Anssi Brederholm

Cracking Phenomena in Welds IV

This is the fourth volume in the well-established series of compendiums devoted to the subject of weld hot cracking. It contains the papers presented at the 4th International Hot Cracking Workshop held in Berlin in April 2014. In the context of this workshop, the term hot cracking refers to elevated temperature cracking associated with either the weld metal or heat-affected zone. These hot cracking phenomena include weld solidification cracking, HAZ and weld metal liquation cracking, and ductility-dip cracking. The book is divided into three major sections based on material type; specifically aluminum alloys, steels, and nickel-base alloys

Weldability of superalloys alloy 718 and ATI (R) 718Plus (TM) - A study performed by Varestraint testing

Abstract In this study, the old and well-known alloy 718 is compared with the newly developed ATI® 718Plus™ from the weldability point of view. This is done in order to gain new information that have not been documented and established yet among the high-temperature materials with high strength, oxidation resistance, thermal stability and sufficient weldability, yet. ATI® 718Plus™ shows a lower sensitivity to hot cracking than alloy 718 with approximately 10 mm total crack length (TCL) difference in Varestraint testing. In the solution-annealed condition at 982°C for 4.5 h followed by air cooling, the crack sensitivity is decreased as compared to the mill-annealed condition. Along the crack path and also ahead of the crack tip, γ-Laves eutectic is present in both alloys. The microhardness measurements showed similar hardness level of 250 HV in the weld metal of both alloys and even in the parent material of alloy 718. ATI® 718Plus™ parent metal had hardness of 380 HV and a small increase of less than 50 HV was observed for both studied alloys in the heat affected zone (HAZ). For the same grain size of ATI® 718Plus™ (8.3 μm) and alloy 718 (15.6 μm), the susceptibility to liquation cracking may increase with increasing grain size. With a small grain size, there is a possibility to accommodate more trace elements (B, S, P) due to the larger grain boundary area. The impurity elements were found in relatively small precipitates, typically borides (0.2 μm), phosphides (0.1 to 0.5 μm) and carbo-sulphides. The solidification sequence of alloy 718 and ATI® 718Plus™ is relatively similar, where the liquid starts to solidify as γ-phase followed by γ/MC reaction at about 1260 °C and then final γ/Laves eutectic reaction at around 1150 °C. Detailed knowledge about weldability of alloy 718 and ATI® 718Plus™ can be used for material selection.

Improved Understanding of Varestraint Testing—Nickel-Based Superalloys

By Varestraint testing, the susceptibility of an alloy to hot cracking during welding can be evaluated on test plates when they are bent at the same time as welding takes place. The strains imposed by welding can thus be augmented by the strains imposed by the bending action to find the strain limits when hot cracks appear and also the sensitivity to hot cracking by counting the number and measuring the length of the individual cracks as a means to differentiate between the weldability of different alloys. Supports are usually recommended to avoid hinging and to use test plates thicker than 10 mm in order to minimize the influence of the compression strains (lower part of the bent specimen) on the weld cracking at the bending. The cracking response of two precipitation hardening Nickel-based superalloys—ATI 718Plus® and Haynes® 282®—was analysed in the context of the actual tensile/compression ratio imposed and measured by strain gauges attached to the upper and lower surface of the test plates. It was found that no influence of the compressive strains on the cracking response in Varestraint testing takes place. It was also seen that the hot cracking susceptibility of Haynes® 282® is lower compared to that of ATI 718Plus®.

Varestraint Weldability Testing of ATI 718Plus®—Influence of Eta Phase

This study investigates the effect of eta phase on hot cracking susceptibility of ATI 718Plus®. Two heat treatment conditions of 950 °C/1 h and 950 °C/15 h having different amounts of eta phase were tested by longitudinal Varestraint testing method. The heat treatment at 950 °C/15 h exhibited the highest amount of cracking. This was related to the higher amount of eta phase precipitation during the long dwell heat treatment which aided to extensive liquation during welding.

Weldability of Ni-Based Superalloys Waspaloy® and Haynes® 282® - A Study Performed with Varestraint Testing

There is a need for materials with high strength, oxidation resistance, thermal stability and adequate weldability in order to facilitate the production of large structural jet engine components. Therefore, the weldability of Waspaloy® and Haynes® 282® have been evaluated using the Varestraint weldability test. The experiments reveal that Waspaloy® has a higher susceptibility to hot cracking compared to Haynes® 282®. This conclusion is supported by increased total crack length (10 mm or more) and larger brittle temperature range (approx. 65°C) for Waspaloy® when compared to Haynes® 282® in Varestraint and Gleeble hot ductility tests, respectively. The cracking in Haynes® 282® seems to be connected with a secondary phase which presumably can be associated with Ti-Mo based MC-type carbide observed in the fusion zone. Also, a surrounding segregated area is present near this secondary phase as well as along the grain boundaries. Furthermore, micro-Vickers hardness results revealed more or less the same weld metal hardness (260- 280 HV) but a difference in the base metal hardness. The weld metal hardness of Waspaloy® was lower than that of the base metal hardness, while Haynes® 282® had a higher hardness in comparison.

Effect of Hot Cracks on EAC Crack Initiation and Growth in Nickel-Base Alloy Weld Metals

The differences in the EAC susceptibility between different weld geometries and weld metals have been distinguished by the doped steam test method. Pure weld metals of Alloy 182 and 82 are clearly more susceptible to EAC than the pure weld metals of Alloy 152 and 52, which did not show any crack initiation. The dissimilar metal welds (DMW) with diluted microstructures are less susceptible than the pure weld metals of Alloy 182 and 82. No crack initiation/extension from hot cracks was observed in any of the studied weld metals. At the hot crack tips no crack growth was observed in any of the studied samples. This is related to the segregated microstructure of the hot crack tips. In accelerated doped steam tests selective dissolution takes place and metallic Ni or NiO forms a continuous layer in the middle of the cracks surrounded by the Cr-rich oxide layer. Selective dissolution typical for EAC was not observed inside the hot cracks or at their crack tips. EAC initiation occurred in the Alloy 600 base metal of the DMWs and selective dissolution inside the EAC cracks in Alloy 600 was extensive. The results are discussed based on the selective dissolution creep model of EAC.