53 Liquid Metal Embrittlement

Liquid metal embrittlement is intergranular penetration (or cracking) of the heat-affected zone (HAZ). Intergranular penetration is penetration of molten metal along the grain boundaries of the base metal that leads to embrittlement of the base metal. Liquid metal embrittlement can occur with specific combinations of base metals and molten metals, usually in the presence of stress. Liquid metal embrittlement appears as a relatively wide, jagged crack revealed under magnification.

Brazes are a common cause of liquid metal embrittlement in susceptible alloys. For example, many nickel alloys, when in a stressed condition, may crack from liquid metal embrittlement in contact with molten brazing metal.

Liquid metal embrittlement may also occur during welding from contamination of a base metal by other metals. For example, when welding austenitic stainless steels to galvanized steels, zinc contamination may cause liquid metal embrittlement of the austenitic stainless steel base metal. The zinc contamination may be introduced by grinding dust or direct contact between the two metals, such as when welding austenitic stainless steel to galvanized carbon steel.

Liquid Metal Embrittlement Prevention

Liquid metal embrittlement is prevented by avoiding susceptible braze base metal couples or by ensuring cleanliness of the joint surfaces before welding or brazing. For example, when welding galvanized steel to austenitic stainless steel, all zinc must be removed by grit blasting a minimum of 2” from the joint face to ensure that the zinc does not melt and mix with the austenitic stainless steel, resulting in liquid metal embrittlement.