Alexander Schwedt

Characterization of dual-phase steel microstructure by combined submicrometer EBSD and EPMA carbon measurements.

Electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) measurements are combined to characterize an industrial produced dual-phase steel containing some bainite fraction. High-resolution carbon mappings acquired on a field emission electron microprobe are utilized to validate and improve the identification of the constituents (ferrite, martensite, and bainite) performed by EBSD using the image quality and kernel average misorientation. The combination eliminates the ambiguity between the identification of bainite and transformation-induced dislocation zones, encountered if only the kernel average misorientation is considered. The detection of carbon in high misorientation regions confirms the presence of bainite. These results are corroborated by secondary electron images after nital etching. Limitations of this combined method due to differences between the spatial resolution of EBSD and EPMA are assessed. Moreover, a quantification procedure adapted to carbon analysis is presented and used to measure the carbon concentration in martensite and bainite on a submicrometer scale. From measurements on reference materials, this method gives an accuracy of 0.02 wt% C and a precision better than 0.05 wt% C despite unavoidable effects of hydrocarbon contamination.

Electron microscopy analysis of structural changes within white etching areas

In the present work, crack networks with white etching areas (WEAs) in cross-sections of bearings were investigated by a complementary use of SEM and TEM with the focus on the use of orientation contrast imaging and electron backscatter diffraction (EBSD). Orientation contrast imaging was used for the first time to give detailed insight into the microstructure of WEA. A significant difference between Nital-etched and polished WEA samples was observed. It was revealed that WEAs are composed of different areas with varying grain sizes. As a result of secondary transformation, needle-shaped grains were observed within WEAs. Using EBSD analysis, evidence was obtained that WEA formation and accompanying crack growth are without relation microstructural features. In addition, an inhomogeneous chemical structure of WEA as a result of carbide dissolution is revealed by analytical investigations.

Failure Initiation in Dual-Phase Steel

This research work aims to model the failure initiation in dual-phase (DP) steel. A microstructure based approach by means of representative volume elements (RVE) is employed to evaluate the microstructure deformation and the failure initiation on the mesoscale. In order to determine cohesive parameters for martensite cracking, a two level approach has been performed experimentally. First, in-situ bending test in SEM with EBSD measurements before and after the test showed that the crack initiation occurs in martensite islands. Then, mini tensile tests with DIC technique were carried out to identify macroscopic failure initiation strain values. RVE modeling combined with extended finite element method (XFEM) was utilized to model martensite cracking on mesoscale. The identified parameters were validated by comparing the predictions with the experimental results.

Study of subsurface initiation mechanism for white etching crack formation

The present paper describes a novel way to detect early stages in the gradual transformation of SAE 52100 bearing steel material to white etching cracks (WEC). The underlying transformation is recorded and investigated by a complementary use of Barkhausen noise measurements, ultrasonic measurements and scanning electron microscopy. While ultrasonic measurements can only be used to detect cracks in a failed component, the recently improved Barkhausen noise measurement technique can be used to detect possible early stages of microstructural transformation. A cross section from a region without ultrasonic signal but with a Barkhausen signal has been investigated by the use of scanning electron microscopes in order to reveal possible pre-stages of WEC formation. These findings support that WEC are locally initiated in subsurface regions. Within those modified regions, carbides start to dissolve in consequence of deformation accumulation, which has been identified as an early state of WEA microstructure formation. This paper is part of a Themed Issue on Recent developments in bearing steels.

Exploiting Process-Related Advantages of Selective Laser Melting for the Production of High-Manganese Steel

Metal additive manufacturing has strongly gained scientific and industrial importance during the last decades due to the geometrical flexibility and increased reliability of parts, as well as reduced equipment costs. Within the field of metal additive manufacturing methods, selective laser melting (SLM) is an eligible technique for the production of fully dense bulk material with complex geometry. In the current study, we addressed the application of SLM for processing a high-manganese TRansformation-/TWinning-Induced Plasticity (TRIP/TWIP) steel. The solidification behavior was analyzed by careful characterization of the as-built microstructure and element distribution using optical and scanning electron microscopy (SEM). In addition, the deformation behavior was studied using uniaxial tensile testing and SEM. Comparison with conventionally produced TRIP/TWIP steel revealed that elemental segregation, which is normally very pronounced in high-manganese steels and requires energy-intensive post processing, is reduced due to the high cooling rates during SLM. Also, the very fast cooling promoted ε- and α’-martensite formation prior to deformation. The superior strength and pronounced anisotropy of the SLM-produced material was correlated with the microstructure based on the process-specific characteristics.

Confirmation of tensile residual stress reduction in electron beam welding using low transformation temperature materials (LTT) as localized metallurgical injection – Part 1: Metallographic analysis

Abstract For the reduction of the distortion and the residual tensile stresses in welded seams on carbon manganese steels, low transformation temperature materials (LTT) were developed. These materials use the volume expansion effect during martensitic transformation. The volume expansion counteracts volume shrinkage during cooling. The positive effects of the low transformation temperature alloys on the residual tensile stresses were demonstrated in various investigations. The low transformation temperature materials were, so far, used as filler material in arc welding processes in large volumes. The use of modular thermal fields of the electron beam welding processes offers the potential of a temporally activated use of compressive stress induction by phase transformation of the low transformation temperature alloys. The aim is to exert influence in situ on the welding residual stress state and thus on the distortion of complex parts. The metallographic analysis of an electron beam welded seam in unalloyed steel with low transformation temperature filler material is demonstrated. The evaluation is made via electron backscatter diffraction (EBSD) and is shown in the first part. The second part will show the effect on near surface residual stresses examined by hole-drilling method in combination with an optical evaluation by electronic speckle pattern interferometry.

Effects of Solution Treatment and Test Temperature on Tensile Properties of High Mn Austenitic Steels

Tensile properties of high Mn austenitic Fe-26.5Mn-3.6Al-2.2Si-0.38C-0.005B (HM1) and Fe-18.9Mn-0.62C-0.02Ti-0.005B (HM2, in mass%) steels after different solution treatments have been investigated. The results show that the solution treatment has a significant influence on microstructure and mechanical properties of the investigated steels. By appropriate solution treatment the product of tensile strength (Rm) and total elongation (A50) of the hot rolled steel can be improved from ˜ 40000-50000 MPa% to ˜ 55000-65000 MPa% depending on the steel chemical composition. A solution treatment with a very high temperature, e.g. at 1100 °C for the Fe-18.9Mn-0.62C-0.02Ti-0.005B steel, results in a significant increase in the ϵ-martensite fraction during quenching. This deteriorates the ductility of the steel. A solution treatment at low temperature in the austenitic range, e.g. at 700 °C for the Fe-18.9Mn-0.62C-0.02Ti-0.005B steel, results in a decrease in the grain size of the steel. This suppresses the ϵ-martensite transformation during cooling. EBSD measurements revealed the mechanisms contributing to the overall plasticity of the investigated steels on the microscale. The plasticity of the 26.5Mn-3.6Al-2.2Si-0.38C-0.005B steel is produced mainly by TWIP mechanism under the examined experimental conditions, whereas for the Fe-18.9Mn-0.62C-0.02Ti-0.005B steel TWIP and TRIP mechanisms occur with different degrees depending on the test temperature of the tensile test.

Influence of Nanodispersions on Properties and Microstructure Features of Cast and T6 Heat-Treated Al Si Hypoeutectic Alloys

Cast light metal alloys have retained their importance and unique characteristics as first candidates when cost-function relationship is considered. Hypoeutectic aluminum silicon alloys as (A356) exhibit several specific and interesting properties that qualify them to be used in many automotive and aeronautical applications. Evidence of significant enhancement in strength in the properties of Al-Si cast alloys by incorporating nano-particles have been recently presented. The present study aims at developing nano-dispersed Al-Si alloys with suitable casting methods that assure the dispersion of the nano-particles. In this work a number of cast samples of A356 were prepared by rheo-casting in a specially designed and built furnace unit allowing for the addition of the nano-particles into the molten Al-Si alloy in the semi-solid state with mechanical stirring. The microstructural features and the mechanical properties of the cast and T6 heat treated samples were investigated. The results obtained in this work showed enhancement in the mechanical strength of the nano-dispersed alloys, accompanied by significant increase in the elongation percentage, supported by evidence of refined dendrite arms length, and inter-lamellar spacing.

EBSD-Analysis of Microstructural Changes Below Wire-EDMed Surfaces

An example for such an application are notched monolithic flexure hinges (cf. Fig. 1), which are analyzed within a larger project for the construction of micro-manipulators at RWTH Aachen University. During the bending of such hinges, the rim zone of the notch is undergoing high tensile strain. In order to predict the operating functionality, the microstructural damage introduced by W-EDM cutting therefore needs to be known in dependence of the cutting parameters.

Chemical characterisation of scale formation of high manganese steels (Fe-Mn23-C0.6) on the sub-micrometre scale: a challenge for EPMA

Fe-23Mn-0.6C steel oxidation in the initial state of annealing in synthetic air and an Ar / 4 % H2 / 7 % H2O atmosphere (600, 800 and 1000 °C for 20 min) was investigated. The chemical structure of the oxide scale and near-surface region of the oxidized sheet was characterized by electron probe microanalysis (EPMA). The results were correlated with the crystallographic structure of the sheet measured by electron backscatter diffraction (EBSD). Special preparations techniques i.e., focussed ion beam (FIB) milling and cross-section polishing by Ar-ions were applied to satisfy the requirements of quantitative X-ray and EBSD analyses. Moreover, carbon concentration profiles were measured on samples prepared by ion beam techniques and the conventional way with lubricants and polishing materials. Finally, a model of the near-surface region was derived by combining the results of quantitative analysis and X-ray mappings and was compared with thermodynamic calculations.