Showing papers by "John G. Speer published in 2015"

Critical Assessment 7: Quenching and partitioning

Abstract: Quenching and partitioning is a relatively new heat treatment concept to generate microstructures containing retained austenite stabilised by carbon partitioning from martensite. Research on quench and partitioning has been conducted by numerous groups, and this critical assessment provides some of the authors’ perspectives on progress and understanding in the field, with particular focus on the physical metallurgy and transformation mechanisms, process variations, mechanical behaviour, and industrial implementation. While much progress has been made, the field provides rich opportunity for further understanding and development.

Retained Austenite Stabilization Through Solute Partitioning During Intercritical Annealing in C-, Mn-, Al-, Si-, and Cr-Alloyed Steels

Abstract: Retained austenite fractions, predicted to be stable at room temperature assuming ortho-equilibrium solute distribution during intercritical annealing, were calculated for “medium-Mn” steels with varying Mn, C, Al, Si, and Cr additions using SSOL 2 and TCFE 7 Thermo-Calc® databases. While Mn additions increase retained austenite levels, increased C levels are not predicted to greatly impact austenite fractions. Additions of Si reduce the levels, whereas opposing trends are predicted for Al additions by the employed Thermo-Calc® databases. Chromium significantly reduces the dependence of retained austenite fraction on annealing temperature. Alloying effects are explained through four critical phase transformation temperatures.

Acicular Ferrite Formation on Ti-Rare Earth Metal-Zr Complex Oxides

Abstract: Acicular ferrite (AF) formation potency of Ti-Rare earth metal (REM)-Zr (TRZ) complex oxide has been investigated in the simulated heat affected zone of low carbon steel. The TRZ complex oxide shows higher AF formation potency than Ti and Al oxides. A TRZ oxide particle is composed of REM-rich and Zrrich phases. AF crystals nucleate on the interface between austenite and the Zr-rich oxide phase, having a crystal structure that is face-centered cubic with lattice parameter of 0.44 nm. The Zr-rich phase and AF have an orientation relationship described by (011)AF//(011)Oxide, [100]AF//[ ]Oxide (the AF-TRZ orientation relationship), which allows good lattice coherency. It is suggested that the formation of this orientation relationship promotes AF nucleation on TRZ complex oxides. The AF also satisfies the Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. It is considered that the coexistence of the AF-TRZ and K-S “three phase” orientation relationships is caused by variant selection of AF in addition to the formation of a rational orientation relationship between the Zr-rich oxide phase and the austenite matrix during the HAZ thermal cycle.

A Composite Modeling Analysis of the Deformation Behavior of Medium Manganese Steels

Abstract: A composite model with different assumed flow behaviors for the individual microstructural constituents and stability parameters for the metastable austenite transformation is presented and shown to provide design insight into the development of third-generation advanced high strength steels with a wide spectrum of tensile properties. The deformation behavior of a Fe-7.09 Mn-0.099 C-0.13 Si (wt%) steel is evaluated with uniaxial tensile testing and the results are correlated with predictions of the composite model. It is shown that the present simple composite model can predict tensile strength and uniform elongation with good accuracy over a range of austenite volume fractions in the steel. The analysis is based on Mn enrichment into austenite during intercritical annealing of the steel resulting in microstructures with significant variations in the amount and stability of austenite. Strategies for controlling the stability and amount of austenite in medium Mn steels are also presented in low carbon, nominally 5–10 wt% Mn-containing steels with/without Al as an alloying addition.

Austenite Grain Growth and Precipitate Evolution in a Carburizing Steel with Combined Niobium and Molybdenum Additions

Abstract: Austenite grain growth and microalloy precipitate size and composition evolution during thermal processing were investigated in a carburizing steel containing various additions of niobium and molybdenum. Molybdenum delayed the onset of abnormal austenite grain growth and reduced the coarsening of niobium-rich precipitates during isothermal soaking at 1323 K, 1373 K, and 1423 K (1050 °C, 1100 °C, and 1150 °C). Possible mechanisms for the retardation of niobium-rich precipitate coarsening in austenite due to molybdenum are considered. The amount of Nb in solution and in precipitates at 1373 K (1100 °C) did not vary over the holding times evaluated. In contrast, the amount of molybdenum in (Nb,Mo)C precipitates decreased with time, due to rejection of Mo into austenite and/or dissolution of fine Mo-rich precipitates. In hot-rolled alloys, soaking in the austenite regime resulted in coarsening of the niobium-rich precipitates at a rate that exceeded that predicted by the Lifshitz-Slyozov-Wagner relation for volume-diffusion-controlled coarsening. This behavior is attributed to an initial bimodal precipitate size distribution in hot-rolled alloys that results in accelerated coarsening rates during soaking. Modification of the initial precipitate size distribution by thermal processing significantly lowered precipitate coarsening rates during soaking and delayed the associated onset of abnormal austenite grain growth.

Analysis of Microstructure in Hot Torsion Simulation

Abstract: Hot torsion is frequently employed to simulate multipass thermomechanical rolling. While flow behavior, observed through shear stress versus shear strain, is typically used to characterize hot deformation and softening behaviors, the resulting microstructures can also provide significant insight into microstructural evolution and strain accumulation during the hot deformation process. A preferred approach for the analysis of microstructural features resulting from hot torsion is presented. Torsional strain paths are reviewed and compared with traditional hot rolling deformations. A tangential sectioning technique, combined with supporting fundamentals, is also presented. Microstructural observation of steels thermomechanically deformed in hot torsion verified the ability to reasonably quantify strain from microstructural analysis. This approach offers a new method for assessing shear strain accumulation within local regions of a body plastically deformed in torsion, and should provide a useful complement to the assessment of mechanical responses in hot deformation studies.

Ab‐Initio Calculation of Solute Effects on Austenite Grain Boundary Properties in Steel

Abstract: Ab-initio density functional theory calculations have been performed to determine the effect of solutes including Cr, Ni, Mo, and Mn on the boundary energy for a variety of coincident site lattice grain boundaries of FCC iron (austenite). The boundaries investigated were of tilt character and both symmetric and asymmetric boundary planes were investigated. Boundary energies were determined for boundaries in pure Fe and for boundaries with single solute atoms at a variety of sites in each boundary. The results are compared to Arrhenius type equations developed from experimental work in the literature and used to hypothesize a mechanistic model for the effects of solutes on boundary mobility based upon the thermodynamic and kinetic effects of solutes at austenite grain boundaries. The predictive capabilities of atomic configurations and bond structures are evaluated and areas for future work are identified. This work provides a new framework for understanding the effects of solutes on atomic scale grain boundary energies and solute drag effects on boundaries.

Mössbauer Spectroscopy and Transmission Electron Microscopy Analysis of Transition Carbides in Quenched and Partitioned Steel

Abstract: Quenching and partitioning (Q&P) is a novel steel heat treatment that produces microstructures of martensite and retained austenite (Fig.1) [1]. Q&P consists of quenching to a temperature (QT) between the martensite start and finish temperatures, partitioning at a temperature the same or greater than the QT, followed by quenching to room temperature (RT). The goal of the heat treatment is to partition carbon (C) from martensite to austenite, thereby stabilizing the austenite prior to the final quench. Competing reactions such as transition carbide formation can reduce the extent of C partitioning, resulting in less retained austenite and mechanical property variations. The small volume fractions, carbide thicknesses below ~50 nm, and numerous overlapping peaks makes X-ray diffraction characterization of transition carbides challenging. In contrast, Mössbauer spectroscopy (MS) with correlative transmission electron microscopy (TEM) is better suited for identifying and quantifying carbides. Most MS studies on transition carbides have focused on quenched and tempered microstructures in binary Fe-C steels with high C, extensive amounts of carbides, and MS spectra primarily comprised of resonance from a limited number of unique Fe sites [2]. Q&P steels with lower C and carbide fractions, alloying additions of Manganese (Mn), Silicon (Si), and other elements, and significant amounts of retained austenite in the microstructures have more complex MS spectra, requiring more precise analysis methods.

Microstructural Developments Leading to New Advanced High Strength Sheet Steels: A Historical Assessment of Critical Metallographic Observations

Abstract: In the past 30+ years significant advancements have been made in the development of higher strength sheet steels with improved combinations of strength and ductility that have enabled important product improvements leading to safer, lighter weight, and more fuel efficient automobiles and in other applications. Properties of the primarily low carbon, low alloy steels are derived through careful control of time-temperature processing histories designed to produce multiphase ferritic based microstructures that include martensite and other constituents including retained austenite. The basis for these developments stems from the early work on dual-phase steels which was the subject of much interest. In response to industry needs, dual-phase steels have evolved as a unique class of advanced high strength sheet steels (AHSS) in which the thermal and mechanical processing histories have been specifically designed to produce constituent combinations for the purpose of simultaneously controlling strength and deformation behavior, i.e. stress-strain curve shapes. Improvements continue as enhanced dual-phase steels have recently been produced with finer microstructures, higher strengths, and better overall formability. Today, dual phase steels are the primary AHSS products used in vehicle manufacture, and several companies have indicated that the steels will remain as important design materials well into the future. Inmore » this presentation, fundamental results from the early work on dual-phase steels will be reviewed and assessed in light of recent steel developments. Specific contributions from industry/university cooperative research leading to product improvements will be highlighted. The historical perspective provided in the evolution of dual-phase steels represents a case-study that provides important framework and lessons to be incorporated in next generation AHSS products.« less