Zhinan Yang

Dislocation-Twin Boundary Interactions Induced Nanocrystalline via SPD Processing in Bulk Metals

This report investigated dislocation–twin boundary (TB) interactions that cause the TB to disappear and turn into a high-angle grain boundary (GB). The evolution of the microstructural characteristics of Hadfield steel was shown as a function of severe plastic deformation processing time. Sessile Frank partial dislocations and/or sessile unit dislocations were formed on the TB through possible dislocation reactions. These reactions induced atomic steps on the TB and led to the accumulation of gliding dislocations at the TB, which resulted in the transition from coherent TB to incoherent GB. The factors that affect these interactions were described, and a physical model was established to explain in detail the feasible dislocation reactions at the TB.

Rolling Contact Fatigue Performances of Carburized and High-C Nanostructured Bainitic Steels

In the present work, the nanostructured bainitic microstructures were obtained at the surfaces of a carburized steel and a high-C steel. The rolling contact fatigue (RCF) performances of the two alloy steels with the same volume fraction of undissolved carbide were studied under lubrication. Results show that the RCF life of the carburized nanostructured bainitic steel is superior to that of the high-C nanostructured bainitic steel in spite of the chemical composition, phase constituent, plate thickness of bainitic ferrite, hardness, and residual compressive stress value of the contact surfaces of the two steels under roughly similar conditions. The excellent RCF performance of the carburized nanostructured bainitic steel is mainly attributed to the following reasons: finer carbide dispersion distribution in the top surface, the higher residual compressive stress values in the carburized layer, the deeper residual compressive stress layer, the higher work hardening ability, the larger amount of retained austenite transforming into martensite at the surface and the more stable untransformed retained austenite left in the top surface of the steel.

Effects of deformation and addition of aluminium on spheroidisation of high-carbon-bearing steel

ABSTRACT The effects of deformation amount, deformation temperature and subsequent holding time on the deformation spheroidising process of high-carbon-bearing steel containing aluminium were investigated. The effects of aluminium contents on the mechanism of spheroidisation were also investigated. Results show that the deformation spheroidising process can shorten the spheroidisation cycles. High deformation temperature and deformation amount induce the coarsening of carbides. However, at low deformation temperature, deformation amount slightly affects the diameters and roundness of carbides. The carbides exhibit a uniform diameter distribution after prolonging the holding time at 650°C to 45 min after deformation. When aluminium content is less than 0.75%, the addition of aluminium inhibits the growth of carbides and improves their roundness.

Effects of alloying elements and cooling rates on the high-strength pearlite steels

ABSTRACT The effects of the alloying elements of Cr, Mn and the cooling rates after hot deformation on the microstructures and mechanical properties of pearlite steels were studied. Results show that increasing Cr and decreasing Mn significantly increase the eutectoid transformation temperature of steel. The grain sizes of prior austenite of the steels after hot deformation are ∼12 µm. However, the high-Cr–low-Mn steel exhibits a finer interlamellar spacing and some better mechanical properties than that of the high-Mn–low-Cr steel. A full pearlite microstructure with an interlamellar spacing of 97 nm was obtained on the former steel, which exhibits a hardness of HRC49, a tensile strength of 1700 MPa and an elongation of 19%.

Research and application progress of nanostructured bainitic steel in bearings

Research into nanostructured bainitic steels has attracted much attention in the past twenty years. The nanostructured bainite was first studied by Bhadeshia and his coworkers. They reported that the nanostructured bainitic microstructure consisting of 20–40nm thick bainitic ferrite plates dispersed in retained austenite matrix exhibited a hardness value in excess of 650HV30, a tensile strength of ~2.3GPa and a toughness of 30–40MPa·m1/2.1–3 The nanostructured bainitic microstructure, which is shown in Figure 1, was formed in high–carbon Si–rich steels by austempering at 125–350°C for a long time. The nanostructured bainite is also known as hard bainite owing to its high hardness, low temperature bainite because of its low transformation temperature, super bainite due to its excellent mechanical properties.

Effect of Aluminum Alloying on the Hot Deformation Behavior of Nano-bainite Bearing Steel

Interest in using aluminum in nano-bainite steel, especially for high-carbon bearing steel, is gradually growing. In this study, GCr15SiMo and GCr15SiMoAl steels are introduced to investigate the effect of Al alloying on the hot deformation behavior of bearing steel. Results show that the addition of Al not only notably increases the flow stress of steel due to the strong strengthening effect of Al on austenite phase, but also accelerates the strain-softening rates for its increasing effect on stacking fault energy. Al alloying also increases the activation energy of deformation. Two constitutive equations with an accuracy of higher than 0.99 are proposed. The constructed processing maps show the expanded instability regions for GCr15SiMoAl steel as compared with GCr15SiMo steel. This finding is consistent with the occurrence of cracking on the GCr15SiMoAl specimens, revealing that Al alloying reduces the high-temperature plasticity of the bearing steel. On the contrary, GCr15SiMoAl steel possesses smaller grain size than GCr15SiMo steel, manifesting the positive effect of Al on bearing steel. Attention should be focused on the hot working process of bearing steel with Al.