An Assessment of Strain-Aging Effect on Weldable Offshore Structural Steel

Abstract

Summary During the fabrication of offshore structures, the steel plate condition changes from the as-received normalized state to a strain-aged state. This paper assesses the effect of 5% prestrain and aging at 250C [482F] for 1 hour on the mechanical properties of normalized low-alloy BS 4360–50D steel plates obtained from two different suppliers. For both plates, strain aging led to an increase in yield strength, ultimate strength, Charpy and crack opening displacement (COD) transition temperatures, and fatigue resistance. The results of fracture toughness tests show that the degree of sensitivity to strain aging may depend on the type of test used; therefore, a test that more closely simulates offshore loading conditions, such as a COD test, is most appropriate. The paper also shows that the impact of strain aging on the integrity of offshore structures is minimized by the competing effects of lower fracture toughness and slower crack-growth rates. Introduction The selection of steel for offshore structures is governed by several factors, including strength, notch toughness, and fatigue strength. These properties are, in general, established on the basis of the as-received, normalized steel condition. Since offshore structures are generally fabricated by forming steel plates into tubular members and connecting these members by welding, a change in the properties would be expected as a result of strain aging. The strain occurs during forming, and aging is accelerated by the temperature rise during welding. Strain aging manifests itself mainly by an increase in yield strength on aging after straining. Strain aging may also change other properties, such as ultimate tensile strength, ductility, ductile/brittle transition temperature, fatigue strength, and electrical and magnetic properties. Nevertheless, the increase in yield stress on unloading and aging is the most universal indication for the occurrence of strain aging. Therefore, strain aging can be detected simply by the tensile test (Fig. 1). The increase in yield stress after aging, is often used as a measure of the strain-aging susceptibility. This change in properties is attributed to the pinning of the free dislocations that are formed during straining by the carbon and nitrogen atoms that are dissolved in ferrite. It is now universally accepted that strain aging occurs in the presence of interstitial elements such as carbon, nitrogen, and boron. Since virtually all offshore structural steels contain carbon and nitrogen and do not contain boron, it is the first two elements that are of greatest interest. It is therefore suggested that the effect of strain aging can be restricted by lowering the concentration of carbon and nitrogen atoms that are dissolved interstitially in the ferrite. The effect of free nitrogen can be reduced adding nitride-forming elements, such as aluminum. To bind the free carbon atoms, elements that form stable carbonitrides and carbides, such as niobium, vanadium, molybdenum, or titanium, are added. Available evidence suggests that a nitrogen content of 0.0001% produces detectable aging, while 0.001 to 0.002% produces severe aging. Increases above this level probably have little further effect. This suggests that the uncombined nitrogen must be reduced to about 0.0001% or less to eliminate strain aging. Wilson and Russell suggested that in moderately deformed steels, the segregation of about 0.0005 to 0.002 wt% carbon or nitrogen solute (depending on the exact dislocation density) is sufficient to complete dislocation locking. Since the solubility of nitrogen in ferrite is about 100 times that of carbon, it is always suggested that strain aging at low temperatures (less than 100C [212F]) is attributed to nitrogen. At high aging temperature >100C [212F]), the solubility of carbon is increased, and strain aging is produced even when nitrogen has been completely removed from solution. Aging at higher temperatures >350C [662F]) enables substitutional solutes, such as phosphorus, niobium, nickel, silicon, and titanium, to cause strain aging. Strain aging is known to affect toughness adversely. Baird recently has shown that the increase in impact transition temperature after 10% prestrain and aging at 250C [482F] for 30 minutes is not very sensitive to composition, prior heat treatment, or type of impact specimen. The overall increase (caused by both straining and aging) lies in the range of 40 to 70C [104 to 158F] for nearly all steels. Recognizing the potential effect of strain aging on fracture toughness of offshore structural steel, some technical standards restrict the use of Charpy V-notch impact test results from the parent plate to the conditions where the forming strain does not exceed 5% and the material temperature does not exceed 650C [1,202F]. Otherwise, Charpy tests shall be done with specimens machined from fabricated plates. The present study is aimed at establishing the effect of strain aging on one class of steel plates, BS 4360–50D, which is used extensively in the fabrication of offshore structures. JPT P. 141^