Creep and low cycle fatigue characterisation of IN718 TIG butt-welds at 620°C

D.W.J. Tanner, A.A. Becker, T.H. Hyde

Research output: Chapter in Book/Report/Conference proceedingConference Contribution (Conference Proceeding)

Abstract

The testing setups, procedures, results and analysis of uniaxial tensile, constant load creep and low-cycle high temperature fatigue tests of both TIG butt-welded and non-welded, thin-section Inconel 718 (IN718) specimens are presented. The main objectives were to determine the effect the welds have on failure time, analyse any differences in their failure behaviour, and obtain material properties for life prediction purposes.

IN718 is a precipitation-hardenable, niobium-modified, nickel-base superalloy often used in critical high temperature creep resistant applications up to 650°C. Data applicable to welded sheet structures is difficult to obtain since significant out-of-plane weld distortions compromise uniaxial testing. Life predictions are also complicated by the effects of residual stresses and material microstructural changes caused by the welding process.

The weld-induced distortions were effectively mitigated in this work by maximizing the relative stiffness and reducing the welding time by only welding small plates that were just large enough to form one test specimen. This solution does not modify the effects of the welding process on the thermal history of the material. Since the plates remained flat after welding, stress-raising weld bead excess could easily be removed by a linishing process.

Tensile tests at temperatures from ambient to 1000°C resulted in both the 0.2% proof stress and the ultimate strength of most of the welded specimens exceeding 90% of the values of the non-welded specimens. However, the strain to failure for the welded specimens was significantly less than that of the non-welded specimens.

Constant load creep tests at 620°C also showed noticeably lower total strain to failure for the welded specimens than the non-welded specimens. The time to failure was also markedly less for the welded specimens. The welded specimen creep failures occurred in a coarse-grained heat-affected zone (HAZ) where large grains were unable to accommodate large straining by grain boundary sliding, leading to premature fracture.

Low cycle fatigue tests at 620°C, with a trapezoidal (1_1_1_1) form, load frequency of 0.25 Hz and 0 stress ratio, resulted in lower failure times for the welded specimens than the corresponding non-welded values for any given nominal stress. This is attributed to the presence of brittle Laves phase at the fine-grained HAZ boundaries, which is known to decrease ductility and life.

In conclusion, although welded IN718 exhibits comparatively little loss of tensile strength, its creep and high temperature fatigue properties are severely decreased.
Original languageEnglish
Title of host publicationFifth International Materials Symposium (MATERIAIS 2009)
Place of PublicationLisbon, Portugal.
Volume(CD)
Publication statusPublished - 2009
EventFifth International Materials Symposium (MATERIAIS 2009) - Lisbon, Portugal
Duration: 5 Apr 20098 Apr 2009

Conference

ConferenceFifth International Materials Symposium (MATERIAIS 2009)
CountryPortugal
CityLisbon
Period5/04/098/04/09

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    Tanner, D. W. J., Becker, A. A., & Hyde, T. H. (2009). Creep and low cycle fatigue characterisation of IN718 TIG butt-welds at 620°C. In Fifth International Materials Symposium (MATERIAIS 2009) (Vol. (CD)).