Continuous Cooling Transformation of Under-Cooled Austenite of SXQ500/550DZ35 Hydropower Steel

The expansion curves of the continuous cooling transformation of undercooled austenite of SXQ500/550DZ35 hydropower steel at different heating temperatures and cooling rates were measured by use of a DIL805A dilatometer. Combined with metallography and Vickers hardness measurement, the continuous cooling transformation diagrams (CCT) of the studied steel under two different states were determined. The results show that in the first group of tests, after the hot-rolled specimens were austenitized at 920 °C, when the cooling rate was below 1 °C·s−1, the microstructure was composed of ferrite (F), pearlite (P) and bainite (B). With the cooling rates between 1 °C·s−1 and 5 °C·s−1, the microstructure was mainly bainite, and martensite (M) formed as the cooling rate reached 5 °C·s−1. When the cooling rate was up to 10 °C·s−1, the microstructure was completely martensite and the hardness value increased significantly. In the second group of tests, after the hot-rolled specimens were quenched at 920 °C and then heated at an intercritical temperature of 830 °C, in comparison with the first group of tests, and except for additional undissolved ferrites in each cooling rate range, the other microstructure types were basically the same. Due to the existence of undissolved ferrite, the microstructures of the specimens heated at intercritical temperatures were much finer, and the toughness values at low temperatures were better.

Numerical Simulation of Full Carburizing Process of an Automotive Gear

The objective of the paper is to present material and numerical models needed to simulate with accuracy the full carburizing process of an automotive gear. The rough dimensions of the gear studied are 120mm in diameter and 45mm in height. From a numerical standpoint, as the carburizing affects only a thin layer under the surface, the mesh discretization must be adapted. Consequently, anisotropic mesh is used to describe accurately this zone. The temporal discretization must be also adapted to follow carbon diffusion and thermal evolution. The material models represent metallurgical phenomena during the complete carburizing process. The initial heating of the part induces phases transformation due to austenization. Subsequently, while holding at carburizing temperature, boundary conditions are applied to diffuse carbon into the part. While carbon content increases next to the surface, austenitic metallurgical grain growth is also modelled. A final cooling sets the properties of the carburized part. The model takes into account the phase changes using phase transformation diagrams locally adapted to chemical compositions and grain sizes. Simulation is used to predict the in-use properties of the gear at the end of the carburizing process as well as important results such as assessment of distortion and residual stresses. Thermal stresses, volume variation due to phase changes, and transformation plasticity all contribute to establish the final mechanical properties of the part. During the complete process, the material is modelled with an elasto-viscoplastic behavior and mixing methods are used to consider the relative contribution of each phase.

A novel physics-based and data-supported microstructure model for part-scale simulation of laser powder bed fusion of Ti-6Al-4V

The elasto-plastic material behavior, material strength and failure modes of metals fabricated by additive manufacturing technologies are significantly determined by the underlying process-specific microstructure evolution. In this work a novel physics-based and data-supported phenomenological microstructure model for Ti-6Al-4V is proposed that is suitable for the part-scale simulation of laser powder bed fusion processes. The model predicts spatially homogenized phase fractions of the most relevant microstructural species, namely the stable $$\beta $$ β -phase, the stable $$\alpha _{\text {s}}$$ α s -phase as well as the metastable Martensite $$\alpha _{\text {m}}$$ α m -phase, in a physically consistent manner. In particular, the modeled microstructure evolution, in form of diffusion-based and non-diffusional transformations, is a pure consequence of energy and mobility competitions among the different species, without the need for heuristic transformation criteria as often applied in existing models. The mathematically consistent formulation of the evolution equations in rate form renders the model suitable for the practically relevant scenario of temperature- or time-dependent diffusion coefficients, arbitrary temperature profiles, and multiple coexisting phases. Due to its physically motivated foundation, the proposed model requires only a minimal number of free parameters, which are determined in an inverse identification process considering a broad experimental data basis in form of time-temperature transformation diagrams. Subsequently, the predictive ability of the model is demonstrated by means of continuous cooling transformation diagrams, showing that experimentally observed characteristics such as critical cooling rates emerge naturally from the proposed microstructure model, instead of being enforced as heuristic transformation criteria. Eventually, the proposed model is exploited to predict the microstructure evolution for a realistic selective laser melting application scenario and for the cooling/quenching process of a Ti-6Al-4V cube of practically relevant size. Numerical results confirm experimental observations that Martensite is the dominating microstructure species in regimes of high cooling rates, e.g., due to highly localized heat sources or in near-surface domains, while a proper manipulation of the temperature field, e.g., by preheating the base-plate in selective laser melting, can suppress the formation of this metastable phase.

Phase transformations occurring during the formation of a welded joint from rail steel

The results of dilatometry, metallography and hardness testing of the decomposition process of supercooled austenite of R350LHT steel are presented. During continuous cooling and in isothermal conditions, continuous cooling transformation diagrams of supercooled austenite decomposition of steel R350LHT are constructed.

Toughness properties at multi-layer laser beam welding of high-strength steels

The material characteristics of high toughness and high strength in steel are usually not available at the same time. However, it would be an advantage if high-strength steels would show high impact toughness also at lower temperatures for applications in critical surroundings. In this paper, an approach of multi-layer welding of high-strength steel is presented in order to increase the weld-metal toughness using wire material in combination with thermal cycle modifications. Promising interlocking microstructures were found after multiple tempering of the previously applied structure at homogeneously distributed material in the weld seam. It was found that short thermal cycles during laser processing lead to insufficient time for carbon diffusion, which leads to remaining ferrite structures in contrast to the prediction of welding transformation diagrams. The additionally applied heating cycles during multi-layer laser welding induce the formation of interlocking microstructures that help to increase the weld seam toughness.

Adjustment of Isothermal Transformation Diagrams Using Finite-Element Optimization of the Jominy Test

A practical method for adjusting and optimizing isothermal transformation (IT) diagrams using the Jominy test is presented. The method is based on a finite-element optimization procedure, which iteratively minimizes the error between the target phase fractions and the corresponding finite-element solutions at the sample points using an optimization tool. A standard Jominy test of AISI 52100 bearing steel is used to investigate the feasibility and reliability of the method. Three optimization parameters for each IT diagram curve are mathematically applied to the modified Kirkaldy model. These parameters are the design variables in the optimization. The curves obtained from the modified Kirkaldy model are used as the initial guesses in the optimization and they approach the experimental IT diagram by minimizing the error. Good agreement is observed between the optimized diagram and the experimental diagram reported in the literature. The predicted phase fractions using the experimental IT diagram, the IT diagram obtained from the modified Kirkaldy model, and those obtained from the optimized model are compared and demonstrate that the adjustment or optimization procedure significantly improves the accuracy of the predicted phase fraction of the model. The applicability of the method is investigated in a practical case study.