Study On The Plastic Deformation Zone Of Metal Materials

Industrial molds prolong their fatigue life through the surface strengthening technology.A variety of surface strengthening techniques make the surface of the metal materials undergo asynchronous plastic deformation to obtain pre-stressing layer,which can slow the crack growth rate and even close the crack.Finally,the metal material is strengthened.The formation of pre-stressing layer with certain depth on the surface via hammering can effectively prolong the fatigue life of metal materials.In this work,we studied the plastic deformation zone of metal materials near tight seam via hammering tight seam.Through the ZEISS Laser Scanning Microscope700,the morphology and range information of the plastic deformation zone were obtained.Then we deeply analyzed and compared the factors affecting the plastic deformation zone.The samples of the hammering experiment are Q235 steel,#45quenched and tempered steel and #45 quenched steel.Their dimensions are 170mm×20 mm×30 mm.The hammer distance are 25,50,75,100,125 and 150 mm,respectively.The punch heads are S?11,S?16,110° cone and ?2 round table.There are single-point hammering and double-point hammering with 1.5 mm pitch.Then,we designed the micro-hardness array method to compare the hardness distribution law of section surface and tightening surface.Based on the data,the hammer strengthening effect of the plastic deformation zone was studied.In addition,the tight grid method can be used to analyze the internal strain phenomenon of the plastic deformation zone.In order to improve the accuracy of results,we designed and assembled a new scribing device.And,the straightness of grid lines and the accuracy of grid points were evaluated.The experimental results show that:(1)When the material and punch head are the same,the area and depth of the plastic deformation zone are proportional to the hammer distance.However,the increase in the depth of the plastic deformation zone is restricted by the plasticdeformation ability of the material.When the material and hammer distance are the same,the area and depth of the plastic zone produced by the 110° cone punch head are the largest,which are 17.95 mm2 and 3.81 mm,respectively.When the punch head and hammer distance are the same,the area and depth of the plastic deformation zone decrease as the hardness of material increases.The raised height increases with the increase of the yield strength of the sample.The area and depth of the plastic zone of#45 quenched steel are the smallest,and the raised height is the highest.Double-point hammering data is the superposition of two single-point hammering deformation vectors.(2)According to the data via the micro-hardness array method,it is confirmed that the section surface has obvious strengthening effect.When the hammer distance is the same,the hardness value of the tightening surface is higher than that of the section surface.The three-dimensional data of the micro-hardness array can be used to distinguish the micro-structure of the sample.The micro-hardness of the pearlite structure in the sample is higher than the ferrite structure.After the hammering,the data of the micro-hardness array in the horizontal and vertical directions on the section surface and the tightening surface is better,and the hardness distribution of the micro-hardness array is in accordance with the law.This method is reasonable and practical,which provides a new method for studying the strengthening effect of the plastic deformation zone.(3)Newly-designed scribing device achieved breakthrough in scribing spacing from 200 ?m to 100 ?m.The straightness of the line and the standard of the grid are greatly improved,and the stability and operation flexibility of the device are also improved.The straightness error of the pure iron sample is 3.4 %,and the accuracy of the new device is 72 % higher than that of the old.The straightness deviation of H13 steel sample is basically controlled within 1 ?m.And the straightness error is about1 %.The accuracy increases by 95 %.As the hardness of sample increases,the smaller straightness offset,the higher straightness accuracy.The grid point offset of the pure iron sample is basically within 5 ?m,and the grid point offset rate reduces by75 % comparing with the old device.The grid standard error is 5 %,and the grid pointposition accuracy increases by 60 %.The grid by the new device is basically closed to square and the line spacing is stable.After evaluation,the new scribing device can provide a relatively high-quality and stable experimental basis for the analysis of strain in the plastic zone by tight grid method.