Experimental Research On Deep Cryogenic Treatment Process Of Cr8 Steel

In recent years,the manufacturing powered strategy of high quality and green development was implemented by the government.For steel industry,higher requirements and more advanced materials were put forward to realize product upgrades and transformation.The roller is the significant part of steel rolling machine in production.Because of the harsh working environment,the requirements of roll hardness,wear resistance,impact toughness and yield strength are very strict.For the purpose of improving rollers' service life and working efficiency,it is necessary to research new roller material and heat treatment process.Deep cryogenic treatment is a n important supplement to heat treatment,which can improve the ultimate performance of the material and has no any damage to the workpiece.The process that nitrogen is used as refrigerant has the advantage of zero-pollution to the environment.Therefore,it is vital to study the deep cryogenic treatment of Cr8 steel roller material.Firstly,the effects of deep cryogenic treatment(-120?)on the microstructure and mechanical properties of Cr8 steel were studied.Meanwhile,this heat treatment was compared with conventional heat treatment(quenching and tempering).The results show that: after deep cryogenic treatment,in the Cr8 sample retained austenite decreases,the martensitic increases and the uniform fine carbide particles ar e precipitated on the substrate;The macro hardness of the sample was reduced by about 0.3HRC,but the toughness was improved by about 46.3%.The fracture morphology of the impact specimen,we can find dimple in a certain area.The micro strain of the crystal was reduced by 0.04 and grain size was also refined.Secondly,the effects of the deep cryogenic temperature,cooling rate and cooling time on the mechanical properties of Cr8 steel were studied.The research results show that the toughness and hardness is in negative correlation.With the temperature of the deep cryogenic treatment of Cr8 steel decreasing,the hardness of Cr8 steel increases but the toughness decreases.Under the rapid cooling condition that the cooling rate is 8?/min or 10?/min,micro cracks is easy to be caused on the sample surface.Moreover,the impact absorbing energy is reduced.In the process of deep cooling,the phase change needs a certain time and the holding time should be prolonged as far as possible.The mechanical properties of Cr8 steel are optimal when the deep cryogenic treatment temperature is-90?~-120?,the cooling rate is 4?/min and the holding time is 2 hours.At last,the process sequence of tempering and deep cryogenic treatment was studied after quenching.The results show that: the process that deep cryogenic treatment is the first procedure can boost more residual austenite transform into martensitic.In addition,the martensitic tempering ability can be enhanced with this process.The hardness and wear resistance of Cr8 steel increases both.However,for the reason of large temperature difference between hot and cold,the residual stress on the sample surface will increase.This process is not suitable for large workpieces;The other process that tempering is in the first order can promote the toughness and yield strength of Cr8 steel because some of austenite have been already transformed after the tempering and fine carbides are precipitated in the deep cryogenic process.