Chromium plating is widely used on gages, cylinder walls, piston rings, and other machine parts where resistance to wear is an important factor. However, the advantages of chromium plating, in the absence of proper precautions, may be offset by the adverse effect of the plating on the fatigue limit of the basis metal. A recent investigation by the National Bureau of Standards provides information of interest not only to aircraft manufacturers, but also to a number of other industries which either produce or utilize chromium-plated machine parts.
Specimens about ΒΌ in. in diameter were machined from different types of steel, and after grinding, polishing, and chromium-plating, the3" were subjected to fatigue tests. Eight or ten specimens usually were required to obtain the fatigue limit for any one set of conditions. In all cases chromium plating was found to reduce the fatigue limits of the steels studied. The fatigue limit generally decreased with increased hardness of the steels and with increased temperature of the plating bath. It was not possible to make generalizations regarding the effect of plate thickness on fatigue limit though little effect was noted.
Two possible causes for the adverse effect of chromium plating on the fatigue limits of the steel were the embrittling effect of hydrogen deposited with the chromium and residual stresses in the chromium.
Chromium-plated objects are often heated with the object of improving their mechanical properties by expelling the hydrogen deposited with the chromium. A study was therefore made of the effect of heating on the fatigue limit of chromium-plated steel. The results showed that the fatigue limits of quenched and tempered specimens heated after plating decreased to a minimum value for some heating temperature between 100 Β° and 300 Β° C. and thereafter increased with increased heating temperatures. Fatigue limits of specimens heated for 1 hr. at 440 Β° C. in some cases were 87.5 per cent of that of the unplated steel, whereas fatigue limits of specimens plated but not heated were only about 68 per cent.
It is generally agreed that residual stresses in a material markedly affect its fatigue limit. Thus, in steel, compressive stresses, such as are produced by shot peening, increase the fatigue limit; tensile stresses have the opposite effect. In order to show the presence of tensile stresses in electrodeposited chromium and the effect of heat treatment on these stresses, thin-walled tubes of annealed steel were chromiumplated and subsequently heated to different temperatures. After observations of heated specimens, it was concluded that the decreased fatigue limit accompanying the heating of the plated fatigue-test specimens was due to increased tensile stresses induced in the chromium plate. This conclusion has been further substantiated by experiments showing that electrodeposited chromium subjected to a heating-and-" (',,Tnmunicated by the Director.
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