P91 Weld Research Articles

Effect of Post Weld Heat Treatment on Weld Hardness Quality of P91 Welded Pipes

Quality of a weld produced is generally evaluated by different parameters such as weld size, bead geometry, deposition rate, hardness and strength. A common problem that has been faced at the construction sites is to obtain a good welded joint having the required strength with minimal detrimental residual stresses to avoid any premature cracking due to high variation of hardness. In order to address this issue and to attain good weld quality, this study has been made to comprehend the effect of the post weld heat treatment on P91 material welds produced by TIG welding. Findings from the studies indicate that the PWHT has significant influence on the weld hardness of Alloy Steel-A335 P91 pipes. It is eminent that during cooling, after welding of P91, quenched martensite was formed in the HAZ that results in an increased hardness to an undesirable level of more than 250HB. PWHT at temperature of 760°C for 2 hours has good influence on mechanical properties as the hardness decreases and turns out to be uniformly distributed. If the PWHT is correctly carried out, the hardness of parent metals, heat affected zones and weld metal can be brought into the required limits to avoid any premature cracking due to high variation of hardness.

Effect of Long-Term Post-Weld Heat Treatment on the Microstmeture and Mechanical Properties of P91 Weld Metal

Modified 9Cr-lMo (P91) steel is used in fossil-fuel fired power plants due to its good thermo-physical, weldability, fabricability and high temperature mechanical properties. Toughness of the P91 weld metal, deposited by the shielded metal arc welding (SMAW) process, is reported to be lower than that deposited by the gas tungsten arc welding (GTAW) process. In spite of this, considering the higher deposition rate and economics, the SMAW process is very popular in industry. Thus, achieving adequate toughness in the P91 weld metal deposited using the SMAW process is an important requirement to qualify the weld joints. Weld joints were prepared using the SMAW process and subjected to post-weld heat treatment (PWHT) at 760°C for durations of 3, 10 and 100 hours. Microstructural observations revealed coarsening of the lath martensite and the precipitates accompanied by an increase in toughness with increase in PWHT duration from 3 to 10 hours. The effect of PWHT duration on subzero toughness was found to be significant, with the subzero toughness increasing with increase in PWHT duration upto 10 hours and then decreasing on PWHT for 100 hours. Increase in sub-zero toughness could be attributed to adequate tempering, while the decrease in toughness on PWHT for 100 hours was attributable to the formation of fresh martensite during cooling from the PWHTtemperature. This variation of toughness of the SMA weld metal with duration of PWHT needs better understanding from the view point of the composition of the weld metal. Detailed microstructural analysis was carried out to understand the reasons for the variations in the mechanical properties. This paper presents and discusses the results of this experimental investigation.

High Temperature, Low Cycle Fatigue Characterization of P91 Weld and Heat Affected Zone Material

The high temperature low cycle fatigue behavior of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 °C and 500 °C. The cyclic behavior of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests. The weld material is shown to give a significantly harder and stiffer stress–strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress–strain response, which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the cross-weld tests.

Creep Crack Growth Behavior of a P91 Steel Weld

Modified 9Cr-1Mo steel (P91) weld joints operating at elevated temperatures are well known to be prone to premature failure due to cracking in the heat affected zone because of the gradients in microstructure, popularly referred to as Type IV cracking. A campaign was undertaken to study the creep crack growth behaviour of modified 9Cr-1Mo weld joints. Creep crack growth (CCG) tests were carried out on compact tension (CT) specimens machined from P91 weld joints prepared using multipass shielded manual metal arc welding procedure. Specimens with two notch locations have been employed, (i) within the weld metal, between the centreline and the fusion line and (ii) in the heat affected zone. Constant load CCG tests were carried out at different applied loads at 798 and 898 K. The C* - da/dt correlations (da/dt =A C*m) were established for both notch locations. At 898 K, a higher A (0.064) and lower m (0.533) for the case with notch placed in the HAZ, compared to those for the case of notch in the weld (0.0399 and 0.75 respectively) were observed indicating the higher creep crack growth in HAZ, confirming type IV cracking. The difference increases at lower C* levels which correspond to long term behaviour. Heavy creep damage was observed in the HAZ region even for the sample with notch in the weld, whereas the weld metal regions showed relatively less damage. As the crack grew, a change in its course to follow the HAZ region was observed.

Determination of creep properties of a P91 weldment by small punch testing and a new evaluation approach

Local creep properties of a P91 weldment consisting of distinct zones, namely base metal, weld metal, as well as the fine-grained and the coarse-grained parts of the heat-affected zone were examined by Small Punch creep tests. Deflection vs. time graphs showed the zones' different creep resistances with that of the Heat Affected Zone-Fine Grained being lowest. Norton and Monkman–Grant type relations were established from Small Punch deflection curves for all zones. The evaluation of the Small Punch results was performed according to a new approach originating from the membrane stretching theory, as well as the method associated with the European Code of Practice for Small Punch Testing. The stress–rupture time results calculated by the new method were found to be very close to the Code of Practice's results with a similar amount of scatter, noting that with the new method, each Small Punch creep test is analyzed individually according to its observed deflection curve contrary to the Code of Practice method which disregards the actual deflection behaviour.

Finite element simulation of welding residual stresses in martensitic steel pipes

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.

Mechanical characterisation of a P91 weldment by means of small punch fracture testing

Small Punch fracture tests were carried out in order to determine low temperature mechanical behaviour of a P91 steel weldment: weld metal, base metal, Heat Affected Zone-Fine Grained and Heat Affected Zone-Coarse Grained. The Heat Affected Zone-Fine Grained showed superior tensile properties in terms of displacement at fracture (ductility) and strength (maximum force) compared to the other zones, which became more pronounced at lower temperatures, whilst the weld metal exhibited inferior behaviour. Ductile Brittle Transition Temperatures were evaluated from Small Punch results for all zones and the results of base metal and weld metal were correlated with the transition temperatures obtained from Charpy V notch tests according to the CEN Code of Practice for Small Punch testing. Heat Affected Zone-Fine Grained with its finest structure exhibited the lowest transition temperature and the weld metal the highest. Heat Affected Zone-Coarse Grained and base metal were found to have quite similar transition temperature values.

Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 2 – Mechanical properties

Three thick pipes of modified 9Cr–1Mo steel were welded using three different levels of heat inputs: low heat input 1·1 kJ mm−1, medium heat input 2·3 kJ mm−1 and high heat input 3·16 kJ mm−1. Two types of heat treatments were employed, namely subcritical post-weld heat treatment and normalising/tempering treatment. The influence of heat input and post-weld heat treatment on the microstructure of boiler steel P91weld joints has been investigated in the previous paper. In the present work hardness, tensile, impact toughness and stress rupture properties were evaluated. Our results show a great influence of heat input and heat treatment on mechanical properties. The best combination of properties was obtained in low to medium range of heat input, between 1 and 2·8 kJ mm−1 for both treatments and a 90% increase in time to rupture was observed for normalising/tempering treatment compared to subcritical post-weld treatment.

Influence of heat input and post-weld heat treatment on boiler steel P91 (9Cr–1Mo–V–Nb) weld joints Part 1 – Microstructure

AbstractSteel P91 is known for its excellent high temperature properties. The achievement of optimum weld metal properties for steel P91 within the course of its extensive applications in power plants has however often caused concern. In the present work, three thick pipes of P91 steel were welded using three different levels of heat inputs within the range of 1·15–3·5 kJ mm−1. A circumferentially multipass butt welded P91 steel pipe, typically used for high temperature applications in power plants was selected for this investigation. The achievements of optimum weld metal properties, which are closely linked to microstructure, are known to cause concern in such weldments. Two types of heat treatments were employed, subcritical post-weld heat treatment and normalising/tempering treatment. The microstructure was evaluated by optical, scanning electron microscope, X-ray diffraction and magnetic permeability. The results have shown a great influence of heat input and heat treatment on the microstructure. Mar...

Identification of Local Viscoplastic Properties in P91 Welds from Full Field Measurements at Room Temperature and 625 °C

A methodology to obtain visco-plastic laws in heterogeneous materials with digital image correlation (DIC) is proposed based on tensile and tensile-relaxation tests conducted at room temperature and at 625 °C. Tested samples are manufactured from a P91 weld in which a microstructural heterogeneity translates into graded mechanical properties. To simplify the problem, a classical decomposition of the weld into five different domains is considered. Strain field in each domain is obtained by means of digital image correlation applied to high magnification pictures recorded with an optical long distance microscope. The conducted identifications exhibit key features in the behaviour of each domain in terms of yield stress, ultimate tensile stress and hardening at both room temperature and 625 °C. Experimental fields are compared to the fields provided by finite element simulations. Eventually, the benefit of accounting for transverse strains in the identification procedure is examined, and the robustness of the procedure with addition of noise (representative of experimental conditions) in the measurement is characterized.

Improvement in Creep Resistance of Modified 9Cr-1Mo Steel Weldment by Boron Addition

The microstructural evolution in the heat-affected zones (HAZs) of boron-added modified 9Cr-1Mo steel (P91B) weldment is distinctly different from that of steel without boron addition (P91). This is accompanied by less reduction in hardness in the inter-critical heat-affected zone (ICHAZ) of the weldment of the boron-containing steel than what is observed in boron-free steel. Though, tensile properties of both the boron-containing steel weldments and boron free-steel weldments do not differ much at room and at high temperatures, there is a significant improvement in the creep strength of the weldment of the boron-containing steel over that of the weldments of boron-free steel. In fact, for creep tests so far completed for weldments of P91B, fracture occurred in the base metal, not in typical Type IV mode (fracture at the ICHAZ or fine grained heat- affected zone) as occurred for the P91 weldment for identical conditions of testing. This improvement is explained based on the role of boron in modifying the HAZ microstructure of the steel.

Understanding room temperature deformation behavior through indentation studies on modified 9Cr–1Mo steel weldments

Deformation behavior of boron free (P91) and boron added (P91B) modified 9Cr–1Mo steel weldments was studied at room temperature using an indentation technique. A sub-structure consisting of sub-grains was observed in the inter-critical heat affected zone (ICHAZ), whereas lath martensitic structure was observed in the coarse grain heat affected zone (CGHAZ) of the P91 weldment. Consequently, weldment showed significant variation in hardness as well as strain rate sensitivity in different regions of the HAZs. On the other hand, P91B steel weldment showed small variation in hardness and strain rate sensitivity, which was commensurate with uniform microstructure consisting of lath martensite. These differences in microstructures as well as hardness values also reflected in large variation in creep depth in P91 compared to the small variation in P91B weldment.

Transition of Crack from Type IV to Type II Resulting from Improved Utilization of Boron in the Modified 9Cr-1Mo Steel Weldment

The roles of boron and heat-treatment temperature in improving the type IV cracking resistance of modified 9Cr-1Mo steel weldment were studied. Two different heats of P91 steel, one without boron, designated as P91 and the other with controlled addition of boron with very low nitrogen, designated as P91B, were melted for the current study. The addition of Boron to modified 9Cr-1Mo steel has increased the resistance against softening in fine-grained heat-affected zones (FGHAZ) and intercritical heat-affected zones (ICHAZ) of the weldment. Creep rupture life of boron containing modified 9Cr-1Mo steel weldment, prepared from 1423 K (1150 °C) normalized base metal, was found to be much higher than that prepared from 1323 K (1050 °C) normalized base metal because of the stabilization of lath martensite by fine M23C6 precipitates. This finding is in contrast to the reduction in creep rupture life of P91 weldment prepared from 1423 K (1150 °C) normalized base metal compared with that of the weldment prepared from 1323 K (1050 °C) normalized base metal. The trace of failure path from the weld metal to ICHAZ in P91B weldment was indicative of type II failure in contrast to type IV failure outside the HAZ and base metal junction in P91 weldment, which suggested that boron strengthened the microstructure of the HAZ, whereby the utilization of boron at a higher normalizing temperature seemed to be significantly greater than that at the lower normalizing temperature.

Preface

Local creep properties of a P91 weldment consisting of distinct zones, namely base metal, weld metal, as well as the fine-grained and the coarse-grained parts of the heat-affected zone were examined by Small Punch creep tests. Deflection vs. time graphs showed the zones' different creep resistances with that of the Heat Affected Zone-Fine Grained being lowest. Norton and Monkman–Grant type relations were established from Small Punch deflection curves for all zones. The evaluation of the Small Punch results was performed according to a new approach originating from the membrane stretching theory, as well as the method associated with the European Code of Practice for Small Punch Testing. The stress–rupture time results calculated by the new method were found to be very close to the Code of Practice's results with a similar amount of scatter, noting that with the new method, each Small Punch creep test is analyzed individually according to its observed deflection curve contrary to the Code of Practice method which disregards the actual deflection behaviour.

Difference of creep crack growth behaviour for base, heat-affected zone and welds of modified 9Cr–1Mo steel

This paper presents creep crack growth behaviour for P91 base material, simulated HAZ material, and welded joint. Creep crack growth tests were carried out using C(T) specimens at 600°C and the same loading condition. The initial crack for P91 welds was inserted into the fine grained HAZ region and therefore Type IV cracking was observed for the P91 weld specimen. The comparison was carried out using time versus crack growth. The CCG life for fine-grained heat affected zones and welded joint is about 45% and 20% of that for base metal, respectively.In addition, FE analysis was conducted using the material parameter. The C* parameter for welded joint is obtained from a numerical line integral and creep load line displacement rate in FE analysis. Under the steady state, the C* value obtained from numerical line integral is little different to that obtained from the load line displacement rate. Therefore ASTM E 1457 provides a suitable estimation for the C* parameter for Type IV cracking such as welded joints of P91.

Difference of creep crack growth behaviour for base, heat-affected zone and welds of modified 9Cr–1Mo steel

AbstractThis paper presents creep crack growth behaviour for P91 base material, simulated HAZ material, and welded joint. Creep crack growth tests were carried out using C(T) specimens at 600°C and the same loading condition. The initial crack for P91 welds was inserted into the fine grained HAZ region and therefore Type IV cracking was observed for the P91 weld specimen. The comparison was carried out using time versus crack growth. The CCG life for fine-grained heat affected zones and welded joint is about 45% and 20% of that for base metal, respectively.In addition, FE analysis was conducted using the material parameter. The C* parameter for welded joint is obtained from a numerical line integral and creep load line displacement rate in FE analysis. Under the steady state, the C* value obtained from numerical line integral is little different to that obtained from the load line displacement rate. Therefore ASTM E 1457 provides a suitable estimation for the C* parameter for Type IV cracking such as weld...

Creep crack growth data and prediction for a P91 weld at 650 °C

Creep crack growth tests have been carried out on compact tension (CT) specimens machined from a P91 weldment. Four of these specimens were cut from the parent material side of the weld and another seven specimens were cut across the weld. For the cross-weld specimens, starter cracks were positioned into (or close to) the Type IV region. The creep tests were carried out under constant loads, at 650 °C. The results obtained showed that, the creep crack growth rates for parent material specimens are about ten times lower than those for the cross-weld specimens and that the scatter in the data is relatively high. In this respect, the accuracy of the crack tip location, in the cross-weld CT specimens, plays an important role. Finite Element (FE) analyses were carried out, on notched bar and CT models, using damage mechanics material behaviour models. These analyses were used to estimate the triaxial stress factor, α, for the parent material (PM), the weld metal (WM) and the heat affected zone (HAZ). FE analyses were then used to predict the creep crack growth in the CT specimens. Results from the FE analyses for both the PM and the cross-weld CT specimens were in good agreement with the corresponding experimental results. The effect of the potential drop versus crack length calibration on the calculated C* values was also investigated.

Qualification of P91 welds through Small Punch creep testing

The deployment of the martensitic alloy P91 in future nuclear plant components operating in the creep regime depends very much on the performance of joints, found occasionally susceptible to failure in fossil fuel fired plant A completely novel approach is proposed to assess the integrity of P91 weldments which relies on the determination of the creep properties of the different zones of the weldment on a micro-scale based on the Small Punch (SP) testing methodology. Results are presented from the Small Punch creep tests carried out exactly following a CEN Code of Practice for the various constituent zones of the weldment and the potential weaknesses are identified within the heat affected zone. In addition, the SP test is applied to determine the fracture properties of the weld compared to the parent metal, showing the expected shift related to the test method in the ductile brittle transition temperature and the inferior performance of the weld metal.

A Simplified Method for Predicting the Creep Crack Growth in P91 Welds at 650 °C

This article uses a numerical method for predicting the creep crack growth in compact tension (CT) specimens, including those containing welds. A series of incremental steady-state finite-element (FE) analyses were performed, using Norton's law to represent the creep behaviour, in order to simulate the progressive creep crack growth process. Validation of the FE predictions was achieved by comparing the creep crack growth test results obtained for parent material (PM) and cross-weld CT specimens removed from a P91 weld; the tests were carried out at 650 °C. Experimental results show that the creep crack growth rates for cross-weld specimens are higher than those for PM specimens by a factor of ˜4, for the same value of C*. Good agreement was obtained between the FE predictions and the experimental results for both the PM and cross-weld CT specimens. The FE results obtained using this approach are also in good agreement with those obtained from continuum damage analyses. The advantage of the proposed approach is that it requires much fewer material constants for use in the numerical predictions, compared with the damage mechanics approach.

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.