SXES VERSUS WDS IN STEEL SCIENCE: A THAI-GERMAN COOPERATION

摘要

High-strength steels play an important role in automotive industry. Especially, the TRIP (transformation induced plasticity) effect delivers remarkable properties, i.e., strain hardening, which is visible in the high energy absorption at high strain rates during a car crash. The TRIP effect is caused by the transformation of retained austenite into martensite during plastic deformation and strongly controlled by the amount and stability of the retained austenite. Carbon partitioning is the main factor influencing both the amount and stability of the retained austenite. The different diffusion and solubility behaviour of carbon in ferrite and austenite phases results in characteristic distributions after the heat treatments and rolling processes of TRIP steels. Viewed from a different angle, studying carbon concentration and its distribution leads to remarkable information about production processes and mechanical properties. Up to now there are only two devices on the market that have the possibility to measure carbon profiles on the sub-micrometre scale: the Schottky emitter electron probe microanalyser (FE-EPMA) and the soft X-ray emission spectrometer (SXES). FE-EPMA is equipped with conventional wavelength-dispersive X-ray spectrometers (WDS) and SXES is installed on a microprobe or a scanning electron microscope (SEM) equipped with a Schottky emitter. In [1] the excellent analytical properties of measuring carbon concentrations at higher resolution were shown. In this work we are focussing more on the comparison between FE-EPMA and SXES and answering the questions: Is it possible to transfer the method of measuring carbon developed by FE-EPMA to SXES? Is there any complementary information since both techniques have their own performances? As test specimens, hot-rolled TRIP steels were processed under different thermomechanical treatments to produce different distributions of carbon. In a first step, mappings of C-Ka were acquired at 15 kV and 100 nA with a JEOL JXA 8530F located in Aachen, Germany. The resulting carbon distribution is shown in Fig. 1. Carbon containing phases are revealed with an enrichment of carbon visible at the boundaries of larger grains. To exclude artefacts due to higher contamination at edges the results were validated by means of phase field simulations described in more detail in [2]. To obtain more precise information about the carbon distribution line scans were performed under the same excitation conditions described above but now at high resolution with a step size of~62 nm and a dwell time of 5 s per point. Data for the first 2 - 3 μm of the line scan were not taken into account due to a non-stabilised situation of the carbon contamination rate [1]. As a result, a characteristic carbon profile is visible in the martensite/austenite structure (middle of the line scan, see Fig. 2 left). With the results of FE-EPMA in mind, experiments were repeated using the same specimen at the National Metal and Materials Technology Center (MTEC) in Thailand but with analytical conditions optimised for the SXES (installed at JEOL JSM-7800F Prime SEM, varied line spacing (VLS) diffraction grating, CCD camera, energy range: 70-210 eV): 5 kV, 190 nA, 20 s and a step size of 125 nm. A carbon profile and associated microstructure are presented in Fig. 2 right. Although the dose (beam current times acquisition time) was a factor of 8 higher, the precision gets worse in comparison to FE-EPMA. The reason might be the use of the 2nd order of C-Ka, the high background intensity, and efficiency of the SXES in general.