Ostenitik paslanmaz çeliklerde tanelerarası korozyonun çekme dayanımına etkisi
The Effect of intergranular corrosion on tensile strength of austenitic stainless steels
- Tez No: 39522
- Danışmanlar: DOÇ.DR. ADNAN DİKİCİOĞLU
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 36
Özet
ÖZET Endüstride yaygın uygulama alanı bulmuş paslanmaz çelik türlerinden en önemlisi, ferritik çeliklere göre daha iyi kaynak kabiliyetinden dolayı ostenitik paslanmaz çeliklerdir. Ostenitik paslanmaz çeliklerin kaynaklı bağlantılarında dikkat edilmesi gereken önemli nokta, kaynak edilecek malzemelemin kimyasal bileşim ve içyapı olarak iyi tanınması ve önlemlerin alınmasıdır. Gerekiyorsa kimyasal analiz cihazları kullanılarak malzemenin alaşımı belirlenmelidir. Ostenitik paslanmaz çeliklerin kaynağında karşılaşılan problemlerden en önemlisi metalin tane sınırlarında oluşan krom karbür çökelmesi ve buna bağlı olarak ortaya çıkan tanelerarası korozyondur. Bu, ostenitik çeliğin 450-850° C arasında belirli bir süre kalması ile oluşur. Çeliğin içerisindeki karbon oranına bağlı olarak bu süre değişmektedir. Oluşan krom karbürün ağırlık olarak %90'ını krom oluşturduğundan, çeliğin içerisinde bulunan az miktarda karbon bile korozyon direncini düşürmeye yeterlidir. Tanelerarası korozyon sonucu malzeme özelliğini yitirerek kullanılmaz hale gelir. Ancak, sözkonusu olan yüksek zorlamalar altında çalışan bir malzeme ise, oluşan bu korozyon sonucu malzemenin dayanımı da düşecektir. Bu yüzden kaynaklı yapı güvenilirliğini yitirecektir. Bu noktada yola çıkılarak, tanelerarası korozyonun ostenitik paslanmaz çeliğin çekme dayanımına etkisi deneysel olarak incelemiştir. Bu amaçla deney malzemeleri temin edilerek bunların kimyasal bileşimleri analiz sonucu belirlenmiştir. Sonra, deney malzemeleri üzerinde tanelerarası korozyon oluşumu için, literatür taraması sonucu seçilen hızlandırılmış korozyon ortamları yaratılmıştır. Bu ortamlarda belirli sürelerde bekletilen deney parçalarına çekme deneyi uygulanmıştır. Alınan deney sonuçlan karşılaştırmalı olarak verilmiş, irdelenerek yorumlanmıştır. Alınan deney sonuçlarına göre, özellikle yüksek karbon oranlı molibdensiz ostenitik paslanmaz çeliklerde, oluşan tanelerarası korozyon sonucu çekme dayanımı düşmektedir.
Özet (Çeviri)
SUMMARY THE EFFECT OF INTERGRANULAR CORROSION ON TENSILE STRENGTH OF AUSTENITIC STAINLESS STEELS Today, the use of austenitic stainless steels in the industry is increasing. With the remarkable advantages to the carbon steels, against the higher price than the classical steels, from the food and refining industries to the manufacturing of vehicles in various applications stainless steels are used. Rather than the ferritic stainless steels, because of their improved voidability, austenitic stainless steels are used commonly. They can generally be defined as steels with at least 11-12% chromium and carbon contents which are normally below 0.15%. With increasing demands on corrosion resistance, however, the chromium contents should be increased to above 16%. Austenitic stainless steels show marked advantages here, particularly with regard to toughness properties and weldability. In stainless steels, the austenitic structure is generally produced by alloying with nickel. Nitrogen, a strong austenitizing element, has also gained increasing importance in recent years. For most types of corrosion an increase in the chromium content of the alloy above 18% will produce an improvement in corrosion resistance. A reduction in the carbon content to 0.030% max. or the stabilization of carbon by alloying with titanium or niobium will increase the alloys resistance against intergranular corrosion. Additions of molybdenum mainly improve the resistance to pitting and crevice corrosion. Increasing the nickel content in an austenitic matrix will increase the resistance to stress corrosion cracking, particularly in chloride containing media, where nickel based alloys with a content of more than 40% nickel give very good corrosion resistance values. On the other hand, ferritic matrix is also sensitive to chloride induced stress corrosion cracking. Finally, the controlled alloying with sulphur, lead or selenium must also be mentioned, a practice which improvesmachinability but greatly reduces weldability. For the improvement of strength and the structural stability, nitrogen may also be added. Unstabilized austenitic stainless steels permit welding with similar filler metals which produce an austenitic but ferrite containing weld metal. Steels with 2-3% molybdenum and grades with a low carbon content of 0.030% max. also belong to this group. These grades normally possess excellent weldability, provided they are welded with filler metals which yield an austenitic weld metal with delta ferrite contents in the range of about 5-15 FN. When welding these steels, however, it is necessary to follow certain procedures in order to achieve sufficient corrosion and cracking resistance and toughness value. It is common knowledge that austenitic steels are always subjected by the steel manufacturer to a solution annealing treatment, carbide, sigma phase and delta ferrite are completely dissolved and the annealing process produces a homogenous fully austenitic structure. With a subsequent quenching treatment, this state is maintained down to ambient temperature which means that austenitic steels show prior to welding an austenitic structure without any delta ferrite. If such a steel grade is welded without a filler metal, a weld metal is produced with a structure which can be determined from its position in the Schaeffler or DeLong diagram by calculating the chromium and nickel equivalents from the chemical composition of the steel grade. To obtain high hot cracking resistance, the weld metal structure produced should not be fully austenitic but rather austenitic with a delta ferrite content in the joint in the range of about 5-15 FN. It is apparent that the chromium content must be kept in the upper range and the nickel content in the lower range to achieve a sufficient delta ferrite content in the steel being fused by welding. For processing and cost reasons, steel manufacturers often set lower values of chromium, nickel and also sometimes of the delta ferrite content within the standard analysis limits of the grade. Therefore, it is recommended that the steel manufacturer should be contacted prior to welding large quantities of austenitic stainless steel if it is intended to weld the steel without the use of filler metal. In general, it is advisable that all welding shops which fabricate large quantities of austenitic stainless steel possess a ferrit measuring device together with the pertinent calibration standards. As previously mentioned, unstabilized austenitic stainless steels should usually be welded with matching filler metals. In some cases, where this is not possible, it is important to ensure that the weld metal is always more“noble”with regard to its chemical composition and corrosion resistance than the base metal. Then if due to the environmental conditions a simple galvanic cell may be formed between the differing compositions of the weld metal and the base metal, galvanic corrosion may occur but will be transferred to the much larger anodic surface area of the parent plate material, where it can quite often be ignored. If on the other hand, the parent plate material is, due to its chemical composition, more“noble”than the IXweld metal then the galvanic corrosion would be transferred to the weld metal which becomes the anodic region. Because the weld metal offers a by far smaller surface area than the rest of the welded component the corrosion here may be more severe. The solution annealing of finished components, can only be succesfully achieved on rare occasions. With the normal cooling rates experienced during electric arc welding for weld metals and base metals in the as welded condition with carbon contents up to 0.060% no sensitivity towards intergranular corrosion will occur. If possible, post weld heat treatment of welded components with the exception of solution annealing treatment should be avoided. If heat treatment cannot be avoided, however, special attention must be paid to the influence of carbide and phase precipitations. For an improvement in the yield strength, nitrogen alloyed austenitic stainless steels with 0.12-0.20% nitrogen can also be employed. In general these materials can be welded with the standard austenitic filler metals since the yield strength and ultimate tensile strength of the austenitic weld metal is sufficiently high in the as welded condition to match those of the nitrogen alloyed base metal. Care must be taken, however, to avoid excessive dilution with the base metal because of the austenitizing effect of nitrogen. These steels are often located relatively far inside the fully austenitic range of the Schaeffler or DeLong diagram which means that the delta ferrite content may drop too low in the diluted weld metal. When welding austenitic stainless steels greater distortion of the welded component must be expected, this is due mainly to the fact that the austenitic stainless steel have a higher coefficient of thermal expansion and lower thermal conductivity than the ordinary ferritic steels. Because of the lower thermal conductivity of austenitic stainless steels below 800° C, dissipation of the welding heat into the base metal takes place more slowly, therefore an accumulation of heat may occur in the weld which may lead to local overheating and severe distortion. Normally high deposition welding processes can also be employed for the welding of austenitic stainless steels, however, for the reasons stated above, an excessive degree of dilution must be avoided by selecting suitable welding parameters. When using these welding processes, the influence of the various shielding gases and fluxes upon the chemical composition of the weld metal must also be taken into consideration. This can be done by plotting the effect of the additional alloying vectors for the shielding gases or welding flux being used into the Schaeffler or DeLong diagram. In principle, the GMAW process should only be carried out using shielding gases with low CO2 contents, in order to keep the carburization of the weld metal by CO2 to a minimum. If carburation is too strong, the severe austenitizing effect of carbon may have a negative influence upon the hot cracking resistance of the weld and in the case of low carbon grades on the intergranularcorrosion resistance. Nitrogen additions to the shielding gas must be avoided because of the possible nitrogen pick up in the weld metal. In order to give the welded components full corrosion resistance, the welding operation must be followed by a suitable post weld surface treatment. This can be done by pickling-passivating, grinding or sandblasting. The pickling is done either by dipping the workpieces into solutions containing hydrofluoric and nitric acid or by more modern methods. The useful blasting method involves treatment with glass beads with diameter of 300 to 400 micrometers at a blasting perssure of 0.4 to 0.6 MPa (4 to 6 bar). Blasting without subsequent pickling results in relatively bad corrosion protection values, similar to the results obtained after grinding. Scale and heat affected zones are best removed by pickling. After pickling, surfaces should always be rinsed with cold water under high pressure, if possible with 10 to 15 MPa (100 to 150 bar). Passivating products increase the thickness of the passive layer. Under stress, the difference in the corrosion resistance depending on the surface treatment applied, can be seen. Modern pickling products are gel-like and thixotrope. Due to their different consistiencies, they can either be sprayed on or applied by brush. The spray pickle method cuts working time by up to 90%, compared to applying the product by brush, gives a uniform surface finish and high corrosion protection ail around. Environmentally, pickling has an advantage, as the harmful substances are collected in the rinse water and can be easily dealt with. For technical as well as economic reasons, pickling should be recommended as a cleaning method for corrosion and acid resistant steels. The most important problem in the welding of austenitic stainless steels is the intergranular corrosion related to the carbide precipitation. Carbide precipitation occurs at 450-850° C during a time of 1-60 minutes. This time is related to the carbon content in the austenitic stainless steel. 90% of the carbide is made by chromium by weight, so a small amount of carbon may drop the corrosion resistance of austenitic stainless steel. The effect of the intergranular corrosion is out of service following the loss of properties of material. This is not dangerous for small pressure storing tanks, pipe lines or similar equipments. But, in case of high pressure vessels, pipelines, the weld is subjected to high stresses. Intergranular corrosion on heat affected zone of those' welds will decrease the strength of material and then, the construction becomes non-reliable. Starting from this point, the effect of intergranular corrosion on tensile strength of austenitic stainless steels was investigated experimentally. For this purpose, experiment material from different austenitic stainless steels was obtained, chemical composition was analyzed. Welds were performed using suitable filler XImetals and correct welding parametres. Then test pieces were prepared by mecanically cutting in accordance to the related Turkish standard TS 287. For determining corrosion mediums to be used on experiments, the literature was studied: the necessary knowledge has been taken from the standards of TSE publications. Using nitric acid of 56% and sulphuric acid/copper sulphate mediums, 2 different accelerated corrosion mediums were created. Staying after 24 and 48 hours in those corrosion mediums, the experiment materials have been tension tested. Obtained results have been studied and commented. According to the results of tensile tests, specially on higher carbon contents in molybdenum free austenitic stainless steels, the occured intergranular corrosion drops the tensile strength. The decrease of tensile strength is related to the type of corrosion medium used in test. Also, time in corrosive medium has an effect on intergranular corrosion but for determining this, tests should be continued. Xll
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