Xinjie Di

Toughening mechanisms of low transformation temperature deposited metals with martensite-austenite dual phases

Four groups of low transformation temperature (LTT) deposited metals with different Ni contents were prepared, and their microstructures were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and electron backscattered diffraction techniques. The relationship between the microstructures of the mixed martensite–retained austenite (RA) phases and their impact toughness were investigated; it was found that the impact toughness of the LTT deposited metals increased with increasing volume fraction of RA. In particular, its magnitude was higher for the specimens containing the lath martensite, interlath RA, and intercellular RA phases than for those composed of the lath martensite and interlath RA. The toughness of the lath martensite–RA mixed microstructure was primarily determined by the presence of the soft RA phase (containing film interlath RA and stringer intercellular RA), while lath martensite phase characterized by a high density of tangled dislocations and relatively small amount of twinned substructures resulted in the embrittlement of the LTT deposited metals. The dislocation absorption by the retained austenite and transformation-induced plasticity (TRIP) effects of RA were found to be main reasons for the improvement in materials toughness during crack initiation stage. The subsequent crack propagation proceeds via the TRIP and the transformation-induced crack termination mechanisms; it is also significantly affected by the increased fraction of martensite/RA boundaries. The optimization of the RA fraction in the martensite–RA dual structure is a potentially effective method for the toughness enhancement of the LTT deposited metals containing martensite–RA dual phases.

Comparison of two types of low-transformation-temperature weld metals based on solidification mode

ABSTRACT Two types of low-transformation-temperature weld metals were devised, one associated with primary austenite solidification, the other primary ferrite solidification. The martensite start temperature of both low-transformation-temperature weld metals was about 125°C. Experimental results showed that low-transformation-temperature weld microstructure associated with primary austenite solidification was martensite with 8.0% retained austenite, whereas that one related to primary ferrite solidification primarily consisted of martensite and δ-ferrite. Accordingly, both welded joints had little distinction between distortion and residual stress, indicating that weld metal associated with primary ferrite solidification played the same function as primary austenite solidification on residual stress reduction. Moreover, the low-transformation-temperature weld metal associated with primary ferrite solidification had higher tensile strength and hardness than that based on primary austenite solidification.

Comparison of Microstructure and Residual Stress Between TIG and MAG Welding Using Low Transformation Temperature Welding Filler

A Cr–Ni type of low transformation temperature (LTT) welding filler was devised in the present study. The LTT weld microstructures of the tungsten inert gas (TIG) and metal active gas (MAG) weldings were investigated by using electron-backscattered diffraction and orientation imaging microscopy. The results showed that the LTT weld microstructures prepared by TIG and MAG weldings were primarily martensite with 17.5% and 8.0% retained austenite, respectively. The LTT weld metal using TIG welding had larger grain size than using MAG. In addition, based on the Taylor factor calculation, the weld metal using MAG welding was more competent in repressing fatigue crack initiation. Meanwhile, the high angle and coincidence site lattice grain boundaries were dominant in the LTT weld metal using MAG welding. Moreover, the hardness of the LTT weld metal using MAG welding was higher than that of using TIG. Based on heat input and phase transformation, finite element method was applied to analyzing the tensile residual stress (RS) reduction in welded joints prepared by both conventional and LTT welding fillers, respectively. The corresponding outcome confirmed that the LTT weld metal using MAG welding was more beneficial to tensile RS reduction.

A bainite transformation kinetics model and its application to X70 pipeline steel

A new continuous-cooling kinetics model for bainite transformation based on diffusionless-displacive mechanism is proposed in this work to predict bainite transformation in the coarse-grained heat-affected zone (CGHAZ) of X70 pipeline steel. The Gleeble-3500 thermal–mechanical simulator was used to analyze the bainite transformation process in CGHAZ of X70 pipeline steel. The features of bainite transformation predicted from the proposed model were compared with the values measured from experiment, and the results show that the predicted bainite volume fraction and transformation velocity are in good agreement with the measured values. This indicates that the proposed kinetics model used to predict the bainite transformation process in the CGHAZ of high-strength low-alloy steel is reasonable. The model could be widely used in prediction of bainite transformation with high cooling rate, such as in welding CGHAZs.

Athermal martensite transformation of modified high Cr ferritic heat resistant steel undergoing different quenching temperatures

Abstract The thermal dilation behaviours of modified high Cr ferritic heat resistant steel quenched at different temperatures were employed to investigate the kinetics of martensite transformation. The martensite fraction and formation rate were obtained as a function of temperature. A model considering ‘spread’ martensites was introduced to explore the influence of quenching temperatures on martensite transformation. Both onset and offset temperatures of martensite formation decrease with the increase in quenching temperatures, and the reaction rate increases rapidly at the beginning of transformation and reaches a peak when a small quantity of martensite forms. The fitted data based on the proposed phase transformation model indicated that the width/length ratio of martensite laths decreases with the increase in quenching temperatures.

Solidification behaviour and microstructure of welding transition zone using low-transformation-temperature welding consumables

The solidification behaviour and microstructure of welding transition zone between low-transformation-temperature deposited metals and high-strength low-alloy steels were investigated. It was found that the steep composition gradient provided driving forces for the diffusion of carbon from base metal to weld metal, leading to the hardened and softened regions near fusion boundary. In weld metal near fusion boundary, there were retained δ-ferrites when the base metal dilution rate below 35% and Creq/Nieq value larger than 2.62. Compared with martensite, the mixed microstructures of martensiteu2009+u2009δ-ferrite obtained less strain localisation, dislocation density and more percentages of large misorientation, which were more liable to resist microcrack initiation and propagation during deformation.

Mechanical properties of low-transformation-temperature weld metals after low-temperature postweld heat treatment

ABSTRACT The as-welded low-transformation-temperature (LTT) weld metal with martensite/retained austenite (RA) dual phase exhibits high toughness and ductility, but the yield strength (YS) is very low. After low-temperature postweld heat treatment at 300°C, the YS, toughness and ductility of dual-phase LTT weld metal increase dramatically, while there is a slight effect on mechanical properties of full martensite LTT weld metal. During the low-temperature postweld heat treatment, carbon atoms diffuse from martensite into RA, which increases the stability of RA. The improvements of mechanical properties for dual-phase LTT weld metal after low-temperature postweld heat treatment are attributed to the increased stability of RA and enhanced transformation-induced plasticity effect.

Enhanced toughness of Fe–12Cr–5.5Ni–Mo-deposited metals through formation of fine reversed austenite

To overcome the strength–toughness trade-off in Fe–12Cr–5.5Ni–Mo-deposited metals, post-weld heat treatment (PWHT) was performed at the intercritical temperature, and the formation mechanisms of reversed austenite were investigated. The microstructures were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and electron backscattered diffraction techniques. It is found that lathy reversed austenite and nanometre-scale carbides are embedded in the martensite matrix after PWHT at 620xa0°C. The reversed austenite prefers to nucleate at multiple lath boundary junctions, and a subset forms adjacent to the M23C6 carbides. The growth of reversed austenite in the manner of martensite–austenite grain boundary migration and austenite–austenite grain boundary mergence is governed by Ni diffusion. The deposited metals exhibit a good combination of strength, ductility and toughness after PWHT at 620xa0°C for 1xa0h. However, the impact toughness and strength do not significantly change with a longer holding time, from 1 to 4xa0h. These phenomena are attributed to the combined effects of reversed austenite toughening, martensite matrix softening and M23C6 carbide precipitation strengthening. Moreover, the formation mechanisms of reversed austenite are discussed and proposed based on two-spherical-cap nucleation model, offering guidance for strength–toughness balance of low-carbon martensitic metals.

Microstructural Evolution and Softening Behavior of Simulated Heat-Affected Zone in 2219 Aluminum Alloy

The effect of peak temperature (Tp) at 200, 300, 400, 500 and 550xa0°C on the microstructural evolution and softening behavior of the simulated heat-affected zone (HAZ) was studied in the 2219-T87 alloy by electron-backscatter diffraction, transmission electron microscopy, X-ray diffraction, micro-hardness and micro-tensile tests. The results showed that the grain size in the HAZs at 200–500xa0°C was comparable, but the number density of the strengthening precipitates (GP zones/θ′) decreased with increasing Tp. At a Tp of 550xa0°C, the grain size significantly decreased and the distribution of the misorientation angles corresponded to the MacKenzie distribution. The GP zones/θ′ phase coarsened and translated into θ phases at Tp values in the range of 200–400xa0°C. Increasing the Tp to 500xa0°C and above, some θ′ phases translated into θ phases and others dissolved into the α-Al matrix which led to an increase in the solid solution strengthening. The reduction of the number density of the GP zones/θ′ was responsible for the softening behavior.

Effect of Post-weld Heat Treatment on the Microstructure and Corrosion Resistance of Deposited Metal of a High-Chromium Nickel-Based Alloy

The evolution of Cr23C6 carbides in the deposited metal (DM) of a high-chromium nickel-based alloy was investigated after the post-weld heat treatment (PWHT) at 650, 750, 850, and 950xa0°C, respectively. With the increase in temperature, the morphology of the Cr23C6 carbides at the grain boundaries was transformed from the continuous lamellar-like to the semi-continuous rod-like and then to the discontinuous granular. Besides, the needle-like Cr23C6 carbides precipitated from γ matrix after PWHT at 850xa0°C. The coarsening kinetics of the needle-like Cr23C6 carbides obeyed the Lifshitz–Slyozov–Wagner law with the growth speed of 4.93xa0μm3/h in length and 5.56xa0×xa010−3xa0μm3/h in width. Moreover, the ratio of the carbide length to width increased rapidly at first and then flattened as the holding time increased to 850xa0°C. The results of electrochemical corrosion experiment indicated that the needled-like Cr23C6 carbides impaired the corrosion resistance of DM due to the formation of chromium depletion around the carbides.