Defect Control And Process Optimization Of 42CrMoS4 Free-Cutting Steel After Quenching And Tempering

After entering into the 20th century,with the development of scientific technology and economy,machinery has gradually become the main labor force.With the increasing use of machine,processing has become the restriction of production.Besides steel quality,the production cost and easy processing are also necessary to customers.Therefore,free-cutting steel rapidly develops.With good processing properties,sulphur free-cutting steel has been one of the main materials to produce componentsSulphur free-cutting steel contains element sulfur which improves the cutting properties of steel.However,sulfur is harmful to product quality and heat treatment.In this paper,defects of a 42CrMoS4 free-cutting steel after quenching and tempering were studied,causes of the defects were discussed.According to the results,a new steel with new compositions was given and a new process of heat treatment was used.The main works are as follows(1)The cracking samples of 42CrMoS4 free-cutting steel after the quenching and tempering were chosen for study.The composition of them and the gas atmosphere around were detected,and the microstructures were observed by scanning electron microscope.The reason for the cracking of the 42CrMoS4 free-cutting steel was confirmed.It can be seen that there was no obvious segregation of composition,and the deviation of different samples was within ±3%.So the cracking was not caused by excessive residual stress due to composition segregation.The contents of hydrogen,oxygen and nitrogen were not obviously different with the gas atmosphere of normal production.The impact energy at half radius of the sample was larger than at the center.It indicates that the tensile stress at the center was larger.It was observed by scanning electron microscopy that the matrix microstructure of the steel was not continuous and there were larges holes at the center which were caused by sulfide inclusions The reason of cracking is that the local stress concentration was formed at the sulfide inclusions during the water cooling of 42CrMoS4 free-cutting steel(2)The heat treatment process was optimized to reduce the defects of the 42CrMoS4 free-cutting steel.According to the detection of compositions and observed microstructure,the compositions and process parameters were adjusted.The content of phosphorus was decreased and the content of sulfur was controlled according to the lower limit of the standard The time for steel from furnace to quenching medium was adjusted to under 3 minutes,time of opening the stirring device of quenching tank was changed 10 minutes earlier,the quenching temperature was lowered,and the placement of the steel was optimized(3)After the optimization of composition and heat treatment process,the quality of 42CrMoS4 free-cutting steel was improved,the qualified rate of properties increased from 83%to 94%,and the comprehensive qualified rate increased from 97%to 99%.With the improvement measures,production costs are reduced and production cycle is shortened which can provide a valuable experience for other sulphur free-cutting steels of the same kind.