Alloy 52M Research Articles

Macroscopic and microscopic element distribution in transition zones of GTAW Alloy 52M weld joint and its PWSCC growth behaviour

The microstructure and element distribution in the transition zone (TZ) of Alloy 52M buttering (52Mb) layer near the fusion boundary to the SA508III low alloy steel in a dissimilar metal weld joint with post-weld heat treatment (PWHT) is characterized. Two macroscopic TZs in the heat-affected zone have lower Cr and Ni contents than the bulk Alloy 52Mb. The first TZ near the fusion boundary has the lowest Cr and Ni compositions, accompanying the highest Fe content. Cr depletion and Ni enrichment along with slight Fe enrichment at the dendrite boundaries in the TZs, are more significant than those in the bulk Alloy 52Mb. The number and the maximum depth of local oxidation penetrations at the alloy-inner oxide interface based on the oxidation tests, the number and the maximum length of locally interdendritic cracks, as well as the average crack growth rate (CGR) in terms of the stress corrosion cracking (SCC) band based on the SCC tests in the first TZ are higher than those in the second TZ. The average SCC CGR is enhanced by macroscopic element dilution, while local oxidation penetration, as well as locally interdendritic SCC, are thought to be enhanced by both the macroscopic dilution of Cr in the TZs and the microscopic Cr depletion at dendrite boundaries.

Effect of thermal ageing on fatigue crack growth behaviour of forged alloy 617M at elevated temperatures

ABSTRACT Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor ( Δ K th ) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The Δ K th value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes.

Microstructural damage assessment in alloy 617M near high cycle fatigue threshold at elevated temperature

AbstractHigh cycle fatigue (HCF) behavior of Ni superalloy 617M is investigated at 973 K and R‐ratio −1 on a resonance‐based fatigue testing system at 85 Hz frequency. The alloy experiences an abrupt drop in fatigue life within a narrow domain of 2.5–5 MPa near its fatigue strength at 320 MPa. Electron microscopy and diffraction techniques were employed to thoroughly analyze the nominal fatigue damage. The characterization revealed the significance of precipitation of secondary phases M23C6, Ti (C, N), and γ′ phase in dictating the HCF strength of the alloy. Cyclic loading at high temperature causes γ‐matrix hardening and secondary phase precipitation synergistically strengthening the material beyond its yield strength. Conjunctively, dynamic strain aging was also seen to play a major role in the evolution of fatigue damage. The work highlights the collective contribution of γ′‐phase precipitation, carbides, and dynamic strain aging and their influence on the HCF behavior of alloy 617M.

Characterization of Fatigue Crack Growth Behaviour of Alloy 617M Base and Weld Material

Alloy 617M is a Ni-based super-alloy made in the form of tubes which are used in high-temperature boilers and super heaters in Advanced Ultra Super Critical (AUSC) power plants. The high-temperature oxidation and mechanical properties of this material have been derived based on chromium (Cr), cobalt (Co) and molybdenum (Mo) as prime alloying elements which cause solid solution strengthening, carbide precipitation (M23C6, M6C) and formation of intermetallic phase Ni3(Al, Ti) (γʹ-phase). The manufacturing route of these components involve bending and welding operations which are critical locations and potential sites for crack nucleation and growth. Also, the conjoint influence of high temperature and flow-induced vibrations in the tubes further aides to crack nucleation and growth. Thus, fatigue crack growth (FCG) properties are important. Therefore, in this work, detailed experimental studies on the FCG behaviour of Alloy 617M base and weld materials have been carried out at different test temperatures (27 °C, 650 °C, 710 °C and 750 °C). The effect of ageing (1000, 2000, 5000 and 20000 hours) at an operating temperature of 710 °C on FCG has also been evaluated. All the FCG tests have been conducted as per ASTM with a constant load ratio (R = 0.1) and frequency of 15 Hz. From the experimental results, a major trend is observed of decreasing threshold stress intensity factor (ΔKTH) with an increase in ageing time and test temperature. The variations in the ΔKTH with test temperature and ageing time have been discussed and the crack growth mechanism has been established through microstructure and fractographic studies validating from existing literatures.

Effect of Temperature and Ageing on Fracture Toughness of Alloy 617M

In recent years, Advanced Ultra Super Critical (AUSC) plants have used Ni-Cr-Co-Mo based alloy 617M for high-temperature components like steam piping and rotors, where material resistance against fracture after prolonged exposure in a hostile environment is a matter of concern to the designers. To this end, fracture toughness characterization of the material in as-received and aged conditions using an elastic-plastic fracture mechanics (J-R curves) approach can provide a significant input towards the component integrity assessments at service conditions. In this study, J-R curves of the alloy 617M have been determined for the as-received and differently aged (up to 20000 h at 710 °C) conditions using compact tension (CT) specimens following ASTM E 1820-13. Repeated tests have been carried out at temperatures 27 °C, 650 °C, 710 °C and 750 °C. The fracture toughness (JIC) thus determined, has shown a declining trend with test temperatures. This attributes to the ease of the crack initiation process owing to the γ′ induced matrix hardening and subsequent ageing-inducing precipitations of M23C6 carbides at the grain boundaries.

Effects of Matrix and Dendrite Boundary Composition on Stress Corrosion Cracking Performance of Alloy 52M Buttering Layer in Simulated Pressurized Water Reactor Primary Water

The oxidation and stress corrosion cracking (SCC) behaviors of Alloy 52M buttering (52Mb) layer were studied in simulated pressurized water reactor (PWR) primary water after microstructure characterization. The 52Mb layer was first postweld heat treatment (1st-PWHT) and follow-up PWHT (FU-PWHT). Two dilution zones (DZs) were found in both the 1st-PWHT and FU-PWHT 52Mb samples. FU-PWHT decreased the Cr content in the first DZ, enhancing the oxidation rate and accelerating the formation of the local oxidation penetration zone at the oxide/substrate interface. Lower Cr content and high Fe content in the DZ were detrimental to the SCC resistance of the 52Mb in PWR primary water.

Effect of material chemical composition on oxidation and stress corrosion cracking of the submerged arc welding Alloy 52M overlay in a simulated PWR primary water

Compact tension (CT) specimens from submerged arc welding Alloy 52 M overlay are used to determine the stress corrosion cracking (SCC) resistance of the dilution zones and bulk overlay metal on the 52 M side in a simulated pressurized water reactor (PWR) primary water. SEM-EDS and X-ray fluorescence spectroscopy (XRF) results show three dilution zones (DZs) on the 52 M side. Oxidation test results show that the thickness of the compact oxide layer and maximum local oxidation penetration depth decrease for 1st DZ, 2nd DZ, and bulk 52 M, which are consistent with the average SCC growth rate and the number of local intergranular SCC cracks in these regions. The dilution zones with low Cr but high Fe content possess lower oxidation and SCC resistance than the bulk 52 M.

Microstructure and mechanical properties characterization of aged and repair welded Alloy 617M

Microstructure and mechanical properties characterization of aged and repair welded Alloy 617M

Effect of Material Inhomogeneity on the Crack Tip Mechanical Field and SCC Growth Rate of 52M/316L Dissimilar Metal Welded Joints

The stress–strain conditions at the crack tip in dissimilar metal welded joints (DMWJ) are a critical factor influencing stress corrosion cracking (SCC) behavior. The processing technology and working environment of DMWJ lead to a randomly inhomogeneous distribution of material mechanical properties, making the crack tip mechanical field more complex. An inhomogeneous model was obtained using a combination of physical experiments and the elastic–plastic finite element method to understand the effect of this inhomogeneous distribution of mechanical properties on the direction of SCC growth and the growth rate in DMWJ and the impact of inhomogeneity on the SCC growth behavior was compared and analyzed. The findings demonstrate that Type I (opening mode) cracks are more likely to form due to the inhomogeneity of mechanical properties and are more likely to deflect toward the Alloy 52M region at the interface between Alloy 52M and 316L stainless steel. Additionally, the strain gradient at the crack tip increases with the degree of inhomogeneity, which has a bigger impact on the accuracy of SCC growth rate predictions.

Development and Qualification of Materials for Indian Advanced Ultra-Supercritical Coal-Fired Power Plant

Abstract India has embarked on an ambitious program of setting up efficient advanced ultra-supercritical (AUSC) technology with steam temperature and pressure of 993 K and 31 MPa, respectively. This article deals with the testing and qualification of indigenously produced materials for the Indian AUSC mission project, the research and development phase of which is currently in the advanced stage of completion. As part of this, exhaustive database pertaining to various mechanical properties including creep, low cycle fatigue (LCF), creep-fatigue interaction (CFI), high cycle fatigue (HCF), fracture toughness, and crack growth has been generated covering important candidate materials such as alloy 617M tubes and forgings, 304HCu stainless steel tubes, and alloy 625 castings. Tubes of both alloy 617M and 304HCu stainless steels are to be used in the boilers, whereas alloy 617M forgings would be used as turbine rotor and alloy 625 castings as turbine casings. The data have been subsequently used to establish design curves identifying safe regimes of operation with respect to creep, LCF, HCF, CFI, and fatigue and creep crack growths. The properties were seen to be comparable with those derived from internationally developed equivalent grades. The bimetallic weld joints between alloy 617M and 10Cr steels were also evaluated to derive the appropriate strength reduction factors under creep and cyclic loadings. Additionally, the influence of thermal aging at service temperature on the microstructural evolution and mechanical properties was investigated based on tensile and fracture properties to account for the service exposure induced degradation and associated embrittlement.

An EBSD based investigation on the deformation mechanisms under HCF-creep interaction in a Ni-based superalloy (alloy 617M)

The study presents an insight into the deformation mechanisms under high cycle fatigue-creep interaction (HCF-creep interaction) in a Ni-based superalloy (Alloy 617 M), using electron backscatter diffraction (EBSD) as an important tool. HCF-creep interaction tests are carried out at 700 °C by imposing periodic holds at mean stress (σm) during HCF cycling. Detailed EBSD investigation using kernel average misorientation (KAM) map indicates a non-uniform distribution of local misorientation especially at the grain boundaries, with increase in σm, which is attributed to the increased creep damage therein. Creep cavitation through grain boundary sliding is found to increase with an increase in the hold temperature to 750 °C owing to a higher dislocation mobility triggered by precipitate coarsening and dislocation climb. Change in twin boundary character from Σ3 to other higher angle boundaries caused by dislocation-twin interaction is noticed under HCF-creep interaction, leading to partial detwinning. A decrease in the fraction of Σ3 annealing twin boundaries through partial detwinning is observed, which is found to be more pronounced when the creep hold is maintained at 750 °C. This caused a loss of strength at the above temperature, through coalescence of secondary intergranular cracks, in turn leading to intergranular failure. In contrast, the hold imposed at 700 °C with similar loading parameters resulted in run out.

Low Cycle Fatigue Behavior of Forged Alloy 617M

Low Cycle Fatigue Behavior of Forged Alloy 617M

Study on hot cracking susceptibility of alloy 617M and an analysis on parameters used for its evaluation

Hot cracking study on nickel base superalloy, alloy 617M, a candidate material for Advanced Ultra Supercritical (AUSC) power plant applications, was carried out. The susceptibility of this alloy to solidification cracking was evaluated using varestraint (variable restraint) test set up and parameters like Brittleness Temperature Range (BTR) and Critical Strain rate per Temperature drop (CST) were estimated from the results. The BTR and CST for alloy 617M indicate higher susceptibility to solidification cracking than stabilized and un-stabilized austenitic stainless steels. However, mock-up welding of 400 mm diameter alloy 617M hollow forgings with weld thickness of 95 mm and alloy 617M boiler tubes of ~12 wall thickness made from this alloy did not reveal any significant cracking in the welds (except some isolated micro fissures) and hence, a detailed analysis of the results from varestraint tests was taken up. The study revealed the dual hot cracking behaviour of alloy 617M and the reasons behind such observation is elucidated. The microstructural characterisation of the hot crack tested specimens of alloy 617M revealed eutectics of austenite + carbides rich in Mo and Cr in the backfilled regions of the hot cracks in the fusion zone. The formation of significant fraction of eutectic phases along the interdendritic regions at the end of solidification is the primary reason for the high BTR exhibited by alloy 617M. Further, backfilling of the solidification cracks by this eutectic liquid is enabled under low augmented strain and the same is insufficient to aid backfilling in cracks at high restraints. The mechanism mentioned above was found to be the underlying reason for the dual behaviour revealed by alloy 617M. Hence, it was deduced that BTR and CST determined at saturated strain cannot be effectively used to predict the actual hot cracking behaviour of alloy 617M. A refined criterion-BTR at threshold strain is proposed to be an appropriate solidification cracking susceptibility index for this alloy.

Saturation stress based models and their applicability to tensile flow behaviour of superalloy 617M

Four different saturation stress based models were employed to investigate the uniaxial tensile deformation behaviour of boron added Alloy 617M. Voce relationship described the tensile flow behaviour of the alloy to a reasonable extent at high strain levels over the temperatures range of 300 K–1023 K. On the contrary, the employed macroscopic Estrin-Mecking model could not provide an adequate description to flow stress versus true plastic strain data at different temperatures. In addition to Estrin-Mecking and Voce relationships, two different saturation models (Model-I and Model-II) were empirically developed and used in the present investigation. The developed relationships provide a better description of Alloy 617M work hardening behaviour compared to the existing models. Relative applicability of the different relationships has been verified based on their predictive capability of the tensile properties such as true yield and ultimate tensile strength and true uniform plastic strain. It was found that the predicted tensile properties using Model-II are comparable with the experimental data than those prediction obtained from the other models. The predicted shear modulus compensated saturation stress exhibited a peak value at 923 K due to the occurrence of dynamic strain ageing as well as precipitation strengthening. In the temperatures ranging from 473 K to 973 K, the observed lower values in the temperature dependent rate constant associated with the Model-II suggested the reduced activity of dynamic recovery at intermediate temperatures compared to 300 K and 1023 K. The rapid decrease in saturation stress, true ultimate strength and uniform plastic strain is the clear indication of the dominance of dynamic recovery effects at 1023 K for the Alloy 617M.

A re-visit to the Haigh diagram with the effect of creep damage on the high cycle fatigue behavior of alloy 617M

The present work is aimed at a re-evaluation of the Haigh diagram incorporating the effect of creep on HCF. HCF testing was carried out at 973 K with varying mean stress (σm) and alternating stress (σa) to generate the Haigh diagram. The influence of creep on HCF was investigated by imposing periodic holds at σm during the course of HCF cycling. Under such HCF-creep interaction tests, specimens were found to fail for certain σa-σm combinations that resulted in run-outs in the absence of superimposed creep holds, leading to a deviation in the Haigh diagram. Such deviation was more prominent with an increase in the σm (increase in R) and the hold time. The test results obtained are correlated with microscopic and fracture surface analysis to understand the mechanism of crack initiation and propagation, as well as the role of precipitation under HCF-creep interaction.

Nanoscale chromium carbides induced abnormal hardness in dissimilar metal welds

This study showed that high-density nano-precipitates, which have not been observed before, formed in the carbon-enriched zone located 20 µm away from the fusion boundary of the dissimilar metal welds of Alloy 52M and A508 steel. The nano-precipitate, which was named as CrCx in this study, was in a rodlike shape and shorter than 30 nm. The atoms of CrCx were arranged as a face-centered cubic (FCC) crystal. The CrCx precipitates have a lattice parameter of 4.08 Å and a cube-on-cube orientation relationship with the austenitic matrix of Alloy 52M. The results will be of great benefit to the development of advanced dissimilar metal welds and new Ni-based alloys serviceable in a harsh environment.

Corrosion behavior of alloy 52M and 52MSS weld surfacing

High chromium nickel-based fillers are used in nuclear dissimilar joints or repairs. Due to the risks of ductility dip cracking of Alloy 52 M weld surfacing, a nickel-based filler with a high niobium and molybdenum content (Alloy 52MSS) is expected to enhance the stability of grain boundary and improve the resistance to stress corrosion cracking. In this study, Alloy 52 M and Alloy 52MSS were used in weld surfacing on A508 steel. The results showed that Nb concentrated in the interdendritic region, and Mo gathered around Nb-rich carbide grains. The hardness profile presented similar trends for both filler metals. In potentiodynamic testing, the corrosion potential was increased while the corrosion current density was lowered for 52MSS.Further cyclic polarization testing indicated that 52MSS had the capability to increase the protection potential and reduce the pitting phenomenon. The morphology of 52 M welding surfacing after testing showed a feature of interdendritic corrosion, while the characteristic of 52MSS′ showed a behavior in general corrosion. The Mo addition resulted in the formation of a metastable oxide scale that improved the alloy's corrosion resistance in the trans-passivity region. The 52MSS had a more remarkable ability to prevent the outer surface of weld surfacing from interdendritic corrosion.

The Weld Microstructure and Mechanical Properties of the Alloy 52 and Its Variants with Applied Electromagnetic Stirring during Welding

This study investigated the impact of electromagnetic stirring (EMS) on nickel-base alloy welds prepared with the gas tungsten arc welding process. Alloy 52 and its variants, Alloy 52M and Alloy 52MSS, were carefully evaluated with their weld microstructure and mechanical properties. The results showed that the welds exhibited a typical microstructure of dendrites, and that the dendrites could be refined by electromagnetic stirring. Meanwhile, with an application of EMS, the precipitates became smaller and more evenly distributed in the inter-dendritic areas. Ti(N,C)s, Nb/(Nb,Si)Cs, and large-scale Laves phase with (Nb,Mo,Ti)Cs were the precipitates present in the Alloy 52, Alloy 52M, and Alloy 52MSS welds, respectively. With the refined microstructure, both Alloy 52 and Alloy 52M welds were observed to have an increase in their tensile strength, with a decrease in their elongations. Comparatively, for the Alloy 52MSS weld, the tensile strength was enhanced along with a slight increase in elongation. Deep and dense dimples were a dominant feature of low-Nb-additions welds, and dendrite-like features were found prevalent among the Alloy 52MSS welds. With EMS, the dimples of Alloy 52 welds and the dendrite-like features of Alloy 52MSS welds became finer, while the dimples of Alloy 52M welds grew coarser.

Mechanisms of Fatigue Endurance in Alloy 617M at Different Temperatures (300-1023 K)

The present study aims at understanding the mechanisms of fatigue endurance through high-cycle fatigue (HCF) behavior of Alloy 617M at varying temperatures, ranging from 300 to 1023 K. Dynamic strain aging (DSA) was a key feature observed in this alloy at high temperatures (853-973 K), which also affects the cyclic life as well as the endurance limit. Furthermore, in situ precipitation of Cr23C6 and γ′ phase (Ni3(Al,Ti)) at 973 K during the course of HCF cycling is found to impart a beneficial effect on the HCF life of the alloy. At T > 973 K, effect of creep damage was found to be prominent, resulting in a deteriorating effect on HCF life. The role of DSA, precipitation and creep on the fatigue damage is assessed through in-depth microstructural characterization of the tested specimens, which is further utilized to develop a damage map across the temperatures 300-1023 K.

Generation of HCF-creep interaction diagram in Alloy 617M at 973 K

The influence of creep damage on high cycle fatigue (HCF) in Alloy 617M at 973 K was investigated in the present study by devising a novel method of incorporating periodic hold at the alternating stress/maximum stress. Under such HCF-creep interaction tests, the endurance limit was found to shift downwards compared to that obtained under HCF cycling. The modified endurance limit was found to be a strong function of HCF block size as well as the hold-time employed. This leads to modification of S-N curve, evolving scopes of re-evaluating the HCF design rules. The test data was further utilized to generate a HCF-creep interaction diagram. The knee-point which marks the allowable damage was found to be lower when a lower block-size is employed, indicating a strong block-size dependency of such diagram. The influence of creep on HCF cycling was also corroborated with fracture surface investigations showing significant intergranular cracking during such tests.