Carbide

Carbides are hard compounds of metal and carbon. In alloyed steels, elements like chromium, molybdenum, vanadium, or tungsten bind the carbon into tiny, hard particles. These make the steel wear-resistant and keep it hard even at high temperatures. During tempering, even finer carbides can be precipitated, which is called secondary hardening. When welding, it's all about distribution. Fine carbides are good, coarse clumps are internal notches and crack starters. With stainless steel, caution is needed: if it gets into a certain temperature range, chromium carbides form at the grain boundaries. The chromium is then missing for corrosion protection, leading to intergranular corrosion next to the weld. So: with stainless, keep the heat low and work quickly, or use stabilized or low-carbon grades. For tool steel, anneal before welding, otherwise the cutting edges are quickly done. And after welding, let it cool slowly and temper, don't just leave it lying around.