Numerical simulation of mechanical controlling parameters for Type IV cracking on the welding joints of martensitic heat-resistant steel

The maximum principal stress, von Mises equivalent stress and equivalent creep strain in the welding joint of martensitic heat-resistant steel (9Cr1MoVNb) are simulated by finite-element method (FEM) under the condition of 600°C and applied stress of 80 MPa. The results show that the maximum principal stress and von Mises equivalent stress are high on the curved points of two sides of the groove face near the fine-grain heat-affected zone (HAZ). The creep strain mainly concentrates in the fine-grain HAZ; the maximum creep strain locates in the bottom of fine-grain HAZ of specimen. The stress triaxiality in the fine-grain HAZ is maximum, and creep cracking occurs because of the intensive constrain of base metal and weld. The simulation result is good in agreement with those of crack initiation site and propagation path by using the stress triaxiality as the mechanical controlling parameter of weld joint of martensite heat-resistant steel. Therefore, it is reasonable that the stress triaxiality is used for analysis initiation and propagation of Type IV cracking in the fine-grain HAZ.     

Effect of multiaxial stress state on creep damage in ASME T92 welded joint

This study reports the type IV fracture process and the influence of multiaxial stress state in ASME T92 welded joints during creep. The type IV fracture occurs at the fine‐grained heat‐affected zone (ie, FGHAZ), involving void initiation, growth, and coalescence, microcrack occurrence, propagation and extension, and eventual macrocrack with consequent joint failure. The creep damage is not uniformly distributed along the thickness direction in the FGHAZ, and the central part of the welded joint is the most seriously damaged region. The equivalent creep strain is higher at the external surface, but the stress triaxiality is larger in the centre section. Large equivalent creep strain could promote creep void initiation, whereas high hydrostatic pressure and stress triaxiality factor accelerate void growth in the FGHAZ of T92 joints. Besides, reducing groove angle and HAZ width of the joints is recommended to delay the occurrence of type IV cracking because of lower equivalent creep strain and stress triaxiality factor.     

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.     

Numerical investigation of factors affecting creep damage accumulation in ASME P92 steel welded joint

The present study mainly investigated Type IV cracking occurring in the fine grained heat affected zone (FGHAZ) in the welded joint of ASME P92 steel at high temperature and low applied stress by numerical simulation method. Based on the modified Karchanov–Rabotnov constitutive equation, the user defined material subroutine (UMAT) was complied and the creep damage accumulation was carried out by finite element method using ABAQUS codes for the welded joint at 650 °C and 70 MPa. Calculated results revealed that the most severe creep damage and the highest equivalent creep strain occurred in the FGHAZ because of high maximum principle stress and high maximum principle stress. Furthermore, the effect of groove angle and HAZ width on the creep damage accumulation was investigated. It indicated that a small groove angle and a narrow FGHAZ width could deteriorate the creep damage accumulation because of the degradation of maximum principle stress and stress triaxiality in the FGHAZ.     

Characterization of incubation time on creep crack growth for weldments of P92

Most structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. In such weldment, it is known that the multi-axial stress fields appear due to the plastic constraint induced by the differences in material micro-structure between the weld metal, heat affected zone (HAZ) and base metal. In the present study, the experiments of creep crack growth tests and the structural mechanical analyses of weldment were conducted to understand the effects of stress multi-axiality of weldment on the characteristics of creep crack growth, creep deformation and creep ductility. Additionally, to characterize the time of creep crack initiation up to the start of a brittle type creep crack growth, the distribution of stress multi-axiality (TF) through the base metal, fine-grain HAZ, coarse-grain HAZ to weld metal were investigated.     

Results of a Japanese round robin program for creep crack growth using Gr.92 steel welds

High-Cr ferritic heat-resistant steels are commonly used for boiler components in ultra-super critical thermal power plants operated at about 600 °C. In the welded joints of these steels, Type-IV cracks initiate in the fine-grained HAZ during long-term use at high temperatures, causing their creep strength to decrease. To assist the standardization of the testing and evaluation method for creep crack growth (CCG) in the welded components, we conducted round robin tests (RRT) using 9Cr-0.5Mo-1.8 W-V-Nb steel (ASME Grade 92 steel) welded joint as part of the VAMAS TWA31 collaboration. The CCG tests were carried out using the CT specimen and the circumferentially-notched round bar specimen for both the base metal and welded joint of Gr.92 steel. Testing was performed at four different laboratories. The effects of specimen configuration, temperature, load, and stress triaxiality conditions on the crack growth rate and fracture life were investigated.     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

Fracture of austenitic steel subject to a wide range of stress triaxiality ratios and crack deformation modes

The stress triaxiality ratio (hydrostatic pressure divided by von Mises equivalent stress) strongly affects the fracture behaviour of materials. Various fracture criteria take this effect into consideration in their effort to predict failure. The dependency of the fracture locus on the stress triaxiality ratio has to be investigated in order to evaluate these criteria and improve the understanding of ductile fracture.This was done by comparing the experimental results of austenitic steel specimens with a strong variation in their stress triaxiality ratios. The specimens had cracks with varying depths and crack tip deformation modes; tension, in-plane shear, and out-of-plane shear. The crack growth in fracture mechanics specimens was compared with the failure of standard testing specimens for tension, upsetting and torsion. By associating the experimental results with finite element simulations it was possible to compare the critical plastic equivalent strain and stress triaxiality ratio values at fracture. In the investigated triaxiality regime an exponential dependency of the fracture locus on the stress triaxiality ratio was found.     

The effects of compressive and tensile prestrain on ductile fracture initiation in steels

It has been recognized that ductility of prestrained steel is inferior to that without prestrain, and the critical equivalent plastic strain of ductile fracture initiation is inversely related to stress triaxiality. In this paper, the effects of compressive and tensile prestrain on ductile fracture initiation in steels are investigated quantitatively by adopting the relationship between stress triaxiality and critical equivalent plastic strain. It is found that compressive prestrain leads to cleavage cracking and reduces ductility. In the case of the TMCP steel, compressive prestrain up to 30% does not decrease the ductility, accompanied by no evidence of cleavage cracks. However, in the case of SM490B steel, 30% compressive prestrain leads to cleavage cracking and reduces ductility significantly.     

Microstructural and failure analysis of welded primary reformer furnace tube made of HP-Nb micro alloyed heat resistant steel

In this paper, the failure of the reformer furnace tube, made of HP-Nb micro alloyed heat resistant steel is investigated. The failure was due to a longitudinal crack developing in the circumferential weld joint close to the outlet end of the tube, where the mean wall temperature had reached the highest value during service. According to the temperature records during the period of 7 years of being in service, the tube had experienced temperatures higher than the tubes designed temperature. This situation besides the occurrence of several sudden shutdowns caused the mechanical properties degradation.Macroscopic and microscopic examinations revealed that the failure had occurred due to the extensive fissuring initiated and developed from the heat affected zone (HAZ) adjacent to the root weld. The results of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) analysis, and the crack growth rate calculations indicated that the crack propagation rate was controlled by the overheating and the secondary thermal stress due to the sudden shutdowns.Moreover, the ductility at the tip of the crack had dwindled due to the nitrogen uptake at high temperature, which promoted the crack growth rate. Quantitative microstructural analysis performed at the uncracked and cracked sides of the weld joint showed that the area fraction of HAZ creep cavities were 0.3% and 0.7%, respectively, which were interpreted in turn as the accumulated creep damage level D and E. Finally, it was concluded that the creep cavities alignment and fissuring that took place at HAZ are the main causes of the cracking and failure of the tube.     

Applicability of C parameter in assessing Type IV creep cracking in Mod. 9Cr-1Mo steel welded joint

A welded joint of Mod. 9Cr-1Mo steel, whose Type IV cracking behavior is an important issue to be assessed, was subjected to a series of creep crack propagation experiments in order to clarify the applicability of existing standard ASTM E1457-98. Standard 1T-C(T) specimens made of base metal (BM) and welded joint (WJ), in which the heat affected zone (HAZ) was set to be the crack plane, were subjected to the experiments under 600, 650, and 700 °C, and with a few load level conditions. While the crack planes of BM specimens were fairly flat, those in WJ specimens showed bumpy surfaces following the shape of multi-path weld beads. The cracks in WJ specimens were of typical Type IV cracking, and their crack passed through nearby the interface of BM and weld metal. There are the fairly good relationships between the creep crack propagation rate (da/dt) and C^* parameter. All the BM and WJ data fallen in each one C^*-da/dt relationship for BM and WJ, respectively, regardless of the temperature and load level. The C^* parameter used here is defined for the homogeneous material and does not give a physically correct C^* for WJ, nevertheless all the WJ data still tends to gather each other on single C^*-da/dt relationship. This fact suggests that the geometrical limitations of E1457-98 standard also can act well as the limitation for the inhomogeneity of weld structure and may eliminate the effect of large scale inhomogeneity due to the combination of BM and weld metal. The da/dt of WJ were about 3-10 times faster than that of BM for the same C^* value. This difference can be attributed as the effect of difference in triaxiality, the relative constraint between the weld metal and the base metal, or the difference in resistance for creep crack propagation in HAZ material.     

WEL-TEN 62CF钢冷裂性能分析

本文对WEL-TEN 62CF高强钢进行了碳当量和冷裂敏感系数分析计算、热影响区最高硬度试验、斜Y坡口焊接裂纹试验和十字接头焊接裂纹试验等一系列焊接试验。从冷裂倾向上分析62CF钢选用日铁L-62CF焊条,采用手工电弧焊时,焊接接头的冷裂敏感性,确定焊接时是否需要预热。通过冷裂敏感性试验,说明62CF高强钢即使在不预热的情况下,冷裂倾向都较小,但预热可降低焊接接头的硬度,有利于提高焊接接头的抗裂性能。     

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M₂₃C₆ precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M₂₃C₆ precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M₂₃C₆ precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.     

Nanoindentation creep behavior of RPV’s weld joint at room temperature

SA508 Gr.3 steel has been widely used in nuclear reactor pressure vessels (RPV). Nuclear components are generally combined through arc welding processes, which always produces heterogeneous mechanical properties in heat affected zone (HAZ) of weld joint. In order to study mechanical heterogeneity of weld joint, HAZ was been divided into five small regions (HAZ1 to HAZ5) based on the distance from the weld center line. The elastic modulus, hardness, and creep deformations of five regions in HAZ were measured through nanoindentation, as well as base and weld metals. According to the experimental results, the HAZ2 region (belonging to the fine-grained HAZ) exhibited a significantly lower hardness and creep behavior. Strain rate sensitivities (SRS) in different regions were then estimated from the steady-state creep, and the HAZ2 region showed a relatively higher value. The influence of grain boundary fraction on the creep behavior of weld joints was discussed later. Furthermore, the results of SRS also indicated that the creep mechanism of tested regions could be dominated by dislocation activities.

     

Determination of critical stress triaxiality along yield locus of isotropic ductile materials under plane strain condition

It is widely accepted that failure due to plastic deformation in metals greatly depends on the stress triaxiality factor (TF). This article investigates the variation of stress triaxiality along the yield locus of ductile materials. Von Mises yield criteria and triaxiality factor have been used to determine the critical limits of stress triaxiality for the materials under plane strain condition. A generalized mathematical model for triaxiality factor has been formulated and a constrained optimization has been carried out using genetic algorithm. Finite element analysis of a two dimensional square plate has been carried out to verify the results obtained by the mathematical model. It is found that the set of values of the first and the second principal stresses on the yield locus, which results in maximum stress triaxiality, can be used to determine the location at which crack initiation may occur. Thus, the results indicate that while designing a certain component, such combination of stresses which leads the stress triaxiality to its critical value, should be avoided.     

Analysis and redesign of failed jacket in a petrochemical reactor

The root cause of repeated water leaks due to cracking in a circumferential weld of the heating jacket of a petrochemical reactor was traced to cyclic steam pressure, not accounted for in the design. Fractographic and metallographic analyses of the jacket base and weld materials revealed widely open cracks, initiation and propagation being due to larger than allowable cyclic stresses. There were no indications of environment induced cracking, neither creep nor stress corrosion cracking. Appendix 2 of API RP 579 was used to verify alternative designs for the substitution of the heating jacket. Due to time restraints, the substituted part had to be built with a lower strength steel. Reconstruction was successfully done using an austenitic SA 316L stainless steel.     

Failure Analysis of a Heat Exchanger Shell

This paper presents a failure analysis performed to investigate cracking observed in a weld joint in the shell of a reheater. Thorough investigation using visual inspection, chemical analyses, metallographic examinations, hardness testing, and creep analysis showed that the weld joint failed by creep. The shell was fabricated from the heat-resistant alloy, Incoloy 800H, and had been in service for 16 years. The shell was exposed to superheated steam at 720 °C. Corrective actions were proposed to repair the shell and prevent recurrence of cracking.     

Creep stress concentrations in mis-matched weldments—starting points for early creep damage and creep cracking

The material creep properties of weldments, i.e. the minimum creep strain rate, the creep rupture strength and the creep ductility, do in general differ from those of the parent material. It is often assumed that if the weld material creep strength is higher than that of the parent, the life of the weldment will also be higher. This is not always true. In fact, when subjected to a prescribed displacement, e.g. a circumferential weldment in a pipe subjected to internal pressure, a creep hard weld develops a stress concentration which may result in a reduced life expectancy of the weldment system. Similarly, the properties of the heat affected zone often differ from those of the parent and weld material properties which may be one reason for early Type IV cracking. In addition, the weld preparation geometry, the width of the HAZ and other geometric parameters influence the stress distribution. The present paper summarizes some of the effects of material property variations in weldments and their impact on the creep life expectancy. It is shown that the stress concentrations resulting from material property mis-matching may be high enough to produce premature creep damage in the weldment, or will cause creep cracking starting from initial defects.     

Evaluation of long-term creep damage in high Cr ferritic steel welds

Abstract

The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.     

Creep resistance of similar and dissimilar weld joints of P91 steel

Abstract

Two experimental weld joints, a similar weld joint of 9Cr–1Mo steel and a dissimilar weld joint of 9Cr–1Mo and 2.25Cr–1Mo steels, were fabricated by the TIG+E method and post-weld heating was applied. Creep testing was carried out at temperatures ranging from 525 to 625°C in the stress range 40–240 MPa. Creep rupture strength was evaluated using the Larson–Miller parameter. Extended metallography including transmission electron microscopy was performed and critical zones were indicated where fractures were concentrated during the creep exposure. At high temperatures rupture of the dissimilar weldment occurred in the heat affected zone (HAZ) of the weld metal while rupture of the similar weldment was located in the HAZ of the parent material. The processes of recovery seem to be the main causes of decrease in creep rupture strength of both weld joints in comparison to the parent materials.