According to industry practices in the heavy machinery sector, a free forging produced using hydraulic presses with a forging capacity of over 1000 tons can be referred to as large forging. Based on the forging capacity of hydraulic presses for free forging, this roughly corresponds to shaft forgings weighing over 5 tons and disc forgings weighing over 2 tons.
The main and fundamental characteristics of large forgings are their large dimensions and heavy weight. For example, the size of a 600MW steam turbine generator rotor forging is φ1280mm×16310mm, weighing 111.5 tons. The size of a 2200-2400MW steam turbine generator rotor forging is φ1808mm×16880mm, weighing 247 tons.
Due to their large size and weight, large forgings must be directly forged from large steel ingots. It is well known that large steel ingots often have serious issues such as segregation, porosity, shrinkage, non-metallic inclusions, and various types of structural non-uniformity. They also tend to have higher gas content, and these defects are difficult to remove during subsequent forging processes. As a result, significant chemical composition non-uniformity, diverse structural defects, and high levels of harmful gas content often exist in large forgings. This makes the heat treatment process for large forgings complex, time-consuming, and expensive. Therefore, careful attention must be paid during the heat treatment.
Furthermore, due to their large size and weight, large forgings have a significantly higher heat capacity, which makes it impossible to achieve high heating and cooling rates during the heat treatment steps. Therefore, for large forgings that require significant changes in the internal structure through tempering or quenching to meet high-performance and quality requirements, highly stable supercooled austenite and high hardenability steels must be used. Examples include Ni-Cr-Mo, Ni-Mo-V, and Ni-Cr-Mo-V series steels. However, steels with high stability of supercooled austenite are prone to structural inheritance, resulting in coarse and uneven grain size in alloy steel forgings. To address this issue, special and complex heat treatment processes are often required.
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Post time: Jan-23-2024
