Browsing by Author "Sarikka, Teemu"

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  • Characterisation of local grain size variation of welded structural steel
    (2016-07-01) Lehto, Pauli; Romanoff, Jani; Remes, Heikki; Sarikka, Teemu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Previously, it has been shown that the grain size distribution plays an important role in the mechanical properties of welded steel. In the previous investigation, the volume-weighted average grain size has been shown to capture the influence of grain size distribution, resulting in a better fitting Hall–Petch relationship between grain size and hardness. However, the previous studies exclude the effects arising from local variation in grain size. In this paper, the grain size measurement methods are extended for the characterisation of the local grain size variation, which is significant for welded joints and can have an adverse effect on mechanical properties. The local gradient of grain size variation and its dependency on measurement direction are considered. In addition, examples of grain size and hardness variation are shown for S355 base metal and two weld metals, and characteristic differences are highlighted and discussed. The coarse-grained areas of a heterogeneous microstructure are found to have lower hardness than fine-grained areas. However, the surrounding microstructure, i.e. local grain size gradient, has an influence on the measured hardness values.
  • Comparison of â Versus a and Hit/Miss POD-Estimation Methods : A European Viewpoint
    (2019-12-01) Virkkunen, Iikka; Koskinen, Tuomas; Papula, Suvi; Sarikka, Teemu; Hänninen, Hannu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    For estimating the probability of detection (POD) in non-destructive evaluation (NDE), there are two standard methods, the so-called a versus a approach and the hit/miss approach. The two approaches have different requirements for the quality and quantity of input data as well as for the underlying NDE method. There is considerable overlap between the methods, and they have different limitations, so it is of interest to study the differences arising from using each methodology. In particular, if the dataset is not ideal, the methodologies may exhibit different problems dealing with various limitations in the data. In this paper, a comparison between a versus a and hit/miss analysis was completed for two different data sets, a manual aerospace eddy-current inspection and a nuclear industry phased array ultrasonic weld inspection using a simplified online tool. It was found that the two standard methods (a vs. a and hit/miss) may give significantly different results, if the true hit/miss decision is based on inspector judgement and not automated signal threshold. The true inspector hit/miss performance shows significant variance that is not attributable to signal amplitude. Model-assisted POD was not able to model the inspector performance due to lack of representative amplitude threshold and difficulties in capturing true signal variance. The paper presents experience from practical cases and may be considered a European viewpoint.
  • Crevice corrosion of stainless steels 904L, 2205 and 2507 in high-temperature sulfuric acid solution containing chlorides: Influence of metal cations
    (2018-02) Lindgren, Mari; Huttunen-Saarivirta, Elina; Peltola, Heljä; Romu, Jyrki; Sarikka, Teemu; Hänninen, Hannu; Pohjanne, Pekka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Crevice corrosion influences the integrity of materials in many industrial sectors, such as hydrometallurgy. The conditions in hydrometallurgical processes are severe, with high concentrations of sulfuric acid leachant as well as high chloride impurity, oxygen and metal cation contents. In this study, crevice corrosion behaviour of highly-alloyed austenitic and duplex stainless steels is investigated in aerated high-temperature sulfuric acid solution with 1 g/l chlorides in the presence of three types of metal cations: Fe3+, Cu2+, and Fe2+. The experimental work contained determination of the oxidizing capacity of the solutions, immersion crevice corrosion tests followed by characterization of the corrosion attack, and potentiodynamic polarization measurements. The results showed that the alloy with the highest PREN-value, 2507, was not susceptible to crevice corrosion under the test conditions. In the case of alloys 904L and 2205 of relatively equal PREN-values, the extent and nature of crevice corrosion attack was different and dependent on the ratio of activities of chlorides to sulfates and the type of metal cations. The results are presented and discussed in the light of IR drop mechanism.
  • Effect of Feedback and Variation on Inspection Reliability
    (2017-12-01) Virkkunen, Iikka; Haapalainen, Jonne; Papula, Suvi; Sarikka, Teemu; Kotamies, Juha; Hänninen, Hannu
     A4 Artikkeli konferenssijulkaisussa
    The reliability of Non-destructive testing (NDT) is an on-going challenge. The consequences of failed inspections can be dire, and thus the requirements for NDT reliability are very high. The work is technically demanding and requires skilled use of the available equipment and keen judgement to properly discern flaw signals from noise. Somewhat paradoxically, the work is also very tedious and repetitive. Most of the inspected targets do not contain any flaws but the inspectors need to be constantly alert for the possibility. The recent studies on human factors have brought advances in (among other things) improved readability of inspection procedures and procedures of reviews and redundant inspections widely used in order to improve overall inspection reliability. In present paper, the effect of feedback and variation on inspector performance is studied. To test this, a small empirical study was completed. An online tool was created with a simplified UT set-up: B-scan image of the data and software gain control and tools to indicate cracks with point and click. The system generates random flawed data images on the fly. The user then analyses the images and indicates found flaws by clicking them. After 150 images have been analysed (many of them without flaws), the system uses the provided hits and misses to compute a POD curve and confidence bounds using standard (ASTM E2862) techniques. Additional "learning" version of the tool was created. In this "learning mode", after user requests next image, the system shows results of the current images (i.e. hits, misses and false calls in the current image). This set-up provides the inspector with direct feedback of his success and better facilitates learning this particular inspection task. The tool was presented to a small group of 9 inspectors in level-III inspector training and results were gathered from trainees both before and after training (and with and without feedback). The results from this small group of inspectors indicated, that the direct feedback on achieved reliability can quickly improve POD values. However, the study group was small and thus the results need further investigation and confirmation.
  • Effect of processing temperatures on the properties of a high-strength steel welded by FSW
    (2018-11-01) Sorger, Gonçalo; Sarikka, Teemu; Vilaça, Pedro; Santos, Telmo G.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The need for weight reduction without compromising load-bearing capacity has driven the development of modern high-strength steels (HSS). The ability to take full advantage of these modern HSS is limited by their weldability. Friction stir welding (FSW) can join HSS at low peak temperatures, better preserving the properties of the base material. This work evaluates the influence of different FSW peak temperatures and cooling rates on the properties of a modern HSS. A HSS produced by a thermo-mechanically controlled process was welded by FSW with peak temperatures, measured within the processed zone, ranging from about 900 to 650 °C. Temperatures were measured using thermocouples positioned at the mid-thickness of the workpiece, at 5 to 10 mm from the joint line. The effect of the different peak temperatures was evaluated by mechanical testing, including bending, tensile testing with digital imaging correlation, Charpy impact test, and hardness measurements. Microscopic analyses, including optical microscopy and electron backscatter diffraction, were used to evaluate the microstructure. Results show that FSW enables welding HSS within the intercritical temperature domain. The impact toughness of the best weld condition overmatched the base material by over 37%, at both − 40 and − 60 °C. The yield strength efficiency was about 70%.
  • Effect of processing temperatures on the properties of high strength steel welded by FSW
    (2017) Lourenco Sorger, Goncalo; Sarikka, Teemu; Santos Vilaca da Silva, Pedro; Santos, Telmo G.
     A4 Artikkeli konferenssijulkaisussa
  • Effect of strength mismatch on fracture behavior of ferrite-austenite interface in Ni-base alloy dissimilar metal welds
    (2016) Sarikka, Teemu
     School of Engineering | Doctoral dissertation (monograph)
    Dissimilar metal welds (DMW) are extensively used in energy production applications, such as nuclear power plants (NPP), to join the ferritic steel nozzles of reactor pressure vessels (RPV) to the austenitic stainless steel pipings using different types of safe-end designs. Traditional, V-groove safe-end designs made using filler metal Alloys 82/182 are being replaced by modern safe-end designs manufactured using narrow gap (NG) welding technique with filler metal Alloy 52/152. The higher Cr-containing Alloy 52/152 provides improved corrosion resistance, especially resistance to stress corrosion cracking (SCC), and NG technique provides technical and economical benefits.  The difference in filler metal and NG technique require thorough understanding of the metallurgical and mechanical properties of the resulting weld as the changes may have an effect on the structural integrity of the DMW. The objective of this thesis was to determine the microstructural properties, mechanical properties, and mechanical mismatch across a ferritic-austenitic interface of a traditional V-groove DMW and a modern NG DMW. The mechanical mismatch was determined using tensile testing and microhardness and nanoindentation testing. The different mismatch states were compared to the fracture mechanical behavior of the ferritic-austenitic interface.  Microstructural properties of the different DMWs were found to differ from each other in terms of the existence of a carbon-depleted zone (CDZ) and a narrow carbon pile-up near the fusion boundaries of the DMWs. The existence of the CDZ and carbon pile-up caused high mechanical mismatch at the fusion boundary region of the NG DMW and affected the fracture propagation behavior near the fusion boundary region of the NG DMW. The mechanical mismatch was observable with nanoindentation testing, but the tensile tests even with small-sized specimen did not reveal the high mechanical mismatch near the fusion boundary region.
  • Effect of thermal ageing at 400 °C on the microstructure of ferrite-austenite interface of nickel-base alloy narrow-gap dissimilar metal weld
    (2020-03-24) Ahonen, Matias; Mouginot, Roman; Sarikka, Teemu; Lindqvist, Sebastian; Que, Zaiqing; Ehrnstén, Ulla; Virkkunen, Iikka; Hänninen, Hannu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Dissimilar metal welds (DMWs) are a key design feature in nuclear power systems, typically involving ferritic low-alloy steels (LAS), stainless steels (SS), and nickel-base alloys. They are, however, a potential concern regarding the structural integrity of nuclear power systems. In particular, the LAS/nickel-base alloy weld metal interface is known to develop a local strength mismatch upon post-weld heat treatment (PWHT). Very limited data is available on the effect of thermal ageing on the DMW interface. The aim of this study was to investigate the effects of thermal ageing at 400 °C for up to 10,000 h on a narrow-gap DMW mock-up representative of the weld between the reactor pressure vessel nozzle and its safe-end after PWHT, with a special focus on the LAS SA 508/nickel-base Alloy 52 weld metal interface. No significant effect of thermal ageing on the appearing microstructure was observed in either LAS base material, LAS heat-affected zone or Alloy 52 weld metal. However, thermal ageing reduced the local strength mismatch at the LAS/nickel-base weld metal interface formed during PWHT. The reduction of the strength mismatch was detected using nanoindentation measurements and was concluded to be associated with a decrease in the carbon pile-up in the weld metal caused by PWHT. Based on the obtained results, thermal ageing promotes carbon diffusion from the weld metal side of the fusion boundary further away into the weld metal and thus slightly decreases the local strength mismatch.
  • High performance carbon-based printed perovskite solar cells with humidity assisted thermal treatment
    (2017) Hashmi, Syed; Martineau, David; Dar, Mohammad Ibrahim; Myllymäki, Teemu T. T.; Sarikka, Teemu; Vainio, Ulla; Zakeeruddin, Shaik Mohammed; Grätzel, Michael J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report humidity assisted thermal exposure (HTE) as a post-treatment method for carbon based printed perovskite solar cells (CPSCs). The method does not only improve the interfaces of different layers of the printed stack, but also provides a pathway to fabricate high performance CPSCs with low hysteresis along with high stability. The HTE treatment directly influences over the associated components in the stack and remarkably improves each photovoltaic parameter of the CPSCs as seen by several characterization schemes presented in this study. The average initial efficiency (9.0% ± 0.2%) of the CPSCs of a batch was significantly improved to 13.1% ± 0.2% i.e. as high as 45% when subjected to HTE treatment for a period of 200 hours. Furthermore, the highest average efficiency obtained from the same batch from reverse scanning was 13.8% ± 0.4% with a CPSC attaining as high as 14.3% when exposed to the same thermo-humid environment for a period of 115 hours. Above all, the stability of the HTE treated CPSCs was also not compromised for over 350 hours under full-sun illumination stress testing at 40 °C. The results presented in this work provide an opportunity to adopt HTE treatment as a complementary step for the fabrication of high-performance carbon-based perovskite solar cells with low hysteresis accompanied by high durability and performance reproducibility.
  • Hydrogen-induced delayed cracking in TRIP-aided lean-alloyed ferritic-austenitic stainless steels
    (2017-06-03) Papula, Suvi; Sarikka, Teemu; Anttila, Severi; Talonen, Juho; Virkkunen, Iikka; Hänninen, Hannu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Susceptibility of three lean-alloyed ferritic-austenitic stainless steels to hydrogen-induced delayed cracking was examined, concentrating on internal hydrogen contained in the materials after production operations. The aim was to study the role of strain-induced austenite to martensite transformation in the delayed cracking susceptibility. According to the conducted deep drawing tests and constant load tensile testing, the studied materials seem not to be particularly susceptible to delayed cracking. Delayed cracks were only occasionally initiated in two of the materials at high local stress levels. However, if a delayed crack initiated in a highly stressed location, strain-induced martensite transformation decreased the crack arrest tendency of the austenite phase in a duplex microstructure. According to electron microscopy examination and electron backscattering diffraction analysis, the fracture mode was predominantly cleavage, and cracks propagated along the body-centered cubic (BCC) phases ferrite and α'-martensite. The BCC crystal structure enables fast diffusion of hydrogen to the crack tip area. No delayed cracking was observed in the stainless steel that had high austenite stability. Thus, it can be concluded that the presence of α'-martensite increases the hydrogen-induced cracking susceptibility.
  • Inspection of Carbon Fibre – Titanium – Carbon Fibre Stepped-Lap Joint
    (2018-08-01) Patronen, J.; Stenroos, Christian; Virkkunen, Mikko; Papula, Suvi; Sarikka, Teemu
     A4 Artikkeli konferenssijulkaisussa
    The optimal combined use of composites and metallic material is essential for the performance of modern tactical aircrafts. At the same time, the resulting structures include complex metal-to-composite joints that may develop failures during use. The inspection of such structures is often highly demanding. The present paper discusses the complex inspection of stepped-lap joint between carbon fibre wing panel and titanium attachment of tactical aircraft. The joint has been found to be susceptible to disbonding both in the outer and inner titanium to carbon fibre interfaces. Due to the multi-material multi-layer structure, the inspection is particularly demanding, especially for the inner interface. Successful inspection can be developed by careful analysis of the ultrasonic signal. A flawless joint on the outer interface will result in phase change at the interface, that can be seen on the RF signal. A disbond prohibits this phase change and thus can be detected by noting the absense of this phase change. Defects bigger than 4 mm can be detected using this effect. Disbond on the outer surface will prohibit sound from traveling to the inner surface. Consequently, disbond on outer surface can be detected by noting a decreased amplitude in the inner-interface echo. A disbond on the inner surface will, likewise, prohibit sound from going through the interface and thus increase reflection from that interface. Consequently, a disbond on the inner surface can be detected by noting an increase in the amplitude of the inner-interface echo. Thus disbonds on both inner and outer surfaces can be detected by monitoring possible decrease or increase in the inner surface echo indicating disbond in the outer or inner surface, respectively.
  • LCC- ja LCA-menetelmien käyttö materiaalinvalinnassa
    (2009) Sarikka, Teemu
     Insinööritieteiden ja arkkitehtuurin tiedekunta | Bachelor's thesis
  • Materials Science and Technology - Nuclear Materials, Advanced Course - Kon-67.5100 Postgraduate Seminar on Engineering Materials - Seminar papers 8 October, 2015
    (2015) Hänninen, Hannu (editor); Kiesi, Timo (editor); Kiesi, Timo; Lindqvist, Sebastian; Leskelä, Esa; Lindroos, Vesa; Pohja, Rami; Rantanen, Ville; Salenius, Elina; Sarikka, Teemu
     School of Engineering | C2 Toimitettu kirja, kokoomateos, konferenssijulkaisu tai lehden erikoisnumero
    The Engineering materials research group of Department of Engineering Design and Production of Aalto University arranged a postgraduate course on nuclear materials. The course consisted of three day long lecture session given in April 20-22, 2015. Lectures were given by professionals from nuclear power related research institutes (Aalto and VTT), nuclear industry and authority. The course also included a seminar session held October 8, 2015. The seminar session was targeted to postgraduate students, who prepared articles from their field of expertise. This proceeding is the collection of these seminar articles.
  • Nanoindentation study of short-range ordering
    (2019-04-01) Mouginot, Roman; Sarikka, Teemu; Ehrnstén, Ulla; Hänninen, Hannu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Thermal ageing of nickel-base alloys can lead to the formation of brittle ordered phases, but the direct study of these phases is challenging. Nanoindentation is used in this study as an alternative technique to determine the extent of short-range ordering (SRO) in thermally aged Alloy 690 TT. Two methods are used for obtaining both qualitative (depth sensitivity) and quantitative (spatial stability) results, which are compared to data from metallography, microhardness and atomic force microscopy in order to discriminate between factors affecting the hardness. When intergranular precipitation and grain size become dominant factors with increasing indentation loads, nanoindentation at low loads enables to distinct within-grain hardness increase related to SRO.
  • Producing a POD curve with emulated signal response data
    (2018-01-01) Koskinen, Tuomas; Virkkunen, Mikko; Papula, Suvi; Sarikka, Teemu; Haapalainen, Jonne
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Ultrasonic testing is an important tool for assessing the structural integrity of pressurised components in nuclear power plants during in-service inspection. Consequently, the reliability of inspection is of high signi cance. In particular, it is important to determine the largest crack that could conceivably be missed during in-service inspection. This information is utilised in order to choose the most e ective method for di erent situations. Probability of detection (POD) curves are used to quantify the e ectiveness of the inspection. However, these POD curves require a lot of data points in order to provide reliable estimates of the lower limit performance. Traditionally, obtaining these curves has been relatively expensive and this is why di erent simulation tools have been used to reduce costs and the number of physical test-pieces needed. In the current study, a novel method based on limited test-pieces is explored to produce a POD curve from the measured data. The present data contains just three arti cial cracks made with thermal fatigue. This is insu cient to produce a POD curve. However, the data was su cient to evaluate the novel approach and to gain preliminary estimates of the expected POD and the most important in uential parameters. In this study, the idea is to emulate the amplitude response from the measured crack in a way that represents an amplitude response from a certain crack size. This amplitude data is converted to a B-scan image from which the inspector will evaluate whether there is a crack or not. Then, a POD curve is generated from the achieved hit/miss data.
  • Properties of ferritic Fe-Cr-Al, alumina-forming austenitic, and Fe-Cr-Si alloy coatings at elevated temperatures
    (2012) Sarikka, Teemu
     School of Engineering | Master's thesis
    Conventional heat-resistant stainless steels used in the critical components of a fossil power plant rely on chromium oxide scales for protection against high-temperature oxidation. In order to achieve higher efficiency of the plants, the operating temperatures of the power plants must increase. The chromia (Cr2O3) scales may no longer provide a sufficient protection against high temperature corrosion attack in temperatures well above 600°C and, thus, new approaches are needed to replace the usage of chromia as the provider of the high-temperature corrosion resistance of the construction materials. Two of the most widely explored replacements to the chromium oxide in iron-based high temperature alloys are aluminium oxide (Al2O3) and silicon oxide (SiO2). The purpose of both alloying elements, Al and Si is to form a healing layer of either aluminium oxide or silicon oxide to protect the structural material against high-temperature corrosion attack. The main goal of this thesis is to study the formation of aluminium and silicon oxide layers on the surface of the aluminium or silicon containing high-temperature alloy and to assess the durability of the oxide scale in chlorine-containing environment. The studied materials in the experimental part were alumina forming, ferritic Kanthal A-1 alloy (nominal Al composition of 5.8 wt. %), manually composed FeCr-Al alloy with a nominal Al composition of 10 wt. %, and silica-forming Fe-12Cr-2Si alloy. Manufacturing of Kanthal A-1 cladding was successful with the Al and Cr concentrations quite evenly distributed throughout the weld. Kanthal A-1 was able to form external, continuous aluminium oxide scale on its surface during the oxidation at 950°C and the peroxidised scale performed well in KCl-tests. Manufacturing of Fe-17Cr-10Al was more challenging. The Al and Cr concentrations had significant variation throughout the weld, i.e., the compositions measured from the first pass of the sample were totally different than those measured from the other pass of the sample. Fe-17Cr-10Al was not able to form continuous aluminium oxide scale during the oxidation test and in the KCl-tests; it formed a thick oxide scale with multiple different oxide layers present. Manufacturing of Fe-12Cr-2Si cladding was also successful; however, due to the lower Cr composition of the welding filler metal it was necessary to weld multiple passes on the surface of the base material in order to achieve the desired amount of Si and Cr addition. Fe-12Cr-2Si cladding formed a thin, continuous scale of chromium oxide with particles of silicon containing oxides on its surface during the oxidation at 950°C. Preoxidation of the Fe-12CR-2Si coating did not seem to have an effect on the performance of the coating in KCl containing environment.