Surface- and volume-based investigation on influences of different Varestraint testing parameters and chemical compositions on solidification cracking in LTT filler metals

Abstract

The subject of this study is how, and to what extent, Varestraint/Transvarestraint test results are influenced by both testing parameters and characteristics of evaluation methods. Several different high-alloyed martensitic LTT (low transformation temperature) filler materials, CrNi and CrMn type, were selected for examination due to their rather distinctive solidification cracking behaviour, which aroused interest after previous studies. First, the effects of different process parameter sets on the solidification cracking response were measured using standard approaches. Subsequently, microfocus X-ray computer tomography (μCT) scans were performed on the specimens. The results consistently show sub-surface cracking to significant yet varying extents. Different primary solidification types were found using wavelength dispersive X-ray (WDX) analysis conducted on filler metals with varying Cr/Ni equivalent ratios. This aspect is regarded as the main difference between the CrNiand CrMn-type materials in matters of cracking characteristics. Results show that when it comes to testing of modern highperformance alloys, one set of standard Varestraint testing parameters might not be equally suitable for all materials. Also, to properly accommodate different solidification types, sub-surface cracking has to be taken into account.

Solidification Cracking Assessment of LTT Filler Materials by Means of Varestraint Testing and µCT

“…Generally, the depth curve for all travel speeds initially increases to a maximum value underneath the weld surface. This observation is consistent with the depth courses of the cumulated crack length shown for example, in References [23,45]. Even though the highest bending strains act at the surface, the graphs reveal a maximum total crack length below the surface.…”

“…As the solubility and diffusion rate of elements like for example, sulphur that promote hot cracking is lower in the face cantered cubic (fcc) structure compared to the body cantered cubic (bcc) structure, primary austenitic solidification favours the formation of hot cracks [29]. Consequently, studies with varying chemical compositions of LTT weld filler materials revealed decreasing hot crack susceptibility with increasing Cr eq /Ni eq ratio [23]. Important welding parameters with regard to hot cracking susceptibility are the arc voltage U, welding current I and the travel speed v w .…”

“…We focused our investigation on a Cr/Ni based LTT alloy (e.g., References [1,41]) that shows a conspicuous hot cracking characteristic [23]. The investigated samples were standardized MVT specimens with dimensions 100 mm × 40 mm × 10 mm.…”

“…However, since LTT alloys are high-alloyed filler materials (e.g., Cr/Ni steels) they may show high hot cracking susceptibilities depending on the chemical composition. This circumstance was extensively investigated by the authors in a previous study [23] using the MVT test. As the surface based MVT standard analysis approach did not produce clear trends and left some open questions, µCT-imaging was carried out in order to examine crack-afflicted areas below the specimen surface.…”

“…It became clear, though, that the µCT results from Reference [23] were not yet exploited to their full potential. The current study therefore aims to develop a method for detailed characterization of sub-surface crack networks with regard to the hot cracking model following Prokhorov .…”