Enhanced Carbon Diffusion in Austenitic Stainless Steel Carburized at Low Temperature
- PDF / 587,525 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 56 Downloads / 214 Views
ork demonstrates that the mechanical properties (including surface hardness, fatigue resistance, and wear resistance) and the corrosion resistance of AISI 316-type austenitic stainless steel (Fe–Cr–Ni alloy) can be significantly improved by a novel, lowtemperature gas-phase carburization process developed by the Swagelok Company.[9–12] Figure 1 illustrates the temperature–time characteristics T[t] of this process (T denotes the absolute temperature and t the time). Prior to the actual carburization stage, it is necessary to ‘‘activate’’ the alloy surface by removing the passivating chromium oxide scale.[13] In the Swagelok process, this occurs in three distinct, empirically developed steps:[9,12] (1) exposing the surface to a gas mixture of HCl and N2 at an activation temperature Ta for an activation time ta1, (2) applying a carburizing gas mixture of CO, H2, and N2 at a processing temperature Tp for a time ta2, and (3) repeating step (1). Subsequently, the actual carburization follows by applying the carburizing gas mixture of CO, H2, and N2 at the processing temperature Tp for a processing time tp. In earlier work,[3–8] typical values for the processing parameters were Ta = 523 K (250 C), ta1 = 7.2 ks (2 hours), Tp = 743 K (470 C), ta2 = 7.2 ks (2 hours), Tp = 743 K, tp = 72 ks (20 hours), as shown in Figure 1. Recently, it [1–8]
F. ERNST, G.M. MICHAL, and A.H. HEUER, Professors, A. AVISHAI, Engineer, H. KAHN, Research Professor, and X. GU, Doctoral Student, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH. Contact e-mail: [email protected] This article is based on a presentation given at the ‘‘International Conference on Surface Hardening of Stainless Steels,’’ which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS. Article published online June 23, 2009 1768—VOLUME 40A, AUGUST 2009
was established that the efficacy of the treatment can be considerably improved by lowering Tp to approximately 723 K (450 C). The Swagelok low-temperature carburization process dramatically improves alloy performance by generating a carbon-rich ‘‘case,’’ a layer with a very high concentration of interstitially dissolved carbon that extends conformally from the original surface into the material below. The case is ‘‘single phase,’’ that is, free of carbide precipitates, and, in the direction z from the surface into the alloy, the profile XC[z] of the carbon fraction XC is smoothly graded.[6,8] Near the surface (z ’ 0), XC typically exceeds 0.1. This corresponds to 105 the equilibrium solubility of carbon at room temperature, which is a ‘‘colossal’’ supersaturation.[6,8] This phenomenon arises for two reasons: (1) The carbon atoms, as they reside in interstitial sites, diffuse much faster than the metal atoms (Fe and solute atoms substituting for Fe) in the alloy. At Tp, the metal atoms are nearly immobile, whereas carbon can diffuse over considerable distances within tp. (2
Data Loading...
