Fracture Toughness and Mechanical Properties of C-Mn Steels Exposed to Wet H2S Environments

Cayard, Michael S. (InterCorr International Inc) | Joia, Carlos (Cidade Universitaria) | Azevedo, F.C.R. (Cidade Universitaria) | Bezzerra, P.S. (FBTS)

OnePetro 

ABSTRACT
C-Mn steel is heavily utilized in the oil and gas industry, primarily in upstream operations and downstream refining. Many of these vessels are exposed to wet hydrogen sulfide environments and as a consequence become damaged. Assessment of these damaged vessels is a key point for continued safe operations. This paper addresses several of the key material properties of C-Mn steels required to perform such assessments. Guidelines on the influence of welding, stress relief treatments, crack location and hydrogen charging on the mechanical properties and fracture toughness are detailed. Results showed elongation and reduction in area were greatly affected by hydrogen charging, however, yield and tensile strength were only mildly affected. Regarding toughness, the parent metal and heat affected regions of hydrogen charged material exhibited a factor of three decrease in toughness compared to baseline values, while the toughness of the weld metal remained relatively unchanged.

The oil and gas industry is required to safely manage equipment exposed to a variety of corrosive environments. In the production~ transport and refining stages, HzS arising from crude oil is one of the principal constituents which leads to the deterioration of equipment. The exposure of conventional carbon steel to an aqueous HzS environment leads to the formation of blistering, hydrogen induced cracking (HIC), stress-oriented hydrogen induced cracking (SOHIC) and/or sulfide stress cracking (SSC). Such defects reduce the life of the equipment and can cause catastrophic failure [1]. To properly assess the integrity of equipment, one has to: (1) evaluate the toughness of both parent metal and weldments, develop non-destructive procedures; and set a criteria for the interaction and acceptance of flaws or discontinuities. In accordance with NACE Standard RP0296 [2], 26 percent of process equipment have HIC and/or SSC. Assessment of damaged equipment is therefore a key point for safe operation in wet H2Senvironments.

Damage produced in wet H2S environments currently is treated in two categories, (1) planar flaws and (2) volumetric flaws. SSC, SOHIC and link-up of HIC damage can all be categorized into planar flaws which can be assessed using conventional fracture mechanics approaches. Two such industry practices which lend themselves nicely to the treatment of planar flaws include BS PD6493 [3] and the proposed API RP 579 [4] assessment procedures. Both utilize a failure assessment diagram incorporating the aspects of brittle fracture and plastic collapse. In order to utilize these approaches effectively, accurate estimations of hydrogen charged material properties are required. With respect to the assessment for brittle fracture, the fracture toughness of hydrogen charged material is needed. With respect to the assessment of plastic collapse, the flow properties of the hydrogen charged/damaged material is required. In the case of the assessment of volumetric flaws, only the later is required.

Hence, the goal of this program was to determine the appropriate values of both mechanical properties and fracture toughness of several grades of hydrogen charged / damaged C-Mn plate steels used in the construction of equipment in the oil and gas industry.