Effects of Ce Addition on Solidification Structure of a Low-Carbon 42CrMo4 Steel

Effects of Cerium (Ce) addition on solidification structure of a low-carbon 42CrMo4 steel was investigated. The addition of up to 0.067 wt.% of Ce in the steel produced greatly improved solidification structure with a suppressed columnar grain zone, finer grain size in an equiaxed grain zone and zero area fraction of casting shrinkage cavity. The added Ce occurred in the steel both in the form of Ce oxy-sulfide inclusions and as dissolved atomic Ce segregated together with other elements at prior austenite grain boundaries and at interdendritic spacing. The Ce oxy-sulfide inclusions were found to play a major role in the observed improved grain structure meanwhile dissolved Ce had pronounced effects on morphology of dendritic networks. The fraction of Ce dissolved in the melt appeared to bring about increase in fluidity of the molten steel, leading to total elimination of interdendritic shrinkage porosity in solidification structure of the steel with added Ce. Ce addition can be considered as a potential solution for grain structure refinement in heavy-weight castings of 42CrMo4 steel grade.

Numerical and experimental simulation of shrinkage porosity closure during hot rolling of bars

Hot rolling of bars issued from continuous-casting aims at refining the material structure and guaranteeing the central soundness of the metallurgical product. The rolling route must be designed to achieve the complete closure of the shrinkage porosity inherent in the continuous casting process. To predict the void evolution, many models exist that can be implemented in the finite element simulation of the process. Nevertheless, these models need parameter adjustments to be adapted to the forming process, the formed material, and the real geometry of the void. Real scale tests being very expensive in the long product rolling mill, an improved representativeness experimental configuration was designed to reproduce at the laboratory scale the key characteristics of the thermomechanical path driving the void closure phenomenon. This testing consists of successive forming stages with shaped anvils applied to samples containing a shrinkage cavity. The shaped anvils and the forming conditions are calibrated to reproduce the levels of strain and the stress triaxiality of rolling stands, and the alternation of the forming direction of the industrial process. The geometry of the voids before and after the forming paths are measured by tomography. The simulation of the test with an explicit modelling of the void is developed parallel to the experiments. The simulation/experiment comparison allows the validation of the numerical model. The obtained model will be used in future works to perform a more extended design of experiments to characterise void closure during hot rolling of bars.

Multiphase Model for the Prediction of Shrinkage Cavity, Inclusion and Macrosegregation in a 36-Ton Steel Ingot

A five-phase model consisting of a liquid phase, columnar dendrites, equiaxed grains, air, and inclusion (discrete phase) is developed to predict the shrinkage cavity, inclusion distribution and macrosegregation simultaneously during solidification of a 36-ton steel ingot. The air phase is introduced to feed the shrinkage cavity and no mass or species exchange with other phases occurs. The transport and entrapment of inclusions are simulated using a Lagrangian approach. The predicted results agree well with the experimental results. The characteristics of inclusion distribution are better understood. A thin layer of inclusions tends to form close to the mold wall, and more inclusions reside in the last solidified segregation channels. The inclusion is easy to aggregate near the riser neck, and it is dragged by the solidification shrinkage. The influence of the inclusion on macrosegregation is comparatively small, while the solidification shrinkage affects the formation of macrosegregation significantly and makes the simulation result more accurate.

Interfacial microstructures and properties of hyper-eutectic Al–21Si / hypo-eutectic Al–7.5Si bimetallic material fabricated by liquid–liquid casting route

The bimetal casting process using the liquid–liquid technique was developed to produce a high-quality hyper-eutectic Al–21Si / hypo-eutectic Al–7.5Si alloy bimetal material. Microstructure and microhardness were investigated as a function of the time interval between pouring hypo-eutectic and hyper-eutectic alloys. A bimetal material was successfully fabricated using a liquid–liquid casting technique with a 10 s time interval in a permanent mould casting. A unique structure comprised of hyper-eutectic Al–21Si, hypo-eutectic Al–7.5Si and a eutectic interface of 70 µm thickness was obtained. This structure totally differs from that obtained using a higher time interval above 10 s that showed an imperfect interface bond due to the shrinkage cavity and formation of oxides. The hardness variation from the upper zone of 117.5 HV to the lower zone of 76 HV corresponded to the variation in Si and the content of other alloying elements. The proposed total solidification time control method is a promising approach for the successful fabrication of liquid–liquid bimetal material.

Effect of Moisture Content of Green Sand on The Casting Defects

In this study, three percentages of moisture content on the green sand were benchmarked to be observed their effects on the casting defects. The metal used is scrap Al-Si with wt% of Al 59.7. The metal was melted in a furnace at 550 ºC then poured into sand molds and cooled for 24 hours before it can be removed, cleaned, and finished. There were prepared three specimens for each benchmark. Qualitative observation of defects was done by physical observations on the surface of the specimens. The observations show that on the moisture content of 2.5%, there is a defect in the form of porosity. Defects in the form of drops, misruns, blowholes, and shrinkage cavity are found on the benchmark 3.5%. The most severe defects are found on the benchmark 4.5% in the form of buckles and severe shrinkage cavities that are found in almost all specimens.

A through process modeling using multi-assistant software applied to radial fatigue life predication of a 356-T6 wheels

In order to solve the problem of isolated design in multi-process using multi-assistant software, a through-software radial fatigue life prediction model was established, the effects of shrinkage cavity, SDAS and mean stress on fatigue life were considered. The casting process of the aluminum alloy wheel was simulated based on ProCast, and the data of SDAS and porosity of different parts were predicted based on the solidification process; The data mapping algorithm between tetrahedral mesh elements was developed to realize the unidirectional transformation of microcosmic data from a cast model to a static mechanical model, the radial loading mechanical analysis model of a wheel containing microcosmic information was established; The fatigue life prediction model was established by Fesafe based on the specific mechanical and fatigue parameters of each node. Based on the self-developed TCD software, the integrated coupling method of the three software prediction models was realized. The application of this method on the virtual fatigue prediction experiment of unidirectional tensile specimen reduce the result dispersion between virtual and physical experiment, and the predicted life result error is reduced from 51% to 16%. The proposed method lays a solid foundation of the optimization design and lightweight design of aluminum alloy wheels.

Causes of the field flowline weld joint rust-through damage

The paper presents results of the flowline pipe analysis in order to determine the causes of the butt weld joint rust-through damage (wormhole). Using Baumann sulfur print technique, the presence of iron sulfides scale on the inner pipe surface was studied. Using X-ray diffraction technique, the phase composition of the corrosion products was determined. It was found that chemical composition and mechanical properties of the metal from the studied flowline fragment comply with standard requirements. The resulting hardness values including those in the weld joint zones indicate that the metal has a certain corrosion-cracking resistance. Results of metallographic studies of longitudinal polished samples with full product thickness show that the residues of the weld capping pass remained on the bottom of the corrosion pit in the area of the observed rust-through hole. They have multiple pores up to 3 mm diameter. The corrosion pit surface is covered with iron sulfide layer 1–3 mm thick. The sulfide layer thickness in the area of the corrosion pit is 10+ times higher than on the rest pipe surface; this indicate that the corrosion process progressed faster here. The authors concluded that the cause for the weld joint rust-through damage was the pit corrosion that occurred under the impact of H2S-containing fluids on the lower generating line of the pipeline in the area of the weld startstop, where the weld root side suckback was observed. Probably, there was a flaw in the first weld pass within the corrosion pit area (shrinkage cavity, incomplete fusion, pore, or other), and the accelerated corrosion was the consequence of H2S-containing liquid slug here. This suggests that there was a flaw in the first weld pass within the area of the corrosion pit that has propagated along the first weld pass start-stop line.