Research On Banded Microstructure Of 18CrNiMo7-6 Gear Steel

18CrNiMo7-6 gear steel is widely used in the automotive industry and wind power generation.This dissertation characterized the banded structure and explored effective methods for reducing or eliminating the structure in 18CrNiMo7-6 gear steel.Both the ingot and the forged blank samples were prepared by a forging company in Jiangsu.The paper studied the effects of high temperature homogenization annealing,normalizing and high temperature tempering on the banded structure.The microstructures of the banded structure were observed by optical microscope?OM?and scanning electron microscope?SEM?.The distribution of the key alloying elements,such as C,Cr,Mn,Si,Ni and Mo in the steel,was scanned by EDS spectrometer.The changes of the phase in the microstructure after heat treatment were analyzed by X-ray diffraction.The hardness distribution of the microstructure before and after the heat treatment was tested by microhardness tester.Gleeble-3800 thermal simulator was used to study the effects of cooling rate and compression deformation along parallel direction of the banded structure on the band microstructure.The main research results are as follows:There was serious primary banded structure in as-cast ingot samples.Its rating is about 3 or 4 grade according to the national standard GB/T 13299-91 and 3.897 grade according to the American ASTM.E1268-2001 standard.Within the band microstructure,the pearlite belt with a bandwidth of 75 to 300?m,an average bandwidth 225?m,an average belt length 800?m,the ratio of length to width 3.55,and an average proportion of the pearlite region 55.3%were mearured.The main segregated elements in the primary band structure were C and Cr,followed by Mn and Mo,and Ni and Si not segregated substantially.Dendritic segregation formed during the cooling process of solidifying steel ingots is the main cause for the formation of the banded structure.There was much severer secondary banded structure in the forged samples.Its rating is about 4.5 grade according to the national standard GB/T 13299-91 and 4.13grade according to the American ASTM.E1268-2001 standard rating.Within the band microstructure,the pearlite belt with a bandwidth 40 to 270?m,the average bandwidth160?m,the average band length 800?m,the ratio of bandwidth to length 5,and the average proportion of pearlite region 67.8%were measured.The main segregated elements in the secondary band structure were C,followed by Cr and Mn,and another elements Si,Ni and Mo not obvious.The remained segregation of alloy elements in the forged structure and slow cooling rate after forging that are beneficial to the diffusion of C element in austenite as well as the growth of pro-eutectoid ferrite were the main reason for the formation of the secondary banded structure.The occurrence of dynamic recrystallization led to grain refinement,which is conducive to the eutectic ferrite grain nucleation,and the directionality of the forging processing aggravating segregation zone contributed to the further development of the secondary banded structure.Homogenization annealing at 1200?¹300?can effectively reduce the secondary band structure in forged samples.High temperature tempering at 720?can further reduce banded structure.Moreover,the longer the tempering holding time is,the more obvious the effect is.The banded structure was nearly eliminated after normalizing at 850?,which corresponds to microstructures of granular bainite,small amounts of residual austenite and undissolved carbides.After tempering at 720?,the microstructure was consisted of granular ferrite and a small amount of M₃C,the hardness further decreased,and the banded structure further reduced.Moreover,the longer the tempering holding time is,the more obvious the effect is.The microstructure with acicular bainite,granular bainite and a small amount of undissolved carbides were obtained from 950?cooling to 600?with cooling rate at0.2k/s or 0.5 k/s and followed by air cooling.Increasing cooling rate can effectively prevent the precipitation of pro-eutectoid ferrite and thereby suppress the occurrence of banded structure.Compression deformation can reduce or eliminate the banded structure.The greater the amount of compression deformation is,the more significant the effect is.