| Average | 257.67 | 450.67 | 102.33 | 202 |
It is obvious from the table that the new process is more prone to heat treatment defects than the traditional process. The large number and size of defects will cause trouble for defect removal and repair welding. For large-area defects, three times of repair welding can not succeed. According to the welding theory, repeat the welding for more than three times. In order to ensure the welding quality, the welding object needs to be properly treated. Multiple repair welding is not only time-consuming and laborious, increasing the cost, but also has an impact on the internal organization of the workpiece and directly affects the internal quality of cast steel gear.