Quenching and Partitioning (Q&P) Processing of Martensitic Stainless Steels
- PDF / 2,596,000 Bytes
- 22 Pages / 593.972 x 792 pts Page_size
- 38 Downloads / 209 Views
I.
INTRODUCTION
QUENCHING and partitioning (Q&P) processing is a method of stabilizing austenite in steels with a martensitic transformation.[1–4] Stabilization of austenite takes place by the diffusion of carbon, from supersaturated martensite substructure, obtained by quenching austenite to a temperature in the Ms to Mf range, into the untransformed austenite. Under properly adopted Q&P processing conditions, the enrichment of austenite with respect to carbon lowers its Ms temperature and leads to the thermal stabilization of the untransformed austenite. Although austenite stabilization can involve nitrogen partitioning too,[5] the low solubility level of nitrogen in low alloy or carbon steels has limited the use of nitrogen as the austenite stabilizer in the Q&P process. It is only by proper alloying or the use of pressurized metallurgy techniques that high nitrogen levels can be achieved in steels.[6–8] Owing to the low temperatures involved in the partitioning step of Q&P, the long-range diffusion of interstitial JAVAD MOLA, formerly Graduate Student with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, South Korea, and is now with the Institute for Iron and Steel Technology, TU Bergakademie Freiberg, 09599 Freiberg, Germany. BRUNO C. DE COOMAN, Director, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted June 25, 2012. Article published online September 15, 2012 946—VOLUME 44A, FEBRUARY 2013
atoms can take place, while substitutional atoms remain stationary. If carbide precipitation is inhibited, then the exchange of carbon between austenite and martensite proceeds until the chemical potential of carbon in both phases equilibrates. The chemical potential of substitutional elements, however, remains different in austenite and martensite. The original thermodynamic description of Q&P processing by Speer et al.,[1,9] commonly referred to as constrained carbon equilibrium (CCE),[10] gives the endpoint of partitioning as the point where the concentration of carbon across a constrained (immobile) austenite/ martensite interface reaches equilibrium. Assuming initial establishment of CCE in a binary Fe-C system, Speer et al.[11] have also demonstrated thermodynamically how the direction of interface migration can be determined if the constraint imposed on the interface is removed. In practice, the problem of interface migration can be circumvented by adding alloying elements which slow down the interface migration thereby limiting such undesirable reactions as the formation of bainite/isothermal martensite. Alternatively, this can be done by choosing a suitable partitioning temperature at which the interface mobility is negligible compared to the partitioning time.[12,13] Examples of Q&P treatments with negligible interface mobility in the partitioning step are Fe-16Cr-0.04C-0.04N (AISI 430),[5] Fe-12Cr-0.1C (AISI 410),[14] and Fe-4.57Mn-1.3Si-0
Data Loading...
