Effect Of Quenching Process On The Microstructure And Properties Of13Cr Super Martensitic Stainless Steels

Super martensitic stainless steels (SMSS) have been developed on the basis oftraditional martensitic stainless steel by redesigning the alloying system i.e. reducingC content (<0.07%), strictly controlling Ni content (3.5%-4.5%) and Mo content(1.5%-2.5%). Compared with conventional stainless steels, SMSS exhibits a moreexcellent combination of strength and toughness, as well as good corrosion resistance.In addition, SMSS demonstrates advantages in overcoming the sulfur induced stresscracking sensitivity during the welding process as well as possessing betterweldability, over the traditional stainless steels. Super martensitic stainless steels areoften used as pipelines for the transport of petroleum products containing corrosiveagents, especially in applications of CO2and a small amount of H2S environment.Recent researches on SMSS have been largely focused on its corrosion resistance andwelding performance, but very little attention has been paid to the investigation of itsphase transformation and heat treatment processes, especially quenching process.In the present work, the experimental specimens were treated with variousquenching parameters, and analyzed with a combination of microscopy, hardnesstesting, scanning electron microscopy (SEM), transmission electron microscopy(TEM) to explore the influence of quenching parameters, i.e. quenching temperatureand cooling rate, on the phase transformation, room-temperature microstructure, andproperties of the steel. Some conclusions were summarized as follows:(1) The prioraustenite grain size increased with the increasing of quenching temperature.(2)Hardness of the room-temperature microstructure increased as the quenchingtemperature increased, which was associated with the coarsening of prior austenitegrains and widening of martensite laths.(3) The13Cr SMSS showed goodhardenability and microstructure consisted of mostly lath martensite were obtained ata very low cooling rate, say,5℃/min; a small amount of retained austenite was alsoobserved in the microstructure.(4) In the designed cooling rate range for this study,the cooling rate showed little influence on the martensitic transformation startingtemperature (Ms). The martensite lath width showed an increase for10℃/min cooledspecimen and remained relatively stable thereafter; the trend of values of hardness(Vickers) versus the specimens cooled at various rates was in agreement with that ofmartensite lath width versus the specimens.