EBSD Study on Proeutectoid Ferrite and Eutectoid Ferrite Refinement Mechanism of D2 Wheel Steel Under a Rolling Condition

In this paper, the SEM (with EBSD system) is used to study the refinement mechanism of proeutectoid ferrite (Pro-f) and eutectoid ferrite (Eut-f) of D2 wheel steel in a rolling contact. The results indicate that with the increase of the shear strain (γ<0.21), the dislocation density in the proeutectoid ferrite increased continuously, and the dislocation cells formed were uniformly distributed in the grains. Subsequently, the dislocation cell boundaries were changed into low-angle boundaries (LABs), and then the low-angle boundaries were gradually changed into the high-angle boundaries (HABs), and the average grain size was refined from the original 8 μm to 710 nm. When the shear strain is at 0.21≤γ≤0.84, dislocation piled up occurred at the ferrite side of the interface of eutectoid ferrite/cementite, and the spatial misorientation between adjacent two eutectoid ferrites increased gradually, then the ferrite lamellar is divided into bamboo-like by the low-angle boundaries, and proeutectoid ferrite the grains are gradually refined into equiaxed grain. When the shear strain is at 0.84<γ<3.314, the number of high-angle boundaries inside the eutectoid ferrite lamellar increased, and it is refined into bamboo-like grains. The two kinds of ferrite grains are repeatedly refined many times by the equiaxial grains "elongation- bamboo like refinement-elongation", which gradually reduced the size difference. As the shear strain further increases, the two kinds of ferrite are completely mixed into the same morphology, the dislocation density is dramatically reduced, and ultra-fine equiaxed grains about 110 nm is formed.

Computational thermodynamics/kinetics aided design for the microstructure of pearlitic steels during cooling process

The effect of cooling rate on the microstructure of SGLX82A steel was systematically investigated by coupling of experimental characterizations and thermodynamic/ kinetic calculations. The as-received casting ingots were heated and hot-deformed using a Gleeble simulator before being subjected to continuous cooling with rates of 5, 10, 15 and 20 K s-1. The microstructure of the SGLX82A steel was analyzed by using laser scanning confocal microscopy and scanning electron microscopy. The homogeneous microstructure with different proeutectoid ferrite fraction, pearlite colony size as well as pearlite lamellar spacing was examined at varied cooling rate. The calculation of the para-equilibrium phase diagram was performed by using Thermo-Calc software, while the DICTRA module was applied to calculate the diffusion-controlled ferrite formation and pearlite growth. Based on the calculations, the deviation between equilibrium and realistic eutectoid point was well explained, and the detected small amount of proeutectoid ferrite was attributable to the short duration before pearlite transformation. For the calculated pearlite lamellar spacings, good agreement was obtained compared with the experimental determinations. In addition, the present thermodynamic/kinetic method was also valid for a Crmodified steel SWRS87B, indicating the composition, processing parameters and microstructure of pearlitic steels can be reasonably designed with the aid of the present computational techniques.

Effects of Molybdenum Addition on Rolling Contact Fatigue of Locomotive Wheels under Rolling-Sliding Condition

Rolling contact fatigue (RCF) damages often occur, sometimes even leading to shelling on locomotive wheel treads. In this work, the RCF damage behaviors of two locomotive wheel materials with different molybdenum (Mo) contents were studied, and the influence of depth of wheel material was explored as well. The result indicates that with the increase in the Mo content from 0.01 wt.% (wheel 1, i.e., a standard wheel) to 0.04 wt.% (wheel 2, i.e., an improved wheel), the proeutectoid ferrite content and the interlamellar spacing of pearlite decreased, the depth and length of the RCF cracks increased and the average RCF live of locomotive wheel steel improved by 34.06%. With the increase in the depth of material, the proeutectoid ferrite content and the interlamellar spacing of pearlite increased, the depth of RCF cracks increased, the length of RCF cracks of wheel 1 increased and then decreased whereas that of wheel 2 decreased, the RCF live showed a decrease trend for wheel 1, while the RCF life increased and then decreased for wheel 2. The processes of shelling can be divided into three patterns: cracks propagating back to the surface, crack connection and fragments of surface materials.

Microstructure evolution of roll core during the preparation of composite roll by electroslag remelting cladding technology

In the present study, the comprehensive analyses based on the numerical simulation, in situ observation, and metallographic detection were carried out for the roll core of GCr15/45 carbon steel composite roll manufactured by the new electroslag remelting cladding (ESRC) method. During the ESRC process, the temperature distributions at the different radial and longitudinal positions of the roll core have great changes due to the different degrees of heat conduction from the slag bath, as a result, various microstructure and properties were obtained at the different positions of the roll core. The results illustrated that the ESRC process tended to be stable as the composite height reached a certain value and the high-temperature austenitization process mainly occurred in the radial regions where R > 60 mm, whereas no significant changes occurred at the areas where R ≦ 60 mm. The coarse grains and few Widmanstatten structures with proeutectoid ferrite were generated in the roll core areas near the bimetallic interface, while the fine grains with more proeutectoid ferrite were obtained in the roll core area away from the interface. Therefore, a higher tensile strength and a better plasticity were obtained for the specimens away from the interface.

Comparative Study of Droplet Transfer Modes on Appearance, Microstructure, and Mechanical Properties of Weld during Pulsed GMAW

Droplet transfer plays a crucial role in welding stability and quality of pulsed gas metal arc weld (GMAW), referring to the process of heat and mass transition. In this work, the appearance, microstructure, microhardness, tensile properties, and impact toughness with three typical modes of droplet transfer in pulsed GMAW (ODMP: one drop per multiple pulses; ODPP: one drop per pulse; MDPP: multiple drops per pulse) were studied and compared. The results showed that the better welding appearance, the deeper penetration, and the less fume covered on the steel plate could be found during the ODPP welding process. At the same wire feeding speed and arc length, the average current was similar in ODPP and MDPP conditions. However, the average current in the ODMP condition was about 15 A larger than the other two, contributing to the higher heat input. Compared with MDPP and ODMP, the longest elongation and impact energy of the welded joint were found in the ODPP condition. Furthermore, the decrease of elongation and impact toughness in the ODMP condition might result from the higher heat input and the coarsen microstructure, like the proeutectoid ferrite and ferrite side plate.

Study on Surface Longitudinal Crack Formation of Typical Hypoeutectoid Steel Produced on a Caster with Billet and Slab

This paper investigates the formation mechanism of typical hypoeutectoid steel with longitudinal cracks produced on a caster with billet and slab. It was found that the microstructure of the proeutectoid ferrite is the most critical factor affecting the longitudinal crack. The formation conditions of the proeutectoid ferrite and the reasonable control direction were determined through experiments and calculations. High-temperature tensile experiments revealed that enhanced cooling did not cause additional cracks. Therefore, the final plan was to reduce the formation of proeutectoid ferrite by strengthening the cooling process. As a result, the optimized surface temperature quickly passed the phase transition region of the proeutectoid ferrite and no cracks were found in the optimized billet.