Unformed austenite

Watch what you're welding: If the steel fully transforms to austenite when heated, the structure has no stable grain boundaries anymore. That stuff is then really tough and metastable, and if you're not careful, you'll get stresses and cracks in the weld when it cools down. Especially with high-carbon steels, it gets critical quickly. The solution is simple: let the weld cool slowly and evenly, then nothing happens. And if you're unsure how the steel reacts, better ask or preheat it.