Nickel-free iron alloy for stents

Abstract

The present invention is directed to a largely nickel-free iron alloy or a nickel-free stainless steel having the following composition: TABLE-US-00001 14.0% by wt.-16.5% by wt. chromium 10.0% by wt.-12.0% by wt. manganese 3.0% by wt.-4.00% by wt. molybdenum 0.55% by wt.-0.70% by wt. nitrogen 0.10% by wt.-0.20% by wt. carbon 0.00% by wt.-2.00% by wt. impurities, such as other metals, semimetals, metal salts and/or non-metals the rest up to 100% by wt. is iron,
which is in particular suitable for the production of stents as well as to stents made of this alloy.

Claims

1. A stent consisting of a steel alloy consisting of the following components based on the total weight of the alloy: TABLE-US-00016 14.0% by wt.-16.5% by wt. chromium 10.0% by wt.-12.0% by wt. manganese 3.0% by wt.-4.0% by wt. molybdenum 0.55% by wt.-0.70% by wt. nitrogen 0.10% by wt.-0.20% by wt. carbon 1 ppm-2.0% by wt. impurities such as other metals, semimetals, metal salts and non- metals the rest up to 100% by wt. is iron.

2. The stent consisting of a steel alloy according to claim 1 consisting of: 1 ppm-2.0% by wt. impurities in form of other metals in a maximal amount of each up to 0.075% by wt. and non-metals from the group S, Si, P in a maximal total amount of 1.2% by wt.

3. The stent consisting of a steel alloy according to one of the claim 1 or 2 additionally consisting of: 0.00% by wt.-0.05% by wt. nickel.

4. The stent consisting of a steel alloy according to claim 1 additionally consisting of: 0.00% by wt.-1.00% by wt. silicon.

5. The stent consisting of a steel alloy according to claim 1 which has been subjected to a heat treatment.

6. The stent consisting of a steel alloy according to claim 1, having a yield strength R.sub.p0.2 between 500 and 600 MPa.

7. The stent consisting of a steel alloy according to claim 1, having a grain size G between 6 and 10.

8. The stent consisting of a steel alloy according to claim 1, for blood vessels, urinary tracts, respiratory tracts, biliary tracts or the digestive tract.

Description

EXAMPLES

Example 1

Production of the Alloys

(1) As raw materials for the manufacture of the master alloy purest starting materials are used and melted in a vacuum melting plant. Herein, all alloy components apart from nitrogen are added to the alloy in the appropriate amounts.

(2) The primary material is remelted by means of DESU-method (pressure electro slag remelting procedure), whereat the nitrogen content is adjusted.

Example 2

Tube Production

(3) From the alloys that have been produced as described in Example 1 a cast blank adapted to the extrusion press was heated before extrusion for 3-6 hours in a reducing atmosphere of nitrogen to 1100 C. to 1250 C. and cooled in air after the extrusion. The produced bars were drilled centrically hollow by means of a precision drilling method. Drawing steps followed, each with a subsequent heat treatment in a reducing atmosphere of nitrogen at 1100 C. to 1250 C., in which the tube was converted to the nominal size.

Example 3

Stent Fabrication

(4) A tube produced according to Example 2 is fixed into an adapter in the laser machine. A pulsed solid state laser (FKL) cuts the contours of the stent design out of the tube. The laser cutting is performed under an inert gas atmosphere.

(5) The stent design is stored in an NC program (numerical control). This provides the laser with the traverse paths, after which the tube is structured. By the laser beam cutting burr formation occurs, especially on the inside of the tube, along the entire cutting contour. This can cause that offcuts and cut-outs remain stuck in the contour after termination of the cutting process. The offcuts and cut-outs are mechanically removed and the stent is cleaned from manufacturing residues. In a first optical visual control an inspection of the cutting contour is performed.

(6) In the following, the stent is electrochemically polished. The stent is anodically connected and immersed in an acid bath. Via a cathode fixed in the bath, an electric circuit is closed. The electric circuit is maintained for several minutes. The electropolishing is an inverted galvanic process where material is removed in a controlled manner from the surface of the anodically connected structural component. Due to the method removal takes preferably place at sharp corners and edges. The stent obtains a smooth surface and rounded edges along the contours. After polishing, the stent is cleaned and freed from acid residues. During the final cleaning all still remaining manufacturing residues are removed from the stent surface. In a last optical visual control the stent geometry is measured and the surface is tested on cleanliness.

Example 4

Determination of the Optimal Chromium Content in an Alloy According to the Invention

(7) For the determination of an optimally adjusted chromium content, the alloys A-I having the following compositions were produced according to Example 1:

(8) TABLE-US-00009 A B C D E F G H I Cr 12.0 13.0 14.0 15.5 16.0 16.5 17.0 17.5 18.0 Mn 11 11 11 11 11 11 11 11 11 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 C 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

(9) The corrosion behavior was determined on the basis of stents by means of potentiostatic tests.

(10) The potentiostatic tests were carried out in an oxygen-free buffered physiological solution at 37 C. At first, the resting potential is determined. This means that a reference electrode and the structural component are immersed in the solution without applied voltage. A potential difference arises, which varies over time. Based on the potential difference, which appeared after one hour, an initial statement about the resistance of the alloy can be made. The more positive the value is the more resistant is the alloy.

(11) Subsequently, a cyclic potentiodynamic polarization was carried out. For this, a potential difference between the structural component and the reference electrode was applied. The initial potential is selected so that it is 0.1 mV smaller than the resting potential arised. The applied potential is increased over time to for example 1.2 V and then reduced to the initial value, wherein the resulting current is measured. Based on the polarization curve, which is a voltage-current curve, the corrosion rates, the minimal currents, the breakdown potentials as well as the repassivation potentials can be determined. The parameters were determined according to ASTM F2129-10 with PBS (phosphate buffered saline).

(12) For the alloys according to the invention corrosion rates between 15 and 25 nm/y (nanometer per year) were measured. For less resistant alloys corrosion rates of more than 50 nm/y were determined.

(13) The determined breakdown potentials for the alloys according to the invention are between 1030 mV and 1070 mV. In contrast to this, the breakdown potential is reached already at 800 mV in the case of less resistant alloy compositions.

(14) Even more important is the difference in the repassivation behavior which has a special meaning in the use as a stent. The alloys according to the invention have a repassivation potential from 940 to 960 mV, wherein alloys with a low repassivation potential reach only a repassivation potential from 100 to 150 mV.

(15) The determined valuesseen in isolationhave only little relevance, only the combination of good individual values result in a good corrosion behavior, wherein the weighting of the individual values depends on the case of application.

(16) The alloys A and B have an uneven surface when polished. Viewed by light microscopy, the surface has slightly matt spots and does not appear high glossy in total. The corrosion resistance and in particular the repassivation potential are reduced compared to the alloys C to E.

(17) The alloys C to E have a very good corrosion behavior. The chemical resistance is much higher than that of the material 1.4441 used for vascular stents. The breakdown and repassivation potentials are comparable with the material 2.4964 (L605).

(18) The alloys C to E have an excellent polishability. A defect-free surface without measurable waviness and without indentations or ridges is produced. Viewed by light microscopy, there is a high glossy surface. The alloy F shows a good polishability, but the surface has isolated indentations, which are partially not polished. The corrosion behavior is still sufficient, similar to the material 1.4441.

(19) The alloys G to I have an increasingly worse polishability with increasing chromium content. Polishing produces a wavy surface with non-polished indentations.

(20) Potentiostatic tests of the alloys G to I show a reduced breakdown potential and a significantly reduced repassivation potential. The unexpected deterioration of the polishability and reduction of the corrosion resistance by increasing the chromium content is attributed to the formation of sigma phases and delta ferrite. Sigma phases and delta ferrite form at temperatures of about 600 C.-800 C. and could be caused by the heat treatment. Since the heat treatment must ensure a low yield strength, as well as a high ductility and a small grain of G>7, the formation of sigma phases and/or delta ferrite cannot be avoided for chromium contents higher than 16.5%. Thus, the chromium content should be limited to 16.5%. Due to the improvement of the corrosion and polishing properties chromium should also represent a minimum proportion of 14% of the alloy.

Example 5

Determination of the Optimal Manganese Content in an Alloy According to the Invention

(21) For the determination of an optimally adjusted manganese content, the alloys A-O having the following compositions were produced according to Example 1:

(22) TABLE-US-00010 A B C D E F G H I J K L Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 10 11 11.6 11.8 12 12.2 12.4 12.6 12.8 13 14 16 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 C 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 M N O Cr 16.0 16.0 16.0 Mn 18 20 22 Mo 3.19 3.19 3.19 N 0.62 0.62 0.62 C 0.15 0.15 0.15 Ni 0.03 0.03 0.03 Si 0.33 0.33 0.33 P 0.01 0.01 0.01

(23) The mechanical parameters yield strength R.sub.p0.2, tensile strength R.sub.m, and elongation at break (A) were determined in the tensile test on tube samples according to DIN EN 10002-1. For this, tube samples were mounted between two brackets. The brackets were fixed on the tensile testing machine and the tensile testing machine stretches the sample over the length up to the break. The mechanical parameters are calculated and put out by the machine based on the measured forces and distances and the given sample geometry.

(24) The alloys A, B, C and D have a very good polishability. A very good surface quality without measurable waviness and without indentations or ridges is produced. Viewed by light microscopy, there is a defect-free high glossy surface. In particular the alloys A and B have in the polished state an excellent and absolutely defect-free high glossy surface viewed light microscopically.

(25) The yield strengths are approximately 550 MPa for the alloy A and rise for the alloy D to approximately 600 MPa. The elongations at break of these alloys are up to over 65%.

(26) The alloy E has a slightly worse polishability. Viewed by light microscopy, there is a glossy surface which is poor in defects. Sporadically, slight waviness of the surface is discernible by light microscopy. In part, there are also individual indentations existent in the structural component. The yield strength is about 610 MPa and the elongation at break of this alloy is around 60%.

(27) With increasing manganese content, the alloys F, G, H and I have a polishability getting worse and worse. Viewed by light microscopy, there is a matt finished surface having defects. The surface is wavy. There are many indentations. Most of all, the alloys H and I have many non-polished indentations. The yield strength of alloy I rises up to approximately 640 MPa. With increasing manganese content the elongation at break is reduced to less than 60%.

(28) The alloys J, K and L do not allow the production of polished surfaces according to the requirements that are put on stents. Viewed with the naked eye, the surfaces appear to be slightly matt, which is attributable to non-polished indentations. The yield strengths have values of up to over 760 MPa and the elongation at break is reduced to about 40%.

(29) The alloys M, N and O do not allow the production of polished surfaces. Viewed with the naked eye, the surfaces appear to be matt, which is attributable to extensively present non-polished indentations. The yield strengths have values of up to over 850 MPa and the elongation at break is reduced to less than 35%.

(30) Furthermore, the alloys P-S having the following compositions were produced according to Example 1:

(31) TABLE-US-00011 P Q R S Cr 16.0 16.0 16.0 16.0 Mn 8.5 9.0 9.6 9.8 Mo 3.19 3.19 3.19 3.19 N 0.62 0.62 0.62 0.62 C 0.15 0.15 0.15 0.15 Ni 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01

(32) The alloys P and Q have a yield strength of approximately 500 MPa. Viewed by light microscopy, the surfaces have smaller indentations and sporadically also ridges after polishing. The elongation at break reaches values of about 50%.

(33) The indentations after polishing suggest precipitation effects in the material. This is in accord with the reduced elongation at break compared to alloy A, because precipitations reduce the elongation at break. Since manganese increases the solubility of atomically dissolved nitrogen, precipitations can occur, if the manganese content is reduced at a constant nitrogen content.

(34) After crimping and dilatation of the stent the alloy P has a slightly ferritic behavior. The chemical resistance of the alloy P is significantly reduced. The alloy Q has a reduced chemical resistance.

(35) The yield strengths of the alloys R and S are about 540 MPa. The elongation at break reaches values of approximately 60%. Viewed by light microscopy, the surface quality of the alloys after polishing is high and poor in defects.

(36) The chemical resistance in particular of alloy S is only reduced to a slight degree compared to alloy A.

(37) The best surface quality and the highest elongation at break as well as the lowest yield strength are achieved with the alloys A-E, so that the manganese content of the inventive steel alloys is set to 10.0-12.0% by wt.

Example 6

Examination of the Impact of Molybdenum in an Alloy According to the Invention

(38) For the examination of the impact of molybdenum on the mechanical and chemical properties of an alloy according to the invention, the alloys A-N having the following compositions were produced according to Example 1:

(39) TABLE-US-00012 A B C D E F G H I J K L M N Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 11 11 11 11 11 11 11 11 11 11 11 11 11 11 Mo 5.0 4.5 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 N 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 C 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

(40) The alloys A and B have a poor polishability. Viewed by light microscopy, there is a matt-finished surface. The surface is wavy. There are both indentations and ridges. The elongation at break is between 35%-40%. The ridges can be explained by the formation of carbides, because these are ablated in a slowed manner during the polishing process. Likewise, the indentations can be attributed to carbides, because these leave indentations when they are detached from the material and fall out of the stent. The strongly reduced elongation at break in comparison to the alloys D to I can be attributed to the notch effect of carbides and to the carbon-depleted material in the surroundings of the carbides.

(41) Potentiostatic tests show a higher flat ablation and a deteriorated repassivation behavior in comparison to the alloy D to I. This can be attributed to galvanic corrosion between carbides and the basic material.

(42) The alloy C has a much better polish compared to the alloys A and B. There is a glossy surface, which has sporadically slight waviness. The elongation at break is above 50%.

(43) The alloys D to I have an excellent polishability. A defect-free surface without measurable waviness and without indentations or ridges is produced. Viewed by light microscopy, there is a high glossy surface. The elongation at break of these alloys is up to over 65%.

(44) The alloy J has a slightly worse polishability compared with alloy I. Viewed by light microscopy, there is a glossy surface which is poor in defects. Sporadically, slight waviness of the surface is discernible by light microscopy. In part, there are also individual indentations existent in the structural component. The elongation at break is around 50%.

(45) Potentiostatic tests of alloy J, as described in example 4, showed a slightly reduced repassivation potential compared to the alloys D to I. The alloys K to N have an increasingly worse polishability with sinking molybdenum content. Polishing produces an uneven surface with non-polished indentations. Potentiostatic tests of the alloys K to N show a reduced breakdown potential and a significantly reduced repassivation potential.

(46) The fundamental impact of molybdenum onto the corrosion resistance is evident from the MARC value. Molybdenum increases the chemical resistance 3.3 fold as much as chromium.
MARC=[% Cr]+3.3[% Mo]+20[% C]+20[% N]0.5[% Mn]0.25[% Ni]

(47) The amount of molybdenum in the alloys according to the invention should, thus, be between 3.0% by wt. and 4.00% by wt.

Example 7

Examination of the Impact of Nitrogen on an Alloy According to the Invention

(48) For the examination of the impact of nitrogen on the mechanical and chemical properties of an alloy according to the invention, the alloys A-L having the following compositions were produced according to Example 1:

(49) TABLE-US-00013 A B C D E F G H I J K L Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 11 11 11 11 11 11 11 11 11 11 11 11 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.36 0.41 0.45 0.49 0.52 0.55 0.58 0.61 0.65 0.70 0.75 0.80 C 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

(50) The alloys A to D have a ferritic behavior and are therefore unsuitable as material for stents. Their corrosion resistances tested as described in Example 4 are extremely low. The alloy E has a much better corrosion behavior but the breakdown and repassivation potential is even lower than that of the samples F to I. An austenitic structure exists.

(51) The alloys F to I have a very good corrosion behavior. The chemical resistance is much higher than that of the material 1.4441 used for vascular stents. The breakdown- and repassivation potentials are comparable with the material 2.4964 (L605).

(52) The alloys A to H have a yield strength increasing with the nitrogen content from approximately 450 MPa to 600 MPa. The elongations at break of the samples A to D reach approximately 55%. The elongation at break of the samples E to J reaches approximately 65%. These parameters were obtained as described in Example 5.

(53) The alloys I to L have a yield strength of up to approximately 640 MPa. The elongations at break reach values of 55%-65%. The alloy L has a higher corrosion rate and a lower repassivation potential. This is attributable to the formation of nitrides, which form at higher nitrogen contents and thus reduce the corrosion resistance by the depletion of chromium and nitrogen arising around the nitrides.

(54) The dependence of the yield strength on the nitrogen content is evident from the following formula:
Yield strength(MPa)=251+33Mn(m %)+313[N+C(m %)]

(55) Since the yield strength must be lower than 600 MPa for the use as a stent, a nitrogen content as low as possible is required in regard to the yield strength. In combination with the requirement of a fine grain of preferably G>7, this can only be produced up to a nitrogen content of maximally 0.7%.

(56) The amount of nitrogen in the alloys according to the invention should thus be between 0.55% by wt. and 0.7% by wt.

Example 8

Examination of the Impact of Carbon on an Alloy According to the Invention

(57) For the examination of the impact of carbon on the mechanical and chemical properties of an alloy according to the invention, the alloys A-L having the following compositions were produced according to Example 1:

(58) TABLE-US-00014 A B C D E F G H I J K L Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 11 11 11 11 11 11 11 11 11 11 11 11 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 0.62 C 0.02 0.05 0.1 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

(59) The alloys A-C have a low corrosion resistance, measured as described in Example 4. Particularly the capability of repassivation is reduced in comparison to the alloys D-H. The alloys have a low proportion of delta ferrite. In the alloy C delta ferrite can be found only sporadically. The elongation at break is approximately 55%-60% and the yield strength is approximately 550-570 MPa.

(60) The alloys D-J have no delta ferrite. The alloy D has a higher corrosion resistance than the alloys A-C. The capability of repassivation is reduced in comparison to the alloys E-H.

(61) The alloys E-H have a very high corrosion resistance with a high repassivation potential. The elongation at break and uniform elongation are increased in comparison to the other alloys. The elongation at break is up to over 65%. The yield strength is approximately 570-600 MPa.

(62) The alloy I has a high corrosion resistance. Most notably, the repassivation potential is reduced compared to the alloys E-H. This is attributable to the formation of scattered chromium carbides.

(63) The alloy J has a significantly reduced corrosion resistance which can be explained by the formation of chromium carbides. The yield strengths of the alloys I-L are approximately at 620-640 MPa. The corrosion resistance of the alloys K and L is reduced even further.

(64) The amount of carbon in the alloys according to the invention should therefore be between 0.10% by wt. and 0.20% by wt.

Example 9

Examination of the Impact of Carbon and Nitrogen on an Alloy According to the Invention

(65) For the examination of the impact of carbon in dependence on the nitrogen content on the mechanical properties of an alloy according to the invention, the alloys A-I1 having the following compositions were produced according to Example 1:

(66) TABLE-US-00015 A B C D E F G H I J K L Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 11 11 11 11 11 11 11 11 11 11 11 11 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.50 0.6 0.7 0.8 0.9 0.50 0.6 0.7 0.8 0.9 0.50 0.6 C 0.08 0.08 0.08 0.08 0.08 0.12 0.12 0.12 0.12 0.12 0.16 0.16 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 M N O P Q R S T U V W Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 11 11 11 11 11 11 11 11 11 11 11 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.7 0.8 0.9 0.50 0.6 0.7 0.8 0.50 0.6 0.7 0.8 C 0.16 0.16 0.16 0.20 0.20 0.20 0.20 0.24 0.24 0.24 0.24 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 X Y Z A1 B1 C1 D1 E1 F1 G1 H1 I1 Cr 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 16.0 Mn 11 11 11 11 11 11 11 11 11 11 11 11 Mo 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 3.19 N 0.50 0.6 0.7 0.8 0.50 0.6 0.7 0.8 0.5 0.6 0.7 0.80 C 0.30 0.30 0.30 0.30 0.36 0.36 0.36 0.36 0.40 0.40 0.40 0.40 Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 Si 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 P 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

(67) The alloys A and B have a low corrosion resistance. Particularly, the repassivation potential is reduced. The elongation at break is approximately 55%-60% and the yield strength is approximately 530-560 MPa. The alloy C has a sufficient chemical resistance with a slightly reduced repassivation potential. The elongation at break is approximately 60% and the yield strength is approximately 600 MPa. The alloys D-E have a good chemical resistance. The elongation at break is approximately 55% and the yield strength is approximately 620-650 MPa.

(68) The alloy F has a good corrosion behavior with a high repassivation potential. In total, the chemical resistance is slightly inferior to the alloys G-H. The elongation at break is approximately 60% and the yield strength is approximately 550 MPa. The alloys G-H have a very good chemical resistance. The elongation at break is approximately 60%-65% and the yield strength is approximately 580-600 MPa. The alloys I-J have a reduced chemical resistance. The elongation at break is approximately 55%-60% and the yield strength is approximately 620-660 MPa.

(69) The alloy K has a good chemical resistance with a high repassivation potential. In total, the chemical resistance is slightly inferior to the alloys L-M. The elongation at break is approximately 60% and the yield strength is approximately 560 MPa. The alloys L-M have a very good chemical resistance with a high repassivation potential. The elongation at break is approximately 65% and the yield strength is approximately 590-610 MPa. The alloy N has a reduced chemical resistance with a reduced repassivation potential. The elongation at break is approximately 55% and the yield strength is approximately 640 MPa.

(70) The alloy O has a clearly reduced chemical resistance with a low repassivation potential. The elongation at break is approximately 50% and the yield strength is approximately 660 MPa.

(71) The alloys P-Q have a sufficient chemical resistance with a high repassivation potential. The elongation at break is approximately 60% and the yield strength is approximately 570-600 MPa. The alloy R has a very good chemical resistance with a high repassivation potential. The elongation at break is approximately 65% and the yield strength is approximately 630 MPa. The alloy S has a higher corrosion rate with a low repassivation potential. The elongation at break is approximately 60% and the yield strength is approximately 670 MPa.

(72) The alloys T-U have a still sufficient chemical resistance with a sufficient repassivation potential. The elongation at break is approximately 60% and the yield strength is approximately 590-620 MPa. The alloys V-W have a reduced chemical resistance with a reduced repassivation potential. The elongation at break is approximately 55% and the yield strength is approximately 660-690 MPa.

(73) The alloys X-Y have a reduced chemical resistance in comparison to alloy G. The repassivation potential is also reduced. The elongation at break reaches values of about 55%. The yield strength is 610-640 MPa. The alloys Z-A1 have an even further reduced chemical resistance. The elongation at break is approximately 50%. The yield strength reaches values of 670-700 MPa. The polished surfaces show indentations to an increased degree.

(74) The alloys B1-C1 have a clearly reduced chemical resistance also in regard to the repassivation potential. The elongation at break reaches values of 50-55%. The yield strength is at 620-650 MPa. The polished surfaces show indentations to an increased degree, which points to fine precipitation events.

(75) The alloys D1-E1 have a low chemical resistance and are thus not to be used as a stent material. The elongation at break is 45-50%. The yield strength reaches values of 680-710 MPa. The polished surfaces, especially of alloy E1, show larger indentations to an increased degree. Precipitation effects occur.

(76) The alloys F1-G1 have a strongly reduced chemical resistance and are thus not applicable as stent material. The elongation at break is 45-50%. The yield strength reaches values of 640-670 MPa. Indentations but also ridges are present to an increased degree in the polished state, which are a consequence of precipitations.

(77) The alloys H1-I1 are chemically not resistant. The alloys have a low breakdown- and repassivation potential. The elongation at break is approximately 40%. The yield strength reaches values of 690-720 MPa. The alloys H1-I1 have after polishing no polished surfaces that are suitable for the use as a stent.

(78) The alloys G and H as well as L and M and R show a special applicability as stent material due to the combination of positive properties. They all have a nitrogen content between 0.6% and 0.7%, a carbon content between 0.12% and 0.2% and a ratio of N:C of 3.50 to 5.83.