RESULTS OF CHARACTERIZATION TESTS OF THE SURFACES OF A COMMERCIALLY CARBURIZED AUSTENITIC STAINLESS STEEL

Farrell, K

Oak Ridge National Laboratory

关键词: Hardening;    Substrates;    Iron Carbides;    Austenite;    Stainless Steels;   

DOI  :  10.2172/885707
RP-ID  :  ORNL/TM-2003/115
RP-ID  :  DE-AC05-00OR22725
RP-ID  :  885707

美国|英语

来源: UNT Digital Library

【 摘 要 】

A commercial surface carburization treatment that shows promise for hardening the surfaces of the stainless steel target vessel of the Spallation Neutron Source against cavitation erosion and pitting caused by the action of pulsed pressure waves in the liquid mercury target has been investigated. To verify promotional claims for the treatment and to uncover any factors that might be of concern for the integrity of a carburized target vessel, some characterization tests of the nature of the surface layers of carburized austenitic 316LN stainless steel were conducted. The findings support most of the claims. The carburized layer is about 35 {micro}m thick. Its indentation hardness is about five times larger than that of the substrate steel and declines rapidly with depth into the layer. The surface is distorted by the treatment, and the austenite lattice is enlarged. The corrosion resistance of the carburized layer in an acid medium is greater than that for untreated austenite. The layer is not brittle; it is plastically deformable and is quite resistant to cracking during straining. Contrary to the provider's assertations, the maximum carbon content of the layer is much less than 6-7 wt% carbon, and the carbon is not simply contained in supersaturated solid solution; some of it is present in a previously unreported iron carbide phase located at the very surface. Large variations were found in the thickness of the layer, and they signify that controls may be needed to ensure a uniform thickness for treatment of the SNS target vessel. Inclusion stringers and {delta}-ferrite phase embraced in the treated layer are less resistant to chemical attack than the treated austenite. From a cavitation pitting perspective under SNS bombardment, such non-austenitic phases may provide preferential sites for pitting. The shallow depth of the hardened layer will require use of protection measures to avoid mishandling damage to the layer during assembly and installation of a target vessel.