Plasma Surface Remelting Of Ductile Iron For Optimization Of Graphite Dissolution And Surface Properties

The application of ductile cast iron(DCI)in agricultural machine parts has been one of the engineering materials innovations,due to high strength-to-weight ratio,wide range of mechanical properties and low cost of the DCI compared to other materials with the like properties.However,its surface properties may not match well with the performance requirements of agricultural machinery parts under abrasive service condition.Consequently,an improvement on the surface hardness and wear resistance are particularly required for it to perform effectively,especially for ferrictic ductile iron.The use of plasma surface remelting technique has been considered an effective method to improve the surface properties of ductile iron by dissolving the soft graphite nodules and a consequence changes in the surface microstructures.In this work,multi-pass plasma surface remelting was adopted and performed on ferritic ductile iron to modify the surface properties,by mitigating the graphite's weaken effect on strength and hardness of the ductile iron.The influence of remelting conditions on quantitative graphite nodules dissolution,surface hardening and wear resistance were evaluated.A number of surface remelting were conducted for the selected combinations of arc current and scanning speed using high-temperature plasma beams.The graphite dissolution was measured and quantified via image analysis approach.The remelted layer was characterized for phase transformation,microstructure,hardness and wear resistance.The combined process parameters effects on the responses were statistically analyzed(ANOVA).Furthermore,modelling based on surface response methodology(RSM)was performed to optimize the dry sliding wear behavior of the modified ductile iron surface,in terms of applied normal load,sliding speed and sliding time.This was done in order to fully understand and characterize the different active factors that influence the wear behavior.Models were developed to define the response relating to wear rate and sliding friction coefficient.The results of surface remelting revealed scanning speed and arc current havingsignificant influence(P<0.01)both on graphite nodules dissolution and surface hardening.The combination of low scanning speed and high arc current gives higher effect,with scanning speed showing the dominance influence.A linear relation was found between graphite nodules dissolution and scanning speed,while quadratic relation was established between remelted surface hardness and graphite nodules dissolution.Remelted layer microhardness was found approximately 3.3 times higher than substrate,and also 2.76 times more than the as-received ductile iron deep tillage point.The maximum penetration depth of 996?m was obtained.Further,microstructures in remelted and solid state hardened zones are mainly consisted of martensite,cementite,retained austenite and few re-transformed ferrites,with martensite fractions becoming more at slower scanning speeds.Consequently,microstructure of the treated layer is modified and the surface exhibited a significant improvement in hardness and wear properties.Results of the RSM analysis showed that all the process parameters were both significant for the sliding wear and friction.Higher effect was shown by the applied normal load.Also the interaction of applied normal load and sliding time showed the most significant effect.Minimum wear rate(1.098 x10^-4mm³/m)and coefficient of friction(0.00528)were obtained at the optimum condition of applied normal load of 20 N,sliding speed of 319 rpm and sliding time of 30 mins.The sliding wear resistance of the plasma remelted surface layer was greatly improved(7.38 times higher than that of the substrate)and showed an oxidative,adhesive wear mechanism.Generally,treatment conditions with slower scanning speed gave the best surface modification of the treated ductile iron.Therefore,the plasma remelting enhanced surface properties of ferrictic ductile iron,which makes it suitable for application in a severe wear condition.