Sertleştirme ve menevişleme ısıl işlem parametrelerinin P91 çeliğinin mekanik özellikleri üzerine etkileri
Effect of quencing and tempering heat treatment parameters on the mechanical properties of P91 steel
Yazar: CANER TÖRE
Danışman: PROF. DR. CEVAT BORA DERİN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Metal malzemeler = Metal materials ; Yüksek sıcaklık malzemeleri = High temperature materials ; Çelik malzemeler = Steel materials ; Üretim metalurjisi = Production metallurgy

Özet:
Sürünmeye dayanıklı martenzitik çelikler termik ve nükleer enerji santrallerinde buhar boruları, kazan tüpleri ve başlıklar gibi yüksek sıcaklıkta çalışan parçalarda geniş kullanım alanı bulmaktadırlar. Bu çelik tipleri yüksek termal iletkenlik, düşük termal genleşme katsayısı, iyi korozyon ve oksidasyon direnci özelliklerine sahiptirler. Günümüzde enerji santrallerinde en çok kullanım alanı bulan çeliklerden bir tanesi P91 çeliğidir. Bu çeliklerin içeriğinde matrisi kuvvetlendirmek için krom (%8,5-9,5), molibden(%1), vanadyum ve niyobyum gibi alaşım elementleri bulunur.
Literatürde belirtilen ısıl işlem çalışmalarının genel olarak düşük kesit kalınlığına sahip boru ve plakalar üzerinde yapıldığı, enerji santrallerinde kullanılan döküm parçaları gibi yüksek kesit kalınlığına sahip parçalar üzerine ise detaylı çalışmaların henüz tam olarak gerçekleştirilmediği görülmektedir.
Bu tez çalışması kapsamında P91 çeliğinin farklı ısıl işlem şartlarındaki mekanik özellikleri incelenmiştir.
9Cr çelikleri olarak da adlandırılan bu çeliklerde mukavemet sağlayan en önemli mekanizma çökelti sertleşmesidir. Alaşıma yüksek sıcaklıkta çözeltiye alma ve ardından su verme işlemi uygulanarak aşırı doymuş katı çözelti elde edilir. Daha sonra uygulanan menevişleme işlemi ile ikincil fazların çökelmesi sağlanır. İnce ve yoğun bir şekilde dağılmış M23C6 ve MX tipi karbür ve karbonitrür çökeltileri çeliğe mukavemet verir. Bu çeliklerin yüksek sıcaklıktaki özellikleri oluşan çökeltilerin şekil, boyut ve dağılımlarına göre belirlenir.
Bu çalışma kapsamında üretilen numune takozları 2 gruba ayrılarak ilk grup numuneler östenizasyon sonrası polimerde soğutulmuş, ikinci grup numuneler ise fanda soğutulmuştur. Daha sonra her iki gruba ait numuneler 760, 780 ve 800 °C olmak üzere 3 farklı sıcaklıkta menevişleme işlemine tabi tutulmuştur. Isıl işlem şartlarının malzeme özelliklerine etkilerini incelemek amacıyla herbir numune takozuna sertlik testi, çekme testi ve darbe çentik testleri uygulanmış ayrıca mikroyapı incelemeleri yapılmıştır.
Sertlik dağılımını görebilmek için numune takozunun tam ortasından yüzeyden içeri doğru 30 mm aralıklarla sertlik ölçümleri yapılmıştır. Yapılan ölçümler sonucunda sertliğin yüzeyden içeri doğru gidildikçe düştüğü görülmüştür. Elde edilen en yüksek sertlik değeri fanda soğutulup 800 °C'de menevişlenen numunede, en düşük sertlik ise polimerde soğutulup 760 °C'de menevişlenen numunede ölçülmüştür. Ayrıca fanda sertleşen numunelerin polimerde sertleşenlere göre daha yüksek sertlik değerlerine sahip oldukları tespit edilmiştir.
Mekanik özelliklerin tespiti için bütün numune takozlarından 4'er adet çekme testi yapılmıştır. Yapılan çekme testi sonuçlarına göre akma ve çekme mukavemetinin menevişleme sıcaklığı artıkça arttığı, uzamanın ise düştüğü görülmüştür. Ayrıca östenizasyon sonrası fanda sertleşen numunelerin polimerde sertleşenlere göre daha yüksek mukavemet değerine sahip olduğu görülmüştür. Östenizasyon sonrası soğuma ortamının uzama değerlerinde dikkate değer bir fark yaratmadığı anlaşılmıştır. En yüksek akma ve çekme mukavemeti fanda sertleşip 800 °C'de menevişlenen numunede, en yüksek uzama değeri ise polimerde sertleşip 760 °C'de menevişlenen numunede elde edilmiştir.
Isıl işlem şartlarının malzeme tokluğuna olan etkisini görmek için bütün numunelerden 4'er adet darbe çentik testi yapılmıştır. Yapılan test sonuçlarına göre darbe mukavemetinin menevişleme sıcaklığı arttıkça düştüğü görülmüştür. Ayrıca polimerde sertleşen numunelerin fanda sertleşenlere göre daha yüksek tokluk değerlerine sahip oldukları tespit edilmiştir. En yüksek tokluk değeri polimerde sertleşip 760 °C'de menevişlenen numunede elde edilmiştir.
Yapılan mikroyapı incelemelerinde döküm sonrası yapının martenzit ve kaba karbürlerden oluştuğu görülmüştür. Hem polimerde hemde fanda yapılan serleştirme işlemleri sonrası yapıdaki bütün karbürlerin matris içersinde çözündüğü, tamamen martenzitik matris elde edildiği görülmüştür. Menevişleme sonrası tane sınırları ve lata sınırlarında ince karbür çökeltileri elde edilmiştir.

Abstract:
Creep resistant martensitic 9Cr steels find wide use in thermal and nuclear power plants in high temperature parts such as steam pipes, boiler tubes and headers. These steel types have high thermal conductivity, low coefficient of thermal expansion, good corrosion and oxidation resistance. P91 steel is one of the most widely used steels in power plants. These steels contain alloying elements such as chromium (8.5-9.5%), molybdenum (1%), vanadium and niobium to strengthen the matrix.
Studies in the literature are mainly focused on the parts which have low section thickness such as pipes and plates, and there is no detailed study on pieces with high section thickness. Since the cast parts used in power plants generally have high section thickness, it was necessary to carry out such a study. In this study high section thickness sampling blocks were subjected to various quenching and tempering operations and effect of these heat treatments on mechanical properties were investigated.
Chemical composition of P91 steel must be optimized to obtain 100% martensitic structure and free from δ-ferrite. The elements present in the composition play an important role in microstructural stabilization during creep exposure. Carbon increases creep strength by forming carbides and carbonitrides. Since high amounts of carbon cause a decrease in toughness values, it is usually added in small amounts. Chromium is one of the main alloying elements in P91 steel. It combines with carbon and creates M23C6 carbide precipitates. The addition of 9-10% of chromium provides good hardenability, high creep strength and good corrosion resistance. Molybdenum is a ferrite stabilising element and improves the mechanical properties of P91 steel by solid solution hardening effect. The addition of molybdenum more than 1% contributes to the formation of laves phase and δ-ferrite, which impairs the high temperature properties of P91 steels. Vanadium and niobium are strong carbide, nitride and carbonitride forming elements of type MX (M: V, Nb; X: C, N). MX precipitates resist the degradation of the microstructure at high temperature by inhibiting dislocation motion and decreasing the rate of dislocation recovery. Nitrogen, like carbon atoms, is present as an interstitial element in the Fe lattice and stabilises austenite. It also contributes to the increase of creep strength by stabilising MX carbonitride precipitates. In the presence of aluminium in steel, nitrogen combines with aluminium instead of vanadium and niobium to form aluminium nitride. Since aluminium nitride is a coarser phase than vanadium and niobium nitrides, it is detrimental to the creep properties of steel. Nickel stabilises austenite in steel and promotes coarsening of the laves phase and carbide phases, resulting in a decrease in creep strength. Therefore, the amount of nickel in steel should be less than 0.4%.
The most important mechanism that provides strength in these steels is precipitation hardening. A supersaturated solid solution is obtained by applying the alloy to solution at high temperature and then quenching. Subsequently, the secondary phases are precipitated by the tempering process. Fine and densely dispersed carbide and carbonitride precipitates of type M23C6 and MX give strength to steel. The high temperature properties of these steels are determined by the shape, size and distribution of the precipitates formed.
M23C6 particles play a very important role in inhibiting grain boundary movements at high temperature. Coarse M23C6 precipitates are formed at the prior austenite grain boundaries and slat boundaries, while fine M23C6 is formed within the laths. Coarse M23C6 precipitates have a detrimental effect on long-term creep properties and should be avoided. The coarsening rate of chromium carbides at high temperature is much higher than that of V and Nb. For this reason, it is crucial to reduce the carbon content in 9Cr steels. This promotes the formation of very fine V, Nb nitrides which are stable at high temperature for a long time.
MX type carbonitrides are formed in the presence of strong carbide and nitride forming elements. V, Nb, C and N elements support the formation of fine MX type precipitates. These precipitates are in the size range of 25-50 nm and have a face-centred cubic structure. These precipitates improve creep strength of the steel by resisting the movement of dislocations, delaying the plastic deformation by inhibiting grain boundary sliding, retaining finer grains during austenitization, and delaying the onset of tertiary creep stage. Therefore, the formation of fine MX-type precipitates is vital for achieving high creep strengths in 9Cr steels.
δ-ferrite is the first solid phase that forms during solidification. This phase is softer than the martensitic matrix structure and has a negative effect on mechanical properties. The formation of δ-ferrite during solidification or solution heat treatment causes a decrease in the strength, ductility and toughness properties of heat resistant 9Cr steels. Therefore, δ-ferrite formation should be avoided. Chromium equivalent is an important parameter used to assess the risk of δ-ferrite formation in high chromium steels. Creq should be <10 to prevent δ-ferrite formation. δ-ferrite may remain in the structure as a result of the inability to disperse microsegregations formed during solidification due to insufficient hardening heat treatment. If the amount of segregation during solidification is too high, the δ-ferrite formed may not be removed by heat treatment.
One of the most important problems experienced by 9Cr steels during high temperature use is Z-phase precipitation. This new phase is formed as a result of the continuous dissolution of MX particles as a result of long-term exposure to high temperature, and the Z-phase precipitates grow very rapidly.The Z phase is a complex nitride in the form of Cr(Nb,V)N. The formation of this phase occurs when the material is kept above 550 °C for a long time. Z phase is stable on cooling at equilibrium conditions and forms at lower temperatures than M23C6 and MX phases. Z phase formation is not observed at high cooling rates. Z phase precipitation is thought to occur by two different mechanisms. The first one is the formation of the Z phase by precipitation on vanadium and niobium rich nitrides at the PAGB and package boundaries. The other mechanism is related to the diffusion of chromium from the matrix structure into MX precipitates. After the Z phase nucleates in the MX precipitate, it grows very fast and swallows the grain. The effect of chromium on Z phase precipitation is very high compared to vanadium and niobium. The high chromium content creates a great driving force for Z phase precipitation. Therefore, Z phase precipitation is much faster in 12Cr steels than in 9Cr steels.
Laves phase is an intermetallic phase with the composition ((Fe,Cr)2(Mo,W)) formed by the combination of Mo and W with Fe and Cr. The Laves phase does not precipitate during the tempering process like the MX and M23C6 phases, but begins to form during creep exposure in the 600-650 °C range. The intermetallic laves phase precipitates during creep. The precipitation of the laves phase reduces the amount of Mo dissolved in the matrix and thus decreases the solid solution hardening effect. Studies have shown that the laves phase has a high growth rate during creep exposure. Laves phase forms at the grain and lath boundaries at high temperature and increases in size with time. In short-term creep exposure, the laves phase has a positive effect on creep strength. However, since the coarsening rate of the laves phase is very high, it has been observed that it negatively affects the creep strength in long term.
The sample blocks produced within the scope of this study were divided into 2 groups, the first group samples were cooled in the polymer pool after austenization, and the second group samples were cooled under the fan. Then, the samples belonging to both groups were subjected to tempering process at 3 different temperatures as 760, 780 and 800 °C. In order to examine the effects of heat treatment conditions on material properties, hardness test, tensile test and impact notch tests were applied to each sample block, and microstructural examinations were also carried out.
In order to see the hardness distribution, hardness measurements were made from the middle of the sample wedge towards the inside of the surface at 30mm intervals. As a result of the measurements, it was observed that the hardness decreased as one went inward from the surface. The highest hardness value obtained was measured in the sample that was cooled in the fan and tempered at 800 °C, and the lowest hardness was measured in the sample cooled in the polymer and tempered at 760 °C. In addition, it was determined that the fan-cured samples had higher hardness values than the polymer-cooled samples.
In order to determine the mechanical properties, 4 tensile tests were performed on all sample blocks. According to the tensile test results, it was observed that the yield and tensile strength increased as the tempering temperature increased, while the elongation decreased. In addition, it was observed that the specimens cooled in the fan after austenization had higher strength values than those cooled in polymer. It was observed that the cooling environment after austenization did not make a remarkable difference in the elongation values. The highest yield and tensile strength was obtained in the sample that was hardened in the fan and tempered at 800 °C, and the highest elongation value was obtained in the sample that was hardened in the polymer and tempered at 760 °C.
In order to see the effect of heat treatment conditions on material toughness, 4 impact notch tests were performed on all samples. According to the test results, it was observed that the impact strength decreased as the tempering temperature increased. In addition, it was determined that the polymer-cooled samples had higher toughness values than the fan-cooled samples. The highest toughness value was obtained in the sample that was hardened in the polymer and tempered at 760 °C.
In the microstructure investigations, it was observed that the post-casting structure consisted of martensite and coarse carbides. It was observed that all carbides in the structure were dissolved in the matrix after the quenching processes in both the polymer and the fan, and a completely martensitic matrix was obtained. After tempering, fine carbide precipitates were obtained at the grain boundaries and lath boundaries.