51 / 2019-03-08 22:37:23

Environmentally-Assisted Cracking (EAC) resistance in martensite-based stainless steel OCTG materials (high-temperature load-applied autoclave test)

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This paper report Environmentally-Assisted Cracking (EAC) resistance, which many engineers may have misunderstood as SCC (Stress Corrosion Cracking), in martensite-based stainless steel OCTG materials in sour condition at room temperature (RT) to 200°C, by applying stress on Four Point Bent Beam (FPBB) inside autoclave.
As a conventional concept, SCC-susceptible temperature range for duplex stainless steel has already been recognized widely, and many engineers often pay attention to 80 to 100°C in material selection duplex stainless steel, and they often check SCC resistance by using autoclave. In this paper, EAC resistance (autoclave test) is targeted for martensitic stainless steel OCTG materials and discussed whether similar SCC-susceptible range exists or not.
Through the comprehensive testing by using FPBB specimens under the conditions of (1)100%AYS@in-situ, (2)saturated gas environment of 5MPa CO2 gas and 0.01MPa H2S gas and (3)the combined environments of both “pH3.0 to pH4.0” and “temperature of 24 to 200°C”, EAC resistance (domain map) for HP2-13CR-110 (13Cr-5Ni-2Mo-based), UHP-15CR-125 (15-6-3-Cu-based), and UHP-17CR-110 (17-4.5-3-Cu-W-based) are reported. The following conclusions are drawn in this paper: (a) tested Martensite-based OCTG materials do not have specific SCC-susceptible temperature range of 80 to 100°C. (b)EAC in martensitic stainless steel is not based on SCC (anodic: Fe-dissolution), but on SSC (cathodic: Hydrogen embrittlement). (c)Domain map (pH-temperature map) in EAC is restricted by either SSC or corrosion rate. (d)As interpretation for SSC resistance, when FPBB test starts to be performed at from RT to higher temperature and then no-cracking is observed at a certain temperature, no-cracking keeps going on at higher temperature, from then on. (e)As interpretation for general corrosion rate, as temperature goes down, general corrosion gets milder. When general corrosion test starts to be performed at from higher temperature to RT and then general corrosion rate is below the criteria at a certain temperature, corrosion rate keeps lower than the criteria.