Interpretation of evaluation indices based on solidification cracking behavior by in situ observation in Trans-Varestraint test

The Varestraint test has been used in various fields for evaluating hot cracking susceptibility and has significantly contributed to studies associated with the hot cracking susceptibility of materials. Evaluation indices obtained from the Varestraint test include number of cracks, total crack length, maximum crack length, minimum nominal strain, brittle temperature range, and critical strain rate for temperature drop; however, the detailed significance of the evaluation indices is yet to be elucidated. In this study, the strain related to the initiation and propagation of solidification cracking is quantitatively evaluated by observing the solidification cracking behavior during the Trans-Varestraint test using an in situ observation method. In practice, the strain loaded onto the weld metal in the Varestraint test increases with the nominal strain. Solidification cracks occur near the solid–liquid interface and then propagate away from the weld pool. The initiation strain to solidification cracking occurrence is constant regardless of the nominal strain, and the strain for the propagation increases with the nominal strain. This suggests that a change in the nominal strain in the Varestraint test affects the solidification crack length; however, the initiation strain of the solidification crack is constant at all nominal strains and is independent of the crack length. Therefore, it is concluded that initiation strain of solidification cracking cannot be evaluated based on the conventional arrangement of the Varestraint test.