hardening by cooling

例句：

1. 1. The metal has been subjectedto a high temperature for several hours,causing it to harden.
   金属在高温下暴露了几个小时，导致它变硬。
2. 2. During the hardening process, thematerial undergoes significant changes in its physicalproperties.
   在硬化过程中，材料会经历其物理性质的重要变化。
3. 3. Thesteel is being hardenedto increase its strength and durability.
   正在对钢材进行硬化处理，以增加其强度和耐用性。