The effect of heat treatment parameters on the structure and fatigue cracks growth in steel titanium bimetal

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1 Inżynieria Materiałowa 2 (210) (2016) DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING The effect of heat treatment parameters on the structure and fatigue cracks growth in steel titanium bimetal Grzegorz Kwiatkowski, Dariusz Rozumek * Department of Mechanics and Machine Design, Opole University of Technology, * d.rozumek@po.opole.pl The paper describes the influence of annealing temperature on the change in the structure and cracks growth and fatigue life in the bimetal titanium carbon steel obtained by explosive welding (cladding). Cyclic bending test was performed for different levels of annealing temperature on specimens of the bimetal having a rectangular cross-section and a fatigue crack growth was measured by microscopy. Structure changes of carbon steel, titanium and explosively welded area induced a different hardness distribution. It discloses the relationship between the level of the annealing temperature, the propagation of fatigue cracks, changes in the structure and hardness level. Key words: cyclic bending, microstructure, explosive welding. 1. INTRODUCTION Titanium is characterized by high corrosion resistance and great strength. Therefore, this material is often used in chemical, petrochemical, power, aircraft or pharmaceutical industry. Because of high costs, titanium is only spread in the form of a thin layer which is laid on the base material in the process of explosive welding. While cladding, the base material and titanium collide with a great velocity of about m/s [1]. As a result of the collision, the materials bond strengthening mainly the area of their joint. The process results in: generating additional stresses in both materials, changing the density and material structure, and increasing hardness. In order to eliminate the changes created in the process of welding i.e. the stresses, the formed bimetal is subject to heat treatment which consists in stress relief annealing. The selection of the temperature level, and the time of heating and annealing of the formed bimetal are aspects that need consideration since they are hindered by various properties of the used materials. This paper [2] discusses the above mentioned issue. The paper [3] presents a description of a bimetal consisting of steel as a base material and titanium as a overlaid material (layer), with a substantially thick interlayer between the joined materials. According to the conducted literature review, the processes of explosion welding were studied by the authors of the works [1, 4 6]. They described the detonation velocity as well as the generated high pressure which accompany explosion welding. Moreover, the paper [4] presents the results of hardness tests for cladding configurations for sheets arranged at various angles. The paper [5] discusses the method of calculation and selection of explosive material parameters for various thicknesses of the cladded materials. The paper [6] involves detailed information on explosion welding itself. The process of welding and the phenomena occurring in the process of cladding such as: welding configurations, detonation wave or plates collision velocity were described in detail. Whereas, the results of the studies on the effect of low values of equivalent thickness melted on the development of fatigue cracks were presented in the works [3, 9]. The purpose of this paper was to study the structural changes and fatigue life after cyclic bending of the steel titanium bimetal subject to a previous heat treatment with various temperature of annealing. 2. MATERIALS RESEARCH The studies involved specimens taken from plates made using the explosion welding method by Z.T.W EXPLOMET S.J., where the base material consisted of SA-516 Gr. 60 steel plate and the clad material of SB-265 Gr. 1 titanium (Fig. 1). The chemical composition of the materials before cladding (obtained from the certificate attached to the material) is presented in Tables 1 and 2 [7]. SA-516 Gr. 60 is a non-alloyed stainless steel also known as carbon steel, mostly used in various constructions, with ferrite-pearlite structure [8]. Gr. 1 titanium is a technical grade titanium with a single-phase α structure. In order to eliminate the internal stresses generated in the process of explosion welding, the bimetal was subjected to annealing at various temperature: 500 C, 600 C, 700 C, 800 C and 900 C and heating velocity of 90 C/h. Then, the materials were annealed for 2 hours and furnace cooled with the velocity of 90 C/h. Such ranges of temperature were selected in order to verify whether and Fig. 1. Join zone of steel titanium bimetal Rys. 1. Strefa złącza bimetalu stal tytan 70 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

2 at what temperature the phenomenon of diffusion occur in the area of bimetal joint. In the case of titanium, the selected temperature correspond to stress relief annealing, recrystallization and allotropic change of αti into βti [3, 7]. All specimens were taken from so called edge of detonation that is the place most distant from the explosion where, theoretically, they are of the poorest strength. 3. METHODOLOGY OF RESEARCH The specimens for metallographic research were taken from a section parallel to the detonation wave. The specimens were grinded by using abrasive paper with the grades between , and then polished using diamond pastes. Finally the titanium was etched in hydrofluoric acid solution, and steel in nital. The specimens for fatigue tests were cut off from sheet parallel to the direction of detonation. Each specimen has external notch with the radius of R = 22.5 mm. Specimen shape was obtained using milling and grinding with abrasive paper. In the tested specimens, the ratio (net) of titanium height to steel height amounted to h 1 :h 2 = 1:2.5. The shape and dimensions of the specimen is presented in Figure 2 [7]. The annealed specimens were subjected to metallographic research using light microscope OLYMPUS IX40 and computer image analysis system LECO IA 32 in order to determine the parameters of joint waves generated by the process of explosion welding. Figure 1 presents the area of joint, without heat treatment, in cross-section (magnification 50 ). The joint has a characteristic wavy structure with a number of penetration areas, microcracks and microvoids (black fields in Figure 1) in the wave hollow. At the top of the image, there is a titanium whereas steel is at the bottom [7]. The presented results are from Master s thesis developed at Opole University of Technology, Department of Mechanics and Machine Design in Opole, and they were performed on the fatigue test stand MZGS-100 (Fig. 3). The stand used in experiment allows to perform cyclic of bending, torsion and both combined [9]. The tests of cyclic bending were conducted within the range of low and high number of cycles with controlled force (in the considered case, the moment amplitude was controlled) and load frequency of 28.4 Hz. Each specimen had stress ratio of R = 1 and constant moment amplitude M a = 14.2 N m. Various rigidity Table 1. The chemical composition of the steel SA-516 Gr. 60, wt % Tabela 1. Skład chemiczny stali SA-516 Gr.60, % mas. C Si Mn P S Al. Cr Cu Ni Mo Nb Ti V / Table 2. The chemical composition of titanium SB-265 Gr. 1, wt % Tabela 2. Skład chemiczny tytanu SB-265 Gr. 1, % mas. Fe O N C H Ti of the materials joint by using explosion welding results in the fact that the neutral axis does not occur on the interface line, it is shifted, and it can be located in steel in the distance of 3.08 mm from the basis. The cracks propagation was observed by light microscope directly in the fatigue test stand (Fig. 3). Fatigue crack growth was measured on the lateral surfaces of the specimens by using a micrometer placed in a portable microscope, with a magnification of 25, accuracy of 0.01 mm, and simultaneous record of the current number of load cycles N. The cracks were measured until reaching the joint, then the authors observed whether the crack runs along the joint or crosses the joint and propagates in the second material [3]. 4. RESULTS OF RESEARCH A wavy form of the joint between the connected materials is a characteristic feature of explosion welding. Figure 1 also presents an interlayer (so called remelted layer) occurring in the wave hollows. During the tests, the authors measured joint parameters along the whole specimens lengths and calculated the equivalent thickness of remelted area (ETR) [6]. Its value should not exceed single-digit values (up to 10). Table 3 presents the measurements results of the joint. According to Table 3, all specimens had a great ETR coefficient. The reason for such great values of the equivalent thickness melted is too great collision velocity of the joining plates while welding. The ETR coefficient was determined based on a relation: n Pi i ETR = = 1 (1) L where: P surface of the melted areas, L interface line lenght. The created steel-titanium joints are characterized by a wavy form and great thickness of the remelted layer occurring on an interface line in the wave hollow. The specimens taken from bimetal were subjected to annealing for stress relief with a constant heating velocity, annealing time and furnace cooling velocity, at various annealing temperature from 500 to 900 C. Next, fatigue tests were conducted using cyclic bending. Moreover, the authors conducted metallographic research and measurements of hardness in a straight line perpendicular to the interface line. The metallographic test results were presented in Figures 4 9, and the distribution of hardness in Figure 10. Heat treatment of bimetal at the temperature of 500 C does not result in considerable changes in the structure of steel and titanium (Fig. 4). In the base material occurs a grow of hardness in the area of joint, about 245 HV0.1. On the other hand, it decreases to 180 HV0.1 when getting more distant from the area of the joint (Fig. 10). In the case of titanium, hardness decreases to 147 HV0.1 relative to the starting material (cladding plate without heat treatment, 202 HV0.1). Fatigue tests with cyclic bending proved that rest Fig. 2. Shape and dimensions of specimens for tests, dimensions in mm Rys. 2. Kształt i rozmiar próbek do badań, wymiary w mm Fig. 3. Fatigue test stand MZGS-100 with a measuring device Rys. 3. Maszyna zmęczeniowa MZGS-100 wraz z urządzeniem pomiarowym NR 2/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 71

3 Table 3. Parameters joint steel titanium Tabela 3. Parametry złącza stal tytan Treatment temperature C Average wavelength Average wave height h Measuring the length line of joint L Sum of the melted areas S 2 Value of the ETR coefficient Basic state , , Fig. 4. The microstructure and specimen crack after a heat treatment at temperature 500 C Rys. 4. Mikrostruktura i pęknięcie w próbce po obróbce cieplnej w temperaturze 500 C Fig. 6. The microstructure and crack specimen after a heat treatment at temperature 700 C Rys. 6. Mikrostruktura i pęknięcie próbki po obróbce cieplnej w temperaturze 700 C Fig. 5. The microstructure and specimen crack after a heat treatment at temperature 600 C Rys. 5. Mikrostruktura i pęknięcie próbki po obróbce cieplnej w temperaturze 600 C the crack was developing from steel and propagated inside the material to the interface line. The crack reached the interface line after cycles, where the appearance of another two cracks was observed, which propagated in titanium. The grow of heat treatment temperature up to 600 C causes titanium recrystallization, nucleation of new grains and their small Fig. 7. The microstructure and crack specimen after a heat treatment at temperature 800 C Rys. 7. Mikrostruktura i pęknięcie próbki po obróbce cieplnej w temperaturze 800 C growth (Fig. 5). In the case of steel, a minor soft skin can be observed in the area of joint and the development of the process of ferrite grains growth. The crack initiation occurred in steel part of bimetal [7]. In both materials, a decrease of hardness in comparison to the state of starting material (cladding plate without heat treatment) was observed, also decrease of fatigue life for bending in 72 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

4 Fig. 8. The microstructure and specimen crack after a heat treatment at temperature 900 C Rys. 8. Mikrostruktura i pęknięcie próbki po obróbce cieplnej w temperaturze 900 C comparison to the specimen after heat treatment at the temperature of 500 C (from cycles to cycles) was stated. The increase of heat treatment temperature up to 700 C resulted in further growth of titanium grains as well as ferrite in steel. Fatigue life for bending decreased to cycles (Fig. 6). The crack, as in the case of previous specimens, also initiate from steel and propagate towards the interface line. The hardness of both materials decrease compared to bimetal in raw state [7] and equals 122 HV0.1 for steel and 128 HV0.1 for titanium. The further increase of heat treatment temperature up to 800 C resulted in a great growth of ferrite grains in steel, where critical degree of deformation is located. In the case of titanium, grains growth can be observed. Fatigue life continues to decrease to cycles. The crack as in the case of previous specimens initiated from steel and propagated towards the joint. When crossing the area of joint, the crack developed along the joint (Fig. 7). The hardness of both Fig. 9. Crack length a vs number of cycles N Rys. 9. Długość pęknięć a w funkcji liczby cykli N materials continues to decrease to 113 HV0.1 (steel) and 123 HV0.1 (titanium) (Fig. 10) [7]. At the temperature of 900 C, a process of normalization with an intense grains refining on the whole structure of the material occurs. An allotropic change, α β and β α also occurs in titanium. Near the interface line, a needle-shaped martensitic-type structure can be observed as well as diffusion layers (Fig. 8). Titanium hardness increases in the area of the joint, probably as a result of diffusion phenomena and the creation of hard intermetallic phases TiC or Ti 2 C type. Further decrease of fatigue life was observed. The crack propagated from steel and reached the joint after cycles producing its separation [7]. Figure 9 presents an example of fatigue cracks development for the tested bimetal and for various heat treatment temperature Fig. 10. Hardness profiles of steel titanium joint after various heat treatment Fig. 10. Profile twardości złącza stal tytan po różnej obróbce cieplnej NR 2/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 73

5 in the number cycles function. In the examined specimens, the authors observed an initiation of fatigue cracks after cycles for the temperature of: 500 C, after cycles for the temperature of 600 C, after cycles for the temperature of 700 C, after cycles for the temperature of 800 C, and after cycles for the temperature of 900 C. Whereas the increase of fatigue cracks was measured from 0.15 mm (500 C), 0.14 mm (600 C), 0.18 mm (700 C), 0.15 mm (800 C) and 0.29 mm (900 C) every 2000 cycles. The cracks developed perpendicularly to the maximum normal stress plane both in steel and titanium. The development of a crack along the interface line was also observed for the annealing temperature of C. Figure 10 presents the results of hardness measurements in the function of distance from the interface line to about 2 mm. In the results of the conducted measurements, the authors found the highest hardness for steel in the starting state (without heat treatment) of 270 HV0.1 and for titanium treated at the temperature of 900 C 431 HV0.1. The highest hardness changes can be observed at a distance of 0.5 mm from the interface line (the place of the materials strengthening). By observing the effect of heat treatment on hardness changes in the area of the interface line, the authors observed that for steel annealed at the temperature of 500 C the hardness increases up to 245 HV0.1 whereas for other parameters of heat treatment, the hardness decreases to about 162 HV0.1. For titanium after heat treatment at the temperature range of ºC, a decrease of hardness to a value of about 150 HV0.1 can be observed. On the other hand, in the case of heat treatment at the temperature of 900 C, there is a clear increase of hardness to the value of 431 HV CONCLUSIONS On the basis of the metallographic research of bimetal consisting of carbon steel and a layer of titanium, in a starting state, after heat treatment including stress relief annealing at various temperature and cyclic bending, it can be stated that: 4. A relation between annealing temperature and fatigue cracks propagation was observed for the same heating conditions, annealing time and cooling conditions. Fatigue life, in the form of the number of cycles bending until failure, decreases along with the increase of annealing temperature. 5. The temperature of annealing greatly affects the hardness of the materials joined in the process of explosion welding. In the case of carbon steel, there is hardness decrease when compared to the material in a starting sate, along with an increase of annealing temperature, whereas, it does not change in titanium, except for annealing at the temperature of 900 C, after which it increases. 6. Annealing temperature causes changes in the metallographic structure of the tested bimetal. With the increase of annealing temperature, it is possible to observe an increase of dynamic recrystallization and an increase of the steel decarburization area within the produced joint. 7. The application of annealing at the temperature exceeding 600 C results in adverse changes in the structure which decrease the fatigue life. REFERENCES [1] Crossland B.: Explosive welding of metals and its application. Claredon press, Oxford (1982). [2] Pocica A., Bański R., Szulc Z., Gałka A., Waindok P.: Wpływ temperatury obróbki cieplnej na strukturę i właściwości połączenia stal tytan. Zeszyty naukowe. Mechanika, Politechnika Opolska (2008) [3] Bański R., Rozumek D.: Wpływ temperatury wygrzewania na trwałość zmęczeniową w bimetalach stal tytan otrzymanych metodą zgrzewania wybuchowego. Przegląd Spawalnictwa 4 (2014) [4] Deribas A. A., Kudinov V. M., Sobolenko T. M.: Shock hardening of low-carbon steel plates at variable impact angle. Combust, Explo, Shock Waves 3 (1967) [5] Nobili A., Masri T., Lafont M. C.: Recent developments in characterization of a titanium steel explosion bond interface. In: Proceedings of reactive metals in corrosive applications conference. Chang Wah, ed. Albany (1999) [6] Walczak W.: Zgrzewanie wybuchowe metali. Wyd. Naukowo-Techniczne. Warszawa (1989). [7] Kwiatkowski G.: Wpływ parametrów obróbki cieplnej na strukturę i właściwości złączy bimetalicznych stal tytan. Praca dyplomowa, Opole (2013). [8] Dobrzański L. A.: Metalowe materiały inżynierskie. Wyd. Naukowo- Techniczne, Warszawa (2004). [9] Bański R., Rozumek D.: Crack growth rate under cyclic bending in the explosively welded steel/titanium bimetals. Materials and Design 38 (2012) INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

6 Inżynieria Materiałowa 2 (210) (2016) DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING Wpływ parametrów obróbki cieplnej na strukturę i rozwój pęknięć zmęczeniowych bimetalu stal tytan Grzegorz Kwiatkowski, Dariusz Rozumek * Wydział Mechaniczny, Politechnika Opolska, * d.rozumek@po.opole.pl Słowa kluczowe: cykliczne zginanie, mikrostruktura, zgrzewanie wybuchowe. 1. CEL PRACY Celem pracy było zbadanie zmian strukturalnych oraz trwałości zmęczeniowej po cyklicznym zginaniu bimetalu stal tytan poddanego uprzednio wygrzewaniu w różnej temperaturze. 2. MATERIAŁ I METODYKA BADAŃ Badaniom poddano próbki pobrane z blach wykonanych za pomocą metody zgrzewania wybuchowego przez Z.T.W EXPLO- MET S.J., w których materiałem podstawowym była płyta stalowa SA-516 Gr. 60, natomiast materiałem nakładanym tytan SB-265 Gr. 1. Skład chemiczny materiałów przed połączeniem (z certyfikatu załączonego do materiału) przedstawiają tabele 1 i 2. W celu eliminacji naprężeń własnych powstałych w procesie zgrzewania wybuchowego plater został poddany wygrzewaniu w temperaturze 500 C, 600 C, 700 C, 800 C i 900 C z szybkością nagrzewania 90 C/h. Następnie materiały wygrzewano przez 2 h i chłodzono z piecem z szybkością 90 C/h. Taki zakres temperatury dobrano w celu sprawdzenia, czy i w jakiej temperaturze wystąpi zjawisko dyfuzji w strefie złącza plateru. Próbki do badań zmęczeniowych wycięto z blachy równolegle do kierunku detonacji. Każda z próbek miała nacięty zewnętrzny karb o promieniu R = 22,5 mm. Kształt i rozmiar próbki przedstawia rysunek 2. Wytrawione próbki badano metalograficznie za pomocą mikroskopu świetlnego OLYM- PUS IX40 z zastosowaniem komputerowego analizatora obrazu LECO IA 32 w celu określenia parametrów fal złącza powstałych w procesie zgrzewania wybuchowego. Rysunek 1 przedstawia w powiększeniu 50 strefę złącza bez obróbki cieplnej w przekroju poprzecznym połączenia bimetalu. Badania cyklicznego zginania wykonano w zakresie niskiej oraz wysokiej liczby cykli przy obciążeniu z kontrolowaną siłą i częstotliwością obciążenia 28,4 Hz. Dla każdej z próbek badania prowadzono przy współczynniku asymetrii cyklu R = 1 oraz stałej amplitudzie momentu M a = 14,2 N m. Propagację pęknięć obserwowano za pomocą mikroskopu świetlnego umieszczonego bezpośrednio na maszynie zmęczeniowej (rys. 3). Przyrost pęknięć mierzono na bocznych powierzchniach próbek za pomocą mikrometru znajdującego się w przenośnym mikroskopie przy powiększeniu 25, z dokładnością 0,01 mm oraz jednoczesnym notowaniu bieżącej liczby cykli obciążenia N. 3. WYNIKI I ICH DYSKUSJA Złącze stal tytan ma charakterystyczną budowę falistą z bardzo dużą liczbą obszarów przetopionych oraz mikropęknięć i mikropustek (czarne pola na rysunku 1) w zawinięciu fal. W górnej części zdjęcia jest widoczny tytan, a w dolnej stal. Na rysunku 1 jest również widoczna warstwa przejściowa (tzw. przetopienia) występująca w zawinięciach fal. W trakcie badań zmierzono parametry złącza na całej długości próbek oraz obliczono równoważną grubość przetopień (RGP), której wartość nie powinna przekraczać wartości jednocyfrowych (do 10). Wyniki pomiarów przedstawia tabela 3. Wynika z nich, że wszystkie próbki miały duży współczynnik RGP. Wyniki badan metalograficznych przedstawiono na rysunkach 4 9, a profil twardości pokazano na rysunku 10. Obróbka cieplna plateru w temperaturze 500 C nie powoduje znacznych zmian w mikrostrukturze stali oraz tytanu (rys. 4). Twardość w materiale podstawowym zwiększa się w pobliżu strefy złącza, ok. 245 HV0,1, a maleje do 180 HV0,1, oddalając się od strefy złącza (rys. 10). W przypadku tytanu twardość ulega zmniejszeniu do 147 HV0,1 w porównaniu z materiałem wyjściowym (plater nieobrobiony cieplnie 202 HV0,1). Badania zmęczeniowe przy cyklicznym zginaniu wykazały, że pęknięcie rozwijało się od strony stali i propagowało w głąb materiału do linii złącza. Pęknięcie dotarło do linii złącza po cyklach, gdzie zaobserwowano powstanie kolejnych dwóch pęknięć propagujących w tytanie. Zwiększenie temperatury obróbki cieplnej do 600 C powoduje rekrystalizację tytanu, zarodkowanie nowych ziaren oraz ich niewielki rozrost (rys. 5). W przypadku stali jest widoczne nieznaczne odwęglenie w obszarze strefy złącza oraz następowanie procesu rozrostu ziaren ferrytu. Inicjacja pęknięcia nastąpiła od strony stali. W obu materiałach następuje zmniejszenie twardości w porównaniu z materiałem w stanie wyjściowym (plateru bez obróbki cieplnej), a także zmniejszenie trwałości zmęczeniowej na zginanie w porównaniu z próbką po obróbce cieplnej w temperaturze 500 C. Zwiększając temperaturę obróbki cieplnej zaobserwowano, że trwałość zmęczeniowa w dalszym ciągu ulega zmniejszeniu. 4. PODSUMOWANIE Na podstawie przeprowadzonych badań przy cyklicznym zginaniu można stwierdzić, że: 1. Ujawniono zależność pomiędzy temperaturą wyżarzania, a propagacją pęknięć zmęczeniowych dla przyjętych tych samych warunków nagrzewania, czasu wygrzewania i warunków chłodzenia. Wraz ze wzrostem temperatury wygrzewania zmniejsza się trwałość zmęczeniowa, będąca liczbą cykli zginania do zniszczenia. 2. Temperatura wyżarzania ma znaczny wpływ na twardość materiałów łączonych w procesie zgrzewania wybuchowego. W przypadku stali węglowej następuje zmniejszenie twardości w porównaniu z materiałem w stanie wyjściowym wraz ze wzrostem temperatury wyżarzania. W tytanie twardość nie ulega zmianie, z wyjątkiem wyżarzania w temperaturze 900 C, po którym następuje jej zwiększenie. 3. Temperatura wyżarzania powoduje zmiany w mikrostrukturze plateru. Wraz ze wzrostem temperatury wyżarzania obserwuje się zwiększenie dynamiki rekrystalizacji oraz zwiększanie się obszaru odwęglenia stali w strefie wykonanego złącza. 4. Zastosowanie wyżarzania w temperaturze powyżej 600 C powoduje niekorzystne zmiany w mikrostrukturze skutkujące zmniejszeniem trwałości zmęczeniowej. NR 2/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 75