| Oil-quenched spring steel wire is the main production material for automotive spring parts,which can be used to produce high-end automotive parts such as valve springs and suspension springs.Its heat treatment processes contain usually oil quenching and lead bath tempering,but lead bath pollutes the environment and takes a long time to operate,while induction heating is fast,efficient and environmentally friendly.oil-quenched spring steel wire tempered by using induction heating which is one of the future development trends can refine grains,Inhibiting decarburization and improving the plastic toughness of steel wire.In this work,oil-quenched 55 SiCr spring steel wire with diameters of 5.4 mm,3.5 mm and 2.2mm is selected as the research object,its microstructure and mechanical properties under different induction tempering processes are studied,and the mechanisms and changes between properties and microstructure were analysized and discussed,and the induction tempering optimized process was obtained.The main conclusions are as follows: oil-quenched 55 SiCr spring steel wires of different specifications all have a take-up speed spinodal,after the take-up speed exceeds the inflection point,the strength of the steel wire will be greatly improved,and the area shrinkage rate will be improved,but there is an ideal take-up speed,the induction tempered microstructure brtween surface and core of spring steel wires are all fine and dense acicular ferrite,with uniform structure distribution and small inter-lamellar spacing,and a large number of tempered acicular carbides are dispersed on the matrix,and there are undissolved small-grained carbides,which have a pinnig effect on dislocations,improving the strength,yield ratio and plastic toughness of the spring steel wire,and the hardness fluctuation of the surface layer and the core is small and stable.Similarly,oil-quenched spring steel wires of different specifications have a tempering temperature inflection point,after the tempering temperature exceeds this inflection point,the strength of steel wires will be greatly reduced,but the area shrinkage rate will increase.The study found that at a tempering temperature of 460 ℃and a matching take-up speed,oil-quenched 55 SiCr spring steel wire after induction tempering has fine and dense acicular ferrite on the surface and core,with blurred martensite lath boundaries,and tempered acicular and granular carbides are dispersed.The hardness range of the core and surface is extremely small,and fluctuation is stable.The tensile strength and yield ratio meet the requirements of the national standard,and the area shrinkage far exceeds the national standard.The comprehensive performance is superior.The greater the drawing total compression ratio of oil-quenched 55 SiCr spring steel wire,the smaller and denser the grains after induction tempering,the more needle-like carbides and the dispersed distribution,the smaller the microstructure spacing,the higher the strength and the area shrinkage rate,but more difficult to control the stability of small diameter wire properties by the induction tempering process.The observation of fracture morphology shows that the fracture of oil-quenched 55 SiCr spring steel wire has obvious tear ridges and a large number of equiaxed dimples,which are typical ductile fractures.With the increase of the take-up speed of induction tempering,the number of small dimples in the steel wire fracture increases,and the number of large dimples decreases;when the take-up speed is too fast,the number of dimples in the steel wire fracture decreases obviously,the depth becomes shallower,and the fracture surface is relatively flat and smooth.With the increase of tempering temperature,the numbers of dimples and the depth of steel wire fractures increase,and the torn fibers are gradually distributed in a "mesh".When the tempering temperature is high,the torn fibers of steel wire fracture surface are "broken". |
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