Intercritically Reheated Coarse-grained Heat-affected Zone Research Articles

Coupled effect of microstructure heterogeneity and hydrogen on local embrittlement of CGHAZ and IC-CGHAZ in X65 pipeline steel

The local embrittlement of the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated CGHAZ (IC-CGHAZ) in X65 pipeline steel was investigated using an in situ crack-tip opening displacement test in air and H2S, combined with microstructure-based simulation. The IC-CGHAZ exhibited significantly lower cracking resistance, with the fracture toughness reduced by 50 % compared to CGHAZ in both environments (0.160 mm in air and 0.017 mm in H₂S for IC-CGHAZ, compared to 0.307 mm in air and 0.034 mm in H₂S for CGHAZ). The martensite/austenite (M/A) constituents showed exceptionally higher intrinsic hardness than the ferrite matrix. The continuous distribution of M/As along the prior austenite grain boundaries resulted in local hardening and the formation of heterogeneous hard zones in IC-CGHAZ. Such microstructure heterogeneity resulted in the unique partitioning of plastic strain localization and stress triaxiality that promotes premature fracture, particularly with the coupled effect of hydrogen. This study provides a novel perspective on the fracture mechanisms and the typical fracture morphology of IC-CGHAZs in pipeline steel under the coupled effect of microstructure heterogeneity and hydrogen.

A new understanding of hydrogen-assisted cracking of coarse-grained heat-affected zone in X65 pipeline steel through {110} lath bainite boundary texture

The susceptibility of subzones of the coarse-grained heat-affected zone (CGHAZ) in X65 pipeline steel to hydrogen embrittlement was studied through an in situ crack-tip opening displacement test in an H2S-containing environment. The intercritically reheated CGHAZ (IC-CGHAZ) exhibited the highest embrittlement factor of 89.4%, compared to 64.0% for the sub-critically reheated CGHAZ (SC-CGHAZ). Hydrogen transportation by dislocations to massive-slender martensite/austenite (M/A) constituents initiated cracks in IC-CGHAZ. Deterioration of lath bainite (LB) boundaries, with separation of M/A-ferrite interface, accelerated crack propagation across prior austenite grains and promoted hydrogen-assisted degradation of fracture toughness. Reheated CGHAZ at a peak temperature of 620 °C led to an increased intensity of the {110} LB boundary texture by up to 52% in SC-CGHAZ. This enhanced texture facilitated dislocation slipping along {110} LB boundary planes, thereby promoting deformation compatibility and preventing the deterioration of LB boundaries in the presence of hydrogen.

Role of M-A constituents in bainitic microstructure on crack propagation behavior in the ICCGHAZ of HSLA steels for offshore applications

The objective of the study is to understand the effect of martensite-austenite (MA) constituents along with the neighbouring bainitic matrix and its effect on low-temperature impact toughness in the thermo-mechanically simulated inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of offshore High Strength Low Alloy (HSLA) steels. ICCGHAZ was simulated using a Gleeble 3500 thermomechanical simulator between the inter-critical temperature near Ac1 (lower ICT) and at a temperature 50 °C higher (upper ICT) for two different steels (steel A and steel B) with predominant microstructures composed of granular bainite (GB) and bainitic ferrite (BF), respectively. MA constituents were observed in the form of necklaces along prior austenitic grain boundaries (PAGB) of simulated ICCGHAZ for both steel A and steel B. Steel A combrised of discrete island MA constituents inside the GB grains and steel B consists of lath shaped MA constituents inside BF grains. Fractographical analysis showed that secondary cracks were initiated at the MA constituents along PAGB and propagated linearly into the GB/BF grains in the ICCGHAZ. The cracks initiated at the bulky MA at grain boundaries of steel A moved towards the pre-cracked or de-bonded discrete MA inside the grains and propagated through the interlinking of microcracks. In contrast, for steel B a notable percentage of cracks were initiated through the necklace-like MA at the grain boundaries and propagated through the BF lath. Pre-cracking of the slender MA present at the lath boundaries facilitated crack propagation in steel B. Furthermore, increasing the inter-critical temperature improved the impact toughness of steel A with a predominantly GB microstructure, whereas impact toughness improvement was limited in steel B with a BF microstructure.

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).

Microstructure and properties of intercritically reheated coarse-grained heat affected zone in X65 pipeline steel with pre-strain

The properties of the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) in X65 pipeline steel with large deformation are studied. The ICCGHAZ is composed of acicular ferrite, granular bainite and lath bainite. With the increasing t8/5 (the time for specimens to cool from 800 °C to 500 °C), the sulfide stress corrosion cracking (SSCC) resistance with and without pre-strain both increases, and the secondary cracks propagate along the necklace-type martensite-austenite (M-A)/matrix interface or near the M-A constituents, and the fracture toughness decrease. When t8/5 = 30 s, ICCGHAZ has the most ideal performance on fracture toughness and stress corrosion resistance.

Microstructural characterisation and micromechanical investigation of the heat-affected zone in multi-pass welding of 9Ni steel pipes

9Ni steels have been recently adopted in supercritical CO2 injection systems in deepwater oil fields. The manufacture of these reinjection systems involves multi-pass welding procedures, which produce an Heat-Affected Zone (HAZ) with a high heterogeneity level regarding the microstructural features and the local mechanical properties. An extensive microstructural and micromechanical characterisation was performed over the HAZ of three welded joints with different heat-input conditions to evaluate the effects of the reheating cycles and the welding parameters on the microconstituents. Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) analyses were performed to identify microstructural features that correlate to the local mechanical responses evaluated through an extensive microhardness mapping. Regarding the Coarse Grained HAZ (CGHAZ), the highest microhardness values for all welding conditions are found at the Supercritically Reheated CGHAZ (SCR-CGHAZ), characterised by its refined microstructure and a quite low area fraction of coarse martensite laths. The Subcritically Reheated CGHAZ (SC-CGHAZ) and the Intercritically Reheated CGHAZ (IC-CGHAZ) – regions where wider martensite blocks and higher coarse martensite lath area fractions were observed – composed the softer zones of the microhardness map. It was also found that reheating at intercritical temperatures induces the formation of supersaturated fresh martensite and may contribute to retained/reversed austenite particles’ C-enrichment, which may degrade the mechanical properties at the IC-CGHAZ.

Study on Corrosion Resistance and Hydrogen Permeation Behavior in Inter-Critically Reheated Coarse-Grained Heat-Affected Zone of X80 Pipeline Steel

We studied the effects of peak temperature and cooling rate in the secondary welding thermal cycles on the martensite/austenite (M/A) constituents’ characteristics (including fraction, average size and distribution), corrosion resistance and hydrogen permeation behaviors in the inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of X80 pipeline steel. We observed that the M/A constituents’ characteristics mainly depend on the secondary peak temperature and cooling rates, while the microstructure style and prior austenite grain size are dependent on the first peak temperature. In addition, the variations in the M/A constituents’ characteristics result in different corrosion resistance and hydrogen permeation behaviors by changing the micro-galvanic effect and the number of hydrogen trapping sites. The high fraction and coarse grain size of M/A constituents are against the corrosion resistance and hydrogen permeation in ICCGHAZs, and their functional relationships are established. Moreover, the effects of the fraction of M/A constituents on the corrosion resistance and hydrogen permeation behaviors are much greater than those of the average size.

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.

The role of copper in microstructure and toughness of intercritically reheated coarse grained heat affected zone in a high strength low alloy steel

This paper presents an investigation on the role of Cu addition in microstructure and toughness of intercritically reheated coarse grained heat affected zone (ICCGHAZ), aiming at elucidating the influencing mechanism of Cu addition on toughness in terms of reverted transformation, alloying element diffusion and variant selection. The element of Cu has a significant effect on reverted structure and Cu-rich precipitates by influencing the diffusion of alloying elements and variant selection. On the one hand, the addition of Cu changed the diffusion kinetics of Ni and Mn, decreasing the segregation of elements at grain boundaries. Accordingly, the reverted structure at grain boundary with an adverse effect on toughness was reduced. Moreover, Cu addition changed the feature of CP group during variant selection, and thus increased the high angle grain boundary density. On the other hand, Cu-rich precipitates were increased after Cu addition, which was easier to originate crack. But, the ductile character of the reverted structure would overwhelm the harmful effect of Cu-rich precipitates on the toughness. As a result, a higher low temperature toughness with impact energy of 72.8 J at −40 °C was achieved in 1.5Cu steel than that in 0Cu steel (36.3 J).

Studying the influence of the interpass temperature on the heat-affected zone of an API 5L X65 steel welded pipe joint through computational and physical simulations

Welding costs can be reduced by increasing the interpass temperature (IT). However, this can adversely impact the mechanical properties of the heat-affected zone (HAZ) of the welded joint; therefore, it is necessary to evaluate the influence of higher ITs on the HAZ. The influence of the IT on coarse-grained HAZ (CGHAZ) and intercritically reheated coarse-grained HAZ (ICCGHAZ) of an API 5 L X65 steel pipe joint welded with gas metal arc was investigated. A microstructure-predicting method (MPM) was built from numerical and thermodynamic simulations to examine the CGHAZ and ICCGHAZ. The results were validated using optical and scanning electron microscopy as well as dilatometry. Vickers microhardness and Charpy V-notch (CVN) impact tests were performed on physically simulated specimens. For all ITs considered herein, granular and lath bainite were observed in the CGHAZ; and bainite, ferrite, and Martensite–Austenite–Carbides (M-A-C) were observed in the ICCGHAZ. Increasing the IT did not significantly change the hardness of the CGHAZ and ICCGHAZ, but it decreased the CVN impact energy of these zones owing to presence of granular bainite and massive M–A. Contrary to that of the ICCGHAZ, the impact energy of the CGHAZ was greater than the DNVGL-ST-F101 standard specified limit (45 J). Based on the MPM and experimental results, it can be concluded that increasing the IT leads to embrittlement of the HAZ.

Microstructure and properties of intercritically reheated coarse-grained heat affected zone in pipeline steel after secondary thermal cycle

As a kind of high strength microalloyed steel, pipeline steel has high strength and toughness. However, the formation of the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) during the welding process is an important limitation affecting its performance. Herein, microstructural transformation in the ICCGHAZ of X100 pipeline steel after a secondary thermal cycle and hydrogen sulfide stress corrosion cracking (SSCC) resistance of the transformed microstructure are investigated. The microstructure of X100 pipeline steel after the secondary thermal cycle is similar to that after the primary thermal cycle, comprising lath bainite and granular bainite. With an increase in t8/5 (time required for the material to cool from 800 °C to 500 °C), the M-A constituents that continuously precipitate along the prior austenite grain boundaries in the second thermal cycle are coarsened and form a necklace-type structure. A variation in t8/5 does not significantly affect the crystallographic characteristics of the ICCGHAZ. The stress corrosion test shows that the resistance to SSCC decreases and cleavage fracture characteristics become more noticeable with an increase in t8/5.

Evolution of microstructure in reheated coarse-grained zone of G115 novel martensitic heat-resistant steel

Based on the thermal simulation method, a systematical analysis was conducted on the effect of welding peak temperature and the cooling time that takes place from 800 to 500 °C on microstructure, precipitates, substructure and microhardness of the reheated coarse-grained heat-affected zone (CGHAZ) of G115 novel martensitic heat-resistant steel. As revealed from the results, the microstructure of un-altered CGHAZ (UACGHAZ) and supercritically CGHAZ (SCCGHAZ) was lath martensite, and structural heredity occurred. Intercritically reheated CGHAZ (IRCGHAZ) exhibited martensite and over-tempered martensite, and subcritical CGHAZ (SCGHAZ) displayed martensite and under-tempered martensite. The austenite in UACGHAZ and SCCGHAZ was transformed with the diffusion mechanism during the first thermal cycle, but with the non-diffusion mechanism during the second thermal cycle. For this reason, Ac1 and Ac3 during the second thermal cycle were significantly lower than those during the first thermal cycle, and Ac1 and Ac3 were reduced by nearly 14 and 44 °C, respectively. Since the content and stability of the austenite alloy during the second thermal cycle of IRCGHAZ were lower than those during the first thermal cycle, Ms increased by nearly 30 °C. There were considerable precipitates in the over-tempered region of IRCGHAZ, and the Laves phase was contained, which was not conducive to high-temperature creep property. Moreover, the dislocation density and the number of sub-grains in the region were lower, resulting in a sharp decrease in the microhardness, and it was the weak area in the reheated CGHAZ.

Improved toughness of double-pass welding heat affected zone by fine Ti–Ca oxide inclusions for high-strength low-alloy steel

In order to improve the mechanical performance of double-pass welding heat affected zone, acicular ferrite (AF) induced by Ti–Ca oxide inclusions was used to enhance the toughness properties of the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ). The microstructural evolution and toughness mechanisms in the coarse-grained heat-affected zone (CGHAZ) and ICCGHAZ containing different oxide inclusions were studied by thermo-mechanical simulator, optical and electron microscopy, electron back-scattered diffraction and mechanical testing. The results indicated that CGHAZ toughness was decreased and Vickers hardness was increased compared with the base metal in Ti–Ca deoxidized and Al–Ca killed steels. However, the toughness loss in Al–Ca killed steel was more significant. In Ti–Ca steel, dominant Ti–Ca–O–Mn–S oxide inclusions of size between ~0.2 and 1.6 μm effectively promoted AF transformation. After the double-pass thermal cycle, AF plates were retained and the prior martensite/bainite structure in CGHAZ transformed into fine ferrite/bainite structure. As a result, the toughness of ICCGHAZ in Ti–Ca steel was significantly enhanced from ~94 J to ~209 J. In the Al–Ca steel, dominant Al–Ca oxide inclusions were ineffective for microstructure refinement and acted as a potential crack initiation site. The formation of coarse granular bainite and lath-like martensite/austenite (M/A) constituents led to deterioration of toughness in CGHAZ (~20 J) and further loss in ICCGHAZ (~13 J).

Microstructure and mechanical characterization of high strength low alloy steel welded joint by hybrid laser arc welding

Hybrid laser arc welding was developed to joint EQ70 steels. Welds with no visible defects were obtained. Investigations on microstructure and mechanical properties showed that: the welded metal (WM) consisted of acicular ferrite (AF), granular bainite (GB) and M-A constituents. The arc zone and laser zone possessed semblable HAZ microstructures. Coarse grained heat affected zone (CGHAZ) and fine grained HAZ (FGHAZ) were both composed of lath martensite (LM), GB and polygonal ferrite (PF). The intercritically HAZ (ICHAZ) consisted of martensite and M-A constituents. The microstructure of the subcritically HAZ (SCHAZ) was tempered sorbite. Partly cracked blocky M-A constituents were found at the grain boundary of intercritically reheated CGHAZ (ICCGHAZ). Some M-A constituents was slender and distributed in the grain of ICCGHAZ. The arc zone and laser zone had similar hardness and micro-shear strength distributions. The highest hardness and micro-shear strength of the backing and double-pass welded joints appeared near CGHAZs, reheated WM and ICCGHAZ. Slightly soften showed up at the SCHAZs. SCHAZ, CGHAZ and ICCGHAZ showed a reduction of micro-shear toughness. -40°C sub-size Charpy impact testing showed that FL specimens possessed the lowest impact values. The coarse CGHAZs and cracked blocky M-A constituents of ICCGHAZ may be the reason caused the toughness deterioration.

Effect of Si content on microstructure and cryogenic toughness of heat affected zone of low nickel steel

Compared with 9%Ni steel, low nickel steel has great advantages in cost and weldability. However, cryogenic toughness of the heat affected zone (HAZ) of conventional low nickel steel deteriorates, which limits its application in the construction of liquefied natural gas tanks. In this work, two low nickel steels with different Si contents were designed, and the relationship between Si content and HAZ cryogenic toughness of these steel samples was investigated. After reducing Si content, the required Charpy impact absorbed energy was obtained in the HAZ at fusion line and 1 mm from fusion line. Microstructural characterization revealed that the reduction in Si content promoted auto-tempering of coarse-grained HAZ (CGHAZ), leading to improved plastic deformation ability of the matrix. In intercritically reheated CGHAZ (ICCGHAZ), martensite-austenite (M-A) constituents were formed, which consisted of twin martensite and austenite. The decrease in Si content reduced the amount and size of M-A constituents and changed their elongated morphology to blocky morphology, which is also believed to be beneficial for ICCGHAZ cryogenic toughness. This work provides a novel way to soften the HAZ microstructure for improving its cryogenic toughness.

Effect of enhanced cooling on mechanical properties of a multipass welded martensitic steel

The effect of forced cooling using heat sinks on the mechanical properties and interpass waiting time of two-pass welds has been studied for a martensitic steel with a yield strength of 960 MPa when the interpass temperature was 100 °C. Cross-weld tensile and − 40 °C Charpy-V impact toughness properties were examined. The use of heat sinks is shown to result in a beneficial increase of the cross-weld yield strength but at the expense of the yield-to-tensile strength ratio. Due to its particularly detrimental effect on the heat-affected zone (HAZ) toughness of multipass welds, special attention was given in the Charpy-V toughness of the intercritically reheated coarse-grained HAZ (ICCGHAZ) by also testing simulated ICCGHAZs. It is shown that forced cooling has a beneficial effect in respect of the toughness of this simulated subzone and on the Charpy-V toughness of the HAZ of the actual welds. The interpass cooling time during the two-pass welding was reduced by 37%. The results indicate that, in the case of high-strength steels, it may be possible to simultaneously improve both welding productivity and mechanical properties by using forced cooling down to 100 °C to reduce waiting time between weld passes.

Effect of Martensite–Austenite Constituent on Low-Temperature Toughness in YS 500 MPa Grade Steel Welds

The effect of martensite–austenite (M–A) constituents and simulated microstructure on low-temperature toughness was investigated in YS 500 MPa grade structural steel welds. The specimens were fabricated using a direct quenching and tempering process. After simulated weld thermal cycles, the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (IRCGHAZ) were produced using a Gleeble tester and real welded joint to support the simulation results. The largest low-temperature toughness was observed in the fine-grained heat-affected zone (FGHAZ) owing to the fine-ferrite microstructure. However, the toughness decreased in the IRCGHAZ because of the slender morphology of the M–A constituents that formed primarily along the prior austenite grain boundaries in the IRCGHAZ.

Hydrogen Embrittlement Susceptibility and Safety Control of Reheated CGHAZ in X80 Welded Pipeline

Coarse-grained heat-affected zone (CGHAZ) exhibits the highest hydrogen embrittlement (HE) susceptibility, which changes under the influence of thermal cycle. In this study, slow strain rate tension (SSRT) tests were conducted to investigate the HE susceptibility of reheated CGHAZs and the critical hydrogen pressure for fracture failure. Results show that intercritically reheated CGHAZ (ICCGHAZ) possesses the lowest HE resistance. Analyses of HE index and fracture indicate that the critical hydrogen pressure is 3.5 MPa. Microstructure analysis reveals that HE susceptibility is associated with multiple factors, such as phase composition, grain coarsening, HAB density, and MA constituent. Blocky necklace-like MA constituent along prior austenite boundaries plays a predominant role in intensifying the HE susceptibility of ICCGHAZ.

Structure and crystallography of martensite–austenite constituent in the intercritically reheated coarse-grained heat affected zone of a high strength pipeline steel

The structure and crystallography of martensite-austenite (M-A) constituent in the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of X100 (690 MPa) pipeline steel weld joint was studied via multi-scale characterization. The results suggested that majority of the necklace-type M-A constituent in the ICCGHAZ preferred to form lamellar lath structure, which primarily consisted of lath martensite (87%). Only small fraction of retained austenite (9%) was found between martensite laths. The retained austenite had K-S orientation relationship with its neighboring martensite: (10-1)M // (11-1)γ and [111]M // [011]γ. Adjacent martensite laths within M-A constituent had large misorientation but no fixed crystallographic orientation relationship which implied that martensite laths may nucleate independently and encounter with each other during growth. M-A constituent and matrix microstructure belonged to different prior austenite grains. The martensite laths within the M-A constituent did not inherit the crystallographic orientation of the parent matrix during the intercritical reheating and subsequent cooling process of the second pass welding.

Effect of morphologies of martensite–austenite constituents on impact toughness in intercritically reheated coarse-grained heat-affected zone of HSLA steel

By using the gleeble-3500 simulator, three different two-pass weld thermal cycles were applied to base metal of a high strength low alloy (HSLA) steel with the purpose of obtaining martensite-austenite (MA) constituents of different morphologies in intercritically reheated coarse-grained heat-affected zones (ICGHAZ). Morphology of each MA constituent was characterized by maximum length, maximum width and aspect ratio (maximum length/ maximum width). Toughness of thermal simulated specimens was examined by using an instrumented Charpy impact tester. Behaviour of cracks was present by Charpy impact curve and typical data. Correlation between behaviour of cracks and morphologies of MA constituents was further analysed by observing crack propagation and microstructure of MA constituent. Fracture modes of MA constituents with slender and massive shape were proposed. Results show that slender MA constituents are more harmful to toughness compared with massive ones.