A study of self‐hardening bainitic cast steel

Bainitic cast steel is a kind of wear resistant material which has high strength and toughness, and can usually be obtained by isothermal quenching or molybdenum alloying. However, isothermal quenching has lower production efficiency and molybdenum alloying has higher production cost. In this paper, according to the characteristics that manganese and boron elements delayed the pearlitic transformation, the authors developed a new type of self‐hardening bainitic cast steel in which manganese and boron were main alloy elements and a small amount of titanium, nitrogen, calcium, barium and yttrium elements were also added in the steel that could refine and purify the solidification structure of steel. On this basis, the author studied the effect of tempering treatment on microstructures, mechanical properties and wear resistance of bainitic cast steel. The results showed that impact toughness of bainitic cast steel increased ceaselessly with the increase of tempering temperature, and there was tempering brittleness while tempering from 450°C–500°C. Moreover, the hardness of bainitic cast steel decreased with the increase of tempering temperature, and hardness decreased slowly and maintained at 55HRC or above when tempering temperature was lower than 300°C. Under the condition of two‐body pin‐on‐disc wear, the wear resistance of bainitic cast steel decreased with the increase of tempering temperature, but bainitic cast steel tempering at 300°C had excellent wear resistance in the condition of impact wear. In the practical use, the bucket teeth of excavator and the hammer of crusher making from self‐hardening bainitic cast steel were safe and reliable, and their service life were increased by 120–150% than Hadfield manganese steel.     

Fe-Ni-C合金中珠光体、贝氏体及马氏体的相变内耗

通过研究三种 Fe-Ni-C 合金中珠光体、贝氏体及马氏体的低频相变内耗,发现马氏体相变内耗遵守相界面位错的静滞后损耗机制,珠光体和贝氏体相变内耗不遵守这种机制。在频率基本不变时,随着冷却速度的增加珠光体和贝氏体的相变内耗峰增高,峰温降低,且 Q(?)和(?)/fm 件·T_m 呈正比线性关系;频率增加时,珠光体和贝氏体的内耗峰明显降低,峰温上串。在连续冷却条下,Fe-Ni-C 合金中开始珠光体及贝氏体相变时并不存在点阵软化现象。     

Characterisation of quasi‐stationary temperature fields in laser welding by infrared thermography

In this work, high‐speed thermography is shown to effectively capture quasi‐stationary temperature fields during the laser welding of steel plates. This capability is demonstrated for two cases, with one involving the addition of a ferritic‐bainitic filler wire, and the other involving the addition of a low‐transformation‐temperature (LTT) filler wire. The same welding parameters are used in each case, but the temperature fields differ, with the spacing between isotherms being greater in the case where the low‐transformation‐temperature filler material is added. This observation is consistent with the differences in the extent of the heat‐affected zone in each sample, and the shape of the weld pool ripples on the weld bead surfaces. The characterization of temperature fields in this way can greatly assist in the development of novel methods for reducing residual stresses, such as the application of low‐transformation‐temperature filler materials through partial‐metallurgical injection (PMI). This technique reduces or eliminates tensile residual stresses by controlling the temperature fields so that phase transformations take place at the optimum times, and success can only be guaranteed through precise knowledge of the temperature fields in the vicinity of the welding heat source in real time.     

Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

A Fe-based coating with nano-scale bainitic microstructure was fabricated using laser cladding and subsequent isothermal heat treatment. The microstructure of the coating was observed and analyzed using optical microscope (OM), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results showed that nanostructured bainitic ferrite and carbon-enriched retained austenite distributed uniformly in the coating. Blocky retained austenite was confined to the prior austenite grain boundaries resulting from the elements segregation. The bainitic microstructure obtained at 250 °C had a finer scale compared with that obtained at 300 °C. The volume fraction of austenite increased with increasing transformation temperature for the fully transformed bainitic coating. The bainitic transformation was accelerated as a result of the fine prior austenite generated during the laser cladding. The evolution of the carbon contents in bainitic ferrite and retained austenite revealed the diffusionless mechanism of the bainitic transformation.     

Sliding wear and low cycle fatigue properties of new carbide free bainitic rail steel

Abstract

A new carbide free bainitic rail steel was prepared, whose comprehensive mechanical properties are equal or superior to current premium pearlitic and bainitic rail steels. The new bainitic rail steel possesses better low cycle fatigue properties and approximate resistance to wear compared with current pearlitic rail steel. The carbon enriched film-like austenite between the ferrite of the new bainitic rail steel can delay crack initiation and propagation in fatigue processes, resulting in a relatively high low cycle fatigue life (about two times) compared to the pearlitic rail steel. Finally, a wear model during sliding wear and deformation model during the low cycle fatigue of pearlitic and bainitic rail steels were established. As a result, the bainitic rail steel with a relatively reasonable combination of wear and fatigue properties compared to pearlitic rail steel is obtained, in which the initiation and propagation of cracks may be partially or entirely removed during the wear process.     

Development of bainitic rail steels with potential resistance to rolling contact fatigue

Rails are a major capital and maintenance cost for railways in North America. While manufacturers produce clean steels with high quality, most rails made today retain the basic carbon–manganese chemistry of traditional pearlitic rails. This paper describes the development of a bainitic rail steel with potential additional resistance to rolling contact fatigue damage. It is shown that rails can be produced in bainitic steel without the need for complex heat treatments after rolling, and that bainitic rails can have higher hardness and fracture toughness than pearlitic rails. Although small‐ and full‐scale tests indicate that the wear performance of bainitic steel depends considerably on test conditions, the indication is that bainitic steel rails can have significantly better rolling contact fatigue performance compared to pearlitic rails. Reasons for the superior fatigue performance are not fully understood, although a number of hypotheses exist. A conclusion is that continued research would be useful to understand quantitatively the physics and metallurgy of wheel/rail contact.     

Transformation kinetics of unalloyed and high Mn austempered ductile iron

Abstract

The kinetics of bainitic transformation was studied in unalloyed and 1%Mn alloyed ductile irons. The samples were subjected to different austempering heat treatment cycles in dilatometry equipment. The results showed the effect of Mn on delaying the start and end of bainitic transformation, supposedly because of Mn segregation to the primary austenite grain boundaries. Decreasing austempering temperature led to precipitation of carbides inside the lower bainitic ferrite plates and therefore, more dilatation during the bainitic reaction. The dilatometric results were also used to calculate n and k in the Johnson-Mehl-Avrami equation and to derive the time-temperature-transformation diagrams. X-ray results were used to calculate the quantitative amount of precipitated carbides within lower bainitic ferrite and the dilatation during austempering.     

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

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.     

Effect of microstructural modification on damage tolerance of 34CrMo4 shaft steel

Overall damage tolerances of the heat‐treated 34CrMo4 steels having ferritic‐pearlitic, bainitic, and tempered‐martensitic microstructures were evaluated based on their threshold stress intensity factor prior to small crack propagation, fatigue strength, and fracture toughness under static loading. Kitagawa‐Takahashi diagrams were constructed to determine the limiting size of small crack propagation. The micromechanical effects of carbide morphology and phase distribution on quasi‐static and dynamic mechanical properties were also elaborated. Fractographic investigations were carried out on the notched fatigue test specimens to distinguish deterioration and deformation mechanism of the microstructure under reversed cyclic loads. Finally, improvements in the damage tolerance were discussed to present the advantages and disadvantages of each heat treatment procedure to minimize in‐service fatigue failures.     

Adaptive Finite Element Simulations for Macroscopic and Mesoscopic Models of Steel

During heat treatment and other production processes, gradients of temperature and other observables may vary rapidly in narrow regions, while in other parts of the workpiece the behaviour of these quantities is quite smooth. Nevertheless, it is important to capture these fine structures during numerical simulations. Local mesh refinement in these regions is needed in order to resolve the behaviour in a sufficient way. On the other hand, these regions of special interest are changing during the process, making it necessary to move also the regions of refined meshes. Adaptive finite element methods present a tool to automatically give criteria for a local mesh refinement, based on the computed solution (and not only on a priori knowledge of an expected behaviour). We present examples from heat treatment of steel, including phase transitions with transformation induced plasticity and stress dependent phase transformations. On a mesoscopic scale of grains, similar methods can be used to efficiently and accurately compute phase field models for phase transformations.     

Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in Steel Weld Metal

Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations.Usually,decreasing the grain size is the only approach by which both the strength and the toughness of a steel are increased.However,low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study.By direct tracking of the morphological evolution during phase transformation,it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite,whereas small austenite grain size leads to grain boundary nucleation of bainite.This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.     

Investigation of bainitic isothermal transformation products in unalloyed ductile cast iron

The properties of the products of isothermal transformation of undercooled austenite into bainite in unalloyed ductile cast iron were investigated using X-ray diffraction. The following parameters were investigated: the fraction of austenite in the cast iron matrix, the crystal lattice parameter, and the width of the diffraction lines of the α and γ phases. The structures were studied using a TEM. It was observed that the temperature T_A and time τ_A of the isothermal transformation significantly influence the nature of the α and γ phases. The transformations are determined by the diffusion of carbon, and the maximum carbon content is approximately twice the equilibrium carbon content at the austenitising temperature. The lattice parameter of the α phase in the range of T_A studied decreases with increasing cooling time but increases in the upper bainite range. The increase in this lattice parameter results from the typical process of bainitic transformation during the retained austenite eutectoidal reaction (stage III). The crystal structure of the γ phase in the upper bainitic region is more perfect than in the lower range. Within the investigated temperature range of T_A, bainitic ferrite continually improves its crystal structure.

MST/3104     

Effect of chemical segregation on nanobainitic transformation in laser cladded coatings

Laser cladding and subsequent isothermal holding were employed to fabricate the Fe-based nanobainitic coatings. The microstructures of the isothermal heat treated coatings were observed and characterized by using the optical microscope (OM), filed-emission scanning electron microscope (FE-SEM), field-emission transmission electron microscope (FE-TEM), energy-disperse X-ray spectroscopy (EDAX), and X-ray diffraction (XRD). The results showed that the elements of Mn, Cr, and Si segregated at the prior austenite interdendritic regions. The segregation of the solutes significantly affected on the kinetics of the nanobainitic transformation and the final microstructures obtained at the different isothermal temperatures. The bainitic transformation incubation time was reduced by the increasing of available driving force for nucleation. The bainitic sheaves grow across the interdendritic regions of the prior austenitic dendrites. The continuity and thickness of blocky austenite distributed at the prior interdendritic regions were sensitive to the transformation temperature, as a result of the threshold of the bainitic transformation varied with the chemical gradients near the segregated interdendritic regions.     

The ferromagnetic transition in MnAs synthesised by mechanical alloying and powder blending techniques

A study of the structure and properties of MnAs synthesized by mechanical alloying and powder blending techniques has been carried out. Mechanical alloying resulted in the formation of MnAs during milling. Following heat treatment samples prepared using both techniques exhibited a magnetostructural transformation from the paramagnetic B31 to the ferromagnetic B8₁ phase with decreasing temperature or increasing magnetic field. The transformation temperatures and fields were found to depend on milling procedure and heat treatment conditions. Powder blended and heat treated samples exhibited excellent stability during transformation cycling.     

Minimizing Stress and Distortion for Shafts and Discs by Controlled Quenching in a Field of Nozzles

A coupled gas‐dynamical and thermo‐mechanical model for simulation of the gas flow, gas and specimen temperature, phase, stress, strain, and displacement transient‐fields during quenching of cutting discs and shafts of steel is introduced. The material properties (e. g. density, conductivity, heat capacity, hardness) are obtained by homogenization procedures. The material behaviour is described as an extension of the classical J₂‐plasticity theory with the extension of temperature and phase fraction dependent yield criteria. The coupling effects such as dissipation, phase transformation enthalpy, and transformation induced plasticity (TRIP) are considered. Simulations were carried out for cutting discs of knives, and for shafts made of steel SAE 52100 with varying diameter. For the validation of the simulations, these work pieces were heated in a roller hearth kiln up to 850 °C, and than quenched in a field of nozzles in which the heat transfer coefficient was known and could be locally adjusted by the volume flow of each nozzle. The phase fractions, surface hardness, distortion, and residual stresses were measured. The simulated and measured results fit quite well. According to optimization‐simulations the shafts were quenched with a certain heat transfer coefficient distribution. The bigger diameter parts of the shaft were more intensively quenched by an increased gas flow so that the hardness profiles were equalized and the residual stresses at the edges were significantly reduced.     

Investigation of rolling contact crack initiation in bainitic and pearlitic rail steels

The stress–strain history and the crack initiation lives of bainitic and head‐hardened pearlitic rail steels were determined under rolling contact loading by implementing the semi‐analytical Jiang–Sehitoglu rolling contact model that incorporates both ratchetting and multiaxial fatigue damage. The calculations revealed that the bainitic steel withstands higher loads than the pearlitic steel at low shear tractions, however; both materials behave in an increasingly similar manner as the shear tractions increase. Furthermore, maximum damage occurs in both steels when ratchetting and fatigue damage coincide on the surface. In addition to shedding light on the rolling contact fatigue (RCF) performance of bainitic and pearlitic rail steels, the current work also establishes a methodology for the realistic prediction of crack initiation under RCF.     

Fabrication and mechanical properties of steel–steel composites

Metal matrix composites based on a low carbon steel matrix reinforced with high carbon steel wires have been fabricated by a combined cold and hot rolling process. Both continuously and discontinuously aligned composites have been produced. A subsequent heat treatment allowed the formation of martenisitc, bainitic or pearlitic wires in a ferrite predominantly matrix. The optimum wire microstructure giving a composite with high strength and reasonable ductility was found to be bainitic — martensitic wires were found to contain microcracks that gave poor composite strengths and ductilities. The discontinuous wire composites produced similar strengths to the continuous composites only when they were deformed to give a wire aspect ratio greater than 20. The strengths of both types of composites showed a good fit to the rule of mixtures as the volume fraction of fibers was increased.     

TRIP and phase evolution for the pearlitic transformation of the steel 100Cr6 under. step‐wise loads

The investigation of complex material behaviour of steel like transformation‐induced plasticity (TRIP) and stress‐dependent phase transformation (SDPT) is a large field of current research. The simulation of the material behaviour of work‐pieces in complex situations requires a knowledge as deep as possible about such phenomena. In addition, there are effects in the case of non‐constant stress which cannot be explained by the widely used Leblond model for TRIP. Therefore, we consider a TRIP model taking into account back stress due to TRIP itself. Based on experimental data for the isothermal pearlitic transformation of the steel 100Cr6 (SAE52100) under step‐wise loads we calculate material parameters for the extended TRIP model. Regardless of the preliminary character of the performed tests, all experiments show a back‐stress effect with a decrease of the TRIP strain after unloading.     

贝氏体钢疲劳性能的研究进展

本文介绍了贝氏体钢的滚动接触疲劳裂纹,比较了贝氏体钢、珠光体钢和高锰钢的抗疲劳性能．     

The effect of hydrogen on the bainite transformation

The effect of hydrogen on the upper bainite transformation in two silicon containing steels has been investigated. For comparison, isothermal transformation at the same temperature has also been performed in a helium atmosphere. In both Fe-0.2C-3Mn-2Si and Fe-0.4C-4Ni-2Si (nominal wt %) alloys it was discovered that the bainite reaction proceeds further towards completion when the transformation is carried out in a hydrogen atmosphere. This can result in the reduction or elimination of the martensite phase which forms from residual austenite upon quenching to room temperature. The resultant microstructure of specimens heat treated in hydrogen was a fine aggregate of upper bainitic ferrite and interlath retained austenite. This effect is discussed in terms of hydrogen interactions in the lattice undergoing bainite transformation via a displacive mechanism. Additionally, it is found that the stability of the retained austenite in the final bainitic microstructure is not markedly influenced by hydrogen.