The effect of specimen size and surface conditions on the local mechanical properties of 14MoV6 ferritic–pearlitic steel

Materials Science & Engineering A 651 (2016) 810–821

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The effect of specimen size and surface conditions on the local mechanical properties of 14MoV6 ferritic–pearlitic steel R.M. Molak a,n, M.E. Kartal b, Z. Pakiela a, K.J. Kurzydlowski a a b

Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland School of Engineering, University of Aberdeen, King's College, Aberdeen AB24 3UE, United Kingdom

art ic l e i nf o

a b s t r a c t

Article history: Received 19 October 2015 Received in revised form 12 November 2015 Accepted 13 November 2015 Available online 1 December 2015

The paper describes multiscale experimental techniques required for determining the mechanical response of a ferritic–pearlitic steel (14MoV6). Accordingly, five different sets of specimens ranging from macro to micro size scales are utilized and whose experimental results have been compared. Digital image correlation is employed to measure deformations on the surface of the test specimens under tensile loading conditions. The influence of the surface conditions on the residual stress distribution was investigated by means of X-ray synchrotron diffraction. In addition, the effects of specimen size, surface conditions and the number of grains in the cross-section of the specimens on mechanical properties are examined. It is observed that key material parameters including the yield stress, tensile strength and elongation to failure are dependent on specimen size. In addition, the results demonstrate that the number of grains in the cross-section of the specimens significantly influence the material response during uniaxial tensile testing. On the other hand, the surface treatment of the micro tensile test specimens bring about reducing differences in mechanical properties between standard and miniature specimens. & 2015 Elsevier B.V. All rights reserved.

Keywords: Size effect Multiscale experiment Digital image correlation Mechanical properties Surface conditions

1. Introduction With the rapid development of micro and nano engineering technologies, there is the quest for miniaturization of engineering components growing considerably over the last decade. Hence, determination of the constitutive properties of engineering components across multi-scales is essential and suitable measurement tools have to be used for evaluation of material responses. It should be realized that scaled down engineering components ranging from micro to nano scales for a given material may profoundly exhibit different properties in relation to their standard counterparts. Therefore, the knowledge of size-dependent properties, especially their mechanical properties, is an important engineering task. Although robust multiscale materials modeling techniques have been developed to predict length scale dependent material behavior, experimental measurements are required to validate the predictions [1]. This is especially true when the material in question has gone through a complicated manufacturing process and/or in service loading. Previous work in the literature indicates that there is the number of different forms of the size effect and it is difficult to n

Corresponding author. E-mail address: [email protected] (R.M. Molak).

http://dx.doi.org/10.1016/j.msea.2015.11.037 0921-5093/& 2015 Elsevier B.V. All rights reserved.

represent all the various parameters with a uniform theory. In the context of mechanical properties, most attention in experimental researches regarding the size effect is given to the tensile strength (sUTS) and yield strength (sY) with the help of various tests such as torsion [2], bending tests [3], hardness tests [4] and fracture toughness [5]. A size dependence on strength can be generally studied in three different approaches. The first approach is based on the concept of the strain gradient [2] in which non-uniform plastic deformation leads to gradients in orientation and strain near a material point [6]. The second approach focusses on the microstructural aspects where the number of grains in the cross-section of the specimen governs the material response [7–10]. The final approach links the size effect with the boundaries of the specimen and employs a surface layer model [11,12]. Generally, all of these three approaches use either the yield strength (sY), the tensile strength (sUTS) or the strain hardening exponent (n) to investigate size dependence. Hence, measuring these mechanical properties to support material modeling methods at different scales is essential. As dimensions of miniature testing specimens are smaller by several orders of magnitude compared with standard specimens, techniques for machining miniature specimens are challenging and influence the strength properties the surface roughness and material structures. Whilst there are a number of cutting tools

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available for precisely machining specimens at the macro-scale, a few machining techniques are applicable down to the microscopic scale. Although Abrasive Jet Machining, Electro-Chemical Machining and Chemical Machining [13] are, for instance, commonly used as cutting tools, a major drawback of these methods is their low speed for material removal and that they cause relatively high tolerance (about 50 μm). Wire Electro Discharge Machining (WEDM) in this field emerges as an effective method used for extracting a miniature specimen from a bulk material. Unlike conventional cutting processes requiring high cutting forces for material removal, WEDM is a noncontact method which uses an electrically charged thin wire in which energy contained in discharge sparks is used to remove material. Hence, the electrode does not directly touch the material which enables to eliminate introduction of significant stress [14,15]. WEDM provides a better shape tolerance and a smaller surface roughness with comparison to the above-mentioned machining methods. The cutting-affected depth with this method is about 125 μm from the free surface [13]. The surface quality following the machining process is characterized by a roughness, presence of surface microcracks and cutting-related residual stresses; and these parameters depend upon discharge current [16], pulse time [17], pulse interval type [16] voltage [18], dielectric pressure [19] and electrode material [20]. During the electric discharge machining process, surface residual stresses may be generated due to the microstructural changes and inhomogeneous heat flow. These residual stresses are usually of the tensile character which frequently leads to microcracks formation on the surface of machined components. It is obvious that the change in the microstructure and residual stresses alters the mechanical properties of the miniature specimens tested. Although there have been numerous articles in the literature in which mechanical properties of the metallic materials using miniature specimens were investigated, the influence of specimen preparation on mechanical properties is very rarely addressed. Numerous studies in the literature rely on the assumption that the grinding process following the cutting process eliminates the effect of the miniature specimen treatment process on strength. However, this assumption has not been verified. In this work, an attempt was made to investigate the relationships between the size dependence, surface conditions and their effects on mechanical properties in a comprehensive manner. The material for investigations is 14MoV6 steel which is widely used as a primary pipe material in power generation applications. To this end, the tensile mechanical properties in standard and miniature specimens were determined with the help of the digital image correlation method. The strength parameters (sY, sUTS) as well as the parameters related to elongation of the specimen (Αgt, ΔAn, A) were compared.

2. Materials and methodology The study was carried out on the 14MoV6 ferritic–pearlitic steel, a structural alloy designed for the use of power generation applications at elevated temperatures. The ‘as received’ material was supplied as a seamless pipe with dimensions of 273 mm in diameter and 400 mm long by Huta Batory inc. (Chorzow, POLAND). According to the certificate supplied by the manufacturer, the pipe had been subjected to normalization and tempering heat treatment. We used EN 6892-1 standard [21] to determine dimensions of the reference specimens (the standard tensile specimen), marked as sample (I). Reduced size tensile specimens, marked as specimens (II–V), were machined with the dimensions

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Table 1 Dimensions of the test specimens. Specimen

Standard tensile specimen Miniature tensile specimen

Thickness [mm]

Width [mm]

Cross section [mm2]

Gage length [mm]

I

6

8

48

50

II III IV V

2.4 1.2 0.6 0.3

3.2 1.6 0.8 0.4

7.7 1.9 0.5 0.1

20 10 5 2.5

given in Table 1. An Agie Charmilles FI 240 SLP WEDM machine with a 0.25 mm diameter brass wire was used for all cutting procedures. In order to analyze the influence of the surface treatments of the miniature tensile specimens on the mechanical properties to be determined by quasi-static tensile tests, four different types of surface treatment were identified. The WEDM surface finish was a reference (initial) surface condition and further treatments were carried out by mechanical polishing, mechanical grinding and polishing or chemical polishing. Table 2 presents four different surface treatments together with their abbreviations. All the mechanical grinding and polishing processes were carried out by using an automatic grinder–polisher Struers. For the grinding process, abrasive papers of grit size 1200 and 4000 were used. Each grinding step took 15 s. For the polishing process, a diamond abrasive with an average particle diameter of 3 μm was used. The polishing duration for each case was 1 min. During each grinding and polishing step, the nominal pressure on the test specimen and the disc rotational speed were 5 N and 120 rpm respectively. During mechanical grinding, the specimen was attached under a designed mount to ensure uniform material removal and a designed moving pusher with a constant pressure of 5 N was used to load the mount against the disc. The surface treatment was applied on both base sides of the specimen. Applying the same process on the lateral planes of the miniature specimens would have been rather difficult without introducing deformation (e.g. edge rounding) and hence were not performed in this work. In order to investigate grains and their morphology, chemical polishing was carried out by immersing a specimen in a solution with the following composition: 70 ml H2O2 (3%) þ5 ml HF (40%) þ40 ml H2O An etching time was experimentally determined and about 8 min on average. The surface of specimens was observed using a Hitachi 3500 scanning microscope in the back-scattered electron mode at various magnifications. In addition the surface was quantitatively characterized using a Wyko NT9300 optical profilometer which enables to determine both 3D surface reconstructions and indicators describing surface topography. Table 2 The surface treatments. Abbreviations Surface treatments W W/G1 W/G1/G4 W/G1/P3 W/CHP

Wire Electro Discharge Machining (WEDM) WEDM þ Abrasive grinding paper of 1200 grit size WEDM þ Abrasive grinding papers of 1200 grit size followed by 4000 WEDM þ Abrasive grinding paper of 1200þ Polished with 3 μm diamond suspension WEDM þ Chemical polishing

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In order to analyse microstructural changes along the depth direction caused by the surface modification, micro-hardness tests were performed along the direction perpendicular to the analysed surfaces. The test load was 10 nN, applied at 1 nN/s loading rate, while the time of full load application was 2 s. The tests were carried out using TI 900 TriboIndenter by Hysitorn equipped with Vickers indenter. Additionally, micro-residual stress profiles along the direction normal to the processed surfaces were measured using the X-ray synchrotron diffraction at the ESRF (European Synchrotron Radiation Facility) in Grenoble, France. For this purpose, two miniature specimens (W and W/G1/P3) with the identical dimensions given as sample (V) in Table 1 were used for ESRF experiments. They were cut in a similar manner that of microhardness tests, perpendicular to the analysed surfaces. A beam with the initial energy of 28 keV was used for each experiment. A monochromatic microbeam was obtained with a double Si monochromator (111) and aluminum lenses. The dimensions of the microbeam used to determine the micro-residual stresses were 5.6  2.6 μm2. The diffractograms collected on the plane perpendicular to the surface were then analysed as a function of the distance from the free surface the specimen. The measurements were carried out every 2.5 μm step size. XRDUA software was used to analyse the results and identify grid parameters [22]. Quasi-static tensile tests were performed using three different testing machines manufactured by MTS Co. All these tests were conducted under the displacement control at a constant strain rate of 1  10  3 s  1. Since dimensions of the miniature test specimens are very small, it would be difficult to use one of the conventional strain measurement methods such as a strain gauge and an extensometer. Hence, digital image correlation (DIC) was employed for surface measurements. DIC is a non-contact optical method based on a correlation coefficient to trace the best matching location of two images thus tracking changes. Since DIC was first developed by Peters, Ranson and Sutton [23,24], the DIC method has been broadly accepted and has grown in its potential to be used for measuring deformation and characterizing related properties such as strength [33], thermal expansion [25], frictional [26] and fracture [27] properties. The advantage of this method is the absence of any physical contact with a test surface and its high resolution of up to 0.01 pixels. Commercial software Vic 2d by Correlated Solutions Inc. [28] was used for determining the elongation and strain measurements. The methodology used in this study had been successfully established by the authors to determine the elongation of miniature specimens in a number of mechanical tests, as described in [29–33]. ISO/IEC Guide 98–3 [34] in the EN ISO 6892 standard was followed to ensure that appropriate statistics for determining mechanical parameters were used. In order to minimize measurement error at least 10 specimens under the same experimental parameters were tested for each test condition. The parameters measured through the experiments are tabulated in Table 3. The parameter definitions in Table 3 are adopted from [35]. The Table 3 Parameters determined during tensile tests. Symbol Meaning sY sYL/sYH s0.2 sUTS A At Agt ΔAn

Yield strength Upper/lower yield strength Proof stress at plastic deformation of 0.2% Tensile strength Elongation to failure Total elongation to failure Total uniform elongation Elongation of strain localization, (At  Agt)

parameter ΔAn is used to characterize the strain localization in the form of a necking, which was defined as the difference between the total elongation to failure (At) and total uniform elongation (Agt).

3. Results and discussion 3.1. The effect of the mini-specimen surface conditions 3.1.1. Microscopic observations Fig. 1(a–e) shows images captured on the free surface of the test specimens by an SEM in which the back-scattered electron detector (SE mode) was used to produce geometrical formations. As can be seen in Fig. 1a, electric discharge cutting brings about discharge craters and a recast layer on the cut surface. After grinding the free surface by means of the abrasive paper of grit size 1200, the layer caused by the WEDM was completely removed except for the scratches observed on the specimen surface in Fig. 1b. Furthering the grinding process with 4000 grit size reduced the remainder of scratches on the surface in Fig. 1c. Polishing the specimen subsequent to mechanical grinding was carried out using 3μm diamond suspension (Fig. 1d). Afterwards, etching was done by a chemical reagent (Fig. 1e). 3.1.2. Surface profile measurements Fig. 2(a–e) shows three-dimensional reconstruction of the surface topography of each surface treatment condition where the region of interest is about 0.5  0.5 mm2. Table 4 presents the roughness parameters experimentally determined on each of the analysed surfaces. In Table 4, Ra is the arithmetic average of the roughness profile, Rq the root mean square surface roughness, Rz the average of the height of the five highest peak plus the depth of the five deepest valley, R t is the vertical distance from the deepest valley to the highest peak. As expected the reference surface condition of the test specimen (W) caused by electric discharge cutting is found to have much greater surface roughness ( Ra=3.59 μm) than other surface conditions. This is a typical surface roughness value obtained for metal components when machined by the WEDM cut. It is also observed that the ground surfaces following to the initial W surface reduce the roughness parameters. For instance, grinding with the abrasive paper of 1200 grit size (W/G1) reduces the surface roughness by more than one order of magnitude (i.e., Ra=0.121 μm) in comparison with the W surface. The surface ground mechanically by grit size 1200 and 4000 (W/G1/G4) displays the lowest average roughness value of Ra (0.0162 μm) and the smooth surface is also achieved by the polishing process with diamond suspension (WG1/P3) in which the average surface roughness is found to be 0.0169 μm. On the other hand, the surface obtained by chemical polishing (W/CHP) is also characterized with significant roughness parameters ( i. e. , Ra=1.33 μm). Based on the three-dimensional reconstructions in Fig. 2e, it can be stated selective surface etching took place in which a certain region of the surface was more heavily etched than the other due to probably the different etching speed of the different phases (ferrite, pearlite) on the 14MoV6 ferritic–pearlitic steel microstructure. Both W and W/CHP surfaces are characterized with a relatively higher value of Rz (28.9 μm and 14.1 μm respectively) in comparison with the other surface conditions and indicate significant differences between the recesses and rises. For the W surface this stems from the presence of the discharge craters while connected with uneven and selective etching for the W/CHP specimen causes high value of Rz .

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Fig. 1. Surface images obtained from the SEM after different surface treatments (a) W, (b) W/G1, (c) W/G1/G4, (d) W/G1/P3 and (e) W/CHP.

3.1.3. Tensile tests Four batches of miniature samples, whose dimensions labeled (IV) in Table 1, were used for the tensile tests together with the surface treatments W, W/G1, W/G1/G4 and W/G1/P3 as defined in Table 2. Digital image correlation was employed to measure strains on the specimens. The chemically polished miniature specimens, W/CHP, were excluded from tensile tests owing to the selective

surface etching reason which led to various recesses and rises. Additionally, macroscopic inspection of the miniature specimens (IV) revealed that there were significant dimensional variations along the entire gauge length stemming from selective melting of the specimen surface as well as the knife-edge effect and rounded specimen edges. These two above-mentioned factors prevented a precise determination of the cross-section of the miniature

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Fig. 2. Surface topographies after different surface treatments (a) W, (b) W/G1, (c) W/G1/G4, (d) W/G1/P3 and (e) W/CHP.

Table 4 Roughness parameters of 14MoV6 steel surface. Surface designation

Ra [μm]

Rq [μm]

Rz [μm]

Rt [μm]

W W/G1 W/G1/G4 W/G1/P3 W/CHP

3.59 0.121 0.0162 0.0169 1.33

4.42 0.156 0.0205 0.0225 1.73

28.9 1.4 0.187 0.246 14.1

28.2 1.4 0.189 0.248 14.1

specimens (IV) and hence they were removed from further analysis. Table 5 presents the measured strength parameters obtained from the miniature specimens (IV) with different surface conditions together with those parameters determined on the standard sample (I). The results in Table 5 are given with possible uncertainty values estimated according the standard [34]. Fig. 3 shows the corresponding stress–strain curves. Based on the results obtained from the tensile testing experiments, it can be claimed that all of the surface treatment methods

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Table 5 Mechanical properties of 14MoV6 steel for various surface treatments. Specimen size

Surface designation

sYH [MPa]

sYL [MPa]

sUTS [MPa]

At [%]

Agt [%]

ΔAn [%]

Rf [MPa]

Standard tensile specimen (I) Miniature tensile specimen (IV)

– W W/G1 W/G1/G4 W/G1/P3

469 7 8 4167 5 4377 6 4377 7 4337 4

454 78 415 75 436 76 437 77 432 74

6277 4 583 7 2 6177 4 6147 4 6137 4

23.2 70.4 20.7 70.5 19.9 70.7 20.5 70.8 20.3 70.7

11.17 0.2 10.2 7 0.1 107 0.4 10.2 7 0.4 10.2 7 0.3

12.2 7 0.4 10.6 7 1.2 10.17 0.6 10.4 7 1.7 10.2 7 0.6

408 7 3 3117 8 3377 15 3277 14 326 7 16

Fig. 3. Representative stress–strain curves of 14MoV6 steel for various surface treatments.

Fig. 4. Microhardness of 14MoV6 steel as function of a distance from the free surface.

consisting in removing the recast layer from the surface of the miniature specimen (IV) caused by the electric discharge cuts result in increasing yield strength (sYH, sYL) and tensile strength (sUTS) towards the values of the standard specimen (I). Parameters characterizing the miniature specimen (IV) elongation, namely elongation to failure (A), uniform elongation (Agt) and elongation due to strain location (ΔAn) remain independent from the surface treatment methods. The results presented indicate the absence of any clear influence of surface deficiencies (cracks, discharge craters) on the specimen deformation process during the tensile tests. Summing up the results obtained, we may state that the specimen cutting process plays a significant role in the strength properties of the specimens. For 14MoV6 steel, removal of the layer created by the electric discharge cutting process from the miniature specimen (IV) surface causes the experimental results to "shifts" towards the reference values obtained for the standard specimens (I) and reduces the differences in the determined mechanical properties between particular specimen types. The effect of surface treatment on mechanical properties can be explained by difference in properties between the recast layer and the substrate material and hence this effect will be analysed further on in this study. In addition, it should be stressed that although the layer created by electric discharge cutting was removed for the miniature specimens, the impact of the specimen size on the strength properties is still present, especially for the lower and upper yield stress values (rYH, rYL) and the parameters relating to the specimen elongation (A, Agt ΔAn ). This effect can be seen by comparing the results for the standard specimen (I) and miniature specimens subjected to different surface treatments (IV) in Table 5. Since tensile testing presented in this study does not provide information about the cut-affected depth, it was attempted to measure material properties of the micro specimens along the

depth direction. For this purpose, two of the miniature specimens labeled W and W/G1/P3 were used for further testing. At this stage the other miniature specimens were excluded from further analysis. 3.1.3.1. Microhardness measurements. To carry out the analysis of the cut-affected depth, microhardness measurements were performed on the cross-sections of the miniature specimens. The obtained results in the form of microhardness profiles versus the distance from the free surface are shown in Fig. 4. The microhardness profile obtained for the W specimen exhibits high values on and near the free surface and it progressively reduces when moving away from the free surface and becomes almost constant at a 20 μm distance remote from the free surface. Hence, it can be seen that the cut-affected depth after the WEDM process is 20 μm . In fact, such a hardness distribution for the materials subjected to the WEDM process can be found in many other studies (e.g. [36–39]). On the other hand, the microhardness profile measured for the W/G1/P3 treated specimen is almost constant in the depth direction and hence the results suggest that the increased hardness near the free surface may be due to the influence of the recast layer created during electric discharge cutting and the surface treatment process eliminates the microhardness growth towards the specimen surface. 3.1.3.2. Micro-stress measurements. To characterize the impact of the surface treatment on the residual strain distribution, X-ray synchrotron diffraction measurements were taken in the depth direction of two different miniature specimens (W and W/G1/P3). The crystal lattice strain profiles along the depth direction from the free surface are shown in Fig. 5a. The corresponding calculated residual stress values are shown in Fig. 5b. The miniature specimen W following the electric discharge

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Fig. 6. SEM images captured on the cross section of (a) W (b) W/G1/P3 specimens.

Fig. 5. Variation in (a) micro-strains and (b) residual micro-stress versus the distance from the free surface.

cutting possesses a tensile residual stress with a peak value of 1100 MPa near the free surface, which constantly reduces away from the free surface. As can be seen in Fig. 5b, the maximum residual stress value exceeds the tensile strength value indicated as sUTS. This proves the occurrence of phase transition and creation of a new layer whose structure is different from the substrate material. Cusanelli et al. [34] analysed the influence of electroerosion treatment of commercial ferritic steel Böhler W300 on the creation of a layer. The microscopic investigation confirmed creation of a new martensite layer and a residual austenite area. This resulted in the occurrence of significant residual stress in the cutaffected layer caused by the WEDM process. Residual stress generation in the cut-affected zone was studied elsewhere [40,41]. In extreme cases where the maximum tensile residual stress value exceeds the material strength, crack formation on the surface of the treated/machined components may occur [42]. Fig. 5b also shows that the grinding and polishing processes on the W/G1/P3 treated specimen result in relaxation of the residual tensile stress due to removing the layer created during the WEDM process. Grinding and polishing introduced a compressive residual stress on and local to the free surface which is balanced by residual tensile stress at the sub-surface. It should be emphasized here that the difference in residual

stress/strain values between two specimens becomes negligible at a 20 μm distance remote from the free surface. This observation is also consistent with the results obtained from the microhardness measurements, concerning the depth of changes resulting from electric discharge cutting. 3.1.3.3. SEM observations. Fig. 6 compares the SEM images captured on the cross-section of the W and W/G1/P3 specimens. For the miniature specimen W, the thickness of the layer created by electric discharge cutting extends to 20 μm from the free surface in which the layer morphology resulting from rapid heat transfer during the specimen cut is distinct. For the miniature specimen W/G1/P3 no obvious microstructure changes were observed which proves a correct process for removal of the layer resulting from electric discharge cutting. Once again, the images confirm previous observations made with the microhardness and diffraction methods. The electric discharge cutting process leads to significant changes in the microstructure and mechanical grinding and polishing processes enable to remove the layer created by the WEDM. 3.2. Effect of the specimen size and its thickness Engineering stress–strain curves measured on specimens of various sizes are shown in Fig. 7. The properties deduced from these stress–strain curves are summarized in Table 6. It was apparent that the strength and the parameters relating

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Table 7 α and φ values for specimens of various sizes. Specimen α

I II III IV V

φ

Thickness of miniature specimen (IV) [mm]

0.011 316 0.028 126 0.6 0.055 63 0.46 0.108 32 0.33 0.210 16 0.2

α

φ

0.108 32 0.123 24 0.152 17 0.241 11

Fig. 7. Engineering stress–strain curves for specimens of various sizes.

to the specimen elongation reduce with a decrease in specimen size. For the standard specimen (I), the upper yield stress, lower yield stress, tensile strength, total uniform elongation and the elongation to failure were 469 MPa and 454 MPa (sYH and sYH), 627 MPa (sΥTS), 11.1% (AGT) and 23.2% (A) respectively while these parameters for the miniature specimen (V) were found to be 398 (Rp0.2) MPa, 544 MPa (sUTS), 8.5% (AGT) and 17.3% (A) respectively. To explain this phenomenon a surface layer model was suggested where the scaled down specimen cannot be considered as a homogeneous continuum, the behavior of which is described by an anisotropic constitutive law. The scaled down specimen is to be regarded as a set of external grains which are unbounded by any other grain on the free surface while the internal grains are entirely surrounded by other grains. This model assumes that external grains show less hardening in comparison to the internal grains, which can be explained by different mechanism of dislocation movement and pile-up and by the fact that they are less subjected to compatibility restrictions. Based on the surface layer model, Kals et al. [43] proposed a parameter (α) describing the ratio of the surface grains to the total number of grains existing on a cross-section of the specimen with the following equation:

α=1−

(w0 − 2dg )(t0 − 2dg ) w0t0

,

(1)

where w0 and t0 are mean width and thickness of the specimen respectively and dg is the average grain diameter. The values of this coefficient for the specimens used in this study are given in Table 7 in which φ is a scaling factor which is used to quantify the influence of specimen size on the elongation related parameters and hence it determines the number of grains in the specimen thickness as:

φ=

t0 . dg

(2)

Fig. 8. (a) Microstructure (b) grain size distributions in 14MoV6 steel.

These values in this study were calculated on the basis of the ferrite grain diameter (dg) measured as 19 μm with the help of the image analysis. A representative microstructure of 14MoV6 steel and grain size distributions are shown in Fig. 8a and b respectively. In Fig. 9, the strength parameters (sYH, sYL, Rp0.2, sUTS) were plotted as a function of α. As can be seen in this figure, linear compatibility is achieved by using the surface layer model in the

Table 6 Summary of the tensile properties for specimens of various sizes (I–V). Specimen

sYH [MPa]

sYL [MPa]

Rp0.2 [MPa]

sUTS [MPa]

A [%]

Agt [%]

ΔAn [%]

I II III IV V

469 7 8 446 7 5 439 7 3 4167 5 –

454 7 8 4417 6 436 7 3 415 7 5 –

– – – – 398 7 8

6277 4 6187 3 605 7 3 583 7 2 5447 7

23.2 70.4 23.4 70.8 20.9 70.9 20.7 70.4 17.3 71.2

11.17 0.2 11.3 7 0.3 10.5 7 0.3 10.2 7 0.1 8.5 7 0.6

12.2 7 0.4 12.3 7 0.6 10.4 7 0.8 10.6 7 1.2 8.9 7 1.1

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Fig. 9. Variation in the strength properties as a function of α coefficient. Fig. 11. Total uniform elongation (Agt), the elongation of strain localization (ΔAn) and the elongation to failure (A) as a function of φ coefficient for specimens of various sizes.

Fig. 10. Engineering stress–strain curves local to the yield stresses for specimens of various sizes. Fig. 12. Engineering stress–strain curves measured on the miniature specimens as a function of the specimen thickness (t0).

Table 8 Relative stiffness of various testing rigs. Specimen I II III IV V

Testing machine MTS MTS MTS MTS MTS

810 858 858 QTest/10 QTest/10

Relative stiffness Km/Ks 3

2.1  10 3.4  10  3 6.7  10  3 4.3  10  3 8.6  10  3

study. One of the interesting findings in this study was that the lower and upper yield stress (sYH/sYL) were not observed in miniature specimens V, as shown in Fig. 10 and Table 6. In addition, a decrease in the stress peaks defining the upper and lower yield stress (sYH, sYL) was found with a decrease in specimen size. This phenomenon has been explained in relation to the stiffness of the testing machines (Km) which were listed in Table 8 together with the specimen stiffness (Ks) [44] which is calculated based on the following relation:

Ks =

s0E , Lc

(3)

in which s0 is the cross section of the testing specimen, E a modulus of elasticity and Lc a gauge length of the specimen. Comparing the data presented in Table 8 with the stress–strain curves in Fig. 10 it was noted that an increase in the relative stiffness of the testing stand (Km/Ks) is accompanied with a decrease of the maximum and minimum stress peaks, defining the upper and lower yield stress (sYH, sYL). The energy related stress relaxation accompanying occurrence of the upper yield strength value (ReH) is not noticeable and hence a value of the maximum peak stress on the stress–strain curve (specimens IV and V) does not drop when a high relative stiffness of the testing machine is used for the tests. On the other hand, the stress relaxation occurs when a less stiff testing machine (high values of Km/Ks) is used and hence the upper or lower yield strengths (ReH, ReL) were visible as clear maximum and minimum stress peaks in the stress–strain curves (specimens I, II, and III). A clear effect of specimen size on the elongation was also found. By analysing the values of the total uniform elongation (Agt), the elongation due to strain localization (ΔAn) and the elongation to failure (A), as summarized in Table 6, it was concluded that there is a critical specimen size below which these values clearly decrease by nearly 30% in comparison to those

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Table 9 A summary of the mechanical properties of the miniature specimens (IV) as a function of the specimen thickness (t0). Thickness of the miniature specimen (IV) (t0) [mm]

sYH [MPa]

sYL [MPa]

sUTS [MPa]

A [%]

n

Agt [%]

ΔAn [%]

0.6 0.46 0.33 0.2

416 75 416 73 417 73 404 73

415 75 414 72 415 74 403 73

583 7 2 582 7 2 583 7 1 5737 1

20.7 7 0.4 18.8 7 0.8 15.6 7 0.8 11.5 7 1.1

0.1597 0.006 0.1697 0.003 0.1637 0.001 0.164 7 0.001

10.2 70.1 9.8 70.5 8.9 70.1 8.3 70.6

10.6 7 1.2 9.17 0.7 6.8 7 0.7 3.4 7 0.88

Fig. 13. Fracture surfaces for the miniature specimens with thickness of 0.6 mm ((a) top and (b) side view) and that of 0.2 mm ((c) top and (d) side view).

obtained for the standard size specimen (I). Fig. 11 shows the variations of the elongation related parameters as a function of φ. It can be seen that the smaller size of the specimens results in a reduction of the uniform elongation (Agt), the elongation of strain localization (ΔAn) and the elongation to failure (A). A similar effect was also observed elsewhere [8,45] for the rolled Cu. As can be seen from Fig. 11, the scaling factor φ is of particular importance for the smallest size specimens. In order to investigate the influence of specimen thickness on the properties, a quasi-static tensile test was performed on the miniature specimens (IV) with various thickness (t0) of 0.6, 0.46, 0.33 and 0.2 mm. Engineering stress–strain curves are shown in Fig. 12 and the mechanical properties are presented in Table 9. Experimental results for the thickness effect indicate that the onset of strain localization in the miniature specimens occurs at a lower total uniform elongation (Agt) and that strain localization (ΔAn) decreases for thicker specimens. However, no detectable evidence was found to indicate that the specimen thickness influences the yield stress (sYH, sYL) and the tensile strength (sUTS).

This effect was also discussed with the surface layer model and the coefficient which was relatively small for all specimens in Table 7. The effect of specimen thickness on the fracture surface was revealed by representative images obtained by the SEM microscopy shown in Fig. 13. It can be seen that for the miniature specimen with a thickness of 0.6 mm the area of strain localization has the shape of a quadrangular pyramid (Fig. 13a) with a mild transition from a uniform elongation to the plane of fracture (Fig. 13b). For the sample with a thickness of 0.2 mm the area of strain localization is assumed the shape of quadrangular pyramid (Fig. 13c) and the knife edge fracture surface is observed (Fig. 13d). The plasticity parameters (Agt, An, A) for the miniature specimens (IV) were plotted as a function of scaling factor φ in Fig. 14. Based on the data given in the plot, it was noted that the parameters related to the deformation of the miniature specimens (IV) change linearly as a function of the φ value which was determined by the number of grains across the specimen thickness (t0).

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References

Fig. 14. Total uniform elongation (Agt), elongation of strain localization (ΔAn) and elongation to failure (A) as a function of φ coefficient.

4. Conclusions The results obtained confirmed the influence of the miniature specimen surface treatments on the strength parameters. It has been observed that the specimen cutting process by the WEDM introduces residual stress into a material and results in changes in microstructure along a certain depth from the free surface of the material. The observed influence of the machining/surface treatment processes on the microstructure of the tested materials reaches several tens micrometers and is of crucial importance for the miniature specimens with the cross-sectional dimensions less than 1 mm. Removal of a recast layer created during the electric discharge cutting process from the miniature specimen surface makes its strength parameters shift towards the reference values obtained for the standard specimens (I). In other words, the observed differences in the measured strength parameters between the miniature and standard specimens dramatically reduce after removal of the layer. This refers particularly to the strength parameters related to the area of the specimen cross-section (sYH, sYH, Rp0.2, Rm). The results of comprehensive tensile tests for standard (I) and miniature specimens (IV) confirmed that sample size has an effect on the strength parameters (sY, sUTS) and parameters related to the elongation (A, Agt, ΔAn) of the 14MoV6 steel. The results can be explained by the so-called boundary, surface effect and surface layer model which refer to the grains in the near-free-surface zone. In order to quantify the observed effect, it is possible to use either the thickness (t0) of the specimens or the scaling factors such as α or φ, the values of which are determined by the size of grains with respect to the specimens' dimensions. Finally, the results presented here clearly demonstrate the effect of machine stiffness, particularly on the values of the upper and lower yield stresses (sYH, sYL).

Acknowledgment The work was carried out under the Project no. ZPB/27/64468/ IT2/10 which is supported by the National Centre for Research and Development within a framework of IniTech programme.

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