hardening in lead bath

例句：

1. 1.The metal is undergoing hardening in the furnace.
   金属正在炉中变硬。
2. 2.During the hardening process, the material's properties change.
   在硬化过程中，材料的性质发生变化。
3. 3. Hardening is a crucial step in the manufacturing process.
   硬化是制造过程中至关重要的一步。