Creep deformation,rupture and ductility of Esshete 1250 weld metal

Abstract

Esshete 1250 is an austenitic steel with high creep strength and is well established as a superheater boiler tube material in UK power stations. In addition, Esshete 1250 is readily welded with either inert gas welding or metal arc welding and has been used for piping and headers in super critical power stations. The latter thick section components are either solution heat treated after welding or enter service in the as-welded condition. Components that enter service in the as-welded condition contain high residual stresses, which pose a threat to the structural integrity of these welds. In order to perform life assessments of as-welded Esshete 1250 welds, it is necessary to have data on the creep deformation, stress relaxation, rupture and ductility of the weld metal, which are described in this paper.     

Hardness–fracture toughness–creep rupture strength relationship for welds in P91 main steam pipe

Abstract

An experimental study on the impact toughness, hardness, fracture toughness and creep rupture strength of P91 steel weld metals under two conditions (R and D) has been carried out. The results show that weld metal having higher hardness, lower impact toughness and lower fracture toughness (sample R) has higher creep rupture strength. The creep rupture strength is closely related to creep life of component. Thus, assessment of the weld hardness of P91 steel is of value in assessing the creep life of pipes in service. The lower toughness of sample R is attributed mainly to the higher welding current and input line energy used. The lower hardness and strength of sample D, which received a second post-weld heat treatment, partially result from too high a secondary tempering temperature being employed.     

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 strength of high chromium steels welded parts under multiaxial stress conditions

Internal pressure creep tests as well as uniaxial creep tests were conducted on 9Cr and 12Cr steels welded joints in order to evaluate creep strength of the high chromium steels welded parts under multiaxial stress conditions, where a welding direction of the internal pressure specimen was longitudinal one. Except under high stress conditions, cracks occurred at fine grained HAZ region for both materials, i.e., the type IV mode fracture. Regarding creep voids distribution along thickness direction of the internal pressure specimens, the creep voids were predominantly observed in the middle of the thickness at the HAZ region while they might be negligible in outer and inner surface of the specimen. Finite element analysis which considers HAZ and weld metal properties revealed that the principal stress (hoop stress) and the stress multiaxiality are the largest in the middle of the thickness at the HAZ region under the internal pressure creep conditions. It was suggested that the creep voids remarkably occurred inside the specimens at the HAZ region due to the severe stress state conditions.     

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.     

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。     

The prediction of creep damage in Type 347 weld metal: part II creep fatigue tests

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In part I of this paper the rupture strength and creep ductility data for a Type 347 weld metal were fitted to provide the material properties that are used to calculate creep damage. Part II of this paper examines whether the time fraction approach or the ductility exhaustion approach gives the better predictions of creep damage in creep–fatigue tests on the same Type 347 weld metal. In addition, a new creep damage model, which was developed by removing some of the simplifying assumptions that are made in the ductility exhaustion approach, was used. This new creep damage model is a function of the strain rate, stress and temperature and was derived from creep and constant strain rate test data using a reverse modelling technique (see part I of this paper). It is shown that the new creep damage model gives better predictions of creep damage in the creep–fatigue tests than the time fraction and the ductility exhaustion approaches.     

Creep strain growth behaviour of Hastelloy X above 800°C under thermal-stress cyclings

Actual structural components at high temperature are usually not only subjected to steady mechanical and cyclic loads but also accompanied by temperature variation. Creep ratcheting due to cyclic thermal stress is one of the typical examples. The creep strain growth behaviours of Hastelloy X above 800°C under thermal stress cyclings have been discussed. (1) Cycle dependent creep ratchet strain growth can be estimated to be ΔT, where is the thermal expansion coefficient and ΔT is temperature difference across a body. (2) Time dependent strain growth with temperature variation can be derived by time fraction, virtual activation energy and creep strain of the base temperature. (3) Decreasing creep rate will appear in the neighbourhood of 15–20% of rupture time and increasing creep rate will occur after this, which is almost linear on a logarithmic time scale and Hastelloy X shows softening behaviour on creep after exposure at the higher temperature.     

HR3C不锈钢采用镍基焊材的焊接性能试验

选用ERNiCrCoMo-1镍基焊材为HR3C不锈钢的焊接接头填充金属,通过焊接接头的常温力学性能、高温瞬时拉伸性能及高温持久性能试验,结果表明焊接接头的常温、高温力学性能均满足标准的规定,完全可以替代进口焊接材料YT-HR3C.     

Analysis of creep in a welded ‘P91’ pressure vessel

The present work considers the uniaxial and multiaxial creep behaviour of the tempered martensite 9%Cr 1%Mo steel ‘P91’ and the creep behaviour of welds in a P91 pressure vessel. The microstructure of a base metal/weld metal transition in a thick section pipe was analysed by means of optical microscopy and hardness measurements. Special emphasis was given to three microstructural states: the base metal (BM), the weld metal (WM) and the intercritical heat affected zone material (IC-HAZ). A significant difference between these three microstructures was their subgrain size, which was measured in the transmission electron microscope and was found to be smallest for the weld metal and largest for the intercritical heat affected zone material. The uniaxial creep behaviour of the three material states was analysed and it was shown that the creep strength increased with decreasing subgrain size. The elastic modulus of P91 was measured and the uniaxial creep behaviour of the three material states was characterized and represented by (i) the Norton law and (ii) in terms of the Robinson model. A welded pressure vessel was creep tested and hoop and axial strains were measured for three welds in the vessel. A creep stress analysis of the welded pressure vessel was performed based on (i) Norton's law and (ii) the Robinson model concentrating on the accumulated hoop and axial strains in the welds. Measured and calculated axial and hoop strains were found to be in good agreement.     

Effect of welding process parameters on embrittlement of Grade P92 steel using Granjon implant testing of welded joints

Precipitation of Cr-rich carbides, diffusible hydrogen content and heterogeneous microstructure formation across the weldments makes heat-affected zone (HAZ) susceptible to intergranular cracking and makes weldability of creep strength enhanced ferritic (CSEF) Grade P92 steel a critical issue. In the present research work, the Granjon implant test and mercury method (for diffusible hydrogen measurement) have been performed on Grade P92 steel welded specimens to study the effect of welding parameters on diffusible hydrogen levels and their subsequent effect on hydrogen-assisted cracking (HAC). The weld metal was deposited by a shielded metal arc welding process on Grade P92 steel samples using P92 matching filler. The three different welding conditions are used to measure the diffusible hydrogen level in the deposited metal. Granjon implant test was performed to evaluate HAZ HAC susceptibility with similar welding conditions which were used in the mercury method. Lower critical stress (LCS) was also evaluated using the Granjon implant test. The higher susceptibility of CSEF Grade P92 steel welded plate towards HAZ HAC was noticed in case of lower heat input or higher diffusible hydrogen content. However, by considering LCS, fracture mode and diffusible hydrogen content, the weld deposited using the highest heat input (condition III) offers great resistance to HAZ HAC.     

The behavior of welded joint in steel pipe members under monotonic and cyclic loading

Most steel pipe members are joined by welding. The residual stress and weld metal in a welded joint have the influence on the behavior of steel pipes. Therefore, to accurately predict the behavior of steel pipes with a welded joint, the influence of welding residual stress and weld metal on the behavior of steel pipe must be investigated.In this paper, the residual stress of steel pipes with a welded joint was investigated by using a three-dimensional non-steady heat conduction analysis and a three-dimensional thermal elastic–plastic analysis. Based on the results of monotonic and cyclic loading tests, a hysteresis model for weld metal was formulated. The hysteresis model was proposed by the authors and applied to a three-dimensional finite elements analysis. To investigate the influence of a welded joint in steel pipes under monotonic and cyclic loading, three-dimensional finite elements analysis considering the proposed model and residual stress was carried out. The influence of a welded joint on the behavior of steel pipe members was investigated by comparing the analytical result both steel pipe with a welded joint and that without a welded joint.     

Modelling of creep in friction stir welded copper

Abstract

Copper canisters for storage of nuclear waste will be exposed to creep. The canisters will be closed with friction stir welding (FSW). To describe the creep behaviour of the welds, uniaxial creep tests have been performed. A previously developed fundamental creep model for parent metal is applied to the different weld zones. The differences in microstructure and yield strength between the weld zones are taken into account. Creep strain versus time curves for the weld zones have successfully been predicted without the use of any adjustable parameters. It should be noted that the temperature range of interest of 50–100°C is deep down in the power law break down regime with Norton exponents between 25 and 100. The constitutive equations are used in FEM computations of creep in the canister weldments.     

The evaluation of weldment creep strength reduction factors by experimental and numerical simulations

The current design rules for welds are usually based upon the uniaxial creep rupture strength data. The effects of the stress multiaxility and the corresponding stress redistribution process of welded components are relatively ignored. As the present high-temperature testing techniques require large resources when testing welded components in full scale, the simulation of the effects will rely more on the numerical modelling. To evaluate the weldment joint efficiency this paper has proposed a general procedure in which the spatial distribution of constitutive parameters is determined by uniaxial testing while the creep process of components is simulated by numerical methods. Finite element methods are employed in the creep analysis of an AISI 316 butt-welded joint in pressurised tubes with a creep soft weld. To interpret the rupture behaviour of the tubes, different criteria are used to predict the rupture life. On the basis of the predicted structural rupture performance equations, the weldment creep reduction factors are evaluated for different design lifetimes. The reduction factors defined by the ASME code principle are found to be non-conservative in this case.     

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.     

FCAW process to avoid the use of post weld heat treatment

Post weld heat treatment (PWHT) is the most common technique employed for relieving residual stresses after general repair welding. Besides, the primary purpose of reducing the effect of stresses induced by welding, PWHT is also intended to temper the metallurgical structure of the heat-affected zone (HAZ). Unfortunately, there are significant difficulties in carrying out post weld heat treatment such as; the complexity of weld geometry, the possibility of distortion in the case of any mechanical loads, difficulty in heating symmetrically, and also PWHT may cause degradation of the material properties (especially creep and tensile strength in the case of multi PWHT cycles). Most of the repairs in industry are performed with manual metal arc welding (MMAW), however, the benefits of the flux cored arc welding (FCAW) process have been appreciated by industry for many years.     

Metallurgical and mechanical properties of hydrogen charged carbide-free bainitic weld metals

Hydrogen embrittlement is a major concern during the welding of high-strength steels. The susceptibility of the welds to hydrogen embrittlement increases with increase in weld strength. The ever-increasing demand to increase the strength of steels necessitates the development of novel welding procedures and fillers to produce welds of high strength and with resistance to hydrogen embrittlement. In this current work, the susceptibility of carbide-free bainitic weld metals to hydrogen embrittlement is studied with varying volume fractions of constituent phases. Using three different weld metal compositions, six different weld metal microstructures of carbide-free bainite were generated. The hydrogen saturation behaviour of the various weld metals was studied by cathodic electrolytic charging and subsequent diffusible hydrogen measurements by the hot extraction method. Tensile tests were conducted on various weld metals with and without hydrogen charging to evaluate their susceptibility to hydrogen embrittlement. The results show that the carbide-free bainite weld metals are highly resistant to hydrogen embrittlement despite their very high strength.     

The prediction of creep damage in type 347 weld metal. Part I: the determination of material properties from creep and tensile tests

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In the case of the time fraction approach the rupture strength is used to calculate creep damage, whereas creep ductility is used in the ductility exhaustion approach. In part I of this paper the methods that are used to determine these material properties are applied to some creep and constant strain rate tests on a Type 347 weld metal. In addition, new developments to the ductility exhaustion approach are described which give improved predictions of creep damage at failure in these tests. These developments use reverse modelling to determine the most appropriate creep damage model as a function of strain rate, stress and temperature. Hence, the new approach is no longer a ductility exhaustion approach but is a true creep damage model.     

Weld repair of low alloy creep resistant steel castings without preheat and post-weld heat treatment

During recent years, a weld repair technique for low alloy CrMoV steel castings, using CrMoV weld metal, which dispenses with preheat and post-weld heat treatment, has been developed. This so-called ‘cold’ weld repair technique is economically attractive because of the potential reduction in down time that it offers. However, its adoption has been delayed because of fears of the risk of brittle fracture at start-up under the action of welding residual stresses and thermal stresses.The present report presents fracture toughness data for untempered CrMoV weld metal, deposited by CrMoV electrodes, so that the risk of fast fracture can be assessed. Additional data on creep ductility of the weld metal are presented and discussed with reference to the longer-term integrity of repairs.     

Comparative analysis of TIG welding distortions between austenitic and duplex stainless steels by FEM

In this study, thermal stress analyses were performed in the tungsten inert gas (TIG) welding process of two different stainless steel specimens in order to compare their distortion mode and magnitude. The growing presence of non-conventional stainless steel species like duplex family generates uncertainty about how their material properties could be affected under the welding process. To develop suitable welding numerical models, authors must consider the welding process parameters, geometrical constraints, material non-linearities and all physical phenomena involved in welding, both thermal and structural. In this sense, four different premises are taken into account. Firstly, all finite elements corresponding to the deposition welding are deactivated and, next, they are reactivated according to the torch’s movement to simulate mass addition from the filler metal into the weld pool. Secondly, the movement of the TIG torch was modelled in a discontinuous way assuming a constant welding speed. Thirdly, the arc heat input was applied to the weld zone using volumetric heat flux distribution functions. Fourthly, the evolution of the structural response has been tackled through a stepwise non-linear coupled analysis. The numerical simulations are validated by means of full-scale experimental welding tests on stainless steel plates. Finally, the results and conclusions of this research work are exposed.