Why the Annealing

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Laser Material Processing

Why the annealing

of 316LVM-Stents

is so important

Dr. C. Meyer-Kobbe

B.H. Hinrichs

Sarstedt, December 2002

MeKo

Laserstrahl-Materialbearbeitung

Kthe-Paulus-Strae 9-11

D-31157 Sarstedt / Germany

www.meko.de


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The Importance of Annealing 316LVM Stents

C. Meyer-Kobbe and B.H. Hinrichs

MeKo Laser Material Processing, Sarstedt, Germany

Annealing has a considerable influence on the quality of stents and is possibly

the most critical process in their manufacture because it determines the

material properties of the stent. Improved methods for verifying the results of

the annealing process are described. These give direct results of grain size and

physical properties to thereby improve the quality of the final product.

More precise verification delivers higher quality

The results of laser cutting and electropolishing can be easily visually inspected by

anyone who possesses a good microscope and has some experience in the final

quality control of stents. Thus, laser cutting and electropolishing have been

recognised as the main quality factors in the manufacture of stents. However,

annealing is another key quality factor in the process chain of stent manufacture. The

annealing process determines the final material properties of the stent such as grainsize, ultimate tensile strength, break elongation, corrosion resistance, fatigue strength

and surface quality. Therefore, it is obviously not sufficient to specify only the

properties of the raw material.

The outcome of the annealing process cannot be determined by visual inspection,

but requires a more costly and demanding analysis of the material properties. In

addition, a detailed understanding of material properties and annealing is necessary.

The main aim of the following investigation was to improve the quality of annealing of

316LVM stents, mainly coronary stents. The final material properties of the stents

were to be improved and a reliable method for verifying the results of the annealing

process was to be identified.

Material properties

The quality of stents starts with the selection of the raw tube material. The two main

factors here are the dimensional accuracy and the material properties of the tubing.


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The dimensional accuracy includes the outer diameter and wall thickness tolerances,

the concentricity, the straightness of the tubing and the surface quality of inner and

outer surfaces. Of these dimensional parameters, only the inner and outer surface

quality can be improved at a later stage of the manufacturing process by

electropolishing.

The important material properties include material composition, purity or

microcleanliness (content of nonmetallic inclusions), grain size and hardness or

grade of strain hardening through cold work. These factors determine the physical

properties of the material including tensile strength, break elongation, ultimate tensile

strength, fatigue strength and other properties such as corrosion resistance. Most of

these material properties are influenced and finally set by annealing.

Stent tubes normally have a grain size of ASTM 7 to 8, which indicates grain sizes of

24-34 m. A typical cross section of a tube is shown in Figure 1. For the finished

stent, it is desirable to obtain the finest possible grain size. Generally, the wall

thickness of the tube should show a minimum of 8 to 10 grains. A finer grain size

reduces the risk of a stent strut breaking. The precipitation along the grain boundary

is also minimised and the fatigue resistance is improved.

Figure 1: Cross sections of a 3/4 hard stent tube

For tubes used for laser-cut stents, 3/4 is preferred because handling causes fewer

problems. Harder tubes are stronger and, therefore, often straighter, and fully heat

treated tubes can be bent easily. The outer surface of hard tubes is more scratch

resistant whereas fully annealed tubes are sensitive to scratching because of the

softer material.


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Figure 2 shows a typical stressstrain diagram of 3/4 hard tubing. The diagram

illustrates an ultimate tensile strength of more than 900 N/mm2 and a break

elongation of only 8-10 %. This low break elongation will almost certainly cause the

stent struts to break during dilatation in the human vessel. Because the struts are not

homogeneously expanded, strut regions with high distortion are critical and prone to

break. Therefore, stents manufactured of 3/4 hard tubes have to be annealed. With

the annealing, break elongations of usually more than 40% can be achieved, which

contribute to preventing the strut from breaking and to achieving high fatigue

resistance. Annealing also removes possible negative influences from the heat-

affected zone of the laser cut. In addition, the annealing process can be better

optimised and controlled for tiny stents than for long tubes.

0 2 4 6 8 10

0

200

400

600

800

1000

strain in %

stressinN/mm

Figure 2: Stress/strain-diagram of hard stent-tubing

Based on this knowledge, the use of 3/4 hard tubing is advocated instead of

annealed tubing as the raw material for stents. The annealing of hard stents must

produce the following material properties: small grain size, high break elongation in

combination, if possible, with excellent ultimate tensile strength. Indirectly, with these

parameters high fatigue strength and corrosion resistance are achieved. To optimise

all these properties an extensive investigation has been performed.


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Investigation of annealing

Until now, the results of the annealing of stents have been indirectly controlled by the

timetemperature curve of the furnace and, from time to time, by microhardness

measurements in the cross sections of annealed stents. Both procedures are

inappropriate. First, even if the timetemperature curve is correct, the result of the

annealing can be unacceptable as a result of tube raw material with bad metallurgical

properties (no absence of free ferrite phase, big gain size, low microcleanliness,).

Second, conforming with EN ISO 6507-1: Metallic materials - Vickers hardness test -

Part 1: Test method, 1997 the microhardness is measured as the area of indentation

of a small prism pressed into the material surface with a specific force. The standard

states a minimum distance, from the middle of the indentation mark to the edge of the

sample, of 2.5 times the diagonal of the indentation. For a tube with, for example, a

wall thickness of 100 m, the maximum permitted diameter of indentation is 20 m, if

the indentation is exactly in the middle of the cross section of the tube [1].

Economically speaking, this small indentation cannot be measured accurately.

Therefore, this testing method is not suitable and should not be permitted for

annealed stents. Finally, there is no reliable formula or diagram to convert the

measured hardness into material strength or break elongation. For these reasons,

the often applied microhardness measurement should be excluded from any stent

inspection.

The methods of choice

The methods the authors apply for analysing stent material properties are

microscopical grain examination and tension tests [2]. These methods give direct and

exact results of the grain size and physical properties such as ultimate tensile

strength and break elongation. To conduct the detailed analysis, stent tubes with

dimensions of 1.8 x 0.14 mm were cut into hundreds of short pieces. These short

tube samples are suitable for the tension tests; complete stents, because of their

flexible structure, cannot be tested in the tension machine. The short tube pieces

were annealed in two different furnaces: furnace 1 was a standard annealing furnace,

and furnace 2 was specially designed for the heat treatment of stents.

Furnace 1 can be considered to be the common method for annealing. It has beenused to anneal tens of thousands of stents. In the investigation, this standard furnace


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was operated according to the validated and certified annealing procedure for stents.

The results of this annealing process are given as a reference.

The aim of the investigation was to improve the results of the annealing process

achieved with the standard furnace 1, specifically with the help of the special furnace

2. For this, hundreds of annealing processes were completed with varying parameter

settings to identify the most suitable settings to obtain the best annealing result. The

main targets were to achieve a reliably high break elongation together with a

homogenously fine grain size, which is not necessarily the same, and a high tensile

strength. After the primary study, involving only one tube type (1.8 x 0.14 mm), other

tube dimensions were investigated.

To certify the annealing results, further experiments were conducted to verify the

temperature homogeneity of the furnace, the reproducibility of the annealing curves,

the influence of the annealing lot size, the influence of the position of single tube

piecesinside the furnace and the reproducibility of the tension tests. These auxiliary

experiments are not described in this article.

Results of the annealing

All results of the investigations are average values and not extremes. Each result has

been proved by several tests with the same annealing parameters.

Figure 3 shows the typical grain structure and size achieved with the standard

furnace 1. The original structure of the cold-drawn material has been totally

eliminated. The grain size of ASTM 8-9 (20-24 m) is already smaller than the

original raw tube material (compared with Figure 1). This grain size is acceptable

provided the tubes wall thickness is not too thin after electropolishing. However, for a

wall thickness or a strut size of less than 200 m, a finer grain size is recommended

to avoid negative effects of too few grains across the wall thickness of the strut,

especially a reduction in average break elongation. This negative effect has been

confirmed by experiments with tubing of thinner wall thicknesses. For a tube-wall

thickness of approximately 80 m, the average break elongation can be decreasedby half.


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Figure 3: Typical structure after annealing with furnace 1

The stressstrain curve of tube sections after standard annealing shows an ultimatetensile strength of approximately 650 N/mm2(Figure 4). The measured average

break elongation is 45% with a standard deviation of 4.6%. The extreme minimum

values for the break elongation are more than 39%. To be on the safe side, for stents

with this annealing, no strut or strut section should ever experience a distortion close

to or above this value.

0 10 20 30 40

0

200

400

600

strain in %

stressinN/mm

Figure 4: Stress/strain-curve after annealing with furnace 1

The results with the standard procedure of furnace 1 clearly show the necessity to

improve the annealing process, especially if stents are filigreed with extremely thin

struts. The main parameters for the optimisation were the time and temperature of

the annealing process. Both parameters strongly influence each other.

The cross section in Figure 5 illustrates the extremely fine grain size of ASTM 11 (9

m) obtained using the optimum process parameters for furnace 2. The grains are

only one third the size of the grains obtained after annealing in furnace 1. With this


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extremely small grain size, higher fatigue strength can be expected [3] and the

corrosion resistance will also be improved. [4] The break behaviour of the material

will be different, but in general improved [5].

Figure 5: Cross Section of the grain after optimised annealing with furnace 2

Figure 6 shows a typical stressstrain curve of annealing with furnace 2. With the

optimised annealing parameters the break elongation has been increased to average

values of more than 50% for stents made of 1.8 x 0.14 mm tubing, which is a

significant improvement. However, it must be stated here that the finest grain size

does not necessarily result in the highest break elongation. A compromise had to be

found during the investigation.

0 20 40 60

0

200

400

600

strain in %

stress

inN/mm

Figure 6: Stress/strain-curve after annealing with furnace 2

The standard deviation of the tension test results was reduced to 2.17 %, compared

with 4.6 % using furnace 1. Because reliability of the processes is essential when

manufacturing medical products, this improvement is of strong significance and


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confirms that the annealing process using furnace 2 and the optimum parameters

produce more stable results.

Finer grain sizes typically increase tensile strength. After annealing in furnace 2, the

ultimate tensile strength was raised by approximately 4 %. Another positive result of

the improved annealing process with furnace 2 is the better surface of the material

after testing in the tension machine. The tube surface after annealing with standard

furnace 1 and tension testing is rough and looks like orange peel (Figure 7). The

optimised annealing with furnace 2 results in a smooth surface after tension testing,

similar to the original, unstrained surface. The better surface is of significance for

fatigue strength, because smoother surfaces reduce the notch effect and crack

initiation during fatigue tests. A higher fatigue strength of expanded stents can be

expected [6].

after annealing with furnace 1 after annealing with furnace 2

Figure 7: Tube surfaces after straining in the tension machine

The better surface is also of significance for the electropolishing process. It makes no

sense to invest considerable effort in a high quality electropolished final surface of

the stent to reduce restenosis (the re-narrowing of a stent-treaded artery) (Figure 8),

if the smooth surface is afterwards destroyed during dilatation in the human vessel.

For stents annealed with furnace 1, strut sections with a high distortion show rough

surfaces after expansion. Stents that were annealed with furnace 2, have a smooth

surface after expansion.


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Recently, efforts have been undertaken worldwide to produce drug-coated stents to

reduce or eliminate restenosis. The achievement of a better surface is another

important benefit when applying thee coatings. First, for the active drug coatings, it

reduces the risk of ablations as a result of deformations of the surface during stent

expansion. Second, the possibility of cracks in passive coatings of stents, which have

to prevent any contact of the 316LVM with blood in the long-term, is minimised.

Figure 8: Electropolished surface of a stent

Importance of fine grain size

Until now, the investigation has concentrated on tube of 1.8 x 0.14 mm in dimension.Under normal circumstances, particularly after electropolishing, the wall thickness for

coronary stents is less. The investigation revealed a decrease in break elongation

after the annealing parts of a thinner wall thickness.

These results show the importance of fine grain size. Figure 9 demonstrates the

relationship between grain size, wall or strut dimensions and the total amount of

grains on the strut. The recommended amount of 10 grains across the cross section

of the strut can be achieved with the standard furnace 1 only if the strut dimensions

are bigger then 200 m.

Similar results were observed for the surface after straining in the tension machine.

With decreasing wall thickness, the surfaces were increasingly rough when the tube

had been annealed using the standard procedure with furnace 1. With the fine grain

size obtained by annealing in furnace 2, the surface roughness increased only

slightly after straining in the tension machine. It is particularly important to take this

effect into account for newer stents made of material with higher strength and


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subsequently less wall thickness. The thinner the wall thickness of the tube or the

smaller the strut dimensions, the more important it is to have a fine grain size.

WALL THICKNESS AND GRAIN SIZE

0,0

5,0

10,0

15,0

20,0

25,0

30,0

0,1 0,12 0,14 0,16 0,18 0,2 0,22 0,24

wall thickness / mm

totalamountofgrainsonwall

standard (grain size ASTM 8)

MeKo (grain size ASTM 11)

recommended > 10 rains on wall

Figure 9: Amount of grains in the strut cross section

Finally, it can be stated that tension testing of tubes is a suitable and reliable method

to inspect the results of the annealing process. Test pieces of tubes, with the same

tube lot number as the laser-cut stents, have to be annealed together with the stents.

The later tension test gives the exact final material properties of the stents. The test

can be performed quickly and is reliable.

The control of the grain size is more time and cost consuming, because cross

sections are required. Providing the curve of the tension test lies in a certain range, it

can be assumed that the grain size is fine and acceptable. The reverse assumption

that a fine grain size automatically provides a high break elongation is not valid.

Summary

The annealing process is an important key step in the manufacture of high quality

and reliable 316LVM stents. The methods commonly applied for verifying the

outcome of the annealing process such as microhardness testing are inappropriate


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and should not be used. The tension testing of tubes, processed together with stents,

provides reliable results of the final material properties of stents. During the course of

the investigation the grain size was reduced significantly and the break elongation

improved. The surface of the strain-tested material shows substantial improvements.

All results are particularly important for thin-wall stents with filigree struts.

References

1. EN 6507-1, Metallic Materials, Vickers Hardness Test, Part 1: Test Method,

1997.

2. EN 100002 Part 1, Metallic Materials, Tensile Testing, Part 1: Method of Test,

1990.

3. H.-J.Bargel and G. Schulze, Hg. Werkstoffkunde, Fifth Edition, VDI-Verlag,

Dsseldorf, Germany (1988).

4. W. Jniche et al., Verein Deutscher Eisenhttenleute. Werkstoffkunde Stahl.

Band 2: Anwendungen, Springer-Verlag, Berlin, Germany (1985).

5. W. Weissbach,Werkstoffkunde und Werkstoffprfung, 14 Edition, Vieweg-

Verlag, Wiesbaden, Germany (2002).

6. E. Hornbogen, Werkstoffe, Aufbau und Eigenschaften von Keramik-, Metall-,

Polymer- und Verbundwerkstoffen., Sixth Edition, Springer-Verlag, Berlin,

Germany (1994).

Dr. C. Meyer-Kobbe is President and Dipl.-Ing. Bernd H. Hinrichs is Senior R&D

Engineer at MeKo Laser Material Processing, Kthe-Paulus Strasse 9-11, D-31157

Sarstedt, Germany, tel. +49 5066-7079-0, fax +49 5066-7079-99, Email:

[email protected], internet: www.meko.de

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