Impact Toughness Of Coarse-grained Heat-affected Zone Research Articles

Revealing the mechanism of crack initiation and propagation in CGHAZ of S690QL welded joints through low-temperature impact tests with gradient setting of pendulum angle

As the most brittle areas of welded S690QL high-strength steel structures, coarse-grained heat-affected zone (CGHAZ) was crucial for the structural integrity of components. This paper designed an innovative low-temperature impact testing method with a gradient setting of pendulum angle to investigate the micro-mechanism of crack initiation and propagation in the CGHAZ of S690QL welded joints. The results reveal that continuous blocky M−A constituents were distributed along prior austenite grain boundaries (PAGBs), leading to stress concentration and crack initiation. During low-temperature impact processes, cracks propagate in a transgranular mode along {100} or {110} crystal planes. Due to the interaction between compressive stress and the crack tip, regions with high kernel average misorientation (KAM) such as PAGBs can divert the crack. The coarsening of prior austenite grains in the CGHAZ leads to a diminished hindering effect of PAGBs on crack propagation. The findings elucidate the complex relationship between microstructure and impact toughness of CGHAZ, providing insights into crack behavior and mechanisms in CGHAZ.

Inclusions and microstructures in coarse-grained heat-affected zone of Al–Ti–Ca deoxidized shipbuilding steels with different Al contents after high-heat input welding

The inclusions, microstructure evolution and impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Al–Ti–Ca deoxidized shipbuilding steel plates with different Al contents after high-heat input welding of 400 kJ cm−1 are investigated. The characteristics of inclusions and their influence on the refinement of microstructure and impact toughness in CGHAZ was elucidated. Increasing the Al content from 0.016 wt% to 0.022 wt% and 0.043 wt% results in an increase in the average size of prior austenite grain and the brittle phase fractions, while decreasing the content of intragranular acicular ferrites (IAFs). The results of the thermodynamic calculations, high-temperature laser scanning confocal microscopy and differential scanning calorimetry all show that increasing the Al content can diminish the austenitic stable region, elevate the transition temperature from austenite to ferrite and suppress the nucleation of IAF. In the present steels, the IAF formation is successfully induced by the specific inclusions, namely Al2O3–CaO–Ti2O3–MnS(-CaS), in which the calcium aluminate inclusions are embedded with Ti-oxides and CaS–MnS precipitates. The effective inclusions have the number density of 93 mm−2 in 16Al steel and 27 mm−2 in 22Al steel, but they are rarely found in 43Al steel. On average, an effective inclusion can induce 3.4 IAFs in 16Al steel and 2.8 IAFs in 22Al steel. Thus, as the Al content increases from 0.016 wt% to 0.22 wt% and 0.043 wt%, the impact toughness of CGHAZ at −40 °C decreases from 134 J to 78 J and 24 J with the welding heat input of 400 kJ cm−1.

Elucidating the heat input on CGHAZ microstructure and its irregular effect on impact toughness for a novel V–N microalloying weathering steel

This study investigated the effect of heat inputs on coarse-grained heat affected zone (CGHAZ) microstructure and impact properties of a novel vanadium (V) and nitrogen (N) microalloyed weathering steel via a Gleeble™ system, revealing the metallurgical essence of the steel suitable for large heat input (Ej) welding. The results indicate that in CGHAZ, the dominant microstructure was composed of lath bainitic ferrite (LBF) at Ej = 15 kJ/cm, granular bainitic ferrite (GBF) at Ej = 35 kJ/cm, and a mixture of intragranular acicular ferrite (IGAF), intragranular polygon ferrite (IGPF) and grain boundary polygonal ferrite (GBPF) at Ej = 55 and 75 kJ/cm, respectively, apart from martensite/austenite (M/A) constituents in each one. The area fraction, fM/A, and mean size, dM/A, of M/A constituents increased monotonously with the Ej. In addition, with increasing Ej from 15 to 35 kJ/cm, the fraction, fMTAs＞15°, of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of grains with HAGBs increased. However, as Ej further increased to 55 and 75 kJ/cm as the two large heat inputs, the precipitation of (Ti, V) (C, N) and thereby the heterogeneous nucleation of polygonal and acicular ferrite were promoted by a lowered degree of undercooling, resulting in the decreased MEDMTA≥15° and the increased fraction fMTAs＞15°. Accordingly, the impact toughness of CGHAZ was degraded with the increase of Ej from 15 to 35 kJ/cm, but enhanced with the increase of Ej from 35 to 75 kJ/cm.

Effect of Mo content on impact toughness of CGHAZ in offshore engineering steels at different low temperatures

The effect of Mo content on impact toughness of the coarse-grained heat-affected zone (CGHAZ) in 420 MPa offshore engineering steels after 15 kJ/cm welding simulations is studied at low temperatures of −40, −60 and −80 °C. With increasing Mo content from 0.08 to 0.16 wt.%, the average size of the prior austenite grain in the CGHAZ is decreased from 150 to 70 µm, which mainly consists of lath bainite (LB). The fraction of high-angle grain boundaries (HAGBs) increases from 45.3% to 58.2%. The EBSD characterizations on the propagation and deflection of the main and secondary cracks in the CGHAZ indicate that the cleavage planes of the main crack mainly comprise of {100}, {110} and {112}. The way of fracture is changed from ductile fracture to quasi-cleavage fracture and cleavage fracture with decreasing the impact test temperature. As the Mo content is increased from 0.08 to 0.16 wt.%, the impact toughness values at −40, −60 and −80 °C are changed from 137, 101 and 40 J to 195, 117 and 88 J, respectively. With increasing the Mo content, the absorbed energies of the CGHAZ is improved because the sizes of PAG (Prior Austenite Grain) and LB are refined, and the fraction of HAGBs is increased to provide larger resistance for propagation of crack in CGHAZ.

The significant impact of introducing nanosize precipitates and decreased effective grain size on retention of high toughness of simulated heat affected zone (HAZ)

Abstract The study of toughening mechanism associated with the heat-affected zone (HAZ) continues to be of significant interest for improving the low temperature (−40 °C) toughness. In this study, the effect of different heat input on microstructure, hardness and low temperature impact toughness of coarse-grained HAZ (CGHAZ) in a high-strength low-alloy (HSLA) plate steel was explored by using welding thermal simulations. After different welding thermal cycles with heat input of 60, 90, 120, and 180 kJ cm−1, average effective grain size of 5.7, 5.9, 6.2, and 6.7 ± 0.2 μm, were respectively obtained. Although the effective grain size marginally increased with the increase of heat input, the values were remarkably smaller than the prior austenite grain size of 54, 93, 104, and 118 μm. Compared to the as-processed HSLA steel, impact toughness of steel welded with a heat input of 60 kJ cm−1 decreased dramatically from 318 ± 20 to 31 ± 8J·cm−2. But interestingly, impact toughness was increased to a peak value of 325 ± 20 J cm−2 when the heat input was increased to 180 kJ cm−1. Toughness was enhanced by grain refinement (D = 6.7 ± 0.2 μm) and through the nano-size precipitates (d = 10 nm), which were induced by medium heat input of 180 kJ cm−1. The values of precipitation strengthening were 203 MPa and 90 MPa while the critical stress values of crack propagation were 32 GPa and 10 GPa for the steels welded at 180 and 270 kJ cm−1, respectively. A critical analysis of fracture toughness and yield strength in terms of theoretical predictions is presented.

Effect of Heat Input on Microstructure and Impact Toughness of Coarse‐Grained Heat‐Affected Zone in Al–Ca and Ti–Ca Killed Steels

The microstructural evolution and toughness properties of simulated coarse‐grained heat‐affected zone (CGHAZ) in Ti–Ca killed steel with different heat input are studied and compared with Al–Ca killed steel. It is found that the impact toughness of CGHAZ in Ti–Ca killed steel with heat inputs of 50, 100, and 200 kJ cm−1 is ≈129, ≈105, and ≈44 J, respectively. However, the toughness of Al–Ca killed steel decreases from ≈28 to ≈23 J and then to ≈18 J. In Ti–Ca killed steel, Ti–Ca–Al–Mn–S oxides with appropriate size of ≈0.3–2 μm effectively promote the nucleation of acicular ferrite (AF). In addition, the high‐density number of ≈1329 inclusions mm−2 is one of the most important reasons for the formation of AF. However, dominant Al–Ca–Mn–S oxides in Al–Ca killed steel cannot effectively promote the nucleation of AF and mainly act as a potential propagation site of impact crack. In Ti–Ca killed steel, predominantly interlocked AF with high‐angle grain boundaries and high‐density of dislocations remarkably improves the toughness of CGHAZ.

Combined effect of M/A constituent and grain boundary on the impact toughness of CGHAZ and ICCGHAZ of E550 grade offshore engineering steel.

The present paper investigated the relationship between low temperature impact toughness and microstructure of bainite in coarse-grained heat affected zone (CGHAZ) and intercritically rehazed CGHAZ (ICCGHAZ) of an offshore engineering steel from both the microstructure morphological and crystallographic aspects. In this work, six groups of samples simulated CGHAZ and ICCGHAZ were designated at three different cooling rates. The Charpy test results showed that the toughness in CGHAZ decreases dramatically with decrease of cooling rate, which was attributed to the microstructural evolution from lath bainite to granular bainite, accompanying with the size increase of Bain zone and the change of M/A morphology from film to block. The increase in hardenability by cooling rate promotes more crystallographic variants from different Bain groups. Meanwhile, the combination with controlled inter-spacing of block boundaries by self-accommodation below the critical Griffith crack length, micro-crack can be arrested by these high angle grain boundaries thereby suppressed brittle fracture initiation and increased fracture properties. However, the variation in toughness of ICCGHAZ is not a concern, since obtaining excellent toughness is scarcely accessible even if the matrix microstructure is analogous to CGHAZ. It was due to the formation of coarse M/A constituents (~2 μm) necklacing at the prior austenite grain boundary. The visualized crystallography suggested that the impact toughness was partially correlated to the configuration manner and the size of Bain zones as well via promoting highly misoriented angle (>45°) boundaries, which in turn effectively deflected or arrested the brittle crack propagation.

Microstructure and Properties of HAZ in Low-Carbon Bainite E550 Steel during Double-Pass Welding Thermal Cycle

Investigations on the microstructure and properties of the Coarse-Grained Heat-Affected Zone (CGHAZ) and intercritical reheated Coarse-Grained Heat-Affected Zone (ICCGHAZ) of a low-carbon bainite E550 steel were carried out using thermal simulation technology in this paper.Double-pass welding thermal cycle were performed on Gleeble-3800 thermal simulator, tempering heat treatment of the critical coarse crystal zone carried out in a box resistance furnace, low impact energies at -40 °C and Vickers hardness determined, and the microstructure were observed. The experimental results show that the microstructure of CGHAZ (Tp1 is 1320 °C) was dominated by coarse granular bainite and Lath bainite Ferrite, the impact toughness of CGHAZ was poor. The toughness of the CGHAZ was improved after second welding heat cycle except intercritical two-phase heating. When the peak temperature of the second thermal cycle(Tp2) was 650 °C, martensite-austenite (M-A) constituent of original CGHAZ wasdecomposed and refined, impact toughness and hardness were all higher than that of CGHAZ; When Tp2 is 750 °C, there was a ” necklace” distribution of massive M-A constituent in this ICCGHAZ, the impact energy at -40 °C prominently decreased and Hardness went up; When Tp2 was in the temperature range of 850 °C ~1100 °C, the microstructure was mainly finer granular bainite, the toughness of CGHAZ could be effectively improved; When Tp2 was over 1100 °C, M-A constituents become coarse, the toughness declined slightly . The changing of hardness was the opposite of toughness but the hardness fluctuation was comparatively small. After tempering at different temperature (520 °C~640 °C) , the grain boundary "necklace" structure of ICCGHAZ was still obvious, some of the M-A constituent were decomposed, the hardness decreased, the lowest hardness was obtained in 610 °C.

Effect of Zr Addition on the Microstructure and Toughness of Coarse-Grained Heat-Affected Zone with High-Heat Input Welding Thermal Cycle in Low-Carbon Steel

Microstructures and toughness of coarse-grained heat-affected zone (CGHAZ) with high-heat input welding thermal cycle in Zr-containing and Zr-free low-carbon steel were investigated by means of welding thermal cycle simulation. The specimens were subjected to a welding thermal cycle with heat inputs of 100, 400, and 800 kJ cm−1 at peak temperature of 1673 K (1400 °C) using a thermal simulator. The results indicate that excellent impact toughness at the CGHAZ was obtained in Zr-containing steel. The Zr oxide is responsible for AF transformation, providing the nucleation site for the formation AF, promoting the nucleation of AF on the multi-component inclusions. High fraction of acicular ferrite (AF) appears in Zr-containing steel, acting as an obstacle to cleavage propagation due to its high-angle grain boundary. The morphology of M-A constituents plays a key role in impact toughness of CGHAZ. Large M-A constituents with lath form can assist the micro-crack initiation and seriously decrease the crack initiation energy. The relationship of AF transformation and M-A constituents was discussed in detail.

Effect of High Heat Input on Toughness and Microstructure of Coarse Grain Heat Affected Zone in Zr Bearing Low Carbon Steel

Microstructure evolution and impact toughness of simulated coarse grained heat affected zone (CGHAZ) in Zr bearing low carbon steel have been investigated in this study. Thermal simulator was used to simulate microstructure evolution of CGHAZ with high heat input welding thermal cycle at 1400°C peak temperature. Microstructure of CGHAZ consisted of high volume fraction of AF inside grain and GBF at prior austenite grain boundaries. Prior austenite grain size of CGHAZ increases with heat input increasing. Excellent impact toughness (more than 100 J) of CGHAZ with heat input of 1000 kJ/cm was obtained in this experiment. Impact toughness of CGHAZ with 400 kJ/cm (230 J) is the highest, because austenite grain size of CGHAZ with 400 kJ/cm favors the development of AF inside grain. Impact toughness is not only related with high angle boundaries but also with effective grain size. High supercooling of CGHAZ provided driving force for the AF transformation during welding thermal cycle, increasing the number of AF.

Mechanical Properties of Arc Welding Heat-Affected Zone of High Nitrogen Steel

Abstract The engineering application of high nitrogen steel (HNS) needs to obtain welded joint with good comprehensive properties. In order to acknowledge the influence of welding thermal cycle on the heat-affected zone (HAZ) of high nitrogen steel (HNS), the microstructure and mechanical properties of HAZ were investigated by using thermo-simulation technique. Experiment results indicate that the microstructure in the HAZ of HNS is composed of austenite and δ-ferrite. Carbide precipitation (Cr 23 C 6 ) occurs in the HAZ where the peak temperature of welding thermal cycle is about 800°C, and nitride (Cr 2 N) is not examined. The parameters of welding thermal cycle can affect the amount of δ-ferrite in the HAZ. Under the experimental conditions, the amount of the δ-ferrite in the coarse-grained heat-affected zone (CGHAZ) increases when the cooling rate increases, and it changes slightly while the peak temperature is lower than 1150°C. The hardness of the CGHAZ increases when the cooling rate increases, and that of the HAZ decreases while the peak temperature decreases. The results show that the hardness of the HAZ is higher than that of the base metal, indicating no softening of the HAZ under appropriate welding conditions. The impact toughness of CGHAZ is improved with the increase of cooling rate, whereas two brittle zones exist in the HAZ.