Carbon makes steel brittle in general, not just at cold temperatures. Over 1 % C, the amount of carbide increases and the carbides form at grain boundaries, reducing the strength of the grain boundary. Wootz blades are heat treated in such a way as to minimize the bad effects of having a lot of excess carbide, to take advantage of their excellent edge enhancing properties.
Sulfur makes the steel more prone to cracking while it is being worked ‘hot’, phosphorus makes it more brittle when ‘cold’ – hot and cold meaning ~1500 F + and room temp, respectively. Phosphorus also collects at grain boundaries, and produces brittleness under shock – not good for a sword blade.
I think phosphorus is the culprit in this case, or the combination of high carbon and phosphorus. Verhoeven said in one of his JOM articles that he thought that Phosphorus was needed for good pattern development in wootz. Which one comes into play more as the temperature goes from +60 F to -20 F, or if it’s the synergy of both - I don’t know if that’s been determined.
Thanks for the heads-up on that book, ‘Medieval Islamic Swords and Swordmaking’ – I think Gilmore hinted that would be on the way in ‘Persian Steel’, but I hadn’t noticed it was out.
Greg - the low temps would get the retained austenite to go to untempered martensite? That would increase breakage, for sure! I think you should go ahead with that wordy post.