Temperature field calculation of rail flash welding

The forging stage of rail flash welding has a decisive influence on joint strength,and the study of the temperature distribution in the process has an important role in further improving joint strength.In this paper,three calculation methods for the temperature field are given.First,the finite element model of the temperature field before forging rail flash welding is established by using the transient heat module of Ansys software and verified by infrared temperature measurement.Second,the temperature distribution of different parts of the rail before flash welding is obtained by using infrared thermal imaging equipment.Third,Matlab software is used to calculate the temperature of the non-measured part.Finally,the temperature distribution function along the rail axis is fitted through the temperature measurement data.The temperature distribution before the top forging of the rail flash welding can be used to analyze the joint and heat-affected zone organization and properties effectively and to guide the parameter setting and industrial production.

Homogenous strength-toughness match for 20Mn2SiCrMoV bainitic large-size parts via processing of VC particles and quenching control

The problem of insufficient hardenability in general large-size parts always occurs in product manufacturing because of their large size,etc.It is restricted mainly by its own alloy composition with micro-alloy and key quenching and partitioning(Q&P) process.The relationship between the cooling rate and properties of small samples was analyzed as the basis of the initial cooling rate at different positions corresponding to large-size parts combined with controlled austenization temperature in this work.Typical as-treated micro structure after a fast cooling rate is mainly composed of lath bainite,martensite,and retained austenite(RA),while bainite and RA after a slow cooling rate.Simulations showed that cooling control via decreasing spray intensity,meeting higher strength at the surface,and good strength and toughness match can be obtained both at the surface and in the center.As-treated large ring part has~1330 MPa tensile strength and~95 J impact energy at the surface,and meantime,~1191 MPa tensile strength and~70 J impact energy in the center,which narrows the property difference.

Microstructural development and wear properties analyses of Fe-based coatings on bainitic steel by laser cladding

Wear-resistant Fe-based coating was prepared by high-speed laser cladding on bainitic steel.The influence of laser scanning speed on microstructure,microhardness,and wear resistance of cladding coating was investigated.Scanning electron microscopy results showed that the coating was mainly composed of dendrite and interdendrite.Scanning electron microscope images were converted by binary algorithm to facilitate statistics of dendrite and interdendrite area.Statistical results indicate that by accelerating the scanning speed,the interdendrite eutectic structure increased.According to energydispersive X-ray spectroscopy and X-ray diffraction results,the interdendrite was enriched with elements Cr,Mo,and B,and main structures in the coating wereα-Fe,γ-Fe,and M(23)C_(6).The hardness of the coating was much higher than that of the substrate.Elements diffused from coating to substrate,resulting in a transition zone of hardness.Moreover,with an increase in the scanning speed,the diffusion of elements at the coating–matrix interface decreased,while the hardness and wear resistance of the coating increased.Grain refinement and interdendrite(γ-Fe,M_(23)C_(6))increasing due to high scanning speed were the major contributors to the increase in hardness and wear resistance.

Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite,retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue(HCF/VHCF)regimes.The subsurface microstructural fatigue crack initiation,referred to as“non-inclusion induced crack initiation,NIICI”,is a leading mode of failure of bainitic steels within the HCF/VHCF regimes.In this regard,there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF.To address this aspect,we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure.This was accomplished by uniquely combining electron microscopy,three-dimensional confocal microscopy,focused ion beam,and transmission Kikuchi diffraction.Interestingly,the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels,including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI,which resulted in the different distribution of fine grains in the crack initiation area.The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries.The formation of fine grains assisted the advancement of small cracks.Another important aspect discussed is the role of retained austenite(RA)during cyclic loading,on crack initiation and propagation in terms of the morphology,distribution and stability of RA,which determined the development of localized cyclic plastic deformation in multiphase structure.