Metal treatment – Stock – Mechanical memory
Patent
1992-02-25
1993-03-30
Yee, Deborah
Metal treatment
Stock
Mechanical memory
148563, C21D 800, C22C 3838
Patent
active
051980410
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a shape memory stainless steel excellent in shape memory effect and a method for enhancing shape memory effect thereof. More particularly, the invention relates to a shape memory stainless steel excellent in resistance to stress corrosion cracking which can advantageously develop its shape memory effect when used as fixing or fastening parts of machines, or as a pipe joint.
BACKGROUND OF THE INVENTION
As alloys exhibiting shape memory effect, there are known nonferrous-metal alloys including Ni--Ti alloys and Cu alloys as well as ferrous metal alloys such as Fe--Pd alloys, Fe--Ni alloys and Fe--Mn alloys. Among others, Fe--Mn alloys are inexpensive, and thus, because of their commercial value, various alloys of this Fe--Mn series are reported in patent literatures, for example, Fe--(15.9-30.0%) Mn alloys in JP A 55-73846, Fe--Mn--(Si, Ni, Cr) alloys in JP A 55-76043, Fe--(20-40%)Mn--(3.5-8%)Si alloys in JP A 61-76647 and Fe--(15-30%)Mn--N alloys in JP A 63-216946. furthermore, JP A 62-112720 discloses a method for enhancing shape memory effect of a Fe--Mn--Si alloy wherein a so-called training effect by repeating a cycle of working at a rate of up to 20% and heating to a temperature of at least 400.degree. C. is utilized.
However, ferrous metal shape memory alloys are generally disadvantageous in low corrosion resistance. JP A 61-201761 discloses examples of Fe--Mn--Si alloys whose corrosion resistance is improved by adding Cr. However, the Cr content taught is too low, i.e. not more than 10.0%, to achieve corrosion resistance well comparable with that of stainless steels. Furthermore, JP A 63-216946 teaches to improve corrosion resistance of ferrous metal shape memory alloys by adding Cr. Again, however, the Cr content disclosed is 10% or less and it is not taught how to realize a desired level of shape memory characteristics with the ferrous metal shape memory alloys having Cr, which is a ferrite former, in excess of 10% incorporated therein.
On the other hand, as to general stainless steels, "Scripta Metallurgica, 1977, vol. 5, pp.663.about.667" reports that SUS304 steel exhibits shape memory effect, if it is deformed at -196.degree. C. and then heated to room temperature, however, its shape recovery is too small to put it to practical use.
OBJECT OF THE INVENTION
An object of the invention is to provide a shape memory alloy containing more than 10% of Cr, which alloy is capable of exhibiting such a shape memory effect that even though the temperature of secondary deformation is not very low, for example, even though the temperature of secondary deformation is slightly below room temperature, when it is heated to moderately elevated temperature after the secondary deformation, it can recover its primary shape prior to the secondary deformation, and which alloy does not substantially suffer from stress corrosion cracking that may be a problem when the alloy is used as a pipe joint or the like.
DISCLOSURE OF THE INVENTION
According to the invention, there is provided a shape memory stainless steel excellent in resistance to stress corrosion cracking, which comprises, by weight, up to 0.10% of C, 3.0 to 6.0% of Si, 6.0 to 25.0% of Mn, up to 7.0% of Ni, more than 10.0% and not more than 17.0% of Cr, 0.02 to 0.3% of N, 2.0 to 10.0% of Co and more than 0.2% and not more than 3.5% of Cu, and optionally at least one selected from up to 2.0% of Mo, 0.05 to 0.8% of Nb, 0.05 to 0.8% of V, 0.05 to 0.8% of Zr, 0.05 to 0.8% of Ti, the balance being Fe and unavoidable impurities, the alloying components being adjusted so that a D value is not less than -26.0, wherein the D value is defined by the following equation: r+1.6.times.Si+Mo+1.5.times.(Nb+V+Zr+Ti)].
When the steel having the above defined chemical composition is processed to an article of a predetermined shape, annealed to memorize the shape, deformed at a temperature of not higher than room temperature, heated to a temperature of at least 100.degree. C. and allowed to cool to room temperature, the mem
REFERENCES:
patent: 4933027 (1990-06-01), Moriya et al.
patent: 5032195 (1991-07-01), Shin et al.
Igawa Takashi
Kinugasa Masayuki
Takemoto Toshihiko
Tanaka Teruo
Nisshin Steel Co. Ltd.
Yee Deborah
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