This group of discontinuities is associated with the various finishing operations, after the part has been rough formed by rolling, forging, casting or welding. Discontinuities may be introduced by machining, heat treating, grinding and similar processes. Secondary processing discontinuities differ from primary processing discontinuities in that these are discontinuities that occur during processes which result in a finished product rather than a part which requires additional processing.
Machining Tears
These are caused by dragging of the metal under the tool when it is not cutting cleanly. Soft and ductile low carbon steels are more susceptible to this kind of damage than are the harder, higher carbon or alloy types. Machining tears are surface discontinuities and are readily found with magnetic particles.
Heat Treating Cracks
When steels are heated and quenched to harden them, or are otherwise heat treated to produce desired properties for strength or wear, cracking may occur if the operation is not correctly suited to the material and the shape of the part. Most common are quench cracks, caused when parts are heated to high temperatures and then suddenly cooled by immersing them in some cool medium, which may be water, oil or even air. Such cracks often occur at locations where the part changes section from light to heavy - or at fillets or notches in the part. The edges of keyways and the roots of splines or threads are likely spots to watch for quenching cracks. Cracks may also result from too rapid heating of the part, which may cause uneven expansion at changes of cross-section, or at corners where heat is absorbed from three sides-more rapidly than in the body of the piece. Corner cracking may also occur during quenching because of more rapid heat loss at such locations. Heat treating cycles can be designed to minimize or eliminate such cracking, but for critical parts, testing with magnetic particles is a safety measure usually applied, since such cracks are serious and their detection presents no difficulty.
Straightening Cracks.
The process of heat treating often causes some warping of the part due to slight unevenness in the cooling during quenching. A hardened shaft, for example, may come from the heat treat operation not quite straight. ln many cases these can be straightened in a press, bcracks the amount of bend required is too great, or if the shaft is very hard, cacks may be formed. These, again, are very readily found with magnetic particles.
Grinding Cracks
Surface cracking of hardened parts as the result of improper grinding is frequently a source of trouble. Grinding cracks are essentially thermal cracks and are related to quenching cracks in more ways than one. They are caused by stresses set up by local heating under the grinding wheel. They are in nearly all cases avoidable if proper wheels, proper cuts and proper coolants are used, and if wheels are properly dressed when required. But since proper grinding requires constant attention and care which is not always provided in practice, these defects do occur. Since they are sharp surface cracks they are easily located with magnetic particles, even if shallow, and locating them is usually of vital importance. Salvage of the cracked part is seldom possible since grinding is usually a final precision finishing operation. The best use of magnetic particle testing in this case is to conduct sampling tests to monitor the grinding operation and thereby control it to avoid the formation of grinding cracks. Hardened surfaces often retain internal stresses from the quenching operation which are not severe enough to cause cracking at the time of quenching. During grinding, however, the relatively small increment of stress set up by local heating under the grinding wheel may cause rupture when added to the residual stress already present. Such surfaces usually crack severely and extensively.
Etching and Pickling Cracks
Hardened surfaces which contain residual stresses may be cracked if they are pickled or etched in acid. Attack by the acid of the surface' fibers of the material gives the internal stress a chance to be relieved by the formation of a crack. Before this action was fully understood, the heat treatment of the part was often blamed for the cracking, when such cracking actually occurred during acid cleaning for plating or other purposes. The heat treat operations did, however, deserve some of the blame, by leaving the part with internal residual stress locked up.
Plating Cracks
When hardened surfaces are to be electroplated, care must be taken to ensure that pickling (or other cleaning operations preparatory to plating) does not produce cracks. Sometimes cracks are formed during the plating operation itself. Residual stresses leading to etching or plating cracks may also be the result of cold work. Spiral springs, cold work, then pickled for plating or hot galvanizing, have also shown such cracks. The hot galvanizing process itself may also produce cracks in surfaces containing residual stresses. Penetration of the hot zinc between the grain boundaries during the hot dip process provides points for relief of such stresses by the formation of cracks. Copper penetration during brazing may result -in similar cracking if the parts contain residual stresses. Mollen alloys from the bearing of a railroad axle journal during a "hot box" will penetrate the surface of the heated journal and provide the starting point for a fatigue crack and axle failure.
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