FATIGUE PROPERTIES OF A T TYPE WELDED JOINT IN A SMALL-SIZED SINGLE PASS BOILER HEADER

Abstract

The design of small-sized single pass boilers is usually based on the construction code for small-sized boilers, in Japan. Design methods in this construction code and other country codes have not been clearly defined, although most of small-sized boiler's headers are constructed from two concentric circular shells and cover plates. Therefore, Finite Element Analysis (FEA) with a PC and measurement by strain gauges are mainly used in strength design of these boilers. In this study, a T type welded joint of SS400 was adopted for the header of small-sized single pass boilers. Fatigue testing and FEA was carried out. The fatigue limit was greater for base metal than for weldment. For the weldment, the S - N curve decreased with decreasing ρ value. The stress concentration at the toe was evaluated by the FEA. S - N curves for different toe curvature radii were re-drawn using maximum elastic stress, Ktσmax. Crack initiation occurred in the vicinity of the toe. Several small cracks were observed on the surface of T type welded joint fatigue specimen at N/N f =0.42. These cracks coalesced together at a crack length of 10mm. Fatigue crack growth rate lineally increased with increasing (crack length)1/2.