Study On Spot Segregation And Conditioning Heat Treatment Of Cr5Forged Steel For Heavy Backup Roll

By intercepting test specimens in Cr5-type forged steel heavy backup roll afterheat treatment, the microstructure, chemical component, phase structure andmechanics performance were studied by SEM, EDS, TEM, XRD, durometer, tensileand impact testing machine. Results show that Cr5steel exist a more serious spotsegregation phenomenon, its essence is the abnormal aggregation of rich Cr, Mo, Mncarbides. The carbides abnormally aggregated have various morphology, larger sizeand irregular distribution. The carbide styles include on complex hexagonal andorthorhombic M7C3, complex cubic M23C6and orthorhombic M3C2. Complexhexagonal M7C3and cubic M23C6have the crystallographic orientation relationship of(110)K//(011)α,[0001]K//[011]αand (511)K//(011)α,[116]K//[011]αwith ferriterespectively. In addition, the micro-hardness of spot segregation area is higher thanedge spot segregation.After quenching, most of carbides are dissolved in the matrix, the carbides of notdissolving with small particles are distributed in the martensite. The dissolutionamount of carbides increase with the increase of temperature and soaking time,carbides are mostly dissolved into matrix when temperature is970℃. At thebeginning soaking stage, carbides dissolve rapidly, but dissolution rate decreases withthe soaking time prolongating, and finally tends to zero when soaking time is60min.The hardness of Cr5steels increase and then decrease with the increase of temperatureand soaking time. The peak hardness is reached after quenching (970℃×60min).Let specimens after quenching (970℃×60min) conduct tempering. With theincrease of temperature and soaking time, the ductility and toughness increase whilethe strength and hardness drop down, when tempering process is540℃×120min, themost ideal comprehensive mechanics performance of Cr5steel can be achieved: tensilestrength, elongation, impact absorbing energy and hardness are1429.88MPa,2.90%,146.1J,44.57HRC respectively, and then microstructure of Cr5steel has not coarsening. When temperature or time remain unchanged, with other parameterincreasing, microstructure and carbides of Cr5steel have coarsening tendency.Soaking time is120min, temperature is540℃, carbides of Cr5steel have singlemorphology, smaller size, their style translated to orthorhombic M7C3and had thecrystallographic orientation relationship of (113)K//(200)α,[512]K//[011]αwith ferrite.As temperature rise to560℃, part of microstructure still keep lathmartensitemorphology in the tempered microstructure, and present glomeroplasmatic distribution,between two adjacent lath martensite has the crystallographic orientation relationshipof (110)M1//(110)M2，[111]M1//[001]M2.The molar enthalpy change (ΔH) and Gibbs free energy change (ΔG) of formationand style transformation of carbides were calculated via thermodynamic theories.Results display that the stability of M7C3is best while M23C6is worse above650K.Compared with M3C2, M23C6is more easily occur to lattice reorganization generateM7C3, in the process of alloy elements homogenization under813K and833K.