Additivity and isokinetic behaviour in relation to particle dissolution

Bjørneklett, B.; Grong, Ø.; Myhr, Ole Runar; Kluken, A. O.

doi:10.1016/s1359-6454(98)00260-2

Cited by 36 publications

(15 citation statements)

References 25 publications

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“…Following a different analytical approach, developed by Ashby and Easterling [14], they derived a particle dissolution model and verified it with experimental results. Their analytical results, which are reported to be in good agreement with the experimental results, are qualitatively consistent with those of Bjorneklett et al [13]. They showed that there was a significant departure from equilibrium under rapid heating condition, in that the temperature of complete dissolution of γ' particles increased with increasing heating rate and the extent of the departure was dependent on the initial particle size.…”

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confidence: 89%

Crack-Free Welding of IN 738 by Linear Friction Welding

Ola

Ojo

Wanjara³

et al. 2011

AMR

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Abstract. Inconel 738 (IN 738), like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during conventional fusion welding processes. The cause of this cracking, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion across one of the solid-liquid interfaces due to on-cooling tensile stresses. In the present work, crack-free welding of the alloy was obtained by linear friction welding (LFW), notwithstanding the high susceptibility of the material to HAZ cracking. Gleeble thermomechanical simulation of the LFW process was carefully performed to study the microstructural response of IN 738 to the welding thermal cycle. Correlation between the simulated microstructure and that of the weldments was obtained, in that, a significant grain boundary liquation was observed in both the simulated specimens and actual weldments due to non-equilibrium reaction of second phase particles, including the strengthening gamma prime phase. These results show that in contrast to the general assumption of LFW being an exclusively solid-state joining process, intergranular liquation is possible during LFW. However, despite a significant occurrence of liquation in the alloy, no HAZ cracking was observed, which can be partly related to the nature of the imposed stress during LFW. IntroductionIN 738 is a nickel-based superalloy with a significant volume fraction of an ordered intermetallic Ni 3 (Al,Ti) γ' phase, possessing an excellent high-temperature strength and remarkable corrosion resistance. This makes it a suitable material for manufacturing of hot-section components in aeroengines and in power generation turbine applications where they are exposed to severe operating conditions. Welding techniques are usually employed for both the fabrication and repair of these components. IN 738, like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heataffected zone (HAZ) cracking during welding and strain age cracking during post-weld heat treatment (PWHT) [1]. The cause of this cracking during conventional fusion welding processes, which is usually intergranular in nature, has been attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid and decohesion across one of the solid-liquid interfaces due to on-cooling tensile stresses. Detailed mechanism of grain boundary liquation cracking in the alloy has been provided elsewhere [2].To address the problem of liquation cracking in weldments, a recent trend has involved the use of supposedly solid-state welding techniques, such as, friction welding for joining crack susceptible structural alloys. Friction welding generally involves the rubbing together of two com...

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confidence: 89%

Crack-Free Welding of IN 738 by Linear Friction Welding

Ola

Ojo

Wanjara³

et al. 2011

AMR

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“…[36] The isokinetic concept can be used if the increments of transformation in infinitesimal isothermal time-steps are additive. Bjorneklett et al [37] have been able to show that second-phase particle dissolution reaction is additive during continuous heating. During continuous heating, the radius of the particle can be expected to start deceasing when the temperature reaches the equilibrium solvus temperature of the particle, T SL.…”

Section: Liquation Of Microconstituent Containing M 23 X 6 Particlesmentioning

confidence: 99%

Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

Ojo

Chaturvedi

2007

Metall Mater Trans A

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The effect of preweld overaging heat treatment on the microstructural response in the heataffected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M 23 X 6 particles and coarsening of primary and secondary c¢ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M 23 X 6 particles and MC-type carbides. In addition, the coarsened c¢ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.

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“…Cahn [22] later showed that transformations which nucleate heterogeneously (as is typically observed during quenching of aluminium alloys) tend to obey the rule of additivity, suggesting that a wide range of reactions are additive. Since then, a broad range of non-isothermal transformations have been successfully modelled under the assumption that the reactions are additive [23][24][25][26][27][28][29][30], despite the fact that the precise conditions for additivity are still disputed [31][32][33][34][35].…”

Section: Introductionmentioning

confidence: 99%

Improvements in quench factor modelling

Rometsch

Starink

Gregson

2003

Materials Science and Engineering: A

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In this contribution, the validity of a number of key quench factor analysis (QFA) assumptions is discussed. It is shown that the incorporation of a square-root dependency of yield strength on precipitate volume fraction provides a sounder physical basis for quench factor modelling. Peakaged strength/hardness prediction accuracies are not affected, but C-curve positions are. It is also demonstrated that transformation kinetics are described more correctly by a modified StarinkZahra equation than by a Johnson-Mehl-Avrami-Kolmogorov type equation, yielding better prediction accuracies when a physically realistic Avrami exponent of 1.5 or greater is used.Finally, a regular solution model is introduced to quantify the influence of the solute solubility temperature dependency on the minimum strength. These improvements are all implemented within the framework of classical QFA.

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Additivity and isokinetic behaviour in relation to particle dissolution

Cited by 36 publications

References 25 publications

Crack-Free Welding of IN 738 by Linear Friction Welding

Crack-Free Welding of IN 738 by Linear Friction Welding

Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

Improvements in quench factor modelling

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