cold-shut

A cold-shut is a surface defect in forged or cast metal where two surfaces have been pressed together while below their fusion temperature, leaving a seam that looks joined but has no metallurgical bond. The surfaces touch and stay in contact, but they do not melt or diffuse into each other. Under load, the joint will crack or separate because there is nothing holding it together except mechanical pressure that has since been removed.

In forging, cold-shuts occur most often when metal is worked too slowly or at temperatures that have dropped below the working range. Die impression forging is particularly prone to this defect: if the two halves of the die come together before the metal has reached plastic flow temperature, the parting line remains as a visible seam with zero tensile strength across it. The defect is often found running along the parting line of the forging or in deep recesses where the last metal to fill the die has cooled during the press stroke.

Castings suffer cold-shuts at gates and junctions where two streams of molten metal meet and solidify before they have time to mix and fuse. This is more common in thicker sections cast in cold molds, or when pouring metal that is only marginally above its liquidus temperature. The defect appears as a crisp line, sometimes with oxide film sandwiched between the two surfaces, rather than the gradual grain structure of a proper fusion.

Detection and prevention

Cold-shuts are difficult to catch in routine inspection because they often lie flush with the surface and are not always visible without sectioning or destructive testing. Radiography may not reveal them clearly if the surfaces are in good contact. The term itself comes from the appearance: the metal has simply shut, or closed, without becoming welded.

Prevention in forging requires maintaining metal above the minimum working temperature throughout the stroke and ensuring dies themselves are hot enough. In casting, faster pouring, hotter metal, and proper gate design to avoid metal meeting at shallow angles all reduce cold-shut risk. Parts known to be at high risk, such as those with deep thin sections or narrow gates, may require destructive sample testing or 100 percent magnetic particle inspection to confirm soundness.