Abstract
A new technique named MISO has been invented by the authors, in which solidification front and/or hot cracking during welding is photographed with camera mounted on optical microscope and analyzed with film analyzer. The MISO technique was applied to welding of stainless steels, Inconel alloy 600 and plain carbon steels. The technique revealed next phenomena: (1) The apparent solidification range where liquid was clearly observed with cellular dendrite growing was only 20 to 35% of the true solidification range measured as brittleness temperature range (BTR). (2) Initiation of solidification crack in a high strain rate with tensile hot cracking test occurred near the solidification front. These were well explained by the calculation of solid fraction vs. temperature relation and fractography. Furthermore, all these suggested that so-called liquid film stage occurs comparatively near the liquidus temperature, and that the about lower half of BTR is liquid droplet stage where the liquid phases locate discontinuously at interdendritic boundary. Based on these, a modification of the "Generalized theory" seems to be necessary in terms of crack susceptibility vs. temperature relation in solid-liquid coexistent region.
