to which he has now returned.The hot ductility behaviour of steels in the temperature range 700-11 oooe when tested in tension at low strain rates is examined. Three distinct ranges of behaviour are displayed: (a) a high ductility, low temperature range which results from the presence of a large volume fraction of the more ductile ferrite phase; (b) a high temperature ductile region covering the range within which grain boundary particles are dissolved and boundaries are able to migrate so that any initiated cracks are prevented from linking up and enlarging; and (c) a trough between these two ranges in which low ductility intergranular failures often occur. These intergranular failures arise as a result of stress and strain concentrations at the austenite boundaries caused either by the presence of thin films of the softer deformation induced ferrite enveloping the austenite grains, or by grain boundary sliding in the austenite. Both failure mechanisms are encouraged by the presence of grain boundary precipitates and inclusions (the finer, the more detrimental), coarser grain sizes, and lower strain rates. The factors which control intergranular failure are analysed, leading to estimates of both the width and depth of the trough. The relation between the hot ductility behaviour in tensile testing and the occurrence of transverse cracking in straightening operations is discussed. It is shown that information from hot ductility tests can be used to predict the likelihood of transverse cracking because the variables that influence the depth of the ductility trough are also responsible for transverse cracking. The steps that can be taken to reduce the incidence of transverse cracking are considered in detail: these include adjustments to the chemical composition, grain size reductions, and control of the volume fraction and size distribution of the inclusions and precipitates. In terms of continuous casting process variables, control of the secondary cooling flow is probably most effective in reducing transverse cracking.

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The dynamic transformation behavior of deformed austenite was studied in four steels of increasing carbon contents that had been deformed over the temperature range 743 -917°C. These experiments were carried out in torsion under an atmosphere of argon and 5% H 2 and the experimental temperatures were above the ortho and para-equilibrium Ae 3 temperatures of the steels.Strains of 0.15 -5 were applied at strain rates of 0.4 -4.5 s -1 . The experimental parameters were varied in order to determine the effects of strain and temperature on the formation of strain-induced ferrite and cementite. The structures observed are Widmanstätten in form and appear to have nucleated displacively.The onsets of dynamic transformation and dynamic recrystallization were detected in the four steels using the double-differentiation method. Two sets of second derivative minima were found to be associated with all the flow curves. It is shown that double minima can only be obtained when the polynomial order is at least 7. The first set of minima corresponds to the initiation of dynamic transformation (DT). The second set is associated with the nucleation of dynamic recrystallization (DRX). The critical strain for DT is always lower than for DRX in this range and increases slightly with temperature. Conversely, the critical strain for DRX decreases with temperature in the usual way.The mean flow stresses (MFS's) pertaining to each experimental condition were calculated from the flow curves by integration. These are plotted against the inverse absolute temperature in the form of Boratto diagrams. The stress drop temperatures, normally defined as the upper critical temperature A r3 *, were determined from these diagrams. These are shown to be about 40°C above the paraequilibrium and about 20 -30°C above the orthoequilibrium upper critical ii transformation temperatures. This type of behavior is ascribed to the occurrence of the dynamic transformation of austenite to ferrite during deformation.The effect of deformation on the Gibbs energy of austenite in these steels was estimated by assuming that the austenite continues to work harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve by making suitable assumptions about two-phase flow. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions (driving forces) are derived that are sufficient to promote transformation as much as 100°C above the Ae 3 . The carbon diffusion times required for formation of the observed ferrite plates and cementite particles are calculated; these are consistent with the occurrence of interstitial diffusion during deformation. Similar calculations indicate that substitutional diffusion does not play a role during dynamic transformation. The Gibbs energy calculations suggest that growth of the Widmanstätten ferrite is followed by C diffusion at the lower carbon contents, while it is accompanied by C diffusion at the higher car...

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Cold spray is a complex process where many parameters have to be considered in order to achieve optimized material deposition and properties. In the cold spray process, deposition velocity influences the degree of material deformation and material adhesion. While most materials can be easily deposited at relatively low deposition velocity (<700 m/s), this is not the case for high yield strength materials like Ti and its alloys. In the present study, we evaluate the effects of deposition velocity, powder size, particle position in the gas jet, gas temperature, and substrate temperature on the adhesion strength of cold spayed Ti and Ti6Al4V splats. A micromechanical test technique was used to shear individual splats of Ti or Ti6Al4V and measure their adhesion strength. The splats were deposited onto Ti or Ti6Al4V substrates over a range of deposition conditions with either nitrogen or helium as the propelling gas. The splat adhesion testing coupled with microstructural characterization was used to define the strength, the type and the continuity of the bonded interface between splat and substrate material. The results demonstrated that optimization of spray conditions makes it possible to obtain splats with continuous bonding along the splat/substrate interface and measured adhesion strengths approaching the shear strength of bulk material. The parameters shown to improve the splat adhesion included the increase of the splat deposition velocity well above the critical deposition velocity of the tested material, increase in the temperature of both powder and the substrate material, decrease in the powder size, and optimization of the flow dynamics for the cold spray gun nozzle. Through comparisons to the literature, the adhesion strength of Ti splats measured with the splat adhesion technique correlated well with the cohesion strength of Ti coatings deposited under similar conditions and measured with tubular coating tensile (TCT) test.