Abstract
Cr-Mn-N austenitic heat resistant steels have bright future in wide applications due to their lower cost, excellent mechanical properties, and outstanding local corrosion resistance after substituting the nickel partly or completely by the Mn, N and C elements. As the content of C or N elements increasing in Cr-Mn-N steels, a large quantity of precipitates form during hot working, resulting in embrittlement of these steels. In the present study, a new Cr-Mn-N austenitic heat resistant steel was aged at temperatures from 600 to 1000 in duration from 10 min up to 6000 min. By OM, SEM and XRD, the microstructure evolution of precipitates and their effects on ductility and toughness of the studied steel were investigated. The results indicated that precipitates formed during aging were mainly Cr-rich M23C6 carbides, whose morphologies changed in a sequence of intergranular films, lamellae in cellular microstructure and intragranular rods or particles with the increase of aging time and/or temperature. The time-temperature-precipitation (TTP) curves for M23C6 carbides were determined, which have a typical C-shaped profile with a nose temperature between 850 and 900 , and an incubation period not more than 1 min. In addition, it was found out that the aging embrittlement of the studied steel is strongly dependent on the morphology of M23C6 carbides. The intergranular film-shaped M23C6 carbides were considered as the main factor to favor cracks rapidly expanding along grain boundaries, finally resulting in brittle intergranular fracture. KEY WORDS Cr-Mn-N austenitic heat resistant steel, M23C6 carbide, cellular microstructure, aging embrittlement
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