Search Results for weld joints

In 9~12% Cr containing martensitic stainless steels, Laves phase usually occurs after long term high temperature exposure, while in the present work, some sparse relatively large particles of (Fe,Cr)2Mo type Laves phase were observed in virgin FB2 steel. It is speculated that the large Laves phase particles formed in casting process due to dendritic segregation. Then the evolutionary behavior of Laves phase during welding thermal cycle was studied and constitutional liquation of Laves phase was found, suggesting a liquation crack tendency in FB2 steel. At last, the hot ductility tests showed that the area where constitutional liquation occurred would act as crack initiation site, and the tested specimen fractured without any obvious plastic deformation. This work provided some guidance for the practical production of welded turbine rotors made of FB2 steel.

The efficiency of power plants is depending on the steam temperature and/or the steam pressure. Efficiency increasing from 35% to 42-45% require increasing of the steam temperature over 600°C and the pressure over 26 MPa. According to the designer opinion it is not profitable to use classical low alloy creep resistant steels 16Mo3, 13CrMo4-5 or 10CrMo9-10 for membrane waterwall construction for these service condition. New modified low alloy creep resistance T23 and T24 (7CrMoVTiB10-10) steels were developed for membrane waterwalls. Welding of these steels with small thickness (around 6.3 mm) should be enabled without preheating and post weld heat treatment (PWHT) due to the lower carbon content below 0.1%. High creep rupture strength (CRS) values are achieved by Ti, N and B elements alloyed to T24 steel. The original expectation that the welding small thickness without preheating was early overcome and was wrong. According to the present experience the T24 steel is welded with preheating at 150-250°C depending on the wall thickness and welded joint toughness in order to achieve required hardness and impact toughness values. Opinions on the T24 welded joints post weld heat treatment (PWHT) requirements are still inconsistent. Especially the membrane waterwalls of the supercritical power plants are still produced without PWHT.

A trial weld joint of COST F and COST FB2 steels was produced using the GTAW HOT-WIRE method in conditions used in industry for production of welding steam turbine rotors. Conventional long-term creep tests (CCT) to the rupture of this weldment and the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa (the longest time to rupture was above 52,000 hours). Creep rupture strength was evaluated using Larson-Miller parameter model. Assessment of microstructure was correlated with the creep strength. Precipitation of Laves phase and structure recovery during creep exposures were the main reasons for the failure which occurred in the heat affected zone of steel COST F. The recently developed simulative accelerated creep testing (ACT) on thermal-mechanical simulator allows the microstructural transformation of creep-resisting materials in a relatively short time to a state resembling that of multiyear application under creep conditions. ACT of samples machined from various positions in the weldment was performed at 600 °C under 100 MPa. Changes in the hardness and the microstructures of the samples, which underwent both types of creep tests, were compared. Small sample creep test (SPCT), another alternative method how to obtain information about the creep properties of materials when only a limited amount of test material is at disposal, were performed. It was shown that the same stress-temperature dependence and relationships are valid in the SPCT as in the CCT. Using a simple load-based conversion factor between the SPCT test and the CCT test with the same time to rupture, the results of both test types can be unified.

Recent years high strength 9Cr1MoNbV steel developed in USA has been major material in boiler high temperature components with the increase of steam parameters of coal fired thermal power plants. As the microstructure of this steel is tempered martensite, it is known that the softening occurs in HAZ of the weldment. In the creep rupture test of these welded joints the rupture strength is lower than that of the parent metal, and sometimes this reduction of strength is caused by TypelV cracking. To develop an effective method to improve the rupture strength of welded joint, advanced welding procedure and normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with thickness of 40-50mm were welded by 10mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1,050°C and tempering at 780°C, material properties of the welded joints were examined. Microstructure of HAZ was improved as before weld, and rupture strength of the welded joints was equal to that of the parent metal. The long term rupture strength of the welded joints was confirmed in the test exceeded 30,000hours. This welding procedure has been applied to seam weld of hot reheat piping and headers in USC boilers successfully.

This study is concerned with the creep damage evaluation for the welded joint of modified 9Cr-1Mo steels. A finite element prediction method based on ductility exhaustion approach has been proposed. Degradation of creep ductility under multi-axial stress state has been formulated from the experimental results of notched bar specimens for the base metal and the fine-grained heat affected zone, and has been taken into the damage model. Creep test of welded joint specimen of modified 9Cr-1Mo steel has been conducted to confirm the accuracy of the damage evaluation method. It has been concluded that the predicted trend of creep damage has good agreement with the experimental results, but the predicted rupture time become longer than the experimental results of rupture time.

Developed 9Cr-3W-3Co-Nd-B heat-resistant steel SAVE12AD (Recently designated as ASME Grade 93) pipes and tubes have higher creep strength in both base metal and welded joints than conventional high Cr ferritic steels such as ASME Grades 91, 92 and 122. The welded joints of SAVE12AD tubes with commercial filler wire for W62-10CMWV-Co (Gr. 92) or Ni base filler wire ERNiCr-3 (Alloy82) also have much better creep rupture strength than those of conventional steels because of suppression of refining in the Heat-Affected-Zone (HAZ). However, the creep rupture strength of weld metal of W62-10CMWV-Co was marginal. Additionally, the hot cracking susceptibility of weld metal using Ni base filler wire ERNiCr-3 was occasionally below the required level. Similar welding consumable for SAVE12AD has been developed to solve these problems. Optimization of nickel, neodymium and boron contents on similar welding consumable enables to obtain both the good long-term creep rupture strength and low enough hot cracking susceptibility of weld metal. Consequently, SAVE12AD welded joint is expected to be applied of piping and tubing above 600°C in USC power plants because of its good properties with similar welding consumable.

In this paper, the performance of T23 and 12Cr1MoVG water wall tubes as well as their welded joints in engineering applications is reported. It was found that the T23 water wall tube may have water leak problems during its operation. In order to make sure the safe operation, leakage reasons of T23 water wall tube were analyzed and improvement measures were taken. Recommendations on the choice of water wall material of 1000MW USC tower boiler are given.

In order to clarify the effect of stress state on microstructural changes during creep, the microstructure was observed in the central part of the cross section of the fine-grained heat-affected zone (FGHAZ) and in the surface region of the FGHAZ in Gr.92 steel welded joint. Creep tests were performed under constant load in air at 650°C, using cross-weld specimens. The creep strength of welded joint was lower than that of base metal. Type IV fracture occurred in the long-term. Creep voids were detected in the FGHAZ after the fracture. Number of creep voids was higher in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. It was checked the multiaxiality of stress during creep was higher in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. The recovery of dislocation structure occurred after creep in the base metal and the FGHAZ. Mean subgrain size increased with increasing time to rupture. However, there was no difference of change of subgrain size during creep in the central part of the cross section of the FGHAZ and in the surface region of the FGHAZ. The growth of M 23 C 6 carbide and MX carbonitrides was observed during creep in the base metal and the FGHAZ. Laves phase precipitation occurred during creep. There was no difference of the change of mean diameter of MX carbonitrides in the central part of the cross section of the FGHAZ and in the surface region of the FGHAZ after creep. However, the growth rate of M 23 C 6 carbide in the FGHAZ was much higher in the central part of the cross section than in the surface region.

The creep rupture properties of welded joints of advanced 9%Cr-Mo-Co-B steel used for 620°C USC steam turbine have been studied. The welded joints were prepared by means of shielded metal arc welding (SMAW). A lot of creep tests have been conducted and the results indicate that fracture usually occurs in the intercritical heat affected zone (ICHAZ) of the welded joint and is typical of Type IV cracking. The microstructure of the HAZ has been investigated by using optical microscopy, SEM and TEM. The degradation mechanism of welded joint of the 9%Cr-Mo-Co-B steel has been explored by analysing the phases of precipitates. Creep voids were observed in the vicinity of the coarse Laves phase particles, resulting in the degradation of the creep rupture properties.

Dynamic development of steels used in power engineering industry for the production of boilers characterised by supercritical parameters poses new welding challenges. The introduction of new combinations of alloying agents aimed at obtaining the best possible mechanical properties, including creep resistance, affects the weldability of new steels. Each of the latter have to undergo many tests, particularly as regards bending and welding, in order to enable the development of technologies ensuring failure-free production and assembly of boiler systems. Martensitic steels containing 9% Cr, used in the manufacturing of steam superheaters, are characterised by good creep resistance and, at the same time, low oxidation resistance at a temperature in excess of 600°C. In turn, steels with a 12% Cr content are characterised by significantly higher oxidation resistance, but accompanied by lower strength at higher temperatures, which translates to their limited application in the production of boilers operating at the highest parameters. The niche between the aforesaid steels is perfectly filled by austenitic steels, the creep resistance and oxidation resistance of which are unquestionable. This article presents experience gained while welding dissimilar joints of advanced steels TEMPALOY AA-1 and T92, with the use of EPRI P87, Inconel 82 and Inconel 617 filler metals. The tests involving the said steel grades belong to the very few carried out in the world.

This paper presents RAFAKO S.A. experience within the field of research and implementation of new group of creep-resistant steel grades with addition of tungsten, including the selection of filler metal, welding procedures and selected results of mechanical properties testing carried out during the implementation of welding process for elements of steam superheaters of the boilers with supercritical parameters. It summarizes the experience and results of research works carried out in RAFAKO, Silesian Technical University in Katowice and Welding Institute in Gliwice within the framework of COST 522 program concerning the influence of simulation of element temperature operating conditions on Chaгpy V notch toughness, HV10 hardness, microstructure of simulated HAZ's, assessment of steel weldability, strength properties. The welded joints structure stability and HAZ after stress relief annealing and after ageing - simulation of element operating conditions are presented in the form of graphs and prints of microstructures.

This paper reports on a study that investigated how low cycle fatigue (LCF) and fatigue crack propagation (FCG) properties of P92, P122, and P23 steels vary between 600°C and 700°C depending on the location relative to a cross weld. Microstructure analysis was also performed on fractured specimens. Due to its higher yield strength, P122 exhibited the best continuous LCF life. However, creep-fatigue interaction (CFI) in the weld heat-affected zone (HAZ) of P122 and P23 steels significantly reduced their lifespans compared to continuous LCF tests. This reduction is attributed to the effect of weld thermal cycles on fine precipitates. FCG tests revealed that the base metal consistently outperformed the HAZ in all tested steels and temperatures. P92 and P122 showed similar FCG rates except for P92's behavior at 600°C, which resembled P23. In both steels, the HAZ exhibited faster FCG rates at 600°C and 700°C compared to the base metal, particularly at lower stress intensity factor ranges (ΔK). Within the HAZ, the region 1 mm from the fusion line displayed the slowest FCG rates, followed by the base metal, while the fusion line and the region 2 mm from it showed the fastest. Fracture surfaces near the fusion line displayed cleavage-like features, while the region 1 mm away exhibited features associated with higher crack growth resistance.

Short characteristics of the out-of-date Polish power generation system are given, which shows that in the near future there will be a shortage of electrical energy and the necessity to build supercritical power units. A lignite-fired boiler will be build at the RAFAKO Boiler Plant. Supercritical operating parameters require new creep resisting steels to be applied for the boiler and pipe systems. Therefore at the Institute of Welding weldability examinations have been performed on selected Cr-W heat resisting steels. Welding thermal cycles have been simulated on steels: HCM2S (T23/P23), T92/P92, E911 and HCM12A. The influence of t 8/5 cooling times on Charpy V notch toughness, HV10 hardness and microstructure of simulated HAZ's is presented in the form of graphs and prints of microstructures. By means of simulation technique the susceptibility to reheat cracking of those steels has been evaluated. At REMAK- Opole (Enterprise for the Modernisation of Power Installations) and RAFAKO tube and pipe test joints were welded, to select proper fabrication conditions. Mechanical properties of the welded test joints, KV notch toughness of weld metals and HAZ’s and microstructures were examined and are presented.

This study investigated the creep rupture strength and microstructure evolution in welded joints of high-boron, low-nitrogen 9Cr steels developed by NIMS. The welds were fabricated using the GTAW process and Inconel-type filler metal on steel plates with varying boron content (47-180 ppm). Creep rupture tests were conducted at 923K for up to 10,000 hours. Despite their higher boron content, these steels exhibited good weldability. Welded joints of the boron steel displayed superior creep properties compared to conventional high-chromium ferritic steel welds like P92 and P122. Notably, no Type IV failures were observed during creep testing. Welding introduced a large-grained microstructure in the heat-affected zone (HAZ) heated to the austenite transformation temperature (Ac3 HAZ). This contrasts with the grain refinement observed in the same region of conventional heat-resistant steel welds. Interestingly, the grain size in this large microstructure was nearly identical to that of the base metal. Analysis of the simulated Ac3 HAZ revealed crystal orientation distributions almost identical to those of the original specimen. This suggests a regeneration of the original austenite structure during the alpha-to-gamma phase transformation. Simulated Ac3 HAZ structures of the boron steel achieved creep life nearly equivalent to the base metal. The suppression of Type IV failure and improved creep resistance in welded joints of the boron steels are likely attributed to the large-grained HAZ microstructures and stabilization of M 23 C 6 precipitates. The optimal boron content for achieving the best creep resistance in welded joints appears to lie between 90 and 130 ppm, combined with minimized nitrogen content.

Grade 92 steel, a creep strength-enhanced ferritic (CSEF) steel, is used in supercritical steam fossil power plants for boilers and piping systems. While its creep strength is crucial, understanding the interaction between creep and fatigue damage is also vital for assessing component integrity under cyclic loading. Despite existing studies on its creep-fatigue behavior, additional data under creep-dominant conditions relevant to plant evaluations are needed. Girth welds, critical to piping system integrity, are particularly important in this context. EPRI and CRIEPI initiated a project to develop a comprehensive database on the creep-fatigue behavior of Grade 92 steel's base metal and welded joints and to establish a suitable life estimation procedure. Key findings include: (i) a thick pipe with submerged arc welding (SAW) was manufactured for testing; (ii) base metal and cross-weld specimens showed similar behavior under short-term creep and cyclic loading; (iii) these specimens had lower creep strengths than average literature values for this steel class in the short time regime, with differences decreasing as stress decreased; and (iv) the fatigue and creep-fatigue behavior of these specimens were similar to those of Grade 91 and 122 steels, with common characteristics in creep-fatigue failure prediction models across the three CSEF steels.

This paper presents comprehensive test results of thick-walled VM12 steel pipes containing 12% chromium, vanadium, and tungsten, with cobalt addition. The primary objective was to verify welding technologies for boiler superheater thick-walled components and characterize the strength, technological properties, and microstructure of welded joints produced at RAFAKO S.A. The extensive research program encompassed a broad range of tests on both parent material and welded joints, including mechanical property assessments at room temperature, creep resistance evaluations, low-cycle fatigue testing at room temperature and 600°C (1120°F), and detailed macro- and microstructural examinations. Furthermore, the investigation included a comprehensive microstructural stability assessment using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), conducted after fatigue resistance testing at room and elevated temperatures, following additional annealing at 700°C (1,920°F), and after 1,000 hours of exposure for both parent material and welded joints. These investigations were conducted as part of the COST 536 Action, representing a collaborative effort to understand and characterize high-temperature creep-resistant steels like VM12 for advanced power generation applications.

The strength of welded joints in high-chromium steels is a critical concern for operators of ultra-supercritical thermal power plants. To investigate this, a series of creep-fatigue tests with tensile strain holds were conducted on welded joints of two widely used high-chromium steels: Grade 91 and Grade 122. The tests revealed that failure consistently occurred in the fine-grain heat-affected zone, even at relatively low temperatures and short durations, whereas in simple creep tests, failure occurred in the plain base metal region. Four different procedures were used to predict failure life, and their results were compared with experimental data. Among them, a newly proposed energy-based approach provided the most accurate failure life estimations, independent of material type and temperature.

Two materials with different purity of 2.25Cr-1Mo steel thick weld joint were prepared and creep rupture behavior was investigated by large sized specimens. For high purity material, two types of challenging heat treatment was tried to modify the original microstructural conditions. Weld joints were made and large sized creep test specimens were machined. Creep tests were performed at 903K, 40MPa. Specimen made from low purity material fractured at fine grained heat affected zone (FGHAZ) and showed so-called Type IV cracking. On the other hand, specimen made from high purity material showed maximum creep damage at weld metal. In the case of specimens applied challenging heat treatment, remarkably high ductility were observed at fracture. Regarding 2.25Cr-1Mo steel, it was confirmed that the suppression of Type IV cracking had been basically achieved by past improvement on purity level. At the same time, improvement of heat treatment condition was found to have further effect. Because of improved creep properties of high purity material, properties of weld metal had rose up to be the next issue to be examined. At least, taking care on layout design of weld beads to avoid creating wide spread fine grained portion is desired.

In the present study, the Inconel 617B superalloy welded trial rotor was fabricated by narrow gap tungsten inert gas (NG-TIG) welding and the effects of temperature on fracture toughness of its welded joint were investigated at 650 ℃ and 730 ℃. Fracture toughness (J0.2) of the base metal was much higher than that of the weld metal at the same temperature, which was attributed to its excellent macroscopical plasticity and the interactions of strain localization, misorientation, and coincidence site lattice (CSL) boundaries. For the base metal, the value of J0.2 was higher at 730 ℃ than at 650 ℃, resulting from the appreciable increase in ductility and decrease in strain localization as the temperature increased. For the weld metal, higher temperature (730 ℃) reduced strength but hardly improved plasticity, and the regions of high strain localization uniformly distributed in the weld metal, resulting in completely tearing the whole interface apart and lower fracture toughness of the weld metal.

In this study, creep rupture behaviors and rupture mechanisms of dissimilar welded joint between Inconel 617B and COST E martensitic steel were investigated. Creep tests were conducted at 600 ℃ in the stress range 140-240 MPa. Scanning electron microscopy (SEM) and micro-hardness were used to examine the creep rupture behaviors and microstructure characteristics of the joint. The results indicated that the rupture positions of crept joints shifted as stress changed. At higher stress level, the rupture position was located in the base metal (BM) of COST E martensitic steel with much plastic deformation and necking. At relatively lower stress level, the rupture positions were located in the fine-grained heat affected zone (FGHAZ) of COST E or at the interface between COST E and WM both identified to be brittle fracture. Rupture in the FGHAZ was caused by type Ⅳ crack due to matrix softening and lack of sufficient precipitates pinning at the grain boundaries (GBs). Rupture at the interface was related to oxide notch forming at the interface.