Failures of Nickel/Copper Bolts in Subsea Application

Abstract

Summary. Slow-strain-rate tests in ASTM seawater using specimens prepared from a failed nickel/copper-alloy bolt have shown that precipitation-hardened UNS N05500 (Monel K-500 trademark) is embrittled by cathodic protection with sacrificial aluminum anodes. Some loss of ductility also occurred when annealed UNS N05500 was coupled to aluminum anodes and when the hardened alloy was coupled to steel. Brittle fractures produced by slow-strain-rate tests were intergranular and were very similar in appearance to the field fractures. While the slow-strain-rate tests were conducted on an alloy from only one source, there is no reason to assume that UNS N05500 alloy from other sources would resist hydrogen embrittlement from standard cathodic protection systems. Introduction Bolts of alloy UNS N05500, a precipitation-hardening nickel/copper alloy commonly referred to as Monel K-500, failed in a brittle manner on North Sea platforms sometime before Aug. 1984. The failures, which originated at thread roots where the hardness was about Rockwell C (HRC) 39, occurred in bolts that had been hardened after threading. The failures were attributed to hydrogen embrittlement that was a result of cathodic protection. It was concluded that hydrogen embrittlement of Monel K-500 would not occur if the hardness was below HRC 35, which is the acceptable hardness of UNS N05500 in sour systems. To ensure proper hardness of the thread roots, it was recommended that annealing and hardening treatments be done after the threading. Since that time, subsea clamp bolts of UNS N05500 alloy with the recommended heat treatment and hardness have failed in a brittle manner on two North Sea platforms. These failures occurred while the bolts were coupled to steel and cathodically protected with aluminum anodes or a combination of aluminum anodes and impressed current. The bolts had been roll-threaded, annealed at 980 to 1050 de-grees C [1,795 to 1,920 degrees F], water-quenched, and precipitation hardened at 500 to 600 degrees C [930 to 1,100 degrees F] for 16 hours. This produced hardnesses of about HRC 25. The majority of the later failures occurred in bolts loaded to 344 MPa [49,890 psi] during installation. This was 59% of the specified minimum yield strength of the material. The bolts began failing after about 1 year of service, and they continued to fail with time. Because failures of the remaining cathodically protected subsea N05500 bolts were anticipated, the broken bolts and other cathodically protected alloy N05500 bolts used in critical subsea applications were replaced with steel bolts. The remainder of this paper discusses bolts that had been annealed and hardened after they were threaded. Results of slow-strain-rate tests of hydrogen-charged specimens from one of the failed bolts are presented, along with studies of fracture surfaces. Subsea Failures Because the more recent failures occurred in bolts that had the recommended heat treatment and were below the critical hardness level the material was traced through the various heat-treating and threading steps to the original alloy supplier to see whether the failures were restricted to the alloy from a particular vendor or processor. It was found that both sets of failed bolts were heat-treated and threaded by the same shops, but the material was supplied by two different companies. A few bolts purchased for use on the platforms had been heat-treated in an oxidizing atmosphere, but the failures were not limited to these bolts. Thus, it is thought that the failures are generic to the alloy and were not caused by improper manufacturing processes. Alloy compositions of the failed bolts were determined by several independent analyses; these analyses, plus heat analyses of all Alloy N05500 fasteners in use on the platforms, were studied to see whether the failures correlated to composition variables or to such tramp elements as sulfur or selenium. The failures occurred in bolts of the proper alloy composition, and were not caused by tramp elements or unusual alloy compositions. Typical microstructure of a failed bolt is shown in Fig. 1. All the failed bolts had grain growth at the thread surfaces, and some had detectable Ni (Ti, Al) precipitates, particularly in the larger grains. Applicable alloy specifications are given in Table 1. The cathodic protection system of the platform on which the most recent bolt failures occurred was designed to produce a current density of 130 mA/m2 using a combination of impressed current and aluminum anodes. Under normal operating conditions, the cathodic protection system delivers a current density of 35 mA/m2 at a potential of - 990 to - 1,140 mV vs. silver/silver chloride. Slow-Strain-Rate Tests Slow-strain-rate tests are popular for determining the susceptibility of materials to stress-corrosion cracking and hydrogen-induced environmental cracking. Tests specimens are subjected to a constant rate of extension until failure occurs. This test method promotes stress-corrosion cracking in alloy/environment systems that do not produce cracking in static tests. The data produced by the tests must be interpreted cautiously, however, because the absence of stress-corrosion cracking in slow-strain-rate tests is not sufficient evidence to eliminate cracking concerns. Slow-strain-rate tests were conducted in air and in circulating ASTM seawater that was exposed to the atmosphere. During the tests. the specimens were contained in plexiglass vessels equipped with two small anodes about 1 cm [0.4 in.] from the gauge sections of the samples. The slow-strain-rate tests were conducted in a 89-kN [20,000-lbf] -capacity load frame specially modified to achieve low strain rates. To calibrate the stiffness of the machine and test fixtures, the specimen strain over a 5-cm [2-in.] gauge length was monitored with an extensometer during the first test. For subsequent tests, the crosshead travel was set at the same speed, but an extensomerer was not used. Strain rates were on the order of 5 X 10-6 per second, For nickel-based alloys, this strain rate reportedly results in the maximum susceptibility to hydrogen embrittlement. Samples for the slow-strain-rate tests had 0.64-cm [1/4-in.] -diameter by 5-cm [2-in.] -long gauge sections. They were machined from the body of a 45-mm [1.8-in.] -diameter Monel K-500 bolt that had failed in the threads while in service on one of the North Sea platforms. This bolt, which had been in service about 1 year, had been threaded, annealed, water-quenched, and precipitation-hardened to HRC 25. Except for two samples that were annealed at 1010 degrees C [1,850 degrees F] and one that was baked at 175 degrees C [350 degrees F] for 6 hours after they were machined, the slow-strain-rate samples had the as-received heat treatment. SPEPE August 1988 P. 282^