Materials Science Forum Vol. 941

Abstract: To predict austenite grain growth behavior in the heat-affected zone (HAZ) in low alloy steels, a new calculation model is proposed herein. This model mainly considers the solute-drag effect and pinning effect, which restrain the austenite grain growth. To calculate the solute-drag effect, the grain boundary concentration of each element is obtained by Hillert’s Law. Calculations are performed by simulating the HAZ with a temperature gradient using the phase field method for two dimensions. This calculation demonstrates the possibility of quantitatively predicting the pinning force for welding heat inputs.

Abstract: Modern steel making and hot rolling processes like CSP^® thin slab technology require precise data on casting and rolling behavior of the produced steel grades. Up today only few data is available for the latest generations of advanced high strength steel (AHSS) grades. AHSS have developed by 3 generations [1, 2]. 1^st Generation AHSS as dual phase (DP), complex phase (CP), martensitic (MS) and transformation induced plasticity (TRIP) steel grades are currently applied in automotive industry. 2^nd and 3^rd Generation AHSS typically have elevated Mn-content as well as Al and Si content. High Mn-content of up to 30% seriously affects casting and forming properties of 2^nd Generation AHSS. In particular, the large solidification range of more than 100 K prevents commercial production of these steel grades by continuous casting [3]. 3^rd Generation AHSS with reduced Mn-content up to about 12% are currently under development [1-4]. Investigations have been carried out to assess the CSP^® thin slab process for the production of such grades. To this purpose solidification and hot forming properties of different alloys having Mn-content up to 10% have been examined by thermodynamic calculations and laboratory testing by hot forming dilatometry. The achieved flow curves match figures achieved on a hot rolling mill.

Abstract: Under loading above the Ae₃ temperature, austenite transforms displacively into Widmanstätten ferrite. Here the driving force for transformation is the net softening during the phase change while the obstacle consists of the free energy difference between austenite and ferrite as well as the work of shear accommodation and dilatation during the transformation. Once the driving force is higher than the obstacle, phase transformation occurs. This phenomenon was explored here by means of the optical and electron microscopy of a C-Mn steel deformed above their transformation temperatures. Strain-temperature-transformation (STT) curves are presented that accurately quantify the amount of dynamically formed ferrite; the kinetics of retransformation are also specified in the form of appropriate TTRT diagrams. This technique can be used to improve the models for transformation on accelerated cooling in strip and plate rolling.

Abstract: The effect of the characteristics of austenite interface with ferrite on the pearlite transformation behaviour after intercritical annealing was investigated. Most austenite grains were situated mainly on ferrite grain boundaries and had the Kurdjumv-Sachs (K-S) or near K-S relationship to one of the neighbor ferrite grains before pearlite transformation. The pearlite transformation started mainly from the austenite grain boundary faced to ferrite. The pearlite transformation showed stasis. This indicates that some austenite is stabilized thermally against the pearlite transformation. The fraction of austenite having only the K-S or near K-S interface to neighbor ferrite grains was correspond to the fraction of austenite grains which does not include pearlite. The pearlite transformation was difficult to start from austenite interface having the K-S relationship to ferrite since the interface between austenite grains and ferrite grains was stabilized energetically in the case of their interface having the K-S relationship.

Abstract: A new combination of laser scanning confocal microscope (LSCM) and electron backscattering diffraction (EBSD) with a field-emission scanning electron microscope (SEM) is utilized to study the mechanism of bainite transformation in reheated low carbon bainitic weld metal. The LSCM observations show that laths grow on the surface at various rates, from 30 μm/s to 240 μm/s, which is greatly larger than those referred in literature for bainite. In order to confirm that the laths are bainite and not surface martensite, additional experiments were performed. The crystallographic characteristics of surface bainite were compared with those of bulk bainite obtained during isothermal treatments and those of bulk martensite obtained by water quenching. By means of a dedicated EBSD data-treatment software, orientation relationship, variant selection and packet groups were identified; it was shown that both the surface laths and bulk bainite share the same misorientation, habit plane, and have similar variant distribution. Experiments are running to compare these features with those of bulk martensite. If the distinction between martensite and bainite is successful, the very high growth rates of the surface laths could be used to discuss the displacive/diffusive nature of bainitic transformations.

Abstract: It’s well known that the vehicle fuel economy is improved by reducing the weight of their structural and mechanical components. Therefore, there is a demand of bearing race with thin thickness for weight reduction. But, this thickness reduction also causes a large oval distortion in bearings after Heat Treatment (HT). The factors affecting the HT distortion are extremely complicated, which makes it difficult to do rectification only by experimental analysis. There is also a limitation in numerical analysis due to the lack of workable model for HT distortion. Therefore, the aim of this study is to identify the effective factors of HT distortion by experimental approach in order to build suitable numerical simulation model that can estimate HT distortion. The material used in this study is a conventional case-hardened steel (Fe-0.4mass%C-Cr-Mn). The ring with the diameter of 100mm is heat treated and quenched. The resulting HT distortion is evaluated by measurement of maximum and minimum diameter of the race. A jig was developed to correct the ring shape during heat treatment, which helped in identifying that the quenching affects more than 50% of HT distortion. From these results, we developed a cooling equipment to reproduce inhomogeneous cooling rate by controlling oil flow toward the exterior of the ring. The quenching with this equipment reveals that inhomogeneous flow with opposite side (0 degree and 180 degree) reproduces a large oval distortion. The data obtained from the experimental analysis is suitable for developing the numerical model for HT distortion.

Abstract: Magnesium components offer an enormous potential to achieve considerable weight savings in the automotive industry. As with any innovative construction material, (e.g. carbon-fiber-reinforced polymer, ultra-high-strength steel) a rigid quality management system needs to be established along the supply chain. For this purpose, the concept of the supply chain quality management is to be introduced for the special case of magnesium components, using the example of the magnesium strip production. By the implementation of the widespread ISO 9001 norm, opportunities can be leveraged by both the creation of upstream, downstream linkages and the defined internal processes to achieve the required and stable quality for the automotive industry. The paper suggests procedures and instructions for process control during the magnesium strip production according to the twin-roll casting technology, giving feedback to the respective production system to avoid insufficient quality. The results serve as a basis for the development of all stages up to the assembly of magnesium components. The individual process stages dependent on the manufacturer should be considered during the procedure.

Abstract: The term “street furniture” includes numerous elements (e.g. street lamps, benches, fountains, gazebos) easing city life, complementing architecture and contributing to decorate urban areas. Despite the massive presence of street furniture in the urban environment, the knowledge about its evolution through the centuries is frequently ignored or neglected. This study analyses the main metallurgical features of twenty cast iron metalworks dating back to the 19^th and early 20^th centuries manufactured in cast iron foundries mainly located in Italy, France and England. The experimental activity was carried out in collaboration with “Fondazione Neri – Museo Italiano della Ghisa” (Longiano, FC, Italy).The microstructure was determined by means of optical microscopy (OM), whereas the semi-quantitative chemical composition was evaluated through scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS).The experimental results highlight the lack of a shared metallurgical culture in steel industry over the last one hundred and fifty years, since non-uniform microstructural features were observed among the specimens produced in that period in the geographical areas under study.

Abstract: In this paper, the microstructural evolution and mechanical properties of the as-cast Fe-25Mn-7Al-1.3C austenitic steel after different heat treatment were investigated. After solution treatment and subsequent aging treatment, the κ-carbides with perovskite structure were found to precipitate coherently within the austenite matrix, which improved the initial hardness and mechanical strength. The experimental steel exhibited an optimal comprehensive performance after being solution treated at 1050 °C for 1 h and then aged at 550 oC for 2 h. The tensile strength was 751 MPa, the yield strength was 581 MPa, the elongation was 48%, the hardness was 252 HB, and the Charpy V-notch impact toughness was 168 J, respectively. The impact wear test was carried out on MLD-10 abrasive wear testing machine, and the worn out surfaces under different heat treatment were characterized by scanning electron microscopy (SEM). The results indicated that the abrasion resistance of the steel under the additional aging treatment was better than that of the as-solutionized steel. The optimal abrasion resistance was obtained after being soluted at 1050 °C for 1 h and then aged at 550 oC for 2 h. However, with the aging time increasing, the coarse κ-carbides precipitating around the grain boundaries would deteriorate toughness, which lead to increase of the abrasive wear volume loss. Besides, obvious micro-cracking and relatively larger peeling pit were observed.

Abstract: Gas metal arc welding (GMAW) process is an indispensable technology in various industrial fields. The phenomena during the welding process such as the temperature history of the base metal strongly affect to the properties and performance of the weld joint. It is very important to predict and control the phenomena in the process, however, the phenomena is very complicated and not completely understood. In this study, numerical models of the heat source and the weld pool formation in GMAW process are constructed to visualize the phenomena and predict the welding results of the GMAW process. The heat source model includes the arc plasma and metal transfer phenomena. The temperature, velocity, pressure, and electromagnetic field in the arc plasma and the molten metal are calculated. Influence of the metal vapor from the surface of high-temperature metal is also considered. Input parameters of the heat source model are the welding condition such as the current and the shielding gas used in the experiment. The properties of the heat source such as the heat input distribution, arc pressure distribution and the temperature of the droplet are calculated by the heat source model. These properties are input to the weld pool model. The weld pool model calculates the flow and temperature fields in the weld pool and can predict the penetration shape and the temperature history in the weld joint. These models are applied to pulsed-MAG welding process, the numerical result of the penetration shape shows good agreement with experimental results.