HAMMER TESTING

A hammer test has been an accepted method of exploring the surface of metal objects to locate areas of reduced wall thickness; however, other NDE techniques have made this technique outdated. When a hammer test is made, the variations in metal-wall thickness are indicated by the feel and rebound of the hammer and by the sound produced. One value of hammer testing is that it is a good way to determine whether the scale on the outside surface of a tube is an oxide due to overheating or a product of fuel combustion. Although combustion deposits may vary in texture depending on the fuel used, the scale that results from oxidation is generally harder, requires a stronger blow to be knocked loose from the tube, and is of a flakier texture than scale from the products of combustion. A magnetic check of the material offers the most conclusive test; oxide scale is magnetic, and scale from the products of combustion is nonmagnetic.

Heater tubes that have been in service may become temper embrittled and have low ductility at ambient temperature. To avoid any possible damage, carbon and alloy steel heater tubes should have a minimum metal temperature of about 60°F (15°C) during hammer tests. In certain cases, the hammer testing of tubes can lead to damage. Austenitic stainless steel tubes may suffer stress corrosion cracking at areas that are cold worked by hammering. Cast austenitic stainless steel tubes and Cr-Mo ferritic tubes should not be hammer tested when tubes are heavily carburized.