Microstructural Features Affecting Tempering Behavior of 16Cr-5Ni Supermartensitic Steel
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supermartensitic stainless steel 16Cr5Ni is a modified version of the CA6NM grade used for hydraulic turbines, compressor rotors, impellers and high-pressure pipes in power generation, and oil and gas industries. It guarantees better resistance to stress corrosion cracking (SCC) in H2S and CO2 environments, as well as high strength and toughness even at low temperatures.[1,2] Sensitivity to sulfide SCC strongly depends on hardness and ultimate tensile strength (UTS) and maximum values of 28 HRC and 880 MPa are actually accepted for industrial applications. The control of hardness MASSIMO DE SANCTIS and RENZO VALENTINI, Associate Professors, GIANFRANCO LOVICU, Research Assistant, and RANDA ISHAK, Technical Staff, are with the Dipartimento di Ingegneria Civile e Industriale, Universita´ di Pisa, Largo Lucio Lazzarino, 2, 56122 Pisa, Italy. Contact e-mail: [email protected]; [email protected] ANTONELLA DIMATTEO, Young Researcher, is with the TeCIP-PERCRO, Scuola Superiore Sant’Anna, Via Alamanni, 13b, 56010 Ghezzano, Pisa, Italy. UMBERTO MIGLIACCIO, Ph.D. Student, and ROBERTO MONTANARI, Full Professor, are with the Dipartimento di Ingegneria Industriale, Universita´ di Tor Vergata, Via del Politecnico, 1, 00133 Rome, Italy. EMANUELE PIETRANGELI, Lead Engineer, is with the Nuovo Pignone Spa, GE Oil & Gas, Via Felice Matteucci, 2 50127 Florence, Italy. Manuscript submitted February 25, 2014. Article published online March 4, 2015 1878—VOLUME 46A, MAY 2015
and UTS requires careful consideration of tempering procedures.[3–5] The presence of Ni depresses the Ac1 temperature and tempering temperature must be kept relatively low [around 873 K (600 C)] with the consequence of retarding the treatment effects. In addition, a certain amount of a¢ fi c reversion has been observed well below 873 K (600 C)[6–9] and this occurrence further limits the maximum allowable tempering temperature with the risk of a limited recovery of martensite. However, the formation of a limited volume fraction of reversed austenite in temperature can also be beneficial providing it is stable on cooling. As a matter of fact, retained austenite can be effective in reducing the hardness of the alloy and to improve both toughness and resistance to sulfide SCC.[10] The amount of retained austenite formed in this steel typically exhibits a peak for tempering at 893 K to 903 K (620 C to 630 C); in correspondence a minimum of hardness is usually observed.[2] It has been proposed that the stability of reversed austenite would increase because of Ni partitioning from surrounding martensitic matrix. As tempering temperature increases the volume fraction of reversed austenite increases too and the consequent solute redistribution is no more able to stabilize the c phase on cooling, thus resulting in the formation of virgin martensite with increased alloy hardness.[11] Experimental diagrams are often used,[12] giving the total content of reversed austenite and of austenite METALLURGICAL AND MATERIALS TRANSACTIONS A
retained at room temperature after various rehe
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