Various Types of Martensitic Stainless Steel Seamless Pipes

To improve the global climate, CO2 capture, storage, and utilization technologies are being promoted and applied in large areas. Since martensitic stainless steel pipes have both high strength and excellent corrosion resistance in high CO2 content environments, they are widely used in oil and gas development in acidic environments and CO2 capture, storage, and utilization. With the deterioration of the mining environment, martensitic stainless steel has also experienced the development process of traditional 13Cr, improved 13Cr, super 13Cr, 15Cr, and 17Cr. The mechanical properties, CO2 corrosion resistance, sulfide stress corrosion resistance, and intergranular corrosion resistance of the materials are constantly improving.
 

Martensitic stainless steel oil well pipes are developed for oil and gas wells containing CO2 corrosive media. The alloy is based on UNS S42000 stainless steel. To continuously adapt to high temperature, high pressure, high chloride ion content, and even the corrosive environment of CO2/H2S coexistence, the alloy design is constantly improved to meet service requirements. It is mainly divided into the following five types of martensitic stainless steel: traditional 13Cr, improved 13Cr, ultra-low carbon SUP13Cr, ultra-low carbon 15Cr, and ultra-low carbon 17Cr.
 

To exploit oil and gas wells containing CO2, foreign countries began application tests of traditional 13Cr martensitic stainless steel oil well pipes in conventional oilfield blocks in 1975. Since the performance of traditional 13Cr martensitic stainless steel is between that of martensitic and duplex stainless steel, it has been rapidly promoted and used. From 1980 to 1993, more than 2,400 km of traditional 13Cr petroleum special pipe products were used in oil and gas wells. Traditional 13Cr martensitic stainless steel, namely the L80-13Cr steel grade in API Spec 5CT "Casing and Tubing Specification," mainly refers to the 2Cr13 alloy system: 1 (C) ≤ 0.22%, w (Cr) 11.5% - 13.5%, w (Ni) ≤ 0.5% alloy design. Traditional 13Cr has better CO2 corrosion resistance than ordinary low-alloy carbon steel and can be used in a 130°C environment at a specific CI concentration. Although ISO 15156-3/NACE MR 0175 "Petroleum and Natural Gas Industry - Materials for HS-containing Environments in Oil and Gas Production Part 3: Cracking-resistant CRAs (Corrosion-resistant Alloys) and Other Alloys" has not yet clarified whether traditional 13Cr has sulfur stress corrosion resistance, numerous studies have shown that traditional 13Cr still has certain corrosion resistance in a specific CO2/H2S coexistence environment.
 

Since the 1990s, foreign countries have developed improved 13Cr materials for three main purposes: first, to obtain higher high-temperature corrosion resistance based on traditional 13Cr, increasing the maximum service temperature from 130°C to 180°C, with better corrosion resistance than traditional 13Cr; second, to increase the strength of the material from the 80-85 steel grade of traditional 13Cr to the 110 steel grade, suitable for high-temperature deep well mining; third, to further improve resistance to H2S stress corrosion cracking compared with traditional 13Cr. The Prps critical value of the improved 13Cr material with a Mo content of 2% at a pH value of 3.5 can be increased to 0.005 MPa. The improved 13Cr is based on the traditional 13Cr and further adds Ni (1%-3%) and Mo (0.5%-2.0%). The protective film formed by Mo improves pitting resistance and significantly reduces hydrogen permeability, enhancing resistance to H2S stress corrosion. ISO 15156-3/NACE MR 0175 specifies that the improved 13Cr material can be used under any combination of CI content and temperature conditions with pH > 3.5 and Pixs critical value of 0.01 MPa, exhibiting reliable corrosion resistance in sweet and weak acid environments.
 

In 1990, while developing improved 13Cr, to achieve the material's resistance to CO2 in higher temperature and CI-concentration environments, ultra-low carbon SUP13Cr martensitic stainless steel was developed. To avoid the reduction of effective Cr content in the material due to the precipitation of Cr carbides, the carbon content of SUP13Cr is reduced to below 0.03%; 5%-6% Ni is added to obtain single-phase martensite, and 2%-4% Mo is added to improve SSC resistance and partial corrosion resistance.
 

In 2000, 125 steel grade 15Cr martensitic stainless steel oil well pipe (0.03C-5Cr-6Ni-2Mo-1Cu) was developed. By adding higher Cr and Mo elements, it has stronger high-temperature CO2 corrosion resistance than SUP13Cr; its uniform corrosion rate at 200°C high temperature and PCO2 of 15 MPa still meets the safety threshold of ≤0.127 mm/a, as shown in Figure 512-3. At the same time, through constant load tensile and C-ring tests, it was found that the critical partial pressure of H2S for 15Cr at pH 3.5 is 0.001 MPa, and the critical partial pressure of H2S at pH 4.5 is 0.01 MPa. The critical partial pressure of H2S for 15Cr is three times that of the improved 13Cr of the same strength level. Additionally, high Ni content can not only enhance the stability of austenite to obtain a complete martensitic structure but also improve the low-temperature impact toughness of the material.
 

To increase the material's CO2 corrosion resistance temperature to 230°C while maintaining high strength for deep well development, ultra-low carbon 17Cr martensitic stainless steel (17Cr-4Ni-2.5Mo-1Cu-IW-low C) has been developed. 17Cr has better CO2, H2S corrosion resistance than 15Cr, SUP13Cr, improved 13Cr, and traditional 13Cr in higher temperature environments. Notably, according to the alloy design of 17Cr, its microstructure is a martensitic matrix + 20%-50% ferrite + a small amount of austenite. The advantage of this microstructure combination is that high strength can be obtained through the traditional quenching and tempering process, rather than through cold working like austenitic/ferritic stainless steel, and the strength will not drop significantly in high-temperature environments. The disadvantage is that, compared with 15Cr, SUP13Cr, and traditional 13Cr, 17Cr inevitably has ferrite in high-temperature hot rolling. This causes the rolling deformation of 17Cr seamless steel pipe to be mainly concentrated on ferrite, which easily causes surface defects.