The supermartensitic stainless steels - also known sometimes as weldable 12%Cr steels, weldable 13%Cr steels, low carbon martensitic stainless steels and soft martensitic stainless steels - are steels with typically between 10-13%Cr, carbon contents of the order of 0.01% and nickel addition in the range 1-6% to stabilise the martensitic microstructures.
These materials have high strength, typically X80 or above, and very good corrosion resistance in brines containing carbon dioxide (CO2) but limited resistance to hydrogen sulphide (H2S). Molybdenum is added in the level 1-3% in some grades to improve resistance to media containing H2S.
The low carbon content and high nickel content ensure that the heat affected zone is virtually fully martensitic with good toughness and that hydrogen crack-free welds can generally be made without pre-heat. Good weld area toughness is achieved without a need for postweld heat treatment.
Due to different alloying and processing requirements, compared with 22%Cr duplex stainless steels, these supermartensitic steels are substantially cheaper than the competing duplex grades for pipeline and flowline applications requiring resistance to CO2 with low levels of H2S. Although the supermartensitic steels are not as corrosion resistant as duplex grades, they have corrosion resistance adequate for many oil and gas fields.