Interstitial solid solution

Small foreign atoms like carbon, nitrogen, or hydrogen lodge in the iron lattice between the atoms. This strains the lattice and makes the steel harder and stronger, but also more brittle. In welding, this is critical: during rapid cooling, hard, brittle martensite can form, and hydrogen quickly leads to cracks in the weld. Therefore, when welding, you must ensure clean, dry fillers and joints. For higher carbon steels, preheating helps slow down cooling and avoid cracks. If you keep the hydrogen content low and control cooling, the weld remains tough and holds.