Carbon Steel Cold Heading Deformation Organizational Characteristics And Evolution

Cold-heading steel is used to manufacture the interchangeable standard parts based on the metal plasticity and it is a kind material used to make fastener such as bolts,nuts, screws,or pins and a variety of cold forging of steel parts.The hot-rolled wire of SWRCH6A,SWRCH15A and SWRCH35 from Hunan Valin Xiangtan Iron and Steel Company Limited were investigated in this study.OM,SEM,TEM,XRD and DSC were used to discuss the microstructure evolution of three kinds of specimens after cold heading.The features of microstructure after cold heading were observed and the detailed microstructure evolutions were studied.Another more severely cold heading tests and heat treatment were carried out after the microstructure studies mentioned above in order to get more information about the new phase related to severe cold heading deformation.Major deformation features and process of microstructure evolution occurring during the cold heading have been studied and the results are as following:(1) The as-received microstructure of three types of steels is consisted of ferrite and pearlite,while the pearlite content varies with the carbon concentration.Ferrite grains with size of about 10-20μm and pearlite lamellae spacing of about 230nm are obtained.The sample after the cold heading is drum like and the microstructures are distinguished into three regions,which is consistent with the distribution of the stress. The center of the sample located in the triaxial compressive stress zone gets the most severe deformation.(2) The ferrite has more powerful deformation capability than pearlite.The ferrite grains at severe deformation region flow obviously,and fiber subgrains with width of around 100-250nm are formed and the deformation strain direction is perpendicular to the cold heading axis.(3) SEM observations reveal that hard pearlite phase deform macroscopically in a similar manner to soft ferrite phase due to a considerable plastic deformation capability of cementite lamellae under the present cold heading conditions.The following features of pearlite deformation phenomenon seem to be important:curled wave,severely necked fragments and closer lamellae.Most of pearlite colonies turn to be vertical to the axis of cold heading.(4) The XRD data is devoted to calculate and depict the inverse pole figures.The main results concern the ferrite grains are that most of whose <110> and <111> direction align with the cold heading axis,and form a kind of <100> and <111> fiber texture.Comparing the lattice parameter of as-received and as-deformed samples,it is found that the lattice parameter don't change that much to reflect the increase of carbon atom solid solution in the ferrite.The proposed interpretation for the distribution of dissolved carbon is:they may be concentrating in the dislocation or grain boundaries instead of solid dissolution in ferrite.(5) Detailed TEM and EDS analysis for ferrite show that the samples have slightly oxidation.Besides the matrixα-Fe diffraction spots and the spots from Fe₃O₄,one can also see some weak diffraction spots and those spots can be indexed to be the diffraction from Fe₇C₃ and M₆C phase in the cold heading samples of SWRCH15A and SWRCH35K.The orientation relationships between the matrix and the Fe₃O₄, Fe₇C₃ and M₆C phase are:(110)_Fe3O4//(001)_α-Fe,[(?)1(?)]_Fe3O4 //[110]α-re;(11(?)0)_Fe7C3 //(1(?)0)_αFe,[0001]_Fe7C3 //[110]_α-Fe;(02(?))_M6C//(1(?)0)α-Fe,[5(?)]_M6C//[110]_α-Fe·(6) It is found that voids are initiated in the specimens of SWRCH35K 1/5 cold-heading.The severely necked fragments and closer lamellae are more severe in the 1/5 cold-heading samples than in the 1/3 ones.At high strain region,the pearlite lamellae trending to be perpendicular to the cold heading axis are observed in colonies of pearlite in the transverse cross-section.(7) The heat treatment of 550℃has little effect on the as-received specimens,because ferrite grains have no evidence of recrystallization and remain the same.The morphology of cementite in pearlite is lamellae structure rather than spherical structure.While the 1/5 cold-heading samples undertook heat treatment,the equiaxed ferrite grains are obtained.Cementite lamella is found to evolve into spheroidized carbides.