Simulated Heat-affected Zone Research Articles

Effect of Sensitization Treatment on Hydrogen Embrittlement Cracking of Duplex Stainless Steel

This study aims to evaluate characteristics of hydrogen embrittlement cracking (HEC) of sensitized duplex stainless steel with simulated heat-affected zone (HAZ). The HEC of the steels, aged for 20hr at 650°C, 750°C and 800°C, were studied using a testing machine which can control the stress intensity factor, K, for the precracked compact type (CT) specimens subjected to hydrogen charging. Fracture mechanisms were studied by acoustic emission (AE) method in conjunction with metallographic observation. Fracture toughness, Kc, for the steel aged at 650°C was higher than those of others. The steel aged at 650°C, with hardened ferrite phase, suffered hydrogen-induced brittle cracks in front of main crack, while austenite phase exhibited ductile fracture. For the steels aged at 750°C and 800°C, both the ferrite and sigma phase suffered hydrogen-induced microcracks.

Microstructures and Properties in Simulated Heat-Affected Zones of 685 MPa Grade Copper-Bearing Steel

Abstract The experiments were carried out upon the determination of SH-CCT diagrams and the characteristic of microstructures and properties in simulated coarse grain heat-affected zone of hot continuously rolled copper-bearing steel under single and double welding thermal cycle using physical simulation. The results show that with the help of SH-CCT diagram of copper-bearing steel, it is proposed to choose t8/5 ranging from 7s to 35s during welding. The copper-bearing steel has a narrow range of heat-input and brittlement is easy to happen in the region of CGHAZ with higher heat-input. Granular bainite transformed from austenite leads to brittlement. Softening starts when t8/5 is more than 7s. The dissolution of ɛ-Cu and coarse lath bainite and more ferrite cause softening. Moreover, the impact toughness decreases dramatically and obvious brittlement happens in the intercritical region of CGHAZ. The reason is pearlite formed on the interface of original austenite and coarse granular bainite, which can reduce the impact toughness.

Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

The effect of preweld overaging heat treatment on the microstructural response in the heat-affected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M23X6 particles and coarsening of primary and secondary γ′ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M23X6 particles and MC-type carbides. In addition, the coarsened γ′ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.

Microstructure and Mechanical Properties of Heat-affected Zone of High Nitrogen Steel Simulated for Laser Welding Conditions

In order to obtain some information on the weldability of high nitrogen steel (HNS), the microstructure and mechanical properties of the heat-affected zone (HAZ) of HNS under laser welding conditions were investigated by using thermo-simulation technique. The experimental results indicate that the microstructure in the simulated HAZ of HNS consists of austenite and d-ferrite that occurs in the grain boundary of austenite. The hardness of the simulated coarse-grained heat-affected zone (CGHAZ) increases when the cooling rate increases, and that of the simulated HAZ decreases while the peak temperature decreases. The results also show that the hardness of the simulated HAZ is higher than that of the base metal, indicating no softening of the HAZ under appropriate welding conditions. The impact toughness of simulated CGHAZ increases at first and then decreases with the increase of the cooling rate, whereas two brittle zones exist in the HAZ.

フェライト系高温用鋼溶接熱影響部のクリープ強度と組織変化に及ぼすC量の影響

The effect of carbon content on creep rupture strength and microstructural change in heat affected zone (HAZ) in high temperature service was investigated in order to alleviate the deterioration of creep rupture strength in HAZ of heat resistant ferritic steel. Simulated weld thermal cycle at 1273K was applied to test specimens machined from 10%Cr-3%Co-3%W-V, Nb ferritic steel plates with carbon content ranging from 0.005% to 0.1%. After heat treatment at 1013K for 1.8ks, simulating post weld heat treatment (PWHT), the creep rupture strength was evaluated at 823K, and the various microstructural examinations were carried out.By decrease of carbon content, the creep rupture time of simulated HAZ got longer and the deterioration of creep rupture strength in HAZ was alleviated. In HAZ with lower carbon content, the density of fine particles in diameter of less than 0.1μm, which were effective for dispersion strengthening, was higher after long term heating. Additionally, the growth rate of particles such as M23C6 type carbide and MX type carbo-nitride was slower in HAZ with low carbon content compared with in that with high carbon content. The growth of particle was roughly in accordance with the theoretical Ostwald ripening considering the proportion of growth of each particle.The mechanism to explain the effect of carbon content was considered as the following. The proportion of M23C6 and MX dominated by chromium diffusion, which has a higher growth rate than that of MX dominated by vanadium diffusion, contributing to Ostwald ripening of particles becomes lower by decrease of carbon content because of the decrease of partially-dissolved M23C6 during weld thermal cycle. As a result, the growth of particle is delayed during heating and the dispersion strengthening by fine particle is maintained for long term in HAZ with lower carbon content.

Suppression of Type IV fracture and improvement of creep strength of 9Cr steel welded joints by boron addition

The microstructure and creep strength of simulated heat-affected zone (HAZ) specimens and welded joints have been investigated for advanced 9–12% Cr steels in order to make clear the mechanisms responsible for Type IV fracture and to improve the creep strength of welded joints at elevated temperature. Creep and creep rupture tests were carried out at 650 °C (923 K) for up to about 2×10 4 h. The creep rupture time of simulated HAZ specimens has its minimum after heating to A C3 temperature, which produces fine-grained martensitic microstructure with high density of dislocations and large M 23C 6 carbides but no lath structure. The welded joints were fractured in fine-grained HAZ at low stresses, indicating Type IV fracture, which resulted in shorter creep life than those of base metal. Reducing the width of HAZ by means of EB welding is effective for the extension of creep life but the brittle Type IV fracture appears at low-stress and long-time conditions. The addition of about 100 ppm boron combined with minimized nitrogen of 10–20 ppm suppresses the formation of fine-grained region in HAZ and hence suppresses the Type IV fracture.

Effects of nitrogen content and weld cooling time on the simulated heat-affected zone toughness in a Ti-containing steel

An increase of nitrogen content in a 0.02 wt% Ti-containing carbon-manganese steel resulted in a low coarsening rate of TiN particles in the heat-affected zone (HAZ), which led to an accelerated ferrite transformation instead of ferrite side plates during weld cooling cycle. The mixed microstructure of ferrite side plate, acicular ferrite and grain boundary polygonal ferrite in the simulated HAZ produced higher toughness. However, the increase of nitrogen content gradually increased the free nitrogen content in the HAZ and deteriorated HAZ toughness. Impact energy of the simulated HAZ (with Δt8/5 ∼60 s) at –20 °C deteriorated by about 97 J per 0.001 wt% free nitrogen, in the free nitrogen range from 0.0009 wt% to 0.0034 wt%, even though the HAZ has the tough mixed microstructure. Cooling time after welding influenced the HAZ microstructure and toughness as well, and maximum toughness was obtained when cooling produced the tough mixed microstructure. Therefore, for a high HAZ toughness, both nitrogen content and cooling time should be controlled to obtain the tough mixed microstructure and to keep the free nitrogen content low. The optimal nitrogen content and cooling time from 800 °C to 500 °C were 0.006 wt% and between 60 s and 100 s, respectively, in this experiment.

Microstructural Characterization and Fatigue Properties of 2195 Al-Li Alloy

The effect of welding, heat-affected zone (HAZ) simulation, and specimen orientation on the microstructure and fatigue properties of 2195 Al-Li alloy was studied. HAZ simulation and GTA welding with a 4043 filler alloy resulted in a significant change in the microstructure. In the HAZ the primary strengthening phase, T1 (Al2CuLi), in the base alloy in T8 temper was replaced by TB (Al7Cu4Li) phase and voids/microcracks, and the fusion zone (FZ) consisted of T (AlLiSi) phase particles in the matrix, which consisted mainly of the filler alloy. The yield strength and fatigue threshold of the 2195-T8 alloy were observed to be dependent upon the specimen orientation. The HAZ simulation and welding led to a reduction in the tensile properties and fatigue strength. While the post-weld heat treatment resulted in the re-precipitation of T1 phase in the HAZ, but no increase in the fatigue strength was observed due to the presence of microcracks. Fatigue crack initiation was observed to occur at the surface in the base alloy in T8 temper, and at the internal defects after HAZ simulation and welding. Fatigue crack propagation exhibited characteristic striations in the T8 alloy, and brittle cleavage-like feature after HAZ simulation and welding.

MICROSTRUCTURE AND MECHANICAL PROPERTY DEVELOPMENT IN THE SIMULATED HEAT AFFECTED ZONE OF V TREATED HSLA STEELS

The simulated heat affected zone (HAZ) of the high strength low alloy (HSLA) steels containing 0%, 0.047%, 0.097% and 0.151% vanadium, respectively, were studied with Gleeble-2000 thermomechanical simulator to determine the influence of vanadium addition on the mechanical properties of the HAZ. The HAZ simulation involved reheating the samples to 1350°C, and then cooling to ambient temperature at a cooling rate of 5°C/s ranging from 800 to 500°C(Δt 8/5 =60s). The mechanical properties including tensile strength and −20°C impact toughness were conducted. The microstructures of the base steel and the simulated HAZs were investigated using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Based on the systematic examination, the present work confirmed that about 0.05% vanadium addition to low carbon low alloy steels resulted in expected balance of strength and toughness of the HAZ. And more than 0.10% levels addition led to detrimental toughness of the HAZ. SEM study showed that the simulated 0.097% and 0.151%V HAZs consisted of more coarse ferrite plates with greater and more M-A constituents along austenite grain and ferrite plate boundaries. The impact fracture surfaces of the simulated 0.097% and 0.151% V HAZs showed typically brittle mode with predominant cleavages. The size of the facet in the fracture surface increased with increasing vanadium level from 0.097% to 0.151%. As a result, the simulated 0.151%V HAZ has the lowest impact toughness of the four specimens.

On the role of liquated γ′ precipitates in weld heat affected zone microfissuring of a nickel-based superalloy

The role of γ′ precipitate particles, which is the principal strengthening phase of most of the precipitation hardened nickel base superalloys, on weld heat affected zone (HAZ) cracking was investigated. The HAZ microstructures around welds in a commercial nickel-based superalloy IN 738LC were simulated using Gleeble thermomechanical simulation system. Microstructural examination of the simulated HAZs and those present in actual tungsten inert gas (TIG) welded specimens showed that γ′ particles persisted during heating to the welding simulation temperatures, where they reacted with the surrounding γ matrix producing liquid film by a eutectic-type reaction, which subsequently infiltrated the grain boundary regions. The on-cooling ductility of the alloy was significantly reduced as shown by the comparatively low ductility recovery temperature. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that HAZ cracking resistance was damaged by liquation reaction involving the γ′ precipitate particles. The results support previously reported observations which indicated that γ′ precipitate particles could contribute to HAZ microfissuring in superalloys through liquation reaction besides the generally reported rapid solid-state re-precipitation of γ′ phase.

Physical Simulation and Metallurgical Evaluation of Heat-Affected Zone during Laser Welding of Ultrafine Grain Steel

Two major challenges in relation to laser welding are abrupt change in metallurgical aspects and actual assessment of the mechanical properties due partly to very narrow heat affected zone (HAZ) and partly to high mechanical properties gradient. The rapid thermal cycle of laser welding imposed on the HAZ was physically simulated using a Gleeble™ dynamic simulator equipped with a special isothermal quenching device (ISO-Q™), and a relatively large volume of HAZ with a homogeneous microstructure was obtained. The thermal cycles were determined from actual laser welding followed by laser tempering. Estimations of microstructure and mechanical properties of the simulated HAZs of an ultrafine grain steel imposed by laser welding with or without post-weld laser tempering were performed. The results indicate that the simulated HAZs, depending on the thermal history, are composed of lathy martensite with different pocket size and dislocation density. The impact toughness of as-welded HAZ is improved in contrast to the base material, but is further degraded by a following laser tempering, which, however, alleviates the abrupt change in hardness of as-welded HAZ.

Fissuration en relaxation des jonctions soudées en aciers inoxydables austénitiques

Cet article represente une synthese des etudes menees au CEA/SRMA, en collaboration avec l'ENSMP, sur la fissuration en relaxation (F.E.R.) des aciers inoxydables austenitiques. Initialement, l'etude a porte sur l'endommagement des Zones Affectees (Z.A.) des soudures en aciers stabilises au titane de type AISI 321. Une demarche de simulation experimentale de Z.A. par des traitements thermo-mecaniques a ete mise en place. Un essai de relaxation sur eprouvette CT a egalement ete mis au point afin de reproduire en laboratoire ce type de fissuration. Outre la validation de cette demarche, il a ete montre que l'ecrouissage joue un role preponderant sur la fragilisation intergranulaire et que le vieillissement attenue cette fragilisation contrairement a ce qui est couramment admis dans la litterature. Cette remise en cause du role premier de la precipitation des carbonitrures de titane permet d'elargir le champ d'investigation aux aciers non stabilises pour lesquels il existe un certain nombre de cas de fissuration similaires a ceux attribues a la F.E.R. En appliquant la demarche precedente a des aciers de type AISI 316 et AISI 304, on montre aussi que l'ecrouissage est un facteur de fragilisation intergranulaire. Parallelement, on etablit, par approche locale de la rupture, un modele d'endommagement intergranulaire utilisable pour l'evaluation du risque de fissuration sur des composants reels.

Fracture resistance of simulated heat affected zone areas in HSLA structural steel

The fracture toughness of some areas in the multi-pass heat affected zone (HAZ) of a high strength low alloy (HSLA) structural steel was analysed in a straightforward way using precracked, cylindrical specimens tested on a conventional tensile machine. The specimens were made from samples with a simulated HAZ microstructure; however, the size of the samples was restricted by the limitations of the Gleeble machine. The brittleness of the samples was an indication of the detrimental effect of welding on their toughness. The specimens were not large enough for a direct KIc measurement over a wide testing temperature range; it was necessary to modify the results. The low fracture toughness and the substantial shift of fracture transition temperatures suggest that welding of the investigated steel could be a delicate procedure.

Effect of δ-ferrite on impact properties of supermartensitic stainless steel heat affected zones

The work presented in this paper focuses on the effect of the presence of non-equilibrium δ-ferrite on the impact properties of a supermartensitic stainless steel. To generate homogeneous δ-ferrite containing microstructures the material was, in the first part of the present study, subjected to a series of high temperature furnace heat treatments. Three microstructures possessing variable ferrite content and different grain sizes were generated. Charpy impact results indicate that the presence of 14% δ-ferrite in a martensitic matrix of 60 μm prior austenite grain size, raises the ductile to brittle transition temperature (DBTT) by about 50°C compared with a fully martensitic microstructure of 80 μm. For a similar grain size, reducing the amount of δ-ferrite from 14 to 2% restored the DBTT to a level comparable to that of the tempered parent material. A Gleeble simulator was used to create the range of microstructures found in a weld heat affected zone (HAZ). Testing of the simulated HAZ indicated that toughness was not significantly affected by the presence of δ-ferrite. The DBTT was comparable to that of the parent material and lower than that of the heat treated specimens containing 14% δ-ferrite.

Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated heat-affected zones of Mn-Mo-Ni low-alloy steels

This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M2C carbides was greatly increased and the number of coarse M3C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M2C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.

Liquid film migration of constitutionally liquated γ ′ in weld heat affected zone (HAZ) of Inconel 738LC superalloy

Microstructural examination of simulated HAZs and those of in an actual TIG welded Inconel 738 superalloy was conducted, and an alternate mechanism through which high volume fraction of constitutionally liquated γ ′ phase contributes to the alloy's low resistance to weld HAZ cracking by limiting liquid film migration process, is proposed.

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.

Microstructure and creep strength of welds in advanced ferritic power plant steels

The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 104 h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.

Influence of Extra Coarse Grains on the Creep Properties of 9 Percent CrMoV (P91) Steel Weldment

With modern welding methods, satisfactory microstructures in 9%CrMoV (P91) steel can be obtained with a modest variation in hardness and prior austenite grain size. However, there is always a risk that significant deviations in the properties can be obtained, if the welding parameters are not optimized. In the present paper the role of extra coarse grains in the heat affected zone (HAZ) has been studied. Creep tests were carried out at 600°C for parent metal, weld metal, cross weld, simulated extra coarse grained HAZ, and simulated intercritical HAZ of a 9%CrMoV (P91) steel. The parent metal, the cross welds, the weld metal, and the simulated intercritical HAZ had about the same rupture strength except at long rupture times, where the values for the cross welds were considerably lower. In the cross welds, rupture took place in the intercritical HAZ at longer times (Type IV cracking). The simulated extra coarse grains gave considerably longer rupture times, lower strain rate and lower creep ductility than the parent metal and the weld metal. The creep strain behavior was successfully analyzed using the Omega model where the log creep strain rate is linear in the creep strain.