Koroner Stent Üretiminde Kullanılan 316 Lvm Paslanmaz Çeliğin Tavlama Özelliklerinin Geliştirilmesi

Abstract

Günümüzde kalp damar hastalıkları en büyük ölüm nedenidir. Kötü beslenme, sigara, stres, şehirleşme bu hastalıkları tetikler. Açık kalp ameliyatı (bypass) gerektirmeyen koroner (kalp ile ilgili) damar tıkanıklıklarını tanılama işlemi olan anjiyografi ve tıkanık damarı açma işlemi olan balon anjiyoplasti ve sonrasında tekrar tıkanma riski olan damarları stentleme işlemleri yıllardır kullanılagelmektedir. Stentleme işleminde kullanılan stentler genellikle 316 LVM paslanmaz çelik, kobalt krom alaşımları, nikel titanyum alaşımları ve soy metallerdir. Bu malzemelerin arasından 316 LVM üstün mekanik özellikleri, biyouyumluluğu, korozyon direnci, östenitik tek fazlı yapısıyla stent malzemesi olarak en çok tercih edilen metalik stent malzemesidir. Günümüzde stent üretimi sırasıyla lazer kesim, asitle temizleme, tavlama, elektroparlatma şeklinde olur. Lazer kesimle içi boş tüpler lazer kesim programına tanıtılan stent tasarımına göre kesilir. Asitle temizleme işlemi ise lazer kesim sırasında oluşan curuf, dros ve diğer kalıntıların giderilmesini sağlar. Tavlama işlemi lazer kesim sırasında oluşan ısı tesiri altında bölgelerin (ITAB) giderilmesini, mikroyapının eski haline döndürülmesini, stentin tüp aşamasında olduğu gibi orijinal yumuşak haline dönmesini sağlar. Bir sonraki aşama olan elektroparlatma ise stentin damar içinde herhangi bir ters etki yaratmayacak şekilde pürüzsüz olmasını ve stent strat (stentte bir tepe noktasından onu izleyen çukur noktaya kadar olan metal parçası) genişliğinin azalmasını sağlar. Son aşamada ise stent üzerinde kalıcı oksit tabakasının oluşturulduğu pasivasyon işlemi yapılır. Bu işlem basamaklarından kuşkusuz en önemli olanı tavlama ısıl işlemidir. Tavlama stentin son mekanik özelliklerini, mikroyapısını, korozyon ve yorulma direncini doğrudan etkileyen bir işlemdir. Tavlama ile stente istenen özellikler kazandırılabilir. Bu çalışmada 316 LVM stent malzemesine farklı tavlama rejimleri uygulanmıştır Böylece farklı tavlama rejimlerinin mikroyapıya ve son mekanik özellikleri etkisi görülmek istenmiştir. Deneylerin sonucunda mevcut tavlama rejimi ile üretilmiş numunelerden daha ince taneli mikroyapıya ve son mekanik özelliklere sahip numuneler elde edilmiştir.Coronary artery diseases (CAD) is one of the common reason of death at the present time. Fatty diet, smoking, stress, civilization lead to CAD. Angiography that is a diagnostic procedure of occluded vessel is needed to coronary artery bypass graft (CABG) or interventional cardiology procedure. Invertional cardiology is much more safe than CABG. Interventional cardiology procedure takes extremely short time, patient could be discharged from a hospital in same day. Pain is also very less than CABG. Angioplasty is a rewidening of occluded vessel with balloon catheter and stenting is a different type of scaffold reduces restenosis risk. These three methods have been used for a long time by interventional cardiologists. Interventional cardiologist could decide to do anjioplasty procedure after angiography considering occlusion situation of vessel. Even in some operations, physician could decide stenting procedure in some vessels that have a strong restenosis risk after a while. Angiography is a diagnostic procedure that is conducted with 0.038” guidewire and diagnostic catheter. Related vessels are displayed with contrast medium injection in x-ray by physician. After diagnostic procedure, physician uses 0,038” guidewire, guiding catheter, 0,014” guidewire and ballon catheter in angioplasty. Balloon catheter is used for rewidening of partially or totally occluded vessel. Stenting is practised if physician predict a restenosis risk in treated vessel. 40%-50% restenosis risk could be seen in angioplasty procedures without stenting. This rate can be reduces to 20%-30% using stents even under 10% with drug eluting stents. Physician should select appropriate stent and balloon catheter diameter and length considering occluded vessel. Coronary stents should provide lots of requirement at same time by reason of such a critical application. The one of the most significant property that stent should have is biocompatibility. Neointimal hyperplazia is extremely important disadvantage of insufficient biocompatible stents. Body tissues can detect stent material as a foreign matter. Tissues could proliferate on stent surface very vigorously. Stent material should be easily seen in presence of x-ray in other words should be radiopaque. Stent material should be MRI (magnetic rezonans imaging) compatible in point of procedure safety. Vessel media is corrosive for stents in spite of the fact that corrosion potential of stent material should be less. Stents are applied sistolic and diastolic pressures of vessel continuously so stent material should has high fatigue resistance. Stent material should also ve more flexible to easily get a shape of vessel. Flexible stent could pass even a tortuous anatomies. Widening rates of stent strat should be equal in all sides of stent. This prevents unexpected vessel dissections. Plastic deformation pressure value of stent material should be high, in other words, radial strength of material should be high to resist radial forces inside vessel. Stent material should be selected considering material property which has a less recoil and foreshortening rates. This phenomenas could be hazardous after implantation. Stent materials should have low yield point for providing operable crimp procedure. In contrast with this, stent material should have high tensile strength to resist physical effects of vessel. Elasticity modulus is another key factor that provides usage of thinner struts. When thinner struts are used, restenosis risk will reduce. Finally, stent materials should have fine and equiaxed grains to provide fatigue and corrosion resistance. 316 LVM stainless steel, Co-Cr alloys, nitinol, precious metals are commonly produced for stent applications. Precious matals can be preferable with their high biocompatibility, radiopacity and MRI compataibility, but they are insufficient in aspect of mechanical properties. Nitinol is used for production of self-expendable stents. This type of stents is implanted to vessel using their phase transformation from room temperature to body temperature. Co-Cr alloys are one of the most widely used stent material with their radial strength, tensile strength and elasticity modulus. 316 LVM is the most significant stent material with its superior mechanical properties, biocompatibility, corrosion resistance and single phase austenitic microstructure. Most of FDA approved stents are made of 316 LVM, this shows the importance of 316 LVM as a stent material. Stent production includes laser cutting, pickling –also called acid descaling-, annealing, electropolishing and passivation respectively. Hollow tubes are cut based on computer aided stent design in laser cutting. Laser cutting is a microfabrication technique that the focused beams are sent to target for local melting/evaporation in the deepness of required section. Melted material is removed from kerf width by using aid gas like oxygen or inert gas. Lazer power, focus position, pulse duration, cutting speed, cutting frequency, type of aid gas are important parameters that should be take into account for well-qualified cutting process. Narrow kerf width and low HAZ formation are one of the necessities after cutting process. Pickling is a necessary step to remove slag, dross and all other contaminants from stent surface. Different acide solutions, temperature and time values could be selected according to producer’s preference. Anneling process is the most important process affects stent functionality deeply. Annealing specifys mechanical properties such as microstructure, grain size, elengation, tensile strength and also corrosion resistance, fatigue resistance. Required final mechanical properties and microstructure could be obtained appropriate heating, holding and cooling temperatures and times. After annealing microstructure should not contain any intermetallic phase and precipitate. Annealing also provides to be removed HAZ that occurs in laser cutting. Electropolishing step continues after annealing. Electropolishing leads to extremely smooth surface for such a critical application. Electropolishing is also efficient about reducing stent strat width and stent weight. Different acide solutions, current values, temperature values, time values, voltage values could be used for obtaining extremely smooth surface not to cause adverse effects such as neointimal hyperplasia. Passivation step after electropolishing increases stability of chromium oxide layer of stainless steel stent material to resist agressive body media. In this study, different annealing regimes have been conducted to 316 LVM stent material then microstructure analysis and tensile tests, which is applied for comparing mechanical properties, have been conducted. Thereby, the effects of different regimes to stent microstructure and mechanical properties have been observed. Moreover, characterization of current stent production has been overviewed. As a first process, temperature mapping study of heat treatment system has been conducted. Temperature inspection in every part of annealing furnace has been checked. After this study which was done with different thermocouples, temperature range was found almost same in all parts of furnace. ICP-AES chemical analysis has been done to be sure chemical content of stent material. This test was done to different batch numbers. All constituents ensued suitable according to related ASTM standard. Different holding temperatures and holding times were applied to stent tubes. Holding temperatures were specified between 950 C°-1100 C°. Holding times were specified between 10 min-60 min. This regimes has been specified considering technical litarature about stent production and stainless steels. Annealed tubes have been applied to metalografic preperation including molding, grinding, polishing and chemical etching and grain size determination. Avarage grain size determination has been done according to grain size estimation test methods that is given in related ASTM standard. After grain size determination; set 1 was selected holding temperature 950 C°, holding time 10 minutes, set 2 was selected holding temperature 950 C°, holding time 30 minutes, set 3 was selected holding temperature 1000 C°, holding time 10 minutes, set 4 was selected holding temperature 1000 C°, holding time 30 minutes. After grain size determination avarage grain size of set 1 is 23, 42 µm, avarage grain size of set 2 is 25,30 µm, avarage grain size of set 3 is 26,19 µm, avarage grain size of set 4 is 30,17 µm. Avarage grain size of samples that were produced with current annealing regime is 34,95 µm. The most important way to see the effects of different annealing regimes to mechanical properties is tensile testing. Ten samples have been taken for eight annealing set. All tensile tests were applied according to related ASTM standard. Snug fitting metal mandrels have been put the lumen of metal tubes to prevent crushing inside load cells of tensile test machine. Test outputs were emerged as ultimate tensile strength, yield strength, uniform elengation and break elengation. As a result, set 1 yielded 585, 1 MPa ultimate tensile strength 67,6% uniform elengation, set 2 yielded 613,9 MPa ultimate tensile strength, 67,1% uniform elengation, set 3 yielded 596,6 MPa ultimate tensile strength, 67,6% uniform elengation. Additionally, metalographic examination of current stent production has been done. Laser cut and pickled samples have similar grain size withal annealed and electropolished samples have similar grain size. Morphological characterization of production steps samples also have been performed. After examination of all test data, set 1-4 have smaller avarage grain size than samples of current production. This situation has been provided with low holding temperature in set 2 and set 4, both low holding temperature and holding time in set 1 and set 3. İndefinite grain boundaries and twin boundaries have been observed in all microstructure visuals which are typical properties of austenitic stainless steel. As a result of tensile testing, tensile strength of set 1-4 samples are higher than current production method samples. Tensile strength values have been increased not to sacrifice uniform elengation in set 1,2 and 3. All tensile test results are consistent to Hall-Petch equation which gives relationship between yield strength and grain size. In the characterization of current production steps, partially cut pattern has been observed in laser cut samples not to use ultra-pulsed laser techniques. Sharp strat edges have been also observed. Dross particles sticked to bakalite in laser cut process has been removed by pickling process. Grain growth has been seen an important issue in annealed samples. Strat widht has been reduced and sharp edges have been eleminated in electropolished samples. When SEM visuals examine carefully, dross and slag that was adhered in laser cutting could be seen easily. Cut zones have white colors because of oxygen assist gas. All dross and slags have been removed after pickling but surface roughness espicially in cut surfaces has been observed. This surface rougness could be removed by electropolishing. Grain struıcture could be seen clearly after annealing. As a consequence, set 1,2 and 3 samples have better properties in point of both grain size/structure and mechanical properties.