Chapter 1 - Defining the problem

Abstract

Hydrogen cracks are normally very fine and difficult to detect, even when they break the surface, which they often do not. For cracks, which are present at the surface, Magnetic Particle Inspection (MPI) is the preferred technique, although dye penetrant should be used if a non-ferritic filler (e.g. an austenitic stainless steel or a nickel alloy) has been used. The MPI technique has the advantage that when using DC, the cracks just below the surface can be detected. The difficulties of detecting buried cracks in as-welded joints make it particularly important to use safe welding procedures, as described subsequently in this book. The identification of hydrogen cracks is not simple, as they can occur in a variety of locations and orientations, and can be trans-granular or inter-granular, the latter being more likely if the steel or weld metal is of the alloyed type, or exceptionally hard. In the HAZ, hydrogen cracks are usually longitudinal to the weld (unless they are extensions of transverse weld metal cracks) and usually have a portion close to the fusion boundary.