TWI Frequently asked questions
by Paul Woollin
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 between10-13%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 (CO 2 ) but limited resistance to hydrogen sulphide (H 2 S). Molybdenum is added in the level 1-3% in some grades to improve resistance to media containing H 2 S.
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 toughnessis 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 applicationsrequiring resistance to CO 2 with low levels of H 2 S. Although the supermartensitic steels are not as corrosion resistant as duplex grades, they have corrosion resistance adequate for many oil and gas fields.
Further information
FAQ: What is current industry standard practice for welding weldable 13%Cr steels?
