chemical surface hardening

例句：

1. 1. The steel was chemically surface hardened to increase its strength.
   该钢材通过化学表面硬化来增加其强度。
2. 2. During the manufacturing process, the steel is chemically surface hardened to improve its toughness.
   在制造过程中，钢铁通过化学表面硬化来提高其韧性。
3. 3. The engineer used a chemical surface hardening technique to enhance the durability of the machine parts.
   工程师使用一种化学表面硬化技术来增强机器部件的耐用性。