surface hardening by flame heating

例句：

1. 1. The steel plate was surface hardened to increase its strength.
   钢板进行了表面硬化处理以增加其强度。
2. 2.During the manufacturing process, the material is surface hardened to improve its durability.
   在制造过程中，材料进行表面硬化以提高其耐用性。
3. 3.The engineer used a chemical treatment to achieve surface hardening of the metal.
   工程师使用化学处理方法实现了金属的表面硬化。