Dilatometry Analysis of Dissolution of Cr-Rich Carbides in Martensitic Stainless Steels
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occurrence of Cr-rich carbides in stainless steels is relevant to technological applications where properties such as corrosion resistance,[1–3] hardness,[4–6] and toughness[7,8] are concerned. The presence of Cr-rich carbides in the microstructure could be beneficial as it increases the wear resistance and causes secondary hardening. One of the negative impacts of Cr-rich carbides, however, is the deterioration of corrosion properties due to the reduced content of solute Cr in the matrix. Common types of Cr-rich carbides are M23C6 and M7C3 (M mainly denotes Cr and Fe), which usually precipitate at temperatures above 773 K (500 °C) in martensitic stainless steels.[5] For a Fe-13Cr-0.22C steel (concentrations in mass-pct), M7C3 carbides formation and a slight secondary hardening have been observed after a holding time of 1 hour at 773 K (500 °C). Softening occurs at higher temperatures where carbides coarsen and partially transform into M23C6 carbides.[9]
QIULIANG HUANG, OLENA VOLKOVA and JAVAD MOLA are with the Institute of Iron and Steel Technology, Technische Universita¨t Bergakademie Freiberg, 09599 Freiberg, Germany. Contact e-mail: [email protected] HORST BIERMANN is with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg Manuscript submitted July 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The same precipitation sequence has been reported for a Fe-13Cr-3Mo-3Ni-0.1C steel.[10] In contrast, M23C6 carbides have been identified by X-ray diffraction (XRD) of the Fe-13.6Cr-0.38C steel tempered at 773 K (500 °C) for 2 hours.[11] In commercial Fe-13Cr-0.45C martensitic stainless steels, the microstructure after soft annealing consists of M23C6 carbides dispersed in a ferritic matrix.[12] The normal treatment routine for martensitic stainless steels involves quenching from solution annealing temperatures ranging from 1173 K to 1423 K (900 °C to 1150 °C) and a subsequent tempering heat treatment at temperatures between 473 K and 973 K (200 °C and 700 °C) to recover the toughness.[13–16] The volume fraction of Cr-rich carbides depends strongly on the choice of solution annealing and tempering conditions. Annealing at higher temperatures leads to the progressive dissolution of Cr-rich carbides.[17–19] The full dissolution temperature of M23C6 carbides in a soft-annealed Fe-13Cr-0.45C steel increases from 1353 K to 1448 K (1080 °C to 1175 °C) as the heating rate increases from 0.05 to 10 K/s.[20] The dissolution of Cr-rich carbides is commonly studied by means of microstructural examinations,[21–27] XRD,[28–32] and hardness measurements.[33–38] The progressive dissolution of carbides at higher temperatures can also be studied by monitoring the associated decrease in the martensite start (Ms) temperature in the cooling step.[39–42] Nevertheless, this method requires multiple dilatometry cycles and cannot be applied if the Ms temperature is below the minimum operation temperature of dilatometer. De Andre´s et al.[12] have applied a dilatometry-based method to study the
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