The 16Cr-5Ni low-carbon supermartensitic stainless steel is a variant of traditional 13Cr-4Ni, with improved corrosion resistance and improved ductility. This steel is mainly used for components of turbines in aggressive service environments (containing C02 and H2S). To be safely used in such an application, the 16Cr-5Ni must fulfill strict requirement in terms of hardness and mechanical resistance. The desired microstructure is composed by a martensitic matrix containing retained austenite (yr). This phase may in fact improve the characteristics of toughness at low temperature, as well as the resistance to Sulfide Stress Cracking (SSC) and Chloride Stress Corrosion Cracking (CSCC) and the tendency to sensitization of the steel. The temperature limit may cause insufficient recovery of the martensitic matrix and thus excessive final hardness, but the presence of 4+5 wt% of Nickel acts decreasing considerably the Ac1 temperature at which the reversion reaction of austenite happens. The cause of an early formation of yrev, may be related to the pre-existence of residual austenite in the as-quenched microstructure and the precipitation, during heating, of chromium carbides (such as M23C6) which depletes the surrounding matrix of chromium and reduce the local Acl. Experimental studies [2] have pointed out that the formation of yrev causes a redistribution of alloying elements that induce an enrichment of Nickel (and other austenite stabilizer elements) in austenitic islands, leading to an increased stability of the fee phase during cooling. This stability, however, decreases with increasing tempering temperature, probably because of the increasing volume fractions of yrev and a consequently lower enrichment of Nickel.
Microstructural evolution during tempering of 16Cr-5Ni stainless steel: Effects on final mechanical properties
LOVICU, GIAN FRANCO;DE SANCTIS, MASSIMO;VALENTINI, RENZO;
2013-01-01
Abstract
The 16Cr-5Ni low-carbon supermartensitic stainless steel is a variant of traditional 13Cr-4Ni, with improved corrosion resistance and improved ductility. This steel is mainly used for components of turbines in aggressive service environments (containing C02 and H2S). To be safely used in such an application, the 16Cr-5Ni must fulfill strict requirement in terms of hardness and mechanical resistance. The desired microstructure is composed by a martensitic matrix containing retained austenite (yr). This phase may in fact improve the characteristics of toughness at low temperature, as well as the resistance to Sulfide Stress Cracking (SSC) and Chloride Stress Corrosion Cracking (CSCC) and the tendency to sensitization of the steel. The temperature limit may cause insufficient recovery of the martensitic matrix and thus excessive final hardness, but the presence of 4+5 wt% of Nickel acts decreasing considerably the Ac1 temperature at which the reversion reaction of austenite happens. The cause of an early formation of yrev, may be related to the pre-existence of residual austenite in the as-quenched microstructure and the precipitation, during heating, of chromium carbides (such as M23C6) which depletes the surrounding matrix of chromium and reduce the local Acl. Experimental studies [2] have pointed out that the formation of yrev causes a redistribution of alloying elements that induce an enrichment of Nickel (and other austenite stabilizer elements) in austenitic islands, leading to an increased stability of the fee phase during cooling. This stability, however, decreases with increasing tempering temperature, probably because of the increasing volume fractions of yrev and a consequently lower enrichment of Nickel.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
