Complex sentence: When quenched, the uninsulated edge contracts, causing the sword to bend towards the edge, but the edge cannot contract fully before the martensite forms, because the rest of the sword remains hot and in a thermally expanded state.
Split into simpler sentences: When quenched, the uninsulated edge contracts, causing the sword to first bend towards the edge. However, the edge cannot contract fully before the martensite forms, because the rest of the sword remains hot and in a thermally expanded state.