Abstract
In situ dilatometry experiments using high energy synchrotron X-ray diffraction in transmission mode were carried out at the high energy material science beamline P07@PETRAIII at DESY (Deutsches Elektronen Synchrotron) for the tempering steel AISI 4140 at defined mechanical loading. The focus of this study was on the initial tempering state () and the hardened state (). Lattice strains were calculated from the 2D diffraction data for different planes and from those temperature-dependent lattice plane specific diffraction elastic constants () were determined. The resulting coupling terms allow for precise stress analysis for typical hypoeutectoid steels using diffraction data during heat treatment processes, that is, for in situ diffraction studies during thermal exposure. In addition, by averaging specific and macroscopic temperature-dependent elastic constants were determined. In conclusion a novel approach for the determination of phase-specific temperature-dependent DECs was suggested using diffraction based dilatometry that provides more reliable data in comparison to conventional experimental procedures. Moreover, the averaging of lattice plane specific results from in situ diffraction analysis supply robust temperature-dependent macroscopic elastic constants for martensite and ferrite as input data for heat treatment process simulations.
Highlights
Manufacturing of technical components is always accompanied by the generation of characteristic residual stress distributions
The different experiments were numbered from no. 1 to 7 according to the applied temperatures between 30 ◦C and 600 ◦C
The much smaller γ peaks are indexed red. These are only observed for experiment no. 1 and 2. They belong to retained austenite after quenching and are neither existent for the initial ( f errite) state nor the investigations of martensite at elevated temperatures, since the intensity of the γ peaks and the amount of retained austenite, decreases at elevated temperatures
Summary
Manufacturing of technical components is always accompanied by the generation of characteristic residual stress distributions. For individual cases values for Cij at elevated temperatures T are given or a so called temperature factor Tij allows for the calculation of Cij ( T ), assuming a linear dependency on T [7] Apart from this, it is well-known that the (macroscopic) elastic behaviour of materials changes with increasing temperature. In literature it is shown that this trend is not entirely linear [8,9] These deviations from linearity in the elastic behaviour may cause significant errors in the determination of stresses at elevated temperatures using DECs based on room temperature single crystal constants. To improve the reliability and validity of high temperature stress analysis, as for example, in our own work about in situ laser surface hardening [10,11] there is the necessity of determining high temperature DECs for proper data evaluation. Averaged macroscopic elastic parameters can directly be used to improve finite element (FE) heat treatment process simulations [14]
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