Save Weld Hardness Levels to Insure Long Service Life in Offshore Pipelines

Abstract

Abstract An investigation of the effect of weld hardness on the cracking susceptibility of pipelines in severe service environments Began when it was discovered that an automatic welding operation to be used in the construction of an offshore pipeline produced hard cap pass heat-affected zones (HAZ). A hardness limit of RC32 for the cap pass HAZ was set by extrapolating data obtained on steels exposed to oil well Brines, The lack of pertinent data in the literature and the trend toward automatic welding systems and high strength steels in pipeline construction necessitated a study of maximum acceptable weld hardness to confirm or improve upon the limit of RC32. Manually welded pipelines have maximum hardnesses of about RC15-20, with occasional maximums of RC26-27. Experience has shown these hardness levels to be relatively immune to stress corrosion and hydrogen cracking problems. Examination of the automatic welds indicated hardnesses of RC35-37 in the cap pass HAZ. We believed this was too hard to insure long service life (20-25 years) of an offshore pipeline without costly repairs due to stress corrosion or HZS cracking. Three-point mild steel bend specimens with the weld at the midpoint were stressed and exposed to corrosive environments for thirty days. The test solutions contained salt water and HZS to represent environments produced by sulfate reducing bacteria found in seawater and marine sediments. The testing indicated that the hardness of welds and heat-affected zones should be kept below RC38-30. I. Introduction Automatic Welding Interest in weld hardness levels has been stimulated by the growing use of high strength steels and automatic field welding for pipeline construction. Automatic welding is definitely here to stay as is indicated by its widespread use and the increasing number of commercially available automatic welding systems. A basic characteristic of all automatic welding systems is that the weld is made rapidly with less energy input than manual stick welding. This tends to produce "hard" heat-affected zones (HAZ) because of the rapid quenching effect of the base material (Figure 1). Since each weld bead is tempered by the following pass, the hard HAZ is generally found only in the last or "cap" pass. Our study of weld hardness was begun when it was discovered that an automatic welding operation to be used in the construction of an offshore gas pipeline produced hard cap pass HAZs. Microhardness testing indicated hardness levels of RC35-37 in the cap pass HAZ of test welds. We felt this was too high to insure a long (35 years), maintenance-free service life when considering stress corrosion cracking in seawater, the possibility of HZS in the gas, and the sulfate reducing bacteria in the marine sediment. A safe hardness limit of Rc32 is observed for steels used in oil wells containing HZS. The data which support the Rc32 limit were extrapolated to obtain a hardness limit of Rc32 for the pipeline field welds. The less severe corrosive environment and stress state were considered in arriving at the Rc32 limit. Establishing a hardness limit and having it accepted by the contractor were made difficult by the lack of pertinent data in the literature.