Tensile and fracture properties of type 316 stainless steel after creep
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I.
THE AISI
INTRODUCTION
type 316 stainless steel has found wide use in high-temperature application because of its good mechanical properties and adequate resistance to corrosion. It is well known that this material suffers significant loss in tensile ductility and fracture toughness after long-term thermal aging at high temperature. Actual service experience reported by Hoke and Eberle I showed a significant reduction in the ambient tensile elongation (38 pct) after 14,281 hours under 14 MPa at 677 ~ but a more serious reduction in the ambient fracture energy (68 pct). Some increase in ambient yield stress (8 pct) and tensile strength (22 pct) was also observed. Stoter~ reported a significant reduction in tensile elongation after 60,000 hours at 650 ~ and thought it was most likely due to the dense M23C6 carbides on the grain boundary plus the intragranular carbides. To avoid ambiguity, thermal aging without stress will be called soaking in this paper. The tensile properties of type 316 stainless steel after high-temperature soaking were reported in several studies. 3-7 In general, the ambient yield and ultimate tensile stresses showed some increase after long-term soaking. The increase can be anywhere from negligible to about 30 pct, depending on temperature, time, and different heats. 5'6'7 However, there is always a significant reduction in ambient tensile elongation (20 - 40 pct). Ary, Scott, and Spruiell4 found little change in ambient yield and ultimate tensile stresses after soaking at 650 ~ for a relatively short time of 1000 hours, though a decrease in tensile elongation was observed. The reduction in fracture energy after high-temperature soaking is a much more serious problem. 6-9 Hoke e t a l 6 reported up to 60 pct reduction in room-temperature Charpy keyhole fracture energy for exposures up to 10,000 hours at temperatures from 677 ~ to 788 ~ Embrittlement occurred most rapidly during the first 1000 hours of exposure.
In general, the reduction is in the range of 55 pct to 75 pct, 6'7'8 but Spaeder and Brickners found the ambient Charpy v-notch fracture energy was reduced by as much as 90 pct after soaking at 815 ~ for 6000 hours. They concluded that though the early stage of reduction in fracture energy is probably due to M23C 6 carbides, sigma and chi phases are responsible for the latter stage. In general, the higher the temperature and the longer the exposure time, the greater the loss in both the fracture energy and the tensile ductility. Although the effect of high-temperature soaking on mechanical properties has been well reported, studies of the effect of creep on the mechanical properties of type 316 stainless steel are scarce. Sikka and Brinkman10 investigated the tensile properties at creep temperature after creep and showed that the reduction in tensile ductility is much greater than that produced by soaking, and the increase in ambient hardness is much more significant. The loss in ductility was attributed to creep cavities and intragranular hardening due to the presence of precipitat
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