Key Engineering Materials Vol. 716

Abstract: Constitutive equations have been used extensively to accurately describe material properties over a wide range of temperatures and strain rates in numerical simulations. In this paper, an algorithmic method of determining the constants of such constitutive equations is presented. The Genetic Algorithm implementation utilising MATLAB is described, and example fits to experimental data are presented.

Abstract: The deep drawability and bendability in hot stamping of ultra-high strength steel parts were examined. Although the cold drawability is greatly influenced by the blank shape, the limiting drawing depths for the square and circular blanks were equal for hot stamping because of small flow stress. In hot hat-shaped bending using draw-and form-type tools, the effect of the blankholder force generated with the draw-type tools on the springback was small, and the seizure for the form-type tools was smaller than that of the draw-type tools. Since both edges in contact with the electrodes are not heated for resistance heating, cracks were caused at the edges for resistance heating in the transversal directions in hot stamping of an S-rail with form-type tools, and thus it is required to control deformation of the non-heating zones.

Abstract: Large-scale forging segments with two different section thicknesses (100 and 250 mm) were separately subjected to water, polymer solution and vegetable oil quenching and then tempered to evaluate the influence of cooling rate on microstructure and mechanical properties under industrial conditions. Regardless of quenching media, the fastest cooling rates are obtained in the thin (100 mm) sections of the ring segments. For the two cross sections, water and vegetable oil generated the faster and slower cooling times, whilst an intermediate cooling timebetween that of water and oil was achieved with polymer solution. Slightly enhanced mechanical properties in the thin sections are associated to the presence of fine mixtures of tempered martensite and tempered bainite (lower type morphology) compared to the thick section microstructure mostly composed of coarse tempered bainite with granular and lower type morphologies and small amounts of tempered martensite. The results obtained in this study also suggest the possibility of using vegetable oil as an alternative quenching media for large-scale forgings with high-specification requirements.

Abstract: The microstructural refinement induced when the holding time between last deformation pass and accelerated cooling is reduced, affects the mechanical properties in low carbon Nb and Nb-Mo microalloyed steels. Plane strain compression tests were performed and mechanical property samples machined in order to quantify this effect using tensile and Charpy impact tests. A complete microstructural characterization was carried out using electron backscattered diffraction (EBSD) measuring unit size distributions and homogeneity of complex microstructures. The synergetic combination of Nb and Mo elements modifies the final microstructures and, therefore, affects the contribution of different strengthening mechanisms, such as substructure, precipitation hardening and dislocation density. Even though strength is not clearly affected by the reduction of the holding time after the last deformation pass, Charpy properties are considerably improved in the case of the Nb steel. The presence of MA islands in the Nb-Mo steel limits the beneficial effect of the microstructural refinement and toughness remains unmodified.

Abstract: Casting of steel with different aluminum content was carried out. Thermo-mechanical process was carried out at 1200°C. A cross-sectional area reduction of 95% was done using hot forging hammer. Two regimes of thermomechanical processes were adopted in this study. First regime was intercritical annealing (between AC₁ and AC₃ to obtain 50% austenite and 50% ferrite) followed by rapid quenching in salt bath to the martensite zone for 15minutes to produce ferrite, martensite and carbon saturated austenite and to prevent the whole transformation of austenite to martensite. The second regime was reheating 50oC above AC₃, followed by rapid quenching in salt bath to the martensite zone (for 15 minutes) to get partially partitioned martensite and super saturated retained austenite. The initial tensile strength after hot forging is 1067MPa and 942MPa for alloys 1 and 2 respectively. The tensile strength after intercritical annealing is 1621MPa while after quenching from austenite to martensite zone with austenite partitioning is 2113MPa. Elongation after intercritical annealing and austenite partitioning is null due to bad shapes and distribution of ferrite with sharp corners.

Abstract: Ladle treatment of molten Hadfield steel considers as the new effective technique for producing Hadfield steel instead of conventional heat treatment process. New eutectic is formed as a result of the ladle treatment process. In this research, the strain-hardening property of the four grades of Hadfield steel containing granular new eutectic phase has been investigated, and compared with the conventional Hadfield steel. Optical and scanning electron microscope reveal that slip bands fractions vary through the five grades of steel as a result of deformation. XRD observation refers to the austenite phase is still the dominant phase through the whole five grades of steel after deformation. The results of the compression and hardness tests clarify that the new granular eutectic phase has a significant effect on the strain-hardening behavior, austenite stability. The nodularity of new eutectic phase is a considerable parameter in the function of yield strength, and hardness increment as well.

Abstract: A detailed analysis of the effect of variable stresses on the intensity of retained austenite transformation into martensite was carried out in the work. Tests were done for three bending stress (σ_max) levels, lower than the value of the yield stress (R_0.2) the material tested. Preliminary tests conducted for one bending stress value have shown that a certain amount of untransformed retained austenite remains in the material in spite of applying as many as 40 000 fatigue cycles. It is therefore necessary to determine whether and, if so, in what extent the magnitude of applied bending stress will increase or decrease that amount of retained austenite. It should be established whether, for different bending stress levels, the curves describing the variation in the volumetric fraction of retained austenite of the tested wire structure, will approximate asymptotically the same value, or the values will, after all, be different.The results of these studies have a practical dimension, since, e.g. a screw made of the TRIP steel, subjected to variable stresses in service, may undergo additional hardening to a varying extent, which will influence its mechanical properties, life and operation safety.The knowledge acquired from the studies will constitute a novelty in this scope of applications, and will be useful from the point of view of both the manufacturer and the user.

Abstract: Dynamic recrystallization (DRX) in 316LN austenitic stainless steel with 0.14wt% nitrogen has been studied using hot isothermal compression tests carried out in temperature range 1073-1423K and strain rate range 0.001 - 10 s^-1. Critical strain and stress for DRX has been characterized using experimental data. Analysis of results shows that for the entire domain the critical stress is directly proportional to peak stress. However, no clear relationship between ε_c and ε_p prevails over the entire tested domain. Dynamic Recrystallized (DRX) grains are quantified by GOS and KAM maps. The four stages of DRX progression have been identified using the correlation between GOS and critical strain at different deformation conditions.

Abstract: The tensile and compressive creep behaviour of aluminium alloy 2050 with T34 initial temper (AA2050-T34) during creep-ageing process has been experimentally investigated and analysed in detail. Both tensile and compressive creep-ageing tests under various stress levels (ranging from 100 MPa to 187.5 MPa) have been carried out at a temperature of 155 °C for 18 hours. The results show that creep strains under tensile stresses are much larger than those under the same levels of compressive stresses and a new “double primary creep feature” with five-stage creep behaviour has been observed in the alloy during the creep-ageing tests. The conventional power-law creep equation was applied to analyse the new creep behaviour of the alloy at the steady-state creep stage. Furthermore, the power-law relationship between the applied stress and the corresponding creep strain rate was found to be effective in all creep-ageing stages of the alloy and was used for further analysis. These analyses indicate that the dislocation and diffusion mechanisms may both contribute to this new creep behaviour and may play different roles in different creep-ageing stages. Moreover, the evolution of the creep resistance or threshold creep stress of the alloy during the creep-ageing process was quantitatively investigated by the proposed relationship. These results help to not only understand the new creep behaviour of AA2050-T34 during the creep-ageing process, but also facilitate further constitutive modelling of this new creep behaviour for its creep age forming applications.

Abstract: The reduction of CO₂-emissions and the lowering of fuel consumption are two main objectives in the automotive industry. To reach these targets, conventional materials like deep drawing steels are substituted by new modern lightweight materials such as aluminum and magnesium alloys. During forming of sheet metal parts, the material experiences a plastic deformation, which can affect the part quality regarding the amount of springback or the occurrence of stretcher strain marks. In this context, a time dependent change of the material behavior can emerge after removing the part from the forming tool. Within this contribution, the influence of pre-strain and the unloading yield effect on the subsequent mechanical behavior of the aluminum alloy AA7020 and the magnesium alloy AZ31B are investigated. Additionally, the time dependency of mechanical properties is analyzed for different aging times from 5 seconds to one week after pre-straining. The results show a significant increase of unloading yield effect with increasing pre-strain during uniaxial tension.