| The creep-rupture behavior of cold-worked Type 316 S.S was investigated in constant-load testing at 649 and 760(DEGREES)C. Prestrain levels of up to 80% were studied with initial stresses ranging from 172 to 259 MPa at 649(DEGREES)C and 68.9 and 172 MPa at 760(DEGREES)C. Microstructures of failed samples were studied by optical metallography and transmission electron microscopy.;A creep-rupture model was developed which incorporates theeffect of (epsilon)(,m) and (epsilon)(,T) on t(,R) and was found to reasonably fit the(' )obtained data:;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI);where C'' is a constant, R is the universal gas constant, m(,2) is anexponent found from the (epsilon)(,m) dependence of t(,R)/(epsilon)(,T), n is the stress(' )exponent, (sigma)(,A) is the applied stress, T is temperature in (DEGREES)K and Q(,A) isthe apparent activation energy for creep.;An "optimum level of cold-work" (OLCW) which results in an^optimized rupture life (t(,R)), up to 34 times that of annealed material,^was found and determined to be temperature and stress dependent.^The OLCW was found to vary from 40 to 50% at 649(DEGREES)C and from 20^to approximately 35% at 760(DEGREES)C. The OLCW increases with^increasing stress and decreasing temperature. A rupture ductility^((epsilon)(,T)) minimum and the lowest minimum creep rates ((epsilon)(,m)) were(' )^observed at the OLCW. CW > OLCW results in decreasing t(,R),^ultimately even less than that for annealed material at the higher temperature. As t(,R) values were observed to decrease, (epsilon)(,T) increased correspondingly.;Q(,A) values were found to range from 230 to 400 kJ/mole, increasing with increasing CW. The higher Q(,A) values are believed to be associated with precipitation hardening and dynamic recrystallization.;The microstructural studies show crystal defect strengthening and/or precipitation hardening consistently throughout the creep-rupture lifetime for OLCW material. Material with CW > OLCW has experienced some or extensive dynamic recrystallization (especially for CW >> OLCW). Material with CW < OLCW exhibits significant in-test strengthening; however, in both cases (CW(NOT=)OLCW) higher initial creep rates result in reduced cross-sectional area to support the constant applied load, resulting in a higher true stress and early failure as compared with OLCW material. |
