Heat Affected Zone Toughness Research Articles

Effect of Ti/N Ratio on TiN Particles, Microstructures, and Toughness of Heat‐Affected Zone After High Heat Input Welding of the Ca‐Treated Steel Plates

The high heat input welding (HHIW) can greatly enhance the welding efficiency of the steel plates. However, coarse grains and brittle microstructures will be formed in the heat‐affected zone (HAZ) during the HHIW process, which deteriorates the toughness of HAZ. In the present study, the effect of the Ti/N ratio on the size and distribution of titanium nitride (TiN) particles, microstructures, and toughness of HAZ after HHIW of 400 kJ cm−1 is investigated for the Ca‐treated steels with different Ti/N ratios of 1.61, 3.79, and 5.00 for TN16, TN38, and TN50 steels. The TN38 steel has the highest number density of the TiN particles with the sizes between 20 and 25 nm and the highest pinning force of the particles in three steels, resulting the smallest grain sizes in its HAZ. In addition, in three steels, the TN38 steel has the main microstructure of intragranular aciculate ferrites with the highest high‐angle grain boundaries density in its HAZ, thereby obtaining the highest value of low‐temperature impact toughness of the HAZ.

Improving the Weld Heat-Affected-Zone (HAZ) Toughness of High-Strength Thick-Walled Line Pipes

The low-temperature fracture toughness of double-V weld seams is a well-known challenge due to the essential increased heat input for heavy-wall pipelines. A thorough investigation was conducted to explore the impact of the heat input on the grain size and precipitate coarsening, correlating the microstructure with the heat-affected-zone (HAZ) toughness. The results indicated that the actual weldments showed a toughness transition zone at −20 °C, with considerable scattering in Charpy V-notch (CVN) tests. Gleeble thermal simulations confirmed the decreased toughness of the coarse-grained HAZ (CGHAZ) with increasing heat input and prior austenite grain size (PAGS). A specially designed thermal treatment demonstrated its potential for enhancing the toughness of the CGHAZ, with the recommended thermal cycle involving peak temperatures of 700 and 800 °C, holding for 1 s, and rapid cooling. The toughness of the intercritically reheated CGHAZ (ICCGHAZ) improved with higher intercritical reheating temperatures and the removal of necklace-type M–A constituents along the PAG. Despite various thermal treatments, no significant improvements were observed in the toughness of the ICCGHAZ. Future work was suggested for optimising the use of tack welds to reduce the effective heat input (HI) associated with double-sided submerged arc welding (SAW).

A study on as-welded microstructure and mechanical properties of thick-walled 9Cr3W3Co1CuVNbBN martensitic steel weldment

A systematic study on the microstructure evolutions and their relationships with mechanical properties of thick-walled 9Cr3W3Co1CuVNbBN martensitic steel pipe joint welded by narrow-gap automatic argon arc welding was carried out. The results showed that a narrow equiaxed grain zone (EGZ) and a significantly wide inter-critical zone (ICZ) were formed in the heat affected zone (HAZ) due to the larger grain size and lower VN precipitates in base metal (BM). The refined grains formed in WM and the higher solid solubility of C, Cr and W in EGZ and ICZ were the main factors leading to their higher hardness than that of BM. The decrease of dislocation density in over-tempered zone (OTZ) immediately adjacent to ICZ not only significantly improved the impact toughness of HAZ, but also led to a significantly lower hardness than that of BM. Nevertheless, the room/high temperature tensile fracture of the welded joints occurred in the BM zone, which was attributed to the cross-sectional shrinkage of the OTZ restrained by the nearby high hardness region ICZ, and the preferential formation of cavities near the coarse Cr23C6 carbides in BM.

Influence of Ti/N Ratio on Inclusions, Microstructures, and Toughness in Heat‐Affected Zone of Shipbuilding Steel Plates with Mg Deoxidation after High Heat Input Welding

The inclusions, microstructures, and toughness in heat‐affected zone (HAZ) of the shipbuilding steel plates with Mg deoxidation and the different Ti/N ratios are studied after the high heat input welding of 400 kJ cm−1. With increasing the Ti/N ratio from 3.00 to 5.67, the average size of inclusions increases, the mass fraction of total TiC inclusions increase from 2.8% to 5.0%, and the average size of MgO–TiN–MnS increases from 2.82 to 5.49 μm, while the number density that can induce intragranular acicular ferrite (IAF) nucleation decreases from 87 to 62 mm−2. These changes in inclusions are detrimental to induce the IAF nucleation. The HAZ microstructures are composed of the IAFs, polygonal ferrites, and upper bainites (UBs). With increasing the Ti/N ratio, the area fraction of the IAFs decreases from 44% to 14%, and the area fraction of UBs increases from 1.8% to 9.5%. The HAZ fracture mode changes from ductile fracture to brittle fracture after the high heat input welding of 400 kJ cm−1, and the HAZ toughness decreases from 183.3 to 48.7 J at −20 °C.

Microstructural evolutions and impact toughness in simulated welding heat affected zones for a high-strength carbide-free bainitic rail steel

Systematic experimental investigations were conducted to study the microstructures and impact toughness of each heat affected zone (HAZ) formed during rail flash-butt welding. A high-strength carbide-free bainitic rail steel was subjected to different thermal simulation cycles to separately reproduce each HAZ subzone by tailoring the peak temperature (PT) with respect to 700, 850, 920, 1000 and 1350 °C, and hence to generate the corresponding microstructures by using Gleeble-3500 simulator. Results show that the HAZ subzones exhibit complicated microstructures depending on the PTs, and with increasing PT the dominant bainitic microstructure type evolves from polygonal bainitic ferrite (700 °C) to a mixture of fine bainitic ferrite and granular bainite (850–1000 °C), and finally to coarse bainitic ferrite and granular bainite (1350 °C). Impact tests demonstrate that the impact toughness initially increases significantly as the PT reaches 920 °C (i.e., fine-grained HAZ), beyond which the impact toughness starts to decrease. The fine-grained HAZ displays optimal impact toughness in HAZs, yet which is lower than the base metal. Moreover, the morphology and distribution of martensite-austenite (M-A) constituents is strongly dependent on the welding PT, and the high fraction blocky and coarse slender M-A constituents is considered to be detrimental for the impact toughness.

Charpy absorbed energy in simulated heat-affected zone of laser-arc hybrid welded joints by high-strength steel for bridge structures

The application of laser-arc hybrid welding (so-called, hybrid welding) to the fabrication of steel bridge members has recently been investigated. One-pass full-penetration butt joints of steels for bridge high-performance structure (SBHS400 and SBHS500) with a thickness of 15 mm were performed by hybrid welding. The sound butt joints by hybrid welding were confirmed by a series of tests. The Charpy impact test was performed on the test pieces extracted from the hybrid welded butt joints with specified test temperatures. A phenomenon known as fracture path deviation (FPD) occurred in most test pieces, due to a large variation in material properties of the heat-affected zone (HAZ), resulting in the difficulty of estimating the toughness of HAZ in hybrid welded joints. Therefore, the Charpy impact test was conducted on the test pieces subjected to the welding thermal cycle tests of hybrid welding, which can exclude the heterogeneity of material properties and obtain the Charpy absorbed energy of the HAZ with high accuracy. The test results indicated that FPD was not observed in all thermal cycle simulated test pieces because the uniform metallographic structures in the vicinity of the notch were formed by the simulated thermal cycle tests, and all thermal cycle simulated test pieces satisfied 47 J at the specified test temperatures, a value that prevents brittle fracture for SBHS. Besides, for investigating the effect of the high Charpy absorbed energy guaranteed by SBHS on the toughness of hybrid welded joints, the Charpy absorbed energy of the thermal cycle simulated test pieces of SBHS and those of conventional steel (SM400B) were compared. The results showed that some of the thermal cycle simulated test pieces of SM400B failed to satisfy 27 J, suggesting that SBHS may ensure a Charpy absorbed energy of 47 J or more in the HAZ of hybrid welded joints.

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.

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.

Weldability evaluation of X80 sour line pipe using a combination of thermal simulation and actual girth welding

The weldability studies of API 5LX80 pipes developed for sour service are very recent and limited information about its heat-affected zone (HAZ) properties can be found. This paper presents a comprehensive evaluation of HAZ using both thermal simulation and the production of actual girth weld joints. The test methodology included microstructural analysis, Charpy impact test, hardness (HV10), microhardness (HV0.1), and sulfide stress cracking tests (SCC). The intercritical coarse grain HAZ presented impact toughness reduction from values above 300 J to less than 50 J at 0 °C when heat input is increased above 1.5 kJ/mm. It can be explained by grain coarsening and martensite-austenite (MA) formation as a ‘necklace’ structure at previous austenite grain boundaries. Hardness and microhardness values for both simulated and actual HAZ were below the maximum limit of 250HV usually specified by industry. Specimens submitted to the sulfide stress cracking (SSC) tests did not present any crack demonstrating that the welded joint is appropriate for sour environments. Finally, a maximum heat input of 1.5 kJ/mm is recommended for field welding. For higher heat inputs, a deeper evaluation of HAZ toughness should be done. The results contribute to supporting the application of this material as an alternative for long-distance high-pressure pipelines with H2S.

Effect of welding heat input on microstructural evolution and impact toughness of the simulated coarse-grained heat-affected zone of Q960 steel

Effect of welding heat input on microstructural evolution and impact toughness of the simulated coarse-grained heat-affected zone of Q960 steel

Effect of Si Content on the Microstructures and Toughness in Heat‐Affected Zone of Offshore Engineering Steels with Mg Deoxidation

Herein, the effect of Si content on the microstructures and toughness in the heat‐affected zone (HAZ) with the heat input of 100 kJ cm−1 for Mg‐deoxidized offshore engineering steels is presented. For the matrix, increasing Si content coarsens the lath bainites (LBs) in HAZ due to the higher Ar3 and lower Ar1. Thus, the high‐angled grain boundary density (HAGBD) decreases from 0.88 to 0.53 μm−1, and the effective grain size increases from 10.6 to 18.3 μm. M–A constituents are transformed from dotted and filmed ones to the stringer and blocky ones, with the area fraction increasing from 2.9% to 5.6%. Among them, the stringer M–A particles are likely to debond from the matrix, while the blocky M–A particles tend to initiate cracks because the interfacial energy of stringer particles is lower than that of blocky ones. Accordingly, increasing Si content from 0.0003 to 0.120 wt% results in both the increased crack initiation probability by more M–A constituents with the larger size and the lowered crack propagation barrier by the smaller HAGBD, thereby reducing the HAZ toughness at −40 °C from 202 to 58 J.

Effect of Ca Deoxidation on Toughening of Heat-Affected Zone in High-Strength Low-Alloy Steels after Large-Heat-Input Welding

Large-heat-input welding can effectively increase the efficiency and reduce the cost of manufacturing a super-large container ship for marine trade worldwide with thick, high-strength low-alloy (HSLA) steel plates; however, it significantly degrades the toughness of the welding heat-affected zone (HAZ). This paper describes the effect of Ca deoxidation on the impact toughness of simulated coarse-grained HAZs (CGHAZs) in HSLA steels after large-heat-input welding at 400 kJ cm−1. The average impact energy of the CGHAZ increases with an increase in Ca content; in particular, the energy of the steel with 25 ppm Ca content is satisfactorily high, owing to the uniform and fine prior austenite grains. In contrast, the grains in the CGHAZs of the steels with relatively low Ca contents are not uniform, leading to large test variabilities at −20 °C. Failure analyses reveal that the major and secondary cracks preferentially propagate along the coarse and brittle grain boundary ferrite (GBF), but their propagation is retarded by the fine and interlocking intragranular acicular ferrite (IAF) as the grain size decreases. It is therefore recommended to increase the Ca content to about 25 ppm during the deoxidation of steelmaking to improve HAZ toughness by forming fine and uniform prior austenite grains and IAF within grains.

Microstructural Effects on J‐Integral Fracture Toughness of Welded High‐Mn Steels at 298 and 77 K

J‐integral fracture toughness tests are conducted on welded high‐Mn steels at room and cryogenic temperatures. The specimens are categorized into weld metal (WM) and heat‐affected zone (HAZ) to consider the different microstructural effects. The microstructure of WM is composed of dendrites formed during solidification, while the microstructure of HAZ consists of coarsened grains from the welding process. Both specimens show twinning behavior after J‐integral fracture toughness tests at 298 K and the superior K JIc fracture toughness of WM (389 MPa√m) due to the formation of plentiful twins at the crack tip. At 77 K, K JIc fracture toughness of HAZ (221 MPa√m) is lower than that of WM (264 MPa√m) due to different stacking fault energies (SFEs). Twinning still prevails in WM at 77 K, while ε‐martensite is formed in HAZ by relatively low SFE. The interface between γ‐matrix and ε‐martensite can act as a crack initiation site during the fracture toughness test, leading to the deterioration of J‐integral fracture toughness in HAZ.

Effect of Microstructure on Impact Toughness and Fatigue Performance in Coarse-Grained Heat-Affected Zone of Bainitic Steel Welds

Effect of Microstructure on Impact Toughness and Fatigue Performance in Coarse-Grained Heat-Affected Zone of Bainitic Steel Welds

Investigation on microstructure and impact toughness of simulated heat affected zone of high strength low alloy steels by laser-arc hybrid welding

The microstructure and impact toughness of the simulated heat affected zone (HAZ) of a high strength low alloy steels by laser-arc hybrid welding (LAHW) were investigated in this paper. Homogeneous simulated HAZ specimens with different grain sizes were prepared using the welding thermal simulating technique. Instrumented impact test was conducted to investigate the toughness of the simulated HAZ. Multi-scale sub-structure characterization of the simulated HAZ specimens was implemented by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD). The instrumented impact test found that the peak temperature T M mainly affected the crack propagation process, the crack propagation energy E p decreased as the increase of T M , while the crack initiation energy E i barely changed, the decrease of crack stable energy E a led to the change of E p . The multi-scale sub-structure characterization showed that the prior austenite grain size (PAGS), packet width, block width both increased with the rise of T M . The block width was similar to the facet size in the crack unstable propagation zone of the simulated HAZ specimens. The block was the microstructure unit controlling the crack propagation process of the LAHW simulated HAZ specimens.

Toughening and Hardening Limited Zone of High-Strength Steel through Geometrically Necessary Dislocation When Exposed to Electropulsing.

The enhancement of both low-temperature impact toughness and the hardness of a high strength steel heat-affected zone (HAZ) is investigated by using high-density electropulsing (EP). The athermal and thermal effects of EP on HAZ microstructure and resultant mechanical properties were examined based on physical metallurgy by electron backscattered diffraction and on tests of hardness and impact toughness at −60 °C, respectively. EP parameters were carefully determined to avoid electro-contraction and excessive pollution of the base metal by using numerical simulation. The EP results show that the mean impact toughness and hardness of HAZ are 2.1 times and 1.4 times improved, respectively. In addition to the contribution of microstructure evolution, geometrically necessary dislocation (GND) is also a contributor with an increase of 1.5 times, against the slight decrease in dislocation line density and dislocation density. The mechanisms behind this selective evolution of dislocation components were correlated with the localized thermal cycle EP, i.e., the competition among thermo- and electro-plasticity, and work-hardening due to local thermal expansion. The selective evolution enables the local thermal cycle EP tailor the martensitic substructure that is most favorable for toughness and less for hardness. This selective span was limited within 4 mm for a 5 mm thick sample. The local thermal cycle EP is confirmed to be capable of enhancing in both toughness and hardness within a millimeter-scale region.

Failure analysis on weld joint of centrifugal pump diffuser for oil and gas pipeline transportation

The failure mechanism of weld joint in centrifugal pump diffuser was analyzed through scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron back-scattering diffraction (EBSD). Meanwhile, the strength and toughness of weld joint, heat-affected zone (HAZ), and matrix are quantitatively compared by geometrically necessary dislocation density (ρGND). The result indicated that the strength and toughness of HAZ are the highest, followed by matrix, and weld joint is the lowest. Defects such as slag inclusion and residual austenite create initial conditions for the corrosion of weld joint. Severe chloride ion corrosion resulted in micro-cracks on the weld surface. Meanwhile, corrosion reduces the surface energy of the micro-cracks. Therefore, the concentrated driving force makes the micro-cracks with reduced surface energy spread rapidly, eventually causing the rapid failure of diffuser weld joint. This study may provide a reference for petroleum engineers and centrifugal pump manufacturers.

Influence of ICCGHAZ on the Low-Temperature Toughness in HAZ of Heavy-Wall X80 Pipeline Steel

Low-temperature embrittlement in the heat-affected zone (HAZ) of heavy-wall X80 weld joints is a primary challenge for arctic oil & gas exploitation. In this paper, the influence of intercritically reheated coarse-grained HAZ (ICCGHAZ) on the low-temperature toughness of the weld joint in a 22 mm thick X80 spiral submerged arc welded pipe was studied through instrumented Charpy V-notch impact test at −80~20 °C and corresponding fracture surface characterization. The results indicated that the influence of ICCGHAZ on the overall toughness of the weld joint is related to temperature. At temperatures below −45 °C, individual and tiny martensite-austenite (MA) constituent debonding can trigger cleavage fracture–which was proved to be nucleation-controlled–and the probability of embrittlement of the ICCGHAZ increases. At temperatures higher than −45 °C, only relatively large or closely distributed MA constituent in ICCGHAZ satisfies the conditions to trigger propagation-controlled cleavage fractures, and the influence of ICCGHAZ on the overall toughness is not remarkable.

Effect of alloy element on microstructure and properties of heat-affected zone

In the present study, the influences of general composition (GC steel), magnesium deoxidation treatment (MT steel) and magnesium deoxidation treatment with high vanadium (MVT steel) content on the inclusions, microstructure and low-temperature toughness of the welding heat affected zone of EH36 steel were studied. The thermal cycle process of the heat-affected zone (HAZ) was simulated by a thermal simulator, and then microstructure and impact properties were tested and analyzed. Obtained results show that Magnesium deoxidation treatment and magnesium deoxidation treatment with higher vanadium content decrease the number and average size of inclusions. Compared with GC steel, the austenite grain size of MT and MVT steel reduces at high temperatures so that the nucleation temperature of acicular ferrite in HAZ microstructure increases. Compared with GC steel, the low-temperature impact toughness of HAZ by MT steel and MVT steel increases.

Effect of Heat-Input on Microstructure and Toughness of CGHAZ in a High-Nb-Content Microalloyed HSLA Steel.

The effect of various heat inputs on the microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAS) of a niobium microalloyed (0.14 wt.%) low-carbon steel was studied. The results showed that higher impact toughness was achieved at a low heat input of 20 kJ/cm, which resulted from the formation of acicular ferrite laths/plates. They sectioned large prior austenite grains into many smaller regions, resulting in smaller crystallographic grains and high-angle grain boundaries. Conversely, when specimens were simulated with larger heat-inputs (100, 200 kJ/cm), the microstructure of the CGHAZ was predominantly composed of granular bainite plus massive MA constituents, thus impairing the impact toughness.