High cycle fatigue behaviors of API X65 pipeline steel welded joints in air and H2S solution environment

To investigate the mutual effect of hydrogen, microstructures and stress concentration on the fatigue failure, fatigue behaviors of X65 steel welded joints in both air and saturated H₂S solution were investigated at high cycle regime. The experimental result demonstrates that due to lower dislocation density observed by electron backscattered diffraction (EBSD), the fine grain heat affected zone (FGHAZ) is prone to induce cyclic strain localization and further lead to fatigue crack propagating along the FGHAZ in air. Furthermore, the quasi-cleavage with brittle-like fatigue striations and secondary crack on the fracture surface in saturated H₂S solution is attributed to hydrogen embrittlement. Moreover, compared with base metal (BM) and FGHAZ, the weld metal (WM) and coarse grain heat affected zone (CGHAZ) are composed of bainite and martensite/austenite (M/A) phase, and more sensitive to hydrogen. Therefore, the fatigue crack is prone to grow along the interface between WM and CGHAZ under the normal applied stress.     

Analysis of cross-weld creep rupture data on the ½Cr½Mo¼V type IV zone

For the ferritic steel ½Cr½Mo¼V, the operating experience of medium and long term creep failures in power station welds has been of cavitation and cracking in the heat affected zone (HAZ) adjacent to the parent material, in what is referred to as the Type IV zone. This experience has led to the generation of uniaxial cross-weld creep rupture data on ½CrMoV weldments and to the development of life assessment procedures such as R5 Volume 7, which uses the rupture strength of the Type IV zone to calculate the life of power station components. Recently, the ECCC Working Group 3A has conducted a collation of the UK and German cross-weld creep rupture data on ½CrMoV weldments. Creep failure in ½CrMoV cross-weld specimens occurs in a variety of weldment zones; typically in the parent, the Type IV zone or the weld metal. Post test examination of the specimens has enabled those tests that failed in the Type IV zone to be identified and a creep rupture data assessment has been performed to derive a new model for the rupture strength of the Type IV zone.     

620℃超超临界机组用9% Cr-Mo-Co-B钢（CB2）焊接接头性能退化机制研究

文章研究了620℃超超临界机组9%Cr-Mo-Co-B钢（CB2）焊接接头的持久性能,焊接接头采用SMAW方法进行焊接,焊材分别选用MTS 5Co1和MTS-616。持久性能实验表明焊接接头的持久强度低于母材,断裂位置发生在焊接接头的热影响区临界区（ICHAZ）,为典型的IV型裂纹。文章采用光学显微镜OM及扫描电镜SEM观察焊接接头的微观组织及析出相,通过对析出相的分析研究了9%Cr-Mo-Co-B钢焊接接头的性能退化机制的成因。     

Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.     

Creep deformation of induction pressure welded 2·25Cr-1Mo steel

In this paper, experimental results involving the effect of stress and temperature on creep behaviour of induction pressure welded (IPW) 2·25Cr-1Mo steel are presented. Creep rupture tests were conducted at 550–700°C in steps of 50°C over a stress range of 112·5–180 MPa. Above 650°C failure of the specimen was enhanced due to the microstructural instability. Failure in the specimens occurred invariably in the heat affected zones (HAZ), and the fracture surfaces indicated ductile failure.     

Difficulties in interpreting data from creep crack growth tests on type 316H weldments

Abstract

Experimental creep crack growth data are generally obtained by following standard methods such as ASTM E1457-07 and subsequently characterised using the C* parameter. These data are then used in assessment procedures, such as R5, together with reference stress estimates of C* in the component, to predict creep crack growth behaviour. Some modifications to the ASTM E1457 creep crack growth testing and analysis methods have already been proposed following a previous analysis of data from long term creep crack growth tests on type 316H parent material. This paper reports the results of creep crack growth tests on type 316H heat affected zone material at 550°C using compact tension (CT) specimens manufactured from non-stress relieved thick section butt welds. It is shown that interpretation of the data from these weldment tests is complicated by both the discontinuous nature of the cracking process and the presence of significant residual stresses in the CT specimens. Further modifications to creep crack growth testing and analysis methods are proposed to address difficulties arising from the discontinuous nature of the cracking process, and further work is identified to investigate the influence of the residual stresses present in the specimens on the observed crack growth behaviour.     

Creep degradation in welds of Mod.9Cr-1Mo steel

Creep life assessment technology has not yet been developed for Mod.9Cr-1Mo steel welded joints due to the unique degradation and failure mechanisms in the heat affected zone. Nevertheless, there is strong demand from power plant operators for the development of nondestructive damage detection and life assessment technology for weldments of this steel. In this study creep rupture testing using a large-size welded joint was conducted, and creep and damage detection tests were carried out to elucidate the degradation mechanism. Also the microstructure and hardness changes in the heat affected zones of the welds were investigated to clarify the degradation mechanisms of such welds in comparison with base metal during creep. In general, the changes in hardness distribution along the welds were difficult to correlate with the life consumption, although degradation in welds due to creep was successfully detected. Accordingly, a new approach to degradation evaluation and creep life assessment by hardness measurement method for Mod.9Cr-1Mo steel welds is proposed.     

Performance of repair welds on aged 2.25Cr–1Mo boiler header welds

In order to clarify the performance of repair welds on power boiler, thick parts such as header and steam piping, an ex-service aged 2.25Cr–1Mo header was repaired using SMAW with postweld heat treatment and the mechanical properties of the repair welded joints were experimentally evaluated.Creep rupture life of the repair welded joint was almost same as that of service-degraded base metal and heat-affected zones. It was proved that the life reduction would not be caused by repair welding. In creep–fatigue tests with strain holding, some type of repair welded joints was fractured at the heat affected zone caused by repair welding. This may be caused by strain concentration at the heat-affected zone under strain holding. Charpy impact toughness of the simulated heat affected zone due to repair welding was much higher than that of service-degraded base metal. It was proved that the toughness would be restored by repair welding.     

Effect of specimen size and shape on creep rupture behavior of creep strength enhanced ferritic steel welds

The creep strength enhanced ferritic (CSEF) steels such as Grades 91, 92, 122, 911, 23 and 24 have become very important key materials for high efficiency fossil-fired power plants for last decades, however the long-term creep rupture strength and strength reduction in welds due to Type IV failure of these steels are serious problem to be urgently resolved. In order to use CSEF steel welds safely setting new weld strength reduction factors have internationally discussed. For instance ASME Boiler and Pressure Vessel Code Committee recently developed a new creep strength reduction factors for CSEF steels intercritically post-weld heat treated to be 0.5 based upon the creep rupture data obtained for standard creep specimens. However it is needed to make further consideration on the specimen size/shape effect on the creep strength of the welds to determine more appropriate weld strength reduction factors and joint influence factors. Present report provides comprehensive creep rupture test results of the specimens with various size and shape, and full size components dealing with creep rupture location/behavior and specimen size/shape effect on the creep rupture strength of CSEF steel welds.     

A metallurgical contribution to the industrial development of 12% chromium martensitic steels for pressure vessels

Some basic metallurgical principles necessary for the processing of steel X20 Cr-Mo-V 12·1 are detailed by particularly CCT curves, forgeability and relaxation curves. These studies allow us to perform welded and bended pipes with results satisfactory with regard to German standard DIN 17175. Ageing tests carried out on this steel show no problem in relation to embrittlement. Low-cycle fatigue tests have been performed in the range of temperatures 350–600°C. For several temperatures, low-cycle fatigue equations have been specified. Finally crack-growth tests, performed at room temperature, show that the cracking growth rates are very similar for the parent metal, heat affected zone and weld.     

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A_c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.     

锅炉F-A异种钢接头早期失效统计分析及防控措施

应用于超(超)临界机组的电站锅炉受热面的铁素体钢-奥氏体钢(F-A)异种钢接头经常发生断裂,断裂形式以铁素体钢侧熔合线开裂为主.对江苏省内超(超)临界锅炉F-A异种钢接头失效情况及检验结果进行了统计分析,发现F-A异种钢接头倾向于在低等级材料、薄壁管、较大附加弯矩、复杂管屏、焊缝根部存在焊瘤等情况下发生失效;且接头失效...     

Creep and fracture behaviour of English Electric Mark III transition welds: Part (b) Ex-service weldments

The microstructure and high temperature behaviour of an ex-service weldment have been assessed. Exposure at elevated temperature resulted in some carbide development in the niobium stabilised buttered layer. However, significant decarburisation in the low alloy steel was not detected. Cross-weld uniaxial creep testing invariably led to low ductility failures in the heat affected zone on the 2.25 Cr1Mo steel side of the weld. These failures were associated with the nucleation, growth and link-up of cavities on prior austenite grain boundaries. Assessment of tests interrupted at selected life fractions indicated that the development of cavitation followed a sensible trend with creep exposure. The cavitation behaviour of the transition weld was in general agreement with data obtained from creep tests undertaken on material heat treated to simulate heat affected zone structures. The level of creep damage in these welds can be assessed by evaluation of local strain accumulation or through monitoring the presence of cavities.     

Creep damage evaluation of 9Cr–1Mo–V–Nb steel welded joints showing Type IV fracture

By conducting long-term creep rupture tests for 9Cr–1Mo–V–Nb (P91) steel welded joints, creep rupture properties and microstructures were examined. Creep rupture tests were conducted at three temperatures of 823, 873 and 923 K, under applied stresses of 160–230, 80–130, and 40–80 MPa, respectively. The rupture locations were found to shift from the weld metal at the higher stress condition to the fine-grained HAZ adjacent to the base metal at lower stress conditions at 873 and 923 K. The relationship between microstructural changes and crack nucleation site and propagation path was clarified. A remarkable decrease of dislocation density and growth of precipitates of M₂₃C₆ and Laves phase during creep was observed in the vicinity of the fine-grained HAZ adjacent to the base metal for the Type IV fractured welded joint specimen. The stress–strain distribution in the welded joint was investigated by the finite element method (FEM) using creep data of the simulated HAZ specimen. It was found that the observed crack initiation site and crack growth path coincided better with the distribution of the stress triaxiality factor than that of the equivalent creep strain.     

Creep fracture behavior of dissimilar weld joints between T92 martensitic and HR3C austenitic steels

The creep fracture of T92/HR3C dissimilar weld joints is investigated. HR3C austenitic steel is welded with T92 martensitic steel to obtain a T92/HR3C weld joint. After welding, creep tests are carried out at 625 °C in the stress range 110–180 MPa. The results indicate that the creep fracture mechanism is dependent on stress. When stresses ≥140 MPa, the fracture location is at the T92 base material and the connection of adjacent dimples results in transcrystalline fracture. For stresses <140 MPa, the fracture location is at the T92 coarse-grained heat affected zone and growth of M₂₃C₆ particles as well as Laves phase (Fe₂(W, Mo)) precipitation on the grain boundaries leads to intergranular fracture.     

Damage assessment method of P91 steel welded tube under internal pressure creep based on void growth simulation

Development of creep damage assessment methods for longitudinal welded piping of P91 steel is important and an urgent subject to maintain reliable operation of boilers in ultra super critical thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage in a heat-affected zone (HAZ) of the longitudinal welded pipe. Failure occurred at a heat-affected zone without significant macroscopic deformation. It was found that initiation of creep voids had concentrated at mid-thickness region rather than surface. Three-dimensional finite element (FE) creep analysis of the creep tested specimens was conducted to identify stress and creep strain distribution within the specimen during creep. Finite element creep analysis results indicated that triaxial tensile stress yielded at the mid-thickness region of the HAZ. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the heat-affected zone resulting in internal failure of the tube specimens. Void growth behavior in the heat-affected zone was well predicted with the previously proposed void growth simulation method by introducing void initiation function to the method. A “limited strain” was defined as rupture criterion and dependency of the maximum stress and multiaxiality on the “limited strain” was derived by the void growth simulation. Creep damage distribution in the HAZ under the internal creep test was calculated by proposed damage assessment method.     

Refurbishment of low pressure rotor last stage blades

Abstract

Work performed to refurbish last stage blades (LSBs) with articulated tip struts on low pressure steam turbine rotors is described. During the late 1990s, a series of erosion shield failures occurred on these high twist last stage blades, resulting in significant damage to tip struts and blades and causing generation downtime. Metallurgical investigation revealed the shield braze bond had degraded during cyclic service, resulting in shield detachment from the blade. Repair options considered included welding of Stellite bar to the blade leading edge, deposition of hardfacing weld metal on the blade leading edge and replacement of the complete blade tip with a laser hardened blade insert. From initial stress corrosion cracking tests, the laser hardened insert was selected. Trials to optimise welding parameters, joint set-up and weld bead deposition sequence are reported. The completed joint requires post-weld heat treatment to reduce residual stress and produce acceptable hardness across the weldment. Further high cycle fatigue and stress corrosion cracking testing of the aerofoil weld confirmed the suitability of this repair method. The susceptibility of the laser hardened insert to fatigue and environmentally assisted cracking was also found to be acceptable. Examples of the successful application of this patented repair technique are given.     

Modelling and experimental studies of alternative heat treatments in Steel 92 to optimise long term stress rupture properties

Abstract

The desire for power plant to give increased generating efficiency and decreased CO₂ emission has led to considerable effort over the last 10–15 years, to develop ferritic–martensitic steels which can be used for steam temperatures up to about 650°C. Examples are the addition of boron and increasing chromium content to 10–12 wt-%. However, high chromium levels have led to problems with long term precipitate stability. One approach which has not been widely explored, is the use of novel heat treatments to optimise the preservice microstructure to give the best long term creep rupture strength. Increased austenitising temperatures and lower tempering temperatures have been examined in Steel 92 (9Cr–0·5Mo–2W) and have produced significant improvements in creep rupture strength at temperatures up to 650°C compared with material given a conventional heat treatment. This has been achieved without any loss in ductility compared with conventional heat treatments. Test data for durations in excess of 40 000 h are presented. Modelling of microstructure evolution based on Monte Carlo simulations has shown important differences especially in the stability of grain boundary M₂₃C₆ and intragranular MX particles, between material with conventional and modified heat treatments. The model predictions are in good agreement with metallographic observations made on material before and after stress rupture testing. Continuum creep damage mechanics modelling based on the microstructural evolution has also been applied to predict creep life of Steel 92 and satisfactory agreement with creep rupture tests has been obtained.     

The role of reverse temper embrittlement on some low and high temperature crack extension processes in low carbon, low alloy steels: A review

The present paper attempts to review the influence or role that reverse temper embrittlement (RTE) plays on the low temperature (<400°C) environmentally assisted cracking (EAC) processes in aqueous environments and the high temperature ( > 400°C) creep dominated crack growth processes prevalent in low carbon, low alloy steels. RTE is a generic problem in iron based alloys and causes a marked reduction in the cohesion strength of grain boundaries due to the segregation of impurity solute atoms.

It has been shown that there is a strong possibility that RTE can promote or enhance EAC in low alloy steels (the heat affected zone in weldments being the most susceptible) subjected to aqueous environments at temperatures below about 350°C.

At higher operational temperatures, viz. above 400°C, the characteristics of creep embrittlement are discussed and three microstructural features which contributed to this phenomena are highlighted. Proposals aimed at mitigating creep embrittlement processes in low alloy steels are forwarded.

In certain steels it was recorded that two embrittlement processes were prevalent at operational temperatures in the range 450–550°C, viz. RTE and an irreversible creep embrittlement which was prevalent when large deformation occurred in service. Indeed both 2·25 Cr1Mo and Cr-Mo-V steels were severely temper embrittled at service temperatures of 430°C while the Cr-Mo-V steels did not exhibit evidence of RTE at higher service temperatures of around 530°C. However, the effects of RTE in promoting creep embrittlement are, as yet, unclear.     

Temperature dependence of low cycle fatigue of 316(N) weld metals and 316L(N)/316(N) weld joints

Abstract

The low cycle fatigue behaviour of 316(N) weld metals and 316L(N)/316(N) weld joints have been investigated in the temperature range of 300–873 K, at a strain amplitude of ±0·6% and a strain rate 3 6 10^–3 s^–1, to study the influence of dynamic strain aging (DSA). The 316(N) weld metal exhibited better fatigue life than the weld joint, though the weld metal has shown higher cyclic stress response and higher plastic strain accumulation than the weld joint. Significant features observed in the temperature regime of 300–873 K include the maximum in fatigue life at 573 K and DSA in the range of 673–873 K. Occurrence of DSA has been manifested through drastic reduction in fatigue life in the range of 673–873 K, associated with anomalous stress response. Dominant DSA effects have been observed at about 773 K in the weld joint and at 823 K in the weld metal. However, the effect of DSA is found to be nominal beyond 823 K where the reduction in fatigue life is attributed to the combined effects of oxidation and DSA. Secondary crack density measurements (in the range of 300–873 K) in the weld joint specimens revealed the severity of the heat affected zone (HAZ) in inducing fatigue damage. Parameters have been identified to determine the temperature corresponding to dominant DSA effects.