Common defects in high-frequency quenching treatment include quenching cracks, soft spots, and delamination. High-frequency quenching should avoid uneven heating that leads to overheating in localized areas, such as sharp corners of workpieces, keyways, and around holes, which are particularly prone to overheating and causing quenching cracks. These issues can be mitigated by using copper sheets to reduce the likelihood of quenching cracks. Another drawback is the uneven hardness of the workpiece surface during quenching, known as soft spots, caused by uneven high-frequency quenching temperatures, clogged water jets, or improper sizes and quantities of holes. The third issue is surface delamination, primarily due to large hardness changes in the cross-section or a too shallow hardening layer, which can often be prevented by preheating to deepen the hardening layer. Why can't stainless steel be tempered between 500°C and 650°C? Most stainless steels, after solution treatment, will exhibit increased hardness and brittleness when held at temperatures between 475°C and 500°C for an extended period, a phenomenon known as 475°C embrittlement. The primary causes are various, including phase decomposition, precipitation of chromium carbides at grain boundaries, and the formation of Fe-Cr compounds, significantly reducing toughness at room temperature and poor corrosion resistance. It is generally advisable to avoid prolonged holding at this temperature range during heat treatment. Additionally, long-term holding at temperatures between 600°C and 700°C can lead to the precipitation of s-phase, which is an Fe-Cr intermetallic compound. This compound is both hard and brittle and depletes a significant amount of chromium from the steel, reducing the corrosion resistance and toughness of stainless steel.
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