| 51CrMnV steel has good mechanical properties and is widely used in the manufacture of various elastic parts.With the booming of the automotive industry,higher demands are placed on the strength and fatigue properties of springs.In addition,the current production of 51 CrMnV steel is quenched + tempered at medium and high temperatures,which can cause a lot of environmental pollution by oil fumes during the quenching process,so with the increasing emphasis on environmental protection and energy conservation in recent years,it is urgent to develop a new clean and environmentally friendly spring preparation process.In this paper,the effects of different heat treatment processes(salt bath isothermal,pre-martensitic phase transformation,double-order isothermal,quenching and tempering)on the bainite transformation,microstructure and mechanical properties of the test steel were investigated using thermal simulation testing machines,metallurgical microscopy,scanning electron microscopy,hardness,tensile and impact tests.The isothermal quenching process was improved,and the heat treatment process of water mist + air + furnace insulation was used instead of salt bath isothermal in order to avoid the pollution and production costs caused by salt bath isothermal.The phase transformation mechanism of bainitic steel was studied,which is important for regulating the bainite content in 51 CrMnV steel and obtaining excellent mechanical properties.The effect of different isothermal quenching parameters on the tissue properties of bainitic spring steels was explored.In addition,the tissue properties of the isothermal treatment process were compared with those of two improved bainitic transformation processes and the conventional quenching and tempering process to provide a reference for the application of lower bainitic organization in spring heat treatment.The main conclusions were obtained as follows:(1)The continuous cooling transformation properties of 51 CrMnV steel were studied by thermal expansion experiments.The results show that the martensitic phase transformation starts at a temperature Ms of 287°C.The critical cooling rate of 51 CrMnV steel to obtain bainite tissue is obtained,and the pearlite transformation can be avoided when the cooling rate is greater than5°C/s.The water mist + air quenching process can meet the critical transformation rate and obtain the same bainitic-martensitic dual-phase organization as the salt bath isothermal quenching process.(2)The effects of different isothermal temperatures and isothermal times on the organization and properties of the test steels were studied.The results showed that the microstructure was bainitic + martensitic,and the lower bainitic structure with parallel arrangement of slats was obtained.The isothermal temperature affected the bainitic morphology and transformation degree,with the increase of isothermal temperature,the bainitic transformation decreased,the bainitic slats became thicker and the carburized size increased,which further affected the mechanical properties.The mixed bainite/martensite tissue with different bainite content was obtained with different isothermal time,and the bainite content increased with the increase of isothermal time,and the increase rate was gradually leveled off,and the bainite transformation was gradually completed after 20 min of isothermal time.the bainite specimens obtained by holding the isothermal temperature at 350°C for 20 min had the highest strength and the best toughness.(3)Comparison of the differences between different heat treatment processes on the organization and properties of 51 CrMnV.Both the pre-martensitic phase transformation process and the double-order isothermal process resulted in a mixture of bainite,martensite and residual austenite,with better tensile and impact properties than the isothermal quenching process.Compared with the traditional spring production process(quenching + tempering at medium and high temperatures),the microstructure after quenching and tempering heat treatment is a single tempered brittleness;the microstructure after heat treatment in the isothermal quenching process is mainly bainite,with a small amount of martensite and residual austenite.Compared with the conventional spring production process,the isothermal quenching of 51 CrMnV steel increases the tensile strength by 74.8 MPa,the elongation by 3.18%,and the impact energy by 9 J.It has good comprehensive mechanical properties. |
