Papers by Author: David K. Matlock

Abstract: A 0.12 wt.% C – 1.26 wt.% Mn steel was studied to evaluate phase transformations that occurred during a specific thermal processing method designed to simulate steel plate surface layer microstructural evolution during processing with intermediate cooling. All process simulations used a Gleeble thermomechanical simulator along with thermal practices developed previously. After intermediate cooling was completed during processing, slight reheating of the plate surface layer region would occur due to heat retained in the plate core. Microstructural evaluation of Gleeble samples quenched at several points along the thermal profile allowed interpretation of microstructural evolution during processing. The microstructure that was present at the point where deformation would be applied consisted of approximately 75% ferrite, 25% austenite and some small, undissolved cementite particles.

Abstract: The novel heat treatment concept of Quenching and Partitioning (Q&P) offers exciting prospects for the production of higher strength steel products with enhanced formability from a microstructure containing retained austenite and martensite. The Q&P process hinges on an interrupted quench and partitioning step at intermediate temperatures whereby the untransformed austenite can be thermodynamically stabilised by enrichment of carbon from the supersaturated martensite. Although the concept is similar to that producing carbide-free bainite in TRIP-assisted steel, Q&P offers the advantage of separating the ferrite formation and austenite enrichment stages of the process. While the concept is readily understood, the details of microstructural evolution during interrupted quenching and partitioning steps are difficult to study and are generally inferred from dilatometry or metallographic examination after a final quench back to room temperature. Consequently, in this study, alloying has been used to develop a model alloy in which the sequential steps of heat treatment can be separated for closer, more direct inspection by neutron diffraction techniques.

Abstract: With the development of new steels and processing techniques, there have been corresponding advances in the fatigue performance of steels. Methods to increase fatigue performance are typically designed to produce gradients in surface properties and are based on heat treating operations, including enhanced carburizing and induction hardening, as well as surface mechanical deformation. In this paper selected examples based on recent work on deep rolling is used to illustrate the importance of the base steel properties on the final performance of surface modified materials. The degree of fatigue improvement by deep rolling, a process to mechanically deform fillet surfaces to improve fatigue resistance in cylindrical components, depends on the deformation response of the substrate to the rolling process. Recent results on the behavior of three medium carbon steel alloys deformed at temperatures up to 360 °C, are discussed. Deep rolling increased fatigue resistance, and the degree of improvement was higher when deep rolling was applied in the dynamic strain aging (DSA) temperature range rather than at room temperature. Observed variations in fatigue performance are interpreted based on fundamental deformation mechanisms and are used to present an overall perspective on approaches to increase the fatigue resistance of conventional and newly developed steels.

Abstract: High strength steels containing significant fractions of retained austenite have been developed in recent years, and are the subject of growing commercial interest when associated with the TRIP phenomenon during deformation. A new process concept “quenching and partitioning” (Q&P) has been proposed by CSM/USA, and the results show the potential to create a new kind of steel microstructure with controlled amounts of retained austenite, enriched by carbon partitioning. Four steels containing C, Si, Mn, Ni, Cr and Mo, were designed with variation in the Ni and C content, aiming to decrease Bs temperature and to suppress carbide formation during the partitioning treatment. Several heat-treatment procedures were performed in specimens previously machined for tensile testing, while x-ray diffraction was used to determine the fraction of retained austenite. The tensile test results showed that except for the high C high Ni alloy, most of the processing conditions resulted in strengths superior to those of advanced high strength steels (AHSS), although it is importantly recognized that higher alloy additions were used in this study, in comparison with conventional AHSS grades.. A variety of strength and ductility combinations were observed, confirming the potential of the Q&P process and illustrating the strong influence of the final microstructure on the mechanical properties. Experimental results for samples partitioned at 400 °C indicate that higher ultimate tensile strength is associated with higher fraction of retained austenite for multiple heat treatments of each alloy investigated. The amount of retained austenite obtained was generally lower than that predicted by the model. Further studies are in progress to understand the influence of alloying and processing parameters (time/temperature) on the partitioning of carbon and precipitation of transition carbides.

Abstract: The microstructure following a new martensite heat treatment has been examined, principally by high-resolution microanalytical transmission electron microscopy and by atom probe tomography. The new process involves quenching to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by ageing either at or above, the initial quench temperature, whereupon carbon can partition from the supersaturated martensite phase to the untransformed austenite phase. Thus the treatment has been termed ‘Quenching and Partitioning’ (Q&P). The carbon must be protected from competing reactions, primarily carbide precipitation, during the first quench and partitioning steps, thus enabling the untransformed austenite to be enriched in carbon and largely stabilised against further decomposition to martensite upon final quenching to room temperature. This microstructural objective is almost directly opposed to conventional quenching and tempering of martensite, which seeks to eliminate retained austenite and where carbon supersaturation is relieved by carbide precipitation. This study focuses upon a steel composition representative of a TRIP-assisted sheet steel. The Q&P microstructure is characterised, paying particular attention to the prospect for controlling or suppressing carbide precipitation by alloying, through examination of the carbide precipitation that occurs.

Abstract: Ti-V and Ti-Nb bake hardenable Ultra Low Carbon (ULC) steels are used to produce hot dip zinc coated steels for automotive applications. An important factor influencing the bake hardenability in such microalloyed ULC steels is the level of solute carbon available to diffuse for pinning dislocations during baking. The level of solute carbon must be controlled carefully during annealing of the steel in the ferritic region. Therefore, this paper summarizes highlights of research conducted to study the effects of chemical composition and annealing temperature (in the ferrite region) on the precipitation (or dissolution) of NbC and VC using a variety of Ti-Nb and Ti-V ULC steels. Carbon diffusivity is another factor that could also influence the bake hardenability through controlling the aging and baking kinetics. Therefore, the paper presents highlights of internal friction measurements performed to assess effects of microalloying elements (Nb,V) and some commonly used solid solution strengthening elements (Mn, P) on carbon diffusivity measured using the internal friction technique.

Abstract: Developments related to the use of microalloy additions, primarily of Ti, Nb, and V, and controlled processing are reviewed to illustrate how steels with tailored microstructures and properties are produced from either bar or sheet steels for new automotive components. Microalloying additions are shown to control the necessary strengthening mechanisms to produce high strength materials with the desired toughness or formability for a specific application. Selected examples of direct cooled forging steels, microalloyed carburizing steels, and advanced high strength sheet (AHSS) steels are discussed.