Maximum Hardness in Heat-Affected Zone

The maximum hardness in the heat-affected zone occurs when the steel gets strongly heated during welding and then cools down quickly. The higher the carbon content, the harder this zone becomes – and the more brittle the structure gets. The risk then is cracking, especially if the component is under stress or impact loaded. So you have to slow down the cooling, for example by preheating or higher heat input. That way you prevent the zone from getting too hard and the part from breaking.