AISI 4340 low-alloy steel (UNS G43400) is frequently used for critical bolting applications on subsea equipment in the oil and gas industry because of its ability to be heat treated to high strength levels while maintaining high toughness, even in large diameter bolting. However, several field incidents resulting from hydrogen embrittlement of AISI 4340 bolting have brought into question the maximum hardness that should be specified for low-alloy steel bolting and whether zinc electroplating can contribute to hydrogen embrittlement of low-alloy steel bolting in subsea applications.
In this evaluation, the relative hydrogen embrittlement resistance of AISI 4340 with different mechanical properties and surface conditions was determined using electrochemical pre-charging followed by step loading per ASTM F1624-121 in a 3.5% NaCl solution. The test specimens were round, notched tensile test bars stressed in uniaxial tension to simulate the load on a tensioned bolt. Testing was performed on material at two different strength levels with no coating, with zinc electroplate (both baked and unbaked), and with a phosphate coating to evaluate the effects of coatings. The tests were performed in air, in the NaCl solution at open circuit potential, and in the NaCl solution at −950 mV, −1050 mV, and −1250 mV (Ag/AgCl) to evaluate the effects of different levels of cathodic protection.
The research presented in this paper was initiated to provide a better understanding of the effect of different strength and hardness levels, surface coatings, and cathodic protection levels on the hydrogen embrittlement resistance of high strength AISI 4340 bolting (UNS G43400) intended for seawater exposure.
High strength AISI 4340 bolting with specified minimum yield strength levels up to 145 KSI has been used on oil and gas subsea drilling equipment for decades. However, three field incidents that occurred in the Gulf of Mexico in 2012, 2014, and 2015 on critical subsea drilling equipment supplied by three oilfield equipment companies lead the US government and the oil and gas industry to question the safe use limits for high strength low-alloy steel bolting. In each of these incidents, the bolts were in subsea service, high strength AISI 4340, coated with zinc electroplate, and hydrogen embrittlement was identified as a contributing factor.