Study On Crack Control Of NM450 Wear Resistant Steel

Low alloy wear resistant steel is a new type of steel in recent years.It has been widely used because of its low alloy content and excellent wear resistance.However,some wear resistant steel appeared cracks in manufacturing process,it severely reduces the production efficiency of enterprise.Therefore,this paper studied cracks of NM450 low alloy wear resistant steel.Optical microscopy and scanning electron microscopy were used for microstructure analysis.The change of stress during the heat treatment process of steel plate was simulated by thermal-force-structure physical model.The hardness test was performed with XHD-2000 TMSC digital vickers hardness tester in the thickness direction.The reasons of cracks were studied systematically and the tempering process was optimized.The main results were as follows:The NM450 wear resistant steel plate produced by a domestic steel mill has delayed crack after flame cutting,and there was a hard segregation band in the position of delayed crack generation,segregation band formed during solidification and it would not be eliminated during subsequent rolling and heat treatment.Therefore,the microstructure and mechanical properties of the steel plate were different in the direction of thickness.During the process of steel plate heat treatment,due to the different cooling rate,the change of the microstructure can result in the phase change stress,and the surface of the steel plate and the core of the steel plate would produce greater thermal stress due to the uneven heating in the cutting process.The phase change stress and thermal stress would be superimposed on the steel plate segregation zone,so that the steel plate would crack in the segregation zone and expand along the thickness.After quenching and cooling,NM450 steel plate has longitudinal crack.The crack was straight and elongated,the opening degree of the crack generally showed a small trend at both ends.There was a radial pattern of extending the length direction of the rolled plate and the direction of the rolling surface.It indicated that the crack originated from the mid-depth of steel plate thickness.There was no oxide spots near the crack,but a little decarbonization.There was thick and tight iron oxide inside the cracks of natural split sample.The crack of NM450 steel was formed during the quenching process and extended under the action of residual stress,and the longitudinal crack extended to the surface.In order to optimized the mechanical properties of wear resistant steel plate,to solve the problems such as uneven hardness and easy to produce cracks,the tempering process was optimized.The tempering temperature is 200~350?,and the holding time is 160~250min.By comparing the microstructure and mechanical properties of NM450 wear resistant steel under different tempering process,it shows that tempering can not completely eliminate the segregation band of the test steel,but the tempering temperature has a very important influence on the hardness of the test steel.With the increase of tempering temperature,martensite gradually decomposition and microstructure of matrix become more uniform,the quenching process of the internal stress release gradually,the hardness of segregation band is falling and the hardness becomes more even.However,there was no significant change in the structure of the matrix with the extension of tempering time,so the hardness did not decrease obviously.When the tempering temperature is 250?,tempering time is 250 min,the mechanical property of NM450 wear resistant steel is excellent.Therefore,the adoption of tempering process can not completely avoid cracks,but it can reduce the rate of cracks to some certain extent.