Abstract
1. The fatigue strengths of all the steels studied (including NN-3B austenitic stainless steel) determined on joints of the nut-and-bolt type tested, in a 3% NaCl solution in rotating bending are similar and relatively low. The conditional fatigue limits of bolted joints of this type corresponding to a life of 10 & 106 cycles do not exceed 12–14 kg/mm2. 2. Surface work-hardening of the thread roots with the aid of a vibrating roller brings about a substantial (67–130%) increase in the fatigue limit of bolted joints operating in sea water; the beneficial effect of this treatment is particularly marked in the of 18KhNVA steel. 3. Zinc plating also increases the corrosion-fatigue limit of the steels studied (with the exception of stainless steel), although to a lesser degree than the surface hardening treatment. 4. A particularly large (107–250%) increase in the corrosion-fatigue limits of bolted joints is attained by a combined treatment, i.e., surface work-hardening followed by zinc plating. The harmful effect of thread-cutting and corrosive media upon the fatigue properties of the steels studied can be almost completely neutralized by this treatment. 5. Zinc plating has no effect upon the corrosion-fatigue strength of bolts made of NN-3B stainless steel: the same fatigue limits were recorded for bolts (both unhardened and surface-hardened) before and after zinc plating. 6. Fatigue tests carried out in air on grade 18KhNVA bolts subjected to fluctuating tensile stresses provided additional evidence of the effectiveness of the surface-hardening treatment which, in this case, brought about a 160% increase in the fatigue limit (related to the stress semi-amplitude). 7. The results of the present investigation provide sufficient grounds for recommending the combined treatment (surface work-hardening followed by deposition of a protective coating) as an efficient method of attaining a substantial increase in the load-carrying capacity of bolted joints operating in sea water.
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