AU1222899A - Pipe joint made of shape memory stainless steel - Google Patents

Description

Shape memory stainless steel and pipe coupling made thereof FIELD OF THE INVENTION The present invention relates to a shape memory stainless steel and pipe coupling made thereof. BACKGROUND OF THE INVENTION Shape memory alloy can be induced to transform from parent phase to martensite phase under stress, meanwhile some shape deformation can be seen macroscopically from corresponding units made thereof. When thus deformed units are heated to a prescribed inverse transformation temperature, the martensite phase transform to parent phase, thus the corresponding components restore to their original shapes. This kind of alloy showing shape memory function is referred to as shape memory alloy. In iron (Fe)-based shape memory alloy, except iron-palladium (Fe-Pd) alloy has fct (face-centered tetragonal) structure of martensite, iron-platinum (Fe-Pd) alloy or nickel-cobalt-titanium (Ni-Co-Ti) alloy has bct (body-centered tetragonal) structure of martensite, generally most martensites of iron-based shape memory alloy are of hcp (hexagonal close packed) type 2H structure, which is generally referred to as c martensite. Recently we found the 4H, 6H, 8H structure of martensite. The parent phase y has fcc type 3R structure, which is also called alloy austenite. The martensitic transformation y-+c is the change of atomic stacking order in nature. The martensite has a combination of different stacking faults compared to parent phase. Stacking fault multiplicates at low temperature and degenerates at high temperatures, which constitutes the basic process of the martensitic transformation and its inverse transformation of martensite. Non-ferrous shape memory alloys, such as nickel-titanium alloys, are expensive although they have already been practically used. However, the lower cost and the moderate operating temperature of the iron-based shape memory alloys predict a wide developing prospect. The related prior patents and the characteristics disclosed therein are described below: CN 106 4319A disclosed an alloy comprising: Mn: 15-35wt.%, Si: 0.2-6.5wt.%, Al: 0.2-8wt.%, Cu: 0-0.5wt.%, one or more elements selected from the group consisting of Pr, Pm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, La, Ce, Nd and Sm in the range of 0.008 0.12wt.%, and the balance being iron and incidental impurities. JP 170 457 disclosed an alloy comprising Mn: 15-40 wt.%, Cr and/or Co: 1-20wt.%, one or more elements selected from the group consisting of Si, Al, Ge, Ga, Nb, V, Ti, Cu, Ni and Mo in the range of 0-15wt.%, one or more elements selected from the group consisting of La, Ce, Nb, Sm and Y in the range of 0-2wt.%. JP 227 0938A (USP 503 2195) disclosed an alloy comprising Mn: 15-20wt.%, Si: 0 3wt.%, Cr: 0-1Owt.%, and the balance being iron and incidental impurities. JP 216 946 disclosed an alloy comprising Mn: 15-30wt.%, Cr and/or Ni: 0-15wt.%, Si and /or Co: 0-6wt.%. JP 201 761 (USP 478 0154) disclosed an alloy comprising: Mn: 2 0-40wt.%, Si: 3.5 8wt.%, one or more elements selected from a group consisting of Cr: 0-1Owt.%, Ni: 0 lOwt.%, Co: 0-1Owt.%, Mo: 0-2wt.%, C: 0-lwt.%, Al: 0-1wt.% and Cu: 0-lwt.%, and the balance being iron and incidental impurities. The weight percent of Mn in the alloys mentioned above are all above 15wt.%, thus the said alloys have high overheating sensibility and low hot workability owing to high Mn percent. In addition, the alloys are liable to rust and their corrosion resistance can not be improved remarkably even when Cr is added. The publications related to the alloys comprising Mn less than 15wt.% are described below: JP 230 1514 disclosed an alloy comprising Cr: 10-17wt.%, Si: 3 .0-6.Owt.%, one or more element selected from a group consisting of Mn: 10-25wt.%, Ni: 0-7.Owt.%, Co: 2.0 10.Owt.%, trace amount of Ti, Zr, V, Nb, Mo, Cu, and the balance being iron and incidental impurities. The alloy disclosed in COEEOSION-NACE (1986), Vol.42, pp.678 by B.E. Wilde comprising Cr: 17-19wt.%, Si: 0.35-4.79wt.%, Ni: 8.83-9.08wt.%, Mn: 1.30-1.53wt.%, Cu: 0.

009 -0.20wt.%, N: 0.011-0.040wt.%, Mo: 0.019-0.2lwt.%. EP 336 157A (JP 203 0734A, USP 492 9289A) disclosed an alloy comprising Cr: 0.1 5.Owt.%, Si: 2 .0-8.Owt.%, Mn 0.1-14.8wt.%, Co: 0.1-30wt.%, Ni: 0.1-2.Owt.%, Cu: 0.1 2 3wt.%, N: 0.0 1-0.4wt.%, and the balance being iron and incidental impurities. USP 493 3027 (EP 336 175A) disclosed an alloy comprising Cr: 5-20wt.%, Si: 2.0 8.Owt.%, one or more element selected from the group consisting of Mn: 0.1-14.8wt.%, Ni: 0.1-2.Owt.%, Co: 0.1-30.Owt.%, Cu: 0.1-3.Owt.% and N: 0.001-0.4wt.%, and the balance being iron and incidental impurities. EP 050 6488AI disclosed an alloy comprising Cr: 1 6 -21wt.%, Si: 3.0-7.Owt.%, Ni: 11-21wt.%, one or more element selected from the group consisting of Mn: 0.1-5.Owt.%, Cu: 0.1-l.Owt.%, N: 0.001-0.100wt.%, Mo: 0.1-3.Owt.%, W: 0.1-3.Owt.%, Ti: 0.01 l.Owt.%, Zr: 0.011-2.Owt.%, Hf: 0.01-2.Owt.%, V: 0.01-1.Owt.%, Nb: 0.01- 2 .Owt.% and Ta: 0.01-2.Owt.%. The shape memory materials for practical utilization are required to have a yield strength above 300 Mpa, a moderate memory recovering temperature (As: 60- 120*C), and are corrosion resistant and workable. However, all the requirements are not meet by any of the alloy disclosed in the above mentioned publications SUMMARY OF THE INVENTION The object of the present invention is therefore to provide a shape memory stainless steel meeting all the above demands, and a pipe coupling made thereof. In order to fulfill the above objective, the present invention provides a stainless steel comprising: Cr: 12-20wt.%, Si: 3-8wt.%, Ni: 0.1-8wt.%, Mn: 0.1-10.0wt.%, Co: 0.1-20wt.%, N: 0.05-0.4wt.%, C: 0-0.03wt.%, and one or more rare earth elements selected from the group consisting of: La, Ce, Sm, Nd, Pm, Eu, Tb, Dy, Pr, Gd, Ho, Er, Tm, Yb and Lu in the range of 0.01-0.15wt.%, preferably in the range of 0.0 2 -0.10wt.%, one or more elements selected from the group consisting of Nb, Ti, V, Zr, Ta, Hf, W, Mo, Al and Cu in the range of 0.05-2wt.%, preferably one or more elements selected from the group consisting of Ti, Zr, Hf, Ta, W, Al and Cu in range of 0.05-2wt.%, and the balance being iron and incidental impurities.. The contents of Cr, Si, Ni, Mn, Co and N in the shape memory stainless steel of the 3 present invention are preferably in the range of Cr: 12.0-18.Owt.%, Si: 3

.

2 -6.lwt.%, Ni: 4.0-8.Owt.%, Mn: 1.9-5.Owt.%, Co: 5.0-20.Owt.%, N: 0.05-0.2wt.%. Compared to CN 106 4319A, JP 170 457, JP227 0938A, JP 216 946 or JP 201 761, the content of Mn in the alloy according to the present invention is less than 10.0wt.%, thereby excellent corrosion resistance of the alloy is* assured. One of the differences between the alloy of the present invention and that of JP 230 1514, paper by B.E. Wilde, EP 336 157A, USP 493 3027 or EP 050 6488A1 is the addition of rare earth element (RE) in the alloy according to the present invention, which reduces dendritic segregation of elements and improves the homogeneity of elements' distribution, thus the effect of shape memory is improved remarkably. Compared to alloys disclosed in JP 230 1514 and paper by B.E. Wilde, the alloy according to the present invention is added 0.05-0.4wt.% of N, which reduces stacking fault energy, regulates phase transformation point, and improves the strength of the alloy. In addition, compared to the alloys disclosed in EP 336 157A and USP 493 3027, the alloy according to the present invention comprises carbonitride forming elements such as Nb, Ti, V, Zr, Ta, Hf, W, Mo, which reduces the material's overheating sensitivity and improves the material's strength. The functions of alloying elements used in the present invention will be described briefly below. Cr: Cr is a ferrite-forming element, mainly functioning to improve corrosion resistance of the alloy. The content of Cr is generally above 12.Owt.% when it is applied to stainless steel. The corrosion resistance is insufficient when the content of Cr is less than 5wt.%. The manner of Cr to effect stacking fault energy and phase transformation point is complicated. When the content of Cr is less than 9wt.%, the stacking fault energy is decreased; when the content of Cr is above 9wt.%, the stacking fault energy is increased, the phase transformation point is decreased remarkably, and the forming of brittle a phase is promoted. The content of Cr according to the present invention is 12-20 wt.%, preferably 12.0-18.0 wt.%. Ni: Ni can promote the forming of austenine and can decrease the yield strength, therefore the content of Ni in the present invention is controlled in the range of 0.1 -8wt.%, and preferably in the range of 4.0-8.Owt.%. As Ni can increase the stacking fault energy, 4 Si, Mn and/or the like which can decrease stacking fault energy is added. Si: As Si can significantly decrease the stacking fault energy, it is generally desirous to increase the content of Si. When the content of Si is less than 3wt.%, the decreasing of stacking fault energy is insufficient; when over 7wt.%, the workability is deteriorated. Therefore the content of Si according to the present invention is in the range of 3-8 wt.%, preferably in the range of 3.2-6.1 wt.%. N: N is an austenite-forming element which can decrease the stacking fault energy remarkably and stabilize Ms point. When N is added, carbonitride can be partically formed partly, which can inhibit hot sensitivity, and improve the yield strength and corrosion resistance of the alloy. However, an excess amount of carbonitride tends to be formed when the content of N is above 0.4 wt.%, which increases the brittleness of material. Therefore the content of N should be controlled in the range of 0.050-0.40 wt.%, and preferably in the range of 0.05-0.2 wt.%. Nb: Nb is a carbonitride-forming elements, which in turn includes Ti, Ta, V, Zr, Hf, W and the like. The elements have function of holding C element, preventing the carbonide of Cr from precipitation, avoiding impoverishment of Cr at grain boundary and intergranular corrosion. Meanwhile the fine carbonide formed can inhibit the growth of crystal grain, prevent the alloy from overheating at high temperature. The intercrystalline brittleness can be resulted when Nb is added excessive by, therefore the content of Nb is selected in the range of 0.05-2 wt.%. Mo: The purpose of adding Mo is to improve intergranular corrosion resistance and stress corrosion resistance. The effect of Mo is neglectable when the content is less than 0.05 wt.%, and the shape memory performance of the alloy is deteriorated when Mo is above 2 wt.%. Therefore the content of Mo should be controlled in the range of 0.05-2 wt.%. Similar effects are found with W. Cu: Cu is a austenite-forming element, which can improve the alloy's corrosion resistance, and increase the stacking fault energy in austenite. Excessive Cu can inhibit the formation of c martensite and deteriorate shape memory performance. Generally Cu is added within the range of 0-2 wt.%. Al: Al can result in grain refinement, decrease stacking fault energy and improve the 5 shape memory performance of the alloy. However, the alloy's workability is deteriorated when Al is above 2wt.%. Therefore, the content of Al should be controlled within the range of 0-2 wt.%. Meanwhile, one or more elements selected from the group consisting of Nb, Ti, V, Zr, Ta, Hf, W, Mo, Al and Cu in the range of 0.05-2 wt.% can be included. The effect and content of individual element is described above qualitatively. The specific composition of the elements should follow three formulas described below: 1. The alloying elements (wt.%) must meet the following relationship to assure that the parent phase of alloy is uniform austenite: (Ni + 0.5Mn +0.4Co + 0.06Cu + 0.002N + 3) [0.67 (Cr + Mo) + 0.804(Si + Ti + Zr + Hf + V + Nb + Ta)] 2. The alloying elements (wt.%) must meet the following relationship to assure the alloy has stress corrosion resistance and pit corrosion resistance of the alloy: Cr + Si >18; Cr + 3.3 Mo + 30N > 18 3. The alloying elements (wt.%) that can decrease stacking fault energy must meet the following relationship to assure excellent shape memory performance and moderate phase transformation temperature of the alloy: 7Si+ Mn + 3Co + lOON > 50 The shape memory stainless steel according to present invention has the following performances: 1. shape memory performance is excellent. shape memory recovery rate can reach 80 % after 3% extension, and linear recovery is above 4% after appropriate thermo mechanical training. 2. Stress corrosion resistance and pit corrosion resistance is comparable to stainless steel 304. 3. The alloy has a yield strength ao 2! 300 MPa, ab 2 650 MPa, which exceeds the levels of standard stainless steel 304. (Compositions (wt.%) of stainless steel 304: C 0.08, Cr: 18-20, Ni: 8-12, Mn: 1-2, N-0.03; mechanical property: aO 2 = 247 MPa, as= 541 MPa, S= 50%). 4. The alloy has excellent cold and hot press workability. 6 5. The alloy has a moderate phase transformation point, -40*CCMs(20*C. The shape memory alloy according to the present invention shows high strength, good workability, moderate phase transformation point, and good corrosion resistance. Therefore, the shape memory pipe sleeve can be made of the shape memory alloy according to the present invention to connect pipes. Currently, similar products have been described, for example, JP 040 69481 A suggested to apply epoxy resin to inner wall of pipe sleeve for sealing when using pipe sleeve made of alloy comprising Fe, Mn, Si to connect pipes; JP 052 15277CN suggested to engrave veins in the inner wall of pipe sleeve in order to strengthen fastening and connecting effect in addition to applying sealing agent to the inner wall of pipe sleeve. Furthermore, CN 211 6140 suggested a closed connecting structure for reliable connection, which can be coated by sealing agent. The above three patents all relate to applying sealing agent to the inner wall of pipe sleeve during connecting, thus results in complicated operation. The present invention provides to a novel, simple and practical pipe coupling illustrated in figure 1, which is composed of pipe sleeve 1 made of shape memory stainless steel or alloy pipe, intermediate sealingfing 2, sealing agent 3 at the both ends of the sleeve and pipe 4 to be connected. The intermediate sealing ring 2, a solid alloying ring of high plasticity or rubber ring or other rings made of high plasticity, is fixed in the middle part of inner wall of the pipe sleeve 1. The cross section of sealing ring can be circular, elliptic, rectangular, trapezoidal or other allotypic. Sealing agent 3, a resin or water glass or moldable inorganic paste, is applied to the proximal end of inner wall section of pipe sleeve 1. The intermediate sealing ring 2 and the sealing agent 3 at the end of pipe sleeve can be used in combination or used separately. The intermediate sealing ring 2 has a function to locate the pipes along axial direction during connection, and is compressed closely for sealing by the pipes of both sides when it is fixed in the middle. The sealing agent 3 is applied only to a part of the surface at the end of inner wall section of pipe sleeve 1, so that the sealing agent 3 can not overflow to the inside of pipes during connecting and assuring sealing effect. The pipe sleeve 1 made of shape memory stainless steel contracts when heated, thus the pipes is closely fastened to achieve connection. The said pipe coupling has the advantage of simplicity, practicability, cost effectivity, reliability and easy to use. 7 BEST MODES FOR CARRYING THE INVENTION The present invention will be further illuminated in detail according to examples described below. Example 1 The alloys comprising various chemical components (wt.%) according to the present invention are produced by using conventional methods, and the performances thereof such as the shape memory performance, corrosion resistance, mechanical property and working performance are measured with conventional test methods. The alloys up to the above mentioned standard are indicated by " o "; the alloys essentially up to the above mentioned standard but inferior in some aspects are indicated by "0 ", the alloys that are not up to the above mentioned standard are indicated by "x". The alloy compositions and performances of 15 alloys are shown in table 1. Comparative Example The alloy steels 16-23 comprising various chemical components are produced by using the same method of example 1, which are used in the Comparative Example. The performances thereof are measured with the same methods. The alloy compositions and performances are shown in table 1. From table 1, we can see the alloys No. 1-15 according to the present invention exhibit excellent shape memory performances, corrosion resistance, mechanical property and workability, while the performances of alloys No.16-23 of comparative Examples are not as good as those according to the present invention. Specifically, alloy No. 16 has poor shape memory performance and corrosion resistance owing to lack of rare earth elements. Alloy No. 17 has poor performances except good corrosion resistance owing to high content of Si element. Alloy No. 18 has poor shape memory performance, mechanical 8 property and corrosion resistance owing to low content of Si element. Alloy No. 19 has poor performances except good working performance owing to lack of N element. Alloy No. 20 has poor corrosion resistance owing to low content of Cr element. Alloy No. 21 has poor shape memory performance and corrosion resistance owing to low content of N element. Alloys No. 22 and 23 have poor corrosion resistance and working performances because the contents of Mn thereof are 15wt.% and 18wt.% respectively, which are higher than the alloys according to the present invention. Example 2 The austenite stainless steel pipes to be connected are + 15.9 x Imm 304 pipes. The pipe sleeve for connecting is made of the shape memory stainless steel No. 4 according to the present invention, whose composition is follow: C: 0.02wt.%, Cr: 12.8wt.%, Si 5.03wt.%, Mo: 1.02wt.%, Ni: 5.1Owt.%, Mn: 14.13wt.%, Co: 3 .Owt.%, N: 0.10wt.%, Ti: 0.20wt.%, Ce: 0.02wt.%, La: 0.03wt.%, the balance being Fe and incidental impurities. The pipe sleeve that was trained has the length of 25 mm, the wall thickness of Imm, and inner diameter of 16±0.05 mm. The memory stainless steel was cast into round ingot after being melted in 25-Kg vacuum induction furnace. The memory stainless steel was heated to 1100*C and kept at this temperature for 6 hours, and then was forged into rectangular bars with a cross section of 50x20 mm. Then the bars were hot-rolled into steel band with a thickness of 3 mm, which is cold-rolled into sheet products with a thickness of Imm after acid washing and light annealing. The sheet products is made into shape memory stainless steel welded tube through high frequency induction welding after edge cutting. The expand test carried out by using a convex cone with angle of 90* confirms that the welding seam has excellent plasticity. The loading in the training of welding tube was carried out through rolling with core rod and two outer plates, and the shape memory recovery of the tube was up to 2%. Sealing agent was applied only to two ends of pipe sleeve made of shape memory stainless steel during connection. The memory pipe sleeve contracted to fasten the connected stainless steel 304 after being heated slowly for 1 minute with a gas burner. The result of water pressure test showed that no abnormal 9 phenomena, such as falling or leaking was observed after water pressure was increased up to 50 kg/cm 2 and keep for 30 minutes. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic diagram showing the structure of connecting pipes made of shape memory stainless steel of the present invention, wherein: Reference number 1 represents a pipe sleeve made of shape memory stainless steel or alloy, reference number 2 represents intermediate sealing ring, reference number 3 represent the sealing agent at both ends of the sleeve, and reference number 4 represents the pipe to be connected. INDUSTRIAL APPLICABILITY With the development of science and technology and improvement of people's living standard, the zinc-plated low carbon steel pipe currently utilized in domestic water supply does not meet the requirements to supply high quality water. The zinc-plated low carbon steel pipe has poor corrosion resistance and rust tends to form in the pipe, which decreases the working life of pipe and deteriorates the water quality. Now there is a tendency of utilizing stainless steel pipe and copper pipe for water supply. The thin-wall stainless steel pipe is preferred because of its low cost. However, it is not suitable for this pipe to be connected by screw coupling. This problem is resolved if the shape memory stainless steel pipe coupling according to the present invention is adopted. This novel type of pipe coupling can be utilized for the connection of hot water pipe (conventionally copper pipe is used). Tight connection can be achieved with easy operation,, the soldering process is avoided. In addition, the stainless steel pipes for protecting cables are difficult to be connected through screw coupling, and welding can not be used so as not to damage the cables. Therefore a low temperature connecting method is needed. In that case the shape memory stainless steel pipe coupling according to the present invention can find usage in this connection. 10 0 - - - - - - - -Q 2

------------------

- - - - - -) Z; 6 6 6 1 --------- -- -- -- - Zq 2 m 2 0a!n 0 6 0 -1 O 2 r4 - :2 2q 2

W,

Claims (6)

1. A shape memory stainless steel, comprising Cr: 1 2 -20wt.%, Si: 3-8wt.%, Ni: 0.l-8wt.%, Mn: 0.1-10.Owt.%, Co: 0.1-20wt.%, N: 0.05-0.4wt.%, C: 0-0.03wt.%, 0.01-0.15wt.% of one or more rare earth elements selected from the lanthanum group consisting of La, Ce, Sm, Nd, Pm, Eu, Tb, Dy, Pr, Gd, Ho, Er, Tm, Yb and Lu, 0.05-2wt.% of one or more elements selected from the group consisting of Nb, Ti, V, Zr, Ta, Hf, W, Mo, Al and Cu, and the balance being iron and incidental impurities.

2. A shape memory stainless steel of claim 1, wherein the content of Cr, Si, Ni, Mn, Co and N is Cr: 12.0-18.Owt.%, Si: 3.2-6.1wt.%, Ni: 4.0-8.0wt.%, Mn: 1.9-5.Owt.%, Co:

5.0-20.Owt.%, N: 0.05-0.2wt.%, respectively. 3. A shape memory stainless steel of claim 1 or 2, wherein the total content of La, Ce, Sm, Nd, Pm, Eu, Tb, Dy, Pr, Gd, Ho, Er, Tm, Yb and Lu is 0.02-0.1 Owt.%. 4. A shape memory stainless steel of claim 1 or 2, wherein the total content of Ti, Zr, Hf, Ta, W, Al, Cu is 0.05-2wt.%. 5. A pipe coupling device comprises a pipe sleeve (1) made of the shape memory stainless steel or alloy of claim 1 or 2, an intermediate sealing ring (2) placed inside of the inner wall of pipe sleeve (1) and between the ends of the pipes (4) to be connected, and a sealing agent (3) applied to the inner surface of the ends of pipe sleeve (1) and outer surface of the pipes (4).

6. A pipe coupling device of claim 5, wherein the intermediate sealing ring (2) is a solid high plasticity alloying ring, rubber ring or other rings made of high plasticity materials, and the cross section of sealing ring is circular, elliptic, rectangular, trapezoidal or other allotypic.

7. A pipe coupling device of claim 5, wherein the sealing agent (3) is a resin, a water glass or a moldable inorganic paste.

8. A pipe coupling device of claim 5, which includes an intermediate sealing ring (2) as well as a sealing agent (3) at the end of the sleeve, or includes either one of them. 12