A.M. Elwazri

Modelling of dynamic recrystallisation kinetics in austenitic stainless and hypereutectoid steels

Abstract Two important parameters for dynamic recrystallisation can be derived from changes in the strain hardening rate: the critical strain for initiation of dynamic recrystallisation and the point of maximum softening. In the present work, these values are determined from stress–strain data obtained by compression testing over the range of 900–1100°C. The resulting strains are used to derive a kinetic model of dynamic recrystallisation for two materials: 304 austenitic stainless steel and a hypereutectoid plain carbon steel. The values of the mechanical parameters used to define the proposed model are confirmed with the aid of metallographic analysis of the recrystallised microstructures.

EFFECT OF SPECIMEN THICKNESS AND PUNCH DIAMETER IN SHEAR PUNCH TESTING

Abstract A good method to directly measure the mechanical properties of a material deformed in compression or torsion is by using the shear punch test. Unlike tensile testing, the shear punch test is not limited by specimen size or shape. However, it is unclear whether there is a limit to the allowable specimen thickness and punch diameter in order to attain (through conversion) tensile properties with reliability and accuracy. According to this study, it was found that the shear punch test was not sensitive to varying specimen thicknesses or punch diameters over a range of 200 to 400 μm and 1.5 to 5 mm, respectively. L’essai de cisaillement par poinçonnage constitue une bonne méthode de mesure directe des propriétés mécaniques d’un matériau déformé en compression ou en torsion. Contrairement à l’essai de traction, l’essai de cisaillement par poinçonnage n’est pas limité par la taille ou la géométrie de l’échantillon. Cependant, la gamme d’épaisseurs de l’échantillon ainsi que de diamètres du poinçon qui permettent d’obtenir (après conversion) des propriétés de traction fiables n’est pas clairement définie. Dans cette étude, nous montrons que l’essai de cisaillement par poinçonnage est indépendant de l’épaisseur de l’échantillon pour des épaisseurs allant de 200 à 400 μm ainsi que du diamètre du poinçon s’il est entre 1.5 et 5 mm.

Critical condition for dynamic recrystallisation of high carbon steels

Abstract A softening mechanism for dynamic recrystallisation has been determined through analysis of the continuous compression flow curves of both hypoeutectoid and hypereutectoid steels, 0·7 and 0·9% carbon respectively, in the range 900 – 1050°C for strain rates 0·01 to 1 s−1. The critical stress and strain for the initiation of dynamic recrystallisation were determined from the inflection points in plots of strain hardening rate against flow stress.

EFFECT OF COOL DEFORMATION IN LOW CARBON MICROALLOYED STEELS

Abstract Cool deformation refers to deformation at very low temperatures (close to the ‘coiling’ temperature) which has been found to increase the strength of Nb microalloyed steels. This paper discusses the microstructure changes and possible strengthening mechanisms associated with this technique. Low carbon microalloyed steel specimens were heated to temperatures of 1200 °C and held at temperature for 20 minutes. After this reheat, some specimens were aged at 400 °C for times ranging from 10 minutes to 10 hours followed by air cooling. Others were subjected to deformation at 400 °C (‘cool deformation’) prior to ageing. The mechanical properties of the heat treatment and cool deformed specimens were evaluated by shear punch tests at room temperature and precipitates were characterized by electron microscopy. By comparing the results of each series of tests, the influence of cool deformation is assessed. Le terme déformation fraiche (cool deformation) fait référence à une déformation performée à basses températures, soit près de la température de bobinage. Certains tests ont démontré que l’utilisation de cette technique augmente la force des aciers micro-alliés au Nb. Cet article étudie les changements de microstructure associés à déformation fraiche et ses mécanismes de renforcement possibles. Des échantillons d’acier bas carbone micro-alliés ont été chauffés et maintenus à 1200 °C pour 20 minutes. Après cette chauffe, certains échantillons ont été vieillis à 400 °C pour un laps de temps variant entre 10 minutes et 10 heures puis refroidis à l’air. D’autres échantillons ont été déformés à 400 °C (déformation fraiche) avant d’être vieillis. Des essais de cisaillement par poinçonnage (” shear punch test ») ont été utilisés pour évaluer les propriétés mécaniques des échantillons à température de la pièce. Les précipités formés lors du traitement thermique ont été observés par microscopie électronique. En comparant les résultats de chaque série de test, l’influence de déformation fraiche est évaluée

Characterisation of precipitation of niobium carbide using carbon extraction replicas and thin foils by FESEM

Abstract Field emission scanning electron microscopy (FESEM) was used to analyse small particles (<50 nm) in ferrite. Analysis of bulk specimens must be carried out at low voltages to preserve volume resolution, resulting in very weak signals. However, thin foils and carbon extraction replicas could be analysed with FESEM using relatively high voltages, because the interaction volume effect is no longer a problem.

Empirical modelling of the isothermal transformation of pearlite in hypereutectoid steel

Abstract The rate controlling mechanism for pearlite growth in a hypereutectoid steel composition was examined through an analysis of the microstructural characteristics of the pearlite structure, namely interlamellar spacing, for different isothermal transformation and austenitising conditions. From a metallographic analysis of the pearlite structure as a function of the austenitisation and undercooling conditions applied to the hypereutectoid steel, the interlamellar spacing was observed to increase with increasing austenitising temperature and increasing isothermal transformation temperature. Through the application and experimental validation of a theoretical model (Zener and Hillert) for the calculation of the pearlite interlamellar spacing as a function of the undercooling, the growth rate of pearlite in the hypereutectoid steel was determined to be controlled by the volume diffusion of carbon in austenite during isothermal transformation in the temperature range of 550–620°C.

Effect of carbon content on dynamic recrystallization behaviour of plain carbon steels

Abstract The occurrence of dynamic recrystallization in hypoeutectoid and hypereutectoid steels of 0.7 and 0.9% carbon was investigated under isothermal conditions. Compression tests were performed using single deformation schedules at temperatures between 900 and 1050 °C and strain rates of 0.01s-1, 0.1s-1 and 1s-1. It was observed that the activation energy for deformation decreased with increasing carbon content in the steels. The peak and steady state strain values determined from the flow curves generated by hot compression deformation were used to examine the possible evolution of grain size by recrystallization. La recristallisation dynamique d’aciers hypoeutectoïdes et hypereutectoïdes à 0.7 et 0.9 % de carbone est étudiée. Des tests de compression sont effectués dans des conditions isothermes à des températures comprises entre 900 et 1050 oC et à des vitesses de déformation de 0.01 s-1, 0.1 s-1 et 1s-1. Il est observé que l’énergie d’activation de déformation diminue lorsque le taux que carbone dans l’acier augmente. Les valeurs de la déformation à la contrainte maximale et au début de l’état stationnaire, déterminées à partir des courbes d’écoulement générées par la déformation à chaud, sont utilisées pour étudier l’évolution possible de la taille des grains par recristallisation.

Influence of Processing Conditions on Obtaining an Ultrafine Grain Structure

Abstract For low carbon steels, thermomechanical processing in the intercritical (austenite–ferrite) temperature region has potential for generating an ultrafine grained structure. By using compressive deformation, the influence of temperature, strain rate and strain on the transformation kinetics was considered for developing a room temperature microstructure of fine ferrite grains. The flow behaviour was also examined to determine the critical conditions for dynamic transformation and/or strain induced transformation using the stress–strain and strain hardening rate curves. Characterization of the microstructure after deformation in the intercritical region confirmed the occurrence of grain refinement with the presence of homogeneous and ultrafine ferrite grains.

Effect of Redundant Strain on No Recrystallization Temperature in Nb Microalloyed Steel

Abstract This work concerns the effect of applied redundant retained strain on the flow behaviour and no recrystallization temperature (Tnr) in Nb microalloyed steel. Unlike conventional torsion testing where multiple unidirectional twisting is applied, in this work, cylindrical specimens were deformed by multiple cycles of alternating uniaxial tension/compression during cooling from a high temperature. Various cyclic-strains of 0.05, 0.1 and 0.2 were applied at a constant strain rate of 0.1 s−1 during cooling from 1200 to 600 °C at a constant cooling rate of 1 °C.s−1. The flow properties and softening capacity were monitored during deformation in order to specify the temperature of no recrystallization. The technique and results will be discussed in this paper.

Comparison of Final Properties between Conventional Trip and Tempered Martensite Assisted Steels (TMAS)

Abstract Recent research in the field of advanced steels has shed some light on new grades of advanced automotive steel such as Tempered Martensite Assisted Steels (TMAS). Processing and analysis of the final properties of TMAS is the scope of this current work. This paper focuses on the influence of tempered martensite volume fraction on the final properties of cold rolled and subcritically annealed Al-containing TRansformation Induced Plasticity (TRIP) steel, otherwise called TMAS. In this work, an X-ray diffraction technique was used to measure retained austenite volume fractions of all samples after TRIP annealing. The steel samples were subsequently cold rolled to obtain strain-induced martensite, deformed ferrite and bainite in the microstructure. The cold rolled TRIP steel samples were subsequently subcritically annealed at 500 °C for 1 hour to obtain TMAS containing tempered martensite, fine ferrite and bainite in the microstructure. Shear Punch testing was used to evaluate the mechanical properties. The properties of the conventional TRIP steel and TMAS are compared and the results are discussed.