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Comparison of material properties: Steel 20MnCr5 and similar steels
Journal of Constructional Steel Research 95 (2014) 81–89
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Journal of Constructional Steel Research
Comparison of material properties: Steel 20MnCr5 and similar steels Josip Brnic ⁎, Goran Turkalj 1, Domagoj Lanc 1, Marko Canadija 1, Marino Brcic 1, Goran Vukelic 1 University of Rijeka, Faculty of Engineering, Department of Engineering Mechanics, Vukovarska 58, 51000 Rijeka, Croatia
a r t i c l e
i n f o
Article history: Received 4 March 2013 Accepted 23 November 2013 Available online 28 December 2013 Keywords: 20MnCr5 steel Material properties Short-time creep behavior Creep modeling Charpy impact energy Fracture toughness assessment
a b s t r a c t Starting from the fact that the experimental data are real data, and that the assessment of their values is of great importance in a design of the structure, this article seeks to draw attention to the designers of experimental data related to steel 20MnCr5 (1.7147, AISI 5120). In this sense, this paper presents the experimental results involving material ultimate tensile strength, yield strength, creep behavior, total fracture strain, reduction in the area as well as Charpy impact energy. All of the mentioned tests were conducted at room temperature and at elevated temperatures. Considerable tensile testing referring to determination of ultimate tensile strength as well as 0.2 offset yield strength resulted in engineering stress–strain diagrams, while those tensile testing regarding creep behavior resulted in creep curves. Also, modeling of material creep behavior using rheological models and an equation proposed by the authors can be found in this paper. Using Charpy impact energy tests, an assessment of fracture toughness was made. In addition to this, the paper presents a comparison of the material properties of 20MnCr5 steel with material properties of other similar (structural/constructional) steels. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Design, manufacture and maintenance of structures and possible failures in their service lives All artifacts that surround us are the result of a specific process of design and production. Decreasing resources both in terms of materials and production costs require optimization of these processes [1]. Optimization may be treated as the act of obtaining the best product under given circumstances [2]. However, differences between engineering analyses, design process and manufacturing process need to be recognized [3]. Namely, analysis process is concerned with determining the behavior of the existing structure, while the design process is intended to calculate sizes, shapes, topology and choice of material of the structure to meet performance requirements. However, each structure is designed and manufactured for a specific purpose, i.e., to carry the load, store a liquid or gas or whatever. When creating a design of the structure, the conditions in which the structure will operate, and the assessment of its service life must be taken into account. During the process of design and production of the structure, some failures may occur. The analysis of failure focused both on its cause and the mode of manifestations is an extremely important aspect of engineering [4]. Failures, among other things, can arise due to a variety of preexisting defects or defects that initiate from imperfections. The causes of failures may be given as the following categories: misuse, design
⁎ Corresponding author. Tel.: +385 51 651 491; fax: +385 51 651 490. E-mail address: [email protected] (J. Brnic). 1 Tel.: +385 51 651 491; fax: +385 651 490. 0143-974X/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcsr.2013.11.024
errors, assembly error, improper maintenance, pre-existing cracks, etc. Many items may be considered in each of the mentioned categories. Fracture can be treated as the last phase of failure, and, especially load bearing structures are always important issues to be considered by designers as well as by those that operate and maintain bridges, airplanes, trains, etc. [5]. Some structural failures can be associated with the fracture. When such events occur, they are usually unexpected and draw attention to the need to reduce the consequences [6]. Despite all the possible errors, the number of successfully designed structures may be treated as satisfactory [7]. Also, it needs to be pointed out that some structures, depending on their functions, operate under special environmental conditions, which can cause some of the failures. In that way, for example, heat treatment or an operation at elevated temperature can produce dimensional changes and residual stresses that in some cases can lead to component cracking, dysfunction due to changes in size or even to the fracture of a considered engineering component. To provide material properties which would meet the design requirements, lots of tests need to be performed. In accordance with the above features referring to the causes of failures, operating the structure at elevated temperature, as it is mentioned, can cause creep. In general, mechanical failure may be defined as any change in shape, dimensions or material properties of an engineering component that makes it incapable of serving the purpose for which it was made. In accordance with this, creep failure may occur whenever the plastic deformation interferes with functional abilities of the component. If creep buckling failure arises, that means that creep process results in an unstable combination between the load and geometry of the component so that the critical buckling limit is exceeded [8]. Metals subjected to high stresses at elevated temperatures exhibit a phenomenon known as creep, a phenomenon that is reflected in the continuous increase of deformation,
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Table 1 Material chemical composition. Material: 20MnCr5 Designation Steel name EN (10084-2008)/DIN (17210): 20MnCr5 AISI SAE 5120 Chemical composition Mass (%) C Mn Cr Si 0.22 1.23 1.11 0.29
Steel number (Mat. No; W. Nr.; Mat. Code) 1.7147
Ni 0.08
Cu 0.06
Nb 0.03
S 0.025
although the stress is kept constant [9]. In engineering practice, usually a few percent, for example, 1–2% of creep strain is allowed, and creep is appreciable only at temperature above 0.4 Tm, where Tm is a melting point [10]. Creep process is usually represented by creep curve consisting of three stages and that, first stage (I—transient creep), second stage (II—steady-state creep) and third stage (III—accelerating creep). It is very important to know how certain material exhibits its response to creep at both prescribed temperature and stress level. To have an insight into creep behavior of some structural and stainless steels as well as in other performances of steels, it is recommended to review Refs. [11–20]. 1.2. Recent works related to considered material The fatigue behavior of similar joints made of 20MnCr5 was studied for two different contact conditions in Ref. [21]. The study dealing with the wear improvements related to 20MnCr5 steel was presented in Ref. [22]. It was found that nano-carbides formed by extended deep cold treatment at (−196)°C significantly improved wear. Some experimental investigations related to evaluation of the power dissipation of gears were carried out and results are presented in Ref. [23]. One of the tested gears was made from 20MnCr5 carburized steel. In Ref. [24] an investigation of different laser beam sources and their effect on the welding distortion in axial welded shaft hub joints made of steel (20MnCr5) is considered. Further, an investigation related to crack propagation was presented in Ref. [25]. In this case, notched specimens were machined from steel 20MnCr5 (SAE 5120). In engineering practice, considering manufacturing methods to produce the counter face for rotary shaft lip seals is of great interest [26]. This investigation includes also 20MnCr5 material. In Ref. [27], the fracture of spiral bevel gear for truck differential produced from case hardening steel is investigated. One of the investigated spiral bevel gears was made of case hardening steel 20MnCr5, which has a low carbon–chromium and was supplied in the rolled condition. The objective of the study in Ref. [28] was the estimation of the cyclic deformation properties of steels from common tensile properties and hardness. In this sense, many relationships among hardness, tensile properties, and cyclic deformation properties were developed for steels, included 20MnCr5. In Ref. [29] the factors that affect micropitting performance of gears were analyzed. The twin disc tests were carried out and analyzed. Two different cases of hardening steels (one conventional case of hardening 20MnCr5 steel and second one micro-alloyed 20MnCr5 steel) were used. In Ref. [30], apart from experimental evaluation of austempered ductile iron (ADI) as a gear material, also some data related to 20MnCr5 steel are given. The study about improvement in the scuffing resistance of toothed gears was presented in Ref. [31]. The test gears were made of 20MnCr5 steel. In this case, gears were carburized, case hardened and tempered. An investigation that has been conducted in order to reach a rational description of the flow stress of structural steels is presented in Ref. [32]. The goal of this investigation was to analyze the deformation behavior of a 20MnCr5 steel deformed in tension under hot-working conditions. Some data related to physical properties of 20MnCr5 can be found in Ref. [33]. An analysis of the influence of high pressure nitride
P 0.021
Ti 0.02
W 0.02
Mo 0.01
V 0.01
Al 0.01
Rest 96.864
steel gears in combination with a bio-lubricant is given in Ref. [34]. In this analysis one of the materials used was 20MnCr5 steel. 2. Information relevant to research 2.1. Material under consideration Material under consideration was the steel 20MnCr5, 18 mm round bar. As reported by the manufacturer it was hot-rolled, delivery condition: annealed. Chemical composition of this material in mass (%) is shown in Table 1. This material is treated as low-carbon, low-chromium steel, sometimes known as special structural steel, which may be recommended for the application where a combination of substantial medium strength, toughness and wear resistance is provided. It is frequently used in mechanical engineering, especially in the manufacture of highly stressed components (parts) in automobile industry, for parts like gears, crankshafts, connection rods, etc. 2.2. Equipment, specimens, testing procedures and standards Tensile tests were performed using a material testing machine, 400 kN. Furnace (900 °C) and a high temperature extensometer were used during examinations at elevated temperatures. The Charpy impact machine was used to determine impact energy. The system consisting of the material testing machine, furnace and extensometer is presented in Fig. 1. Test specimens, Fig. 2, used in tensile testing were machined from 8 mm round bar and their sizes were defined according to ASTM: E8M-11. The ends of the specimens outside the gage length were threaded to match the holders of the testing machine.
Fig. 1. Testing system.
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Fig. 2. Specimen for tensile testing (mm).
Standards for all testing procedures implemented in these researches are defined in Ref. [35]. 3. Experimental results and discussion As previously mentioned, material properties such as tensile strength, yield strength and creep behavior of the material are determined by using uniaxial tensile tests, while the impact fracture energy is developed by Charpy impact machine. The results are presented in the form of appropriate diagrams and tables. 3.1. Material properties 3.1.1. Engineering stress–strain diagrams In any structural engineering design, ultimate tensile strength and 0.2 offset yield strength are of particular importance. To be able to monitor changes in the value of material properties as a function of temperature change, a number of tests have been performed by the authors of this paper. Test results are presented as engineering stress–strain diagrams, Fig. 3. Based on engineering stress–strain diagram, ultimate tensile strength and total strain of considered material can be seen. 3.1.2. Material properties versus temperature Numerical values of material properties, like ultimate tensile strength (σm), 0.2 offset yield strength (σ0.2), modulus of elasticity (E), total strain (εt) and reduction in the area (ψ) of the specimen not only for steel considered in this paper but also for particular structural steels are presented in Table 3. Temperature dependence of material properties for considered steel is shown in Fig. 4. These data were experimentally obtained. It is worth mentioning that in engineering practice there are real engineering processes, for example, the process of stretching the specimen to rupture or, for example, the process of creep for certain material which is subjected to a certain stress at certain temperature. These processes may be recorded, i.e. displayed in the form of certain curves. On the other hand, these processes can be simulated (modeled, predicted). Also, in addition to it, a set of real data can
Fig. 4. Material properties versus temperature for steel 20MnCr5: set of real data and the curves of approximation. a) Mechanical properties: ultimate tensile strength (σm), 0.2 offset yield strength (σ0.2). b) Modulus of elasticity (E). c) Total strain, i.e. strain at break (εt) and reduction in the cross-sectional area of the specimen (ψ).
Fig. 3. Engineering stress–strain diagrams for 20MnCr5 steel at room and elevated temperatures.
also be approximated (fitted) by a certain curve. In the case when the real process (shown by curve, or by real set of data) is simulated by a curve, the actual situation is said to be simulated. Subsequently, an analysis between a real (actual) and simulated situation (process) or between a real set of data and a fitted curve can be made. In this way, the accuracy with which the simulated process can replace the real process can be determined. A coefficient, called the coefficient of determination (R2) is established, which is a measure of accordance of measured and approximated (predicted, simulated) values. The R2 is a statistic that gives some information about how fit a model is [36].
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In Fig. 4, except the set of experimental data shown with symbols (♦,■) related to the changes of material properties versus temperature, the curves are shown by which these changes are approximated. As shown in previous figures, Figs. 3, 4a, both the tensile strength and 0.2 offset yield strength of this material at room temperature are quite high. With the rise of the temperature the values of these properties decrease. The exception is visible only at a temperature of 300 °C where both of the mentioned properties are a little higher than at temperature of 200 °C. Modulus of elasticity decreases with a temperature increase. 3.1.3. Comparison of particular structural steels with respect to material properties A comparative analysis referring to the comparison of the material properties of some structural steels will be carried out based on the review of the level of the main ingredients in the chemical composition of the considered materials and the examination of the tensile tests performed on them. In this sense, the results are presented in two tables. One of them, Table 2, shows the ratio of some ingredients in materials, and the second one, Table 3, shows the levels of the material properties. Ratio (ξ) is given in such a way that the values of items in considered materials are compared to the values of items relating to material 20MnCr5, i.e., ξX(Y) = XY/XA, where Y means material (A, B, C, D, E) and X means considered element of chemical composition. Information relating to the chemical composition and material properties of materials that are the subject of analysis can be found in Refs. [12,14,15,20] as well as in this article. Although all of considered steels are named structural steels, it would be appropriate to say that ASTM A618 steel, S355JO steel and S275JR steel are treated as structural steels, while 20MnCr5 steel is named special structural steel and 50CrMo4 steel is named constructional steel. All the mentioned steels have very wide use in mechanical and civil engineering. In that way, for example, applications of steels ASTM A618, S355JO and S275JR can be associated to areas such as bridges, cranes, high loaded structures, etc. Also, steels like 20MnCr5 and 50CrMo4 may be associated with design and production of statically and dynamically stressed larger cross-section structural components like craft, gears, connect roads, etc. In accordance with the data presented in Table 2, which were obtained from experimental research, it is evident that the steel 50CrMo4 has the highest percentage of carbon in its composition, while the lowest percentage of carbon has S275JR steel. The same goes for the percentage of chromium, taking into account that this element is not included in the chemical composition of ASTM A618 steel and S355JO steel. Material 20MnCr5 has the highest percentage of manganese. These data are presented because it is known that the properties of steels are linked to the chemical composition, resulting microstructure and processing path [37]. Based on data given in Table 3, it is visible that 50CrMo4 steel has the highest levels of ultimate tensile strength of all testing temperatures. Ultimate tensile strengths of the other steels for each considered temperature are much smaller than that of steel 50CrMo4, and it is
Table 2 Structural steels (A, B, C, D, E): Participation of some elements in the chemical composition of particular structural steels (material: A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4). Element of chemical composition (X)
C Cr Si Mn
Material A
Ratio of participation of elements in chemical composition of materials: ξX(Y) = XY/XA; material (Y) = B, C, D, E
X (%)
ξX(B)
ξX(C)
ξX(D)
ξX(E)
0.22 1.11 0.29 1.23
0.3636 0.09 0.7586 0.4634
0.9636 – 1.162 1.065
0.8904 0.0581 0.6696 0.4478
2.2136 0.9 0.886 0.5975
Table 3 Structural steels (A, B, C, D, E): Material properties (σm; σ0.2; E). (A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4). Material
Temperature (°C)
σm (MPa)
σ0.2 (MPa)
ζ = σ0.2/σm
E (GPa)
A B C D E A B C D E A B C D E A B C D E A B C D E A B C D E A B C D E A B C D E
20
561.6 451.9 550.6 506.46 1146.9 513.6 485.4 507.5 517 1095.4 482.8 605.3 580.7 601 1102.1 499.3 488 565.4 530.46 1014.2 392.5 341.7 420.8 365.97 799.7 265.6 203.8 237.0 210.96 495.2 146.8 90.2 111.8 97.53 165.6 96.6 – – 86.7 97.4
397.6 322.7 385.7 336.78 1090.2 373.1 309.1 354.8 355.23 1005.1 349.2 267.4 329.4 285.77 912.2 362.2 211.6 250.7 234.14 850 315.9 191.8 233.9 220.54 693.9 253 143.4 174.8 147.7 437.2 141.5 71 96.2 60.95 113.8 67.4 – – 37.4 64.3
0.707 0.714 0.7 0.665 0.951 0.726 0.637 0.7 0.687 0.918 0.723 0.442 0.567 0.4755 0.828 0.725 0.434 0.443 0.4414 0.838 0.805 0.5613 0.556 0.603 0.868 0.953 0.704 0.738 0.7 0.883 0.964 0.797 0.860 0.625 0.687 0.698
219 211 209 205 203.9 206 204 208 – – 192 194 – – – 184 183.4 – – – 177 175 – – – 129 153.8 – – – 76 101.6 – – – 58 – – – –
100
200
300
400
500
600
700
0.431 0.66
visible that they did not differ significantly between each of considered steel for considered temperature. It is also visible that the ratio between 0.2 offset yield strength and ultimate tensile strength is the highest at 50CrMo4 steel for temperature range of 20 °C–400 °C. In the temperature range of 500 °C–700 °C the highest ratio between 0.2 offset yield strength and ultimate tensile strength has 20MnCr5 steel. Regarding modulus of elasticity, it can be said that the differences in them for these steels are not so obvious. 3.2. Short-time creep behavior and simulation 3.2.1. Short-time creep tests Several creep tests were performed at different stress levels and different temperatures. The data about these processes are given in Table 4. Creep behavior of this steel in the form of creep curves is presented in Figs. 5–7. Table 4 Creep tests—Temperatures and stress levels. Material: 20MnCr5 steel
Constant stress level σ (MPa)
Constant temperature T (°C) 400
500
600
158 = 0.5 σ0.2 172.6 = 0.546 σ0.2 221 = 0.7 σ0.2
38 = 0.15 σ0.2 71.7 = 0.283 σ0.2 126.5 = 0.5 σ0.2
14.2 = 0.1 σ0.2 42.5 = 0.3 σ0.2 –
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Fig. 7. Creep behavior of 20MnCr5 steel at temperature of 600 °C.
Fig. 5. Creep behavior of 20MnCr5 steel at temperature of 400 °C.
3.2.2. Creep test simulation Real creep processes are the best indicators for the behavior of materials at elevated temperatures. Conducting creep processes, especially long-term processes, can be expensive and quite difficult. For these reasons, it is very convenient to find the way to predict the behavior of material in the case of creep. A number of ways to implement the prediction of material behavior in the case of creep are known. This paper will point out two rheological models and an analytical formula that is recommended by the authors. The mentioned simulation will be shown in a case of short-time creep processes. Rheological models which can be used are, for example, Burgers model and Standard Linear Solid (SLS) model. When model is considered, then the following equation related to strain may be used [15]: εðt Þ ¼ σ
1 1 t −E =η t þ 1−eð 2 1 Þ þ : E1 E2 η2
85
ð1Þ
In Eq. (1), items ε(t), σ, E1 and t are: strain, stress, modulus of elasticity and time related to the considered creep curve, while E2, η1 and η2
are parameters which are determined on the basis of matching the considered curve and Eq. (1). It should be noted that this model is suited for modeling the first and second stages of the creep curve but not for accelerating stage of creep. Also, the quality of the simulation depends on the stress range and on the shape of the primary creep stage. The simulation is not of required quality when the primary stage is markedly parabolic and the range of stress is quite wide. In any case, a better result is achieved when considering each curve separately. In terms of the Standard Linear Solid model, the equation used to describe the strain is [15]: εðt Þ ¼
σ 1 1 −ðEE1þEE2 tÞη þσ − e 1 2 : E1 E1 þ E2 E 1
ð2Þ
In Eq. (2), items ε(t), σ and t are strain, stress and time related to the considered creep curve, while E1, E2 and η are parameters. The recommended formula (equation) for deformation monitoring at creep process is [12]: εðt Þ ¼ D
−T
p r
σ t :
ð3Þ
In Eq. (3) there are: ε—strain, T—temperature, σ—stress and t—time and D, p and r are parameters which are to be determined. Several creep curves were modeled. Modeling of creep curves are presented in Figs. 8–10, while Table 5 shows data relating to the time, temperature, stress levels as well as the conditions under which each of the models is applied.
Fig. 6. Creep behavior of 20MnCr5 steel at temperature of 500 °C. a) Stress levels of 38 and 71.7 MPa. b) Stress level of 126.5 MPa.
Fig. 8. Simulation of short-time creep behavior of steel 20MnCr5 at temperature of 400 °C. (Burgers and SLS models and Eq. (3)).
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3.2.3. Comparison of particular structural steels with respect to creep resistance In any design, because of the use of the structure, it is very important to know how the material behaves at elevated temperatures. In accordance with experimentally obtained curves representing creep behavior of particular structural steels [12,14,15,20], certain statements about their creep resistance are presented in Table 6. Based on the available experimental data, Table 6, used in the processes of creep, it is possible to say that the materials A, B and C may be treated as creep resistant for the anticipated levels of stress at a temperature of 400 °C. The same goes not only for these materials but also for 500 °C, while at 600 °C they are considered to be resistant but only for very low levels of stress. Material E should be treaded as non-resistant to creep for the prescribed conditions given in Table 5. 3.3. Material microstructure Three specimens were selected for this microstructure analysis. An optical micrograph of as-received material (one specimen), used in this work, is presented in Fig. 11a, while the optical micrograph of material after its use in creep process (second specimen), which was performed at a temperature of 500 °C/71.7 MPa/1200 min, is presented in Fig. 11b, and third one, the optical micrograph of material after its use in the creep process, which was performed at a temperature of 500 °C/126.5 MPa/400 min, is presented in Fig. 11c. Fig. 11 shows a mixture of ferrite and cementite without significant segregation. Also, it can be said that there were no significant changes between as-received material and that used in creep processes. No significant changes in grain sizes.
Fig. 9. Simulation of short-time creep behavior of steel 20MnCr5 at temperature of 500 °C. (Burgers and SLS models and Eq. (3)).
3.4. Charpy V-Notch (CVN) impact energy tests and fracture toughness calculation For certain structure, depending on its purpose and operating conditions, the material needs to be chosen so as to meet the requirements. It actually means that the material must have the properties that meet the requirements of load and conditions under which the structure operates. In this sense, there are certain design philosophies as stated in the further part. According to one of them, for many engineering applications, it is sufficient to ensure that the operating stress, which may arise in the structure, remains below acceptable limits. Acceptable limits are usually determined according to yield point, i.e., according to maximum stress that material can withstand without plastic deformations. In this case, an engineering component is considered to be free of failure after
Fig. 10. Simulation of short-time creep behavior of steel 20MnCr5 at temperature of 600 °C. (Burgers and SLS models and Eq. (3)).
From Figs. 8–10 it is visible that creep processes for anticipated conditions of temperatures and strain levels are well simulated by all models. However, since the Burgers model and SLS model were applied for exactly defined conditions, it might be said that the application of the proposed formula actually suits the best because it covers the entire test area with respect to temperatures and stress levels.
Table 5 Creep modeling data. Material
20MnCr5
Constant temperature T (°C)
400
Constant stress level σ (MPa) Time (min) Burgers model (Eq. (1)): ε = ε(t) σ = const T = const SLS model (Eq. (2)): ε = ε(t) σ = const T = const Eq. (3) ε = ε(σ,T,t)
σ = 0.5 σ0.2 = 158
σ = 0.546 σ0.2 = 172
σ = 0.15 σ0.2 = 38
σ = 0.283 σ0.2 = 71.7
σ = 0.1 σ0.2 = 14.2
1250 E1 = 177 GPa E2 = 1.05321 · 109 Pa η1 = 1.92354 · 1010 Pa min η2 = 5.62405 · 1011 Pa min
1250 E1 = 177 GPa E2 = 9.91823 · 108 Pa η1 = 5.05636 · 108 Pa min η2 = 3.70674 · 1011 Pa min
1250 E1 = 129 GPa E2 = 5.41781 · 108 Pa η1 = 2.23984 · 1010 Pa min η2 = 3.05688 · 1011 Pa min
1250 E1 = 129 GPa E2 = 1.95099 · 108 Pa η1 = 1.74847 · 1010 Pa min η2 = 5.76558 · 1012 Pa min
1250 E1 = 76 GPa E2 = 1.45662 · 108 Pa η1 = 6.95669 · 109 Pa min η2 = 3.62828 · 1010 Pa min
500
600
E1 = 2.38904 · 108 Pa E1 = 1.48577 · 108 Pa E1 = 1.48164 · 108 Pa E1 = 2.25189 · 107 Pa E1 = 9.24786 · 106 Pa E2 = 1.29101 · 109 Pa E2 = 1.17091 · 109 Pa E2 = 7.79256 · 108 Pa E2 = 4.29195 · 108 Pa E2 = 2.04516 · 108 Pa η = 2.27115 · 1011 Pa min η = 1.98224 · 1011 Pa min η = 1.02987 · 1011 Pa min η = 3.26403 · 1010 Pa min η = 2.60171 · 1010 Pa min Valid for: t = 0–1250 min; T = 400 °C–600 °C; σ = ((0.5–0.6) σ0.2/400 °C; (0.15–0.3) σ0.2/500 °C; (0.1–0.15) σ0.2/600 °C)) D = 4.45248 · 10−9 · T4 − 8.95915 · 10−6 · T3 + 6.70134 · 10−3 · T2 − 2.21 · T + 272.414 p = 1.01532 · 10−7 · T4 − 2.05778 · 10−4 · T3 + 0.1547146 · T2 − 51.19273 · T + 6297.352 r = −5.4484 · 10−10 · T4 + 9.25929 · 10−7 · T3 − 5.74739 · 10−4 · T2 + 0.155267 · T − 14.97694
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Table 6 Creep resistance of particular structural steels. (A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4). σ0.2—corresponds to considered material. Material
Creep resistance at temperature T(°C) and time of 1200 min (Yes; Not) Criterion: Creep strain εcr ≤ 2% 400
A B C Da E a
500
600
σ = 0.5σ0.2
σ = 0.6σ0.2
σ = 0.1σ0.2
σ = 0.3σ0.2
σ = 0.1σ0.2
σ = 0.2σ0.2
Yes Yes Yes – Not
Yes Yes Yes – Not
Yes Yes Yes – Yes
Not Yes Yes – Not
Yes Yes Yes – Not
Not Not Not – Not
Material D (ASTM A618) was exposed to creep for a shorter time period than that given in Table 6.
manufacturing; it is designed to remain failure-free during service life. According to the second one, the component is designed to withstand the maximum operating stress for a period of time under assumption that possible failure would not be catastrophic, and the third one is that the component is designed to withstand the maximum operating stress for a period of time when even flaws, cracks or other failures are present. All the material properties are important, but usually two of them stand out and those are: yield strength and fracture toughness. Usually, it can be said that yield strength (σ0.2) is used to design structure against plastic deformation, while fracture toughness (KIc) is used to design structure against fracture. In the introductory part of this article, various failures are mentioned which can occur in the structure during its service life that might disable the structure to serve the purpose for which it was intended. In that way, failure analysis has become a very important task of engineering in discovering the causes of failures and the ways of their expressing. Namely, this analysis can help us to
establish why and how a certain component has failed because the particular failure has its cause of origin and the form of its expression. Structural loading, corrosion, wear as well as some other defects are usually mentioned as general causes of failures. Also, misuse, design errors and assembly errors, inadequate choice of material and inadequate maintenance, operating conditions, temperature effect, etc. may be treated as causes of failures [38,39]. In engineering practice a lot of numbers of failure modes may arise, for example: yielding, creep, buckling, thermal shock, and fracture, force induced deformation, etc. In addition, it needs to be said that structure life prediction is also an important detail of design procedure. However, many of the mentioned failures can lead to the fracture of engineering element. Therefore, it is necessary to characterize the fracture toughness of the material from which the structure is made, i.e., design process should include some knowledge about sufficient level of resistance of the material against crack propagation. Although several parameters exist to determine
Fig. 11. Optical micrograph of steel 20MnCr5 (cross-section of the specimen), 4% nital. a) As—received material, considered in this work. b) After creep process conducted at 500 °C/71.7 MPa/1200 min. c) After creep process conducted at 500 °C/126.5 MPa/400 min.
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fracture resistance, usually it is done by fracture toughness. Fracture mechanics, as a part of applied mechanics, is a subject concerned with predicting the failure containing crack-like defect [40]. Sometimes, fracture mechanics is said to be a field of solid mechanics dealing with responses of cracked bodies. So called stress intensity factor (SIF or K) is a parameter by which stress distribution around the crack tip is described. If the plastic zone is small compared with specimen dimensions, and if the plane-strain condition is also fulfilled, a critical value of the SIF is called fracture toughness (KIc). It is the minimum value of fracture toughness. This will be achieved when the specimen thickness exceeds the critical dimension. Three modes of crack tip loading exist, i.e., three modes of crack opening. Mode “I”, where the direction of the largest tensile loading is normal to the crack plane that is the most encountered mode. The relationship between SIF and KIc is quite similar to the relationship between uniaxial stress and tensile stress (ultimate tensile strength). Fracture toughness (KIc) can be measured by a number of standard tests [41] and it can be used as an appropriate parameter of material resistance to crack extension. Of course, it needs to be said that, experimental determination of fracture toughness has also some disadvantages. First, it requires quite a large specimen and the manufacturing of the specimen is rather complicated because of its geometry. Also, the geometry of the crack, especially crack tip, may well vary from the actual crack in the structure. However, this investigation can be made in laboratory, because it is impossible to take this specimen from the operating component. To avoid the mentioned problems, some other experimental methods exist to determine fracture resistance of the structure. One of them is, for example, using Charpy impact test. Impact energy which is obtained by Charpy test may be used as a basis for fracture toughness assessment (calculation). This method has also several limitations, namely, the sizes and geometry differ considerably with reference to the real structure and the machined tip of the crack is blunt while the one used in fracture mechanics laboratory examination is sharp. In spite of that, the obtained results by Charpy tests and those obtained by standard tests can be compared [42–44]. Many authors have investigated the fracture toughness of the material using standardized tests and Charpy tests. Also, a comparative analysis of the results was made. Frequently used formula for calculating the fracture toughness based on Charpy tests, which can be applied regardless of the level of energy and test temperature, is Roberts–Norton formula [42]: 0:63
KIc ¼ 8:47ðCVNÞ
:
ð4Þ
According to this formula, fracture toughness was calculated for several materials, Table 7. A specimen related to Charpy impact test is presented in Fig. 12. Table 7 Charpy V-notch impact energy and fracture toughness for particular structural steels. (A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4, T— temperature; ν—Poisson ratio; Ac—cross-sectional area of the specimen at the place of notch). Parameter data
Material: A; B; C; D; E pffiffiffiffiffi KIc (MPa m); KIc = 8.47 (CVN)0.63
CVN (J) T (°C)
ν
A2c mm
A
B
C
D
E
A
B
C
D
E
−20 −10 0 20 50 70 80 100 120 150
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
80 80 80 80 80 80 80 80 80 80
160 – 170 178 181 185 188 192 198 210
170 182 195 210 221 224 225 230 232 254
60 – 65 94 – – – 170 182 200
– – – – – – – – – –
– – – 69 – – – – – –
207.2
215.3 224.7 234.7 245.9 254 256 256.8 260.5 261.9 277.3
111.7 – 117.5 148 – – – 215.3 224.8 238.5
– – – – – – – – – –
– – – 117.5 – – – – – –
221.6 222.4 227.1 229.4 232.5 237 246
Fig. 12. Specimen for Charpy test (mm).
3.4.1. Comparison of particular structural steels with respect to impact energy and fracture toughness Based on experimental investigations, some data about Charpy impact energy as well as calculated fracture toughness for particular structural steel are given in Table 7. On the basis of the given data, Table 7, it is visible that structural steels A, B and C have quite similar values of fracture toughness, while steel E, which has the highest tensile strength, has the lowest value of the fracture toughness at room temperature. Besides, it is visible that fracture toughness of each steel increases with the temperature increase. 4. Conclusions Experimental investigations related to steel 20MnCr5 were carried out. As a result, material properties like ultimate tensile strength, 0.2 offset yield strength, and modulus of elasticity as well as Charpy impact energy at different temperatures were determined. Creep behavior of this material was also considered. For certain cases of creep behavior related to steel 20MnCr5, simulations were presented. All of the mentioned properties of this steel were compared with identical properties of similar steels. The mentioned comparisons related to material properties and creep behaviors are presented in appropriate tables. References [1] Papalambros PY, Wilde DJ. Principles of optimal design. 2nd ed. Cambridge: Cambridge University Press; 2000. [2] Rao SS. Engineering optimization. 4th ed. New Jersey: John Wiley & Sons; 2009. [3] Arora JS. Introduction to optimal design. 2nd ed. San Diego: Elsevier Academic Press; 2004. [4] Brooks CR, Choundry A. Failure analysis of engineering materials. New York: McGraw-Hill; 2002. [5] Saxena A. Nonlinear fracture mechanics for engineers. New York: CRC Press; 1988. [6] Shukla A. Practical fracture mechanics in design. 2nd ed. New York: Marcel Dekker; 2005. [7] Stephens RI, Fatemi A, Stephens RR, Fuchs HO. Metal fatigue in engineering. U. S. A.: John Wiley & Sons; 2001 [8] Collins JA. Failure of materials in mechanical design. 2nd ed. New York: John Wiley & Sons; 1993. [9] Solecki R, Conant RJ. Advanced mechanics of materials. New York: Oxford University Press; 2003. [10] Raghavan V. Materials science and engineering. 5th ed. New Delhi: Prentice-Hall of India; 2004. [11] Brnic J, Turkalj G, Canadija M, Lanc D. AISI 316Ti (1.4571) steel—mechanical, creep and fracture properties versus temperature. J Constr Steel Res 2011;67(12):1948–52. [12] Brnic J, Turkalj G, Niu J, Canadija M, Lanc D. Analysis of experimental data on the behavior of steel S275JR—reliability of modern design. Mater Des 2013;47:497–504. http://dx.doi.org/10.1016/j.matdes.2012.12.037. [13] Pepelnjak T, Petek A, Kuzman K. Analysis of the forming limit diagram in digital environment. Adv Mater Res 2005;6–8:697–704. [14] Brnic J, Turkalj G, Canadija M, Lanc D. Creep behavior of high strength low-alloy steel at elevated temperatures. Mater Sci Eng A 2009;499(1–2 Special Issue):23–7.
J. Brnic et al. / Journal of Constructional Steel Research 95 (2014) 81–89 [15] Brnic J, Canadija M, Turkalj G, Lanc D. Structural steel ASTM A709—behavior at uniaxial tests conducted at lowered and elevated temperatures, short-time creep response, and fracture toughness calculation. J Eng Mech 2010;136(9):1083–9. [16] Klobcar D, Kosec L, Kosec B. Thermo fatigue cracking of die casting dies. Eng Fail Anal 2012;20:43–53. [17] Brnic J, Turkalj G, Canadija M, Lanc D, Krscanski S. Martensitic stainless steel AISI 420—mechanical properties, creep and fracture toughness. Mech Time-Depend Mater 2011;15(4):341–52. [18] Brnic J, Niu J, Canadija M, Turkalj G, Lanc D. Behavior of AISI 316 L steel subjected to uniaxial state of stress at elevated temperatures. J Mater Sci Technol 2009;25(2): 175–80. [19] Pepelnjak T, Gantar G, Kuzman K. Numerical simulations in optimization of product and forming process. J Mater Process Technol 2001;115(1):122–6. [20] Brnic J, Canadija M, Turkalj G, Lanc D. 50CrMo4 steel-determination of mechanical properties at lowered and elevated temperatures, creep behavior, and fracture toughness calculation. J Eng Mater Technol, Trans ASME 2010;132(2):0210041–6. [21] Stoschkaa M, Thalera M, Huemerb H, Eichlsedera W. Contribution to the fatigue assessment of laser welded joints. Procedia Eng 2011;10:1785–90. [22] Stratton P, Graf M. The effect of deep cold induced nano-carbides on the wear of case hardened components. Cryogenics 2009;49(7):346–9. [23] Magalhães L, Martins R, Seabra J. Low-loss austempered ductile iron gears: Experimental evaluation comparing materials and lubricants. Tribol Int 2012;46(1):97–105. [24] Buschenhenke F, Hofmann M, Seefeld T, Vollertsen F. Distortion and residual stresses in laser beam weld shaft-hub joints. Phys Procedia 2010;5(Part B):89–98. [25] Burkart K, Bomas H, Zoch HW. Fatigue of notched case-hardened specimens of steel SAE 5120 in the VHCF regime and application of the weakest-link concept. Int J Fatigue 2011;33(1):59–68. [26] Kunstfeld T, Haas W. Shaft surface manufacturing methods for rotary shaft lip seals. Sealing Technol 2005;2005(7):5–9. [27] Sekercioglu T, Kovan V. Pitting failure of truck spiral bevel gear. Eng Fail Anal 2007;14(4):614–9. [28] Lopez Z, Fatemi A. A method of predicting cyclic stress–strain curve from tensile properties for steels. Mater Sci Eng A 2012;556(30):540–50. [29] Ahlroos T, Ronkainen H, Helle A, Parikka R, Virta J, Varjus S. Twin disc micropitting tests. Tribol Int 2009;10(42):1460–6.
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[30] Martins R, Seabra J, Magalhaes L. Austempered ductile iron (ADI) gears: power loss, pitting and micropitting. Wear 2008;264:838–49. [31] Tuszynski W, Michalczewski R, Szczerek M, Kalbarczyk M. A new scuffing shock test method for the determination of the resistance to scuffing of coated gears. Arch Civ Mech Eng December 5 2012;12(4):436–45. [32] Puchi-Cabrera E, Guérin JD, Barbier D, Dubar M, Lesage J. Plastic deformation of structural steels under hot-working conditions. Mater Sci Eng A January 1 2013;559:268–75. [33] Brandão JA, Jorge HO, Castro SJ. Surface initiated tooth flank damage. Part II: prediction of micropitting initiation and mass loss. Wear January 4 2010;268(1–2): 13–22. [34] Martins RC, Cardoso NFR, Bock H, Igartua A, Seabra JHO. Power loss performance of high pressure nitrided steel gears. Tribol Int December 2009;42(11–12):1807–15. [35] Annual Book of ASTM Standards. Metal test methods and analytical procedures, vol. 03.01. Baltimore: ASTM International; 2012. [36] Draper NR, Smith H. Applied regression analysis. 3rd ed. USA: Wiley—Interscience Publications; 1998. [37] Bramfitt BL. Effects of composition, processing and structure on properties of irons and steels. In: Dieter GE, editor. ASM handbook, materials selection and design, vol. 20. USA: ASM international; 1997. p. 357–82. [38] Feld J, Kenneth. Construction failure. USA: John Wiley & Sons; 1997. [39] Farahmand B, Bockrath G, Glassco J. Fatigue and fracture mechanics of high risk parts. Chapman & Hall; 1997. [40] Zhang L. Failure assessment of thin-walled structures with particular reference to pipelines. Southampton: WIT Press; 2010. [41] Anderson TL. Fracture mechanics. 3rd ed. Boca Raton: CRC Press; 2005. [42] Roberts R, Newton C. Interpretive report on small scale test correlation with KIc data. Weld Res Counc Bull 1981;265:1–16. [43] Shekhter A, Kim S, Carr DG, Crocker ABL, Ringer SP. Assessment of temper embrittlement in an ex-service 1Cr–1Mo–0.25 V power generating rotor by Charpy V-notch testing; KIc fracture toughness and small punch test. Int J Press Vessels Pip 2002;79(8–10):611–5. [44] Chao YJ, Ward JD, Sands RG. Charpy impact energy, fracture toughness and ductile–brittle transition temperature of dual-phase 590 steel. Mater Des 2007;2:551–7.
Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Comparison of material properties: Steel 20MnCr5 and similar steels Josip Brnic ⁎, Goran Turkalj 1, Domagoj Lanc 1, Marko Canadija 1, Marino Brcic 1, Goran Vukelic 1 University of Rijeka, Faculty of Engineering, Department of Engineering Mechanics, Vukovarska 58, 51000 Rijeka, Croatia
a r t i c l e
i n f o
Article history: Received 4 March 2013 Accepted 23 November 2013 Available online 28 December 2013 Keywords: 20MnCr5 steel Material properties Short-time creep behavior Creep modeling Charpy impact energy Fracture toughness assessment
a b s t r a c t Starting from the fact that the experimental data are real data, and that the assessment of their values is of great importance in a design of the structure, this article seeks to draw attention to the designers of experimental data related to steel 20MnCr5 (1.7147, AISI 5120). In this sense, this paper presents the experimental results involving material ultimate tensile strength, yield strength, creep behavior, total fracture strain, reduction in the area as well as Charpy impact energy. All of the mentioned tests were conducted at room temperature and at elevated temperatures. Considerable tensile testing referring to determination of ultimate tensile strength as well as 0.2 offset yield strength resulted in engineering stress–strain diagrams, while those tensile testing regarding creep behavior resulted in creep curves. Also, modeling of material creep behavior using rheological models and an equation proposed by the authors can be found in this paper. Using Charpy impact energy tests, an assessment of fracture toughness was made. In addition to this, the paper presents a comparison of the material properties of 20MnCr5 steel with material properties of other similar (structural/constructional) steels. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Design, manufacture and maintenance of structures and possible failures in their service lives All artifacts that surround us are the result of a specific process of design and production. Decreasing resources both in terms of materials and production costs require optimization of these processes [1]. Optimization may be treated as the act of obtaining the best product under given circumstances [2]. However, differences between engineering analyses, design process and manufacturing process need to be recognized [3]. Namely, analysis process is concerned with determining the behavior of the existing structure, while the design process is intended to calculate sizes, shapes, topology and choice of material of the structure to meet performance requirements. However, each structure is designed and manufactured for a specific purpose, i.e., to carry the load, store a liquid or gas or whatever. When creating a design of the structure, the conditions in which the structure will operate, and the assessment of its service life must be taken into account. During the process of design and production of the structure, some failures may occur. The analysis of failure focused both on its cause and the mode of manifestations is an extremely important aspect of engineering [4]. Failures, among other things, can arise due to a variety of preexisting defects or defects that initiate from imperfections. The causes of failures may be given as the following categories: misuse, design
⁎ Corresponding author. Tel.: +385 51 651 491; fax: +385 51 651 490. E-mail address: [email protected] (J. Brnic). 1 Tel.: +385 51 651 491; fax: +385 651 490. 0143-974X/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcsr.2013.11.024
errors, assembly error, improper maintenance, pre-existing cracks, etc. Many items may be considered in each of the mentioned categories. Fracture can be treated as the last phase of failure, and, especially load bearing structures are always important issues to be considered by designers as well as by those that operate and maintain bridges, airplanes, trains, etc. [5]. Some structural failures can be associated with the fracture. When such events occur, they are usually unexpected and draw attention to the need to reduce the consequences [6]. Despite all the possible errors, the number of successfully designed structures may be treated as satisfactory [7]. Also, it needs to be pointed out that some structures, depending on their functions, operate under special environmental conditions, which can cause some of the failures. In that way, for example, heat treatment or an operation at elevated temperature can produce dimensional changes and residual stresses that in some cases can lead to component cracking, dysfunction due to changes in size or even to the fracture of a considered engineering component. To provide material properties which would meet the design requirements, lots of tests need to be performed. In accordance with the above features referring to the causes of failures, operating the structure at elevated temperature, as it is mentioned, can cause creep. In general, mechanical failure may be defined as any change in shape, dimensions or material properties of an engineering component that makes it incapable of serving the purpose for which it was made. In accordance with this, creep failure may occur whenever the plastic deformation interferes with functional abilities of the component. If creep buckling failure arises, that means that creep process results in an unstable combination between the load and geometry of the component so that the critical buckling limit is exceeded [8]. Metals subjected to high stresses at elevated temperatures exhibit a phenomenon known as creep, a phenomenon that is reflected in the continuous increase of deformation,
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Table 1 Material chemical composition. Material: 20MnCr5 Designation Steel name EN (10084-2008)/DIN (17210): 20MnCr5 AISI SAE 5120 Chemical composition Mass (%) C Mn Cr Si 0.22 1.23 1.11 0.29
Steel number (Mat. No; W. Nr.; Mat. Code) 1.7147
Ni 0.08
Cu 0.06
Nb 0.03
S 0.025
although the stress is kept constant [9]. In engineering practice, usually a few percent, for example, 1–2% of creep strain is allowed, and creep is appreciable only at temperature above 0.4 Tm, where Tm is a melting point [10]. Creep process is usually represented by creep curve consisting of three stages and that, first stage (I—transient creep), second stage (II—steady-state creep) and third stage (III—accelerating creep). It is very important to know how certain material exhibits its response to creep at both prescribed temperature and stress level. To have an insight into creep behavior of some structural and stainless steels as well as in other performances of steels, it is recommended to review Refs. [11–20]. 1.2. Recent works related to considered material The fatigue behavior of similar joints made of 20MnCr5 was studied for two different contact conditions in Ref. [21]. The study dealing with the wear improvements related to 20MnCr5 steel was presented in Ref. [22]. It was found that nano-carbides formed by extended deep cold treatment at (−196)°C significantly improved wear. Some experimental investigations related to evaluation of the power dissipation of gears were carried out and results are presented in Ref. [23]. One of the tested gears was made from 20MnCr5 carburized steel. In Ref. [24] an investigation of different laser beam sources and their effect on the welding distortion in axial welded shaft hub joints made of steel (20MnCr5) is considered. Further, an investigation related to crack propagation was presented in Ref. [25]. In this case, notched specimens were machined from steel 20MnCr5 (SAE 5120). In engineering practice, considering manufacturing methods to produce the counter face for rotary shaft lip seals is of great interest [26]. This investigation includes also 20MnCr5 material. In Ref. [27], the fracture of spiral bevel gear for truck differential produced from case hardening steel is investigated. One of the investigated spiral bevel gears was made of case hardening steel 20MnCr5, which has a low carbon–chromium and was supplied in the rolled condition. The objective of the study in Ref. [28] was the estimation of the cyclic deformation properties of steels from common tensile properties and hardness. In this sense, many relationships among hardness, tensile properties, and cyclic deformation properties were developed for steels, included 20MnCr5. In Ref. [29] the factors that affect micropitting performance of gears were analyzed. The twin disc tests were carried out and analyzed. Two different cases of hardening steels (one conventional case of hardening 20MnCr5 steel and second one micro-alloyed 20MnCr5 steel) were used. In Ref. [30], apart from experimental evaluation of austempered ductile iron (ADI) as a gear material, also some data related to 20MnCr5 steel are given. The study about improvement in the scuffing resistance of toothed gears was presented in Ref. [31]. The test gears were made of 20MnCr5 steel. In this case, gears were carburized, case hardened and tempered. An investigation that has been conducted in order to reach a rational description of the flow stress of structural steels is presented in Ref. [32]. The goal of this investigation was to analyze the deformation behavior of a 20MnCr5 steel deformed in tension under hot-working conditions. Some data related to physical properties of 20MnCr5 can be found in Ref. [33]. An analysis of the influence of high pressure nitride
P 0.021
Ti 0.02
W 0.02
Mo 0.01
V 0.01
Al 0.01
Rest 96.864
steel gears in combination with a bio-lubricant is given in Ref. [34]. In this analysis one of the materials used was 20MnCr5 steel. 2. Information relevant to research 2.1. Material under consideration Material under consideration was the steel 20MnCr5, 18 mm round bar. As reported by the manufacturer it was hot-rolled, delivery condition: annealed. Chemical composition of this material in mass (%) is shown in Table 1. This material is treated as low-carbon, low-chromium steel, sometimes known as special structural steel, which may be recommended for the application where a combination of substantial medium strength, toughness and wear resistance is provided. It is frequently used in mechanical engineering, especially in the manufacture of highly stressed components (parts) in automobile industry, for parts like gears, crankshafts, connection rods, etc. 2.2. Equipment, specimens, testing procedures and standards Tensile tests were performed using a material testing machine, 400 kN. Furnace (900 °C) and a high temperature extensometer were used during examinations at elevated temperatures. The Charpy impact machine was used to determine impact energy. The system consisting of the material testing machine, furnace and extensometer is presented in Fig. 1. Test specimens, Fig. 2, used in tensile testing were machined from 8 mm round bar and their sizes were defined according to ASTM: E8M-11. The ends of the specimens outside the gage length were threaded to match the holders of the testing machine.
Fig. 1. Testing system.
J. Brnic et al. / Journal of Constructional Steel Research 95 (2014) 81–89
83
Fig. 2. Specimen for tensile testing (mm).
Standards for all testing procedures implemented in these researches are defined in Ref. [35]. 3. Experimental results and discussion As previously mentioned, material properties such as tensile strength, yield strength and creep behavior of the material are determined by using uniaxial tensile tests, while the impact fracture energy is developed by Charpy impact machine. The results are presented in the form of appropriate diagrams and tables. 3.1. Material properties 3.1.1. Engineering stress–strain diagrams In any structural engineering design, ultimate tensile strength and 0.2 offset yield strength are of particular importance. To be able to monitor changes in the value of material properties as a function of temperature change, a number of tests have been performed by the authors of this paper. Test results are presented as engineering stress–strain diagrams, Fig. 3. Based on engineering stress–strain diagram, ultimate tensile strength and total strain of considered material can be seen. 3.1.2. Material properties versus temperature Numerical values of material properties, like ultimate tensile strength (σm), 0.2 offset yield strength (σ0.2), modulus of elasticity (E), total strain (εt) and reduction in the area (ψ) of the specimen not only for steel considered in this paper but also for particular structural steels are presented in Table 3. Temperature dependence of material properties for considered steel is shown in Fig. 4. These data were experimentally obtained. It is worth mentioning that in engineering practice there are real engineering processes, for example, the process of stretching the specimen to rupture or, for example, the process of creep for certain material which is subjected to a certain stress at certain temperature. These processes may be recorded, i.e. displayed in the form of certain curves. On the other hand, these processes can be simulated (modeled, predicted). Also, in addition to it, a set of real data can
Fig. 4. Material properties versus temperature for steel 20MnCr5: set of real data and the curves of approximation. a) Mechanical properties: ultimate tensile strength (σm), 0.2 offset yield strength (σ0.2). b) Modulus of elasticity (E). c) Total strain, i.e. strain at break (εt) and reduction in the cross-sectional area of the specimen (ψ).
Fig. 3. Engineering stress–strain diagrams for 20MnCr5 steel at room and elevated temperatures.
also be approximated (fitted) by a certain curve. In the case when the real process (shown by curve, or by real set of data) is simulated by a curve, the actual situation is said to be simulated. Subsequently, an analysis between a real (actual) and simulated situation (process) or between a real set of data and a fitted curve can be made. In this way, the accuracy with which the simulated process can replace the real process can be determined. A coefficient, called the coefficient of determination (R2) is established, which is a measure of accordance of measured and approximated (predicted, simulated) values. The R2 is a statistic that gives some information about how fit a model is [36].
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In Fig. 4, except the set of experimental data shown with symbols (♦,■) related to the changes of material properties versus temperature, the curves are shown by which these changes are approximated. As shown in previous figures, Figs. 3, 4a, both the tensile strength and 0.2 offset yield strength of this material at room temperature are quite high. With the rise of the temperature the values of these properties decrease. The exception is visible only at a temperature of 300 °C where both of the mentioned properties are a little higher than at temperature of 200 °C. Modulus of elasticity decreases with a temperature increase. 3.1.3. Comparison of particular structural steels with respect to material properties A comparative analysis referring to the comparison of the material properties of some structural steels will be carried out based on the review of the level of the main ingredients in the chemical composition of the considered materials and the examination of the tensile tests performed on them. In this sense, the results are presented in two tables. One of them, Table 2, shows the ratio of some ingredients in materials, and the second one, Table 3, shows the levels of the material properties. Ratio (ξ) is given in such a way that the values of items in considered materials are compared to the values of items relating to material 20MnCr5, i.e., ξX(Y) = XY/XA, where Y means material (A, B, C, D, E) and X means considered element of chemical composition. Information relating to the chemical composition and material properties of materials that are the subject of analysis can be found in Refs. [12,14,15,20] as well as in this article. Although all of considered steels are named structural steels, it would be appropriate to say that ASTM A618 steel, S355JO steel and S275JR steel are treated as structural steels, while 20MnCr5 steel is named special structural steel and 50CrMo4 steel is named constructional steel. All the mentioned steels have very wide use in mechanical and civil engineering. In that way, for example, applications of steels ASTM A618, S355JO and S275JR can be associated to areas such as bridges, cranes, high loaded structures, etc. Also, steels like 20MnCr5 and 50CrMo4 may be associated with design and production of statically and dynamically stressed larger cross-section structural components like craft, gears, connect roads, etc. In accordance with the data presented in Table 2, which were obtained from experimental research, it is evident that the steel 50CrMo4 has the highest percentage of carbon in its composition, while the lowest percentage of carbon has S275JR steel. The same goes for the percentage of chromium, taking into account that this element is not included in the chemical composition of ASTM A618 steel and S355JO steel. Material 20MnCr5 has the highest percentage of manganese. These data are presented because it is known that the properties of steels are linked to the chemical composition, resulting microstructure and processing path [37]. Based on data given in Table 3, it is visible that 50CrMo4 steel has the highest levels of ultimate tensile strength of all testing temperatures. Ultimate tensile strengths of the other steels for each considered temperature are much smaller than that of steel 50CrMo4, and it is
Table 2 Structural steels (A, B, C, D, E): Participation of some elements in the chemical composition of particular structural steels (material: A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4). Element of chemical composition (X)
C Cr Si Mn
Material A
Ratio of participation of elements in chemical composition of materials: ξX(Y) = XY/XA; material (Y) = B, C, D, E
X (%)
ξX(B)
ξX(C)
ξX(D)
ξX(E)
0.22 1.11 0.29 1.23
0.3636 0.09 0.7586 0.4634
0.9636 – 1.162 1.065
0.8904 0.0581 0.6696 0.4478
2.2136 0.9 0.886 0.5975
Table 3 Structural steels (A, B, C, D, E): Material properties (σm; σ0.2; E). (A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4). Material
Temperature (°C)
σm (MPa)
σ0.2 (MPa)
ζ = σ0.2/σm
E (GPa)
A B C D E A B C D E A B C D E A B C D E A B C D E A B C D E A B C D E A B C D E
20
561.6 451.9 550.6 506.46 1146.9 513.6 485.4 507.5 517 1095.4 482.8 605.3 580.7 601 1102.1 499.3 488 565.4 530.46 1014.2 392.5 341.7 420.8 365.97 799.7 265.6 203.8 237.0 210.96 495.2 146.8 90.2 111.8 97.53 165.6 96.6 – – 86.7 97.4
397.6 322.7 385.7 336.78 1090.2 373.1 309.1 354.8 355.23 1005.1 349.2 267.4 329.4 285.77 912.2 362.2 211.6 250.7 234.14 850 315.9 191.8 233.9 220.54 693.9 253 143.4 174.8 147.7 437.2 141.5 71 96.2 60.95 113.8 67.4 – – 37.4 64.3
0.707 0.714 0.7 0.665 0.951 0.726 0.637 0.7 0.687 0.918 0.723 0.442 0.567 0.4755 0.828 0.725 0.434 0.443 0.4414 0.838 0.805 0.5613 0.556 0.603 0.868 0.953 0.704 0.738 0.7 0.883 0.964 0.797 0.860 0.625 0.687 0.698
219 211 209 205 203.9 206 204 208 – – 192 194 – – – 184 183.4 – – – 177 175 – – – 129 153.8 – – – 76 101.6 – – – 58 – – – –
100
200
300
400
500
600
700
0.431 0.66
visible that they did not differ significantly between each of considered steel for considered temperature. It is also visible that the ratio between 0.2 offset yield strength and ultimate tensile strength is the highest at 50CrMo4 steel for temperature range of 20 °C–400 °C. In the temperature range of 500 °C–700 °C the highest ratio between 0.2 offset yield strength and ultimate tensile strength has 20MnCr5 steel. Regarding modulus of elasticity, it can be said that the differences in them for these steels are not so obvious. 3.2. Short-time creep behavior and simulation 3.2.1. Short-time creep tests Several creep tests were performed at different stress levels and different temperatures. The data about these processes are given in Table 4. Creep behavior of this steel in the form of creep curves is presented in Figs. 5–7. Table 4 Creep tests—Temperatures and stress levels. Material: 20MnCr5 steel
Constant stress level σ (MPa)
Constant temperature T (°C) 400
500
600
158 = 0.5 σ0.2 172.6 = 0.546 σ0.2 221 = 0.7 σ0.2
38 = 0.15 σ0.2 71.7 = 0.283 σ0.2 126.5 = 0.5 σ0.2
14.2 = 0.1 σ0.2 42.5 = 0.3 σ0.2 –
J. Brnic et al. / Journal of Constructional Steel Research 95 (2014) 81–89
Fig. 7. Creep behavior of 20MnCr5 steel at temperature of 600 °C.
Fig. 5. Creep behavior of 20MnCr5 steel at temperature of 400 °C.
3.2.2. Creep test simulation Real creep processes are the best indicators for the behavior of materials at elevated temperatures. Conducting creep processes, especially long-term processes, can be expensive and quite difficult. For these reasons, it is very convenient to find the way to predict the behavior of material in the case of creep. A number of ways to implement the prediction of material behavior in the case of creep are known. This paper will point out two rheological models and an analytical formula that is recommended by the authors. The mentioned simulation will be shown in a case of short-time creep processes. Rheological models which can be used are, for example, Burgers model and Standard Linear Solid (SLS) model. When model is considered, then the following equation related to strain may be used [15]: εðt Þ ¼ σ
1 1 t −E =η t þ 1−eð 2 1 Þ þ : E1 E2 η2
85
ð1Þ
In Eq. (1), items ε(t), σ, E1 and t are: strain, stress, modulus of elasticity and time related to the considered creep curve, while E2, η1 and η2
are parameters which are determined on the basis of matching the considered curve and Eq. (1). It should be noted that this model is suited for modeling the first and second stages of the creep curve but not for accelerating stage of creep. Also, the quality of the simulation depends on the stress range and on the shape of the primary creep stage. The simulation is not of required quality when the primary stage is markedly parabolic and the range of stress is quite wide. In any case, a better result is achieved when considering each curve separately. In terms of the Standard Linear Solid model, the equation used to describe the strain is [15]: εðt Þ ¼
σ 1 1 −ðEE1þEE2 tÞη þσ − e 1 2 : E1 E1 þ E2 E 1
ð2Þ
In Eq. (2), items ε(t), σ and t are strain, stress and time related to the considered creep curve, while E1, E2 and η are parameters. The recommended formula (equation) for deformation monitoring at creep process is [12]: εðt Þ ¼ D
−T
p r
σ t :
ð3Þ
In Eq. (3) there are: ε—strain, T—temperature, σ—stress and t—time and D, p and r are parameters which are to be determined. Several creep curves were modeled. Modeling of creep curves are presented in Figs. 8–10, while Table 5 shows data relating to the time, temperature, stress levels as well as the conditions under which each of the models is applied.
Fig. 6. Creep behavior of 20MnCr5 steel at temperature of 500 °C. a) Stress levels of 38 and 71.7 MPa. b) Stress level of 126.5 MPa.
Fig. 8. Simulation of short-time creep behavior of steel 20MnCr5 at temperature of 400 °C. (Burgers and SLS models and Eq. (3)).
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3.2.3. Comparison of particular structural steels with respect to creep resistance In any design, because of the use of the structure, it is very important to know how the material behaves at elevated temperatures. In accordance with experimentally obtained curves representing creep behavior of particular structural steels [12,14,15,20], certain statements about their creep resistance are presented in Table 6. Based on the available experimental data, Table 6, used in the processes of creep, it is possible to say that the materials A, B and C may be treated as creep resistant for the anticipated levels of stress at a temperature of 400 °C. The same goes not only for these materials but also for 500 °C, while at 600 °C they are considered to be resistant but only for very low levels of stress. Material E should be treaded as non-resistant to creep for the prescribed conditions given in Table 5. 3.3. Material microstructure Three specimens were selected for this microstructure analysis. An optical micrograph of as-received material (one specimen), used in this work, is presented in Fig. 11a, while the optical micrograph of material after its use in creep process (second specimen), which was performed at a temperature of 500 °C/71.7 MPa/1200 min, is presented in Fig. 11b, and third one, the optical micrograph of material after its use in the creep process, which was performed at a temperature of 500 °C/126.5 MPa/400 min, is presented in Fig. 11c. Fig. 11 shows a mixture of ferrite and cementite without significant segregation. Also, it can be said that there were no significant changes between as-received material and that used in creep processes. No significant changes in grain sizes.
Fig. 9. Simulation of short-time creep behavior of steel 20MnCr5 at temperature of 500 °C. (Burgers and SLS models and Eq. (3)).
3.4. Charpy V-Notch (CVN) impact energy tests and fracture toughness calculation For certain structure, depending on its purpose and operating conditions, the material needs to be chosen so as to meet the requirements. It actually means that the material must have the properties that meet the requirements of load and conditions under which the structure operates. In this sense, there are certain design philosophies as stated in the further part. According to one of them, for many engineering applications, it is sufficient to ensure that the operating stress, which may arise in the structure, remains below acceptable limits. Acceptable limits are usually determined according to yield point, i.e., according to maximum stress that material can withstand without plastic deformations. In this case, an engineering component is considered to be free of failure after
Fig. 10. Simulation of short-time creep behavior of steel 20MnCr5 at temperature of 600 °C. (Burgers and SLS models and Eq. (3)).
From Figs. 8–10 it is visible that creep processes for anticipated conditions of temperatures and strain levels are well simulated by all models. However, since the Burgers model and SLS model were applied for exactly defined conditions, it might be said that the application of the proposed formula actually suits the best because it covers the entire test area with respect to temperatures and stress levels.
Table 5 Creep modeling data. Material
20MnCr5
Constant temperature T (°C)
400
Constant stress level σ (MPa) Time (min) Burgers model (Eq. (1)): ε = ε(t) σ = const T = const SLS model (Eq. (2)): ε = ε(t) σ = const T = const Eq. (3) ε = ε(σ,T,t)
σ = 0.5 σ0.2 = 158
σ = 0.546 σ0.2 = 172
σ = 0.15 σ0.2 = 38
σ = 0.283 σ0.2 = 71.7
σ = 0.1 σ0.2 = 14.2
1250 E1 = 177 GPa E2 = 1.05321 · 109 Pa η1 = 1.92354 · 1010 Pa min η2 = 5.62405 · 1011 Pa min
1250 E1 = 177 GPa E2 = 9.91823 · 108 Pa η1 = 5.05636 · 108 Pa min η2 = 3.70674 · 1011 Pa min
1250 E1 = 129 GPa E2 = 5.41781 · 108 Pa η1 = 2.23984 · 1010 Pa min η2 = 3.05688 · 1011 Pa min
1250 E1 = 129 GPa E2 = 1.95099 · 108 Pa η1 = 1.74847 · 1010 Pa min η2 = 5.76558 · 1012 Pa min
1250 E1 = 76 GPa E2 = 1.45662 · 108 Pa η1 = 6.95669 · 109 Pa min η2 = 3.62828 · 1010 Pa min
500
600
E1 = 2.38904 · 108 Pa E1 = 1.48577 · 108 Pa E1 = 1.48164 · 108 Pa E1 = 2.25189 · 107 Pa E1 = 9.24786 · 106 Pa E2 = 1.29101 · 109 Pa E2 = 1.17091 · 109 Pa E2 = 7.79256 · 108 Pa E2 = 4.29195 · 108 Pa E2 = 2.04516 · 108 Pa η = 2.27115 · 1011 Pa min η = 1.98224 · 1011 Pa min η = 1.02987 · 1011 Pa min η = 3.26403 · 1010 Pa min η = 2.60171 · 1010 Pa min Valid for: t = 0–1250 min; T = 400 °C–600 °C; σ = ((0.5–0.6) σ0.2/400 °C; (0.15–0.3) σ0.2/500 °C; (0.1–0.15) σ0.2/600 °C)) D = 4.45248 · 10−9 · T4 − 8.95915 · 10−6 · T3 + 6.70134 · 10−3 · T2 − 2.21 · T + 272.414 p = 1.01532 · 10−7 · T4 − 2.05778 · 10−4 · T3 + 0.1547146 · T2 − 51.19273 · T + 6297.352 r = −5.4484 · 10−10 · T4 + 9.25929 · 10−7 · T3 − 5.74739 · 10−4 · T2 + 0.155267 · T − 14.97694
J. Brnic et al. / Journal of Constructional Steel Research 95 (2014) 81–89
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Table 6 Creep resistance of particular structural steels. (A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4). σ0.2—corresponds to considered material. Material
Creep resistance at temperature T(°C) and time of 1200 min (Yes; Not) Criterion: Creep strain εcr ≤ 2% 400
A B C Da E a
500
600
σ = 0.5σ0.2
σ = 0.6σ0.2
σ = 0.1σ0.2
σ = 0.3σ0.2
σ = 0.1σ0.2
σ = 0.2σ0.2
Yes Yes Yes – Not
Yes Yes Yes – Not
Yes Yes Yes – Yes
Not Yes Yes – Not
Yes Yes Yes – Not
Not Not Not – Not
Material D (ASTM A618) was exposed to creep for a shorter time period than that given in Table 6.
manufacturing; it is designed to remain failure-free during service life. According to the second one, the component is designed to withstand the maximum operating stress for a period of time under assumption that possible failure would not be catastrophic, and the third one is that the component is designed to withstand the maximum operating stress for a period of time when even flaws, cracks or other failures are present. All the material properties are important, but usually two of them stand out and those are: yield strength and fracture toughness. Usually, it can be said that yield strength (σ0.2) is used to design structure against plastic deformation, while fracture toughness (KIc) is used to design structure against fracture. In the introductory part of this article, various failures are mentioned which can occur in the structure during its service life that might disable the structure to serve the purpose for which it was intended. In that way, failure analysis has become a very important task of engineering in discovering the causes of failures and the ways of their expressing. Namely, this analysis can help us to
establish why and how a certain component has failed because the particular failure has its cause of origin and the form of its expression. Structural loading, corrosion, wear as well as some other defects are usually mentioned as general causes of failures. Also, misuse, design errors and assembly errors, inadequate choice of material and inadequate maintenance, operating conditions, temperature effect, etc. may be treated as causes of failures [38,39]. In engineering practice a lot of numbers of failure modes may arise, for example: yielding, creep, buckling, thermal shock, and fracture, force induced deformation, etc. In addition, it needs to be said that structure life prediction is also an important detail of design procedure. However, many of the mentioned failures can lead to the fracture of engineering element. Therefore, it is necessary to characterize the fracture toughness of the material from which the structure is made, i.e., design process should include some knowledge about sufficient level of resistance of the material against crack propagation. Although several parameters exist to determine
Fig. 11. Optical micrograph of steel 20MnCr5 (cross-section of the specimen), 4% nital. a) As—received material, considered in this work. b) After creep process conducted at 500 °C/71.7 MPa/1200 min. c) After creep process conducted at 500 °C/126.5 MPa/400 min.
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fracture resistance, usually it is done by fracture toughness. Fracture mechanics, as a part of applied mechanics, is a subject concerned with predicting the failure containing crack-like defect [40]. Sometimes, fracture mechanics is said to be a field of solid mechanics dealing with responses of cracked bodies. So called stress intensity factor (SIF or K) is a parameter by which stress distribution around the crack tip is described. If the plastic zone is small compared with specimen dimensions, and if the plane-strain condition is also fulfilled, a critical value of the SIF is called fracture toughness (KIc). It is the minimum value of fracture toughness. This will be achieved when the specimen thickness exceeds the critical dimension. Three modes of crack tip loading exist, i.e., three modes of crack opening. Mode “I”, where the direction of the largest tensile loading is normal to the crack plane that is the most encountered mode. The relationship between SIF and KIc is quite similar to the relationship between uniaxial stress and tensile stress (ultimate tensile strength). Fracture toughness (KIc) can be measured by a number of standard tests [41] and it can be used as an appropriate parameter of material resistance to crack extension. Of course, it needs to be said that, experimental determination of fracture toughness has also some disadvantages. First, it requires quite a large specimen and the manufacturing of the specimen is rather complicated because of its geometry. Also, the geometry of the crack, especially crack tip, may well vary from the actual crack in the structure. However, this investigation can be made in laboratory, because it is impossible to take this specimen from the operating component. To avoid the mentioned problems, some other experimental methods exist to determine fracture resistance of the structure. One of them is, for example, using Charpy impact test. Impact energy which is obtained by Charpy test may be used as a basis for fracture toughness assessment (calculation). This method has also several limitations, namely, the sizes and geometry differ considerably with reference to the real structure and the machined tip of the crack is blunt while the one used in fracture mechanics laboratory examination is sharp. In spite of that, the obtained results by Charpy tests and those obtained by standard tests can be compared [42–44]. Many authors have investigated the fracture toughness of the material using standardized tests and Charpy tests. Also, a comparative analysis of the results was made. Frequently used formula for calculating the fracture toughness based on Charpy tests, which can be applied regardless of the level of energy and test temperature, is Roberts–Norton formula [42]: 0:63
KIc ¼ 8:47ðCVNÞ
:
ð4Þ
According to this formula, fracture toughness was calculated for several materials, Table 7. A specimen related to Charpy impact test is presented in Fig. 12. Table 7 Charpy V-notch impact energy and fracture toughness for particular structural steels. (A = 20MnCr5; B = S275JR; C = S355JO; D = ASTM A618; E = 50CrMo4, T— temperature; ν—Poisson ratio; Ac—cross-sectional area of the specimen at the place of notch). Parameter data
Material: A; B; C; D; E pffiffiffiffiffi KIc (MPa m); KIc = 8.47 (CVN)0.63
CVN (J) T (°C)
ν
A2c mm
A
B
C
D
E
A
B
C
D
E
−20 −10 0 20 50 70 80 100 120 150
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
80 80 80 80 80 80 80 80 80 80
160 – 170 178 181 185 188 192 198 210
170 182 195 210 221 224 225 230 232 254
60 – 65 94 – – – 170 182 200
– – – – – – – – – –
– – – 69 – – – – – –
207.2
215.3 224.7 234.7 245.9 254 256 256.8 260.5 261.9 277.3
111.7 – 117.5 148 – – – 215.3 224.8 238.5
– – – – – – – – – –
– – – 117.5 – – – – – –
221.6 222.4 227.1 229.4 232.5 237 246
Fig. 12. Specimen for Charpy test (mm).
3.4.1. Comparison of particular structural steels with respect to impact energy and fracture toughness Based on experimental investigations, some data about Charpy impact energy as well as calculated fracture toughness for particular structural steel are given in Table 7. On the basis of the given data, Table 7, it is visible that structural steels A, B and C have quite similar values of fracture toughness, while steel E, which has the highest tensile strength, has the lowest value of the fracture toughness at room temperature. Besides, it is visible that fracture toughness of each steel increases with the temperature increase. 4. Conclusions Experimental investigations related to steel 20MnCr5 were carried out. As a result, material properties like ultimate tensile strength, 0.2 offset yield strength, and modulus of elasticity as well as Charpy impact energy at different temperatures were determined. Creep behavior of this material was also considered. For certain cases of creep behavior related to steel 20MnCr5, simulations were presented. All of the mentioned properties of this steel were compared with identical properties of similar steels. The mentioned comparisons related to material properties and creep behaviors are presented in appropriate tables. References [1] Papalambros PY, Wilde DJ. Principles of optimal design. 2nd ed. Cambridge: Cambridge University Press; 2000. [2] Rao SS. Engineering optimization. 4th ed. New Jersey: John Wiley & Sons; 2009. [3] Arora JS. Introduction to optimal design. 2nd ed. San Diego: Elsevier Academic Press; 2004. [4] Brooks CR, Choundry A. Failure analysis of engineering materials. New York: McGraw-Hill; 2002. [5] Saxena A. Nonlinear fracture mechanics for engineers. New York: CRC Press; 1988. [6] Shukla A. Practical fracture mechanics in design. 2nd ed. New York: Marcel Dekker; 2005. [7] Stephens RI, Fatemi A, Stephens RR, Fuchs HO. Metal fatigue in engineering. U. S. A.: John Wiley & Sons; 2001 [8] Collins JA. Failure of materials in mechanical design. 2nd ed. New York: John Wiley & Sons; 1993. [9] Solecki R, Conant RJ. Advanced mechanics of materials. New York: Oxford University Press; 2003. [10] Raghavan V. Materials science and engineering. 5th ed. New Delhi: Prentice-Hall of India; 2004. [11] Brnic J, Turkalj G, Canadija M, Lanc D. AISI 316Ti (1.4571) steel—mechanical, creep and fracture properties versus temperature. J Constr Steel Res 2011;67(12):1948–52. [12] Brnic J, Turkalj G, Niu J, Canadija M, Lanc D. Analysis of experimental data on the behavior of steel S275JR—reliability of modern design. Mater Des 2013;47:497–504. http://dx.doi.org/10.1016/j.matdes.2012.12.037. [13] Pepelnjak T, Petek A, Kuzman K. Analysis of the forming limit diagram in digital environment. Adv Mater Res 2005;6–8:697–704. [14] Brnic J, Turkalj G, Canadija M, Lanc D. Creep behavior of high strength low-alloy steel at elevated temperatures. Mater Sci Eng A 2009;499(1–2 Special Issue):23–7.
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