In-situ Studies with Photons, Neutrons and Electrons Scattering

The microstructures of welds are formed through various thermal cycles depending on the details of the welding path, including composition, heat input, post-weld heat treatment and so on. Thus, it is not easy to estimate the process of microstructure formation during the thermal cycle of welding. However, the origin of the microstructure exists at high temperature. For instance, the crystal structure of low carbon steel transforms twice (δ → γ → α) from liquid phase to room temperature. This provides an opportunity to obtain various desirable properties of a weld (the weld metal and the heat-affected zone (HAZ)) through microstructural control. Therefore, understanding the behaviour of phase transformation in the welding process is essential. As these phenomena occur during rapid heating and cooling cycles (several hundred degrees/second) at high temperature, it is not easy to observe the phase transformation during welding in detail.

Novel martensitic filler materials with specially adjusted martensite start temperatures (M_s) can counteract the cooling specific shrinkage due to expansion effects of the weld metal associated with phase transformations. That can be exploited to create compressive residual stresses in the weld and adjacent areas, i.e. beneficial for increasing the fatigue strength. The M_s temperature is shifted via the chemical composition, mainly by the alloying elements nickel and chromium, resulting as well in different retained austenite contents. Comparative investigations were made using a Low Transformation Temperature (LTT) alloy and a conventional high strength steel. The resulting phase transformation temperatures were—for the first time—detected using high energy synchrotron diffraction. Compared to angle dispersive diffraction, energy dispersive diffraction offers the possibility to measure residual stresses of the martensite and austenite phase parallel fast in one experiment. Furthermore, the high energy allows for obtaining information from the material volume by measuring in transmission geometry. For that purpose a special welding setup was designed applicable at different beam-lines and diffraction setups, allowing for diffraction experiments under realistic welding conditions. In particular the setup gives the possibility to observe and correlate localized phase transformations and thermo-mechanical stress/strain evolution during and after welding specific, rapid heat treatments. Additionally, due to local melting, solidification processes can be investigated. First results, presented here, show the correlation of local residual stress distributions affected by lowered transformation temperature.

An in situ high temperature-straining test associated to scanning electron microscopy has been implemented to study at the submicron scale different phase transformation and failure mechanisms phenomena in structural materials. This setup has been used to study the solid state cracking phenomenon known as Ductility Dip Cracking (DDC), which plagues some fcc metallic materials when strained at high temperatures. The Ni-base alloys AWS A5.14 ERNiCrFe-7 and ERNiCr-3 behavior were evaluated in situ at temperatures ranging from 700 to 1000°C.

The DDC susceptibility for both alloys was quantified using the threshold strain for cracking initiation (ε_min). The in situ results obtained at the sub-micron scale were compared with strain-to-fracture test results available in the literature, which are obtained at the macro scale using a thermo-mechanical simulator Gleeble^®. The ε_min measured by the in situ test for the ERNiCrFe-7 and ERNiCr-3 alloys was 7.5 and 16.5%, respectively, confirming the better resistance of ERNiCr-3 to DDC. In addition to the quantitative DDC susceptibility information, and most important, the in situ approach made possible the real-time observation of such failure phenomenon at the sub-micron scale. The grain boundary sliding associated to DDC was verified and quantified. Two differentiated components of grain boundary sliding: pure sliding (Sp) and deformation sliding (Sd) were quantified. Thus, a direct and quantitative link between grain boundary morphology (tortuosity), grain boundary sliding, and DDC resistance has been established for the ERNiCrFe-7 and ERNiCr-3 alloys.

The mechanical properties of steel strongly depend on the microstructure, which is formed during the production and processing of steel. Understanding the underlying mechanisms of the nucleation and growth kinetics during solid-state phase transformations in steel is of vital importance to control the microstructure of steel. The kinetics of individual grains in the bulk of steel can be measured in situ with the three-dimensional X-ray diffraction microscopy (3DXRD) at the European synchrotron radiation facility (ESRF). Simultaneously the fraction transformed, the nucleation rate, and the growth rate of individual grains can be measured. A furnace was developed to match with the 3DXRD-technology with the aim to map the three-dimensional microstructure of steel at elevated temperatures and to follow the kinetics of individual grains in more detail. Unique in situ measurements of nucleation and growth rates of individual austenite and ferrite grains are presented.

Time-resolved X-ray diffraction experiment was carried out to track the solidification mode of stainless steel during welding in situ. It was discussed the relation between the solidification mode and the results of hot-cracking test. The analyzing method of solidification mode was described in detail and it was shown that a halo pattern was important mark to trace the solidificaton mode during welding.

Inclusion in the weld pool in Al–Ti–Si–Mn deoxidized steel was in situ analyzed by using time-resolved X-ray diffraction (TRXRD) technique. The effect of aluminum content on the inclusion distribution in the weld pool was clearly analyzed. The inclusion was also directly analyzed by using FIB-TEM technique in ex situ. By comparing the results of TRXRD analysis at high temperature with the results of FIB-TEM analysis, the reaction between alumina and glassy phase was figured out.

Arc welding processes involve cooling rates that vary over a wide range (1–100 K/s). The final microstructure is thus a product of the heating and cooling cycles experienced by the weld in addition to the weld composition. It has been shown that the first phase to form under weld cooling conditions may not be that predicted by equilibrium calculations (Babu et al. 2002a). The partitioning of different interstitial/substitutional alloying elements at high temperatures can dramatically affect the subsequent phase transformations. In order to understand the effect of alloying on phase transformation temperatures and final microstructures time-resolved X-ray diffraction technique has been successfully used for characterization (Babu et al. 2002a, 2005; Stone et al. 2008; Babu 2002). The work by Jacot and Rappaz (1999) on pearlitic steels provided insight into austenitization of hypo-eutectic steels using a finite volume model. However there is very little work done on the effect of heating and cooling rates on the phase transformation paths in bainitic/martensitic steels and weld metals (Thiessen et al. 2007). Previous work on a weld with higher aluminum content, deposited with a FCAW-S process indicated that even at aluminum levels where the primary phase to solidify from liquid should be delta ferrite, non-equilibrium austenite was observed (Babu 2002). The presence of inhomogeneity in composition of the parent microstructure has been attributed to differences in transformation modes, temperatures and microstructures in dual-phase, TRIP steels and ferritic welds (Jimenez-Melero et al. 2009; Wang and Van Der Zwaag 2001; Babu et al. 2002b; Palmer and Elmer 2005).

The technique for single sensor differential thermal analysis (SS DTA) that has recently been developed at the Welding Engineering Laboratory of the Ohio State University is described in this paper. The application range of SS DTA is discussed in terms of engineering materials, phase transformations and structural changes, heating and cooling rates, and temperatures.

The paper summarizes results of recent in situ SS DTA studies on non-equilibrium phase transformations in high strength low alloy (HSLA) steel welds, on non-equilibrium solidification in high chromium Ni-base welding consumables, and on the effect of friction stir processing (FSP) and non-equilibrium heating and cooling on the beta-transus temperature in alloy Ti5111. These studies were conducted in actual conditions of gas-metal arc welding (GMAW), FSP, and heat treatment, and in simulated conditions of gas-tungsten arc welding. As a result of these studies in situ CCT diagrams in the heat affected zone and weld metal of HSLA100 steel welds have been developed, the effect of the filler metal composition on the solidification process and the susceptibility to solidification cracking in high chromium Ni-base filler metals has been evaluated, and processing conditions for above beta-transus processing in alloy Ti5111 have been determined. An approach for improved identification and quantification of phase transitions associated with small enthalpy changes is proposed. This is based on simultaneous application of SS DTA with other in situ techniques as Confocal Scanning Laser Microscopy, Synchrotron X-ray Diffraction, and SEM and TEM hot stage and straining techniques.

In situ phase transformation behaviour of aluminium-containing transformation induced plasticity steels, while subjecting them to heat affected zone weld thermal cycles have been studied. Experiments were carried out at ID11 of the European Synchrotron Radiation Facility, Grenoble, France. A specially designed oven was used to simulate the weld thermal cycles. Time–temperature resolved 2D synchrotron diffraction patterns were recorded and used to calculate volume fractions and lattice parameters of the phases. Results show that during heating, the retained austenite starts to decompose to ferrite and iron carbides once the temperature reaches 290°C. The lattice parameter of austenite increases linearly up to 290°C, followed by an increase in slope due to the formation of iron carbides. The combined effect of carbon concentration and thermal expansion causes scatter in the lattice parameter of austenite once the temperature reaches the inter-critical (α + γ) region. It is also observed that a significant amount of austenite (6–7%) was found to be retained at room temperature despite a high cooling rate (>20°C s⁻¹). Even after cooling the samples to room temperature, austenite was found to continue decomposing upon further holding and the volume fraction of retained austenite decreased continuously with time at room temperature.

New X-ray sources of unmatched brilliance, like the superconducting undulator device at ESRF high-energy beamline ID15A, allow for micro-radioscopic investigations with time-resolution up to the micro-second range. Here we present first results of two recent in situ experiments: the visualization of semi-solid metal flow at an acquisition speed 500 frames/s (fps) and the collapse of pore walls in liquid metallic foams investigated at 40,000 fps. Both applications reveal important qualitative and quantitative facts about the dynamic processes in liquid and/or semi-solid metals which were inaccessible until now because of either the limited spatial and/or the limited time-resolution of conventional X-ray devices. Thus, semi-solid slurry is observed to break into small particle clusters when injected at high speed. The event of cell wall collapse in metal foams is found to take ~1–2 ms time, indicating that the dynamics of this system is inertia controlled.

In the power generation industry it is necessary to carry out structural integrity assessments on welded components in order to ensure that a power plant will operate (or can continue to operate) safely. Welds are of particular concern because they are associated with complex thermal cycles, microstructure gradients, and high levels of residual stress. It is in this context that in situ diffraction techniques can offer significant benefits to the performance and safety of thermal power plants. For example, improvements in our understanding of the manner in which a material responds to complex thermo-mechanical cycles will lead to more reliable predictions for residual stresses in welds, and hence improvements in overall safety. Meanwhile, through new insights to material behaviour, in situ experiments may reveal opportunities for improvements in the performance of welded joints, and hence improvements in the performance of power plants as a whole.

This article describes several areas in which in situ diffraction techniques can play an important role in advancing materials and welding technology. It is hoped that, in doing so, it will help to stimulate interest in the application of in-situ diffraction techniques to the behaviour of materials during welding thermal cycles, as well as to some topical engineering challenges.

Neutron diffraction has been employed to study microstructural evolution in situ during heat treatment or thermo-mechanically controlled process for low alloy steels. Transformation kinetics, texture, carbon enrichment and internal stresses can be tracked and these data are useful to develop new processing to realize optimum microstructures.

The effects of plastic deformation and cyclic loading on residual stress distribution have been studied at welds in high strength 800 MPa steel. Effects of residual stress levels on fatigue life are also presented. Tensile loading was used to induce precisely controlled plastic deformation at weld toes. Residual stress distributions were measured by neutron diffraction. The influence of fatigue loading on the residual stress level was much smaller than that of plastic deformation. A large drop of residual stresses in the vicinity of the welds was recognised after local straining at the level of 2%. Stresses decreased with up to 550 MPa in the region around the weld toe. The effect of local plastic deformation on fatigue properties was also found to be significant. Fatigue strength more than doubled compared to the non-strained as-welded condition.