Fracture Toughness and Related Characteristics of the Cryogenic Nickel Steels (1232)
A large volume of data concerning the 2 1/4, 3 1/2, 5 and 9% nickel steels, corresponding to ASTM specifications A203 Grades A and D, A353, A553 Type I, and A645, has been collected from the open literature and private sources. The mechanical-property data collected include tensile-test properties at ambient and cryogenic temperatures, notch toughness, fracture toughness and fatigue strength. A brief description of each of the testing methods used and their significance to cryogenic service are included. Tables and figures summarizing the data are presented. The strengths and toughnesses required by the respective ASTM steel specifications and ASTM specification A593 are normally met or exceeded by the steels. For the lower nickel steels, gains in toughness can be obtained by quenching and tempering rather than normalizing.
The effects of fabrication operations such as cold work, heat treatment and welding on the mechanical properties are considered. In general the effects of cold work and welding on the tensile properties of the cryogenic nickel steels are small. In the case of 2 1/2 and 3 1/2 nickel steels, the notch toughness is adversely affected by cold forming and aging but little influenced by welding operations. Subsequent thermal stress-relief treatments will restore the original toughness. Conversely, the notch and fracture toughness of the higher nickel steels are relatively insensitive to cold forming, but are reduced in weld heat-affected zones produced by high-heat inputs. The fatigue properties of the cryogenic steels appear to fit the typical scatter band obtained for structural steels, and cryogenic crack-growth rates are not significantly different from those at room temperature.
An attempt has been made to evaluate the critical flaw sizes at design-sress levels for the steels as a function of temperature and stress concentration. For the lower nickel steels, it was concluded that A203 Grade A steel should be limited to service temperatures above -75oF (-59oC) and A203 Grade D should be limited to service temperatures above -110oF (-73oC). When quenched and tempered, the service temperature for these steels can be reduced by about 25oF (14oC). For the higher nickel steels, leak-before-break behavior may be predicted for most expected cryogenic applications. For example, the A353 and A553 Type I steels will meet this criterion at -320oF in 1.5 in. (3.8 mm) plate for a design stress of 25ksi (172 MPa) and a stress concentration of Kt=2. The A645 steel will meet these same requirements at a service temperature of -275oF (-170oC).