A physics-based model coupling the evolution of dislocation density in the austenite and strain-induced martensite phases with composite flow strength and kinetics of martensite transformation was ...developed to describe ambient temperature uniaxial tensile deformation behavior of mill annealed and pre-deformed 304 SS. The applicability of the model was evaluated by fitting the experimental stress-strain data and validated with the experimentally estimated dislocation density in the austenite phase of the steel. The modeling results revealed that the initial microstructure of the steel significantly influences the dislocation accumulation and annihilation rates as well as the load partitioning between austenite and martensite phases. Despite a lower initial dislocation density than the pre-deformed conditions, the mill annealed condition revealed a higher capacity (approximately two orders of magnitude) for dislocation accumulation in the austenite phase. The initial volume fraction of strain-induced martensite phase and the initial dislocation density of pre-deformed 304 SS are governed by the degree of pre-deformation and pre-deformation temperature, either below or above Md temperature (where Md is defined as the temperature above which plastic deformation does not result in austenite to martensite transformation). It is observed that storage, as well as annihilation rates in the austenite phase, are higher for the steel having a higher initial volume fraction of martensite and dislocation density. Though the pre-deformation altered the evolution of martensite volume fraction, the steel exhibited a minor variation in dislocation accumulation capability in the martensite phase for different conditions.
This paper critically assesses phase transformations occurring after welding and subsequent post weld heat treatments in simulated sub-heat affected zones (HAZ) of P91B steel. Samples for weld-HAZ ...simulation were produced corresponding to grain-coarsened HAZ, grain-refined HAZ and inter-critical HAZ. Analyses revealed diverse phase transformation mechanisms (for GCHAZ = pipe-diffusion and for GR/ICHAZ = GB-diffusion) owing to manipulation in grain size and boron-enriched nanosized particles as regards virgin steel after welding. However, after PWHT, same phase transformation mechanism (interface diffusion) in all simulated sub-HAZs is observed. Hardness evaluations and prior austenite grain boundaries dissolution confirm GB embrittlement after welding. Boron segregation, the presence of borides and boron-enriched particles heads to ~ 50% drop in hardness deviations enhancing GB hardening after PWHT. Particle refinement is observed after PWHT which is further validated by numerical modelling. In addition, particle evolution during cooling from peak temperature of weld thermal cycle and isothermal holding of PWHT is analysed. Apparent activation energy of nucleation/growth follows descending order, i.e. GC/GR/ICHAZ for nanosized particles during welding.
This study aims to quantify the improvement in the creep crack growth (CCG) resistance in the heat-affected zone (HAZ) of boron added modified 9Cr–1Mo (P91B) steel weld at 898 K. The crack growth ...rates in the HAZ of P91B weld have been compared with the crack growth rates in HAZ of without boron P91 steel weld. The creep crack growth rates are estimated as per ASTM E1457 using 7 mm thick compact tension specimens with a notch in the HAZ and test duration 248h-11,000 h. The CCG rate (a˙) vs. C* correlation, commonly expressed as a˙=D(C∗)ϕ, has been established. In comparison, the crack growth rate in the HAZ of P91 weld is found to be threefold as compared to HAZ of P91B weld; with the difference in CCG rate increasing towards a higher C* range. Creep damage assessment study has been done in the vicinity of creep crack in HAZ of a P91B weld specimen tested for 11,000 h. The extent of cavitation, cavities link-up, and micro-crack formation in HAZ of P91B weld is found to be relatively lower than HAZ of P91 weld. Most of the cavities appear to be discrete in nature. This testifies improvement in the creep crack growth resistance in the HAZ of P91B weld due to controlled boron addition of about 100 ppm to modified 9Cr–1Mo steel.
This study reports the enhanced chemical resistance of a blended concrete mix (CFNI) made with 40 wt.% fly ash, 2 wt.% nanoparticles, and 2 wt.% sodium nitrite inhibitor as partial replacement of ...cement against calcium leaching, acid and sulfate attacks. The concrete test specimens of four different compositions were fabricated and immersed in natural seawater, 3% sulfuric acid solution, and 10% magnesium sulfate solution for 120 days. Long-term chemical deterioration of the concrete systems is evaluated by assessing visual changes of the specimens and solutions along with the changes in percentage mass loss, compressive strength of the concrete, pH of the solution, and dimensions. The results indicate that CFNI concrete exhibits a superior resistance against chemical attack under all the three aggressive environments. Detailed chemical characterization of the specimens, carried out using XRD, FTIR, and thermogravimetric analysis, reveal a reduced CaO content, absence of deterioration phases like ettringite, brucite, and enhanced C-S-H content in the CFNI concrete. The addition of nanoparticles and inhibitors into fly ash concrete has lowered w/c ratio, increased surface pH, enabled conversion of soluble calcium hydroxide into insoluble calcium silicate hydrate, filled pores/voids, and reduced shrinkage and cracking. The compact microstructure of the CFNI prevented leaching and reduced the ingress of aggressive chemical ions into the concrete. Our results demonstrate that incorporation of nanoparticles and inhibitor into the fly ash concrete composition is ideally suited for the design of high-quality, low-permeable concrete structures that is the key for enhanced chemical resistance in natural and industrial environments.
Type 304 stainless steel is a metastable austenitic stainless steel that undergoes strain induced transformation from austenite to α′ martensite during deformation at room temperature. Role of prior ...deformation, by rolling at room temperature and at 200 °C (a temperature at which the strain induced martensitic transformation does not occur), on the subsequent room temperature tensile deformation behaviour of the austenite phase is investigated. The changes in dislocation density of the parent phase and the volume fraction of the martensite formed due to deformation were estimated by X-ray diffraction line profile analysis of data obtained from Beam Line - 12 of the Indus 2 synchrotron. The contrast caused by dislocations in the austenite phase has been addressed using the modified Williamson-Hall method. The character of dislocations changes from screw to edge above a critical equivalent strain of ~0.14. The influence of martensite formation on the increase in dislocation density in the austenite phase is confirmed from XRDLPA. An alternative approach to determine dislocation density is proposed which is independent of elastic constants of the material and is found to correlate well with the values determined by the Williamson-Smallman approach. The results obtained from XRDLPA are corroborated with EBSD analysis. It is also seen that the composite strength of the steel, which has undergone different levels of deformation, is related to the changes in dislocation density of austenite and volume fraction of martensite based on Taylor's equation enabling correlation of structure-property.
•Deformation induced change in dislocation density (ρ) and character are studied by XRDLPA.•Strengthening due to α′ martensite and ρ in austenite are related through rule of mixtures.•SIM contributes to strengthening by introducing extra dislocations into γ phase.•Results from EBSD analysis corroborate the findings of XRDLPA.•Alternative approach to determine ρ from different forms of mWH equation is proposed.
SMAW (Shielded Metal Arc Welding) and GMAW (Gas Metal Arc Welding) are two of the most prominent welding processes commonly utilized in almost all types of modern industries. Among various aspects of ...these processes, some of the important parameters that govern the quality of the final weld product are the skill level of welders, welding consumables, and the role of shielding gases (in GMAW). Currently, the role of these parameters in determining the quality of the welded product is examined by evaluating the final weld produced and not by investigating how these factors affect the welding process. This is an indirect way to evaluate such welding parameters, which are both time-consuming and expensive. During the actual welding process, random variations in arc signals (voltage and current) take place. These dynamic variations are so short and rapid that ordinary ammeters and voltmeters cannot monitor the rate of such variations. However, the reliable acquisition of such variations and its subsequent analysis can provide very useful information in determining the quality of the final weld product. In this study, arc voltage and current were acquired at 100,000 samples/sec, filtered and subsequently analyzed using Continuous Wavelet Transform based on Fast Fourier Transform (CWT-FFT) technique to evaluate welding skill, welding electrodes (in SMAW process), and the effect of shielding gases (in GMAW process). Results thus obtained clearly differentiated the skill level of different trainee welders and welding electrodes in the SMAW process and the effect of shielding gases and arc current in the GMAW process. Very good correlation among the obtained results, its weld bead and its weld pool images were observed. Hence, this research proposes a simple yet effective methodology to evaluate the arc welding process parameters using CWT-FFT analysis of the welding signals.
The present study systematically varied the concentrations of B (0–100 ppm) and N (20–500 ppm) in modified 9Cr-1Mo steel to understand the combined effect of B and N addition on the microstructural ...stability and the creep resistance (650 °C, 120 MPa). The Auger Electron Spectroscopic analysis reveals the enrichment of B within the M23C6 precipitates at the vicinity of prior-austenite grain boundaries in B added steels both in normalized and tempered specimens and also in creep tested specimens. The 70 ppm B steel with 108 ppm N showed the best creep resistance (rupture time as high as 1536 h and minimum creep-rate as low as 1 × 10−5/h), followed by 90 ppm B, 90 ppm N steel. Too low B content (25 ppm or less) or too high N content (500 ppm) affected precipitate stability and consequently the creep resistance of the steel. Finally, it can be recommended that it is necessary to maintain optimum concentrations of B (70–100 ppm) and N (90–110 ppm) to improve the creep resistance of modified 9Cr-1Mo steel.
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•The effect of B and N on the evolution of microstructure and precipitate stability in normalized and tempered specimens (for different austenitization temperatures, 1000−1100 °C) and the creep resistance of 9Cr-1Mo grade steels (5 steels varying B (0–100 ppm)and N(20–500 ppm) were discussed.•Auger electron spectroscopy revealed the enrichment of boron within the M23C6 precipitates at the vicinity of PAG boundaries in B added steels.•The 70 ppm B steel with 108 ppm N shows the best creep resistance (rupture time of 1536 h and minimum creep-rate of 1 × 10−5/h at 650 °C, 120 MPa) followed by 90 ppm B with 90 ppm N steel and 100ppm B with 20ppm N respectively.•Too high N concentration in B-free steel, on the other side, promotes the coarsening of MX precipitates that is detrimental to the creep resistance. Too low boron content (25 ppm B here) is also not a preferred combination.•It can be recommended that it is necessary to maintain optimum concentrations of B (70–100 ppm) and N (90–110 ppm) to improve the microstructural stability and creep resistance of modified 9Cr-1Mo steel.
A chemistry-controlled variant of nickel-based Alloy 617 (UNS N06617) designated as Alloy 617M is the candidate structural material for superheater header and tubing and high pressure and ...intermediate pressure steam turbines for Indian advanced ultra-supercritical (AUSC) coal-fired thermal power plant mission program. This study evaluates the effect of ageing due to service exposure on microstructure, mechanical properties and repair welding behaviour of Alloy 617M boiler tube material. Initially, Alloy 617M tube weld joints (52 mm diameter and 11.9 mm thick) were fabricated by multi-pass manual gas tungsten arc welding (GTAW) process using ERNiCrCoMo-1 welding wire. Service-exposed material is simulated by subjecting the base metal and welded tubes to ageing heat treatment at 700 °C and 750 °C for 1000 h and 4000 h. Extensive carbide precipitation along the grain boundaries and γ′-Ni
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(Al, Ti) within the grains were observed in the aged base metal and weld metal. The yield strength and tensile strength of Alloy 617M base metal and weld metal after ageing heat treatment increased by 150–200 MPa in comparison to as-received/as-welded condition. In contrast, the Charpy V-notch toughness showed significant reduction after ageing, viz. weld metal toughness reduced from 125 J in the as-welded condition to 50 J after ageing. Partial penetration repair weld joints fabricated from the aged welded tubes showed presence of micro-fissures in the heat-affected zone (HAZ) and are intergranular in nature. However, the application of solution annealing heat treatment at 1160 °C for 1 h to aged Alloy 617M before repair welding could help in producing repair welds without micro-fissures.
Integrity assessment of 10Cr ferritic steel/alloy 617M dissimilar metal weld joint (DMWJ) fabricated from hot-wire narrow-gap TIG (NG-TIG) welding process using alloy 617 filler wire (ERNiCrCoMo-1) ...was carried out under creep testing at 888 K. Microstructural constituents and hardness across the weld joint (10Cr steel-alloy 617 butter layer-alloy 617 weld metal-alloy 617 M) were found to vary significantly. Alloy 617 weld metal and butter layer have possessed higher hardness as compared to base metals of alloy 617M and 10Cr steel. However, lower hardness was observed in the alloy 617 butter layer, which is adjacent to 10Cr steel, and in the outer edge of HAZ in the 10Cr steel. The carbon migration was predominantly observed across the interface between 10Cr steel and alloy 617 butter layer. Creep tests performed on 10Cr steel and DMWJ at 888 K have revealed the lower creep strain accumulation in DMWJ than the 10Cr steel base metal. An early onset of tertiary creep deformation and consequent premature failure of DMWJ were noticed as compared to the 10Cr steel. Creep rupture strength of the DMWJ was about 31% lower than the 10Cr steel. Fracture in the weld joint has occurred at the interface between 10Cr steel and alloy 617 buttered layer with significant reduction in ductility. The formation of coarse M
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C
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precipitates and Laves phase (enriched by Mo and W), oxidation, heterogeneity in strength across the interface have facilitated the extensive cavitation at the interface, thereby leading to premature failure of the DMWJ. Weld strength reduction factor of about 0.69 at 888 K for 10
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h has been obtained for the DMWJ.
•A novel technique for determining appropriate weld data acquisition rate has been proposed.•High speed imaging results of actual process are correlated with the welding signals.•Welding data and the ...imaging results was used to evaluate different welding consumables.•Potential application to overcome present technique to analyze welding parameters.•When compared to Current practice proposed technique is both time and cost effective.
Shielded Metal Arc Welding (SMAW) Process utilizes a constant current type power source with drooping characteristics. Due to complex nature of welding arc and metal transfer that occurs during welding, there is a lot of random variations in welding current and voltage which cannot be recorded directly by normal ammeter or voltmeter. However, acquisition of welding data while welding is in progress and subsequent analysis of this data can be very useful to evaluate various welding parameters (i.e. welding consumables etc.). For this purpose, high speed of data acquisition is essential. As noise level in the data will be high hence, before performing any meaningful analysis filtering of this data is also important. In the present study, a technique is proposed for the reliable acquisition of welding data to acquire all the possible variations in arc voltage while welding is in progress using a Digital Storage Oscilloscope (DSO). Various signal processing methods were used for selecting the appropriate filtering technique. Filtered data thus obtained were used to evaluate arc welding electrodes with different flux coating using probability density distributions. The results thus obtained were correlated with the images obtained using high speed camera setup. This clearly brings out the differences in the arc characteristics for welding consumables. They also indicate that the proposed technique can be developed as a tool to compare the performance of different welding electrodes.
