Assessment of the high-frequency mechanical forging mode effect on fatigue strength of welded joints

Abstract

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...

Effect of High-Frequency Mechanical Peening Conditions on Increase in the Fatigue Resistance of Butt-Welded Joints

“…4. This figure also shows the experimentally determined endurance limits of butt-welded joints in the initial state (s R in = 200 MPa) and at HFMP velocity of 0.06 m/min (s R = 375 MPa) at the working tool vibration amplitude a =19 mm [4], as well as the values of hardened layer depth corresponding to them. In this case, the calculated relation between the endurance limit of welded joints and the current depth of the hardened layer, l h , is of the form:…”

“…This is due to the fact that HFMP can provide the maximal improvement of the fatigue resistance of products and increase in their life [2,3], at low climatic temperatures as well. However, the large-scale introduction of HFMP is limited by the absence of the reliable control criterion of its efficiency, insofar as the criterion now in use, which is based on the velocity of working tool movement along the weld, which is 0.5 m/min [2], does not ensure the required increase in fatigue resistance and treatment efficiency control both in the manufacture of welded metallic structures and in repair and restoration works.In [4] it was proposed to use as a HFMP efficiency criterion the depth of the groove formed after the peening of the zone of fusion of weld and base metal. The optimal depth value was determined to be 0.14 mm, and HFMP velocity parameters as a function of working tool vibration amplitude were proposed.…”

“…In [4] it was proposed to use as a HFMP efficiency criterion the depth of the groove formed after the peening of the zone of fusion of weld and base metal. The optimal depth value was determined to be 0.14 mm, and HFMP velocity parameters as a function of working tool vibration amplitude were proposed.…”

“…As to the influence of residual stresses, it can be pointed out that the conditions of fatigue tests of welded specimens, which have been carried out earlier [4], indicate their almost complete relaxation during the first several thousands of loading cycles.…”

“…It is known from practice that the welded members of metallic structures are treated most frequently in the range of a values from 19 to 26 mm. Using plots of groove depth against HFMP velocity obtained earlier [4] at a =19 and 26 mm, we constructed plots of groove depth against the endurance limit of welded joints taken from Table 2 and of groove depth against peening velocity at different working tool vibration amplitude, which are combined in Fig. 5.…”