Two types of new martensitic stainless steel with good weldability and superior corrosion resistance have been developed for line pipe application. Both steels are suitable for welding without preheating owing to lowering C and N contents, and they show good low temperature toughness in welds without PWHT. One is applied to sweet environments. It gives better resistance to C02 corrosion than the 13Cr martensitic stainless steel for OCTG. Lowering C and addition of Ni contribute to reduction of general corrosion rate in the C02 environment. The addition of Cu improves the pitting resistance. The other is applied to light sour environments. It gives good SSC resistance in welds owing to the improvement of the pitting resistance due to Mo addition. The seamless pipes of these martensitic stainless steels are applicable as substitutes for a part of duplex stainless steel flow lines.
Carbon dioxide (CO2)corrosion is a very serious problem in oil and gas pipe lines. The widely used countermeasures against C02 corrosion are injection of inhibitor and usage of duplex stainless steel. The operation of inhibition is expensive, especially in offshore application, and the effectiveness of inhibitor declines under high temperature environment. Duplex stainless steels are very expensive materials. Martensitic stainless steels are generally so poor in weldability that they have scarcely been used for line pipe. However, they show good resistance to C02 corrosion and are less expensive for line pipe application than the carbon steel with inhibitor and the duplex stainless steel. From these backgrounds, a new martensitic stainless steel seamless pipe with good weldability and C02 corrosion resistance has been developed for line pipe application in sweet environment.
Moreover, many wells containing a small amount of hydrogen sulfide (H2S) are being developed recently. Therefore, a good resistance to sulfide stress cracking (SSC) is required for the steel in such environment. Then, another new martensitic stainless steel seamless pipe has been developed for line pipe application in light sour environment. The steel has good SSC resistance as well as good weldability and C02 corrosion resistance.
This paper describes the development of the two types of martensitic stainless steel seamless pipe.
Targets for the properties of the steel for sweet environment are as follows:
Thorough weldability without preheating and post welding heat treatment (PWHT)
High CO2 corrosion resistance equivalent to 0.2C-13Cr martensitic stainless steel for oil country tubular goods (CXTG)
More than 100J of Charpy absorbed energy in base metal and welded joint at -40ºC
More than 360MPa of yield strength (more than X52 grade)
Thorough hot workability for manufacturing of seamless pipes by Mannessman process
Concept of alloy design
The material design of the new steel is based on the fundamental knowledge about the effects of alloying elements on weldability, hot workability and corrosion resistance of martensitic stainless steels.
A martensitic stainless steel that contains less than 0.04% in total amount of C and N is weldable without preheating and PWHT1).
