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

Super martensitic stainless steels (SMSS) have been widely used as pipelinematerials in the applications of oil and gas exploration in CO2–containingenvironment. It has been developed on the basis of traditional martensitic stainlesssteel by redesigning the alloying system i.e. reducing C content (<0.07%), strictlycontrolling Ni content (3.5%-4.5%) and Mo content (1.5%-2.5%). Compared withconventional stainless steels, SMSS exhibits a more excellent combination of strengthand toughness, as well as good corrosion resistance. In addition, SMSS demonstratesadvantages in overcoming the sulfur induced stress cracking sensitivity during thewelding process as well as possessing better weldability, over the traditional stainlesssteels. Super martensitic stainless steels are often used as pipelines for the transport ofpetroleum products containing corrosive agents, especially in applications of CO2anda small amount of H2S environment. Recent researches on SMSS have been largelyfocused on its corrosion resistance and welding performance, but very little attentionhas been paid to the investigation of its phase transformation and heat treatmentprocesses, especially single-stage tempering 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) Martensitelath refinement is noted in the specimens tempered above austenite transformationstart temperature,650oC. The lath refinement is strongly dependent on the temperingtime in the intercritical region since it is caused by reverse transformation, which isthought to occur following a diffusional mechanism.(2) Contrary to the softeningeffect of normal tempering process, intercritical tempering induced lath refinementnotably improved the hardness of the steel. This enhancement effect is stronger for700oC tempering than for650oC tempering.(3) Retained austenite content exhibits asteady increase with increasing tempering time at650oC while no retained austeniteis detected in the specimens tempered at700oC.