An EBSD Study of the Deformation of Service-Aged 316 Austenitic Steel
- PDF / 2,016,730 Bytes
- 18 Pages / 593.972 x 792 pts Page_size
- 55 Downloads / 196 Views
UCTION
AISI-TYPE 316 austenitic stainless steel is widely used in power plants because of its good high-temperature strength, oxidation, and creep resistance[1,2] but longterm exposure to temperatures above 823 K (550 C) during service can degrade its mechanical properties[3] and plastic strain can increase its susceptibility to stress corrosion cracking.[4–6] Predictions of a plant’s remaining safe operating life require reliable methods of determining the inelastic strain levels already reached in safety critical 316 steel components and good models of the operative deformation mechanisms. Measurements of strain distributions around welds made in already aged material are of particular interest because welded repairs may be necessary after several years of plant operation. Electron backscatter diffraction (EBSD) has been used to measure plastic strain in various metals including solution-annealed (SA) 316 steel[7–10] and several different EBSD metrics have been observed to scale with strain in the same way for both compressive and tensile strain. The metric grain average misorientation (GAM) has shown DAVID N. GITHINJI, Ph.D. Research Student, SHIRLEY M. NORTHOVER, Research Fellow, and P. JOHN BOUCHARD, Professor in Materials for Energy, are with the Materials Engineering, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K. Contact e-mail: [email protected] The late MARTIN A. RIST, formerly Senior Lecturer with Materials Engineering, The Open University, is now deceased. Manuscript submitted January 12, 2013. Article published online May 15, 2013 4150—VOLUME 44A, SEPTEMBER 2013
promise for evaluating creep damage in SA material[7–10] but there have been no EBSD studies on ex-service (aged) material where copious intragranular and intergranular precipitation might be expected to change the deformation behavior. There have also been no studies of whether EBSD measurements are sensitive to changes in deformation that may arise through changes in temperature or strain rate. This article describes the results of an experimental study of deformation of AISI Type 316H steel which had previously seen long-term service at high temperatures [~789 K (516 C)] using EBSD to follow the build up of intragranular misorientations with applied strain (both tensile and compressive) at different temperatures and strain rates. It compares the application of three different established EBSD metrics to the measurement of plastic strain in steel with well-developed precipitation and proposes a simple new metric that can be applied to both aged and non-aged material. A. Strain Measurement Using EBSD In EBSD, a fine beam of electrons is focused sequentially on a grid of points on the surface of a crystalline material. Diffraction of the electrons backscattered from the top 10 to 100 nm[11,12] forms patterns from which the crystallographic orientation of the material under each spot can be determined. The generation and accumulation of dislocations during metal’s plastic deformation results in local changes to the lattice orie
Data Loading...
