Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Aging Behavior and Precipitation Analysis of Cu-Ni-Co-Si Alloy

Cu-Ni-Si alloy with a different Co content was prepared by inductive melting and hot rolling. The alloy was solution treated at 950 °C for 1.5 h and aged at 450 °C, 500 °C, and 550 °C for different times. The phase diagram calculation and transmission electron microscopy was used to investigate the effect of Co addition on the aging precipitation behavior of the Cu-Ni-Si alloy. The phase transformation kinetics equation was calculated as well. The results show that, with the increase of aging temperature, the two-phase region of Fcc + Ni2Si in the Cu-Ni-Si ternary diagram would get wider. Some NixSiy phases would also form in the Cu-rich isothermal section. The addition of Co would replace part of Ni to form the (Ni, Co)2Si phase, which inhibits the spinodal decomposition process of the Cu-Ni-Si alloy during the aging process. The precipitated phase of the Cu-Ni-Si alloy with a high content of the Co element is more likely to grow with the extension of aging time. The phase transformation kinetic equations of the Cu-Ni-Si alloy at 450 °C and 500 °C showed good agreement with the experimental results. Furthermore, it can be seen from the precipitation kinetic curve the addition of the Co element accelerates precipitation in the aging process.

Comparative Characteristics of GaAs and InAs Langmuir Evaporation - Monte Carlo Simulation

The process of GaAs and InAs substrates high-temperature annealing under the Langmuir evaporation conditions is studied by Monte Carlo simulation. The temperature range of gallium arsenide and indium arsenide congruent and incongruent evaporation are determined. It was demonstrated that the congruent evaporation temperature Tc is sensitive to the vicinal surface terrace width. The decrease of the terrace width results in a decrease in the congruent evaporation temperature. The Ga and In diffusion lengths along the (111)A and (111)B surfaces at congruent temperatures are estimated. The surface morphology transformation kinetic during high-temperature annealing is analyzed.

Interface Segregation and Nitrogen Measurement in Fe–Mn–N Steel by Atom Probe Tomography

Improved understanding of the interactions between solutes and the austenite/ferrite interface can benefit modeling of ferrite growth during austenite decomposition, as the transformation kinetic is significantly affected by solutes that influence interface mobility. Solute-interface interactions dominate solute segregation at the interface in binary systems, but in multi-component alloys, solute–solute interactions may also affect segregation. In this study, interface segregation in Fe–Mn–N is examined and compared with Fe–Mn–C, to reveal the extent to which C affects the segregation of Mn. Atom probe tomography (APT) is well-suited to analyze solute concentrations across the interface, as this technique combines high spatial resolution and compositional sensitivity. Measurements of Mn show that segregation is only observed for Fe–Mn–C. This demonstrates that Mn segregation is primarily driven by an affinity for C, which also segregates to the interface. However, the measurement of N in steels by APT may be affected by a variety of experimental factors. Therefore, in verifying the Fe–Mn–N result, systematic examination is conducted on the influence of pulsing method (voltage versus laser), sample preparation (ion milling versus electropolishing), and vacuum storage on the measured N concentration. Both laser pulsing and focused ion beam sample preparation are observed to decrease the apparent N concentration.

Size Effects in Residual Stress Formation during Quenching of Cylinders Made of Hot-Work Tool Steel

The present work investigates the residual stress formation and the evolution of phase fractions during the quenching process of cylindrical specimens of different sizes. The cylinders are made of hot-work tool steel grade X36CrMoV5-1. A phase transformation kinetic model in combination with a thermomechanical model is used to describe the quenching process. Two phase transformations are considered for developing a modelling scheme: the austenite-to-martensite transformation and the austenite-to-bainite transformation. The focus lies on the complex austenite-to-bainite transformation which can be observed at low cooling rates. For an appropriate description of the phase transformation behaviour nucleation and growth of bainite are taken into account. The thermomechanical model contains thermophysical data and flow curves for each phase. Transformation induced plasticity (TRIP) is modelled by considering phase dependent Greenwood-Johnson parameters for martensite and bainite, respectively. The influence of component size on residual stress formation is investigated by the finite element package Abaqus. Finally, for one cylinder size the simulation results are validated by X-ray stress measurements.

Vacancy Diffusion under a Stress and Kinetic of Nanovoid Growth in Cubic Metals

Elastic fields, generating by defects of the structure, influence the diffusion processes. It leads to the alteration of the phase transformation kinetic. One of the chief aims of our work is to obtain general equations for the diffusion fluxes under strain that give the possibility for using these equations at low temperatures, as in this case the strain influence on the diffusion fluxes is manifested in maximal degree. Our approach takes into consideration, that the strains can alter the surrounding atom configuration near the jumping one and consequently the local magnitude of the activation barrier and a rate of atom jump. The rates of atom jumps in different directions define the flux density of the vacancies. Now we take into account, that strain values are different in the saddle point and in the rest atom position, in differ from our consideration that was done by us earlier. As a result in the development of our approach the general equations for the vacancy fluxes are obtained for fcc and bcc metals. In our paper we discuss the main features of the theory of diffusion under stress and its applications. In particular we examine how elastic stress, arising from nanovoids, influence the diffusion vacancy fluxes and the growth rate of voids in metals.

High Silicon Addition in 780 MPa Cold-Rolled Dual Phase Steel for Carbon Reduction and Plastic Reinforce

Two types of DP steels which contain high carbon and high silicon respectively were produced on industrial production line. The microstructures and mechanical properties were investigated. Based on the thermodynamics and kinetic analyses, the intercritical austenization was researched. The results show that the high silicon and low carbon composition used to DP steel can avoid martensite band structure and decrease the martensite fraction, which will improve the elongation and work hardening ability. Phase transformation kinetic analysis indicates that high silicon content can make manganese enrich in austenite and stabilize austenite in intercritical region. Assisted by the strengthening role of silicon in austenite, the mechanical properties of high silicon and low carbon DP can fully meet the standard.