Research On The Effect Of Heating Methods On The Mechanical Properties And Microstructure Of Oil Quenched And Tempered Spring

Oil-quenched and tempered spring steel wire,as an important component of the automobile shock absorption system,is mainly used to absorb vibration and ensure the driving safety and comfort.At present,two methods of induction heating and traditional tube furnace heating are adopted in the production of oil-quenched and tempered spring steel wire.In order to explore the influence of different heating methods on the property and microstructure of the steel wire.Therefore,55 Cr Si spring steel wire is taken as research object in this thesis.The induction heating and tube furnace heating continuous production lines for quenching and tempering were used to obtain four strengths levels of 1800 MPa,1900MPa,2000 MPa and 2100 MPa.The influence of induction heating and tube furnace heating on the mechanical properties of the spring steel wire were investigated using a tensile testing machine and a microhardness tester.The microstructure,carbides,dislocation density,retained austenite and carbon contents of induction wire and tube furnace wire of different strength levels were measured and analyzed by using field emission electron microscopy(SEM)and X-ray diffraction(XRD).The evolution of the microstructure and strengthening and toughening mechanism of the spring steel wires under different heating methods were also discussed.The main work and research conclusions are as follows:(1)There are obvious differences in the homogeneity performance of the spring steel wires under different heating methods.The homogeneity performance of induction heating steel wire is gradually improved with the increase in strength,while the homogeneity performance of the tube furnace heating steel wire is gradually deteriorated.At the level of 1800 MPa and 1900 MPa,the tube furnace heating wire is better than the induction heating wire.Above the strength level of 2000 MPa and2100MPa,the induction heating wire shows a more stable homogeneity performance.In terms of the area reduction rate,the induction heating wire is much higher than the tube furnace heating wire,the area reduction rate is basically maintained above 50%,and shows a certain monotonic decrease with the increase of strength,while the tube furnace heating wire fluctuating near 42%,the microhardness distribution along the diameter of tube furnace heating steel wire is more stable than the induction heating wire at the strength levels of 1800 MPa and 1900 MPa,and the hardness distribution of the tube furnace heating wire and the induction heating wire are similar at the strength levels of 2000 MPa and 2100 MPa.(2)Compared with the microstructure of the same strength steel wire,the induction heating wire is smaller than the tube furnace heating wire and has no decarburization.The tube furnace heating wire has an obvious decarburization layer with a depth of about 14～41μm on the lower layer of each strength.With the increase of strength,the segregation degree of tube furnace heating wire is getting more and more serious,but the induction heating wire shows a decreasing trend.SEM analysis shows that the tube furnace heating steel wire of the same strength level has more carbides and larger size.The induction heating steel wire also behaves like this at the similar tempering temperature.XRD analysis shows that the retained austenite and carbon content of the induction heating steel wires of 1800 MPa and 1900 MPa strength levels are lower than those of the tube furnace steel wire,but the induction wire of 2000 MPa and 2100 MPa strength level exhibits higher retained austenite and carbon content.(3)The strengthening and toughening mechanisms of spring steel wires mainly includes fine-grain strengthening,dislocation strengthening,precipitation strengthening and dispersion strengthening.Among them,the fine-grain strengthening increment of induction heating steel wires is bigger than that of tube furnace heating steel wires.The dislocation strengthening increase of tube furnace heating steel wires is bigger than that of Induction heating steel wires.The precipitation strengthening increment of induction heating steel wires is bigger than that of tube furnace heating steel wires at the strength levels of 1800 MPa and 1900 MPa,while tube furnace heating steel wires is bigger than that of induction heating steel wire for 2000 MPa and2100MPa level.For the dispersion strengthening,the discrete distribution of particles bear the matrix load.The strengthening stress of the tube furnace heating steel wires is larger than the induction heating steel wires,but the induction heating steel wires up to the 2100 MPa level is slightly higher than of the tube furnace heating steel wires.(4)Based on the influence of the comprehensive heating methods on the oil-quenched and tempered spring steel wire and the analysis of the process characteristics,under the strength levels of 1800 MPa and 1900 MPa,the tube furnace heating process can be preferentially used,which can make the tube furnace line show better comprehensive performance.At the 2000 MPa and 2100 MPa levels,induction heating production can be given priority,it maintains a high level of strength while exists better plastic toughness performance,which will be a research trend in the future.