| Grinding process is commonly used to machine the work-piece surfaces of the hard metal materials which are difficult to process. Grinding burn and grinding cracks can easily appear on such surface of work-pieces, which are usually harmful for the performance of the work-pieces. The traditional grinding cracking theories are relatively mature for the cases related to secondary quenching and hydrogen embrittlement involving apparent hydrogen absorption such as acid treatment. However, these theories can not explain the grinding cracking phenomenon in many cases with the normal grinding process but without apparent hydrogen absorption and secondary quenching involved. The macro-and micro-characteristics and mechanism of this grinding cracking phenomenon caused by grinding wheels on GCrl5 steel have been studied in this work, in order to contribute to grinding cracking theory and to avoid such problems in the industry.Firstly, the characteristics of grinding cracking was studied. The results showed that the cracking had delayed characteristics with the appearing time being 5min-3h. The cracks mainly appeared on the shallow surface layer with the depth range from 0.14 mm to 0.55 mm. Most of the grinding cracks were parallel to each other, and perpendicular to the grinding direction. For all conditions with different gringding parameters, grinding burned layer was always produced on the surface. Maximum tensile stress always appeared in some zone in the shallow surface layer. Cracking mode was mainly intergranular fracture, plus some cleavage and quasi-cleavage, which is similar to the characteristics of hydrogen-induced cracking of this material.The affecting factors of the grinding cracking were further studied by changing the tempering temperature, performing deep-cooling treatment with liquid nitrogen and acid treatment. Results showed that grinding cracking was easy to appear for the condition that the tempering temperture was 170℃and the gringding speed was more than 0.01mm per time. With the rise of tempering temperature, the cracking tendency decreased. Both acid treatment and the deep-cooling treatment promoted the cracking tendency. It is considered based on the analysis with above experimental results that the grinding cracking should be related to hydrogen-induced cracking.The hydrogen-induced delayed cracking behavior of the steel was characterized by performing slow strain rate tests (SSRT) on smooth specimens and crack growth tests on pre-cracked specimens, to help studying the mechanism of the grinding cracking. SSRT results showed that the steel tempered at 170℃was highly susceptible to hydrogen-induced cracking, which was further higher after acid treatment. Crack growth test results showed that the steel even tempered at 400℃also exhibited high susceptibility to hydrogen-induced cracking in deionized water. The crack growth was fast and the threshold stress intensity K1th was measured as in 12.0-13.0MPa-m1/2.During the grinding processing, grinding was all right, there was no apparent hydrogen absorption and no secondary quenching. Rusults showed that high tensile stress existed on the machined surface and the hydrogen content in the raw steel was 0.1-0.4ppm, which were enough for hydrogen-induced cracking to occur in the condition. The characteristics of delay and fracture morphology of grinding cracking showed that the mechanism should be hydrogen-induced cracking. |
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