We propose an equation for the evaluation of the fatigue limits of welded joints of low-carbon and low-alloy steels with high residual stresses based on the results of testing of small specimens without residual stresses. The independence of the amplitude of stresses on the mean stress in a cycle is proved for welded joints with high residual stresses.Keywords: diagram of ultimate stresses in a cycle, fatigue limit, residual stresses, yield strength, ultimate strength, amplitude of stresses in a cycle, mean stress in a cycle.

We present results of fatigue testing of strengthened butt-welded joints of St. 3sp steel subjected to various modes of high-frequency mechanical forging. As the treatment effectiveness criterion, we propose to use the groove depth in the work-hardened area. The optimal values of groove depth and high-frequency mechanical forging rates have been determined at various vibration amplitudes of the forging tool, which ensure the maximum fatigue life of butt-welded joints. The effect of high-frequency mechanical forging on fatigue strength of butt-welded joints with account of their particular manufacturing technologies is discussed.Introduction. Service life of metal structures operating under repeated alternating loading conditions is conditioned, as a rule, by the endurance of welded joints, whose fatigue strength is lower than that of the base metal. Welded joint strength reduction is attributed to metallurgical flaws, as well as to presence of stress raisers and residual tensile stresses. Therefore, the requirements for increasing service life of metalware components with simultaneous reduction of metal consumption imply application of additional strengthening techniques. Weld joint strength studies [1], including those performed under harmonic and repeated impact loading conditions at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine [2], have demonstrated that the hardening of welded joints improves their fatigue strength.The following welded joint treatment techniques are used at present: thermal and mechanical weld dressing, argon-arc microexplosion treatment, vibration treatment, etc. [1,3]. Application of certain techniques leads to reduction of stress concentration factor due to increased curvature radius of the weld/base metal transition zone [4], while other techniques reduce tensile residual stresses [5]. The high-frequency mechanical forging (HFMF) of weld/base metal fusion line, which combines high efficiency in strengthening of various types of welded joints with relatively low labor input, has found a wide application since 1990's [6]. In this case, the improvement of fatigue strength and life of welded joints is attained by combined reduction of the stress concentration factor (increased curvature radius of the weld/base metal transition zone), strain-hardening of metal surface layer and residual compressive stresses induced within the stress raiser area [6]. The HFMF disadvantage is that it implies a manual execution, which is sensitive to the human factor, since low-quality treatment is likely to nullify the expected positive effect. The latter is controlled by operator skill and ability to select proper forging rate, insofar as high forging rates fail to provide the required forged surface quality and expected efficiency, while treatment with lower forging rates is not cost-effective.One of the main criteria of HFMF efficiency evaluation is travel speed of forging tool along the weld, which varies over a rather wide range, according to various referen...

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