At first, most metal products need various heat treatment processes to achieve the desired effect of customers. In heavy industry, heat treatment process is also indispensable. Take large steel castings for example, the heat treatment process is extremely troublesome. Let's explain why large steel castings need special heat treatment process:
The as cast structure of large-scale steel castings is prone to serious crystal segregation and uneven structure. For example, the surface of the casting section is often fine equiaxed crystal, the second layer is columnar crystal, the middle is equiaxed crystal, as well as the widmanstatten structure and network cementite. It is necessary to eliminate or reduce its harmful effects by heat treatment (solid recrystallization). The microstructure of steel castings is controlled by heat treatment to meet the needs Performance.
In addition, due to the difference of wall thickness and structure of large-scale steel castings, each part of the same casting has different structure state, and a considerable residual stress needs to be eliminated. The shape of large steel castings is complex, and the difference of wall thickness is large. It is necessary to consider the difference of microstructure and properties between the sample and the actual casting caused by cross-section effect.
According to different heating and cooling conditions, the main heat treatment methods of steel castings are annealing, normalizing, homogenizing treatment, quenching, tempering, solution treatment, precipitation hardening, stress relief treatment and hydrogen removal treatment. Main characteristics and application scope of various heat treatment. Due to the characteristics of casting structure, the heat treatment of large-scale steel castings is often different from that of forged steel or rolled steel with similar chemical composition, and its characteristics are as follows:
In the as cast structure of steel castings, there are often coarse dendrites and segregation. During heat treatment, the heating temperature of steel castings is slightly higher than that of forged steel castings with similar composition, and the heat preservation time of austenitizing should be appropriately prolonged. Some alloy steel castings have serious as cast structure segregation. In order to eliminate its influence on the final heat treatment of castings, homogenization treatment is needed.
After reading the above explanation, do you understand the reason why the special heat treatment process should be carried out for large steel castings? If you have any questions or doubts about this, you can contact us, or call the company's contact number for professional troubleshooting, and look forward to your call.
On 2018-08-08 11:45:46
