Vladimir Kostov

Time- and temperature-resolved synchrotron X-ray diffraction: observation of phase transformation and strain evolution in novel low temperature transformation weld filler materials

Solid-state phase transformations and the evolution of thermal and elastic strains in novel low temperature transformation (LTT) weld filler materials in the near surface region are monitored in real time by means of an innovative experimental set-up at the PDIFF (powder diffraction) beamline at the synchrotron light source ANKA (Angströmquelle Karlsruhe) at the KIT (Karlsruhe Institute for Technology). The key components of the diffraction set-up are two fast microstrip line detectors, which enables the strain evolution to be followed as a function of time and temperature for a 0.5 s counting time. During controlled heating and cooling cycles, as well as during near welding cycles, the martensite–austenite–martensite phase transitions are analysed. The transformation kinetics are monitored during resistance heating of small chips of the pure LTT alloys and during gas tungsten arc welding of simplified LTT welds using a specially designed welding rig for in-situ studies on the diffraction instruments. Under the mechanically unconstrained condition allowing free thermal expansion and shrinkage, the LTT alloys are found to exhibit decreasing transformation temperatures Ac and MS and increasing phase fraction of retained austenite for increasing Ni content. The strain evolution during welding reveals increased compressive stresses upon welding, which is attributed to the martensite formation upon cooling, which counteracts the thermal contraction strains. Comparison of the transformation temperatures reveals higher values than in the pure LTT alloys, but no variation between the different alloys. On the one hand, this is attributed to preferred grain orientation affecting the diffraction measurements and the determination of the transformation temperatures. On the other hand, it is possible that with the different chemical compositions of the LTT alloys and the mechanical constraints during welding, the evolution of the residual strain and stress may vary and result in counteracting affects with respect to lowered martensite start temperatures.

Fast in situ phase and stress analysis during laser surface treatment: A synchrotron x-ray diffraction approach

An in situ stress analysis by means of synchrotron x-ray diffraction was carried out during laser surface hardening of steel. A single exposure set-up that based on a special arrangement of two fast silicon strip line detectors was established, allowing for fast stress analysis according to the sin(2)ψ x-ray analysis method. For the in situ experiments a process chamber was designed and manufactured, which is described in detail. First measurements were carried out at the HZG undulator imaging beamline (IBL, beamline P05) at the synchrotron storage ring PETRA III, DESY, Hamburg (Germany). The laser processing was carried out using a 6 kW high power diode laser system. Two different laser optics were compared, a Gaussian optic with a focus spot of ø 3 mm and a homogenizing optic with a rectangular spot dimension of 8 × 8 mm(2). The laser processing was carried out using spot hardening at a heating-/cooling rate of 1000 K/s and was controlled via pyrometric temperature measurement using a control temperature of 1150 °C. The set-up being established during the measuring campaign allowed for this first realization data collection rates of 10Hz. The data evaluation procedure applied enables the separation of thermal from elastic strains and gains unprecedented insight into the laser hardening process.

Laser Surface Hardening of Steel: Effect of Process Atmosphere on the Microstructure and Residual Stresses

The effect of processing atmosphere on the microstructure and residual stresses are studied for laser surface hardening on steel samples of grade AISI 4140. Samples were hardened in air, vacuum and inert gas atmosphere (Helium) by means of a stationary laser beam. A high-power diode laser (HPDL) system was used in combination with a custom-designed process chamber. Residual stress distributions in lateral and in depth direction were analysed after laser processing by means of X-ray diffraction according to the well known sin² - method. X-ray residual stress analyses were supplemented by microscopic investigations of the local microstructure. The results indicate a widening of the compressive stressed region in lateral as well as in depth direction by surface hardening in inert gas atmosphere compared to laser surface hardening in air or vacuum atmosphere. This is due to the local heating flux distribution during the laser assisted heat treatment which is strongly affected by the processing atmosphere an leads to an extension of the hardening zone when using helium as inert gas.

Local Residual Stress Distributions Induced by Repeated Austenite-Martensite Transformation via Laser Surface Hardening of Steel AISI 4140

The effects of laser surface hardening of steel samples on the microstructure and residual stresses were determined for single as well as multiple laser pulses. Samples made of steel grade AISI 4140 were hardened by means of a high-power diode laser (HPDL) system using either single or multiple laser pulses resulting in single as well as repeated austenite-martensite transformations. The hardening was carried out in a specially designed process chamber allowing laser surface treatment in inert atmosphere in order to avoid oxide scale formation. The residual stress distributions in lateral and in depth direction were analysed by means of X-ray diffraction for samples hardened by up to 27 laser pulses. Residual stress analyses were carried out by means of the sin²y- method. The results indicate the extension of the hardened zone in lateral and in depth direction with an increase in the number of applied laser pulses. This evolution is connected with significant changes in the local residual stress distributions.

Time-Resolved X-Ray Diffraction Stress Analysis during Laser Surface Hardening of Steel - Impact of the maximum process temperature

Abstract The time- and temperature-dependent phase specific stress evolution, the residual stress distribution as well as the resulting microstructure of the heat treatable steel AISI 4140, generated by laser surface hardening was investigated. Using a measuring approach presented in [1] fast in-situ synchrotron X-ray stress analysis with sampling rates up to 10 Hz were carried out during laser surface hardening. Tests were carried out with maximum process temperatures of 850 °C, 1150 °C and 1300 °C. The results show that compressive stresses are induced inside the laser spot during the heating-up step. During the subsequent self-quenching step, these compressive stresses are reduced and tensile stresses develop. As martensite transformation starts, the tendency is reversed again and finally compressive residual stresses are obtained at room temperature. The in-situ stress analyses give a deep insight into the stress evolutions for the three maximum process temperatures. Results also show that, the higher the maximum process temperature, the smaller are the resulting compressive residual stresses.

Real Time Monitoring of the Strain Evolution during Rapid Heat Treatment of Steel Samples by Means of Synchrotron X-Ray Diffraction

An innovative experimental set-up for fast X-ray diffraction analysis on polycrystalline materials has been established at the synchrotron radiation facility ANKA (Karlsruhe, Germany). Key components of the set-up are two fast microstrip line detectors arranged symmetrically around the incident beam in the backscatter region. The capabilities of the set-up are tested by means of in-situ heat treatment experiments on SAE 4140 steel samples. In this feasibility study the heat was introduced by means of either a heating stage or by means of a gas tungsten arc welding torch. It will be shown that the evolution of thermal and elastic lattice strains can be monitored at a sampling rate of up to 4 Hz at a bending-magnet synchrotron beamline. Since the sampling rate may be increased further at a insertion device synchrotron beamline providing higher photon flux, our setup appears to be feasible for monitoring laser treatments in real time.

Load Partitioning Study in a 3D Interpenetrating AlSi12/Al2O3 Metal/Ceramic Composite

Internal load transfer in an interpenetrating metal/ceramic composite has been studied in this work using energy dispersive synchrotron X-ray diffraction. One of the samples was loaded in tension and the other one in compression. In each case, the sample was first loaded into the elastic-plastic regime, unloaded to zero stress, and reloaded beyond the prior maximum stress. Results show that at stress amounts greater than 100 MPa aluminum deforms plastically and the load is transferred to alumina and silicon. Unloading and reloading typically show reverse plastic deformation, Bauschinger effect and strain hardening in aluminum.