Coarse-grained Heat-affected Zone Research Articles

Effect of Welding Heat Input on the Mechanical Properties of Mild Steel at Coarse Grain Heat Affected Zone (CGHAZ)

Effect of Welding Heat Input on the Mechanical Properties of Mild Steel at Coarse Grain Heat Affected Zone (CGHAZ)

Galvanic Corrosion Behavior of the X80 Steel Welded Joint

Wire beam electrode techniques and classical electrochemical techniques were used to investigate the effect of the area ratio of each part of the welded joint on the galvanic corrosion behavior. The results showed that the order of the corrosion current density of the different regions in the simulated X80 steel welded joint was as follows: coarse-grained heat-affected zone > fine-grained heat-affected zone > intercritical heat-affected zone > base metal > weld metal. As the area ratio of weld metal increased, the galvanic potential shifted positively, the maximum anode galvanic current density increased and the main anode galvanic effect increased. On the other hand, as the area ratio of the base metal and the heat-affected zone decreased, the coupled potential shifted negatively and the maximum anode galvanic current density decreased. The galvanic corrosion intensity of the simulated X80 steel welded joint increased as the area ratio of the weld metal decreased and increased as the area ratio of the heat-affected zone and base metal increased. To enhance corrosion resistance, it is advisable to choose a shape with a larger groove to increase the WM area in the welded joint. Additionally, selecting a welding method with lower heat input and a higher energy density can help reduce the HAZ area in the welded joint.

Physical simulation of the underclad heat affected zone in a reactor pressure vessel to study intergranular cracking

Underclad cracking in nuclear pressure vessels was of significant concern in the 1970s and 1980s before mitigating adjustments were made both to steel compositions and manufacturing practice. Unfortunately, the cracking mechanisms are still not well understood, and this can undermine confidence when changes to cladding operations are under consideration. In this work, Charpy-sized test coupons of 18MND5 steel were subject to a range of thermal cycles that can be experienced by the substrate immediately adjacent to the interface with the overlay. The Charpy test results are considered in combination with analysis of the samples through field emission gun (FEG) scanning electron microscopy (SEM) and wavelength dispersive X-ray spectroscopy (WDXS). The findings suggest that the heat affected zone subject to the coarse grained plus intercritical thermal cycle, before post weld heat treatment, has a peculiar and potentially brittle microstructure. However, in a steel with no obvious macrosegregation, no clear evidence of intergranular cracking in the early stages of the post weld heat treatment could be found. The second part of the work focuses on the effects of segregation regions in large forgings, more specifically ghost lines, on the tendency for underclad cracking. Ghost line regions that subsequently form a coarse grained heat affected zone (CGHAZ), and are then heated to just under the A1 temperature, seem to be the most prone to intergranular cracking. Cracking susceptibility appears to be associated with elevated concentrations of alloying and impurity elements at grain boundaries.

Effect of Wire Preheat and Feed Rate in X80 Steel Laser Root Welds: Part 1 — Microstructure

Laser welding with cold versus hot wire feed was employed as a root pass to weld X80 pipeline steel. The influences of wire feed rate and preheat on the fusion zone microstructure were investigated. Increasing the wire feed rate helped generate acicular ferrite in the weld metal, and preheating the wire further suppressed the formation of bainite. The acicular ferrite in the upper region of the fusion zone was finer than that in the lower region, which was due to an increase in nucleation sites available. Five fill and cap passes were applied by gas metal arc welding to fill the remaining top part of the groove. Compared to arc welding with a higher heat input, laser welding led to finer prior austenite grains and smaller bainite packet size in the coarse-grained heat-affected zone and limited the formation of martensite-austenite constituents in the intercritically reheated coarse-grained heat-affected zone.

Microstructure and Low-Temperature Toughness of Reheated Coarse-Grained Heat-Affected Zone of 06Ni9DR Steel

Microstructure and Low-Temperature Toughness of Reheated Coarse-Grained Heat-Affected Zone of 06Ni9DR Steel

Effect of welding state on the re-precipitation behavior of Cu-rich and NiAl nanoparticles in HAZ of 1100 MPa grade low carbon ultra-high strength steel

The influence of multi-layer multi-pass welding and post-weld heat treatment (PWHT) on the co-precipitation behavior of Cu-rich and B2–NiAl, microstructural characteristics, grain size and mechanical properties of the coarse grain heat affected zone (CGHAZ) were systematically investigated for low-carbon ultra-high-strength steel with different Ni/Cu ratio. The results show that the CGHAZ microstructure of both high (1.5Cu) and low (2.0Cu) Ni/Cu ratio steel are composed of LB and GB, the grain size increases from 10.1 ± 1.1 μm and 9.5 ± 0.7 μm to 32.1 ± 1.0 μm and 32.2 ± 2.7 μm, and the nanoparticles are dissolved and subsequently re-precipitated during the welding process, with an average radius 1.56 nm and 1.71 nm, respectively. The hardness profile of the joint shows softening at CGHAZ and the tensile strength reaches 1218 MPa and 1056 MPa respectively. After PWHT, the re-precipitated behavior of the nanoparticles was enhanced, with average radius of 1.53 nm and 1.55 nm, and the number density increases from 1.55 × 1023 m−3 and 1.06 × 1023 m−3 to 4.01 × 1023 m−3 and 3.33 × 1023 m−3, respectively. The co-precipitation evolution mechanism shows that the typical core-shell structure is transformed into individual 9 R–Cu and B2–NiAl particles, and a large number of small-sized B2–NiAl particles were re-precipitated during the aging process. The strengthening mechanism shows that martensite strengthening and grain size strengthening are weakened in the as-welded CGHAZ, and only part of the nanoparticles was re-precipitated leading to the softening of CGHAZ during welding. After PWHT, a large number of small-sized nanoparticles were re-precipitated to restore the contribution of precipitation strengthening, while small-sized nanoparticles interacted with dislocations and entangled near the M-A constituents to promote the formation of microvoids and microcracks, thus seriously deteriorating the ductility.

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.

Characterization of As-Welded Microstructure in a P91 Steel

The microstructure of the heat affected zone (HAZ) and fusion zone (FZ) in the as-welded condition of a single-pass weld performed by the flux-cored arc welding (FCAW) process was studied in a P91 steel using microhardness measurements, field-emission scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The evolution of precipitates in each region of the single-pass weld was analyzed, and particular attention was paid to the presence of retained austenite (RA). Experimental observations indicated that M23 C6 carbide played a major role in the thermal cycle associated with the weldment. On one hand, the dissolution of M23 C6 led to the precipitation of spherical NbCN in the finegrained HAZ (FGHAZ) near the coarse-grained HAZ (CGHAZ). On the other hand, the total or partial dissolution of M23 C6 carbides changed the chemical composition of the initially formed austenite. In the regions where the M23 C6 carbides were completely dissolved (i.e., the CGHAZ and FZ), retained austenite was observed as a thin, continuous film along primary austenite grains and lath boundaries. Instead, a shell of retained austenite was observed around some partially dissolved M23 C6 of the intercritical HAZ (ICHAZ) and FGHAZ.

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

Influence of Boron on Stress-Relief Cracking Susceptibility of T23 Steel

The effect of boron on stress-relief cracking (SRC) sensitivity in the coarse grain heat-affected zone (CGHAZ) of ASME SA213-T23 was studied by thermo-mechanical simulation. Then, the fracture mode, microstructure, carbide evolution, and boron segregation were observed by an optical microscope, a scanning electron microscope, a transmission electron microscope, and an electron probe microanalyzer. Finally, the mechanism of increasing boron content to improve SRC resistance was described. The results showed that when the boron content is lower than 0.0038 wt.-%, T23 steel is sensitive to SRC at 550–750°C (1022–1382°F), and the sensitive temperature range narrows as the boron content increases. When the boron content increases to 0.010 wt.-%, SRC can be eliminated. Moreover, boron addition did not improve grain boundary (GB) strength nor did it fundamentally change the fracture mode at high temperatures, but it significantly improved intergranular ductility. This is because the boron segregation at the GB inhibits the precipitation of M23C6 carbides, which reduces the void nucleation and alloy element depletion near the GB, thus significantly improving intergranular plasticity. The improvement of intergranular plasticity gives the grain sufficient time to deform and greatly improves the overall plasticity of the CGHAZ. As a result, the SRC resistance of the welded joint is significantly improved because the stress can be released through enough plastic deformation during postweld heat treatment or service.

Enhancing Microstructural and Mechanical Properties of Ferrous Medium-Entropy Alloy through Cu Addition and Post-Weld Heat Treatment in Gas Tungsten Arc Welding.

This study investigates the impact of a high-entropy alloy filler metal coated with copper (Cu) and post-weld heat treatment (PWHT) on the weldability of a ferrous medium-entropy alloy (MEA) in gas tungsten arc welding. The addition of 1-at% Cu had an insignificant effect on the microstructural behaviour, despite a positive mixing enthalpy with other elements. It was observed that a small amount of Cu was insufficient to induce phase separation into the Cu-rich phase and refine the microstructure of the as-welded specimen. However, with an increase in the PWHT temperature, the tensile strength remained mostly consistent, while the elongation significantly increased (elongation of as welded, PWHT700, PWHT800, and PWHT 900 were 19, 43, 55 and 68%, respectively). Notably, the PWHT temperature of 900 °C yielded the most desirable results by shifting the fracture location from the coarse-grained heat-affected zone (CGHAZ) to base metal (BM). This was due to significant recrystallisation and homogenised hardness of the cold-rolled BM during PWHT. However, the CGHAZ with coarse grains induced by the welding heat input remained invariant during the PWHT. This study proposes a viable PHWT temperature (900 °C) for enhancing the weldability of cold-rolled ferrous MEA without additional process.

Microstructural study of microwave welded austenitic stainless steel

The joining of austenitic stainless steels through microwave hybrid heating has gained much attention due to its advantages, like uniform and volumetric heating. However, to establish a microwave joining process for promising applications, a microstructural study of the welded joint is still an area to explore. In the present study, microstructural analysis of austenitic stainless steel (SS304) joints is explained through optical microscopy, scanning electron microscopy (SEM), elemental mapping, X-ray diffraction (XRD) and Vickers' micro-hardness tester. Further, detailed microstructural analysis of interlayer and heat-affected zone (HAZ) is studied through electron back-scattered diffraction (EBSD). XRD results reveal the development of various Ni, Cr and the M23C6 type carbides at the joint interlayer region. SEM results point toward non-epitaxial grain evolution along the fusion zone. The mean microhardness of 350 ± 20 HV was determined at the joint interlayer region. A relatively fine grain structure was observed along the fusion boundary. However, moving away from the joint, two different forms of coarse grain heat-affected zone (CGHAZ) were observed. The presence of two different forms of CGHAZ is confirmed through inverse pole figure maps and grain size plots. The presence of high-angle boundaries (HABs) at 60° rotation describes the twin boundaries characterized along the <111> axis. The existence of improved fractions of HABs in the microwave-joined specimen of SS will favor the microstructure to retard bulk deformation. The joint specimen endures a maximum force of 6.98 kN, and the ultimate tensile strength of 613 MPa with 4.75% of elongation. It is observed that microhardness observations and the microstructural studies in the research help establish a solid microstructure-property correlation for the joined material.

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).

Effect of Mo microalloying on impact toughness of X80 pipeline steel in SCGHAZ

The microstructure and impact toughness of subcritically reheated coarse-grained heat-affected zone (SCGHAZ) of X80 pipeline steels with different Mo content (0Mo and 0.11 wt%Mo) were investigated. Gleeble 3500 thermal simulation experiment was used to simulate the heat-affected zone (HAZ) of X80 pipeline circumferential weld, and the two samples were characterized by a series of material testing techniques such as SEM, TEM, EBSD and XRD. The research showed that: Appropriate addition of Mo element can improve the hardenability of the matrix, increase the content of bainitic ferrite (BF) in the microstructure, refine the BF lath, cause plastic deformation, and reduce local stress concentration. The increase of BF content greatly increases the proportion of high angle grain boundary (HAGB) in order to effectively inhibit crack propagation. On the other hand, Mo element influences precipitation with solid solution, reduces the early energy barrier for nucleation, delays the disappearance of dislocation lines at high temperature, forms high-density dislocation grid, so that it can increase the nucleation site for NbC precipitation, prevent the coarsing of NbC, increases the threshold of external stress required for the generation of micropores induced by precipitation, and hinder crack initiation. The impact toughness of SCGHAZ was significantly improved by adjusting the microstructure and second phase morphology of HAZ by using Mo element microalloying.

Microstructure and performance of NiCrBSi coatings prepared by modulated arc currents using powder plasma transferred arc welding technology

This study examines the impact of varying plasma arc currents 70 A, 80 A, 90 A, and 100 A on NiCrBSi coatings applied to 15HM steel substrates using powder plasma transferred arc welding (PPTAW). Finite element analysis identifies two thermal cycle zones: the coating middle and coating/substrate transition. Microstructural analysis reveals dendritic structures in all samples, primarily rich in Cr. At the transition zone, various solidification modes are observed, accompanied by a coarse-grained heat-affected zone and minor macrosegregation. Energy-dispersive X-ray spectroscopy (EDS) confirms metallurgical bonding, while electron backscatter diffraction (EBSD) and X-ray diffraction indicate grain refinement as current decreases, with γ-Ni, Cr23C6, and Cr3C2 strengthening phases. PTA current of 70 A emerged as the optimal fabrication current within the tested range of currents, producing coatings exhibiting superior properties over the others. The results show improved corrosion resistance and tribological properties as current drops from 100 A to 70 A. Notably, corrosion current density decreases from 23.6 µA/cm2 to 12.8 µA/cm2, and average microhardness increases from 488 HV to 832 HV, reducing wear rate from 2.9 × 10−2 m3/m to 1.8 × 10−2 m3/m.

Microstructure and mechanical behaviour study of the dissimilar weldment of ‘IN82 buttered’ P92 steel and AISI 304L steel for ultra super critical power plants

This study examines the influence of the ERNiCr-3 (Inconel 82) butter layer on the microstructure and mechanical feature of the dissimilar weld joint (DWJ) of ferritic grade P92 and austenitic grade AISI 304 L steel, manufactured using gas tungsten arc welding (GTAW) method with IN82 filler. The butter layer aims to prevent the formation of a hard martensitic layer adjacent to the fusion line by mitigating the carbon migration from the P92 steel to the welding pool. The optical microscope, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron probe microanalyzer (EPMA) were used to examine the microstructures of the buttered welded plate. The mechanical characteristics of the joints were assessed using the standard and sub-sized tensile and Charpy tests. The flat and round specimens underwent the tensile test to determine their relative tensile properties at room temperature and high temperature. The high-temperature tensile properties were assessed at 600 °C and 650 °C at 1 mm/min extension rates. Along the weldments, hardness profilometry was also performed. Microstructural analysis validated that the application of a buttering layer successfully eliminated the extensive martensitic layer on the P92 side. Nevertheless, in the vicinity of the interface between the buttering layer and P92, there was evidence of a narrow unmixed zone and macrosegregation. SEM/EDS and EPMA studies confirmed that the predominant phases identified in IN82 butter and weld metal are NbC and TiC. The butter layer, overall, improved elongation/ductility by 25 % without a noticeable reduction in tensile strength. During tensile testing, the specimen fractured from the IN82 butter layer, revealing the presence of dimples and voids on the fracture surface, along with the TiC/NbC phases, as confirmed by SEM analysis. Tensile specimens subjected to high-temperature testing exhibited failure at the base materials of AISI 304 L (tensile strength=404 MPa) and P92 (307 MPa), specifically at 550 °C and 650 °C, respectively, rather than at the buttering layer or weld metal. The results of the tensile tests met the minimum criteria specified for Ultra-Supercritical (USC) boilers. More importantly, the IN82 butter layer successfully serves as a protective barrier between the weld pool and the P92 HAZ. As a result, the dual hardness peaks on either side of the welding pool are eliminated. After adding the buttering layer, the peak hardness in the coarse-grained heat-affected zone (HAZ) of P92, which ranged from 450 to 500 HV in the unbuttered sample, dropped by 200 MPa. The relationships between microstructural characteristics and mechanical properties have been thoroughly examined and discussed.

Effect of PWHT on metallurgical and mechanical characterization of dissimilar welded joint of P91 and P92 steels

P91 and P92 steels are widely used in nuclear and thermal power plant’s high-temperature (650–700 °C) operating components. The welding procedure, base plate composition, filler material composition, and post-weld heat treatment (PWHT) significantly affect the metallurgical characterization of weldments at room temperature. This work focuses on joining P91 and P92 steel by the TIG welding with filler ER90S-B9. The study analyzes the materialization of coarse-grained HAZ (CGHAZ), inter-critical HAZ (ICHAZ), and fine grained HAZ (FGHAZ) under welded and PWHT. PWHT was conducted at 840 °C for 2, 4 and 6h. An optical microscopic (OM) scanning electron microscope (SEM) was employed to characterize the welded and PWHT samples. It was perceived that the tensile strength of PWHT increased as the heat treatment times increased from 2 to 6 h, while the hardness value decreased accordingly. The maximum tensile strength (594.71 MPa) was observed in PWHT_6 h, while the minimum tensile strength (537.52 MPa) was perceived in the as-welded sample. The PWHT_6h reveals the minimum hardness values (271.12 HV) at the fusion zone among all the conditions. The WFZ consisted of an untampered lath martensitic grain structure, demonstrating an average hardness value of 314.15 HV in the PWHT_2h condition. The hardness value in CGHAZ samples for as-welded, PWHT_2h, PWHT_4h, and PWHT_6h was observed at 456.85, 436.81, 368.63, and 327.41 HV, respectively.

Thermal Simulation and Fracture Resistance Evaluation of Coarse-Grain Heat-Affected Zone in Welds of Q355D Steel

Thermal Simulation and Fracture Resistance Evaluation of Coarse-Grain Heat-Affected Zone in Welds of Q355D Steel

Effect of hydrogen on impact fracture of X80 steel weld: Various heat inputs and coarse grain heat-affected zone

In this work, the impact fracture of coarse grain heat-affected zone (CGHAZ) of an X80 steel weld with various heat inputs was investigated in the absence and presence of hydrogen (H)-charging by metallographic characterization, microhardness and impact testing, electrochemical H permeation measurements. The results show that, as the heat input (HI) during welding decreases, the microstructure of the CGHAZ changes from granular bainite (GB) to lath bainite (LB). When the LB content exceeds 70%, the microhardness of the material increases remarkably. For H-free specimens, the fracture surface transforms from ductile to brittle fracture at the HI of 29.2 kJ/cm. Upon H-charging, the fracture surfaces show brittle transgranular cleavage fracture when the HI is 18.1 kJ/cm. The H atoms decrease the threshold HI where a brittle fracture occurs. Compared with the GB microphase, the LB-containing microstructure requires a higher H concentration to cause transgranular impact brittle fracture.

Influence of boron segregation on the hot ductility of T23 steel CGHAZ

Boron segregates in a non-equilibrium segregation mode at lath, packet, block boundaries as well as prior austenite grain boundaries (PAGBs) in 2.25Cr1.6WVNb steel coarse grain heat-affected zone (CGHAZ) during the thermal cycle, and tends to segregate only at PAGBs in the same mode during the high-temperature tensile test. Boron segregation along PAGB increases GB plasticity but not inter-granular strength. This is because the boron segregation at PAGBs occupies the vacancies and other defect positions, which inhibits the nucleation and growth of micro-voids, and reduces plasticity consumption. As a result, the hot ductility of CGHAZ is enhanced.