Improvement of surface properties of stainless steel used in the oil and gas industries through plasma assisted thermochemical treatment

Resumo

Stainless steels are being used in a variety of applications within the refining and petrochemical industry, where high corrosion resistance and mechanical properties are required. Surface properties of these materials may be improved in order to reach a better performance in highly stressed tribological systems. Results obtained by the two research groups have shown that low temperature DC-pulsed plasma diffusion surface treatments, dose to 400oC, where nitrogen and/or carbon are introduced in stainless steels promoting "colossal supersaturation" leads to strong hardening without loss of corrosion resistance. The hardening mechanism changes from nitride or carbide precipitation to lattice distortion of the FCC austenitic lattice leading to formation of expanded austenite, or S-Phase, inducing high compressive stresses in the surface layer and leading to strong hardening between 1000 HV and 1500 HV, with no loss of corrosion resistance. The research work carried out by the Brazilian group focuses on S-phase formation and duplex treatment of austenitic, martensitic and duplex stainless steels and the benefits that S-phase can bring to the improvement of erosion-corrosion, cavitation-erosion and hydrogen embrittlement of stainless steel valves and parts to be used at high depths in the pre-salt exploitation wells. The Birmingham group has done systematic work on S-phase including: active-screen plasma S-phase surface engineering, S-phase surface engineering of duplex and precipitation stainless steels, duplex surface engineering involving S-phase, erosion-corrosion & corrosion-wear, mechanical and/or thermal stability of S-phase. (AU)