Microstructure and creep behaviour of pressure boundary welds from Grade 91 steel tee

Abstract

New power generation equipment firing either coal or natural gas is required to operate at high levels of efficiency with greater operational flexibility than has been true for such equipment in the past. Because of their unique combination of superior creep strength and favourable thermal–physical properties, Grade 91 and the other creep strength enhanced ferritic (CSEF) steels are materials of choice for components operating in the elevated temperature sections of this equipment. To fully realise the benefits of these alloys, it is essential that careful control be maintained during all phases of processing of the material, from design, through material production, to fabrication and installation. Absent such control, rates of damage accumulation can be much greater than expected from ‘Code’ minimum data. Problems have been associated with incorrect heat treatment: failure to develop the necessary structure during the normalising heat treatment or breakdown of that structure during subsequent sub-critical heat treatments. Heat treating schedules must consider not only the specific chemical composition of the base and weld filler metal, but also the cumulative impact of multiple heat treatments on the final material structure and its properties. The present work has examined the performance of welds in an ex-service component. The results suggest that, even when less than optimal heat treatment is carried out, the susceptibility for Type IV cracking in weld heat affected zones may be the life limiting factor for most CSEF steel components.     

Characterisation of dissimilar P91 and P92 steel welds joint

In the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised.     

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract

Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld.     

Single pass laser joining of Inconel 718 superalloy with filler

Abstract

Microstructure and mechanical property of CO₂ laser beam welded IN 718 superalloy were studied by electron microscopy and hardness testing. The use of a welding filler wire produced a sound fusion zone with no cracking but grain boundary microfissuring occurred in the heat affected zone (HAZ) and was observed to be significantly influenced by pre-weld heat treatment and laser welding speed. Crack-free weld was produced by a pre-weld heat treatment that minimised non-equilibirum grain boundary boron segregation and inhibited grain growth. While post-weld heat treatment (PWHT) reduced the difference between the hardness values of the base alloy, HAZ and the fusion zone, it resulted in increased HAZ cracking, which was likely aided by pre-existing cracks. The PWHT cracking was, however, avoided by subjecting pre-weld material to the heat treatment condition that produces crack-free weld during welding process.     

Development of EPRI P87 solid wire

Abstract

Dissimilar metal welds (DMW’s) between ferritic and austenitic materials at elevated temperatures have concerned boiler manufacturers and operators for decades because of the proven potential for premature failure. The industry has desired an improved filler metal that would minimize or eliminate DMW failures and, with the current trend toward higher boiler steam pressures and temperatures, have suitable creep strength for joining higher strength materials such as Grade 91 steels After years of research, the Electric Power Research Institute (EPRI) concluded the development and commercialization of a nickel-based filler metal, EPRI P87, for application in shielded metal arc welding (SMAW). This work describes the subsequent development of an EPRI P87 solid wire welding product for application in gas tungsten arc and gas metal arc welding (GTAW and GMAW) processes, and the initial research into the performance of DMWs produced with the new solid wire P87 product. A 135 kg heat of solid wire was produced and tested using various welding processes and evaluation methods to ensure that the material would meet required weldability and design specifications. Welding methods included GMAW-P, GTAW and hot-wire GTAW in welds up to 50mm in thickness. The weld joint tested was a dissimilar metal weld of grade 91 to 347H, which was assessed using microstructure evaluation, creep testing, hot tensile testing, circular patch, and edge build-up investigations to examine hot-cracking susceptibility. This paper summarizes the research completed to date on the EPRI 87 filler wire which supports the acceptability of this material for its intended use in high-temperature power generation applications.     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

Correlation between microstructure and creep performance of martensitic/austenitic transition weldment in dependence of its post-weld heat treatment

This paper deals with the influence of post-weld heat treatment (PWHT) of T92/TP316H martensitic/austenitic transition weldment on the resulting microstructure and creep characteristics. Experimental weldments were fabricated by gas tungsten arc welding using a nickel-based weld metal (Ni WM). After the welding, two individual series of produced weldments were heat-treated according to two different PWHT procedures. The first “conventional PWHT” was carried out via subcritical tempering (i.e. bellow Ac₁ temperature of T92 steel), whereas the other one, the so-called “full PWHT” consisted of a complete reaustenitization of the weldments followed by water-quenching and final tempering. The use of “conventional PWHT” preserved microstructural gradient of T92 steel heat-affected zone (HAZ), consisting of its typical coarse-grained and fine-grained subregions with tempered martensitic and recrystallized ferritic–carbidic microstructures respectively. In contrast, the “full PWHT” led to the complete elimination of the original HAZ via transformation processes involved, i.e. the reaustenitization and back on-cooling martensite formation. The observed microstructural changes depending on the initial PWHT conditions were further manifested by corresponding differences in the weldments’ creep performance and their failure mode. The weldments in “conventional PWHT” state ruptured after long-term creep tests by premature “type IV failure” within their recrystallized intercritical HAZs. On the contrary, the long-term creep behavior of the weldments processed by “full PWHT” was characterized by their remarkable creep life extension but also by the occurrence of unfavorable “decohesion failure” along T92/Ni WM interface.     

Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

Abstract

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.     

Material behaviour and plant experience of P91/P92 components

In several lignite-fired power plants of the ‘Vattenfall Generation AG’ in Germany components of P91/P92 material are used in long-time operation. About this experience in operation of selected components will be reported. In this context own experimental results of a research project in the damage evolution will be presented. The project ‘Damage development III’ was edited together with the MPA Stuttgart and was supported by Vattenfall and AVIF. The aim of the project was to improve the knowledge about the process of creep damage by experimental tests and additional numerical calculations. An instruction was given for planning, implementation and analysis of recurrent investigations on components consisting of 9% Cr steels which are subjected to high operation loading. Finally, the damage phenomena are presented by two case studies, a damage in a pipe bend due to faulty heat treatment and the creep-crack assessment of a lack of side-wall fusion in a reheater weld by fracture mechanics.     

Creep properties of 12% Cr and improved 9% Cr weldments

Martensitic Cr-alloyed high-temperature materials offer interesting opportunities for design and construction of advanced power plants. An extensive research programme has been carried out at the Research Centre of the Belgian Welding Institute and Laborelec on martensitic 12% Cr steel for gaining a better understanding of the failure mode and the deformation mechanism of welded joints under uniaxial and multiaxial loads. A large number of pipe girth welds were realized by three Belgian manufacturers (Cockerill Mechanical Industries, Fabricom and Mannesmann-Carnoy). Different filler metals were used and the influence of the welding regime (austenitic and martensitic), the post-weld heat treatment (PWHT) (single or double) and the base metal wall thickness on the high-temperature properties of the different weldments was evaluated. It was found that the creep properties of a 12% Cr weldment are not influenced by the welding regime and the base metal wall thickness. As would be expected, the creep strengths of the original 12% Cr base metal as well as the temperature of the PWHT have some effect. The existence of a typical failure in the intercritical zone (type IV region) is demonstrated and explained. The consequences for the design of welded 12% Cr components are indicated. More recently the research was extended towards improved 9% Cr steel (T91). A rather small preliminary programme for the orientation of further research showed a similar failure location as for 12% Cr steel, although the observed loss in strength of the weldment compared to the base metal tended to be considerably lower. The so-called ‘half-tempering’ treatment was tried out and the effect on the creep strength of the weldment is shown. A more fundamental national research programme on P91 steel has been established and is actually running.     

Some key effects on the failure assessment of weld repairs in CrMoV pipelines using continuum damage modelling

Some typical results obtained from finite element (FE) creep and continuum damage mechanics analyses, for assessing weld repair performance, are presented. The work outlines some developments in failure analyses of repaired welds in pressurised, thick-walled, main steam circumferential pipe weldments made of CrMoV steels. Methods involved in determining the material properties for constitutive equations are briefly described. Results presented cover a range of analyses, taking account of the effects of repair profiles/dimensions, geometry change during creep, end (system) loading, reheating effects in the weld metal of partial repair welds and initial damage level, etc., on the failure life of the repaired welds. The potential uses and limitations of the damage analysis for weld repair performance assessment are discussed.     

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.     

Laser weld pool management through diffractive holographic optics

Abstract

Conduction laser welding involves initiating a melt pool by exposure to high power laser induced light and controlled thermal conduction. Existing welding techniques generally provide enough energy to join the component but have no real control over the melt pool. This process can invariably lead to overheating in adjacent areas or even the melt pool itself, often causing unavoidable effects, such as ‘burn through’. The present work presents a procedure in which a desired melt pool shape is conceived, and a bespoke beam irradiance distribution is designed to match. The beam is shaped not by conventional lenses but by a diffractive holographic optical element (DHOE). The DHOE utilises holography to wholly create highly complex three-dimensional energy distributions through constructive and destructive interference. This technique allows novel beam irradiance distributions to be applied to conduction mode laser welding, with the melt pool transverse profile being shaped to a specific design. Holographic conduction laser welding has been shown to be successful and represents a significant step forward in the industry, as demonstrated in this case in both mild and stainless steels. The fusion zone is shown to be particularly influenced by the shape of the illuminating laser beam profile, and many of the welds demonstrate a highly novel weld profile because of this. The use of a bespoke beam irradiance distribution allows control of the heat flow to the workpiece, and this allows greater control over material migration due to surface tension effects. Many of the welds demonstrate unique surface solidification patterns directly linked to the beam profile used. The DHOE also presents a number of additional advantages, such as an increased usable depth of field, allowing for less stringent set-up tolerances. Comprehensive metallography has been performed on samples of these welds through the use of optical microscopy, electron microscopy, electron backscatter diffraction and energy dispersive (X-ray) spectroscopy. These techniques offer in depth analysis of crystal size, shape, orientation and phase. By incorporating DHOEs into a laser welding process, not only does the melt pool shape become controllable, but also the crystal growth is highly influenced. Many of the undesirable attributes of a conventional laser weld are reduced by using a beam distribution created by a DHOE, bringing the microstructure of the weld pool closer to that of the parent material.     

Stress relieving and its effect on life of welded tubular joints

This paper describes investigations into the fabrication and repair of large welded tubular joints and how the processes used affect fatigue life. Research has been conducted to determine the benefit of local post weld heat treatment (PWHT) of the connections in terms of hardness and residual stress. The use of hole drilling and trepanning techniques for residual stresses measurements in the clusters are described. The results are compared with fitness-for-service assessments and considered in relation to the fatigue performance of the welded connections. Very substantial theoretical improvement in fatigue life can be expected after PWHT.     

The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints

In this study, the effect of post-weld heat treatment (PWHT) on the mechanical properties of friction stir-welded 2024 aluminum alloys in the T4 temper state was investigated. Solution heat treatment and various ageing treatments were given to the welded joints. The PWHT procedures caused abnormal coarsening of the grains in the weld zone, which resulted in a drop in micro-hardness at the weld zone compared to the base material of the joints. T6 (190 °C – 10 h) ageing treatment after welding was found to be more beneficial than the other heat treatments in enhancing the mechanical properties of the 2024-T4 joints. However, the T6 (190 °C – 10 h) heat treatment led to significant ductility deterioration in the joint.     

Weldability and properties of martensitic/austenitic stainless steel joints

Abstract

A series of studies has been carried out to examine the weldability and properties of dissimilar steel joints using martensitic and austenitic stainless steels F6NM (OCr13Ni4Mo) and AISI 347, respectively. This type of joint requires good mechanical properties, corrosion resistance, and a stable magnetic permeability in addition to a good weldability. Weldability tests include weld thermal simulation of the martensitic steel to investigate the influence of weld thermal cycles and post-weld heat treatment (PWHT) on the microstructure and mechanical properties of the heat affected zone (HAZ); implant testing to examine the tendency for cold cracking of martensitic steel; and rigid restraint testing to determine hot crack susceptibility of the multipass dissimilar steel joints. The simulation results indicated that the toughness of the martensitic steel HAZ did not change significantly after the weld thermal cycles. The implant test results indicated that welds produced using nickel based filler show no tendency for cold cracking, whereas welds produced using martensitic or ferritic filler show such a tendency. Based on the weldability tests, a welding procedure (tungsten inert gas welding for root passes with HNiCrMo-2B wire followed by manual metal arc welding using ENiCrFe-3B coated electrode) was developed and a PWHT at 600°C for 2 h was recommended. Joints produced using the developed welding procedure are not susceptible to hot and cold cracking. After PWHT the joints exhibit both satisfactory mechanical properties and stress corrosion cracking resistance.

MST/1955     

On the weldability of grey cast iron using nickel based filler metal

Shielded metal arc welding process using nickel based filler metal was used to join grey cast iron. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was studied. PWHT included heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. By using nickel based filler metal, formation of hard brittle phase (e.g. carbides and martensite) in the fusion zone is prevented. Before PWHT, heat affected zone exhibited martensitic structure and partially melted zone exhibited white cast iron structure plus martensite. Applied PWHT resulted in the dissolution of martensite in heat affected zone and graphitization and in turn the reduction of partially melted zone hardness. Results showed that welding of grey cast iron with nickel based filler metal and applying PWHT can serve as a solution for cast iron welding problems.     

Microstructures and mechanical properties of creep resistant steel for application at elevated temperatures

The subject of this paper is the microstructural and mechanical characterisation of regions of the heat-affected zone (HAZ) in steels containing 9–12% Cr that are used for operation at elevated temperatures. Tests were performed on regions in the HAZ, which was created by physical simulation using a thermal welding simulator. Half of the simulated samples (SSs) were tested at room temperature (RT) and at an operating temperature (OT) of 600 °C immediately after simulation/welding, while the rest of the simulated samples were tested at RT and at the OT after heat treatment following the welding, i.e., post-weld heat treatment (PWHT). In addition to the results from mechanical testing, the results from microstructural analysis using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are also presented. The manner in which PWHT contributes to the creep resistance of the HAZ in P91 steel is demonstrated.     

An experimental study of the residual stresses,and their alleviation,in tube to tube-sheet welds of industrial boilers

Micro-crack development in welds, including repair welding, of the tube to tube-sheet region of industrial boilers is not an uncommon problem. Often such cracking is exacerbated by stress corrosion cracking or thermal fatigue, in which the residual stresses associated with welding can play a major role. In this paper, a custom built, but robust, air abrasive centre hole drilling facility was used; which induced negligible machining stresses. This system was modified to attach to the (vertical) tube sheets of boilers on site, and to sample residual stresses in the tube to tube-sheet welds, of both conventional circumferential fillet welds of protruding tubes as well as recessed ‘J’ configuration type. The experimental test programme evaluated transverse and circumferential profiles of the residual stresses. Global post-weld heat treatment (PWHT) of a whole boiler, indicated substantial (75%) reductions in the tensile peak residual stress. A localised PWHT technique, using a 1 kW incandescent light source inserted into the tube, easily achieved the requisite temperatures and heating rates, but led to significant increases in local residual stresses. The implications of such localised PWHT techniques on residual stress and consequent service life, are therefore of vital importance with regard to plant integrity and safety.     

Effect of laser beam welding on fracture toughness of a Ti-6.5Al-2Zr-1Mo-1V alloy sheet

Investigation of fracture toughness on Ti-6.5Al-2Zr-1Mo-1V alloy thin sheet and its laser-welded joints has been carried out. In the test compact tension (CT) specimens and single specimen technology were used. In addition, hardness distribution and microstructure of the welded joints were examined. Fracture test indicates that brittle unstable fracture occurs after slow crack propagation for all the specimens, except that one heat affected zone (HAZ) specimen is brittle crack initiation. It is found that rolling directions have no obvious effect on fracture toughness of base metal. Moreover, fracture toughness of weld metal is obviously decreased in comparison with base metal whatever in as-welded condition or in stress relief condition. Post-weld heat treatment (PWHT) leads to fracture toughness of the welds further decreasing. Fractography observation shows that the fracture mode is predominantly dimpled in base metal. However, there exists intergranular fracture in the weld metal. Thus, the transition of fracture mode from both base metal and HAZ to weld metal may lead to dramatic decrease in fracture toughness. Microstructure examination reveals that the microstructure of weld metal consists of large grains with fine acicular structure. The formation of fine α acicular structure is due to rapid cooling during laser welding. After PWHT, the acicular structure is coarsened.