Modern structural design, commonly called optimal design, is based on the knowledge of material behaviour, numerical structural analysis and high computer capacity. Engineering structure is designed, manufactured, maintained and controlled in order to guarantee that it does not exhibit any failure. The designer, the manufacturer and the user of the product expect that the structure will operate without any problem and that no failure will appear. However, many failures can occur during engineering practice and each of them has its cause of origin and form (mode) of manifestation (expression). It is of importance to determine why and how an engineering component has failed. The analysis of failures is an important aspect of engineering because it deals with discovering the causes of failures. When the cause of the failure is specified and the mode of failure manifestation is recognized, then an answer to the question why and how and what component has failed is known. The knowledge based on failure analysis provides better understanding of the causes of the occurrence of failures and the cost of such failures. At the same time, this knowledge presents a basis for design and manufacture improvements, preventing us thus from repeating similar mistakes in the future. Many decisions in structural design are based on engineering judgement as a result of theoretical understanding of the problem and engineering experience. Namely, some of engineering decisions come from successful designs, and also from the known historical failures in design. Commonly observed failure modes in engineering practice are  force/temperature-induced elastic deformation, yielding, creep, wear, fatigue, corrosion, and so on. Common causes of failure may include misuse, improper material, design errors, manufacturing defects, assembly errors, improper maintenance, unforeseen operating conditions (e.g. high temperatures), poor communications between designers, fabricators and professionals involved in maintenance, compressed design time, and so on [2, 3]. In any case, an aspect of engineering dealing with the ability of a structure to support a designed load without failure has become a new engineering discipline known as structural integrity. It is no doubt that structural design is based on the knowledge of both material properties and material behaviour at given environmental conditions. Some of the material properties such as yield strength or hardness, for example, are called structure-sensitive properties and depend on the microstructure affected by means of its development and control. Cold or hot rolling are some of the mentioned means and by using these processes the microstructure will result in its final form. Since that load can cause plastic deformation of the structural element, the knowledge about yield strength is of importance to avoid this failure. Also, another possible plastic deformation may occur and it can be caused by creep. Creep is usually defined as time-dependent behaviour/phenomenon, where deformation continuously is increased while the stress (load) is kept constant . Creep as one of possible failure modes was experimentally studied in this investigation. The knowledge about material resistance to creep is of importance to materials to be used in high-temperature conditions. In engineering practice, only 1–2 % of creep strains is allowable. The creep occurrence can be expected at the temperature above 0.4 Tm where Tm is melting temperature . The aim of this research is to provide engineering information about behaviour of the material subjected to different temperature conditions. Data we are dealing with are related to mechanical properties and material creep resistance. In general, creep behaviour of metallic materials is usually represented by a creep curve consisting of three prominent fields classified into three major stages: (a) transient or primary creep stage, (b) steady-state or secondary stage and (c) accelerating or tertiary creep stage.
Some of recent information related to behaviour of investigated material
Some information related to behaviour of studied material can be found in literature. In Ref. , fatigue properties were determined by means of conventional fatigue tests and the data were collected to S–N curves. Since reactive air brazing is treated as an advanced joining process in the field of joining ceramics to metal, such processes between selected braze alloys and base materials were investigated . Based on the obtained results, the reaction mechanisms as well as selection of filler metals and brazing temperature can be explained. In these investigations, steel 1.4841 was also used as base material. In Ref. , the oxidation behaviour of 1.4841 steel with the content of Cr and Si similar to material investigated in this paper was studied during isothermal oxidation at 1,287 K in air. In engineering practice, the problem of connecting steel and ceramic parts is widely considered. In these cases, as a solution, only the gastight joint can be considered. Due to operating conditions, this joint has to be stable in any manner. These demands can be met with brazed joints produced by reactive air brazing. Joining of BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3−δ) to 1.4841 steel was considered in Ref. . The perovskite (BSCF/Ba0.5Sr0.5Co0.8Fe0.2O3−δ) is a ceramic that can be used as oxygen transport membrane (OTM) material for oxygen supply in high-temperature applications. For the mentioned membrane, reactive brazing in air atmosphere may be considered as technically applicable solution for a heat-resistant and gas-tight sealing to metals. In this case, when BSCF is brazed to the heat-resistant steel 1.4841, some changes in BSCF microstructure arise . However, investigations conducted in this study differ from previously mentioned ones and provide information that can be of importance to designers dealing with structural design made of 1.4841 steel. To compare the material properties of the studied material with the properties of other materials, possibilities of material use and material behaviour at different temperatures, thus providing information about the best choice of material, it is of interest to have a look at studies that are presented in Refs. [11–34]. Also, some information of the deformation behaviour of this material for different environmental conditions (temperatures) can be found in Ref. .
Fundamental information about considered material and testing
Material and its possible applications
Material under consideration was austenitic heat-resistant steel 1.4841, known as X15CrNiSi25-20 steel according to EN or as X15CrNiSi25-21 steel according to DIN, or as AISI 314 steel. As received material it was annealed and cold worked in 16 mm round bar. Material chemical composition in mass (%) is as follows: C (0.0271), Cr (24.06), Ni (20.24), Si (2.73), Mn (1.61), Cu (0.244), V (0.209), Mo (0.2), Sb (0.0971), Al (0.0391), Nb (0.0259), P (0.0254), S (0.01) and rest (50.4824). High-temperature stainless steels (heat-resistant steels) have been usually designed for temperature up to 1,423 K, and this resistance has been achieved by the addition of a number of alloying elements in the steel. In this sense, performances of these steels can be ensured across a wide range of high-temperature applications. The mentioned performances include appropriate mechanical properties at high temperatures, resistance to high-temperature corrosion, creep strength, etc. Austenitic steels are commonly employed in applications where temperatures exceed 823 K.
Applications of considered steel (1.4841) are in areas such as petrochemical, shipbuilding, cement and aerospace as well as automotive industry. Specific areas such as engineering conversion plants, industrial furnaces, steam boilers, etc., are also domains of its applications. However, it is widely used in high temperatures and corrosion environments. Tendency of this steel towards the embrittlement can be manifested in the temperature range of 873–1,123 K.
Testing equipment, procedures and standards
Results of the research presented in this paper were obtained on the basis of two types of tests: the uniaxial tensile tests and the impact energy tests. The basic material properties were obtained by using uniaxial tensile tests performed with tensile testing machine capacity of 400 kN while with Charpy impact machine the impact energy was obtained. When high-temperature uniaxial tests were conducted, furnace (1,183 K) and high-temperature extensometer were used. Round specimens of 5 mm in diameter manufactured from 16 mm round bar were used in uniaxial tests. Tensile tests were performed as follows: at room in accordance with the ASTM: E 8M-11 standard and those related to high temperature in accordance with the ASTM: E21-09 standard while uniaxial creep tests in accordance with the ASTM: E 139–11 standard. Charpy impact tests were carried out according to the ASTM: E23-07ae1 standard. Aforementioned standards can be found in Ref. .
Experimental results and analysis
Engineering stress–strain diagrams
Engineering stress–strain diagrams were obtained as a result of uniaxial tensile tests. Based on these tests, values of mechanical properties of materials as tensile strength, yield strength and modulus of elasticity were determined. Several tensile tests were performed for each of the test temperatures. The shapes of the diagrams for considered temperature as well as values of considered properties do not differ from each other significantly, and because of this reason, in Figure 1 the diagram of the first test for each test temperature is presented. The results as numerical values are presented in Table 1.
Graphical representation of material properties versus temperature
In Figure 2(a)–(c), experimental results are shown using special characters (■, ♦), while curves as polynomial approximation of these experimental results are shown using a solid or dashed line. The coefficient of determination (R2) is used as a measure of matching between experimental results and polynomial approximation and it serves as statistics that gives information about how fit a model is . Graphical representation of ultimate tensile strength and yield strength versus temperature is presented in Figure 2(a); representation of modulus of elasticity is presented in Figure 2(b), while representation of elongation and contraction in cross-sectional area of the specimen is presented in Figure 2(c).
As it is known, on the basis of experimentally obtained results, the designer can assess whether the material with such properties may be a good choice for design of structural element which is to be subjected to a specific load and temperature conditions. In this sense, since this material is optimized to possess good mechanical properties, it is visible, based on experimental results that its tensile strength and yield strength are of high level in the temperature range from room temperature till 973 K. Also, it is visible that both strengths and modulus of elasticity constantly are decreased with an increase in temperature.
Strength versus strain rate
Based on the experience, for different materials, the applied strain rates (areas) of stress–strain diagram are prescribed for different phases. However, it is also known that the mechanical properties change with the strain rate. Taking into account the possibilities of the used testing machine, some uniaxial tests were conducted in such a way that at constant considered temperature strain rate was changed. Based on these tests, the changes in tensile strength and yield strength depending on the strain rate are presented in Figure 3. The differences in the level of strength are more visible at the temperature of 1,123 K.
Based on these tests, it is visible that both of considered properties (tensile strength and yield strength) are slightly increased with increasing strain rate.
Short-time creep and creep modelling
In engineering practice, two types of processes may be mentioned: real processes and predicted/modelled/simulated processes. Real processes such as technological processes, experimental processes or similar are always the best data that can be used in assessment of the behaviour of considered structure, machine or process. Since in many cases obtaining such information is expensive or time-consuming, based on known data about structure behaviour, it is possible to assess the structure behaviour in new environmental conditions. In this case, it is said that a new process is predicted/modelled/simulated on the basis of previously known data/behaviour. In this sense, using some known models, the creep behaviour of steel 1.4841 at the creep process defined in Figure 4(b) is modelled. In Figures 4–6 creep behaviour of steel 1.4841 at different temperatures and stress levels is presented.
On the basis of experimentally obtained short-time creep curves, it is visible that this material is creep resistant when stress level is below 50 % of yield point at the temperature of 823 K, and less than 30 % of yield point at the temperature of 923 K. Even at the temperature of 1,023 K, it is quite creep resistant when stress level is low (10–15 % of yield point). As an example of creep modelling, the creep curve shown in Figure 4(b) is modelled using several models such as Burgers model, standard linear solid (SLS) model and an analytical model, Table 2. Burgers model  is defined as (1)where and t are strain, stress, modulus of elasticity and time related to the considered creep curve, while and are parameters that need to be determined.
SLS model  is defined as (2)where and t are strain, stress and time related to the considered creep curve, while and are parameters.
Analytical expression  is defined as (3)where T is the temperature, the stress, t the time and D, p and r are parameters that are to be determined. All of the mentioned models can be suitable for modelling of first and second creep stages. Burgers model is most suitable for modelling of the second stage of creep curve, while for modelling the first creep stage when creep curve has quite highlighted parabolic shape it is not adequate choice for modelling. As it can be seen, analytical expression is a very good choice in creep modelling. In addition, modelling is here performed in such a way that only one creep curve is considered. Much appropriate procedure is when a range of temperature and a range of stress level are considered, i. e. when . Such a procedure is here not performed since experimental data do not provide that.
As it is visible from Figure 7, all models are used to describe the creep curve quite well.
Basic analysis of microstructure
In order to analyse the microstructure of the steel 1.4841, an optical microscope is used. Two states of the material are considered: the state of as-received material and the state which is caused by performed creep process. Optical micrographs are presented in Figure 8 and are based on longitudinal sections of the specimens. The specimen that represents material subjected to a creep process was previously subjected to creep at the temperature of 823 K/167 MPa/1,200 min. As it is seen from Figure 8(a), the basic microstructure consists of polygonal austenite grains. At elevated temperature and after creep process, the grains take better polygonal shapes and are increased in size, and this indicates that material has been exposed to load, i. e. the load exerts an influence on the deformation process.
Impact energy and fracture toughness calculation
In engineering practice, two material properties are usually mentioned as the main representatives of design criteria and that yield point (yield strength) and fracture toughness. Yield strength is used as a parameter or a criterion in structural design against plastic deformation, while fracture toughness is a parameter or a measure of material resistance to crack extension/propagation . However, fracture toughness as a critical value of stress intensity factor (SIF) is designated as KIc and is plane strain fracture toughness. The relationship between SIF and fracture toughness (KIc) may be compared with the relationship between uniaxial stress and tensile stress (ultimate tensile strength). Fracture toughness can be determined by a number of methods . It needs to be said that the determination of fracture toughness can belong to the elastic behaviour of material but also to the elastic–plastic behaviour. However, as a test result, a single value of fracture toughness or a resistance curve can be obtained. If toughness parameters such as K, J and CTOD are plotted versus crack propagation, then appropriate resistant curve can be obtained. K (SIF) is stress intensity factor, J is path-independent contour integral and CTOD means crack tip opening displacement. Mentioned single values are KIc (fracture toughness), JIc (critical value of J-fracture toughness parameter), etc. Determination of plane strain fracture toughness (KIc) may be linked to many problems such as manufacturing of a specimen, credibility of the specimen and crack in relation to the real crack in the structure, etc. To avoid such problems, a very useful method for impact energy determination can be used and such obtained results can serve as input data for the correlation with fracture toughness. Charpy impact energy (CVN) can be determined with Charpy impact machine. Several correlations between Charpy impact energy and fracture toughness can be mentioned. Here, the following formula is used : (4)Equation (4) is known as Roberts–Newton formula and it is valid independent of temperature range and stress level. The Charpy tests were carried out in accordance with the ASTM: E23-07ae1 standard. Specimens of 10×10×55 mm with manufactured 2V notch are made from 16 mm rod of considered material. Results of obtained impact energy and calculated fracture toughness are presented in Table 3.
In Table 4, a comparison of impact energies for different materials are presented. Testing the impact energy for each considered material has been done by the same procedures.
This investigation has resulted in the obtained results and analyses of ultimate tensile strength, yield strength and modulus of elasticity at different temperatures. These data can be very useful for designers of structures that can be made from considered material. In addition, material creep tests provide the information about material creep resistance, while impact energy provides also useful information about fracture toughness. Based on these results, mechanical properties of considered material are said to be appropriate for the material at high temperatures and also fairly creep resistant if the stress level with respect to temperature level is acceptable. All the data we are dealing with are visible from the presented testing results.
Authors are grateful to prof. Jitai Niu and prof. Qiang Li from School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo, China, for analysing the microstructure of the material. In addition, authors are grateful to prof. Roman Sturm and to the company Acroni, Jesenice, Slovenia, for the experimental determination of material impact energy.
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About the article
Published Online: 2016-03-12
Published in Print: 2017-02-01
Funding: This work was supported by the Croatian Science Foundation under the project 6876 and also partially supported by the University of Rijeka within the project 13.09.1.1.01.