Fucheng Zhang

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.

Effect of Martensite Morphology on Impact Toughness of Ultra-High Strength 25CrMo48V Steel Seamless Tube Quenched at Different Temperatures

A 25CrMo48V steel for ultra-deep oil/gas weil casings was quenched at 900–1200 °C and tempered at 650 °C. The lath martensitic structures were characterized by optical microscope (OM), field emission scanning electron microscopy (FESEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the trans verse impact energy at 0 °C was measured from the as-quenched and tempered specimens. The results show that with the quenching temperature decreased, the prior austenite grain, martensitic packet and block are refined, while the lath width seems to remain unchanged. The enhancement of impact toughness with the decreasing quenching temperature can be attributed to refinement of the martensitic structure with high-angle boundaries, and the block is the minimum structure unit controlling impact toughness. The trans verse impact energy [ECVN (0 °C) ≥ 100 J] required for seamless casings with ultra-high strength (Rp0.2 ≥ 932 MPa) has been finally achieved with the experimental steel quenched at 900–1000 °C and tempered at 650 °C.

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.

Novel method for refinement of retained austenite in micro/nano-structured bainitic steels

ABSTRACT A comparative study was conducted to assess the effects of two different heat treatments on the amount and morphology of the retained austenite in a micro/nano-structured bainitic steel. The heat treatments used in this work were two-stage bainitic transformation and bainitic-partitioning transformation. Both methods resulted in the generation of a multi-phase microstructure containing nanoscale bainitic ferrite, and/or fresh martensitic phases and much finer retained austenite. Both heat treatments were verified to be effective in refining the retained austenite in micro/nano-structured bainite and increasing the hardness. However, the bainitic transformation followed by partitioning cycle was proved to be a more viable approach than the two-stage bainitic transformation due to much shorter processing time, i.e. ∼2 h compared to ∼4 day, respectively.

Forging of High-Manganese Steel Crossing

The thermoplastic characteristics of high-manganese steels with different phosphorus and sulfur contents were investigated. The zero-ductility temperature and plasticity-temperature range of the high-manganese steels were reduced significantly with increase in phosphorus or sulfur content. The relationship between the initial forging temperature of the high-manganese steel and the content of phosphorus and sulfur can be expressed as Tift = 1200 – 1530wS – 1650wP. The final forging temperature is 900°C, and the heating rate during the forging of the high-manganese steel crossing must be slow. The lifetime of the forged high-manganese steel crossing was almost doubled compared with conventional cast high-manganese steel crossing.

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.

Sulfide Stress Cracking Behavior of a Martensitic Steel Controlled by Tempering Temperature

A medium-carbon Cr–Mo–V martensitic steel was thermally processed by quenching (Q) at 890 °C and tempering (T) at increasing temperatures from 650 °C to 720 °C and the effect of tempering temperature, Tt, on sulfide stress cracking (SSC) behaviors was estimated mainly via double cantilever beam (DCB) and electrochemical hydrogen permeation (EHP) tests and microstructure characterization. The results indicate that the threshold stress intensity factor for SSC, KISSC, increased with increasing Tt. The overall and local H concentration around the inclusions decreased with increasing Tt, due to reductions in the amounts of solute atoms, grain boundaries and dislocations, which effectively prevented SSC initiation. Also, increasing Tt caused an increased fraction of high-angle boundaries, which evidently lowered the SSC propagation rate by more frequently diverting the propagating direction and accordingly restricted SSC propagation. The overall SSC resistance of this Q&T–treated steel was therefore significantly enhanced.

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.

Al and Si Influences on Hydrogen Embrittlement of Carbide-Free Bainitic Steel

A first-principle method based on the density functional theory was applied to investigate the Al and Si influences on the hydrogen embrittlement of carbide-free bainitic steel. The hydrogen preference site, binding energy, diffusion behaviour, and electronic structure were calculated. The results showed that hydrogen preferred to be at the tetrahedral site. The binding energy of the cell with Si was the highest and it was decreased to be the worst by adding hydrogen. The diffusion barrier of hydrogen in the cell containing Al was the highest, so it was difficult for hydrogen to diffuse. Thus, hydrogen embrittlement can be reduced by Al rather than Si.