Hydrogen Induced Cracking of Low Strength Steels in Geothermal Fluids

Abstract

Typically, low strength steels and annealed stainless steels are specified for geothermal energy applications because of the risk of Sulfide induced Stress Corrosion Cracking (SSCC) and Hydrogen Induced Cracking (HIC) in the presence of H2S containing fluids. Recent experience has demonstrated the risk of SSCC and HIC, sometimes known as Hydrogen Embrittlement (HE), of low strength steels subject to high residual stress derived from fabrication techniques. Unexpected cracking, discovered in two geothermal pressure vessels, was attributed to HE cracking in the welds. Fitness-for-purpose assessments completed on the two vessels found one suitable for operation, with a temporary repair, while a replacement vessel was being fabricated but the second required immediate removal from service for repair. The cause of cracking was attributed to use of submerged arc welding leading to high residual stress in the welds of the 32 mm thick vessel walls. The vessels were made in compliance with the ASME VIII Div 1 design code that allows vessels up to 32 mm wall thickness to be fabricated without Post Weld Heat Treatment. The vessels also met the requirements of NACE MR1075 for resistance to sulfide stress cracking, having hardness less than HRC 22. The experience gained suggests some general "rules of thumb" for avoidance of HE cracking in vessels used for geothermal service:
1. The number of weld passes must be as many as the wall thickness in millimeters ie for a 32 mm wall at least 32 passes should be applied.
2. All vessels should be stress relieved unless it can be proven that this is not necessary by the manufacturer for the welding procedure chosen.
3. Care should be taken to ensure any closing welds are correctly stress relieved.