hardened in air

例句：

1. 1. The metal has been hardened bythe heat from the furnace.
   金属因炉火的高温而变硬。
2. 2. The oldwooden chair had hardened over time.
   旧木椅随着时间变得坚硬了。
3. 3. Theground was hardened by the frost.
   地面因霜冻而变硬。