Seawater Corrosion Fatigue of 2¼ Cr-1 Mo and 4130 Steels for Marine Riser Application

Abstract

ABSTRACT This paper reports on a corrosion fatigue study of 4130 and 2 1/4 CR-1 Mo steels immersed in seawater which addressed the reduction of fatigue life for these steels in a seawater atmosphere, and developed fatigue life design curves. The study has evaluated the effect of shot peened and nitrided surface treatments on sea water corrosion fatigue, and demonstrated that a significant loss of fatigue life is caused when unprotected 4130 and 2 1/4 Cr-1 Mo steels are fatigued in seawater. Shot peening of the material surface improves the seawater fatigue life for these steels, but does not restore it to the full atmospheric fatigue life of these steels in air. The test results for the effect of nitriding material surfaces prior to seawater fatiguing were not conclusive, but nitriding may be beneficial. Manufactures of subsea hardware need to take appropriate precautions to minimize or negate the life shortening effect of seawater corrosion fatigue to 4130 and 2 1/4 Cr-1 Mo steels. INTRODUCTION The study of the effect of seawater Immersion on 4130 and 2 1/4 Cr-1 Mo fatigue life was divided into two parts, a literature view, and speech tests which were used to develop specific design limits. These two steels were selected for evaluation because of their frequent use in the manufacture of oil tool, and because their use has ken considered for marine drilling risers. The study was expanded to include an examination of the effect of shot peen and nitride surface treatments on the seawater fatigue life of the materials. These surface treatments are recognized as proving fatigue properties of steels in air atmosphere, but their effectiveness in proving fatigue life of steel immersed in seawater is unknown and hence, was the reason for inclusion in this study. In reviewing the literature on the effect of corrosion on steel fatigue life, the authors found no literature pertaining directly to corrosion fatigue of 2 1/4 Cr-1 Mo steel, but did find literature pertaining to corrosion fatigue on other low alloy steels. Because 2 1/4 Cr-1 Mo steel is a number of the low alloy steel grouping, it is expected to follow the same general set of rules by which other family numbers abide. Hence, the following discussion of steel corrosion fatigue is considered applicable to both steel seven though the literature survey possessed no information specifically on 2 1/4 Cr-1 Mo steel. Fatigue of unprotected steel immerse in Seawater will always possess a shorter fatigue life than fatigued steel in an air atmosphere. Hence, all air fatigue data should be down graded to account for the loss of properties when subjected to seawater fatigue. However, it is important to note that there are certain variables, when appropriately altered, may maintain fatigue properties of steel immersion in seawater equivalent to those exhibited within air. Table 1 (1) summarizes the basic variables of fatigue in seawater and how they effect fatigue life of steel.