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.

Particles, Microstructures, and Impact Toughness of CGHAZ of Ca Deoxidation Shipbuilding Steel Plates with Different Nb Contents

Herein, the effects of the Nb content on the particles, microstructure evolution, and mechanical properties of base metal (BM) and high‐heat input coarse‐grained heat‐affected zone (CGHAZ) of shipbuilding steel plates with Ca deoxidation are studied. With increasing Nb content, the strength and microhardness of the BM increase without significant loss of impact toughness. However, in CGHAZ, the average grain size increases from 168 to 244 μm, and the brittle phase fraction increases from 0.33% to 11.76%. The high‐angled grain boundary (HAGB) density in microstructures reduces from 0.35 to 0.19 μm−1, and the average effective grain sizes increase from 10.97 to 12.48 μm. The particles in 40 Nb‐BM and 40 Nb‐CGHAZ are mainly cubic TiN particles. In 170 Nb, except for cubic particles, there are approximately ellipsoid particles with high value of Nb content, which tend to dissolve into matrix at the elevated temperature. With increasing Nb content, the average size of particles in CGHAZ increases from 18.33 to 32.29 nm, and the number density decreases from 13.44 to 10.35 μm−2. Therefore, the impact toughness of CGHAZ decreases from 197 to 96 J at −20 °C under the high‐heat input welding of 400 kJ cm−1.

Effect of Boron Content on Microstructure and Impact Toughness in CGHAZ of Shipbuilding Steel Plates with Ca Deoxidation

Herein, the effect of boron (B) content on the microstructure and impact toughness in the coarse‐grained heat‐affected zone (CGHAZ) after large heat input welding of 400 kJ cm−1 for the EH40 shipbuilding steel plate with Ca deoxidation is studied. As the B content is increased from 0.0002 to 0.0024 wt%, the average width of grain boundary ferrites (GBFs) is decreased from 14.6 to 11.3 μm, the formation of acicular ferrite (AF) is promoted, and the formations of GBF and Widmanstatten ferrite (WF) are hindered in CGHAZ. The heat‐affected zone (HAZ) impact toughness is increased from 39 to 97 J at −20 °C. The fraction of low‐angle grain boundaries (LAGBs) is decreased from 40 to 29%, and the high‐angle grain boundaries (HAGBs) is increased from 60 to 71%. When the B content is 0.0024 wt%, the B atom segregates on the grain boundaries, and the widths of the segregation lines range from 2.3 to 3.6 μm. GBF is found to be the weakest zone in the CGHAZ. B addition can effectively reduce the formation of GBF, enhancing the critical crack stress and improving the HAZ toughness.

Research on the mechanism of Cu addition on the grain boundary characteristics and toughness in CGHAZ of low carbon low alloy steel

In this study, the coarse grained heat affected zone (CGHAZ) of Cu-containing/free steels was simulated at various heat inputs by thermal simulator. A comparative study was conducted to reveal the role of Cu addition and heat input in toughness variation by microstructural observation and crystallographic analysis. The results showed that the toughness exhibited a plateau-like variation with heat input increasing, which was mainly determined by the distribution of high angle grain boundary (HAGB). Also, a considerably higher toughness was achieved in Cu-containing steel irrespective of heat input. This attributed to the grain boundary distribution and variation selection. Cu addition weakened the variation selection and promoted the formation of more Bain groups, which helped to increase the fraction of HAGB and improve the impact toughness. In addition, Cu addition resulted in a higher fraction of boundary length in V1/V2, V1/V3, V1/V5 variant pair, which was beneficial to accommodate the transformation strain and release the residual stress, leading to a higher toughness in Cu-containing steels.

Role of Carbon Content on Microstructure Evolution and Impact Toughness in Coarse-Grained Heat-Affected Zone of High-Strength Steel

The effect of carbon content in the base metals of high-strength steel on the microstructure and impact toughness of simulated welding focusing on a coarse-grained heat-affected zone (CGHAZ) at different heat inputs was systematically investigated by using scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test confirmed that there was an optimal heat input, which caused the CGHAZ to obtain the highest impact toughness. The optimal heat input is ~20 kJ/cm and remains unchanged with an increase in carbon content from 0.04 to 0.12 wt.%. However, the impact toughness of the CGHAZ decreases with the increase in carbon content at each heat input. Microstructure characterization showed that a CGHAZ with 0.04 wt.% carbon gradually changed from lath bainite (LB) to granular bainite (GB) with an increase in heat input, while it changed from lath martensite (LM) to LB and then to GB for a CGHAZ with 0.12 wt.% carbon. Although the density of high-angle grain boundaries (HAGBs) obtained at 20 kJ/cm in the high-carbon sample is higher than that of the low-carbon sample, its impact toughness is lower, which is related to the parallel structure of the lath bundles and the morphology the austenite penetration.

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.

Effect of heat input on the microstructure and mechanical properties of the simulated coarse-grained heat-affected zone (CGHAZ) of novel twining-induced plasticity(TWIP) steel

The study of strengthening mechanisms associated with the HAZ is of significant interest for improving the mechanical properties. In this study, the effect of different heat inputs (6.21, 13.89, 19.64, 27.78, and 36.75 KJ·cm -1 ) on the microstructure, carbide precipitation, and mechanical properties of CGHAZ in a novel FeCMnSi TWIP steel was explored by using welding thermal simulations. The results showed that the CGHAZ organization did not experience phase transformation after different welding thermal cycles. As the heat input increased, and the grain size was 95.87, 107.03, 109.65, 98.68, and 136.95 μm, respectively. The factor of grain size variation with heat input is the dissolution of AlN particles and the generation of carbides at grain boundaries. The dramatic deterioration of CGHAZ toughness at a heat input of 27.78 KJ·cm -1 is mainly attributed to the enrichment of carbon and silicon elements at the grain boundaries in the CGHAZ, which leads to cracks along the grain boundaries. At this time, the fracture mode of CGHAZ shifts from ductility to intergranular brittleness. It is proposed that fine grain strengthening is the main factors in improving strength, and carbide precipitation is responsible for the significant decrease in toughness in the novel FeCMnSi TWIP steel. It provides an effective guide for the application of FeCMnSi TWIP steels.

Crystallographic study on microstructure and impact toughness of coarse grained heat affected zone of ultra-high strength steel

The significant effect of welding heat input on microstructure evolution and toughness of coarse grained heat affected zone (CGHAZ) in ultra-high strength steel was studied from the aspect of crystallography. Charpy impact test confirmed the optimum toughness of CGHAZ was obtained at the heat input of 20 kJ/cm, which was related to the cumulative contribution of its well-refined bainitic structure and the highest density of high angle grain boundaries (HAGBs). Analysis on crystallography shows that the weakest variant selection occurs at the nose temperature of bainite transformation curve, inducing the staggered configuration of different Bain groups, and thus obtaining the highest density of HAGBs, especially the block boundary dominated by V1/V2 variant pair. From the establishment of the relationship between impact toughness and crystallographic structure, the variation in toughness of CGHAZ is primarily correlated to the block size.

Effect of post-heat treatment on microstructure and properties of EH47 steel CGHAZ

To clarify the effect of post-heat treatment on the microstructure and mechanical properties of the coarse-grained heat-affected zone (CGHAZ) for EH47 steel, simulated CGHAZ specimens were subjected to post-heat treatment at 250°C for 1.0 h. The results showed that the microstructure of CGHAZ was composed of granular bainite (GB) and lath bainite (LB), and a large number of iron-carbon particles (∼81 nm) were precipitated after post-heat treatment. The relationship between dislocations, low-angle grain boundaries and iron-carbon particles was revealed by KAM values. The iron-carbon particles hindered the movement of sub-grain boundaries and dislocations, thereby increased the yield strength (∼120 MPa). The precipitation of iron-carbon particles caused the cracking of bainite ferrite and reducing the fracture toughness of CGHAZ.

Effect of Nb on microstructure and mechanical properties between base metal and high heat input coarse-grain HAZ in a Ti-deoxidized low carbon high strength steel

In this paper, a Ti-deoxidized low carbon high strength steel plate with different Nb content was obtained by thermal mechanical control processing (TMCP) methods. The coarse-grain heat-affected zone (CGHAZ) under high heat input of 300 kJ/cm conditions was simulated. The effects of Nb on microstructure and mechanical properties in base metal (BM) and CGHAZ were analyzed. The results showed that high content of Nb addition is not conducive to the improvement of toughness, especially in high heat input CGHAZ. A certain amount of Nb inhibites the formation of ferrite, while promotes the bainite transformation with coarse carbides during high heat input welding process. Although high-angle grain boundary has a certain hindrance to the crack propagation, the coarse carbides caused by the enhanced hardenability in Nb-bearing steels stimulates the crack initiation sensitively. Besides, the uneven prior austenite grains may also take responsibility to the deterioration of toughness in Nb-bearing CGHAZ. In general, the content of Nb should be carefully added when impact toughness of CGHAZ is considered during high heat input welding in Ti-deoxidized low carbon high strength steel.

Mechanism of BN-Promoting Acicular Ferrite Nucleation to Improve Heat-Affected Zone Toughness of V-N-Ti Microalloyed Offshore Steel.

This study examined the effect of boron (B) on the microstructure and toughness of the simulated coarse-grained heat-affected zone (CGHAZ) of normalized vanadium microalloyed offshore steel by using the welding thermal simulation method under different heat inputs for welding. The results showed that when t8/5 (the cooling time from 800 to 500 °C) increased from 14 s to 24 s, In the range of t8/5 of 24–44 s, the impact energy of the CGHAZ rose initially and then remained constant at around 125 J at −40 °C, and dropped to 79 J when t8/5 increased to 64 s. The particles (Ti,V)(C,N)-BN and BN contributed in the generation of acicular ferrite, which minimized the loss of CGHAZ toughness due to the presence of carbon. Furthermore, the microstructural parameters controlling CGHAZ toughness were the contents of the high misorientation grain boundaries and effective grain size at a tolerance angle of 15° at varied heat inputs.

Significant improvement in CGHAZ toughness of HSLA steel via welding with trailing mechanical treatment

The Coarse-grained heat-affected zone (CGHAZ) is well known as the local brittle zone in the welding joints of HSLA steels. In the present work, compressive strain is applied to the specimen during the CGHAZ welding thermal simulation process to achieve microstructure refinement and modification for toughness improvement. The strain is applied to the specimen when the temperature of the simulated thermal cycle decreased to 1100 °C, with the purpose of achieving dynamic recrystallization in the CGHAZ. Parallel experiments without the compressive strain are also conducted as a reference. The microstructure of the CGHAZ with and without the compressive strain was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used for observation of the matrix and microstructure of martensite-austenite (M-A) constituents. Electron back-scattered diffraction (EBSD) techniques were used to get the information of boundary misorientation distribution. The toughness of the specimens under each condition was assessed through instrumented Charpy impact test at −20 °C. Results showed that the compressive strain refined prior austenite grain (PAG) size and packet size, while the block width and lath width were almost unaffected. Refinement of the PAG also modified the microstructure of M-A constituents. The microstructure refinement and modification of M-A constituents improved crack initiation energy and crack propagation energy, resulting in significant promotion of toughness from 60.20 J to 163.10 J.

Effect of C and Si contents on microstructure and impact toughness in CGHAZ of offshore engineering steel

The combined effect of the C and Si contents on the microstructure and low-temperature impact toughness of heat-affected zone (HAZ) of the offshore engineering steel. As the C and Si content is decreased from 0.09 to 0.07 wt.% and 0.12 to 0.03 wt.%, the HAZ toughness at −40 °C is increased from 36 to 180 J, and the hardness in the coarse-grained heat-affected zone (CGHAZ) is decreased from 325 to 297 HV0.05. The higher C and Si content promote the precipitation of M23C6 and cementite, but restrain the formation of MC and M2C carbide in the present developed steels. The microstructures in the CGHAZ are comprised of a mixture of martensite and bainite after the welding with the heat input of 50 kJ/cm and are similar with the different C and Si contents. As the C and Si contents are decreased, the density of the geometrically necessary dislocation (GND) is increased. The fracture is changed from brittle to ductile manner, and the cleavage facet size is decreased from 23 to 11 µm. Reducing the C and Si contents significantly decreases the packet size from 25 to 17 µm, thereby leading to the improvement of HAZ toughness.

Effect of undercooled austenite ausforming on the role of the M–A constituents in the CGHAZ toughness of the HSLA steels with bainite structure

The brittle martensite-austenite (M-A) constituents in the coarse-grained heat-affected zone (CGHAZ) of the bainite steel are known to have negative effect on toughness. To modify the microstructure of the M–A constituents and then improve the CGHAZ toughness, this work applied ausforming to the bainite steel during its CGHAZ simulating process when the temperature of the thermal cycle decreased to 600 °C. Morphology of the M–A constituents in the CGHAZ with and without the ausforming was observed by optical microscopy (OM) and scanning electron microscopy (SEM). The microstructure of the M–A constituents was characterized by transmission electron microscopy (TEM), the toughness of the specimens was assessed through instrumented Charpy impact test, and the crack behavior beneath the fracture was revealed by SEM. Results show that the ausforming contributes to the austenite formation within the M–A constituents, which helps to improve the CGHAZ toughness from 10.50 J to 33.70 J. This work proposed a nontraditional method that enables the microstructure modification of the M–A constituents and the toughness improvement in the CGHAZ.

Effect of Cooling Rate on The Microstructure And Cryogenic Impact Toughness of HAZ in 9% Ni Steel

The effects of cooling rate on the microstructure and cryogenic impact toughness of coarse-grained heat-affected zone (CGHAZ) and inter-critically reheated coarse-grained HAZ (IC CGHAZ) in 9% Ni steel were investigated. CGHAZ and IC CGHAZ specimens were prepared from 9% Ni steel by controlling the cooling rate of the simulated welding process. The microstructure of the CGHAZ specimens consisted of autotempered martensite and lath martensite. As the cooling rate increased, the volume fraction of the autotempered martensite and the effective grain size decreased. A large amount of fine carbides was distributed inside the auto-tempered martensite, the dislocation density was low, and high angle grain boundaries were not observed. The microstructure of the IC CGHAZ specimens consisted of tempered martensite and lath martensite. As the cooling rate increased, the volume fraction of the tempered martensite and effective grain size decreased. Finer carbides were distributed inside the tempered martensite than in the auto-tempered martensite, the dislocation density was low, and high angle grain boundaries were not observed. Cryogenic fracture revealed that ductile fracture occurred in the auto-tempered martensite and tempered martensite, and brittle fracture occurred in the lath martensite. The crack propagation path was zig-zag in the high angle grain boundaries of the lath martensite. The volume fraction of auto-tempered martensite and tempered martensite and the effective grain size in the HAZ specimens had a significant effect on cryogenic impact toughness. In the IC CGHAZ specimens, cryogenic impact toughness decreased and then became constant as the cooling rate increased, due to a decrease in the volume fraction of the tempered martensite and effective grain size.

Study on the mechanism of heat input on the grain boundary distribution and impact toughness in CGHAZ of X100 pipeline steel from the aspect of variant

The variant selection and pairing directly affect the grain boundary distribution in the coarse-grained heat-affected zone (CGHAZ) of pipeline steel and consequently impact toughness. In this study, we attempted to establish a direct correlation between heat input, variant, grain boundary distribution, and impact toughness in CGHAZ of pipeline steel by means of electron backscatter diffraction (EBSD) analysis. The results showed that the variant selection is strong at 4 kJ/cm, such that V1/V1 becomes the dominant variant pair in CGHAZ. As the heat input increases, the transformation strain decreases, such that the mean variant pair in CGHAZ is V1/V2 and V1/V4/V8 at heat inputs of 9 and 21 kJ/cm, respectively. Compared with V1/V1 and V1/V4/V8 variant pairs at 4 and 21 kJ/cm, respectively, V1/V2 variant pair promotes the formation of high-angle grain boundaries (HAGB) and decreases the residual stress. When the heat input is 9 kJ/cm, the microstructure of CGHAZ has maximum resistance to crack propagation and lower crack nucleation ability, leading to the highest impact energy.

Effect of Initial Microstructure on the Toughness of Coarse-Grained Heat-Affected Zone in a Microalloyed Steel

Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.

Effect of step cooling process on microstructures and mechanical properties in thermal simulated CGHAZ of an ultra-high strength steel

Ultra-high strength steel (UHSS) has been widely applied in a variety of industries to satisfy lightweight requirements. For now, it is still a challenge to realize both high strength and excellent toughness in welded joints of UHSS, especially in the coarse grain heat affected zone (CGHAZ). In this research, an innovative step cooling process was proposed to control microstructures and mechanical properties in the thermal simulated CGHAZ of UHSS via thermal simulation test. The results show that the proposed step cooling process can realize the collaborative improvement of strength and toughness. With the cooling rate of 150 °C/s, the yield strength, the ultimate strength and the toughness are comparable to those of base metal (BM), even the total elongation exceeds that of BM. The combination of high strength and toughness of CGHAZ is primarily attributed to the synergetic effect from martensitic transformation, microstructure refinement and transformation-induced plasticity effect induced by the retained austenite.