表面硬化：表面硬化的紧固件是黑色金属，其表面经过处理后比内心硬度要高。
Case hardened: a case hardened fastener is a fastener of ferrous material having a surface which has been made harder than the core.
表面硬化：表面硬化的紧固件是黑色金属，其表面经过处理后比内心硬度要高。
Case hardened: a case hardened fastener is a fastener of ferrous material having a surface which has been made harder than the core.
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