Simulated Post-weld Heat Treatment Research Articles

Precipitation behaviour of 20MnMoNi55 RPV steel in the temperature range of 630–670 °C

The precipitation behaviour of 20MnMoNi55 grade RPV steel while tempering and PWHT in the temperature range of 630–670 °C was characterised through microstructural observation and CALPHAD based thermodynamic calculations. Steel samples were tempered at 630 °C from 1 to 6 h and further subjected to simulated post weld heat treatment (PWHT) between 650 °C and 670 °C temperatures, which is relatively higher than the conventionally employed PWHT temperature range. Microstructural observations revealed that tempering and subsequent simulated PWHT of 20MnMoNi55 grade RPV steel resulted in precipitation of Fe3C and Ksi carbides as Mo lean and Mo rich precipitates, respectively. CALPHAD based computations also suggested that Ksi carbide and Mo2C could form as metastable carbides, whereas Fe3C and M7C3 as thermodynamically stable carbides while heat treatment in the temperature range of 630–670 °C.

Study on mechanical properties at different test temperatures of SA-738 Gr.B plate in two states

This paper describes the study on mechanical properties at different test temperatures of SA-738 Gr.B plate in both delivery state and simulated post-welding heat treatment state. And the properties of the plate were characterized by tensile tests, Charpy impact tests and drop weight tests. We found that as temperature increasing, the strength of the SA-738 Gr.B plate decreased when temperature was below 200°C. While as temperature decreasing, the Charpy impact absorb energies decreased, that is, toughness of the plates became worse gradually. Through drop weight tests, we obtained the nil-ductility transition temperature for the plate. The tests data indicated the SA-738 Gr.B plate possessed good strength and toughness, which can meet the design requirements for the containment vessel and some other components of nuclear plant. The tests data also showed that post-welding heat treatment may influence both strength and toughness, so attention should be paid to simulated post-weld heat treatment when purchasing materials for plant construction. By optical microscope and scanning electron microscope, microscopic pictures have been obtained for the plate to help analyze the properties of this material, and the microstructure was the ensurance of the strength and toughness.

Unexpected Charpy Impact Toughness Spike of 1.25Cr-0.5Mo Steel After Simulated Postweld Heat Treatment

The present study elaborates on the effect of simulated postweld heat treatment (SPWHT) on microstructure and mechanical property variations of 1.25Cr-0.5Mo steel. With the increase in SPWHT holding time, lath ferrite tends to morph into blocky ferrite, and carbides aggregate along grain boundaries. Although tensile strength keeps decreasing, Charpy impact toughness undergoes a spike, followed by a sharp decrease. Such an interesting phenomenon is explained by the competition between high-angle grain boundary population and carbide segregation, and bodes well to the pressure vessel industry for optimized PWHT.

Effects of PWHT on Microstructure and Mechanical Properties of A516 Gr.70

The article describes the effect of simulated post weld heat treatment (PWHT) on the properties and microstructure of high performance pressure vessel steel plate A516 Gr.70. The research indicates that the strength decreased sharply and the toughness is decreased slightly with increasing simulated PWHT times. And the strength of plate with high carbon content decrease more than which with low carbon content. The main reason of strength decrease is the ferrite degenerated after simulated PWHT. However, the strength can still satisfy the standard requirement with the reasonable composition design.

Toughness of SA738Gr.B steel used for nuclear containment vessel

Toughness of structural steels is as important as the strength, but much more complicated partly due to the diversity of the characterization methods and the inconsistency among the obtained results. In the present work, the toughness of SA738Gr.B steel plates of 55 and 130 mm thicknesses used for building the containment vessels (CV) of third-generation nuclear power plants, in the quenched and tempered (QT) and simulated post-weld heat treatment (SPWHT) states, was thoroughly characterized by measuring the Charpy impact energy, nil-ductility temperature, fracture toughness and fatigue crack growth rate. Attention was paid to the distinction between these toughness indicators in terms of crack initiation and propagation, and on their correlations in an attempt of proper and combined use of the test values. It is shown that the toughness of the 55 mm thick plate is excellent and does not change significantly across the thickness in either heat treatment condition. Therefore the plate can be reliably used for building the head and shell of the CV. The 130 mm thick plate exhibits larger variation in toughness across the thickness, and is only suitable for fabricating the accessory parts of the CV. Moreover, the existing relationships between the toughness indictors gained from Charpy impact tests and drop weight tests or fracture toughness tests are no longer valid for modern steels such as SA738Gr.B, as the harmful elements and inclusions that significantly influence the initiation of cracks have been notably removed in processing the steel.

Evolution of Carbide Precipitates in 1.25Cr-0.5Mo Steel During Simulated Postweld Heat Treatment

In the present work, the effect of simulated postweld heat treatment (SPWHT) on the microstructure of 1.25Cr-0.5Mo steel was systematically studied. Microstructure is mainly composed of ferrites and carbides after quenching-tempering and SPWHT. Outstanding features of carbides, including distribution, composition, morphology, and size, are identified by means of an electron-probe microanalyzer and a transmission electron microscope. The results show that the carbides coarsen and gather along grain boundaries and M7C3 transforms into M23C6 in situ.

Effects of Postweld Heat-Treatment on Girth Weld Tensile Property and Microstructure of High-Frequency Electric Resistance Welded Pipe for API X80 Grade Casing Pipe

The girth weld tensile properties of API X80 grade high-frequency electric resistance welded (HFW) steel pipe for surface casing with the chemical composition of 0.05C–1.6Mn–0.06Nb (mass %) and the diameter of 558.8 mm and wall thickness of 25.4 mm were investigated by simulated postweld heat-treatment (PWHT). The tensile specimens taken from girth butt welded pipe were heat-treated under the conditions of 625 °C × 2 h and 675 °C × 2 h in an air furnace in order to simulate PWHT of casing products. The result of the girth weld tensile test of the heat-treated specimens showed that yield strength and tensile strength decreased very little and these properties sufficiently satisfied the API X80 specification. The change in strength due to heat treatment was discussed based on microscopic observation of the submicrostructures of the base metal by the electron back-scattered diffraction (EBSD) technique, transmission electron microscopy, X-ray diffraction (XRD), and the extraction residue precipitate classification method. The authors concluded that the fine NbC with a diameter of 12–18 nm, which precipitated during the heat treatment, prevented the decrease of strength due to the slight grain growth and dislocation recovery associated with PWHT. Additionally, the effect of PWHT conditions was evaluated by using small-scale laboratory specimens obtained from the base metal. Tensile properties were summarized as a function of the tempering parameter. As a result, strength remained almost constant at the tempering parameter equivalent to the PWHT conditions of 625 °C × 16 h.

Applicability of improved Dyson–McLean approach to creep deformation behaviour of tempered martensitic P9 steel

Modelling creep deformation of tempered martensitic P9 steel in quenched and tempered, and simulated post weld heat treatment conditions has been performed in the framework of improved Dyson–McLean approach for wide range of stresses at 873 K. In this approach, kinetic creep law coupled with the set of first-order differential equations representing the evolution of microstructural internal-state-variables with strain/time has been employed to describe creep deformation behaviour of tempered martensitic steel. The optimised material constants associated with the model such as dislocation storage parameter (Kd) and rate constants associated with the precipitate coarsening (Kp) and solute depletion (Ks) reflect the influence of two different heat treatments on creep characteristics examined in the present investigation. At all test conditions, good agreement between the predicted and experimental creep strain/strain rate-time data at 873 K has been observed. Further, good correlations have been obtained between the experimental and predicted steady-state creep rates and time to reach the specified strain levels for both the heat treatment conditions.

Constitutive description of primary and steady-state creep deformation behaviour of tempered martensitic 9Cr–1Mo steel

The model based on the coupled sine hyperbolic creep rate relation with the evolution of internal stress as a function of strain provides better understanding of primary and secondary creep behaviour of tempered martensitic 9Cr–1Mo steel. The predicted evolution of internal stress as an increase in the internal stress value (or, decrease in effective stress) with strain/time appropriately described the observed decrease in creep rate during primary creep in the steel. The applicability of the model has been demonstrated by comparing experimental and predicted creep strain–time and creep rate–strain/time data of 9Cr–1Mo steel at 793 and 873 K for quenched and tempered and simulated post-weld heat treatment conditions. Irrespective of prior heat treatment and test temperature, the optimised parameters associated with the internal stress values exhibited linear variations with applied stress. The influence of prior heat treatment on primary and secondary creep characteristics of the steel is reflected on the rate constant values associated with the model. At all temperatures and heat treatment conditions, good agreement between the experimental and predicted steady-state creep rates demonstrate the further applicability of the model.

Correlation of precipitate stability to increased creep resistance of Cr–Mo steel welds

An order of magnitude decrease (from 16.0×10−4 to 4.1×10−4% h−1) in steady-state creep rate was observed in the fine-grained heat-affected zone (HAZ) of a Cr–Mo steel weld by the reduction of the pre-weld tempering temperature from 760°C (HTT) to 650°C (LTT). The microstructure during each stage of the manufacturing path, including pre-weld temper, thermal cycling and post-weld heat treatment, was characterized using a suite of characterization techniques. The techniques included simulated thermal cycling, dilatometry and electron microscopy, as well as time-resolved X-ray diffraction using Synchrotron radiation. Both LTT and HTT steels before welding contain M23C6 (M=Cr, Fe) and MX (M=Nb, V; X=C, N) precipitates in a tempered martensite matrix. During simulated HAZ thermal cycling with different peak temperatures, changes in M23C6 carbide characteristics were observed between the HTT and LLT conditions, while MX precipitates remained stable in both conditions. Simulated post-weld heat treatment samples show larger M23C6 in the HTT condition than in the LTT condition. The results provide a solution to extending the life of Cr–Mo steel welded structures used in power plants.

Potential Detrimental Consequences of Excessive PWHT on Pressure Vessel Steel Properties

During fabrication of Pressure Vessels, steels undergo several heat treatments that aim to confer the required properties on the entire equipment, including welds and base metal. Indeed, the production heat treatment of the base material, which leads to achieve the target properties, is most of the time followed by post weld heat treatment (PWHT). The aim of such treatments is to insure a good behavior of the welded zones in terms of residual stresses and obviously properties such as toughness. Generally, many simulated PWHT (up to 4 or more) are required for the testing of the base material, which can affect its properties and even lead to unacceptable results. In some cases for fabrication purposes an intermediate Stress relieving treatment can be required. Special attention is paid on C-Mn steels (e.g., SA/A516 from ASME BPV Code) with the effect of thickness and Ceq (International Institute of Welding Carbon equivalent formula: see page 3) requirements on the final compromise between properties and heat treatments. In particular, toughness and ultimate tensile strength (UTS) are the critical parameters that will limit the acceptance of too high PWHT. Although micro-alloying is a mean to increase the resistance to PWHT, this leads to difficulties in softening the heat affected zones. This solution is therefore not the best one considering the whole equipment optimization. Finally, the manufacturing process can play a major role when specifications are stringent. Quenching and tempering (Q&T) can indeed provide better flexibility in terms of PWHT and improved toughness for given Ceq and thickness. The case of Cr-Mo(-V) steels, which are widely used in the energy industry, is also addressed. Indeed, PWHT requirements for increasing the toughness in the weld metal can lead to decrease the base metal properties below the specification limits. For example, the case of SA/A387gr11 is very typical of metallurgical changes that can occur during these high PWHT leading to a degradation of toughness in the base metal. Another focus is made on the Vanadium Cr-Mo grade SA/A542D that must withstand very high PWHT (705 °C and even 710 °C) because of welds toughness issues. Optimization has therefore to be done to increase the resistance to softening and to guarantee acceptable microstructure, especially in the case of thick wall vessels. Some ways for improvement are proposed on the basis of the equivalent Larson–Miller parameter (LMP) tempering parameter concept. The basic philosophy is to fulfil the need for discussion between companies involved in pressure vessels fabrication so that the best compromise can be found to ensure the best and safest behavior of the equipment as a whole. In particular, the tempering operation can sometimes be done at lower temperature than PWHT in order to offer the best properties to the final vessel.

Creep-rupture behavior of forged, thick section 9Cr-1Mo ferritic steel

Detailed investigations have been performed to examine the creep-rupture behavior of a 1,000-mm diameter and 300-mm-thick tube plate forging of 9Cr-1Mo ferritic steel in quenched and tempered (Q + T), simulated postweld heat treatment (SPWHT), and thermally aged (TA) conditions. Creep tests were conducted over a wide stress range (50 to 275 MPa) at 793 and 873 K. The alloy exhibited well-defined primary, steady-state, and extended tertiary creep stages at all test conditions. At 793 K, no significant difference in the creep-rupture properties was noted between Q + T, SPWHT, and TA conditions. On the other hand, SPWHT specimens exhibited lower creep-rupture strength than that of Q + T specimens at 873 K. Applied stress ({sigma}{sub a}) dependence of rupture life (t{sub r}) exhibited two-slope behavior. Both the Monkman-Grant ({dot {epsilon}}{sub s}.t{sub r} = C{sub MG}) and modified Monkman-Grant ({dot {epsilon}}.t{sub r}/{epsilon}{sub f} = C{sub MMG}) relationships were found to be valid for 9Cr-1Mo steel, where {dot {epsilon}}{sub s} is the steady-state creep rate and {epsilon}{sub f} is the strain to failure. The two-slope behavior was also reflected as two constants in the Monkman-Grant relationship (MGR) and modified Monkman-Grant relationship (MMGR) in the two stress regimes. Further, two creep damagemore » tolerance factors ({lambda} = 1/C{sub MMG}) of 5 and 10 were also observed in the high and low stress regimes, respectively. The alloy exhibited high creep ductility, which was retained for longer rupture lives at low stresses, and the creep ductility increased with increase in test temperature. The failure mode remained transgranular under all test conditions. The extensive tertiary creep in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure. The creep-rupture strength of the forging was found to be lower than that of thin section bars and tubes.« less

Effects of strain rate and temperature on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel

Tension tests have been conducted to evaluate the influence of strain rate (6.3 × 10 −3 to 3.2 × 10 −5 s −1) and temperature (300–873 K) on tensile deformation and fracture behaviour of 1000 mm diameter and 300 mm thick 9Cr-1Mo tube plate forging in simulated post weld heat treatment condition. Yield strength and ultimate tensile strength decreased gradually up to around 723 K. Beyond 723 K a rapid fall in both the strength values was observed. In the intermediate temperature range (523–673 K), the alloy exhibited higher yield and tensile strength values with decreasing strain rate, indicating negative strain rate sensitivity. In contrast, at temperatures of 723 K and above, the strength values decreased with decrease in strain rate. Serrated flow, a characteristic of dynamic strain ageing, was observed in the temperature range 523–673 K. The upper end temperature of serrated yielding decreased with decrease in strain rate. A detailed analysis of the dependence of critical strain for the onset of serrations on temperature and strain rate yielded an apparent activation energy of 83 kJ mol −1 for serrated flow at temperatures between 523 and 623 K. Ductility measured in terms of percentage elongation and reduction in area after fracture showed a gradual decrease up to about 673 K and a general increase at high temperatures at all the strain rates, exhibiting a ductility minima in the intermediate temperature range. The alloy has undergone predominantly transgranular ductile fracture at all strain rates and temperatures investigated. Yield and tensile strength of the forged tube plate material is consistently lower than the thin section bar material data. However, the strength values were still higher than the minimum values proposed in the ISO specification for the thin section material. Lower strength values of the forged tube plate material have been attributed to its coarse grain size compared to that of thin section bar material.

Development of fatigue design curves for thick-section 9Cr-1Mo ferritic steel forgings

Strain-controlled low-cycle fatigue (LCF) data have been generated on 9Cr-1Mo thick-section (300 mm) tube plate forgings under simulated postweld heat treatment conditions at 723, 773 and 793 K employing a constant strain rate of 1 × 10 −3 s −1. LCF tests were performed over strain amplitudes in the range ±0.25 to 1.00%. The fatigue life was found to decrease with increasing temperature. The effect of temperature was more pronounced at low strain amplitudes. Within the range of strain amplitudes examined, the experimental data on fatigue life at elevated temperatures were found to match well with the life predicted by the Tomkins crack growth model. Tentative fatigue design curves for elevated temperatures have been formulated for 9Cr-1Mo based on information generated in the laboratory using the philosophy outlined in the ASME code case N-47. A fatigue design curve applicable for the temperature range 300–644 K was computed based on room temperature tensile properties, incorporating the concept of fatigue limit and mean stress.

High temperature low cycle fatigue properties of a thick-section 9wt.%Cr-1wt.%Mo ferritic steel forging

Total-axial-strain-controlled fatigue tests have been conducted in air to ascertain the influence of temperature (723, 773 and 793 K) on the low cycle fatigue behaviour of thick-section (300 mm) 9Cr-1Mo tube plate forging (where the chromium and molybdenum contents are in approximate weight per cent). The alloy was tested in a simulated post-weld heat treatment codition. A symmetrical triangular waveform and a constant strain rate of 1 × 10 −3 s −1 were employed for all the tests performed over strain amplitudes in the range from ±0.25% to ±1.00%. The crack initiation and propagation modes were studied. Deformation and damage mechanisms which influence the stress response and endurance have been identified. A reduction in fatigue life was observed at all the strain amplitudes with increasing temperatures. The temperature effect on life was more pronounced at low strain amplitudes. The reduction in fatigue life at elevated temperatures was attributed to the combined effects of increased inelastic strain and fatigue-oxidation interactions. Thick-section forged 9Cr-1Mo steel exhibited inferior fatigue resistance compared with the hot-rolled material either in normalized-plus-tempered or in simulated thick-section heat treatment conditions. The poor fatigue resistance of tube plate forging was ascribed to its coarse grain size. The cyclic stress response of tube plate forging varied as a complex function of temperature and strain amplitude. The cyclic stress-strain behaviour could be described by a power law relationship at 773 and 793 K. At 723 K, the alloy exhibited a two-slope cyclic stress-strain curve. The crack initiation and propagation modes remained transgranular at all the conditions investigated.

Effect of potential on the corrosion fatigue crack growth rate of FeAlMn alloy in 3.5% NaCl solution: Duh, J.-B., Tsai, W.-T., Lee, J.- T. and Chang, H. Corrosion Dec. 1990 46, (12), 983–988

Strain-controlled low-cycle fatigue (LCF) data have been generated on 9Cr-1Mo thick-section (300 mm) tube plate forgings under simulated postweld heat treatment conditions at 723, 773 and 793 K employing a constant strain rate of 1 × 10−3 s−1. LCF tests were performed over strain amplitudes in the range ±0.25 to 1.00%. The fatigue life was found to decrease with increasing temperature. The effect of temperature was more pronounced at low strain amplitudes. Within the range of strain amplitudes examined, the experimental data on fatigue life at elevated temperatures were found to match well with the life predicted by the Tomkins crack growth model. Tentative fatigue design curves for elevated temperatures have been formulated for 9Cr-1Mo based on information generated in the laboratory using the philosophy outlined in the ASME code case N-47. A fatigue design curve applicable for the temperature range 300–644 K was computed based on room temperature tensile properties, incorporating the concept of fatigue limit and mean stress.

The fatigue behaviour of HSLA non-load-carrying spot-welded joints: Hamel, F.G. and Masounave, J. Can. Metall. Quart. Oct.⇌ec. 1990 29, (4), 313–318

Strain-controlled low-cycle fatigue (LCF) data have been generated on 9Cr-1Mo thick-section (300 mm) tube plate forgings under simulated postweld heat treatment conditions at 723, 773 and 793 K employing a constant strain rate of 1 × 10−3 s−1. LCF tests were performed over strain amplitudes in the range ±0.25 to 1.00%. The fatigue life was found to decrease with increasing temperature. The effect of temperature was more pronounced at low strain amplitudes. Within the range of strain amplitudes examined, the experimental data on fatigue life at elevated temperatures were found to match well with the life predicted by the Tomkins crack growth model. Tentative fatigue design curves for elevated temperatures have been formulated for 9Cr-1Mo based on information generated in the laboratory using the philosophy outlined in the ASME code case N-47. A fatigue design curve applicable for the temperature range 300–644 K was computed based on room temperature tensile properties, incorporating the concept of fatigue limit and mean stress.

Fatigue crack propagation and cryogenic fracture toughness behaviour in powder metallurgy aluminium-lithium alloys: Rao, K.T.V. and Ritchie, R.O. Metall. Trans. A Jan. 1991 22A (1), 191–202

Strain-controlled low-cycle fatigue (LCF) data have been generated on 9Cr-1Mo thick-section (300 mm) tube plate forgings under simulated postweld heat treatment conditions at 723, 773 and 793 K employing a constant strain rate of 1 × 10−3 s−1. LCF tests were performed over strain amplitudes in the range ±0.25 to 1.00%. The fatigue life was found to decrease with increasing temperature. The effect of temperature was more pronounced at low strain amplitudes. Within the range of strain amplitudes examined, the experimental data on fatigue life at elevated temperatures were found to match well with the life predicted by the Tomkins crack growth model. Tentative fatigue design curves for elevated temperatures have been formulated for 9Cr-1Mo based on information generated in the laboratory using the philosophy outlined in the ASME code case N-47. A fatigue design curve applicable for the temperature range 300–644 K was computed based on room temperature tensile properties, incorporating the concept of fatigue limit and mean stress.

Prediction of stress relaxation and stress relief cracking in SEN testpieces

AbstractA computer model has been described for the prediction of stress relaxation and stress relief cracking (in coarse grained, heat affected zone microstructure) during the heating period of a simulated post-weld heat treatment in four-point bend single edge notched (SEN) testpieces. Stress relaxation by plastic deformation was calculated from isothermal creep strain rate data. The two main mechanisms of stress relief cracking were considered, i.e. higher temperature, intergranular microvoid coalescence and lower temperature, low ductility intergranular fracture. Existing theoretical models for ‘creep cavitation’ were used to estimate crack growth by the intergranular microvoid coalescence mechanism. An experimental relationship between crack propagation rate, stress intensity, and temperature was derived for the estimation of crack growth by the low ductility intergranular fracture mechanism. A comparison was made between the model predictions and experimental results from a recently developed, rehea...

Caustic stress corrosion cracking of Inconel-600, Incoloy-800, and Type 304 stainless steel

High-temperature electrochemical tests have resulted in the stress corrosion cracking of Inconel-600 and Incoloy-800 (registered trademarks, International Nickel Company), and Type 304 stainless steel in caustic solutions. Results show that stress corrosion cracking of these alloys can be prevented or accelerated by varying their electrochemical potential. To a certain extent, the same effect can be achieved by altering the gas atmosphere above the test solution from a pure nitrogen cover gas to a mixture of 5 percent H$sub 2$ and 95 percent N$sub 2$. The effect of the cover gas can then be negated by adjusting the specimen's electrochemical potential either to cause or to inhibit stress corrosion cracking. Some specifics of the test results reveal that in deoxygenated caustic solutions, Inconel-600 cracks intergranularly at mildly anodic potentials; Incoloy-800 cracks transgranularly at reduced potentials (at or near the open circuit potential) and intergranularly at highly oxidizing potentials; and cracking is mixed (transgranular/intergranular) for Type 304 stainless steel at or near the open circuit potential. The severity of cracking for both Inconel-600 and Incoloy-800 in deoxygenated caustic solutions is reduced by giving the materials a simulated post-weld heat treatment (1150$sup 0$F for 18 h). Test results on Inconel-600 show that high-carbonmore » (0.06 percent) material cracks less severely than low-carbon (0.02 percent) material, in both the simulated post-weld heat-treated condition and the mill-annealed condition. (auth)« less