| 摘要 |
The invention relates to free-machining steels which do not rely on lead as a means of enhancing machinability. Instead, the steels of the invention employ concentrations of tin at ferrite grain boundaries to replicate a role of lead, which the inventors have discovered, in enhancing machinability. This role is to cause an embrittlement at the localized cutting zone temperatures by changing the fracture mode from transgranular to intergranular at those temperatures. The invention's use of concentrations of tin at ferrite grain boundaries of the steel permits the machinability-enhancing effect to be obtained while employing bulk tin contents below levels at which hot tearing becomes problematic. The invention improves over lead-bearing, free-machining steels in that the machinability-enhancing embrittlement produced by concentrating tin at ferrite grain boundaries is both controllable and reversible. The invention also relates to methods of producing the described tin-bearing, free-machining steels and the products of those processes.
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