Metal treatment – Stock – Ferrous
Patent
1997-03-07
2000-10-17
Yee, Deborah
Metal treatment
Stock
Ferrous
148327, 148611, C22C 3848, C21D 600
Patent
active
061325252
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns the manufacturing of austenitic grade materials for radiation exposure applications.
BACKGROUND ART
The starting point is an austenitic steel whose alloying constituent quantities are largely standardized, e.g., steel carrying the German Stock Number 1.4550 which require a carbon content under 0.1%, a niobium content higher than the eight fold of the carbon content, as well as a chromium content of 17 to 19 wt. %, and a nickel content from 9 to 11.5 wt. %. Impurities level limits are set at 2.0% Mn, 1.0% Si, 0.045% P and 0.03% S by weight.
The properties of iron are modified by the prescribed amounts of the alloying components with the upper limits on impurities dictated by the specified application zone. Higher impurity limits are generally allowed to make it possible to manufacture alloys from standard, inexpensive source materials which conform to commercial impurity standards. The upper limits of many impurities are the result of optimized manufacturing processes. Concentration limits on other alloying constituents are determined through the optimization of pertinent material properties. Steel qualities 1.4301 and 1.4401, for example, contain niobium as an impurity, but otherwise correspond to the usual impurities of 1.4550 steel. In the U.S., the corresponding steel qualities approximately correspond to markings AISI types 348, 304, and 316.
The microstructure of these materials depends upon their composition, thermal treatment and other procedural steps during the manufacturing process. If for example, the material is subjected to high temperatures for extended periods, large grains will form. Impurities and/or the use of lower temperatures during manufacturing discourages grain growth. The formation of coarse grains can be promoted in some cases during forging, where extensive deformation of grains at elevated temperature causes larger grains to be formed when the forging cools. These grains can be reduced through recrystallization. Grain structure affects material properties such as ductility and strength.
Austenitic steels distinguish themselves from other steels because they have suitable mechanical properties while simultaneously possessing a high level of stability in the face of general corrosion, the even removal of material from the surface of a component, a fact which led to early use of austenitic steels as the material of choice for high stress nuclear reactor internal structural components. Industry experience and laboratory testing has show that these materials fail when exposed to low stress, a matter which can be traced back to selective corrosion at grain boundaries ("intergranular stress corrosion cracking", IGSCC). This selective attack on the grain boundaries can be examined outside the reactor in laboratory tests ("outpile test") by conducting corrosion tests under special aggressive conditions. The results of such tests, show that austenitic steel which is resistant to IGSCC when not exposed to radiation, does fail during inpile testing where radiation is present. The in-reactor failure mechanism is therefore called "irradiation assisted stress corrosion cracking ("IASCC"). It is suspected that phosphorus and silicon are forced to the grain boundaries leading to a susceptible site for the onset of corrosion. Supported by outpile IGSCC tests, the articles "Behavior of Water Reactor Core Materials with Respect to Corrosion Attack" by Garzarolli and Rubel and Steinberg's "Proceedings of the International symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors", Myrtle Beach, S.C., Aug. 22-25, 1983, Pages 1 through 23, recommend that the silicon content be maintained under 0.1 wt. % and the phosphorus content be kept under 0.01 wt. %, while pointing out that irradiation in a reactor enhances the occurrence of selective corrosion.
In "Deformability of Austenitic Stainless Steel and Ni-Base Alloys in the Core of a Boiling and a Pressurized Water Reactor", Proceedings of the 2nd International Symposium on
REFERENCES:
patent: 4836976 (1989-06-01), Jacobs
patent: 4863682 (1989-09-01), Coates et al.
Alter Dietrich
Dewes Peter
Garzarolli Friedrich
Hahn Roland
Nelson J. Lawrence
Electric Power Research Institute Inc.
Yee Deborah
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