Georgina Kelly

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

Abstract The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited to 5–10 μm using conventional industrial approaches. Nowadays, though, several thermomechanical processes have been developed to produce ferrite grain sizes of 1–3 μm or less, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. The present paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. This requires deformation within the Ae 3 to Ar 3 temperature range for a given alloy. Here, the formation of ultrafine ferrite (UFF) involves the dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation (DSIT) arises from the introduction of extensive intragranular nucleation sites that are not present in conventional controlled rolling. The DSIT route has the potential to be adjusted to suit current industrial infrastructure. However, there are a number of significant issues that have been raised, both as gaps in our understanding and as obstacles to industrial implementation. One of the critical issues is that it appears that very large strains are required. Combined with this concern is the issue of whether a combination of dynamic and static transformation can be used to achieve an adequate level of refinement. Another issue that has also become apparent is that grain sizes of 1 μm can lead to low levels of ductility and hence many workers are attempting to obtain 2–3 μm grains, or to introduce a second phase to provide the required ductility. There are also a number of areas of disagreement between authors including the role of dynamic recrystallisation of ferrite in the production of UFF by DSIT, the reasons for the low coarsening rate of UFF grains, the role of microalloying elements and the effects of austenite grain size and strain rate. The present review discusses these areas of controversy and highlights cases where experimental results do not agree.

Predicting the critical strain for dynamic recrystallization using the kinetics of static recrystallization

School of Engineering and Technology, Deakin University, Pigdons Rd, Geelong, VIC 3217,Australia(Received November 12, 1999)(Accepted March 9, 2000)Keywords: Steels-austenite; Dynamic recrystallization; Recrystallization1. IntroductionOne view of dynamic recrystallization is that it comprises static recrystallization occurring in the timescale of deformation. While this is clearly not true for geometric dynamic recrystallization [1] orcontinuous dynamic recrystallization [1], there is some metallographic support for it with respect to thebeginning of conventional dynamic recrystallization. The microstructure after a few volume percent ofrecrystallization by this mechanism appears very similar to that seen after a small degree of staticrecrystallization [2,3].A number of workers have constructed mathematical equations for the beginning of conventionaldynamic recrystallization (DRX) using a formulation based on the nucleation mechanisms of staticrecrystallization (SRX) [2,4,5]. These models fit the observed trends in DRX quite well. However, theydo not allow any firm conclusions to be drawn with respect to the relative kinetics of the two processes.In the present work, conventional equations for the kinetics of SRX are modified to allow “SRX” tobegin prior to the end of deformation. In this manner a critical strain for the beginning of “SRX” duringdeformation is derived. This value is then compared with observations of the critical strain required forthe initiation of DRX in steel.2. BackgroundThe static recrystallization kinetics following the hot deformation of steel are often expressed in termsof the time taken after deformation for 50% recrystallization (t

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

Abstract In the present study, wedge-shape sa mples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.

THE DEVELOPMENT OF ULTRAFINE GRAINED STEELS THROUGH THERMOMECHANICAL PROCESSING

Abstract The formation of ultrafine ferrite by strain induced transformation is assessed using rolling and hot torsion experiments. These experiments are used to examine the impact of thermomechanical processing conditions and steel chemistry on strain induced austenite to ferrite transformation and the formation of ultrafine ferrite. The critical strain for dynamic strain induced transformation increased with increasing carbon equivalence, deformation temperature and austenite grain size. The deformation structure in the austenite grains changes with the thermomechanical processing conditions. Drawing on these results and the current literature, the important factors for the production of ultrafine ferrite are described and a mechanism is proposed. On évalue la formation de ferrite ultrafine par transformation induite par déformation en utilisant des expériences de laminage et de torsion à chaud. On utilise ces expériences pour examiner l’impact des conditions de traitement thermomécanique et de la chimie de l’acier sur la transformation de l’austénite en ferrite induite par déformation et sur la formation de ferrite ultrafine. La déformation critique pour la transformation dynamique induite par la déformation augmentait avec une augmentation du carbone équivalent, de la température de déformation et de la taille de grain de l’austénite. La structure de déformation des grains d’austénite change avec les conditions de traitement thermomécanique. À partir de ces résultats et de la littérature courante, on décrit les facteurs importants dans la production de ferrite ultrafine et l’on propose un mécanisme.

Exploring Molecular Changes at the Surface of Polypropylene after Accelerated Thermomolecular Adhesion Treatments

A central composite rotatable design (CCRD) method was used to investigate the performance of the accelerated thermomolecular adhesion process (ATmaP), at different operating conditions. ATmaP is a modified flame-treatment process that features the injection of a coupling agent into the flame to impart a tailored molecular surface chemistry on the work piece. In this study, the surface properties of treated polypropylene were evaluated using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). All samples showed a significant increase in the relative concentration of oxygen (up to 12.2%) and nitrogen (up to 2.4%) at the surface in comparison with the untreated sample (0.7% oxygen and no detectable nitrogen) as measured by XPS. ToF-SIMS and principal components analysis (PCA) showed that ATmaP induced multiple reactions at the polypropylene surface such as chain scission, oxidation, nitration, condensation, and molecular loss, as indicated by changes in the relative intensities of the hydrocarbon (C(3)H(7)(+), C(3)H(5)(+), C(4)H(7)(+), and C(5)H(9)(+)), nitrogen and oxygen-containing secondary ions (C(2)H(3)O(+), C(3)H(8)N(+), C(2)H(5)NO(+), C(3)H(6)NO(+), and C(3)H(7)NO(+)). The increase in relative intensity of the nitrogen oxide ions (C(2)H(5)NO(+) and C(3)H(7)NO(+)) correlates with the process of incorporating oxides of nitrogen into the surface as a result of the injection of the ATmaP coupling agent.

An investigation of tribological properties of CN and TiCN coatings

Today the tool industry on a worldwide basis uses hard, wear-resistant, and low-friction coatings produced by different processes such as electrochemical or electroless methods, spray technologies, thermochemical, chemical-vapor deposition (CVD), and physical vapor deposition (PVD). In the current work, two different coatings, nitrocarburized (CN) and titanium carbonitride (TiCN) on M2-grade tool steel, were prepared by commercial diffusion and PVD techniques, respectively. Properties such as thickness, roughness, and hardness were characterized using a variety of techniques, including glow-discharge optical emission spectrometry (GD-OES) and scanning electron microscopy (SEM). A crossed-cylinders wear-testing machine was used to investigate the performances of both coatings under lubrication. The effect of coatings on the performance of lubricants under a range of wear-test conditions was also examined. Degradation of lubricants during tribological testing was explored by Fourier transform infrared (FTIR) spectroscopy.