Surface Enhancement Of SKD11 Mold Steel By Hydrostatic Burnishing

SKD11 is cold-work mold steel with relatively high strength,toughness and heat resistance and is widely used in the field of mold manufacture.Burnishing is a newly developed processing technology with many unique advantages in terms of the enhancement of mold steel.In this thesis,the hydrostatic ball burnishing process was studied by the experimental and theoretical methods.Burnishing parameters sets suited for SKD11 mold steel were presented.Burnishing process was modeled to illustrate its enhancement mechanisms.Orthogonal experiments were performed to illustrate the effects of the burnishing parameters(burnishing direction,force,feed,number of passes,velocity)on the surface properties including surface hardness,roughness and residual stresses.Results showed that burnishing force played the most significant role in the properties of surfaces.The surface hardness and residual stresses increased with the increase of burnishing force while the surface roughness followed an opposite trend.Burnishing feed and number of passes had less significant influence on the surface properties than burnishing force.Surface hardness and roughness increased with the increase of burnishing feed.With the increase of the number of passes,the surface hardness increased and the roughness decreased.The influences of burnishing feed and number of passes on the surface residual stresses were rather complicated,with the influence mechanism varing with the directions of measurement.Burnishing direction had a great influence on the distribution of the residual stresses while its influence on the surface hardness and roughness could be neglected.The effects of relatively low burnishing velocity on the surface properties could always be ignored.Based on the orthogonal experiments,single-factor experiments of burnishing feed and number of passes were carried out respectively and the results corresponded well with the orthogonal experiment in general.The single-factor experiments also showed that with the increase of burnishing feed,the surface hardness firstly experienced steady decrease and then decreased dramatically.However,the surface roughness firstly experienced steady increase and then increased dramatically.The surface hardness increased initially,but then followed a decreasing trend with the increase of the number of passes.The distribution of residual stress in the direction at an angle of 45° to the burnishing direction was related with the parity of the number of passes.Based on the experimental results,empirical model on surface roughness was fitted by using the regression method.The analysis of significance of the empirical model showed that the surface roughness after burnishing was mostly dependent on the roughness before burnishing despite the influence of the burnishing parameters.FEM simulation of the burnishing process was performed and the results showed that the compressive residual stresses were introduced on the burnished surfaces by the piling-up effect.