Research On Improving Mechanism Of Metal Bar Surface Quality Under Low-Stress Cropping

Compared with traditional cropping,low-stress cropping technology of metal bar has the advantages of energy saving,material saving,high efficiency and environmental protection.However,it is difficult to determine the crack initiation force effectively in the process of cropping,which leads to the difficult control of the crack initiation stage,resulting in the increase of the area of the bar instantaneous fracture zone.In addition,due to the extrusion contact between the hammer head and the bar material,the surface of the circumference of the bar material is seriously worn.Therefore,this thesis adopts the method of combining theoretical analysis and experimental research to focus on the mechanism of improving the feeding surface quality from the two aspects of bar cross section and circular surface quality during the feeding process.The main research contents of this thesis are as follows:（1）The working principle of low-stress cropping is briefly described,and an experimental platform for simultaneous measurement of acoustic emission and stress and strain is built.Using acoustic emission technology to crack the time to study,the acoustic emission characteristic parameters of ring count divided into four stages,the use of two stage"energy saving area","plastic failure zone"to explain the bar V groove edge crack initiation process,and kurtosis coefficient and combined with multiple parameter correlation analysis method is used to judge the bar t₀ crack.At the same time,the surface strain of the bars at the same time is calculated with the stress-strain technique,and the value of the crack initiation force of the bars with different diameters and groove depths is obtained.On this basis,micro-analysis and vertical crack line cutting were used to obtain the initial crack depth under different loading forces,so as to verify the crack initiation process accurately.（2）The law of crack initiation at the tip of V-shaped groove was studied,and the new load curve of crack initiation force was fitted.On the test platform of simultaneous measurement of acoustic emission and stress and strain,the curve is used to test the cutting section.The results show that the instantaneous area of bar cross section is reduced by 37%and the cropping time is shortened by 8%compared with the traditional cropping results.A new crack parameter,a_m,was used to characterize the initial crack depth of the initiation process.Based on this,the crack depth-initiation force formula at the tip of V-shaped groove was obtained for low-stress feeding.The error between the crack depth data obtained by the formula and the experimental crack initiation force was less than 5%.（3）Hammer head cropping process-bar is studied on the surface of the friction theory,put forward to overcome the tip wear hammer head of new circle structure,according to the micro convex body deformation and acoustic emission model to determine the acoustic emission characteristic parameters,the corresponding relationship between the wear degree of friction pair is proposed using RMS,ASL,ae characteristic parameters such as amplitude and energy to the lubrication effect were characterized,and combining the temperature rise to measure the degree of wear and tear of bar circumferential surface.（4）The crystal structure and lubrication mechanism of Mo S₂ were analyzed,and the method of adding Mo S₂ to improve the circular surface of the bar material was proposed.Using synchronous measurement experimental platform to bar circumferential surface wear test,the results show that ae characteristic parameters and temperature rise with the increase of Mo S₂ content first decreases,and the change of increase,quantitatively determine the best under different load Mo S₂ additive mass fraction is 0.1%,under the content of the wear intensity decreased by more than 12.26%,effectively improve the process of cropping bar surface quality on the surface of the circumference of a circle.