Heat Affected Zone Regions Research Articles

Heat conduction analysis of bidirectional multipass welding with short bead lengths

A heat conduction analysis is considered for multipass welding conducted in a forward–reverse manner. When the welding is conducted continuously and the bead is short, the interpass temperature sometimes exceeds 500°C. To predict the thermal histories, a heat conduction model is examined using the analytical solution of a moving point heat source in a wide plate of finite thickness. In the model, point heat sources, some of which are moving in one direction and others in the opposite direction, are considered. Comparison of the calculated and experimental results reveals that the proposed model can predict the thermal histories fairly well. This is followed by calculation of the equivalent arc energy, defined as the apparent arc energy which gives either the same cooling time from 800 to 500°C or the same kinetic strength of the heat cycles with no preheating (the concept of kinetic strength has been developed to estimate austenite grain growth in the heat affected zone region), and examination of the effect of maximum interpass temperatures on the equivalent arc energy. When the interpass temperature is greater than 500°C, its effect can be to increase the equivalent arc energy from 2.5 kJ mm-1 to between 6 and 14 kJ mm-1.

A Comparison of Creep Rupture Behaviour of 2.25Cr-1Mo and 9Cr-1Mo Steels and Their Weld Joints

Creep rupture behaviour of 2.25Cr-1Mo and 9Cr-1Mo base metals and weld joints has been studied at 823 K over a stress range of 100-250 MPa. In the stress range examined, 2.25Cr-1Mo steel showed better creep rupture strength compared to 9Cr-1Mo steel. Both the weld joints displayed a creep rupture strength inferior to those of their respective base metals. Failure occurred in the respective intercritical region of the heat affected zone (HAZ) in both the steel weld joints. At a given applied stress, the difference in rupture life of base and weld joint was more pronounced in 2.25Cr-1Mo steel than in 9Cr-lMo steel and this difference increased with decrease in applied stress. As the presence of Mo 2 C precipitate imparts creep resistance to 2.25Cr-1Mo steel, its absence in the intercritical HAZ region of the joint is held responsible for its poor creep life. On the other hand, 9Cr-1Mo steel which is strengthened by solid solution hardening and dislocation substructure, is less prone to degradation in the intercritical region. Also, the presence of δ-ferrite in the coarse grain region of HAZ of 9Cr-1Mo steel reduces its grain growth tendency and also its susceptibility to intergranular creep cavitation compared to 2.25Cr-1Mo steel.

Role of gaseous environment and secondary precipitation in microstructural degradation of Cr-Mo steel weldments at high temperatures

This study is an attempt to understand the combined role of variations in oxidizing environment and secondary precipitation, in the microstructurally different regions of a standard Cr-Mo steel weldment, on the intensity of internal oxidation during high-temperature oxidation in air and steam environments. Samples of the weld-metal, heat-affected zone (HAZ), and base-metal regions were separated from the weldment of 2.25Cr-1 Mo steel and oxidized in the environments of air and steam at 873 K. The oxide scales and underlying subscales were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). Extensive internal oxidation and oxidation-induced void formation in the subscale zone and grain-boundary cavitation in the neighboring region were found to occur during oxidation in the steam environment. However, the internal oxidation and void formation were much more extensive in the subscale regions of the HAZ than in the subscales of the weld-metal and base-metal regions. As a result, the alloy matrix in the area neighboring the subscale region of the HAZ specimen suffered extensive grain-boundary cavitation. This behavior has been attributed to a rather specific combination and complex interplay of the environment, alloy microstructure, oxidizing temperature, and nature of the resulting external scale in causing and sustaining internal oxidation. The article also discusses the role of internal oxidation-assisted microstructural degradation in deteriorating the service life of components of 2.25Cr-1Mo steel.

Determination of dynamic fracture toughness of heat affected zone of 9Cr–1Mo steel from instrumented drop weight tests

The dynamic fracture toughness (K1d) of the heat affected zone (HAZ) of 9Cr–1Mo steel at and below the nil ductility transition temperature has been estimated from instrumented drop weight test results. The presence of a significant microstructural gradient in the HAZ, comprising coarse, fine, and intercritical regions with sharp toughness differences, is reflected in the presence of distinctive load peaks in the load–time traces; the K1d estimates for the coarse, fine, and intercritical regions are 53, 85, and 128 MPa m1/2 respectively. The results from the 9Cr–1Mo steel were compared with those for AISI grade 403 martensitic stainless steel. The lack of distinctive multiple load peaks in the load–time traces for the latter is attributed to the absence of a steep toughness gradient, owing to a more or less uniform martensitic microstructure in the HAZ.

Characterization of the Weld Structure in a Duplex Stainless Steel Using Color Metallography

A three-step etching technique, utilizing a Glyceregia etch, a 10% oxalic acid electrolytic etch, and a boiling Murakami's reagent etch, was employed to reveal the weld structure in a commercial duplex stainless steel alloy. Color metallography indicated that chromium segregation, which is residual from significant phase composition differences in the original ferrite-austenite base metal structure, existed in all areas of the weld heat-affected zone. Evidence of fine chromium-rich precipitates distributed homogenously within the ferrite grains and heterogenously at ferrite subgrain boundaries in the middle and near heat-affected zone regions was observed and correlated with local variations in chromium content.

Microstructural evolution of simulated heat affected zone in ASTM A533 type B steel plates

In order to investigate the weldability of ASTM A533 type B steel plates, simulated heat affected zone (HAZ) experiments with heat inputs of 20, 50, and 80 kJ cm−1 were carried out. The thermal cycles used corresponded to the actual thermal cycles that occur in the coarse grained region of the real HAZ. The microstructural development and the toughness of the simulated HAZ were studied. It was found that with a heat input of 20 kJ cm−1 the simulated HAZ microstructure gives larger amounts of lower bainite with significant amounts of auto tempered martensite. In the cases of heat inputs of 50 and 80 kJ cm−1 both the simulated HAZ microstructures were composed chiefly of upper bainite. The specimen obtained from the simulation of a heat input of 20 kJ cm−1 possesses higher toughness than those obtained from the simulations of heat inputs of 50 and 80 kJ cm−1 Furthermore, the results also indicate that the tempering response is more sensitive in autotempered martensite than in bainite.

Heat-Affected Zone Toughness of a TMCP Steel Designed for Low-Temperature Applications

The objective of this investigation was to provide a detailed evaluation of the heat-affected zone (HAZ) toughness of a high-strength TMCP steel designed for low-temperature applications. The results from both Charpy-vee notch (CVN) and cracktip-opening displacement (CTOD) tests conducted on two straight-walled narrow groove welds, produced at energy inputs of 1.5 and 3.0 kJ/mm, show that significantly lower toughness was exhibited by the grain-coarsened HAZ (GCHAZ) compared with the intercritical HAZ (ICHAZ) region. This is explained based on the overall GCHAZ microstructure, and the initiation mechanism which caused failure. For the particular TMCP steel investigated in this study very good ICHAZ toughness properties were recorded using both HAZ Charpy and CTOD tests. In general, this was attributable to the low hardness, relatively fine ferrite microstructure, and the formation of secondary microphases that were not overly detrimental to the toughness. The lower-bound GCHAZ CTOD results obtained for both welds (KA W-L and KA W-H) did not meet the targeted requirement of δ = 0.07 mm at −50°C. It was found in both welds that low CTOD toughness was associated with the initiation of fracture from nonmetallic inclusions, which were complex oxides containing Ce, La, and S. The sites were located in the subcritical GCHAZ (SCGCHAZ) region in the case of the 1.5 kJ/mm weld and in the GCHAZ for the 3.0 kJ/mm weld. Some variation in CVN toughness was observed at different through-thickness locations. Toughness was lowest for the GCHAZ of the weld deposited at 3.0 kJ/mm and was related to the proportion of GCHAZ being sampled, which was ~55 percent for the bottom compared to 25–30 percent for that of the top location. Recommendations are proposed on the preferred practices and criteria that should be used in establishing guidelines and specifications for evaluating the HAZ toughness of candidate steels for construction of Arctic class ships.

Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding

The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone (HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region, the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer. The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent upon peak temperature(i.e., position within the HAZ), although the cooling rate(i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions.

Effect of liquation on welding of a continuous cast sheet of an Ni 3Al alloy containing Zr

Autogenous gas tungsten arc welding was performed on a continuous cast sheet (about 1 mm thick) of an Ni 3Al alloy (Ni 77.83Al 21.73Zr 0.34B 0.10) known as IC-50. The material was highly susceptible to cracking in the heat-affected zone (HAZ). The cracks in the HAZ were liquation cracks and occurred along the grain boundaries. There were occasional solidification cracks in the fusion zone (FZ) and they occurred along the main dendrite boundaries. Microporosities were observed in both the FZ and HAZ. Microprobe analysis showed Zr enrichment in HAZ regions where melting had taken place. Annealing the alloy at 1100 °C for 5 h produced Zr-rich particles. Welding after the annealing showed similar behavior to that without annealing, except that the partially molten region in the HAZ was reduced in size. The role of Zr segregation at the grain boundaries in causing partial melting is discussed.

Effect of Strength Mismatch on Crack Tip Stress Fields of HAZ-notched Joints Subjected to Bending and Tension

A numerical investigation for a crack located in the boundary between the weld metal and the heat affected zone (HAZ) of a welded joint is presented. Three-dimensional finite element analyses are performed on a center cracked panel (CCP) subjected to tension and single edge notch bend (SENB) specimens. The numerical results show that the stresses that develop in the HAZ region of the overmatched welded joint are more intense than those in the undermatch and evenmatch cases. It is noted that highly stressed near-tip zones are detrimental to fracture resistance of HAZ regions containing local brittle zones. A discussion also follows on the significance of weld strength overmatch on fracture behavior of HAZ-notched welded joints, which is based on the local approach. The analyses indicate that the relative load-carrying capacity of a overmatched joint may be reduced, especially in the case of the high constrained deep notch bend specimen. Such features may be particularly deleterious to cleavage fracture resistance of welded joints containing cracks close to the HAZ region.

Stack cracking by hydrogen embrittlement in a welded pipeline steel

Arrays of cracks, parallel to the original plate rolling direction, were produced in a X65 microalloyed steel by hydrogen embrittlement of pipeline sections containing a weldment. A region of the heat-affected zone of the weldment was shown to have a lower yield strength (“soft” zone) than the surrounding material and cracking was concentrated in this throughthickness zone to produce the effect known as stack cracking. In situ cathodic hydrogen charging of tensile specimens under load led to failure by linking the rolling-plane cracks with transverse cleavage cracks, which were often initiated at inclusions. All cracking was predominantly by cleavage and failure occurred in tension in short times by hydrogen embrittlement when the applied tensile stress was above about half the uncharged yield stress. The influence of microstructure, hydrogen pressure and tensile loading conditions on the location of stack cracks and the mode of fracture is discussed.

Detection of bending fatigue life of steel by magnetic method: Endo, K., Yoshinaga, A., Nakano, M. and Okazaki, Y. Hihakai Kensa (J. NDI) Aug. 1989 38, (8), 671–676 (in Japanese)

In this study, a series of optical microscope examinations, constant loading rate indentation (CLR), and creep indentation (CI) tests were performed on room temperature to investigate the strain rate sensitivity behavior of the SM490 structural steel weld zone. Microstructures of weld metal (WM), fine-grain heat-affected zone (HAZ), and base metal (BM) in the weld zone were observed using optical microscopy. Numerous CLR tests were performed at a wide range of strain rate indentation from 0.02 s−1 to 0.2 s−1 in the middle regions of WM, fine-grain HAZ, and BM across the weld zone, and as a result strain rate indentation effects on both indentation hardness and yield strength were investigated. The results showed that both yield strength and indentation hardness of WM, fine-grain HAZ, and BM exhibit the strain rate-dependent behavior, in which yield strength and indentation hardness tend to increase with increasing strain rate indentation. Strain rate sensitivity (SRS) values of WM, fine-grain HAZ, and BM were determined by using three different models: indentation hardness, yield strength, and creep indentation models. The SRS behavior of SM490 structural steel weld zone was investigated. The results indicated that, while both yield strength and indentation hardness of BM are lower than those when compared with HAZ and WM, BM has the highest SRS value than does fine-grain HAZ and WM. Furthermore, WM has the lowest SRS value compared with the SRS values of other components in the weld zone. The results of this study were used to assess and understand the strain rate sensitivity behavior of SM490 structural steel weld zone.

Thermal cycle and microstructure of heat affected zone(HAZ) of flash butt welded Mn-Cr-Mo dual phase steel.

Flash butt welding of 3.7 mm thick Mn-Cr-Mo dual phase steel is carried out at different final jaw distances, where various post weld cooling systems such as the normal machine cooling, forced air cooling and water spray cooling are used. The energy input during welding such as the current and voltage are kept constant. The intention of this investigation is limited to study the influence of various weld thermal cycle on the behaviour of phase transformation at weld centre and different regions of HAZ. Under different conditions of welding the weld thermal cycle is analysed and the behaviour of phase transformation at different regions of the weldment is determined with the help of welding CCT diagram. A detail microstructural study is carried out under optical and scanning electron microscopes. A special attention is paid to study the behaviour of tempered martensite region of HAZ, which is identified earlier as the weakest region of HAZ, under different welding conditions. The weakening of tempered martensite region is estimated by measuring its microhardness under different conditions of welding.

HAZ fracture toughness of titanium containing steels

AbstractSteels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm−1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.MST/887

Fracture toughness, fatigue crack propagation and creep rupture behaviour in thick section weldments of 3Cr-Mo pressurevessel steels developed for high-temperature/high-pressure hydrogen service

AbstractFracture toughness, fatigue crack propagation and creep rupture properties have been examined in parent plate, weld metal and heat affected zone regions of thick section, commercial size weldments of a 3Cr-11/2Mo-1/2Ni pressure vessel steel, newly developed for improved performance in hydrogen and coal conversion environments. Mechanical tests were performed on the steel, both before and after prolonged (WOO h) exposure to hightemperature/ high-pressure gaseous hydrogen, and results compared to similar data obtained on another 3Cr-lMo 1/4V-Ti-B steel developed by the Japan Steel Works for coal conversion applications. Both plane-strain fracture toughness and creep rupture lives were found to be relatively unchanged in the weldment microstructures compared with parent plate, and they were affected little by environmental damage (hydrogen attack) induced by the prior hydrogen exposure. Creep properties, however, were found to be sensitive to post-weld heat treatment. Similarly, fatigue crack growth ...

Corrosion of two similar austenitic stainless steels when welded

Two similar grades of austenitic stainless steel (317 L and 317 LM) were welded and their corrosion behaviour noted potentiostatically. It was found that, because of its higher Mo content, 317LM exhibited less active current densities and passive ranges than 317L when the heat affected zone (HAZ) microstructures were examined. This finding held good for a range of δ–ferrite contents representing the whole width of the HAZ. In the weld bead, however, the behaviour of both materials was essentially similar. Electron optical studies were performed on polarized samples and it was found that in both HAZ and weld bead regions there was a distinct Mo denudation zone at the δ/γ boundary. It was found that in HAZ material the reduction of Mo was associated with the production of very small particles on the δ/γ grain boundary, believed to be metallic (mainly Mo) carbides. From these findings it was concluded that it is unwise to discuss the corrosion resistance of welded austenitic stainless steels without detailing the method of formation of the δ-phase. To aid this, solid–phase transformation δ is termed δs and liquid–phase formed δ,δL.MST/115

Corrosion of two similar austenitic stainless steels when welded

Two similar grades of austenitic stainless steel (317 L and 317 LM) were welded and their corrosion behaviour noted potentiostatically. It was found that, because of its higher Mo content, 317LM exhibited less active current densities and passive ranges than 317L when the heat affected zone (HAZ) microstructures were examined. This finding held good for a range of δ–ferrite contents representing the whole width of the HAZ. In the weld bead, however, the behaviour of both materials was essentially similar. Electron optical studies were performed on polarized samples and it was found that in both HAZ and weld bead regions there was a distinct Mo denudation zone at the δ/γ boundary. It was found that in HAZ material the reduction of Mo was associated with the production of very small particles on the δ/γ grain boundary, believed to be metallic (mainly Mo) carbides. From these findings it was concluded that it is unwise to discuss the corrosion resistance of welded austenitic stainless steels without detailing the method of formation of the δ-phase. To aid this, solid–phase transformation δ is termed δs and liquid–phase formed δ,δL.MST/115

Characterization of the weld structure in a duplex stainless steel using color metallography

A three-step etching technique, utilizing a Glyceregia etch, a 10% oxalic acid electrolytic etch, and a boiling Murakami's reagent etch, was employed to reveal the weld structure in a commercial duplex stainless steel alloy. Color metallography indicated that chromium segregation, which is residual from significant phase composition differences in the original ferrite-austenite base metal structure, existed in all areas of the weld heat-affected zone. Evidence of fine chromium-rich precipitates distributed homogenously within the ferrite grains and heterogenously at ferrite subgrain boundaries in the middle and near heat-affected zone regions was observed and correlated with local variations in chromium content.

溶融亜鉛中における鋼溶接部の時間依存性破壊の検討

The liquid metal embrittlement cracking (LMEC) of heat affected zone (HAZ) of steel in hot dip galvanizing was investigated. The experiment was conducted using synthetic HAZ of JIS G 3444 STK55 and Sustained Load Test (SLT) at 370°C-470°C heating range in liquid (molten) and solid Zinc.The results obtained are as follows.(1) LMEC behavior is of thermal activated process and LMEC sensitivity is affected by hardness of HAZ.(2) The coarse grain region in HAZ showed the highest LMEC sensitivity.(3) The interfacial energy of steel is reduced by the existence of Zinc.(4) The reaction rate process of LMEC of steel is controlled by grain boundary diffusion of Zinc.(5) The nucleation and the propagation of LMEC in steel are likely to have a close relation to the reduction of interfacial energy due to the grain boundary diffusion of Zinc.

Local melting of nodular cast iron by plasma arc

The microstructural changes in the fusion boundary area of nodular cast iron (NCI) were investigated by plasma arc local-melting with and without a filler metal. The NCI cylinder specimen was locally melted by the static plasma torch for a very short time, adopting the non-keyhole method. The microstructures produced in the fusion region, fusion boundary region and heat-affected zone (HAZ) were examined metallographically. In the absence of filler metal, the fusion boundary area is composed of ledeburite in the fusion region, and martensitic, troostitic—ferritic layers for the short arc-time and sorbitic, sorbitic—ferritic layers for long arc times in HAZ. Although the formation of white iron in the fusion region may be caused by a relatively high cooling rate, the major cause is due to supercooling effects produced by the decrease of magnesium content in the molten iron directly under the plasma arc. Using nickel and Ni—Fe filler metals, Nimartensite appears in the deposited fusion boundary, and the HAZ region is composed of ledeburitic and martensitic or sorbitic layers. The ledeburitic layer thickness varies with the melting point of the filler metal. The diffusion of nickel from the deposit metal to the HAZ occurs at least until the fused base metal in the HAZ.