镀锌管螺纹加工方式对管牙显微组织和性能的影响

Effect of processing methods on the microstructures and properties of galvanized pipe thread

对采用滚压和切削两种成形方式加工的镀锌管螺纹管牙的显微组织、力学性能和极化曲线进行了对比研究。结果表明:滚压加工成形方式明显改善了管牙的表面质量,管牙表层和浅表层的显微组织因强烈塑性形变而明显细化且呈细长纤维状;严重的加工硬化使显微硬度显著升高,最高可达约240 HV;单管牙扣的最大抗拉力约为29.88 k N;此外,滚压加工方式使管牙表面镀锌层得以部分保留,从而提高了螺纹的耐蚀性。相比之下切削加工成形方式所加工的管牙表面粗糙,内部显微组织仍为等轴晶,表层和次表层显微硬度一般在184~194 HV之间,力学性能相对较差,单管牙扣的最大抗拉力为21.56 k N,仅为滚压成形的72.2%;管牙表面镀锌层被切除,螺纹耐蚀性显著下降。

The microstructures,mechanical properties and polarization curves of the galvanized pipe thread processed by cold rolling and cutting methods were investigated respectively. The results show that after cold rolling,the surface quality of the thread is improved obviously and the microstructures in the surface and sub-surface layer of the thread are obviously refined and evolve into fibrous structure because of severe plastic deformation. Meanwhile,the microhardness increases significantly because of intense work-hardening and the maximum value of 240 HV is obtained. The tensile test results of single pipe thread show that the maximum force is about 29. 88 k N; moreover,the galvanized layer on the thread surface is partially retained after rolling,therefore the corrosion resistance property of the thread is improved. In contrast,the surface of thread formed through cutting is rough,the microstructures still remain equiaxed grains and the microhardness of surface layer and sub-surface layer generally ranges from 184 to 194 HV,the mechanical property is relatively poor and the maximum force is about 21. 56 k N,just about 72. 2% of that of thread formed through cold rolling; the corrosion resistance property decreases significantly due to the galvanized layer is cut during the cutting process.