Factors Affecting Impact Toughness in Stabilized Intermediate Purity 21Cr Ferritic Stainless Steels and Their Simulated
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LATELY, several stainless steel producers have introduced and standardized 20 to 24 pct by mass Cr ferritic stainless steel grades.[1–5] These cost-efficient high-Cr steels have similar corrosion resistance to the common AISI 304 and 316 austenitic stainless steels and are not to be mixed with the 26-29Cr + Mo superferritic steels of the 1970s that have been used in more demanding corrosive environments. Typically, these recently developed high-Cr ferritic grades have low Ni and Mo contents to ensure stable and predictable alloying costs as well as stabilizing elements Ti, Nb, etc. that trap interstitial C and N so that sensitization to intergranular corrosion is suppressed. Unlike austenitic steels, ferritic steels undergo a ductile-to-brittle transition in their fracture behavior. Therefore, replacement of austenitic grades with ferritic grades requires care to be taken in design to ensure that sufficient resistance to brittle fracture is maintained in the structures even after welding.[6] The total content of
SEVERI ANTTILA and DAVID A. PORTER are with Materials and Production Engineering, University of Oulu, Oulu, 358, Finland. Contact e-mail: severi.anttila@oulu.fi TUOMAS ALATARVAS is with Process Metallurgy, University of Oulu, Oulu, 358, Finland. Manuscript submitted May 16, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
interstitial C and N is generally agreed to be the most critical factor for determining the toughness of fully ferritic stainless steels that do not undergo phase transformation to austenite and/or to martensite.[7–12] Low solubility of interstitials in ferrite leads to rapid, virtually inevitable, precipitation of carbonitrides at low temperatures. Ultimately, these brittle particles solely, in clusters, or in particle networks such as in grain boundaries may induce microcracks that then in turn provide nucleation sites for cleavage cracks.[13–17] Intermediate purity steels with interstitial levels of 250 to 500 mass ppm are typically obtained when the steels are produced using argon–oxygen decarburization (AOD). Ultrapure interstitial levels of about 100 ppm and below are needed if superior toughness is desired, but costly vacuum steelmaking techniques have to be used to achieve these levels.[7,9,18,19] In intermediate purity stabilized steels, comparatively large inclusion and precipitate clusters can exist in the ferritic matrix. During casting, stable oxide inclusions such as Al2O3 and TiO2 provide substrates for Ti(C,N) nucleation and growth from the molten state. Typical hot working and annealing temperatures in the range of 1073 K to 1523 K (800 C to 1250 C) allow further coarsening of the carbonitrides and large conglomerations of particles are seen after cold working and annealing. In the weld heat-affected zone (HAZ) of ferritic stainless steels, some of the precipitates dissolve and re-precipitate intergranularly and intragranularly.[20] This process
1973 K (500 C to 1700 C) using the elements Fe, C, Mn, Cr, Cu, N, Nb, and Ti. A Gleeble 3800 thermomechanical simulato
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