在这种情况下不管零件尺寸如何，其表面总归有一定深度被硬化。

Under these circumstances there would always be some finite depth of surface hardening regardless of size.

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因此，辊子需要较高的表面硬度和较深的硬化层深度。

Thus, these rolls require a higher surface hardness and must be hardened to a greater depth.

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金属表面硬化层深度的检测及控制是目前机械工业部门急需解决的一个问题，它属于材料检测的范畴。

Nondestructive testing and quality control of metal hardened-depth is an important problem to be solved in machinery industry. It belongs to material property testing.

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