Study On Denatured Layer In Electrochemical Grinding Of Chromium Alloy

Electrochemical grinding?ECG?is one of the non-traditional manufacturing methods.It has the characteristics of small grinding force,wide application range,and good surface quality.It is widely used in the processing of various chrome alloys such as 304 stainless steel,GCr15,316 stainless steel and 420 stainless steel,etc.It has been considered that a thin passive film is formed on alloys during the electrochemical process and much research has been done on it.However,it is found that a kind of denatured layer?which is much thicker than passive film?is formed on 304 stainless steel,GCr15,316 stainless steel and 420 stainless steel surface during the process while the characteristics and influence factors of the denatured layer have been rarely focused.The understanding of the characteristics of the denatured layer and the control of the thickness are conducive to the subsequent mechanical removal.Therefore,it is instructive and necessary to study the physicochemical characteristics of the denatured layer generated in the electrochemical process.In this paper,an experimental study as well as a computational method were implemented to investigate characteristics of denatured layer on 304 stainless steel and GCr15.By using a three-electrode test system to perform polarization curve experiments,the polarization curves of four common passive electrolytes,NaNO₃,NaNO₂,NaClO₃,and Na₂CO₃,were compared and the 20%mass fraction of NaNO₃ solution was finally selected as the electrolyte for electrochemical experiments.Energy dispersive spectrometer?EDS?with SUPRA 55 model field emission scanning electron microscope?SEM?was used to observe surface morphologies and to analyze elemental composition of denatured layer.With increasing voltage,current density and time,chromium dissolves or passivates first,the denatured layer produced in the electrochemical reaction consists of chromium and iron oxides.The surface morphology of the chromium denatured layer gradually becomes uniform and dense,but excessive voltage will make it more corrosion pits due to corrosion.FM-700 model micro hardness tester was used to measure the hardness of the denatured layer.After electrochemistry,the hardness of 304stainless steel is reduced by 70%,and the hardness of GCr15 is reduced by about 65%.Reduced hardness of workpiece is conducive to avoiding tool damage and facilitating the subsequent mechanical removal process.In this paper,the electrochemical experiments of voltage,current density and time parameters of 304 stainless steel,GCr15,316 stainless steel and 420 stainless steel were conducted respectively to study the influence mechanism of electrochemical parameters on the thickness of the denatured layer.After angle polishing measurement method,the thickness of the denatured layer of chromium alloy was observed under an ultra-deep depth microscope,and the experimental data was fitted using Matlab.The thickness of the four chromium alloy denatured layers are affected by different electrochemical parameters,and third-order equations for predicting layer thickness under different electrochemical parameters were proposed.The thickness of the 304 stainless steel denatured layer can reach 10 to 12?m;the thickness of the denatured layer of the GCr15 can reach about 20?m;the thickness of 316 stainless steel denatured layer is about 20?m;and the thickness of the denatured layer of the 420 stainless steel can reach 8 to 10?m.The obviously reduced hardness and controllable thickness of the denatured layer are expected to be of great significance in machining of chromium alloy.