Low-cycle fatigue of welded joint from steel of X70 strength class

Steels of X70 strength class are particularly widely used in the field of heavy engineering. One of the most important issues when choosing steel for structures is its behavior under cyclic loads. It is difficult to find a description of the behavior of all zones of the welded joint under fatigue. The purpose of this study was to determine the fatigue characteristics of welded joints made of the Russian analogue of S690QL steel with fixation of acoustic and magnetic parameters for their use in the diagnosis. The objects of the study were the samples from domestic steel of X70 strength class. The chemical composition was determined using optical emission spectrometry. The grinds for microstructural analysis were prepared according to the standard technique with etching in the metal. The fatigue test was carried out on a specialized test bench. The authors used the acoustic system AIS NRK-3 for acoustic measurements and the acoustic parameter D – as an informative parameter. A MA-412MM coercitimeter was applied to evalua­te the magnetic characteristics. The following were evaluated: residual magnetization B r , coercive force H c , H c / B r ratio. The smallest number of cycles corresponds to the deposited metal zone. The decrease in amplitude showed a significant variation in the behavior of the material depending on the junction zone. However, the curves for heat affected zone (HAZ) and the deposited metal are practically the same. HAZ differs to a lesser extent from the base metal than the deposited metal zone. The graph of the acoustic parameter in its form is the reverse of the magnetic characteristics graph. Thus, there is a minimum for the acoustic parameter, depending on the operating time, and a maximum for the magnetic characteristics. For both graphs, the extremum is the point corresponding to the operating time of 0.6.