Initiation and growth kinetics of solidification cracking during welding of steel

Abstract

Solidification cracking is a key phenomenon associated with defect formation during welding. To elucidate the failure mechanisms, solidification cracking during arc welding of steel are investigated in situ with high-speed, high-energy synchrotron X-ray radiography. Damage initiates at relatively low true strain of about 3.1% in the form of micro-cavities at the weld subsurface where peak volumetric strain and triaxiality are localised. The initial micro-cavities, with sizes from 10 × 10−6 m to 27 × 10−6 m, are mostly formed in isolation as revealed by synchrotron X-ray micro-tomography. The growth of micro-cavities is driven by increasing strain induced to the solidifying steel. Cavities grow through coalescence of micro-cavities to form micro-cracks first and then through the propagation of micro-cracks. Cracks propagate from the core of the weld towards the free surface along the solidifying grain boundaries at a speed of 2–3 × 10−3 m s−1.

A Three-Stage Mechanistic Model for Solidification Cracking During Welding of Steel

“…More details about the experimental procedure have been described in our previous publication. [36] In order to enhance the contrast of the cracks by means of X-ray inline phase contrast, a propagation distance of 7.3 m between sample and detector was realized. [37] All post-processing of the captured image data was performed using the public domain Java image processing program ImageJ, [38] with the assistance of a simple processing routine detailed in Reference 36.…”

“…At this stage, the volume fraction of liquid is higher and inter-dendritic fracture is observed from the separation of primary dendrite arms as a result of tensile deformation as shown in Figure 8 and evidenced in Figure 7. As the propagation is linked to the solidifying dendrites, the propagation path is related to that of the growing dendrites in terms of direction (towards the heat source) and in a velocity of~2.3 mm/ s [36] . Strain is the driving force for crack propagation and continues to develop with solidification.…”

“…In a previous study, an in situ synchrotron X-ray imaging experiment [36] was presented to observe and quantify the initiation and growth kinetics of solidification cracking during welding of steel. In the current study, the mechanisms for solidification cracking are elaborated by identifying the thermodynamic state and localized structure features at nucleation sites and the resultant fracture surfaces.…”

Getting the Strain Under Control: Trans-Varestraint Tests for Hot Cracking Susceptibility

“…Recent developments that have been presented in 2017 and 2018 by Aucott et al include the three-stage mechanistic model for solidification cracking during welding of steel [14] and the initiation and growth kinetics of solidification cracks during welding. [15] Solidification in general is affected by the temperature gradients that are forming during the process and the direction of heat flow. [16] During welding the heat flow is not the same as it would be during the free solidification of materials because of the movement of the heat source and the existence of solid material at the edge of the weld pool.…”

Critical assessment 26: Forensic metallurgy – the difficulties

“…Two processes that particularly introduce residual stresses into components are forming operations and welding. The residual stresses that are introduced into metals from solidification processes during welding [48] can be mitigated against by performing post-weld heat treatment after welding [49]. In forensic metallurgy, examination of the microstructure or chemistry of the component alone will not show whether or not these stresses are present and other techniques such as laboratory based XRD will be required [19,50,51].…”