HAZ Specimens Research Articles

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

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

Prediction of HAZ width and toughness of HY85 steel using artificial neural network

ABSTRACT Physical weld simulation of single pass welding of HY 85 steel was performed using Gleeble® 3800 thermo-mechanical simulator at a peak temperature of 1300ºCwith heating rate of 200ºCs−1, and cooled with cooling rates ranging from 0.5 to 120ºCs−1 and corresponding heat inputs ranging from 1137 to 4.7 kJ/cm. The impact toughness of coarse grain heat-affected (CGHAZ) zone of simulated HAZ specimens was determined at a temperature of −50°C and related with their microstructures. Back propagation neural network (BPN) model was used to model heat-affected zone width and impact toughness of simulated heat-affected zone specimens, using heat input and cooling rate as input parameters. Predicted data of HAZ width and impact toughness is compared with the experimental data and it is observed that the back propagation neural network model with 4 hidden neurons adequately predicts the impact toughness and HAZ width. Calculated relative percentage error for HAZ width is ±3.15, whereas for impact toughness it is ±7.93%.

Near-weld creep crack growth behaviour in Type 316H steel ex-service components

ABSTRACT Creep crack growth is known to be the dominant failure mechanism in high-temperature components. Particularly in welded structures operating at elevated temperatures, cracks are often found to initiate and propagate in the vicinity of the weld region which can eventually penetrate into the base material after a long period of operation. In this study, creep crack growth tests have been performed on specimens extracted from an ex-service 316 H welded component to examine the crack initiation and growth behaviour in near-weld regions. The results show that the cracking behaviour of the base metal in near-weld specimens is similar to the as-received 316 H data set, suggesting that the material inhomogeneity would not influence the crack propagation behaviour in service-exposed components. Moreover, the test results show that the crack initiation and growth behaviour of the HAZ specimens can be estimated in much shorter time scales by performing tests on pre-compressed material.

Investigation of local tensile strength and ductility properties of an X100 submerged arc seam weld

Tensile testing is the most common method to determine the mechanical properties of a weld joint, yet the local tensile properties can be different from the global properties when stress localization occurs. In the current study, Digital Image Correlation was used to explain the difference in local and global tensile behavior in different sections of a submerged arc specimen. Tensile tests are used to compare the base metal and weldment properties of a two-pass submerged-arc welded X100 pipe steel in terms of strength and ductility. The base metal exhibits superior tensile strength compared to specimens machined from the second pass (Pass 2) or the overlap weld region spanning between Passes 1 and 2, corresponding to the reheated heat affected zone. Both cross-weld tensile specimens fractured in the fine grained heat affected zone (FGHAZ). Although the overlapped tensile specimen achieved higher global strain compared to the Pass 2 weld specimen, it sustained lower local strain, which promoted fracture in the HAZ. In addition, the overlapped tensile specimen exhibited a higher degree of strain hardening. The martensite-austenite (MA) microconstituents in the FGHAZ of Pass 2 contained higher carbon than the MA in the overlapped FGHAZ, which led to increased MA hardness as measured by nanoindentation. A higher fraction of MA with a finer spacing in the FGHAZ of the overlapped HAZ zone provides higher strength and strain hardening but lower ductility. The tensile fracture was dominated by void formation from the hard MA/ferrite interfaces in all tensile specimens, and so an increased fraction of MA led to accelerated microvoid coalescence, and reduced ductility in the overlapped HAZ specimens. In-situ strain monitoring technology via Digital Image Correlation was used to identify the potential fracture location during tensile test. X-ray tomography clearly revealed microvoid formation and coalescences of voids.

Evaluation of the mechanical properties of laser cladded hypereutectoid steel rails

Local material properties of the cladding layer, heat affected zone and substrate of laser cladded hypereutectoid steel rails were evaluated by uniaxial tensile tests on miniature specimens. Three laser cladding materials, i.e. 410 L, SS420 and Stellite 6, and two different heat treatment conditions were considered. To examine any possible anisotropic behaviour, specimens machined along the longitudinal and transverse laser cladding directions were tested. All the cladding layers showed similar or higher yield and ultimate tensile strengths, but lower elongation than the non-clad rail. Application of post-heat treatment significantly improved the elongation of all the cladding layer specimens. Elongation increments of 70%, 192% and 90% were respectively obtained from the 410 L, SS420 and Stellite 6 post-heat treated cladding layers. The post-heat treatment also marginally increased the yield and ultimate tensile strengths in this region. The improvement in tensile properties was associated with a more favourable microstructure, which resulted in a more dimpled morphology as depicted in the SEM images of the fractographic analysis of the tested specimens. While other properties are similar, both cladding layer and HAZ specimens showed significant different elongation values between the longitudinal and transverse directions in some cases, which indicates certain anisotropy in those materials’ ductility.

Investigation on the effect of ball burnishing on fracture toughness in spiral API X70 pipeline steel

The present work is an investigation on the effect of mechanical surface treatment using ball burnishing technique on fracture toughness in spiral API X70 pipeline steel. The burnishing operation has been carried out on three series of impact API 5L 45 Ed specimens that have been prepared according to the location of the notch towards the weld bed: BM notched specimens, FZ notched specimens, and HAZ notched specimens. In the untreated specimens, the absorbed energy in BM specimens is higher than the energy found in FZ and HAZ through the testing temperature range of 50 to −40 °C. In surface-treated specimens, there has been a significant change in the three curves of absorbed energy versus temperature. This is observed when testing temperature decrease from 20 to −40 °C since the energy in BM specimens dropped drastically reaching values lower than that obtained for FZ and HAZ specimens. At the temperature range 50 to 20 °C, the absorbed energy in ball-burnished specimens is relatively higher. At lower testing temperatures below −40 °C, the absorbed energy converged to the same value. At room temperature, hardness tests indicate that where the absorbed energy is higher, hardness is lower.

Experimental and ductile fracture model study of single-groove welded joints under monotonic loading

Tests and finite element (FE) analyses of smooth flat bar, U-notch and V-notch specimens are presented to demonstrate the application and validation of proposed three-stage and two-parameter ductile fracture model for evaluating the ductile crack initiation, propagation and final failure in steel welded joints under monotonic loading. Modeling concepts and procedures for characterizing the material parameters of ductile fracture model using smooth flat bar and U-notch tests are described. Accuracy of the model is validated through a series of tensile tests of U-notch, V-notch and welded smooth flat bar specimens. Three types of materials used in welded structures including base metal, weld metal and HAZ are investigated. Furthermore, the effect of notch position on ductile fracture behavior of HAZ specimens and the effect of mesh size on ductile fracture behavior of U-notch and V-notch specimens are studied. Detailed finite element analyses that employ the ductile fracture model are shown to predict ductile fracture behavior with good accuracy across the specimen geometries and material types in terms of ductile crack initiation point, ultimate load point and load–displacement curve.

Relationship between Creep Crack Propagation Rates of P92 Steel and Welding HAZ and High Temperature Creep Fracture Parameter C* Based on Reference Stress Method

Uniaxial creep tests on micro-regions of P92 steel base metal and welding HAZ specimens were carried out at 923 K and under different stress levels. The creep equation of index of B and n was obtained from the test data fitted by using least square method. Then, creep crack growth tests based on the reference stress method on P92 steel base metal and welding HAZ compact tensile samples were carried out at 923 K and under stress intensity factor K of 18 MPa·m1/2. From the calculated and experimental data, it was found that a good relationship existed between the creep crack growth rate da/dc and high temperature creep fracture parameter C*. The reference stress method was verified to calculate the C* of compact tension specimens and can predict the creep crack growth rate of the P92 steel base metal and welding HAZ materials under steady state creep conditions. At the same time, high temperature defect evaluation parameters of D0 and Φ were obtained by fitting the linear parts in the relation curves using the log-log method. This study provides tests data on the defects evaluation and life prediction of P92 steel welding materials.

Crack Tip Opening Displacement of X100 SAWL Pipe

The fracture toughness of different zones of X100 SAWL pipe was studied by three-point bend test method under different temperature. At the same temperature, the crack propagation resistance and tear modulus of X100 SAWL pipe distributed as: base metal>HAZ>weld bead, and the distribution of the CTOD value of X100 SAWL pipe is :base metal >weld bead >HAZ. Further investigation was conducted to analyze the decrease of fracture toughness (especially low temperature fracture toughness) and the occurrence of local unstable phenomenon in HAZ specimens.

Suppression of Type IV fracture in welded joints of advanced ferritic power plant steels – effect of boron and nitrogen

The effect of boron and nitrogen on the grain refinement in HAZ during heating and on the Type IV fracture in welded joints has been investigated for P92 and 9Cr–3W–3Co–VNb steel containing boron and or nitrogen. Creep tests were carried out at 650°C (923 K) for up to about 3 × 104 h for base metals, simulated HAZ specimens and welded joints. The addition of boron of 47 to 139 ppm to 9Cr – 3W–3Co–VNb steel containing minimized nitrogen as low as 10–20 ppm suppresses the grain refinement in HAZ and hence suppresses the Type IV fracture in welded joints at low stresses. The addition of excess nitrogen of 0.06% causes the grain refinement during heating at around Ac3 temperature, in both conditions of no boron and high boron of 130 ppm. A combination of high boron of 130 ppm and low nitrogen of 80 ppm causes no grain refinement in HAZ during heating at around Ac3 temperature as well as significant improvement of creep rupture strength of base metal by fine MX nitrides without any formation of boron nitrides.

Investigation of Dissolution and Precipitation Behaviours of Precipitates in Welded Joints of High Cr Ferritic Heat Resistant Steels

This paper is aimed at investigating dissolution and precipitation behaviour in fine grained heat-affected zones (FGHAZ), coarse grained heat-affected zones (CGHAZ), and base metals for the welded joints of high Cr ferrite heat resistant steels. The simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1 173 and 1 523 K to represent FGHAZ and CGHAZ, respectively. The behaviours of dissolution and precipitation during welding, PWHT and ageing processes were observed. SEM observation showed that after long time ageing the precipitates in FGHAZ specimens (1 173 K) were fewer and larger than those in CGHAZ (1 523 K) specimens and base metal specimens. This phenomenon implied that growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep property and Type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates were M23C6, M7C3, MX, Laves phase, and μ phase. By using software of Thermo-Calc and Dictra, a 3-D model was performed to simulate the dissolution process of M23C6 during welding. It was found that the dissolution of M23C6 can lead to high Cr and high W concentration in a local region. On the other hand, the precipitation behaviour of M23C6 was simulated, and it was found that the growth of M23C6 was fast and in the period of PWHT and creep process it can reach to the equilibrium concentration in a short time of about 2 000 seconds. At last, the growth of Laves phase was simulated and it was found that it grew very slowly and it reached the equilibrium concentration in about 3 000 hours. As an important point, the simulation result showed that Laves phase was easy to grow in the region where M23C6 had dissolved due to the high residual concentration of Cr and W.

Improvement of Creep Strength of Advanced Ferritic Steel Welded Joints

The microstructure and creep strength of simulated 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 crack and to improve the creep strength of welded joints at high temperature. Creep and creep rupture tests were carried out at 650 °C (923 K) for up to about 104 h. Creep crack growth tests were also carried out using CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained microstructure. 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 even in the EB welded joints at low stress and long time conditions. The welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metal due to the formation of creep voids and cracks. Using a specially-designed FEM code for Type IV crack growth behaviour, the effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated. The Type IV fracture is suppressed by the addition of boron, which suppresses the formation of fine-grained region in HAZ.

多層溶接継手のクリープ変形挙動に及ぼす溶接金属部の不均一な高温強度特性の影響

Evaluation of creep properties of the welded joint through taking local fluctuation of the properties into consideration is experimentally or analytically seldom carried out. In this context, the purposes of the present study are to examine the surface strain distribution in the weld metal of a full thickness welded joint specimen and subsequently to investigate the local variation in the properties of the all-weld metal part of the joint using miniature specimens. A welded joint was prepared for 316FR steel plates by gas tungsten arc welding process using Mod. 316 filler wire. Creep tests were conducted at 823K in air using full thickness large welded joint specimens, HAZ and all-weld metal miniature specimens. By interrupting the creep test for the full thickness welded joint specimen, we observed the change in the uniaxial creep strain distribution in the weld metal in the thickness direction of the plate. The last layer of weld metal showed considerably larger creep strain than the central layers of the weld metal. The elastic-plastic creep analysis of the welded joint specimen by FEM was performed using these data. Finite element simulation performed for full-thickness welded joints provided creep strain distribution that is consistent with measured values. From the results above, it is concluded that the creep properties of weld metal in multi-pass, multi-layer welded joints strongly depend on the location of specimen sampling. It is necessary to take the fluctuation of material properties of a weld metal part into consideration when we perform simulation of creep deformation of a welded joint with sufficient accuracy.

Investigation of precipitation behavior in a weld deposit of 11Cr-2W ferritic steel

This article is aimed at investigating the difference in precipitation behavior in the fine-grained heat-affected zone (FGHAZ), coarse-grained heat-affected zone (CGHAZ), and base metal for the welded joint of high Cr ferritic heat-resistant steel (11Cr-0.4Mo-2W-1Cu-V-Nb, normalized 1323 K×1 h and tempered 1033 K×1 h). Simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1523 and 1173 K to represent CGHAZ and FGHAZ, respectively. Based on scanning electron microscopy (SEM) observation, it looks that the precipitates in FGHAZ specimens (1173 K) were fewer and larger than those in CGHAZ (1523 K) specimens and base metal specimens. This phenomenon implied that the growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep properties and type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates are the same for the 1173 K specimens and base metal specimens, including M23C6, MX, Laves phase, and μ phase. Further, the elemental analysis of the extraction showed that the mass percentages of Cr, W, and Mo in the precipitates to specimen mass were higher in the FGHAZ specimen than those in the base metal specimen, especially during the period between 600 and 2464 hours. Finally, a two-dimensional (2-D) model was proposed to simulate the precipitation behavior of the Laves phase.

Effects of temperature and shielding gas mixture on fatigue life of 5083 aluminum alloy

A fatigue crack develops as a result of stress concentration and extends with each load cycle until failure occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal.This is a study of fatigue strength of Al5083-O aluminum alloy to investigate influence of fatigue life on welding zone according to the mixing shield gas ratio and the test temperature. The GMAW was carried out for weld specimens with various mixing shield gas ratios (Ar100%+He0%, Ar67%+He33%, Ar50%+He50%, and Ar33%+He67%). The test temperatures for the fatigue test were +25°C (+77°C), −30°C (−22°C), −85°C (−121°C), and −196°C (−321°C). The welding zone was divided into four subzones for analysis; weld metal, fusion line, HAZ, and base metal according to the notch position. Regarding the influence on the bead shape of the mixing shield gas ratio, the bead width was greatest in Ar100%+He0% mixture. But the bead depth and area were greatest in Ar33%+He67% mixture. The dilution was also best in the mixing shield gas ratio. In the influence of mixing gas ratio, fatigue life of weld metal specimens decreased with higher argon gas ratio, but fusion line and HAZ specimens were not influenced greatly. Also, the fatigue life increased at lower temperatures, particularly at −196°C.

Effects of microstructural change on fracture characteristics in coarse-grained heat-affected zones of QLT-processed 9% Ni steel

This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.

Investigation of dynamic fracture toughness (JId) behaviour of strength mis-matched Q & T steel weldments using instrumented Charpy impact testing

Abstract Instrumented Charpy impact testing has been used for evaluating the dynamic fracture behaviour of mis-matched Q & T steel weldments. The effect of strength mis-match on the dynamic fracture toughness (JId) and dynamic crack resistance curves (Jd–R) of HAZ and weld metals specimens are discussed. The crack propagation and mode of failure due to impact loading are analyzed through SEM and correlated with dynamic resistance curves.

Temper embrittlement in the HAZ of 2¼ Cr-1Mo steel induced by reheating after SR treatment: Study of temper embrittlement in HAZ of Cr-Mo steels (4th report)

Summary The temper embrittlement affecting the HAZ of 2 1/4Cr-1Mo steel subjected to SR treatment and subsequent reheating is investigated with synthetic HAZ specimens and compared with the embrittlement observed in specimens tempered in the same temperature range without SR treatment (direct tempering). The synthetic HAZ specimens are de-embrittled on application of suitable SR treatment (975 K × 5 hours). Subsequent reheating at 775 to 925 K, however, produces four different types of embrittlement in the specimens. These are the second, third, fourth, and fifth types of embrittlement already identified in the direct-tempered specimens. The first type of embrittlement which occurs during direct tempering in the shortest time range does not appear after SR treatment. The second type (the short-term type), which occurs over a wide time-temperature range during direct tempering, only appears in a narrow range after SR treatment. The third and fourth types (the long-term types) occur just as intensively as during direct tempering. The fifth type (the high-temperature type) also appears after SR treatment. This suggests that embrittlement in the short-term range is to some extent suppressed by SR treatment, but the types of embrittlement arising in the long-term range and high-temperature range occur after SR treatment as well. The ‘step cooling process’ can be applied for short-term detection of temper embrittlement in this steel. However, there is considered to be little or no embrittlement promotion effect due to step cooling per se.

Creep rupture properties of welded joints in W-containing 9Cr ferritic heat-resistant steel

Summary This paper describes an investigation of the creep rupture properties of welded joints produced from W-containing 9Cr-Mo-W steel. The creep rupture properties of the HAZ are also studied using simulated HAZ specimens subjected to PWHT (post-weld heat treatment). The effect of W on the creep rupture strength of the welded joints is examined. Creep rupture tests of GTA (TIG) welded joints are conducted. The longest creep rupture time is around 20 000 hours. In the creep rupture tests, the welded joints rupture in the base metal at higher applied stresses, rupturing in the low-ductility fine-grained HAZ adjacent to the base metal at lower applied stresses. When the welded joints rupture in the base metal, their creep rupture strength is as high as that of the base metal. When the welded joints rupture in the HAZ, however, their creep rupture strength is lower than that of the base metal. The cracking which occurs in the HAZ is TYPE IV cracking which tends to affect the welded joints of ferritic heat-resistant steel. TYPE IV cracking is the type which occurs in the fine-grained HAZ at a lower stress than the creep rupture strength of the base metal without being associated with any heavy deformation. In the creep rupture tests, the simulated HAZ specimens heated to a temperature around Ac1 and Ac3 give a lower creep rupture strength than that of the base metal. The simulated HAZ specimens heated to the Ac3 temperature give the lowest creep rupture strength. A comparison of the creep rupture strengths of welded joints produced from 9Cr-Mo-W steel and 9Cr-1 Mo-Nb-V (mod. 9Cr-Mo) steel suggests that W improves the creep rupture strength of both welded joints and base metal.

Cr-Mo鋼再現熱影響部の水素侵食に及ぼすAl,Ti,B及びNの影響

Effects of Al, Ti, B and N on hydrogen attack were examined by using simulated HAZ specimens of Cr-Mo steel. The hydrogen attack resistance was evaluated by the ratio of Charpy absorbed energy measured after exposure to hydrogen atmosphere and that measured after aging in air. The hydrogen attack resistance increased with sol.Al content more than 0.01% for simulated HAZ of 10 ppm B bearing Cr-Mo steels. The addition of Ti and B also improved the resistance to hydrogen attack. It was found that M23C6 type carbide precipitated preferentially in the simulated HAZ of B bearing Cr-Mo steel. On the other hand, only the carbide of M7C3 type was observed when B was not added. The carbide of M23C6 type precipitation was promoted by free B in solution when N was fixed by Al and Ti.It was recognized that M23C6 was more stable than M7C3 at high temperature and high pressure in hydrogen atmosphere. The carbide of M7C3 type reacts with hydrogen to form methane and metal. The reason why the additions of Al and B improve the resistance to hydrogen attack is that the precipitation of M23C6 type carbide is accelerated by free B in solution which is more stable at high temperature and high pressure of hydrogen.