metals

Article Comparasion of and Tensile Properties of High-Strength Steel

Željko Alar and Davor Mandi´c* ID

Faculty of Mechanical Engineering and Naval Architecture, Ivana Luˇci´ca5, 10000 Zagreb, Croatia; [email protected] * Correspondence: [email protected]; Tel.: +38-598-909-0117

 Received: 15 June 2018; Accepted: 2 July 2018; Published: 3 July 2018 

Abstract: Tensile and impact properties of API 5L X80 steel were investigated and compared in order to evaluate their relation. and maximum forces under dynamic impact testing were obtained from force-displacement curves. Dynamic yield strength was estimated using the von Mises yield criterion. A different approach was taken in order to estimate the dynamic tensile strength; the instrumented RKP 450 Zwick/Roell machine was used for this specific purpose. From the 10-mm-thick steel plate, Charpy V notch (CVN) samples were cut off in longitudinal orientation considering the rolling direction. For this research, CVN specimens were machined according to the ISO 148:2016 standard. Tensile specimens were machined according to the ISO 6892-1:2016 standard. It was found that the results between the tensile and impact properties were very close and within 5%.

Keywords: dynamic loading; instrumented impact machine; tensile test; force

1. Introduction Since it was first introduced in material testing, the Charpy impact testing method has provided significant information about the behavior of material under impact conditions. Throughout history, the classic and instrumented methods have significantly improved, although there is still a wide range of unknowns that need to be solved—which is especially the case for the instrumented method [1,2]. Accurate measurement of displacement and force because of the high oscillations which occur during the testing on the force-displacement diagram is one of those unknowns (see Figure1)[ 3]. Alongside the physical values, there are also difficulties in evaluating the mechanical properties from the data obtained from the impact testing machine. In this work, the authors propose a method for estimating the dynamic tensile strength. An important issue in industry today is the need to reduce the costs of material testing. The driving force behind this research was the idea of whether there was a way to only use a single method—the instrumented Charpy, in order to obtain mechanical property values that are usually obtained by another method—with the aim of reducing time and expense. The purpose of this paper was to establish a connection between certain mechanical properties obtained by the instrumented impact and tensile tests, including the constraint factor values calculated at the maximum force. In published papers, the connection between impact and tensile tests have been referred to as the yield strength, where the constraint factor is well known [4–6]. During this investigation, tensile and instrumented impact Charpy tests were carried out and mechanical properties were estimated at the yield and maximum force.

2. Background The typical force-displacement diagram for ductile steel materials is shown in Figure1. From the force-time or force-displacement diagram obtained from instrumented impact testing, the yield force should be estimated by fitting the slope to the elastic part of the curve. The elastic part of the

Metals 2018, 8, 511; doi:10.3390/met8070511 www.mdpi.com/journal/metals Metals 2018Metals, 8 ,2018 511, 8, x FOR PEER REVIEW 2 of 82 of 7

Metalsforce 2018 should, 8, x FOR be PEERestimated REVIEW by fitting the slope to the elastic part of the curve. The elastic part of2 of the 8 slope consistsslope consists of the of elastic the elastic compliances compliances of the ofspecimen the specimen and and machine. machine. To estimateTo estimate the the maximum maximum force, the curveforceforce, shouldshould the curve be fittedestimated should to be the by fitted generalfitting to the the yield general slope part to yield the of the partelastic diagram. of thepart diagram. of Thethe curve. intersection The intersection The elastic of these ofpart these twoof the two curves yieldsslopecurves the FconsistsGY yields. ofthe the elastic. compliances of the specimen and machine. To estimate the maximum force, the curve should be fitted to the general yield part of the diagram. The intersection of these two

curves yields the .

Figure 1. Force-displacement diagram with the elastic slope and curve fitted to the remaining part of Figure 1. Force-displacement diagram with the elastic slope and curve fitted to the remaining part of the diagram. the diagram. Figure 1. Force-displacement diagram with the elastic slope and curve fitted to the remaining part of theIn diagram.Figure 1, is the maximum force and is the yield force. To estimate the yield strength, In Figure1, FM is the maximum force and FGY is the yield force. To estimate the yield strength, the slip-line solution was introduced. Kotter proposed the theory in 1903, which was later developed the slip-line solution was introduced. Kotter proposed the theory in 1903, which was later developed by PrandtlIn Figure in 1,1920, where is the maximumhe introduced force the and slip-line is solution the yield for force. plastic-rigid To estimate bodies the [7].yield The strength, solution by Prandtlthewas slip-line later in 1920, developed solution where was by he Hencky, introduced. introduced Hill ,Kotter Prager the slip-line propos et al. Toed solution calculate the theory for yield in plastic-rigid 1903, strength which under bodieswas dynamic later [7 ].developed The loading, solution was laterbyit Prandtlis developedessential in 1920, to calculate by where Hencky, he the introduced constraint Hill, Prager thefactor, etslip-line al. f. Green To solution calculate calculated for yield plastic-rigid thestrength constraint bodies under factor, [7]. dynamic fThe, during solution pure loading, it is essentialwasbending later todeveloped with calculate a notch by theconsisting Hencky, constraint Hill of a, Pragervanishingly factor, etf .al. Green Tosmall calculate calculatedroot radius yield theofstrength r ≈ constraint 0 mm, under which factor,dynamic is shownf, loading, during clearly pure bendingitin is with[8].essential Green a notch to and calculate consisting Hundy the calculated constraint of a vanishingly f underfactor, three-point f. smallGreen rootcalculated bending radius the with of constraintr ≈ a 45°0 mm, notch factor, which and f, aduring is vanishingly shown pure clearly in [8].bendingsmall Green root with and radius a Hundy notch of consisting r calculated ≈ 0 mm of[9]. af vanishinglyunderLianis and three-point Fordsmall introducedroot bending radius ofawith general r ≈ 0 amm, 45 solution◦ whichnotch isto and shown calculate a vanishingly clearly f for smallin rootdifferent [8]. radiusGreen types and of of rHundy notches≈ 0 mm calculated [10]. [9 ].Alexander Lianis f under and and three-point FordKomo introducedly usedbending CVN with asamples general a 45° in notch solutiontheir and work a to vanishingly[11], calculate but they f for differentsmalldid types not root include ofradius notches the of rstriker [≈10 0 ].mm Alexanderradius [9]. Lianisin their and and calculatio Komoly Ford introducedns. used To estimate CVN a general samples the constraint solution in their factor,to work calculate Ewing [11], f but for[12] they different types of notches [10]. Alexander and Komoly used CVN samples in their work [11], but they did notincluded include the the striker striker radius. radius The in st theirandard calculations. ISO striker has To a estimate radius of the2 mm. constraint According factor, to Alexander Ewing [12] didand not Komoly, include Ewing the striker replaced radius the in radius their calculatioof the strikens. rTo with estimate a flat-topped the constraint punch factor,with a Ewingwidth of[12] 2b included the striker radius. The standard ISO striker has a radius of 2 mm. According to Alexander includedand analyzed the striker the slip-line radius. field.The st Inandard Figure ISO 2, Ewing’s striker has schematic a radius slip-line of 2 mm. field According is presented. to Alexander and Komoly,and Komoly, Ewing Ewing replaced replaced the the radius radius of theof the striker strike withr with a flat-toppeda flat-topped punch punch withwith aa width of 2b 2b and analyzedand analyzed the slip-line the slip-line field. In field. Figure In Figure2, Ewing’s 2, Ewing’s schematic schematic slip-line slip-line field field is presented.is presented.

Figure 2. One half of the plain strain slip-line field, reproduced from [12], with permission of Elsevier, 1968.

FigureFigure 2. One 2. One half half ofof the plain plain strain strain slip-line slip-line field, field,reproduced reproduced from [12], from with [permission12], with of permission Elsevier, of 1968. Elsevier, 1968.

In Figure2, p is the hydrostatic pressure and k is the plain strain yield shear stress. Metals 2018, 8, 511 3 of 7

3. Estimation of Dynamic Mechanical Properties

In order to estimate the dynamic yield strength, σGY, the constraint factor, f, is introduced. By R. Hill’s definition, the constraint factor is the factor by which the mean axial stress in the minimum section exceeds the true yield stress of 2k and is greatest when the notch has parallel sides [13]. For a CVN notch with the ISO striker radius, the value of the constraint factor is equal to 1.274 [12]. To calculate the constraint factor, the next equation is introduced [9]:

2 ∗ M f = (1) k ∗ (W − a)2 where the M is the yield-point moment, W is the specimen width, and a is the depth of the notch. The bending moment M at the general yield (for span to width ratio of 4) is equal to (2) [14].

F ∗ W M = GY ; for CVN − notch (2) GY B By introducing the ISO 148:2016 designations, W and B are the width and thickness of the CVN specimen. For the purpose of calculating the dynamic yield strength, the von Mises yield criterion is used (3),

σGY τGY = √ (3) 3 where τGY is the yield strength of the material in pure shear. If we substitute k for τGY in Equation (1), then it is easy to derive the equation for the dynamic yield strength from Equations (1) and (3) [14].

3.464 ∗ MGY σGY = ; for CVN, ISO 148 striker type (2 mm) (4) f ∗ (W − a)2

Replacing MGY in Equation (4) with Equation (2), we can calculate:

3.464 ∗ FGY ∗ W σGY = ; for CVN, ISO 148 striker type (2 mm) (5) f ∗ B ∗ (W − a)2

After introducing the constraint factor, f, we can include the parameter ηGY, which is defined by [4]: 3.464 η = = 2.719 (6) GY f Equation (6) will become:

2.719 ∗ FGY ∗ W σGY = ; for CVN, ISO 148 striker type (2 mm) (7) B ∗ (W − a)2

To estimate the dynamic tensile strength, σTS, it is essential to know the value of the constraint factor. An attempt to derive the value of the constraint factor at maximum force is introduced in this paper by the authors. The reason for this is that in available literature, its value has not been found. Also, it is questionable to use the von Mises yield criterion. According to R. Hill, the constraint factor should decrease as the bending of the specimen increases, i.e., the cstraint factor rises steadily from the value unity wonith increasing a/r ratio, where r is notch radius [13]. With a vanishingly small root radius (r/a → 0), the constraint factor increases [9,11,12]. However, if we include the width of the striker, as suggested in [11], the value of the constraint factor rises as the bending of the Charpy specimen increases. The constraint factor at maximum force was estimated using Ewing’s six equations, which were introduced in [12]. Metals 2018, 8, 511 4 of 7

Using to Ewing’s six equations [12], it was attempted to estimate the constraint factor, f, at the maximum force value. The average value of the constraint factor for all specimens in longitudinal rolling direction is given in Table1. After the estimation of the constraint factor, an equation for the dynamic tensile strength was developed based on the procedure for dynamic yield strength. According to the common approximate value of the ratio between tensile shear strength and tensile strength of 0.75 [4], the following equation was derived (8):

ηTS ∗ FM ∗ W σTS = ; for CVN, ISO 148 striker type (2 mm) (8) Metals 2018,, 8,, xx FORFOR PEERPEERB REVIEWREVIEW∗ (W − a)2 4 of 8 dynamic tensile strength was developed based on the procedure for dynamic yield strength. According to the commonTable approximate 1. Constraint value factor of the values ratio atbetween maximum tensile force. shear strength and tensile strength of 0.75 [4], the following equation was derived (8): Parameter Rolling Direction, RD-L ∗ ∗ = ; ; for CVN, for CVN, ISO 148 striker type ISO 148 striker type (2 mm) (8) ∗f (−) 1.283 As stated, the valuesη ofTS the constraint factor, f, are2.0779 obtained from Equation [12], and the

parameter is then calculated in the same way as the parameter , in Equation (6), taking into consideration the ratio of 0.75 between tensile shear strength and tensile strength. As stated, the values of the constraint factor, f, are obtained from Equation [12], and the parameter ηTS is then calculated in the sameTable way 1. ConstraintConstraint as the parameter factorfactor valuesvalues ηatatGY maximummaximum, in Equation force.force. (6), taking into consideration the ratio of 0.75 between tensile shearParameter strength Rolling and tensile Direction, strength. RD-L f 1.283

4. Experimental Work and Results 2.0779

An investigation4. Experimental wasWork carriedand Results out on five Charpy samples. These samples were machined according theAn ISO investigation 148-1:2016 was standard. carried out Anon instrumentedfive Charpy samples. RKP These 450 Zwick/Roellsamples were machined impact machine with U-typeaccording hammer the ISO and 148-1:2016 a 2 mm standard. radius strikerAn instrumented in accordance RKP 450 withZwick/Roell ISO 148-2:2016 impact machine was with used. In this research, theU-type velocity hammer at and impact a 2 mm was radiusv0 = striker 5.234 in m/s. accordance Samples with wereISO 148-2:2016 tested at was a roomused. In temperature this of 21.5 ◦C. Theresearch, material the velocity used in at thisimpact research was was = 5.234 pipeline m/s. Samples steel were API tested 5L X80. at a Theroomorientation temperature of of the CVN 21.5 °C. The material used in this research was pipeline steel API 5L X80. The orientation of the CVN samples issamples shown isin shown Figure in Figure3. 3.

FigureFigure 3. Orientation3. OrientationOrientation ofof of CVNCVN CVN samples.samples. samples.

In Figure 4, the genuine force-displacement speed diagram of a single specimen is presented. In FigureIn4, Figure the genuine 4, the genuine force-displacement force-displacement speed speed diagram diagram of ofa single a single specimen specimen is presented. is presented.

20000 5

4 15000

3 10000

2 Speed in m/sSpeed Speed in m/sSpeed Standard forceStandard in N Standard forceStandard in N 5000

1

0

0 0 5 10 15 20 25 Standard trav el in mm

FigureFigure 4. Force-displacement4. Force-displacementForce-displacement diagram.diagram. diagram.

Metals 2018, 8, 511 5 of 7

The average values of estimated force measured on the instrumented Charpy machine are given in Table2. The forces were estimated according to ISO 14556:2015.

Table 2. Results of force under dynamic conditions at 21.5 ◦C.

Force, kN Rolling Direction, RD-L

FGY 13.36 FM 19.67

Dynamic strengths were estimated according to Equations (8) and (9). The results of dynamic yield strength and dynamic tensile strength of the specimens tested by the instrumented Charpy method are given in Table3.

Table 3. Results of strength under dynamic conditions at 21.5 ◦C.

Strength, N/mm2 Rolling Direction, RD-L

σGY 567.59 σM 638.63

Tensile tests were carried out according to ISO 6892-1:2016 on a Messphysik Beta 250 kN machine, class 0.5, equipped with Messphysik Laser extensometer class 1. The samples were tested at an ambient temperature of 21.5 ◦C. The crosshead speed was 1.1 mm/min. The average results of the mechanical properties of four API 5L X80 samples are given in Table4.

Table 4. Mechanical properties obtained from a tensile test at 21.5 ◦C.

Orientation of the Sample Considering Strength at 0.2% Tensile Strength Elongation A, % the Rolling Direction Rp 0.2, MPa Rm, MPa RD-L 539.35 ± 5.65 664.75 ± 6.96 23.66 ± 0.36

In addition to mechanical tests, the measurement of chemical composition of API 5L X80 was carried out on the Leco Glow Discharge Spectrometer GDS 500 machine. The samples were ground with 500 SiC paper under water. The average values of three measurements are given in Table5.

Table 5. Chemical composition, weight %.

C Mn Si P S Ni Cr Al Nb 0.055 1.84 0.346 0.0086 0.0005 0.0096 0.147 0.035 0.034

5. Discussion As pointed out earlier, in the available published papers, the value of the constraint factor at maximum force obtained during the instrumented impact test was not known. Therefore, the comparison based on the constraint factor between the tensile test and the instrumented impact test was only done at the yield strength. An analysis of the yield strength shows that the values of the dynamic yield strength obtained by the instrumented impact method are about 5% higher than the ones from the tensile test; Figure5. Metals 2018, 8, 511 6 of 7

Metals 2018, 8, x FOR PEER REVIEW 6 of 8

Metals 2018, 8, x FOR PEER REVIEW Yield strength 6 of 8 580 570 Yield strength 560580

2 550570 540560

N/mm 530 2 550 520540

N/mm 510530 500 520 1234 510TT 534.9 533.9 546.1 542.5 500 CVN 561.641234 566.74 573.11 568.87 TT 534.9 533.9Number of specimen 546.1 542.5 CVN 561.64 566.74 573.11 568.87 Figure 5. YieldNumber strength of specimencomparison. Figure 5. Yield strength comparison.

Fang et al. [15] stated that the higher results are due to the effect of the loading rate increase. In Figure 5. Yield strength comparison. Fangtheir et al. research, [15] stated the difference that the is about higher 20%. results are due to the effect of the loading rate increase. In their research,OppositeFang the et al. difference to [15] the stated yield isthatstrength about the higher values, 20%. results at maxi aremum due stress,to the effectthe dynamic of the loading tensile ratestrength increase. values In Oppositeobtainedtheir research, to theby the yield the instrumented difference strength is impact about values, 20%.testing at were maximum about 4% stress, lower than the the dynamic ones from tensile the tensile strength test values values; Figure 6. obtained by theOpposite instrumented to the yield impact strength testing values, wereat maxi aboutmum stress, 4% lower the dynamic than thetensile ones strength from values the tensile test obtained by the instrumented impact testing were about 4% lower than the ones from the tensile test values; Figure6. values; Figure 6. Tensile strength 690 680 Tensile strength 670 690 660 2 680 650 670 640

N/mm 660 2 630 650 620 640

N/mm 610 630 600 620 1234 610TT 663.5 654.8 674.9 665.8 600 CVN 633.431234 637.01 643.17 640.90 TT 663.5 654.8Number of specimen 674.9 665.8 CVN 633.43 637.01 643.17 640.90 Figure 6. TensileNumber strength of specimen comparison.

The reason for the higher tensile test values could be that the striker of the impact machine Figure 6. Tensile strength comparison. decelerates (315.11 m/s2) duringFigure the 6.testTensile (bending strength the Charpy comparison. specimen) from its velocity at impact ( =The 5.234 reason m/s) tofor the the speed higher at maximumtensile test strength values coofuld 4.842 be m/s. that Furthermore, the striker of the the appearance impact machine of the The reasoncrackdecelerates at the for (315.11root the tip higher m/scontributes2) during tensile to the the test lower (bending values loading the could rate. Charpy be specimen) that the from striker its velocity of the at impact impact machine 2 decelerates( (315.11 = 5.234 m/s) m/s to) the during speed theat maximum test (bending strength theof 4.842 Charpy m/s. Furthermore, specimen) the from appearance its velocity of the at impact 6. Conclusions crack at the root tip contributes to the lower loading rate. (v0 = 5.234 m/s) to the speed at maximum strength of 4.842 m/s. Furthermore, the appearance of the The results of the yield and tensile strength values obtained in this research indicate a good crack at the root tip contributes to the lower loading rate. relation6. Conclusions between the tensile and instrumented Charpy impact test. In both measured mechanical properties, the difference is around 4%, i.e., 5%. This research also contains additional results for the 6. ConclusionsThe results of the yield and tensile strength values obtained in this research indicate a good relation between the tensile and instrumented Charpy impact test. In both measured mechanical

The resultsproperties, of the the difference yield and is around tensile 4%, strengthi.e., 5%. This values research obtained also contains in thisadditional research results indicate for the a good relation between the tensile and instrumented Charpy impact test. In both measured mechanical properties, the difference is around 4%, i.e., 5%. This research also contains additional results for the constraint factor. From the values of the constraint factor at maximum force, the dynamic tensile strength was estimated. The influence of the width of the striker was included during the evaluation of the constraint factor [11]. In this paper, the authors used Ewing’s six equations to estimate the value of the constraint factor at the maximum force. It was found that the constraint factor value increases as the bending of the Charpy specimen continues. For a better understanding of material behavior, Metals 2018, 8, 511 7 of 7 further extensive research is required on the constraint factor and the plastic deformation of the Charpy specimen above the yield point.

Author Contributions: Conceptualization, D.M.; Data curation, Ž.A.; Formal analysis, D.M.; Investigation, D.M.; Methodology, Ž.A.; Project administration, Ž.A.; Resources, Ž.A.; Supervision, Ž.A.; Validation, Ž.A. and D.M.; Visualization, D.M.; Writing—original draft, D.M.; Writing—review & editing, Ž.A. Funding: This research received no external funding. Acknowledgments: The authors are very grateful to Donald R. Ireland for his advice on the instrumented method and Davor Kolednjak for supplying the material for testing. Conflicts of Interest: The authors declare no conflict of interest.

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