Thermal shock
Thermal shock is the load from a quick, strong temperature change: hot metal expands the surface, cooling suddenly contracts it again. This alternating stress creates cracks – typical for die-casting molds.
Physicochemical Mechanism of Action
Every rapid temperature jump puts the surface under stress while the core stays unaffected, or vice versa. When hot metal hits the die, the outer layer wants to expand, the cold core holds it back, and it gets compressed. During cooling, this reverses and it gets stretched. Each cycle does this once. None of these stress peaks alone is enough to break anything. But a thousand times in a row fatigues the steel, and then the surface starts to crack in fine networks.
Metallurgical Relevance & Welding Risks
You see this as on the die surface, in , and on hot forming tools. How long a die lasts depends on its and its . If the steel loses hardness in service, the surface sinks in. If it's set too hard, it cracks instead. Repair welding on such a die is tricky, because a cold weld in a hot environment builds up exactly what you wanted to get rid of.
Workshop Practice & Action Guidelines
Never bring a die up cold; instead, heat it evenly to operating temperature and keep it there. Distribute release agents and cooling evenly, and don't spray one spot, or you'll have a cold spot there. Repair only on a preheated tool, with matching filler and afterward. Work out fine crack networks early. If you wait, you'll be grinding away half the die insert later.