Investigation on effect of welding parameters on solidification cracking of austenitic stainless steel 314

This study investigates the solidification cracking susceptibility of the austenitic stainless steel 314. Longitudinal Varestraint testing was used with three different set of welding test parameters. Weld speed, current and voltage values were selected so that the same heat input resulted in all the test conditions. From the crack measurements it was seen that the test condition with the lowest current and welding speed value also produced the least amount of cracking with very good repeatability

Influence of Hot Isostatic Pressing on the Hot Ductility of Cast Alloy 718: The Effect of Niobium and Minor Elements on the Liquation Mechanism

The influence of two hot isostatic pressing (HIP) treatments on liquation behavior was investigated and compared with regard to the extent of heat-affected zone liquation cracking in cast Alloy 718. The extent of liquation was seen to increase after HIP treatment at 1190 \ub0C due to solute changes caused by the homogenization of Nb, which contributed to extensive grain boundary melting. The HIP treatment at 1120 \ub0C exhibited lower liquation with contributions from particle liquation of the Laves phase and constitutional liquation of NbC carbides. This was also reflected in a lower ductility recovery temperature, with slower recovery for the former due to the extensive liquation. Interestingly, the nil ductility temperatures were both below the predicted equilibrium solidus of the alloy, which suggests that the ductility drop is related to liquation caused by solute segregation at the grain boundaries

Influence of homogenisation treatments on the hot ductility of cast ATI\uae 718Plus\uae: Effect of niobium and minor elements on liquation characteristics

The hot ductility of cast ATI\uae 718Plus\uae was assessed using the Gleeble thermo-mechanical simulator after being subjected to different homogenisation heat treatments. The hot ductility deteriorated significantly after long-dwell homogenisation heat treatments for 24\ua0h\ua0at temperatures of 1120 and 1190\ua0\ub0C as compared with those treated at a short dwell time of 4\ua0h\ua0at the same temperatures. The observed ductility deterioration was related to more extensive liquation along the grain boundaries caused by different mechanisms, e.g., liquation by solute segregation mechanism, Laves melting, constitutional liquation of MC carbides and supersolidus grain boundary melting, with the effect and extent depending on the solute changes after the homogenisation heat treatments. Furthermore, the role of Nb as the solute element and as the precipitate former, as well as the effect of minor alloying elements segregating along the grain boundaries, is discussed in connection to grain boundary liquation, which contributes to a better understanding of heat-affected zone liquation cracking susceptibility of cast ATI\uae 718Plus\uae

Varestraint weldability testing of cast ATI\uae 718Plus™—a comparison to cast Alloy 718

Varestraint testing of the newly developed cast ATI\uae 718Plus™ after pseudo-HIP (hot isostatic pressing) heat treatments showed that the extent of solidification cracking was independent of the heat treatment condition. The susceptibility towards heat-affected zone (HAZ) liquation cracking was found to be related to the heat treatment dwell time rather than the temperature. The heat treatments at 1120 and 1190 \ub0C for 24 h were the most susceptible to cracking. On the other hand, heat treatments at 1120, 1160 and 1190 \ub0C for 4-h dwell time exhibited the least amount of cracking. The solidification cracking was found to be similar whereas the HAZ liquation cracking was lower for ATI\uae 718Plus™ after the heat treatment at 1120 and 1190 \ub0C for 4-h dwell time in comparison to cast Alloy 718

Measurement of the thermal cycle in the base metal heat affected zone of cast ATI\uae718PlusTM during manual multi-pass TIG welding

This paper presents a method to acquire thermal data in the base metal heat affected zone (HAZ) during manual multi-pass TIG welding of ATI\uae718PlusTM, representing conditions close to an actual repair welding operation. Thermocouples were mounted in different locations along side walls of linear grooves to record temperature data. The thermal cycling was found to be largely independent of location within the HAZ. The recorded temperatures were below the incipient laves melting temperature, indicating that the current test setup requires optimisation to study HAZ liquation. Based on the results of this study, a modified thermocouple mounting technique is proposed

Solidification Cracking of Alloy Allvac 718Plus and Alloy 718 at Transvarestraint Testing

Allvac 718Plus is a newly developed superalloy with a good potential for use in fabricatedaircraft engine structures when service temperatures exceed 650\ub0C; the limit for standard alloy718. Fabrication involves welding, where cracking usually is a prohibitive factor. Comparison ofthe solidification cracking behavior between these two alloys was made by Transvarestrainttesting. It was shown that the total amount of cracking was significantly lower for 718Plus alloy,which was associated with a lower amount of γ/Laves or γ/NbC eutectic-type constituents. Thisin turn is believed to be due to the lower iron content of the Allvac 718Plus material. Theanalysis of alloying element segregation both within primary dendrite and interdendriticconstituents clearly indicates that the segregation of Nb is the controlling factor in determiningthe final solidification microstructure in these alloys. The Nb concentrations of these constituentsare significantly higher compared to the bulk alloy concentration. This was confirmed by theSEM-EDS analysis and calculations assessed with JMatPro, the latter also indicated that alloy718 is more prone to form Laves and NbC eutectic-type constituents compared to Allvac 718Plusmaterial

Hot Cracking and Grain Boundary Weaknesses of Nickel - Based Superalloys

Abstract Grain boundary weaknesses of nickel-based superalloys not only materialize during service in the hot turbine engine sections but also during the weld manufacturing. The performance, in this respect, of the most commonly used alloys at Volvo Aero Corporation (VAC), 718 and the higher temperature capacity Waspaloy, are therefore the main subjects for this thesis. The emerging Allvac 718\uae PLUSTM (718 Plus) is also included due to the prospect of replacing the expensive Waspaloy. The overall emphasis is on weldability through Varestraint testing but limited work was initially carried out on notch rupture sensitivity of 718 Plus.The notch rupture testing of 718Plus emphasized the importance of the secondary phases like delta-phase in the grain boundaries.Hot cracks were observed in all alloys with re-solidified products on the liquated and re-solidified grain boundaries. Grain boundary liquation and segregation of alloying elements was considered to be the primary reason for liquation of HAZ grain boundaries which caused cracking.Evidence of constitutional liquation of Nb rich MC type carbides in alloy 718 and 718 Plus were found in the region adjacent to the fusion zone and also on the cracked grain boundaries. The resolidified products along the hot cracks consisted of γ + MC type carbide and γ + Laves eutectics

Weldability of Precipitation Hardening Superalloys – Influence of Microstructure

Superalloys and in particular the precipitation hardened Ni-based superalloys havealways been used extensively in the hot sections of jet engines. Large hot structuralengine components with complex geometry have preferably been cast as single piececomponents since the large scale vacuum investment casting process becameavailable about fifty years ago. However, a recent trend is to cast smaller pieces whichcan be joined with sheet or forged parts to fabricate structural components. Therationale for this fabrication strategy is the possibility to save weight by the use of higherstrength wrought material, where geometry allows, and join these wrought parts withcast material where complex geometry is needed and where the demand for strength ismoderate. One of the major challenges using this strategy is the obvious fact thatnumerous welds must be made which requires the fundamental understanding, notleast metallurgical, of how different materials may be joined by specific weldingprocesses.The main objective of this research has, for this reason, been to examine and interpretthe weldability of precipitation hardened superalloys from a metallurgical standpoint.Two newly developed superalloys Allvac\uae 718PlusTM and Haynes\uae 282\uae are comparedwith the two well established Alloy 718 and Waspaloy. The understanding of theinfluence of secondary phases such as carbides and δ phase in the microstructure wasaddressed by systematic hot ductility testing (Gleeble) and by weldability testing(Varestraint). The effect of secondary phases were also analysed through practicalwelding as by electron beam welding (EBW), and by gas tungsten arc welding (GTAW).The research showed that all the techniques used (Varestraint testing, Gleeble testing,DSC thermal analysis and welding (GTAW repair and EBW)) in studying the weldabilityindependently provided important knowledge and most importantly that a combination ofthe results from these different techniques were necessary for the understanding of theweldability of these four alloys. From a microstructural point of view it has been possibleto show that δ phase contrary to what has generally been assumed improves theweldability due to its ability to inhibit grain growth and to assist in the healing of cracks.For future research, a new modified weldability testing method was developed where itis possible to perform Varestraint, Transvarestraint and spot-varestraint testing at ramspeeds from 15 to 300 mm/s using GTAW, plasma arc welding and laser welding

Weldability of Precipitation Hardening Superalloys – Influence of Microstructure

Superalloys and in particular the precipitation hardened Ni-based superalloys havealways been used extensively in the hot sections of jet engines. Large hot structuralengine components with complex geometry have preferably been cast as single piececomponents since the large scale vacuum investment casting process becameavailable about fifty years ago. However, a recent trend is to cast smaller pieces whichcan be joined with sheet or forged parts to fabricate structural components. Therationale for this fabrication strategy is the possibility to save weight by the use of higherstrength wrought material, where geometry allows, and join these wrought parts withcast material where complex geometry is needed and where the demand for strength ismoderate. One of the major challenges using this strategy is the obvious fact thatnumerous welds must be made which requires the fundamental understanding, notleast metallurgical, of how different materials may be joined by specific weldingprocesses.The main objective of this research has, for this reason, been to examine and interpretthe weldability of precipitation hardened superalloys from a metallurgical standpoint.Two newly developed superalloys Allvac\uae 718PlusTM and Haynes\uae 282\uae are comparedwith the two well established Alloy 718 and Waspaloy. The understanding of theinfluence of secondary phases such as carbides and δ phase in the microstructure wasaddressed by systematic hot ductility testing (Gleeble) and by weldability testing(Varestraint). The effect of secondary phases were also analysed through practicalwelding as by electron beam welding (EBW), and by gas tungsten arc welding (GTAW).The research showed that all the techniques used (Varestraint testing, Gleeble testing,DSC thermal analysis and welding (GTAW repair and EBW)) in studying the weldabilityindependently provided important knowledge and most importantly that a combination ofthe results from these different techniques were necessary for the understanding of theweldability of these four alloys. From a microstructural point of view it has been possibleto show that δ phase contrary to what has generally been assumed improves theweldability due to its ability to inhibit grain growth and to assist in the healing of cracks.For future research, a new modified weldability testing method was developed where itis possible to perform Varestraint, Transvarestraint and spot-varestraint testing at ramspeeds from 15 to 300 mm/s using GTAW, plasma arc welding and laser welding

Solidification Cracking of Alloy Allvac 718Plus and Alloy 718 at Transvarestraint Testing

Allvac 718Plus is a newly developed superalloy with a good potential for use in fabricatedaircraft engine structures when service temperatures exceed 650\ub0C; the limit for standard alloy718. Fabrication involves welding, where cracking usually is a prohibitive factor. Comparison ofthe solidification cracking behavior between these two alloys was made by Transvarestrainttesting. It was shown that the total amount of cracking was significantly lower for 718Plus alloy,which was associated with a lower amount of γ/Laves or γ/NbC eutectic-type constituents. Thisin turn is believed to be due to the lower iron content of the Allvac 718Plus material. Theanalysis of alloying element segregation both within primary dendrite and interdendriticconstituents clearly indicates that the segregation of Nb is the controlling factor in determiningthe final solidification microstructure in these alloys. The Nb concentrations of these constituentsare significantly higher compared to the bulk alloy concentration. This was confirmed by theSEM-EDS analysis and calculations assessed with JMatPro, the latter also indicated that alloy718 is more prone to form Laves and NbC eutectic-type constituents compared to Allvac 718Plusmaterial