hardening

例句：

1. 1.The metal is undergoing hardening as it cools.
   金属正在冷却过程中变硬。
2. 2.The ground is beginning to harden after the rain.
   雨后地面开始变硬了。
3. 3.The steel is being hardened in a furnace.
   钢铁正在炉中变硬。