Fatigue Comparison Of Two High Strength Heat Treated Structural Steels For Low Temperature Service

Abstract

ABSTRACT Quenched and tempered carbon steels are used in the construction of offshore platforms, refrigerated gas cargo tanks, storage tanks, pipelines, pressure vessels, container ships, ice breaker ships, and other structures which require a high degree of notch toughness at low temperatures or where the increased strength with minimized weight is the dominant factor in design. This paper summarizes the results of 40 plate specimen fatigue tests on two quenched and tempered carbon steels:U. S. Coast Guard specification CG-A-537M, andASTM specification A-537B. Bending stress and axial stress test results are reported for stress ratios of 0, ½ and -1, representing zero-totension, half-tension-to-tension, and fully reversed loading, respectively. Conventional S-N curves are presented for both steels The A-537B bending S-N curve at a stress ratio of zero corresponds with that for axial loading taken from the literature. For lives up to about 3 × 105 cycles at any positive stress ratio the fatigue strength of non-welded CG-A-53'7M exceeds the yield strength. The fatigue strength of CG-A-537M' is higher than that of A-537B when comparing ratios of fatigue strength to tensile strength. At a stress ratio of the fatigue strength of CG-A-537M exceeds the yield strength for lives of two million cycles. Data for both steels with the tensile strengths are plotted as modified Goodman Diagrams. INTRODUCTION In recent years hurricanes have caused great damage to offshore operations in the Gulf of Mexico. In other parts of the world platforms have been extended into deep water and into severe cold regions, such as Alaska and the North Sea. It has become necessary to study the problems of brittle fracture at low temperatures (1). The failures of offshore drilling platforms under severe weather conditions emphasize the need for increased reliability in these structures to protect the large capital investments and human lives. To solve the problems of brittle fracture at low temperature and to meet the need for stronger offshore platforms with minimum structural weight, designers are now choosing heat-treated carbon steels. These steels not only provide high static strength, good weldability and excellent notch toughness at low temperatures (2), but also have high fatigue strength as reported in this paper. The purpose of this paper is to summarize the results of fatigue tests on two kinds of quenched and tempered structural steel:a relatively new grade tailored for artic ice breaker and tanker service which is currently designated U. S. Coast Guard specification CG-A-537M, andan older grade ASTM specification A-537B which has been used successfully for the past ten years in offshore platforms as well as in storage tanks for low temperature refrigerated gases.