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现有文献,对渗硼层的脆性,或用声发射法测试其脆断强度,或用砂轮磨削法测试其剥落倾向。关于热处理对脆性的影响,用这两种方法测得的结果正好相反。作者的试验表明,渗硼层有两种不同的失效形式:脆断和剥落,必须加以区分。脆断是在拉应力下硼针之间生成裂纹并向内扩展;剥落则是硼针在压应力下折断,形成碎块与崩落,一般没有向内扩展的裂纹。表面残余压应力有利于提高脆断强度,但促进剥落倾向。作者认为,不能用一个统一的“脆性”来表征渗硼层,应该包含“脆断脆性”和“剥落脆性”两个内容。前者可用声发射法测量,后者可用砂轮磨削法测定。提高回火温度,基体的硬度及比容降低,表面残余压应力增大,会提高脆断强度,但增大剥落倾向。零件在使用中既不允许脆断也不允许剥落,作者建议,多数零件以处理到基体硬度HRC45-50为佳,如服役中因脆断失效,可适当提高基体硬度,如因剥落失效,则反之。
Existing literature, the brittleness of boronized layer, or by acoustic emission method to test the brittle fracture strength, or wheel grinding method to test its flaking tendency. On the impact of heat treatment on the brittleness, the results obtained by the two methods are the opposite. The authors’ experiments show that there are two different forms of failure of the boronizing layer: brittle fracture and peeling, which must be distinguished. Brittle fracture is under the tensile stress between the boron needles to generate cracks and inward expansion; stripping boron needle is broken under compressive stress, the formation of fragments and collapse, generally no inward expansion of the crack. Surface residual compressive stress is beneficial to improve the brittle fracture strength, but to promote the tendency of flaking. The author believes that a uniform “brittle” can not be used to characterize the boronized layer, which should include both “brittle brittleness” and “flaking brittleness”. The former can be measured by acoustic emission method, which can be measured by grinding wheel method. Increasing the tempering temperature, the substrate hardness and specific volume decreased, the surface residual compressive stress increases, will increase the brittle fracture strength, but increases the tendency of flaking. The part does not allow brittle fracture and does not allow exfoliation. The author suggests that most parts should be treated to the hardness HRC45-50 of the substrate, such as failure due to brittle fracture during service, the hardness of the substrate may be properly increased, and if the exfoliation fails, on the contrary.