materials

Article Analysis of the Mechanical Behavior, Creep Resistance and Uniaxial Strength of Martensitic Steel X46Cr13

Josip Brnic 1,*, Sanjin Krscanski 1, Domagoj Lanc 1, Marino Brcic 1, Goran Turkalj 1, Marko Canadija 1 and Jitai Niu 2,3 1 Department of Engineering Mechanics, Faculty of Engineering, University of Rijeka, Vukovarska 51, 51000 Rijeka, Croatia; [email protected] (S.K.); [email protected] (D.L.); [email protected] (M.B.); [email protected] (G.T.); [email protected] (M.C.) 2 School of and Engineering, Harbin Institute of Technology, XiDaZhi Street 92#, Nangang District, Harbin 150001, China; [email protected] 3 School of Materials Science and Engineering, Henan Polytechnic University, No 2001, Century Avenue, Jiaozuo 454003, China * Correspondence: [email protected]; Tel.: +385-51-651-491; Fax: +385-51-651-490

Academic Editor: Nicola Pugno Received: 14 January 2017; Accepted: 4 April 2017; Published: 6 April 2017

Abstract: The article deals with the analysis of the mechanical behavior at different temperatures, uniaxial creep and uniaxial fatigue of martensitic steel X46Cr13 (1.4034, AISI 420). For the purpose of considering the aforementioned mechanical behavior, as well as determining the appropriate resistance to creep and fatigue strength levels, numerous uniaxial tests were carried out. Tests related to mechanical properties performed at different temperatures are presented in the form of engineering -strain diagrams. Short-time creep tests performed at different temperatures and different stress levels are presented in the form of creep curves. Fatigue tests carried out at stress ratios R = 0.25 and R = −1 are shown in the form of S–N (fatigue) diagrams. The finite fatigue regime for each of the mentioned stress ratios is modeled by an inclined log line, while the infinite fatigue regime is modeled by a horizontal line, which represents the fatigue limit of the material and previously was calculated by the modified staircase method. Finally, the toughness has been calculated based on the Charpy V-notch energy.

Keywords: analysis; mechanical properties; short-time creep; fatigue; X46Cr13 steel

PACS: 81.05.Bx; 81.70.Bt; 81.40Np; 62.20.Hg; 62.20.me; 62.20.-x

1. Introduction The properties of the materials used in structural design need to be correlated with the purpose for which the structure is designed [1]. A structural design procedure, the manufacture of the structure, as well as structure maintenance must be able to ensure smooth and safe operation without any failure. However, many failures can occur during structure service life, and they can be caused by factors, such as , inadequate loading, , failure presence in the material, the improper use of material, poor design, poor structural assembly, manufacturing defects, unforeseen operating conditions, inadequate maintenance, untimely and inadequate control, etc. [2,3]. The failure modes are usually listed as fatigue, creep, fracture, , corrosion, elastic , etc. [4]. This study primarily considers two of the mentioned failure modes; the former is the creep behavior at different temperatures and different stress levels, and the latter is uniaxial fatigue at prescribed stress ratios. The creep phenomenon is usually defined as a time-dependent behavior where strain continuously

Materials 2017, 10, 388; doi:10.3390/ma10040388 www.mdpi.com/journal/materials Materials 2017, 10, 388 2 of 18 increases while the stress (load) is kept constant. It is usually appreciable at temperatures above 40% of the melting temperature of the material [5]. The following part of this paper is concerned with some of the recent research dealing with the steel X46Cr13. The work in [6] focuses on the lifetime reduction of cyclically-loaded X46Cr13 steel constantly exposed to highly corrosive CO2-saturated hot thermal water at 60 ◦C. Furthermore, the corrosion behavior of X46Cr13 martensitic depends strongly on the applied heat treatment, which was considered in [7]. Since steel X46Cr13 is usually used in cutlery fabrication requiring corrosion resistance and high hardness, an analysis is presented in [8] with reference to the identification of the heat treatment parameters influencing the corrosion resistance of the martensitic stainless steel. Furthermore, the influence of the microstructure and surface treatment on the corrosion resistance of martensitic steel was studied, and the results are consequently presented in [9]. A brief research work of the influence of process parameters on the quality of thermally sprayed material coating is given in [10] since the final coating quality is strongly related to the spray parameters’ definition, such as oxygen pressure, fuel gas type, etc. Sliding wear properties and corrosion resistance related to X46Cr13 steel were tested, and this problem is treated in [11]. Mechanical data on this steel are very scarce in the literature. The motivation for this investigation and preparing of the paper is to provide an insight into the behavior of materials under different temperature conditions. These data can be of interest for designers of the structures since this steel can be used for pump parts, roller bearings, etc.

2. Experimental Procedures

2.1. Tested Material Experimental results elaborated in this paper are related to the chromium martensitic stainless steel X46Cr13. The as-received material, as stated in the certificate of the supplier, was annealed and is a 16-mm cold drawn round bar, whose chemical composition is shown in Table1. However, the certificate of the material does not provide insight into the details that can explain how the delivered state of the material is achieved. This steel is commonly recognized as a high hardness material in conjunction with good corrosion resistance. Due to its high hardness, it is also well suited for the production of roller bearings, cutting tools, etc. Usually, it is most widely used in the quenched and tempered condition. As mentioned, this material may be used in mechanical, civil and industrial engineering, means for transport, etc. In particular, it can be used in the manufacturing of pump parts, machined parts, shaft, gears, tie rods, screws, etc. Pumps can be exposed to high corrosive conditions, such as corrosive thermal water present in geothermal power plants. The results of this research related to uniaxial fatigue testing provide an insight into the material fatigue behavior under normal environmental conditions at room temperature. However, this material can also be used for the production of valves and molds for the production of plastics, the cutting tools industry, etc.

Table 1. Material chemical composition: X46Cr13 steel.

Material: X46Cr13 Steel (Chromium Martensitic Stainless Steel) Designation Steel name Steel number (Mat. No; W. Nr; Mat. Code) EN 10088-3-2005/DIN 17440: X46Cr13; AISI 420; BS: 420S45. 1.4034 Chemical composition of considered material Mass (%) C Si Mn P S Cr V Rest 0.442 0.375 0.381 0.0121 0.0192 13.05 0.201 85.2997 Mo Cu Al Ti Pb Sn Co 0.0493 0.108 0.0026 0.0044 0.0004 0.0243 0.031 Materials 2017, 10, 388 3 of 18

Materials 2017, 10, 388 3 of 18

2.2.Materials Equipment, 2017, 10 Specimens,, 388 Testing Procedures and Standards 3 of 18 2.2. Equipment, Specimens, Testing Procedures and Standards This research includes several types of tests, testing procedures and standards. However, the 2.2. Equipment,This research Specimens, includes Testing several Procedures types of and tests, Standards testing procedures and standards. However, the standards define the geometry of the specimen used in the test, as well as the testing procedure. standards define the geometry of the specimen used in the test, as well as the testing procedure. UniaxialThis tests research that includedincludes several the determination types of tests, of testing the material procedures properties and standards. and the However, material the creep Uniaxial tests that included the determination of the material properties and the material creep behaviorstandards were define performed the geometry using of a 400-kNthe specimen capacity used material in the test, testing as well machine. as the Measurementstesting procedure. were behavior were performed using a 400-kN capacity material testing machine. Measurements were conductedUniaxial tests with that a macro included extensometer the determination and a high of temperaturethe material extensometer.properties and Inthe fatigue material testing, creep the behaviorconducted were with performed a macro extensometer using a 400-kN and capacitya high te materialmperature testing extensometer. machine. InMeasurements fatigue testing, were the servopulser of ±50-kN capacity was used, while in impact energy determination, a Charpy impact conductedservopulser with of ±50-kN a macro capacity extensometer was used, and whilea high in te impactmperature energy extensometer. determination, In fatigue a Charpy testing, impact the machine (150 J; 300 J) was used. The type of the specimen (shape and geometry) used in uniaxial servopulsermachine (150 of J;±50-kN 300 J) wascapacity used. was Th eused, type while of the in specimen impact energy (shape determination, and geometry) a used Charpy in uniaxial impact testing in order to determine the stress-strain diagrams at room and elevated temperatures, as well as machinetesting in (150 order J; 300to determine J) was used. the Thstress-straine type of thediag specimenrams at room (shape and and elevated geometry) temperatures, used in uniaxial as well material creep behavior is shown in Figure1. testingas material in order creep to behavior determine is shownthe stress-strain in Figure diag1. rams at room and elevated temperatures, as well as material creep behavior is shown in Figure 1.

Figure 1. Specimen’s geometry (mm): tensile test. Figure 1. Specimen’s geometry (mm): tensile test. Figure 1. Specimen’s geometry (mm): tensile test. Specimens were machined from the 16-mm steel rod in accordance with ASTM: E 8M-15a. Specimens were machined from the 16-mm steel rod in accordance with ASTM: E 8M-15a. Tensile TensileSpecimens tests (uniaxial were machined tensile testing/procedures) from the 16-mm steerelatedl rod to in the accordance determination with ofASTM: the stress-strain E 8M-15a. testsTensilediagram (uniaxial tests at room tensile(uniaxial temperature testing/procedures) tensile testing/procedures)were carried related out in accordance torelated the determination to the with determination the mentioned of the stress-strainof ASTM:the stress-strain E 8M-15a diagram atdiagram roomstandard, temperature at whileroom temperaturethose were related carried were to out carriedhigh in accordance temperatur out in accordance withes were the with mentionedperformed the mentioned ASTM:in accordance EASTM: 8M-15a Ewith 8M-15a standard, the whilestandard,ASTM: those E while related21-09 standard.those to high related temperaturesThe tomodulus high temperatur wereof performedes werewas in determinedperformed accordance inin with accordanceaccordance the ASTM: withwith E thethe 21-09 standard.ASTM:ASTM: EE The 11121-09 modulusstandard. standard. ofCreep elasticity The tests modulus were was carried determined of elasticity out in inaccordance accordancewas determined with with the ASTM: thein accordance ASTM: E 139-11 E 111 standard.with standard. the CreepASTM:Subsequently, tests E 111 were standard. the carried geometry Creep out in testsof accordance the were specimen carried with out used the in accordance ASTM:in Charpy E 139-11with impact the standard. ASTM:energy E determination139-11 Subsequently, standard. is the geometrySubsequently,presented of in the Figure the specimen geometry 2. Charpy used oftests inthe Charpywere specimen carried impact usedout energyon in the Charpy Charpy determination impact V-notch energy specimens, is presented determination machined in Figure isin 2. Charpypresentedaccordance tests in werewith Figure the carried 2. ASTM: Charpy out E ontests23-12c the were Charpystandard. carried V-notch out on specimens,the Charpy V-notch machined specimens, in accordance machined with in the ASTM:accordance E 23-12c with standard. the ASTM: E 23-12c standard.

Figure 2. Charpy V-notch specimen (mm). Figure 2. Charpy V-notch specimen (mm). Finally, specimens used inFigure fatigu 2.e Charpytests, Figure V-notch 3, were specimen machined (mm). in accordance with the ASTM: E 466-15Finally, standard. specimens All mentioned used in fatigu standardse tests, Figurecan also 3, be were found machined in the annual in accordance book [12]. with the ASTM: E 466-15Finally, standard. specimens All usedmentioned in fatigue standards tests, can Figure also3 ,be were found machined in the annual in accordance book [12]. with the ASTM: E 466-15 standard. All mentioned standards can also be found in the annual book [12].

Materials 2017, 10, 388 4 of 18 MaterialsMaterials 2017, 201710, 388, 10 , 388 4 of 184 of 18

FigureFigure 3. Specimen’s 3. Specimen’s geometry geometry (mm): (mm): fatigue fatigue test. test. Figure 3. Specimen’s geometry (mm): fatigue test.

3. Experimental3. Experimental Results Results and andDiscussion Discussion 3. Experimental Results and Discussion ResultsResults of this of thisexperimental experimental research research related related to the to mechanicalthe mechanical behavior behavior of material of material X46Cr13 X46Cr13 are are Results of this experimental research related to the mechanical behavior of material X46Cr13 presentedpresented in the in formthe form of diagrams of diagrams and/or and/or tables tables. These. These experimental experimental resu results includelts include mechanical mechanical are presented in the form of diagrams and/or tables. These experimental results include mechanical properties,properties, creep creep behavior, behavior, impact impact energy energy and anthed fatiguethe fatigue of the of consideredthe considered material. material. properties, creep behavior, impact energy and the fatigue of the considered material. 3.1. Uniaxial Tensile Tests 3.1.3.1. Uniaxial Uniaxial Tensile Tensile Tests Tests

EngineeringEngineeringEngineering Stress-Strain Stress-Strain Stress-Strain Diagrams Diagrams Diagrams and and andMechanical Mechanical Mechanical Properties Properties Properties versus versus versus Temperature Temperature Temperature ForFor theFor the results the results results of of uniaxial uniaxial of uniaxial testing testing testing of of the the of consider the considered considered material materialed material at at different different at different temperatures, temperatures, temperatures, engineering engineering engineering stress-strainstress-strainstress-strain diagrams diagrams diagrams were were were obtained, obtained, obtained, as as in inas Figure Figurein Figure 4.4. It It 4.is Itworthwhile is worthwhile to point to point out thatout that several several tests tests werewerewere made made made at at each each at each testing testing testing temperature, temperature, temperature, but but thebut the rethe resultingsulting resulting diagrams diagrams diagrams did did notdid not differnot differ differ much; much; much; thus, thus, thus, Figure Figure Figure 1 1 showsshowsshows the resultsresultsthe results ofof the the of first firstthe test firsttest for testfor each eachfor of each the of testedth ofe testedth temperatures.e tested temperatures. temperatures. Furthermore, Furthermore, Furthermore, in the samein the in manner, samethe same manner,Tablemanner,2 evaluates Table Table 2 evaluates the 2 evaluates numerical the numericalthe values numerical of values the values mechanical of th ofe mechanicalthe propertiesmechanical properties related properties torelated the related first to the test to firstthe at each firsttest test attesting eachat each testing temperature. testing temperature. temperature.

Figure 4. Engineering stress-strain diagrams at different temperatures: X46Cr13 steel. FigureFigure 4. Engineering 4. Engineering stress-strain stress-strain diagrams diagrams at different at different temperatures: temperatures: X46Cr13 X46Cr13 steel. steel.

Materials 2017, 10, 388 5 of 18

MaterialsTable2017, 10 2., 388Material properties versus temperature and reduction factors: X46Cr13 steel. (, ultimate 5 of 18 tensile strength; . , 0.2 offset strength; E, modulus of elasticity). Material Properties Reduction Factors Table 2. Material properties versus temperature and reduction factors: X46Cr13 steel. (σm, ultimate Temperature tensile strength; σ0.2, 0.2 offset yield strength; E, modulus of elasticity). T (°C) (MPa) . (MPa) E (GPa) =() =.() =() /() /.() /() Temperature Material Properties Reduction Factors ◦ 20 781.7 657.5 220 1 1 1 T( C) σm (MPa) σ0.2 (MPa) E (GPa) k1 = σm(T)/σm(20) k2 = σ0.2(T)/σ0.2(20) k3 = E(T)/E(20) 20100 781.7720.6 657.5 618.4 220 210 1 0.922 10.940 0.955 1 100150 720.6695.7 618.4 578.5 210 205 0.922 0.889 0.9400.879 0.932 0.955 150200 695.7664.8 578.5 570.1 205 200 0.889 0.850 0.8790.867 0.909 0.932 200 664.8 570.1 200 0.850 0.867 0.909 300300 613613 588.8 588.8 190 190 0.784 0.784 0.8960.896 0.864 0.864 400400 549549 510 188510 188 0.702 0.702 0.7760.776 0.855 0.855 500500 352.3352.3 323.3 323.3 155 155 0.451 0.451 0.4920.492 0.705 0.705 600 186.5 158 80 0,239 0.240 0.364 700600 125.7186.5 83 158 60 0.16180 0,239 0.1260.240 0.364 0.273 700 125.7 83 60 0.161 0.126 0.273

Figure5 presents the temperature dependencies of the mechanical properties. Experimental Figure 5 presents the temperature dependencies of the mechanical properties. Experimental resultsresults are are shown shown by by using using special special characters characters ((▪,, ♦),), while while the the continuous continuous dependencies dependencies of ofthe the consideredconsidered properties properties are are shown shown by by approximation approximation curves (solid line line or or dashed dashed line). line). Since Since these these approximationsapproximations describe describe the the temperature temperature dependencedependence of experimentally-obtained results results with with some some degreedegree of of accuracy, accuracy, the the so-called so-called coefficient coefficient ofof determinationdetermination RR22 is is introduced. introduced. It Itis isa ameasure measure of of accordanceaccordance between between experimental experimental results results and and mentionedmentioned polynomial approximation, approximation, and and it itserves serves as as thethe statistics statistics that that gives gives the the necessary necessary information information aboutabout how fit fit a a model model is is [13]. [13].

8 4 5 3 2 2  m (T )  2.08432 10 T  2.85903 10 T  1.12297 10 T 1.99985 T  822.77 8 4 5 3 2 2  0.2 (T )  2.57882 10 T  3.63968 10 T 1.48273 10 T  2.27857 T  704.893 (a)

Figure 5. Cont.

Materials 2017, 10, 388 6 of 18 Materials 2017, 10, 388 6 of 18

7 3 5 2 2 E(T)  4.21774 10 T  2.67629 10 T  5.07072 10 T  216.957 (b)

 (T )  6.16603 1012 T 5 1.13929 108 T 4  6.73962 106 T 3 

1.7431103T 2 1.83656 101T  52.3365 11 5 8 4 5 3  t (T )  1.01799 10 T 1.7061310 T 1.00116 10 T 

 2.59757 103T 2  2.98047 101T 13.0389 (c)

Figure 5. Material properties versus temperature: X46Cr13 steel. (a) Mechanical properties Figure 5. Material properties versus temperature: X46Cr13 steel. (a) Mechanical properties (σm, ( , ultimate tensile strength; . , 0.2 offset yield strength) versus temperature; (b) modulus of ultimate tensile strength; σ0.2, 0.2 offset yield strength) versus temperature; (b) modulus of elasticity (E)  versuselasticity temperature; (E) versus (c) totaltemperature; elongation (c ()ε t)total and reductionelongation in ( areat ) ( ψand) versus reduction temperature. in area (  ) versus temperature. Based on the experimental results, it is visible that the considered steel possesses high ultimate tensileBased strength on the (781.7 experimental MPa) and results, high yield it is visible strength that (657.5 the considered MPa) at thesteel room possesses testing high temperature. ultimate tensile strength (781.7 MPa) and high yield strength (657.5 MPa) at the room testing temperature. Furthermore, this steel exhibits a high level of the modulus of elasticity (220 GPa/20 ◦C). Furthermore, this steel exhibits a high level of the modulus of elasticity (220 GPa/20 °C). Furthermore, with an increase in temperature, all of the mentioned properties decrease. However, Furthermore, with an increase in temperature, all of the mentioned properties decrease. even at a temperature of 500 ◦C, the ultimate tensile strength (352 MPa) and yield strength (323 MPa) However, even at a temperature of 500 °C, the ultimate tensile strength (352 MPa) and yield strength may be treated as properties of a high level. In addition, it is visible that after the temperature of 500 ◦C, (323 MPa) may be treated as properties of a high level. In addition, it is visible that after the thetemperature strength decreases of 500 °C, quite the quickly. strength On decreases the other quite hand, quickly. the strains On the that other are hand, reduced the betweenstrains that the are room ◦ temperaturereduced between and the the temperature room temperature of 500 andC increase the temper afterature this of temperature 500 °C increase very after quickly. this temperature A drop in the strainsvery inquickly. the mentioned A drop in range the strains of temperatures in the mentioned may be range treated of astemperatures a consequence may ofbe dynamic treated as strain a

Materials 2017, 10, 388 7 of 18 aging, which is a hardening phenomenon. When the effect of serrations at the stress-strain curve is not visible, then a strain aging phenomenon can be marked by lower strain rate sensitivity. Dynamic strain aging causes a minimum variation of with temperature changes, and a plateau in strength can be seen. If this material is compared with the 42CrMo4 steel, [14], which can also be used for manufacturing of the shaft, then it can be said as follows. Steel X46Cr13 is martensitic stainless steel (0.442 C; 13.05 Cr), while steel 42CrMo4 (0.45 C; 1.06 Cr) is a low alloy steel, and thus, steel X46Cr13 is very resistant to corrosive environmental conditions. In terms of the strength of the material, steel X46Cr13 (ultimate tensile strength: 781.7 MPa/20 ◦C; yield strength: 657.5 MPa/20 ◦C) is much better than 42CrMo4 steel (ultimate tensile strength: 617 MPa/20 ◦C; yield strength: 415 MPa/20 ◦C).

3.2. Uniaxial Short-Time Creep Tests and Creep Modeling Generally, testing of the materials to creep can be classified into two groups: short-term and long-term testing. In any case, long-term testing is very expensive. In accordance with the possibilities of available equipment and the cases in which the short-term tests may be of interest, this study considers short-term creep. On the other hand, it is useful to simulate/model material creep behavior for those materials for which the behavior of creep in certain conditions is already known. When creep modeling is considered, then the following cases for strain calculation are possible:

(a) ε = ε(t); T = const, σ = const; (b) ε = ε(σ, t); T = const (i.e., T = const : σ1(T) = const, σ2(T) = const ) ε = ε(σ T t) T = const σ = const σ = const T = const , ... .. ; (c) , , (i.e., 1 : 1(T1) , 2(T1) , ... ..; 2 : = const = const ) σ1(T2) , σ2(T2) ,.... . Modeling can be performed using rheological models [15] or with certain equations. In the case of this investigation, the following equation will be used [16]:

ε(t) = D−Tσptr (1)

In Equation (1), the notations stand for: T, temperature; σ, stress; t, time; and D, p and r are parameters that need to be determined. Equation (1) is intended to model the first stage of the creep (transient creep) and can be applied for any of the above-mentioned cases (a, b, c). In the case of this research, creep modelling is performed in such a way that:

ε(t) = ε(σ, T, t) (1a)

Thus, the last of the above-mentioned cases of modeling is performed (time-temperature-stress dependence). In accordance with Equation (1a), Equation (1) provides modelling of any considered (desirable) creep process that belongs to any of the considered stress levels within the considered temperature levels. For the purpose of modeling in the literature, it is known as the Bailey–Norton equation [17,18]: p r εc = kσ t , → εc = F(σ) f (t) (1b) The parameters (material constants) in this equation need to be obtained on the basis of the considered creep curve. Equation (1b) can be generalized as the product of two functions, as is shown above. Function f (t) represents any suitable time function, and it was observed that power function tr is in good agreement with the experiments. The exponent “r” is in general smaller than 0.5 [17]. In this equation, an instantaneous strain can be involved. If Equations (1) and (1b) are compared, it is visible that Equation (1) can be taken as the rearranged Bailey–Norton equation, and in this case, the factor (D−T) can take the meaning of “k” in the Bailey–Norton equation. On the other hand, when the temperature range and range of stresses within any of the prescribed temperature levels are considered (above-mentioned case “c”), then factor “D” is obtained on the basis of both ranges. The obtained value of factor “D” in this way (the same is valid for other factors) provides modeling of any creep curve within both of the mentioned ranges (temperature and stress), as is visible from Table3 and Figure 9a. Furthermore, for creep modeling the following equation can be used [18]: Materials 2017, 10, 388 8 of 18

−A/T p r MaterialsMaterials 20172017,, 1010,, 388388 ε(t) = ae σ t 88 (1c) ofof 1818 Materials 2017, 10, 388 8 of 18 Since this equation is similar to the previously-used Equation (1), the obtained result using it is SinceSinceSince this equationthisthis equationequation is similar isis similarsimilar to the toto thethe previously-used previously-usedpreviously-used Equation EquationEquation (1), (1),(1), the thethe obtained obtainedobtained result resultresult using usingusing itit isis shown in Table 3 and Figure 9b. In Equation (1c), ,, , , and are material parameters. shownshownshown in Table inin TableTable3 and 33 Figure andand FigureFigure 9b. In 9b.9b. Equation InIn EquationEquation (1c), (1c),(1c),a, A,, ,p and andandr are are are material materialmaterial parameters. parameters.parameters. Creep tests carried out at different temperature levels are presented in Figures 6–8. CreepCreepCreep tests teststests carried carriedcarried out out atout different atat differentdifferent temperature tempertemperatureature levels levelslevels are areare presented presentedpresented in inin Figures FiguresFigures6– 6–8.6–8.8.

FigureFigure 6.6. Short-timeShort-time creepcreep teststests ofof steelsteel XX46Cr1346Cr13 atat thethe temperaturetemperature ofof 400400◦ °C.°C. FigureFigure 6. Short-time 6. Short-time creep creep tests tests of steelof steel X46Cr13 X46Cr13 at at the the temperature temperature of of 400 400 C.°C.

FigureFigure 7.7. Short-timeShort-time creepcreep teststests ofof steelsteel XX46Cr1346Cr13 atat thethe temperaturetemperature ofof 500500 °C.°C. FigureFigure 7. Short-time 7. Short-time creep creep tests tests of steelof steel X46Cr13 X46Cr13 at at the the temperature temperature of of 500 500◦ C.°C.

FigureFigure 8.8. Short-timeShort-time creepcreep teststests ofof steelsteel XX46Cr1346Cr13 atat thethe temperaturetemperature ofof 600600 °C.°C. Figure 8. Short-time creep tests of steel X46Cr13 at the temperature of 600 °C. Figure 8. Short-time creep tests of steel X46Cr13 at the temperature of 600 ◦C. On the basis of the presented creep curves obtained in short-time creep conditions, some OnOn thethe basisbasis ofof thethe presentedpresented creepcreep curvescurves obobtainedtained inin short-timeshort-time creepcreep conditions,conditions, somesome conclusions related to the creep resistance of the tested material may be given. Obviously, this conclusionsconclusions relatedrelated toto thethe creepcreep resistanceresistance ofof thethe testedtested materialmaterial maymay bebe given.given. Obviously,Obviously, thisthis material may be treated as creep resistant at a temperature of 400°C when the applied stress does not materialmaterial maymay bebe treatedtreated asas creepcreep resistantresistant atat aa temptemperatureerature ofof 400°C400°C whenwhen thethe appliedapplied stressstress doesdoes notnot

Materials 2017, 10, 388 9 of 18

On the basis of the presented creep curves obtained in short-time creep conditions, some conclusions related to the creep resistance of the tested material may be given. Obviously, this material may be treated as creep resistant at a temperature of 400◦C when the applied stress does not exceed 50% of the yield stress at this temperature. Furthermore, it is creep resistant at the temperature of 500 ◦C when the applied stress does not exceed 30% of the yield stress and, finally, at the temperature of 600 ◦C when the applied stress does not exceed 20% of the yield stress at this temperature. However, it needs to be said that these test results can give some orientation relating to material creep behavior, but only in the short-time regime of exploitation. If the comparison of this steel with previously-mentioned 42CrMo4 steel [14] is also made with respect to creep resistance, then, in general, it can be said that no significant difference can be seen in the range between 400 ◦C and 600 ◦C. However, it can be interesting to note that at the temperature of 600 ◦C, steel X46Cr13 subjected to stress of 31.6 MPa exhibits the strain of 2.5%, while steel 42CrMo4 subjected to stress of 34 MPa (a little different from 31.4 MPa) exhibits the strain of 5%. This shows that in these conditions, steel 42CrMo4 is less resistant to creep than X46Cr13. In this research, some of the creep curves within the temperature range from 400 ◦C–600 ◦C are modeled using the earlier mentioned Equation (1). Selected creep curves are visible in Table3. This means that using the model presented by Equation (1), any of the creep processes belonging to the mentioned range of temperatures and stress levels can be modeled using the obtained parameters given in Table3. Of course, using the model presented by Equation (1), any of the mentioned creep curves may also be modeled separately, but in this case, other parameters are valid. In Table3, creep modeling data are given, while in Figure9, the creep modeling for selected creep curves is shown.

Table 3. Creep modeling data.

Material X46Cr13 (1.4034) Creep strain-time dependence models: ε(t) = D−Tσptr (Equation (1)); and ε(t) = ae−A/Tσptr (Equation (1c)) where ε(t) = ε(σ, T, t) Time (min) = 1200, for all creep processes Creep processes were carried out at the temperatures and stresses listed below Constant 400 500 600 temperature (T ◦C) Constant stress level 153 255 64 97 31 σ(MPa)

σ = x·σ0.2 x = 0.3 x = 0.5 x = 0.2 x = 0.3 x = 0.2 Parameters (D, p, r) and (a, A, p, r) valid for: Parameters x = 0.1–0.6 x = 0.1–0.4 x = 0.1–0.3 D, p, r D(T) = 1.4300566 · 10−5T2 − 1.3106973 · 10−2T + 4.0672118 In accordance with p(T) = 4.0013503 · 10−4T2 − 3.5889003 · 10−1T + 81.625941 Equation (1) r(T) = 1.5594627 · 10−6T2 − 1.0455052 · 10−3T + 4.9960179 · 10−1 a(T) = −9.3283076 · 10−3T2 + 9.3283076T − 2.2387938 · 103 a, A, p, r A(T) = −2.3021939 · T2 + 2.2964057 · 103T − 5.4910214 · 105 In accordance with ( ) = − · −4 2 + · −2 − Equation (1c) p T 1.024534 10 T 9.7985856 10 T 21.166019 r(T) = 1.7800633 · 10−6T2 − 1.2881659 · 10−3T + 5.6578198 · 10−1 Materials 2017, 10, 388 10 of 18 Materials 2017, 10, 388 10 of 18

(a)

(b)

Figure 9. Experimentally-obtained creep curves and modeled creep curves. (a) Modeling performed Figure 9. Experimentally-obtained creep curves and modeled creep curves. (a) Modeling performed by Equation (1);(1); ((bb)) modelingmodeling performedperformed byby EquationEquation (1c).(1c).

On the basis of the shapes of the modeled creep curves, it can be said that both models (EquationsOn the (1) basis and of (1c)) the shapes give quite of the well-modeled modeled creep cu curves,rves. However, it can be said based that on both experience models (Equations in this kind (1) andof modeling, (1c)) give the quite following well-modeled should curves. be noted. However, Since the based modeling on experience covers a in range this kind of temperatures of modeling, and the followinga range of shouldstresses, be a noted. very large Since number the modeling of (experimental) covers a range points of temperatures of all of the andcurves a range is a problem of stresses, in adata very processing. large number Furthermore, of (experimental) the forms pointsof the primar of all ofy stages the curves of creep is a curves problem differ in datafrom processing.each other, Furthermore,and the common the formsmodel of is thenot primaryequally stageseffective of for creep all curvesprimary differ phases from ofeach the creep other, curves. and the In common spite of modelthat, the is model not equally provides effective the simulation for all primary of the phasesdesired of curve, the creep which curves. is an advantage In spite of of that, such the a model. model providesIt should thebe pointed simulation out of that the sometimes, desired curve, it is whichdifficult is anto distinguish advantage ofhow such far a the model. primary It should stage beof pointedcreep extends. out that sometimes, it is difficult to distinguish how far the primary stage of creep extends. 3.3. Charpy V-notch Impact Energy and Fracture Toughness Calculation 3.3. Charpy V-notch Impact Energy and Fracture Toughness Calculation The service life of the structure depends on operating conditions and material properties. The service life of the structure depends on operating conditions and material properties. Concerning this, many of the material properties, the design methodology, the maintenance of the Concerning this, many of the material properties, the design methodology, the maintenance of the structure, etc., can be included in the design procedure. The structure is usually assumed to have been structure, etc., can be included in the design procedure. The structure is usually assumed to have designed and manufactured so that no failure in the material exists. However, many failures can arise been designed and manufactured so that no failure in the material exists. However, many failures in structure service life due to loading, creep, fatigue, etc. In accordance with the design strategy, two can arise in structure service life due to loading, creep, fatigue, etc. In accordance with the design of the material properties are usually mentioned, that is yield strength (σ0.2) and fracture toughness strategy, two of the material properties are usually mentioned, that is yield strength (.) and fracture (KIc). Yield strength serves as a criterion for the design of the structure against plastic deformation, toughness (). Yield strength serves as a criterion for the design of the structure against plastic deformation, while fracture toughness serves as a criterion for the design of structure against fracture.

Materials 2017, 10, 388 11 of 18 whileMaterials fracture 2017, 10 toughness, 388 serves as a criterion for the design of structure against fracture. Both11 of of 18 the mentioned properties can be experimentally determined. Since the experimental investigations related Both of the mentioned properties can be experimentally determined. Since the experimental to fracture toughness can be expensive and require some efforts in manufacturing specimens, there is a investigations related to fracture toughness can be expensive and require some efforts in method that is simpler, but that may also serve in the fracture toughness assessment of the considered manufacturing specimens, there is a method that is simpler, but that may also serve in the fracture material. This method consists of deriving the fracture toughness from the fracture impact energy of toughness assessment of the considered material. This method consists of deriving the fracture thetoughness material. from The impact the fracture energy impact can be energy determined of the usingmaterial. the CharpyThe impact test energy method. can Fracture be determined toughness canusing be calculated the Charpy with test the well-knownmethod. Fracture Roberts–Newton toughness equationcan be calculated [19–21]: with the well-known

Roberts–Newton equation [19-21]: 0.63 KIc = 8.47 (CVN) (2) 0.63 = 8.47 (CVN) (2) ◦ ◦ ◦ TheThe measured measured impact impact Charpy Charpy energies energies are are as as follows: follows: 6, 6, 6, 6, 6.5/ 6.5/−1010 °C;C; 7, 7, 7, 7, 8/0 8/0 °C;C; 8, 8,9, 9,8/20 8/20 °C; C; ◦ ◦ ◦ ◦ 17,17, 17, 17, 17/50 17/50 C;°C; 25, 25, 24, 24, 24.5/80 24.5/80 °C;C; 30,30, 29,29, 29/10029/100 °C;C; 32, 32, 33, 33, 32/120 32/120 5 °C, 5 C, and and they they are are shown shown in inthe the FigureFigure 10 10.. Although Although the the Charpy Charpy tests tests areare much simp simpler,ler, the the results results obtained obtained by by the the fracture fracture toughness toughness teststests are are much much more more reliable. reliable. However, However, fracture fracture toughnesstoughness testing is is also also a a laboratory laboratory method. method.

K (T )  4.6598 10 5 T 3  7.49795 10 3 T 2  1.66731 10 1T  27.4741 Ic CVN(T ) 2.5065 105 T 3 4.41944 103T 2 4.48903 102 T 6.28307         FigureFigure 10. 10.Charpy CharpyV-notch V-notch impactimpact energy an andd fracture fracture toughness toughness calculation. calculation.

3.4.3.4. Fatigue Fatigue Tests Tests and and Fatigue Fatigue Limit Limit Calculation Calculation Fatigue is known as one of the possible mechanical failure modes of engineering structures. Fatigue is known as one of the possible mechanical failure modes of engineering structures. Therefore, it is of interest to investigate the resistance of the material to such mechanical failures. Therefore, it is of interest to investigate the resistance of the material to such mechanical failures. Namely, the material subjected to a repeated load (cyclic load) may experience rupture at a stress Namely, the material subjected to a repeated load (cyclic load) may experience rupture at a stress level significantly lower than the rupture stress corresponding to the same type of monotonic load. levelUsually, significantly the fracture lower of an than engineering the rupture element stress is correspondingdefined as the process to the of same damage type accumulation of monotonic due load. Usually,to cyclic the loading fracture during of an engineeringits service life element [22]. This is defined research as is the focused process on of the damage axial cyclic accumulation load. Tests due towere cyclic carried loading out during at room its temperature service life [ 22and]. Thisat two research mutually-different is focused on stress the axialratios cyclic in accordance load. Tests with were carriedthe ISO out 12107:2012 at room temperature(2012) standard and [23]. at The two results mutually-different of this investigation stress offer ratios some in accordance good information with the ISOthat 12107:2012 can be used (2012) in structural standard design [23]. Theagainst results fracture of this and investigation belong to the offer so-called some stress-life good information model. thatUnnotched can be used specimens in structural of the type design shown against in Figure fracture 3 were and subjected belong toto thedifferen so-calledt levels stress-life of axial loads model. Unnotchedfor two prescribed specimens stress of the ratios. type shownFor each in of Figure the applied3 were subjectedstress levels, to different several tests levels were of axial carried loads out for twosince prescribed scatter in stress the ratios.number For of eachthe cycles of the to applied failure stressmay occur. levels, Experimentally-obtained several tests were carried data out are since scatterplaced in in the the number coordinate of the system; cycles see to failureFigure may11. On occur. the ordinate, Experimentally-obtained data related to the data maximum are placed stresses in the coordinate(/MPa system;) are plotted, see Figure while 11 on. On the theabscissa, ordinate, data data related related to the to number the maximum of the cycles stresses (N) (toσmax failure/MPa ) areare plotted, plotted. while In this on way, the abscissa, so-called dataS–N relateddiagram to (str theess number versus ofthe the number cycles of (N) cycles to failure to failure), are plotted. also Inknown this way, as so-calledthe Wohler S–N curve, diagram or the (stress fatigue versus life diag theram, number can be of created, cycles toand failure), in this alsocase, known this is as presented in Figure 11. These diagrams are drawn separately for two stress ratios. Each diagram

Materials 2017, 10, 388 12 of 18 the Wohler curve, or the fatigue life diagram, can be created, and in this case, this is presented in FigureMaterials 11. 2017 These, 10, 388 diagrams are drawn separately for two stress ratios. Each diagram represents12 of 18 the fatigue behavior of the considered material for one of the prescribed stress ratios. In addition, each diagramrepresents consists the offatigue the two behavior areas: theof the one considered that belongs material to the for finite one fatigue of the lifeprescribed (region) stress and theratios. second In oneaddition, that belongs each todiagram the infinite consists fatigue of the life. two Sinceareas: both the one parts that of belongs the diagrams to the finite are presentedfatigue life in (region) a linear form,and the the diagram second consistsone that of belongs one inclined to the lineinfinite and onefatigue horizontal life. Since line. both This parts horizontal of the diagrams line represents are thepresented fatigue limit in a (endurance linear form, limit). the diagram In engineering consists of practice, one inclined for steel line alloys, and one the horizontal number ofline. cycles This to horizontal line represents the fatigue limit (endurance limit). In engineering practice, for steel alloys, failure at which the specimen remains unfailed (unbroken) is usually adopted as 107 cycles. The fatigue the number of cycles to failure at which the specimen remains unfailed (unbroken) is usually adopted limit (fatigue strength in the infinite fatigue life region, endurance limit), presented in Figure 11 as as 10 cycles. The fatigue limit (fatigue strength in the infinite fatigue life region, endurance limit), a horizontal line, can be calculated using a modified staircase method. In both cases, i.e., in the case presented in Figure 11 as a horizontal line, can be calculated using a modified staircase method. In of stress ratio R = −1, as well as in the case of stress ratio R = 0.25, testing was carried out under both cases, i.e., in the case of stress ratio =−1, as well as in the case of stress ratio =0.25, testing a decreasingwas carried stress out under regime. a decreasing Data related stress to regime. the specimens Data related that to have the specimens failed and that those have that failed have and not failedthose are that given have in Tablenot failed4, and are they given are in needed Table 4, for and the they analysis are needed in the for staircase the analysis method. in the An staircase analysis of themethod. staircase An dataanalysis for of the the derivation staircase data of the for constants the derivation is presented of the constants in Table5 is. Thepresented calculation in Table of the5. constantsThe calculation in accordance of the withconstants the ISO in accordance standard is wi presentedth the ISOin standard Table6. is presented in Table 6.

(a)

(b)

Figure 11. Stress versus number of cycles to failure: experimental data and S–N curve. (a) Stress ratio Figure 11. Stress versus number of cycles to failure: experimental data and S–N curve. (a) Stress ratio = 0.25; (b) stress ratio = −1. R = 0.25; (b) stress ratio R = −1. Data in S–N diagram (curve) related to the failed specimens (♦) and those that did not fail (○) are placed at the appropriate locations according to the number of cycles. Tests were conducted in such a way that the specimen has failed or the test has passed 10 million cycles. However, the S–N

Materials 2017, 10, 388 13 of 18

Data in S–N diagram (curve) related to the failed specimens () and those that did not fail ( ) are placed at the appropriate locations according to the number of cycles. Tests were conducted in such# a way that the specimen has failed or the test has passed 10 million cycles. However, the S–N diagram in Figure 11 consists of two regions, namely the finite fatigue region represented by the inclined line and the infinite fatigue region represented by the horizontal line.

Fatigue Limit Calculation Fatigue strength in the infinite fatigue life region, known as the fatigue limit (endurance limit), can be calculated in accordance with the modified staircase method. This quantity is drawn as the horizontal line in Figure 11. As was said, analysis in the modified staircase method is also completed on the basis of a decreasing stresses regime. On the basis of the S–N data, the modified staircase method analysis is completed for a particular stress ratio. From the S–N diagram (also Table5), stress levels used in the modified staircase method are visible. It is also visible that when the staircase modified method for determining the fatigue limit (i.e., in the infinite fatigue regime) is used, the highest level of the stress is σ2, and this coincides with the lowest stress measured in the finite fatigue regime. In general, the “σi” (MPa) stress level corresponds to the “i-th” level of testing and to the failure events that occurred, designated as “ fi”. The step of the stress applied at stress ratio (in the modified staircase method) R = 0.25 was d = 5 MPa, and d = 2.5 MPa in the case of R = −1. Data related to the failed and non-failed specimens used in the modified staircase method are shown in Table4.

Table 4. Data for the modified staircase method related to failed () and non-failed ( ) specimens obtained from the fatigue tests. #

R = 0.25 Number of Specimens Stress/max σi MPa 1 2 3 4 5 6 7 690    685  680 ## # R = −1 Number of Specimens Stress/max σi MPa 1 2 3 4 5 6 7 330    327.5   325 # # Table 5. Data analysis for the modified staircase method (f = failed).

R = 0.25 2 Stress/max σi (MPa) Number of the level of stress i fi ifi i fi 690 2 3 6 12 685 1 1 1 1 680 0 0 0 0 2 ∑ fi, i fi, i fi 4 7 13 R = −1 2 Stress/max σi (MPa) Number of the level of stress i fi ifi i fi 330 2 3 6 12 327.5 1 2 2 2 325 0 0 0 0 2 ∑ fi, i fi, i fi 5 8 14 Materials 2017, 10, 388 14 of 18

Table 6. A, B, C and D constants calculated in accordance with the ISO standard.

R = 0.25 Equation Material: X46Cr13 (1.4034)

A = ∑ i· fi 7 2 B = ∑ i · fi 13 C = ∑ fi 4 2 = B·C−A 0.1875 D C2 R = −1 Equation Material: X46Cr13 (1.4034)

A = ∑ i· fi 8 2 B = ∑ i · fi 14 C = ∑ fi 5 2 = B·C−A 0.24 D C2

These data are analyzed in a further procedure (Table5), and constants A, B, C and D are determined (Table6). The fatigue limit (endurance limit) is defined in accordance with the ISO standard as follows:

σf (P,1−α) = µy − k(P,1−α,do f )·σy (3)

In Equation (3), the following notations are used:

- µy, the mean fatigue strength, which is defined as: A 1 µ = σ + d( − ) (4) y 0 C 2 where “d” is the stress step (i.e., the difference between the neighboring stress levels) (Table5).

- k(P,1−α,ν), the coefficient for the one-sided tolerance limit for a normal distribution - σy, the estimated standard deviation of the fatigue strength, which is calculated as:

σy = 1.62·d(D + 0.029). (5)

In accordance with the standard recommendation of ν = n − 1 = 6, where n is the number of items in a considered group. Further, for a desired probability of P = 10% and a confidence level

(1 − α) = 90%, in accordance with Table B1 [23], it is: k(P,1−α,ν) = k(0.1;0.9;6) = 2.333. In accordance with Equation (4), it is:

A 1 R = 0.25 → µ = σ + d( − ) = 680 + 5 (7/4 − 1/2) = 686.25 MPa y 0 C 2 A 1 R = −1 → µ = σ + d( − ) = 325 + 2.5 (8/5 − 1/2) = 327.75 MPa y 0 C 2 or this can be obtained as (Table4):

R = 0.25 → µy = (680 + 685 + 690 + 685 + 690 + 685 + 690)/7 = 686.4 MPa,

R = −1 → µy = (325 + 327.5 + 330 + 327.5 + 330 + 327.5 + 330)/7 = 328.2 MPa, whose amount is similar to the previously-obtained ones. In accordance with Equation (5), it is:

R = 0.25 → σy = 1.62·d(D + 0.029) = 1.625·5 (0.1875 + 0.029) = 1.75 MPa Materials 2017, 10, 388 15 of 18

R = −1 → σy = 1.62·d(D + 0.029) = 1.62·2.5 (0.24 + 0.029) = 1.09 MPa Finally, the fatigue limit is (3): Materials 2017, 10, 388 15 of 18

R = 0.25 → σf (0.1;0.9;6) = µy − k(P,1−α,ν)·σy = 686.25 − 2.333 × 1.75 = 682.16 MPa = 0.25 → (.;.;) =̅ −(,,) ∙ = 686.25 − 2.333 × 1.75 = 682.16 MPa Materials 2017, 10, 388 15 of 18 R = −1 → σf (0.1;0.9;6) = µy − k(P,1−α,ν)·σy = 327.75 − 2.333 × 1.09 = 325.2 MPa =−1 → (.;.;) =̅ −(,,) ∙ = 327.75 − 2.333 × 1.09 = 325.2 MPa One of the fractured specimens is presented in Figure 12. One of the = fractured 0.25 → (.;.;) specimens= is̅ −presented(,,) in∙ Figure = 686.25 12. − 2.333 × 1.75 = 682.16 MPa

=−1 → (.;.;) =̅ −(,,) ∙ = 327.75 − 2.333 × 1.09 = 325.2 MPa One of the fractured specimens is presented in Figure 12.

Figure 12. Fractured specimen (from the fatigue process, R = 0.25). Figure 12. Fractured specimen (from the fatigue process, R = 0.25). All fractured specimens at stress ratio = 0.25 and stress ratio =−1 show the same kind of Allfracture. fractured The results specimens obtainedFigure 12. at stressFracturedby means ratio specimen of Rthe= fatigu (from0.25e andthe tests fatigue stress give process, an ratio insight RR == 0.25). regarding−1 show the the level same of the kind of fracture.fatigue The limit results in comparison obtained with by means the ultimate of the streng fatigueth of tests the givematerial an insightobtained regarding in static conditions. the level of the fatigue limitAll in fractured comparison specimens with at the stress ultimate ratio strength = 0.25 and of stress the material ratio =−1 obtained show inthe static same conditions.kind of 3.5.fracture. Microstructure The results Analysis obtained by means of the fatigue tests give an insight regarding the level of the fatigue limit in comparison with the ultimate strength of the material obtained in static conditions. 3.5. MicrostructureThe basic Analysismicrostructure analysis of the investigated material was performed on several specimens. An optical microscope was used to investigate the microstructure of as-received material The3.5. basic Microstructure microstructure Analysis analysis of the investigated material was performed on several specimens. and the microstructure of the material that had previously been exposed to creep. Furthermore, SEM An optical microscope was used to investigate the microstructure of as-received material and the (scanningThe electronbasic microstructure microscopy) wasanalysis used of for the the in invevestigatedstigation material of the microstructurewas performed of onthe several material microstructureunderspecimens. fatigue. of An the Anoptical material optical microscope micrograph that had was previously usedof the to investas-r beeneceivedigate exposed the material microstructure to creep.and optical Furthermore, of as-received micrographsSEM material of (scanning the electronmaterialand microscopy) the microstructurethat had was previously used of the for materialbeen the exposed investigation that had to pr creviouslyeep of theare microstructurebeenpresented exposed in toFigure creep. of the 13. Furthermore, material Finally, underthe SEM SEM fatigue. An opticalmicrograph(scanning micrograph electron of the material microscopy) of the as-receivedexposed was toused unia material forxial the fatigue inve andstigation is optical presented of micrographs the in microstructure Figure 14. of the of materialthe material that had previouslyunder beenfatigue. exposed An optical to creepmicrograph are presented of the as-r ineceived Figure material 13. Finally, and optical the SEMmicrographs micrograph of the of the material that had previously been exposed to creep are presented in Figure 13. Finally, the SEM material exposed to uniaxial fatigue is presented in Figure 14. micrograph of the material exposed to uniaxial fatigue is presented in Figure 14.

(a) (b)

(a) (b)

(c)

Figure 13. Optical micrograph: steel X46Cr13 (cross-section of the specimen), Aqua regia, 1000×. (c) (a) As-received material; (b) after the creep process performed at 400 °C, 153 MPa, 1200 min; (c) after Figure 13. Optical micrograph: steel X46Cr13 (cross-section of the specimen), Aqua regia, 1000×. Figurethe 13. creepOptical process micrograph: performed at steel500 °C, X46Cr13 97 MPa, (cross-section1200 min. of the specimen), Aqua regia, 1000×. (a) As-received material; (b) after the creep process performed at 400 °C, 153 MPa, 1200 min; (c) after a b ◦ c ( ) As-receivedthe creep process material; performed ( ) after at 500 the °C, creep 97 MPa, process 1200 performed min. at 400 C, 153 MPa, 1200 min; ( ) after the creep process performed at 500 ◦C, 97 MPa, 1200 min.

Materials 2017, 10, 388 16 of 18 Materials 2017, 10, 388 16 of 18

(a)

(b)

Figure 14. SEMSEM and and EDS EDS analysis: X46Cr13 steel under uniaxialuniaxial fatigue, 776776 MPa/194 MPa, MPa, stress stress ratio R = 0.25, (average value of cycles to failure: 330,000). ( a)) Fractured surface of the specimen; ( b) EDS analysis of Points A, B and C.

Based on the micrograph of the as-received material (Figure 13a), it is visible that the basic Based on the micrograph of the as-received material (Figure 13a), it is visible that the basic microstructure (main phase) is ferrite with spherical pearlites uniformly dispersed. Regarding the microstructure (main phase) is ferrite with spherical pearlites uniformly dispersed. Regarding the material subjected to creep (Figure 13b,c), no visible changes were made. With an electron scanning material subjected to creep (Figure 13b,c), no visible changes were made. With an electron scanning microscope, the images of a sample (fracture surface of a sample/specimen) can be produced by microscope, the images of a sample (fracture surface of a sample/specimen) can be produced by scanning it with a focused beam of electrons interacting with the atoms in the sample (sample scanning it with a focused beam of electrons interacting with the atoms in the sample (sample fractured fractured surface). Signals contain information of the sample’s surface topography and composition. surface). Signals contain information of the sample’s surface topography and composition. EDS EDS (energy dispersive spectroscopy) on the SEM allows for identifying particular elements and their (energy dispersive spectroscopy) on the SEM allows for identifying particular elements and their relative proportions (atomic %, for example). In this case, from the insight gained by EDS analysis of relative proportions (atomic %, for example). In this case, from the insight gained by EDS analysis of Points A, B and C, a synthesis may be deduced that the white phases in the and the Points A, B and C, a synthesis may be deduced that the white phases in the grain boundary and the white spherical particles inside the grain are composed of Fe-Cr-C, which is carbide. They just differ white spherical particles inside the grain are composed of Fe-Cr-C, which is carbide. They just differ in in their contents. their contents. 4. Conclusions These investigations were focused on observing the mechanical behavior of the steel 1.4034. In this respect, the mechanical properties of the material in the temperature range of (20 °C–700 °C) are determined, the material creep behavior in the temperature range of (400 °C–600 °C) examined, as

Materials 2017, 10, 388 17 of 18

4. Conclusions These investigations were focused on observing the mechanical behavior of the steel 1.4034. In this respect, the mechanical properties of the material in the temperature range of (20 ◦C–700 ◦C) are determined, the material creep behavior in the temperature range of (400 ◦C–600 ◦C) examined, as well as the fatigue at room temperature for stress ratios R = 0.25 and R = −1. The obtained data related to the mechanical properties at room temperature and the highest test temperature of 700 ◦C are as follows: ultimate tensile strength (781.57 MPa/20 ◦C; 125.7 MPa/700 ◦C), yield strength (657.5 MPa/20 ◦C; 83 MPa/700 ◦C) and modulus of elasticity (220 GPa/20 ◦C; 60 GPa/700 ◦C). These data may serve as good indicators for possible applications of the material for appropriate environmental conditions. Comparing the obtained data related to the mechanical properties of this material with those of steel 42CrMo4, which is also usually used in the manufacturing of shafts, as shown in the text, it is possible to say that this material has better mechanical properties and, besides, is very corrosion resistant. In addition, this material shows a very low total elongation in the temperature domain of 100–400 ◦C. The obtained data related to the Charpy impact energy (8 J/20 ◦C; 127 J/200 ◦C) may also serve for the assessment of fracture toughness. In terms of creep behavior, several creep tests were carried out at different temperatures (400 ◦C, 500 ◦C and 600 ◦C) and at different stress levels within the mentioned temperatures. Some of the results, shown in the form of creep curves, are as follows: (σ = 357 MPa, ε = 6%, t = 700 min/T =400 ◦C; σ = 52 MPa, ε = 20%, t = 500 min/T = 600 ◦C). Fatigue testing carried out at different stress ratios and the calculated fatigue limit give an insight into the possible use of the material in the cases of fatigue loadings (R = 0.25 : σf = 682.16 MPa; R = −1 : σf = 325.2 MPa).

Acknowledgments: This work has been financially supported by the Croatian Science Foundation under Project 6876 and partly supported by the University of Rijeka under Project 13.09.1.1.01. The authors are also grateful to the staff of the School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, China, for their efforts in the experimental analysis and explanation of the microstructure of the material. Author Contributions: Josip Brnic led the experimental investigations, analyzed the data related to the material properties, creep and fatigue and wrote the paper. Sanjin Krš´canskidrew the diagrams and modeled the behavior of creep. Domagoj Lanc performed the fatigue tests. Marino Brcic performed and analyzed the tensile tests. Goran Turkalj analyzed the fatigue data. Marko Canadija analyzed the creep and tensile data. Jitai Niu performed the microstructure tests and the analysis. Conflicts of Interest: The authors declare no conflict of interest.

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