An order of magnitude decrease (from 16.0×10−4 to 4.1×10−4% h−1) in steady-state creep rate was observed in the fine-grained heat-affected zone (HAZ) of a Cr–Mo steel weld by the reduction of the ...pre-weld tempering temperature from 760°C (HTT) to 650°C (LTT). The microstructure during each stage of the manufacturing path, including pre-weld temper, thermal cycling and post-weld heat treatment, was characterized using a suite of characterization techniques. The techniques included simulated thermal cycling, dilatometry and electron microscopy, as well as time-resolved X-ray diffraction using Synchrotron radiation. Both LTT and HTT steels before welding contain M23C6 (M=Cr, Fe) and MX (M=Nb, V; X=C, N) precipitates in a tempered martensite matrix. During simulated HAZ thermal cycling with different peak temperatures, changes in M23C6 carbide characteristics were observed between the HTT and LLT conditions, while MX precipitates remained stable in both conditions. Simulated post-weld heat treatment samples show larger M23C6 in the HTT condition than in the LTT condition. The results provide a solution to extending the life of Cr–Mo steel welded structures used in power plants.
Submerged arc weld metal of a high strength low alloy steel was subjected to friction stir welding (FSW) at a higher rotation rate of 400rpm (FSW-a) and a lower rotation rate of 125rpm (FSW-b), ...respectively. The microstructures and mechanical properties of three typical phase structures, coarse bainite phase in the weld metal, refined bainite phase and ferrite phase in the nugget zones (NZs) of FSW joints were investigated. Compared to the weld metal, enhanced mechanical properties were achieved in the NZs of both FSW joints. Large cracks apparently propagated along the bainite lath in the coarse grains of the weld metal, which would cause the brittle quasi-cleavage fracture. However, large crack propagation was inhibited in the refined bainite phase structure in the NZ of FSW-a joint, and enhanced strength and toughness with dimple fracture were achieved. Meanwhile, enhanced mechanical properties, including strength and ductility, as well as toughness, were obtained in the NZ of FSW-b joint, because of the refinement of the ductile ferrite structure.
Ultrafine-grained (UFG) ferrite phase was the desired structure in the weld metal of high strength low alloy (HSLA) steel joints. In this study, submerged arc weld metal of a HSLA steel was subjected ...to friction stir processing (FSP) under a very low rotation rate of 80rpm. The original coarse bainite structure was changed into UFG ferrite structure after FSP, and the grain sizes were refined to about 500 and 300nm in the upper and lower parts of the processed zone, respectively. This study provides an effective strategy to preparing UFG ferrite structure in the weld metal of HSLA steel joints, and also a potential welding method.
•A weld metal of high strength low alloy steel was friction stir processed.•Ultrafine-grained (UFG) ferrite structure was obtained in the processed zone (PZ).•The grain sizes were 500 and 300nm in upper and lower parts of PZ, respectively.•This study provides an effective strategy to prepare UFG ferrite in steel weld metal.
Owing to the good thermal response of an infrared image furnace, the thermal cycle of welds (weld metal or heat affected zone) can be reproduced, while a laser scanning confocal microscope has a ...suitable light source and optical geometry for observing metal irradiated at high temperatures. The combination of an infrared image furnace and a laser scanning confocal microscope provides a useful tool for determining the microstructural changes in metals at the micrometre scale during the thermal cycle of welding. The technique of using such a combined system is called high temperature laser scanning confocal microscopy. Both macro- and microviews expand the understanding of the microstructural formation of welds and are useful in developing microstructure control methods. This is the first part of a report on the application of laser scanning confocal microscopy to observe microstructural changes for various types of steel samples and thermal cycles. The micro- or macroviews of different microstructural formations are presented, including the solidification cell, austenite from δ-ferrite, pearlite, martensite, Widmanstätten ferrite and bainite.
The structure and stability of Nb(C,N) precipitates were direct observed during reheating of an austenitic heat-resistant steel using in situ hybrid observations. The coarsened δ-Nb(C,N) and finer ...ε-Nb(C,N) were present in as-received steel. During the coarsening process, the d-spacing of cubic δ-Nb(C,N) exhibits a sharp decrease with increasing temperature, while that of hexagonal ε-Nb(C,N) maintains a steady increase. Compared to δ-Nb(C,N), the ε-Nb(C,N) was quickly dissolved into the austenite during heating, and it would no longer reprecipitate in the subsequent cooling.
This paper describes the effect of Al content on the formation of acicular ferrite in low carbon steel welds. Inclusions and the microstructures of welds with three levels of Al content were ...analysed. Acicular ferrite was formed in the low and medium Al content samples, but not in the high Al content sample. In the low and medium Al samples, a TiO layer, which encourages acicular ferrite nucleation, was formed around the inclusions. Grain boundary ferrite was formed only in the low Al content sample. Inclusions in the low Al sample were predominantly composed of the amorphous phase, whereas those in the medium Al sample were composed of spinel structures. It was confirmed by electron energy loss spectroscopy analysis that boron existed in the amorphous phase. The formation of the amorphous phase enhances grain boundary transformation by absorbing boron, which stabilises austenite grain boundary, whereas the formation of spinel structures enhances intragranular transformation by suppressing the boron absorption.
Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations.Usually,decreasing the grain size is the only ...approach by which both the strength and the toughness of a steel are increased.However,low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study.By direct tracking of the morphological evolution during phase transformation,it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite,whereas small austenite grain size leads to grain boundary nucleation of bainite.This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.
The theory of solidification of steels and the kinetics of austenite to α-ferrite phase transformation were extensively studied; however, comparatively, little information is available concerning the ...kinetics of the δ-ferrite to austenite transformation due to the difficulty of making in situ observations. In the present study, a laser scanning confocal microscopy with an infrared image furnace was implemented with which the in situ observations at the high temperature of the dynamic behaviour of the δ/γ grain nucleation and growth and interphase boundaries of the steels are made possible. The solidification mode of the carbon steel and the austenitic stainless steel during welding can be directly observed, and the definitive sequence of phase transformation that led to the final microstructure was detected in real time. Finally, new experimental results will be presented and compared with previous studies.
In order to better understand the microstructural evolution in steel welds, in-situ experiments using time-resolved X-ray diffraction (TRXRD) and laser scanning confocal microscopy (LSCM) are ...performed and the phase transformations that occur during welding of C-Mn-Al steels are directly observed. Ferrite and austenite phases are identified and quantified in the fusion zone (FZ) using the real time TRXRD and LSCM data. The results show non-equilibrium peritectic transition during rapid cooling, in contrast to the equilibrium δ-ferrite solidification that occurs under slow cooling conditions. Austenite nucleates along δ-ferrite grain boundaries during the δ→γ transformation, whereas during the γ→α transformation, the microstructures change from Widmanstatten ferrite (WF) to intragranular bainitic ferrite in the rapidly and slowly cooled welds, respectively. Using these real time observations, important kinetic information about phase transformations during steel weld solidification can be determined.
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