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.     

Effect of flux addition on the microstructure and tensile strength of dissimilar weldments involving Inconel 718 and AISI 416

This research work articulates the microstructural features of the dissimilar weldments involving Ni based superalloy Inconel 718 and martensitic stainless steel, AISI 416 that is difficult to weld. Autogenous tungsten inert gas (TIG) welding was carried out with and without using flux to fabricate these bimetallic combinations. Microstructure at the fusion zones and the interfaces were characterized using optical and scanning electron microscopy. The results depicted the presence of fine martensite at the heat affected zone (HAZ) of AISI 416 and the formation of unmixed zone containing secondary phases at the HAZ of Inconel 718. Similarly, the formation of Nb rich eutectics along with sulfides was witnessed at the fusion zone of both the weldments. Owing to the low heat input witnessed during the flux assisted TIG welding, the Nb segregation was found to be minimal. Tensile studies reported that the fracture occurred at the fusion zone in both the cases. It was inferred from the tensile studies that the joint strength of the weldments with flux addition was greater than the ones without flux. This study demonstrated that dissimilar joints with complete penetration could be achieved in single pass using the TIG welding process with the aid of flux.     

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.     

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     

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.     

Influence of heat treatments on microstructural parameters and mechanical properties of P92 steel

Abstract

The microstructural parameters (dislocation density, martensite lath width, precipitate diameters, and volume fractions) have been measured for the 9%Cr steel P92 (NF616) after different heat treatments. The austenitising temperatures were 970, 1070, and 1145°C and the tempering temperatures 715, 775, and 835°C. Increasing the austenitising temperature led to an increase in the austenite grain size and in the martensite lath width, but no significant effect on the tensile properties at 20, 600, and 650°C was observed. The creep strength was, however, reduced by tempering at 835°C due to rapid recovery of the martensitic structure with a sharp decrease in dislocation density. The lowest creep strength was found for the P92 steel subjected to a heat treatment that produced a fully ferritic microstructure; the secondary creep rate was four orders of magnitude higher than that of the steel in the usual martensitic condition.     

Formation and evolution of layered structure in dissimilar welded joints between ferritic-martensitic steel and 316L stainless steel with fillers

Dissimilar high-energy beam (HEB) welding is necessary in many industrial applications. Different composition of heat-affected zone (HAZ) and weld metal (WM) lead to variation in mechanical properties within the dissimilar joint, which determines the performance of the welded structure. In the present study, appropriate filler material was used during electron beam welding (EBW) to obtain a reliable dissimilar joint between reduced-activation ferritic-martensitic (RAFM) steel and 316 L austenitic stainless steel. It was observed that the layered structure occurred in the weld metal with 310S filler (310S-WM), which had the inferior resistance to thermal disturbance, leading to severe hardening of 310S-WM after one-step tempering treatment. To further ameliorate the joint inhomogeneity, two-step heat treatment processes were imposed to the joints and optimized. δ-ferrite in the layered structure transformed into γ-phase in the first-step normalizing and remained stable during cooling. In the second-step of tempering, tempered martensite was obtained in the HAZ of the RAFM steel, while the microstructure of 310S-WM was not affected. Thus, the optimized properties for HAZ and 310S-WM in dissimilar welded joint was both obtained by a two-step heat treatment. The creep failure position of two dissimilar joints both occurred in CLAM-BM.     

Creep deformation and failure of E911/E911 and P92/P92 similar weld-joints

This paper deals with characterisation of microstructure and creep behaviour of similar weld-joints of advanced 9% Cr ferritic steels, namely E911 and P92. The microstructures of the investigated weld-joints exhibit significant variability in different weld-joint regions such as weld metal (WM), heat-affected zone (HAZ), and base metal (BM). The cross-weld creep tests were carried out at 625 °C with initial applied stresses of 100 and 120 MPa. Both weld-joints ruptured by the “type IV cracking failure mode” in their fine-grained heat-affected zones (FG-HAZ). The creep fracture location with the smallest precipitation density corresponds well with its smallest measured cross-weld hardness. The welds of P92 steel exhibit better creep resistance than those of E911 steel. Whereas the microstructure of P92 weld after creep still contains laths, the microstructure of E911 weld is clearly recrystallized. The creep stress exponents are 14.5 and 8 for E911 and P92 weld-joints, respectively. These n-values indicate the “power-law creep” with dislocation-controlled deformation mechanism for both investigated weld-joints.     

Microstructural characterization of shielded metal arc weldments of a copper-bearing HSLA stee

Abstract

A systematic microstructural characterization in the heat-affected zone (HAZ) of two ASTM A710 grade A steel weldments (one preheated and the other pre–cooled), employing identical shielded metal arc welding conditions, has been performed. The microstructure in both the HAZ and the weld metal of both welds has been characterized by optical, scanning, and transmission electron microscopy in conjunction with microhardness traverses. No difference in microstructure was observed in the HAZ on comparing the preheated and non-preheated weldments. The only significant difference observed in the two weldments was the width of the HAZ, which is about 1 mm wider for a preheated weldment. Examination by transmission electron microscopy revealed the following microconstituents in the HAZ of both the weldments: polygonal ferrite, acicular ferrite, ferrite–carbide aggregates, ε-copper and fine cementite precipitates, martensite, tempered martensite, retained austenite, and transformation-twinned martensite. The microhardness traverse revealed almost identical hardness gradients in the two welds. The microstructural and microhardness data are discussed with regard to the preheating requirements for this alloy.

MST/118     

Enhancement of mechanical properties and failure mechanism of electron beam welded 300M ultrahigh strength steel joints

In this study, four post-weld heat treatment (PWHT) schedules were selected to enhance the mechanical properties of electron beam welded 300M ultrahigh strength steel joints. The microstructure, mechanical properties and fractography of specimens under the four post-weld heat treatment (PWHT) conditions were investigated and also compared with the base metal (BM) specimens treated by conventional quenching and tempering (QT). Results of macro and microstructures indicate that all of the four PWHT procedures did not eliminate the coarse columnar dendritic grains in weld metal (WM). Whereas, the morphology of the weld centerline and the boundaries of the columnar dendritic grains in WM of weld joint specimens subjected to the PWHT procedure of normalizing at 970 °C for 1 h followed by conventional quenching and tempering (W-N2QT) are indistinct. The width of martensite lath in WM of W-N2QT is narrower than that of specimens subjected to other PWHT procedures. Experimental results indicate that the ductility and toughness of conventional quenched and tempered joints are very low compared with the BM specimens treated by conventional QT. However, the strength and impact toughness of the W-N2QT specimens are superior to those of the BM specimen treated by conventional QT, and the ductility is only slightly inferior to that of the latter.     

On the impact of post weld heat treatment on the microstructure and mechanical properties of creep resistant 2.25Cr–1Mo–0.25V weld metal

Creep resistant low-alloyed 2.25Cr-1Mo-0.25V steel is typically applied in hydrogen bearing heavy wall pressure vessels in the chemical and petrochemical industry. For this purpose, the steel is often joined via submerged-arc welding. In order to increase the reactors efficiency via higher operating temperatures and pressures, the industry demands for improved strength and toughness of the steel plates and weldments at elevated temperatures. This study investigates the influence of the post weld heat treatment (PWHT) on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V multi-layer weld metal aiming to describe the underlying microstructure-property relationships. Apart from tensile, Charpy impact and stress rupture testing, micro-hardness mappings were performed and changes in the dislocation structure as well as alterations of the MX carbonitrides were analysed by means of high resolution methods. A longer PWHT-time was found to decrease the stress rupture time of the weld metal and increase the impact energy at the same time. In addition, a longer duration of PWHT causes a reduction of strength and an increase of the weld metals ductility. Though the overall hardness of the weld metal is decreased with longer duration of PWHT, PWHT-times of more than 12 h lead to an enhanced temper resistance of the heat-affected zones (HAZs) in-between the weld beads of the multi-layer weld metal. This is linked to several influencing factors such as reaustenitization and stress relief in the course of multi-layer welding, a higher fraction of larger carbides and a smaller grain size in the HAZs within the multi-layer weld metal.

     

Effect of Ni Contents on the Microstructure and Mechanical Properties of Martensitic Stainless Steel Guide Roll by Centrifugal Casting

A novel process based on centrifugal casting was developed to produce martensitic stainless steel for guideroll materials. Centrifugal casting provides a lower production cost and less of the thermal cracking defects which normally occur in the overlaid welding process. In this study, the effects of Ni on the microstructure and mechanical properties of martensitic stainless steel were investigated. The results show that the addition of Ni resulted in a decrease in the volume fraction of delta ferrite and an increase in the volume fraction of the retained austenite, respectively. Moreover, a tensile strength of 1600 MPa with an elongation of 4% were obtained after tempering at 500℃ for 2 h. These values were higher than those obtained by using the conventional overlaid process.     

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.     

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.     

Role of butter layer in low-cycle fatigue behavior of modified 9Cr and CrMoV dissimilar rotor welded joint

The present work aims at studying the role of butter layer (BL) in low-cycle fatigue (LCF) behavior of modified 9Cr steel and CrMoV steel dissimilar welded joint. The significant difference of the chemical composition of base metals (BMs) makes it a challenge to achieve sound welded joint. Therefore, buttering was considered to obtain a transition layer between the dissimilar steels. The LCF tests of two kinds of specimens without and with butter layer were performed applying strain-controlled cyclic load with different axial strain amplitudes. The test results indicated that the number of cycles at higher strain amplitudes of welded joint without butter layer was greatly higher than that of the joint with butter layer, while the fatigue lifetime to crack initiation (2N_f) became closer to each other at low and middle strain amplitudes. The failure was in the tempered heat affected zone (HAZ) at the CrMoV side for specimens without BL, while the fracture occurred at the tempered HAZ in the BL for specimens with BL. The microstructure details of BM, BL, HAZ and weld metals (WMs) were revealed by optical microscopy (OM). It was found that the tempered martensite was major microstructure for welded joint and much more carbides were observed in tempered HAZ than other parts due to the repeated tempering. Microhardness test indicated a softest zone existing tempered HAZ of BL and also there was a softer zone in tempered HAZ at the CrMoV side due to repeated tempering during welding and post weld heat treatment (PWHT). And scanning electron microscopy (SEM) was applied to observe the fractography. It was indicated that the fatigue crack initiation occurred from the specimen surface and all specimens were ductile–brittle mixed fractures. It is deemed that the softening behavior in BL caused by twice tempering correspondingly decreased the LCF lifetime at higher strain amplitudes. So suitable welding parameters and heat treatment processes became a key measure to ensure LCF property without losing other properties for welded joint with BL.     

Metallurgical investigations and corrosion behavior of failed weld joint in AISI 1518 low carbon steel pipeline

In this paper, failure analysis was carried out based on the available documents, metallographic studies and corrosion behavior of the welded joint pipe sample made of AISI 1518 low carbon steel. Nondestructive evaluations including penetration test (PT) and radiographic test (RT) were performed on the as-received pipeline and results indicated the presence of micro- and macro-cracks. Optical microscopic images and scanning electron microscopy (SEM) micrographs revealed various microstructures in the base metal (BM), heat affected zone (HAZ) and weld metal (WM). The microstructural variations may result in galvanic feature and lead to failure and rupture of the weld joint during the service. Microhardness measurements showed that hardness value was about 260 HV in the WM, while it declined in the HAZ and BM. Qualitative chemical analyses such as X-ray diffraction pattern (XRD) and SEM equipped with energy dispersive spectroscopy (EDS) confirmed the presence of corrosive media during weld joint rupture. Additionally, SEM and optical investigations indicated that micro-cracks were formed in HAZ due to residual stress as a consequence of improper welding condition. Surface fracture studies showed that the crack initiation, crack growth and finally crack propagation took place in the WM/HAZ interface. Electrochemical studies were conducted on the BM, HAZ and WM to investigate corrosion behavior of the failed joint sample. Finally, a proper corrosion mechanism is proposed based on the failure analyses and electrochemical studies.     

High-temperature tensile and creep deformation of cross-weld specimens of weld joint between T92 martensitic and Super304H austenitic steels

The high-temperature mechanical behavior of cross-weld specimens prepared from a dissimilar weld joint between T92 martensitic and Super304H austenitic heat-resistant steels incorporating Ni-based weld metal was evaluated at temperatures up to 650 °C. For both high temperature tensile and creep tests, failure took place in T92 due to its faster degradation with temperature increase. The heat-affected zone of T92 played a critical role during creep deformation, resulting in type IV failure under the long-term creep condition. For the creep specimens, the location of failure shifted from the base metal region to the fine-grained heat-affected zone as the creep duration time increased from the short-term to the long-term condition. The massive precipitation of Laves phase on the grain boundaries of the fine-grained heat-affected zone during creep deformation was observed and found to be responsible for the accelerated void formation in the area leading to the premature failure.     

Mechanism of post-weld heat treatment cracking in Rene 80 nickel based superalloy

Abstract

Microstructural studies carried out on Rene 80 (approximate composition 60Ni–14Cr–9.5Co–4Mo–5Ti–3Al–0.17C–Zr–B, wt-%) weldments before and after post-weld heat treatment (PWHT) revealed abundant evidence of constitutionally liquated and resolidified grain boundaries extending from the mushy zone into the heat affected zone (HAZ). While total dissolution of γ' occurred along such grain boundaries, a much lesser degree of γ' dissolution was noted in the adjacent material. During the PWHT, a high density of γ' precipitated out both within the mushy zone and in the constitutionally liquated and resolidified grain boundary regions in the HAZ. As the dissolution and reprecipitation of γ' occurred fairly uniformly throughout the mushy zone, the ensuing aging contraction stress/ strain was fairly uniformly distributed in the region. In contrast, in the adjacent part of the HAZ, an extreme volume of γ' precipitation occurred locally along the grain boundary regions, a result of the highest concentration of γ' forming solutes and the complete dissolution of γ' during welding in these regions. This, combined with the much stronger adjacent grain matrix, caused the aging contraction stress and strain to become highly concentrated along the grain boundary regions in the HAZ. This promoted the formation of PWHT cracks along such grain boundaries, which then propagated along the grain boundary into the mushy zone and beyond.     

船用吊杆0Cr16Ni5Mo不锈钢的焊接

概述了与0Cr16Ni5Mo马氏体不锈钢配套的焊接材料、焊后热处理制度以及适用的焊接工艺。试验选定采用M831A焊条,双U型坡口双面对称焊,焊后进行950℃/2h油淬+500℃/10h回火,得到的焊接接头性能完全达到指标要求。     

Effect of TIG welding on corrosion behavior of 316L stainless steel

316L stainless steel (SS) is one of the most consumable materials in orthopedic implants. Certain types of orthopedic implants such as mono-bloc hip stems are often made of two elements welded together. In this study, effect of TIG welding on corrosion behavior of 316L stainless steel in physiological solution was investigated. In this method, filler metal wasn't used due to the small thickness of samples and it was welded to lap form. Corrosion behavior in physiological solution at 37 °C was investigated with potentiodynamic polarization curves. Microstructure of base metal (BM) and weld metal (WM) was studied with scanning electronic microscopy (SEM). The corrosion behavior of weld metal, base metal and couple (BM and WM together) was compared together. For detecting microstructure and phases in BM and WM, X-ray diffraction analysis was done. Finally, post-weld heat treatment (PWHT) was performed on as-welded samples. Results indicated that corrosion behavior of WM was better than the BM. This phenomenon was attributed to secondary phases that were present in the BM. Secondary phases in the weld metal are dissolved when the base metal is melting due to the welding process. Based on the results of electrochemical analysis, it was determined that the corrosion rate of a couple was more than of other parts. Heat affected zone (HAZ) is responsible for this phenomenon. The adjacent zones of the weld metal are classically less corrosion resistant, thereby being attacked preferentially when the steel is exposed to corrosive environments. PWHT decreased the corrosion rate of the couple.