Tensile and Creep Testing of Sanicro 25 Using Miniature Specimens

materials Article

Tensile and Creep Testing of Sanicro 25 Using Miniature Specimens Petr Dymáˇcek 1,2, *, Milan Jarý 1 , Ferdinand Dobeš 1 and Luboš Kloc 1 1 2

*

Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-61662 Brno, Czech Republic; [email protected] (M.J.); [email protected] (F.D.); [email protected] (L.K.) CEITEC IPM, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-61662 Brno, Czech Republic Correspondence: [email protected]; Tel.: +420-532-290-411

Received: 21 December 2017; Accepted: 12 January 2018; Published: 16 January 2018

Abstract: Tensile and creep properties of new austenitic steel Sanicro 25 at room temperature and operating temperature 700 ◦ C were investigated by testing on miniature specimens. The results were correlated with testing on conventional specimens. Very good agreement of results was obtained, namely in yield and ultimate strength, as well as short-term creep properties. Although the creep rupture time was found to be systematically shorter and creep ductility lower in the miniature test, the minimum creep rates were comparable. The analysis of the fracture surfaces revealed similar ductile fracture morphology for both specimen geometries. One exception was found in a small area near the miniature specimen edge that was cut by electro discharge machining, where an influence of the steel fracture behavior at elevated temperature was identified. Keywords: Sanicro 25; austenitic steel; miniature tensile test; creep; fracture

1. Introduction Mechanical testing on miniature specimens is becoming increasingly important for several reasons. It can be used for testing irradiated materials to minimize the radiation dose, determining the local properties of weld zones or the remaining life of service exposed parts, the development of new materials available in limited amounts, etc. Different specimen geometries have been introduced in the past for tensile, fracture, and creep properties determination [1–6], including the well-known small punch test (SPT) and small punch creep test (SPC) [7–20]. The typical shape of the small punch specimen is most often a disc of 8 mm diameter or a square 10 × 10 mm and 0.5 mm thickness. The advantage of SPT or SPC is the need of higher forces for penetration of the disc (due to higher effective cross-section) in comparison to forces needed for testing a uniaxial miniature tensile specimen of cross-section in range of 0.5 to 2 mm2 . A certain disadvantage of SPT and SPC is the equibiaxial stress state that makes correlation with uniaxial tests more complicated. The use of tensile specimens of similar size to an SP disc called a micro-tensile test (M-TT) was demonstrated in [21–23] with promising results. It has also been demonstrated that the flat specimens can represent the tensile curve well up to the necking point, and the ductility is influenced only by the post-necking region [1]. The use of a protective inert atmosphere is inevitable for testing miniature specimens at high temperatures to prevent specimen oxidation. The application of creep tests to miniature specimens is primarily intended for accelerated testing—mainly remaining life or local properties determination. It would hardly replace conventional long-term creep tests, and therefore a reasonable length of such an experiment is within the 1000 h range. The aim of this study is to obtain tensile and creep properties using miniature tensile specimens prepared from the same Ds = 8 mm, similar to the SPT disc size, and correlate the results with measurements on standard specimens for the prospective austenitic steel Sanicro 25. This should Materials 2018, 11, 142; doi:10.3390/ma11010142

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measurements on standard specimens for the prospective austenitic steel Sanicro 25. This should demonstrate miniature specimen specimen testing testing methods methods to to investigate investigate the the mechanical mechanical demonstrate the the potential potential of of miniature properties of of new new materials materials at at room room and and elevated elevated temperatures. temperatures. properties 2. Materials and Methods Austenitic steel of grade UNS S31035, Sanicro 25 [24], was selected as an experimental material (Sandviken, Sweden) Sweden) in the form of a seamless tube of for the study. It was produced by Sandvik (Stockholm, and lotlot no.no. 24476. It was supplied to 38 mm in in diameter diameterand andwall wallthickness thicknessofof8.8 8.8mm, mm,heat heatno. no.527207, 527207, and 24476. It was supplied Doosan Babcock Energy Ltd.Ltd. (Renfrew, UK) and theof tube provided to Institute of Physics to Doosan Babcock Energy (Renfrew, UK) part and of part thewas tube was provided to Institute of of Materials, ASCR (IPM) for creep testing withintesting the European Institute on Knowledge-based Physics of Materials, ASCR (IPM) for creep withinVirtual the European Virtual Institute on Multifunctional Materials (KMM-VIN) research activities [25]. The chemical composition shown Knowledge-based Multifunctional Materials (KMM-VIN) research activities [25]. The is chemical in Table 1. The experimental was used inmaterial the as-received after solution annealing composition is shown in Tablematerial 1. The experimental was usedstate in the as-received state after ◦ C/5 1220 min/cooled water. A fine twinned microstructure of microstructure the steel is shown in solution annealing 1220in°C/5 min/cooled in water.austenitic A fine twinned austenitic of the Figure It contains grains average size of about 25 µm, butofgrains sizebut of up to 200 µmsize are steel is 1a. shown in Figure 1a. with It contains grains with average size aboutwith 25 μm, grains with also in the in Figure 1b. The grain boundaries are decorated with of uppresent to 200 μm aremicrostructure, also present in as theshown microstructure, as shown in Figure 1b. The grain boundaries Nb-rich carbonitrides typicalcarbonitrides size 200 nm, asofreported [26,27]. present in are decorated with of Nb-rich typicalinsize 200 The nm,carbonitrides as reportedareinalso [26,27]. The the interior of larger grains, andintheir chemistry also discussed in their [26,27]. Sanicro 25 shows very good carbonitrides are also present the interior of is larger grains, and chemistry is also discussed in [26,27]. Sanicro 25resistance shows very goodtemperatures creep and fatigue high temperatures up to 700 °C, creep and fatigue at high up to resistance 700 ◦ C, as at well as oxidation resistance [24,28]. as well as oxidation resistanceand [24,28]. strain hardening and cyclic hardeningdue of the Significant strain hardening cyclicSignificant strain hardening of the steel has beenstrain demonstrated to steel has been demonstrated interaction the dislocations with about 50-nm nanoclusters at interaction of the dislocationsdue withtoabout 50-nmof nanoclusters at high temperatures [24,29]. high temperatures [24,29]. Table 1. Chemical composition of Sanicro 25. Element

Element wt % wt % Element Element wt wt %%

C

C 0.064 0.064 Nb Nb 0.49 0.49

(a)

Table 1. Chemical composition of Sanicro 25. Ni Cr W Co

Ni 25.36 25.36 N N 0.23 0.23

Cr 22.35 22.35 Si Si 0.18 0.18

W 3.37 3.37 N N 0.23 0.23

Co 1.44 1.44 P P 0.016 0.016

Cu

Cu 2.98 2.98 B B 0.0035 0.0035

Mn

Mn 0.51 Fe Fe balance balance 0.51

(b)

Figure 1. Sanicro austenitic microstructure, hydrochloric acid and hydrogen hydrogen peroxide peroxide Figure 1. Sanicro 25 25 austenitic microstructure, etched etched in in hydrochloric acid and water solution, magnification (a) 50× and (b) 500×. water solution, magnification (a) 50× and (b) 500×.

All creep tests were performed in purified Ar 4.6 atmosphere using a lever arm (10:1) 8 kN creep All creep tests were performed in purified Ar 4.6 atmosphere using a lever arm (10:1) 8 kN creep machine of IPM design for testing standard specimens. The machine was additionally equipped with machine of IPM design for testing standard specimens. The machine was additionally equipped special grips designed for miniature specimens. The elongation was recorded via linear variable with special grips designed for miniature specimens. The elongation was recorded via linear variable differential transformer (LVDT), and the force was monitored by a 10 kN load cell located on the differential transformer (LVDT), and the force was monitored by a 10 kN load cell located on the bottom bottom side of the load train. The LVDT is located below the load train of the creep machine and it side of the load train. The LVDT is located below the load train of the creep machine and it measures measures the specimen elongation indirectly from the movement of the upper grip using two the specimen elongation indirectly from the movement of the upper grip using two transmitting transmitting pullrods. The machine was equipped with a stepping motor connected with the pullrods. The machine was equipped with a stepping motor connected with the deadweight table deadweight table to enable constant rate experiments. In this case, a deadweight of the full capacity

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to enable constant rate experiments. In this case, a deadweight of the full capacity of the machine of(80 thekg) machine (80 kg)The wasloading applied.rate The0.25 loading rate 0.25 was used for miniature tensile was applied. mm/min was mm/min used for miniature tensile tests. Uniaxial tests. Uniaxial tensile tests on standard specimens were performed in a 50 kN electromechanical creep tensile tests on standard specimens were performed in a 50 kN electromechanical creep machine machine (Zwick KAPPA LA-Spring, Ulm, Germany) with Maytec furnace in Ar at (Messphysik KAPPA 5050 LA-Spring, Fürstenfeld, Austria) with Maytec furnace in 5.0 Ar atmosphere 5.0 atmosphere 1 . In strain raterate 10−3 10 s−1−. 3Ins− this case Maytec high-temperature extensometer (Singen, Germany) was used at strain thisa case a Maytec high-temperature extensometer (Singen, Germany) was toused measure the elongation of the specimen’s cylindrical gauge gauge length.length. to measure the elongation of the specimen’s cylindrical The Thestandard standardspecimen specimenisisshown shownininFigure Figure2a. 2a.Miniature Miniaturespecimens specimenswere were88mm mminindiameter diameterand and 11mm ± 0.005 mm thick. They were prepared from a cylinder by electro discharge machining (EDM) mm ± 0.005 mm thick. They were prepared from a cylinder by electro discharge machining (EDM) toto1.2 1.2mm-thick mm-thickslices slicesand andground groundfrom fromboth bothsides sidestotofinal finalthickness thicknessunder underwater wateron onmetallographic metallographic papers they were cut byby EDM toto thethe required shape according to to Figure 2b.2b. papersup uptoto2500 2500grit. grit.After Afterthis, this, they were cut EDM required shape according Figure

(a)

(b)

Figure Figure2.2.(a) (a)Standard Standardand and(b) (b)miniature miniaturetensile tensilespecimen. specimen.

The tensile tests as well as creep tests were performed one time per test condition. A larger set of The tensile tests as well as creep tests were performed one time per test condition. A larger set of specimens for testing repeatability or statistical treatment was not at our disposal. The microstructure specimens for testing repeatability or statistical treatment was not at our disposal. The microstructure of the studied steel was well homogenous and the consistency of the tensile and creep results of the studied steel was well homogenous and the consistency of the tensile and creep results presented presented in following indicates that the scatter should be at a low level. in following indicates that the scatter should be at a low level. 3.3.Results and Discussion Results and Discussion 3.1. 3.1.Tensile TensileProperties Properties ◦ C)were Tensile Tensileproperties propertiesatatroom roomtemperature temperature(RT) (RT)and andelevated elevatedtemperature temperature(700 (700°C) weremeasured measured using both specimen geometries. As shown in Figure 3, the tensile curves differ mainly using both specimen geometries. As shown in Figure 3, the tensile curves differ mainlydue duetotothe the method methodofofdeformation deformationmeasurement measurement(extensometer (extensometeron onspecimen specimenvs. vs.extensometer extensometeron ongrips). grips).This Thisisis most mostevident evidentininthe theslope slopeofofthe theelastic elasticpart partofofthe thecurves. curves.The Theminiature miniaturespecimen specimenhad hada arelatively relatively small head and it visibly plastically deformed (pulled in) during the tensile test at room small head and it visibly plastically deformed (pulled in) during the tensile test at roomtemperature, temperature, ◦ C.Fracture asasshown shownininFigure Figure4a. 4a.Such Suchaadeformation deformationofofthe thespecimen specimenhead headwas wasnot notfound foundatat700 700°C. Fracture ◦ surfaces of both specimens tested at RT and 700 °C showed clear signs of ductile transgranular surfaces of both specimens tested at RT and 700 C showed clear signs of ductile transgranular fracture. most of of the thefracture fracturesurface, surface,asasshown shown Figure 4b,c (test at RT) fracture.Dimples Dimpleswere were present present in in most inin Figure 4b,c (test at RT) and ◦ and Figure 5a,b (test at 700 °C). However, signs of an intergranular fracture were also found as shown Figure 5a,b (test at 700 C). However, signs of an intergranular fracture were also found as shown in inFigure Figure a small location the specimen was machined by Different EDM. Different 5c,5c, in in a small location nearnear the specimen side, side, whichwhich was machined by EDM. fracture fracture morphology about 40isμm depth is visible in Figure 5c, which is generally morphology in zone 1inofzone about140ofµm depth visible in Figure 5c, which is generally considered as the considered as the zone influenced by high energy accumulation by EDM, but another zone 2 at about zone influenced by high energy accumulation by EDM, but another zone 2 at about 80–100 µm deep 80–100 deep clearly shows intergranular fracture morphology. could point to a higher25 clearlyμm shows intergranular fracture morphology. This could point to aThis higher sensitivity of Sanicro sensitivity of Sanicro 25 to EDM than other steels, further. and should be studied further. to EDM than other steels, and should be studied

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900 900 900 800 800 800

Stress [MPa] Stress [MPa] Stress [MPa]

700 700 700 600 600 600 500 500 500 400 400 400 300 300 300

Sanicro 25 25 Sanicro Sanicro 25 L0 == 25 25 mm, mm, RT RT L0

200 200 200

L0 L0 = 25 mm, mm, RT 700 °C °C L0 == 25 25 mm, 700 L0 == 25 mm,RT 700 °C L0 4 mm, L0 = 4 mm, RT L0 L0 = mm, RT 700 °C °C L0 == 444 mm, mm, 700

100 100 100 00 0 00 0

L0 = 4 mm, 700 °C 0.05 0.05 0.05

0.1 0.1 0.1

0.15 0.15 0.15

0.2 0.2 0.2

0.25 0.25 0.25

0.3 0.3 0.3

0.35 0.4 0.35 0.4 0.35 0.4 Strain [-]

Strain [-] Strain [-]

0.45 0.45 0.45

0.5 0.5 0.5

0.55 0.55 0.55

0.6 0.6 0.6

0.65 0.65 0.65

0.7 0.7 0.7

0.75 0.75 0.75

Figure 3. Tensile curves Figure 3. 3. Tensile Tensile curves of of Sanicro Sanicro 25 25 steel. steel. Figure curves of Sanicro 25 steel. Figure 3. Tensile curves of Sanicro 25 steel.

(a) (b) (c) (a) (b) (c) (a) (b) (c) Figure 4. 4. SEM fractographs fractographs of miniature tensile tensile specimen tested tested at room room temperature (RT): (RT): Figure Figure 4. SEM SEM fractographs ofof miniature miniature tensile specimen specimen tested at at room temperature temperature (RT): Figure 4. SEM fractographs of miniature tensile specimencenter tested500×. at room temperature (RT): (a) magnification magnification 25×; (b) (b) specimen specimen fracture 100×; 100×; (c) specimen specimen (a) 25×; fracture (c) center 500×. (a) magnification 25×; (b) specimen fracture 100×; (c) specimen center 500×. (a) magnification 25×; (b) specimen fracture 100×; (c) specimen center 500×.

(a) (b) (c) (a) (b) (c) (a) (b) (c) Figure 5. 5. SEM SEM fractographs fractographs of of miniature miniature tensile tensile specimen specimen tested tested at at 700 700 °C: °C: (a) (a) specimen specimen fracture fracture 100×; 100×; Figure Figure 5. SEM fractographs of miniature tensile specimen tested at at 700 700 ◦°C: (a) specimen specimen fracture fracture 100 100×; (b) specimen center 500×; (c) specimen right side 1000×. Figure 5. SEM fractographs of miniature tensile specimen tested C: (a) ×; (b) specimen center 500×; (c) specimen right side 1000×. (b) specimen center 500×; (c) specimen right side 1000×. (b) specimen center 500×; (c) specimen right side 1000×.

Despite these these observations, observations, the the agreement agreement of of ultimate ultimate tensile tensile and and yield yield strength strength values values obtained obtained Despite Despite observations, agreement of tensile ultimate tensile and yield was strength values identical. obtained is very very good,these as shown shown in Table Tablethe 2. The The ultimate strength obtained practically is good, as in 2. ultimate tensile strength obtained was practically identical. is very good, as shown in Table 2. The ultimate tensile strength obtained was practically identical.

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Despite these observations, the agreement of ultimate tensile and yield strength values obtained is very good, as shown in Table 2. The ultimate tensile strength obtained was practically identical. The yield strength was identical at room temperature and less than 10% higher at 700 ◦ C. The maximum forces Fm obtained in both types of tensile tests are shown in Table 3. The ratio of Fm for standard/miniature specimen was ~10, which is approximately the ratio of the two specimen type cross-sections (19.635 mm2 /2 mm2 = 9.82). Table 2. Results of uniaxial tensile tests at RT and 700 ◦ C. Method/Condition Temperature Miniature tensile test Standard tensile test Inspection certificate tensile test Difference miniature vs. standard

Ultimate Tensile Strength Rm (MPa) RT 804 787 786 +2.2%

700 ◦ C 500 514 514 −2.7%

Proof Yield Strength Rp0.2 (MPa) RT 376 375 369 +0.3%

700 ◦ C 238 217 202 +9.7%

Table 3. Maximum forces Fm corresponding to Rm in the uniaxial tensile tests at RT and 700 ◦ C. Method/Condition Temperature Miniature tensile test Standard tensile test Ratio Fm standard/Fm miniature

Maximum Force F m (N) RT 1551 15,450 9.96

700 ◦ C 955 10,086 10.56

A further detailed optimization of SPT sized miniature tensile specimen geometry, especially its gauge length, cross-section, and head size, would be beneficial by finite element method and experimentally. The M-TT specimen geometry described in [21–23] seems preferable for static testing in order to avoid the excessive plastic deformation of the specimen head. Broad discussion is needed to initiate a process that would lead to the standardization of this miniature test. 3.2. Creep Short-term creep testing was performed on both specimen geometries at 700 ◦ C. The creep curves and strain rate dependence on creep strain for stress between 200 to 400 MPa are shown in Figures 6–8. Except for the test at 200 MPa, all of the tests were conducted above the yield strength of the steel. The initial strain obtained instantly after loading was subtracted, so only creep strain is plotted in the creep curves. It can be seen from Figures 6a, 7a, 8a and 9a that for all tests the time to rupture was lower for miniature specimens. Figure 9a also shows that the time to rupture at 200 MPa is well comparable with results obtained in [24] and [27]. The creep strain rate (derivative of creep strain) is shown in Figures 6b, 7b and 8b. There was a higher noise for miniature specimens compared to standard specimens. This is attributed to the specimen length and given sensitivity and noise of the induction sensor recording the deformation. The stress exponent n is approximately 8 for standard specimens and 7 for miniature specimens, as shown in Figure 9b. The rupture strain dependence on stress is plotted in Figure 10a. It is shown that the rupture strain steadily increases with decreasing stress. For miniature specimens, it increased less than for standard specimens. This can be attributed to small gauge length. The strain at minimum creep rate dependence on stress is plotted in Figure 10b. It is systematically higher for miniature specimens compared to standard specimens. Creep rupture time of the miniature specimens was in all cases approximately 30–40% lower compared with standard specimens, and the difference slightly decreased with lower stress. This represents a certain level of conservative factor if the miniature specimen testing method is applied instead of the standard test.

Materials 2018, 11, 142 Materials2018, 2018,11, 11,142 142 Materials 0.1 0.1

1.E-04 1.E-04

Sanicro25 25700 700°C, °C,400 400MPa MPa Sanicro

Sanicro25 25700 700°C, °C,400 400MPa MPa Sanicro creep creep strain rate [1/s] creepstrain strainrate rate[1/s] [1/s]

L0==25 25mm mm L0 L0==44mm mm L0

0.08 0.08

creep creep strain [-] creepstrain strain[-] [-]

6 of 10 of10 10 66of

0.06 0.06

0.04 0.04

L0==25 25mm mm L0 L0==44mm mm L0

1.E-05 1.E-05

0.02 0.02

00

00

0.5 0.5

11

1.5 1.5

22

2.5 2.5

33

3.5 3.5

1.E-06 1.E-06 00

44

0.01 0.01

0.02 0.02

0.03 0.03

0.04 0.04

0.05 0.05

0.06 0.06

0.07 0.07

0.08 0.08

0.09 0.09

creepstrain strain[-] [-] creep

time[h] [h] time

(a) (a)

(b) (b)

◦ C,400 Figure 6. (a) Creep curves; (b) creep rate vs. strain relations of Sanicro 25 at 700 °C, 400 MPa. Figure Figure6. 6.(a) (a)Creep Creepcurves; curves;(b) (b)creep creeprate ratevs. vs.strain strainrelations relationsof ofSanicro Sanicro25 25at at700 700°C, 400MPa. MPa. 0.35 0.35

1.E-04 1.E-04


0.3 0.3

creep creep strain rate [1/s] creepstrain strainrate rate[1/s] [1/s]

0.25 0.25



L0==25 25mm mm L0 L0==44mm mm L0

0.2 0.2 0.15 0.15 0.1 0.1

L0==25 25mm mm L0 L0==44mm mm L0 1.E-05 1.E-05

1.E-06 1.E-06

0.05 0.05 00 00

10 10

20 20

30 30

40 40

50 50

60 60

70 70

80 80

1.E-07 1.E-07 00

90 90

0.05 0.05

0.1 0.1

0.15 0.15

0.2 0.2

0.25 0.25

0.3 0.3

0.35 0.35


time[h] [h] time

(a) (a)

(b) (b)

◦ C,300 Figure 7. (a) Creep curves; (b) creep rate vs. strain relations of Sanicro 25 at 700 °C, 300 MPa. Figure Figure7. 7.(a) (a)Creep Creepcurves; curves;(b) (b)creep creeprate ratevs. vs.strain strainrelations relationsof ofSanicro Sanicro25 25at at700 700°C, 300MPa. MPa. 1.E-04 1.E-04

0.5 0.5


Sanicro25 25700 700°C, °C,200 200MPa MPa Sanicro L0==25 25mm mm L0 L0==44mm mm L0

creep creep strain rate [1/s] creepstrain strainrate rate[1/s] [1/s]


0.4 0.4

0.3 0.3

0.2 0.2

1.E-06 1.E-06

1.E-07 1.E-07

1.E-08 1.E-08

0.1 0.1

00

L0==25 25mm mm L0 L0==44mm mm L0

1.E-05 1.E-05

1.E-09 1.E-09 00

200 200

400 400

600 600

800 800

time[h] [h] time

(a) (a)

1000 1000

1200 1200

1400 1400

00

0.1 0.1

0.2 0.2

0.3 0.3

0.4 0.4


(b) (b)

Figure8. 8. (a)Creep Creep curves;(b) (b) creeprate rate vs.strain strain relationsof of Sanicro25 25 at700 700 °C, 200MPa. MPa. Figure ◦ C,200 Figure 8.(a) (a) Creepcurves; curves; (b)creep creep ratevs. vs. strainrelations relations ofSanicro Sanicro 25at at 700°C, 200 MPa.

0.5 0.5

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10000

1.E-05 1.E-05

Sanicro 25, 700°C Sanicro 25, 700°C

UCT L0 = 25 mm UCT L0 = 4 25mm mm UCT L0 = 4[24] mm Chai et al. Chai et al. [24] Zieliński25, et al. [27] Sanicro 700°C Zieliński et al. [27] UCT L0 = 25 mm UCT L0 = 4 mm Chai et al. [24] Zieliński et al. [27]

10000 1000 1000

1000 100 100

100 10 10

Minimum creep rate [1/s] rate [1/s] Minimum Minimum creep ratecreep [1/s]

Time to rupture Time to[h] rupture [h][h] Time to rupture

10000 Materials 2018, 11, 142

-8.4

y = 5E+22x-8.4 y = 5E+22x -9.3 y = 3E+24x-9.3 y = 3E+24x

10 1 1100 100

-8.4

y = 5E+22x Stress [MPa] Stress [MPa] y = 3E+24x-9.3

(a) (a)

1 100


1.E-05 1.E-06 1.E-06

7 of 10

6.8

y = 8E-24x6.8 y = 8E-24x

UCT L0 = 25 mm UCT L0 = 25 mm UCT L0 = 4 mm UCT L0 = 4 mm

7.8

y = 1E-26x7.8 y = 1E-26x

Sanicro 25, 700°C

6.8

y = 8E-24x

UCT L0 = 25 mm

7.8

y = 1E-26x

UCT L0 = 4 mm 1.E-06 1.E-07 1.E-07

1.E-07

1000 1000

1.E-08 1.E-08100 100

1000

1.E-08 100

1000 1000

Stress [MPa] Stress [MPa]

(b) (b)

1000

Stress [MPa]

Stress [MPa] Figure 9. (a) Time to rupture dependence on stress; (b) minimum creep rate dependence on stress. Figure 9. (a) to rupture dependence (b)minimum minimum creep rate dependence on stress. Figure 9. Time (a) Time to rupture dependenceon on stress; stress; (b) creep rate dependence on stress. (a) (b) 0.6

0.20

0.6 Figure 9. (a) Time to rupture dependence on stress; (b)0.20 minimum creep rate dependence on stress. 0.5 0.5 0.6


0.4 0.4 0.5

Strain min. rate Strain atat min. creep rate [-][-] Strain at min. creep rate [-]creep

True rupture strain True rupture [-][-] True rupture strain [-] strain



Sanicro 25, 700°C UCT L0 = 25 mm UCT L0 = 4 mm

0.3 0.3 0.4 0.2 0.2 0.3 0.1 0.1 0.2 0.0 0.0100 0.1 100

150 150

200 200

0.0 100

150

200

250 250

300 300

350 350

400 400

450 450

300

350

400

450


(a) (a)

250

Stress [MPa]

0.20 0.15 0.15


Sanicro 25, 700°C UCT L0 = 25 mm

0.15 0.10 0.10

UCT L0 = 4 mm

0.10 0.05 0.05

0.05 0.00 0.00100 100

150 150

200 200

0.00 100

150

200

250 250

300 300

350 350

400 400

450 450

(b) 250 (b)

300

350

400

450


Stress [MPa]

Figure 10. (a) True rupture stress dependence on applied stress; (b) strain at min. creep rate dependence Figure 10. (a) True rupture stress dependence on applied stress; (b) strain at min. creep rate dependence

Figure (a) True rupture(a) stress dependence on applied stress; (b) strain at(b) min. creep rate dependence on10. applied stress. on applied stress. on applied stress. Figure 10. (a) True rupture stress dependence on applied stress; (b) strain at min. creep rate dependence The fractographic analysis does not show any significant differences between the standard and The fractographic on applied stress. analysis does not show any significant differences between the standard and miniature creep specimen fracture. Fracture morphology is mainly ductile transgranular, shown and The fractographic analysis does not show any significant differences between the as standard miniature creep specimen fracture. Fracture morphology is mainly ductile transgranular, as shown in Figures 11 and 12. There is the presence of dimples and quasi-cleavage fracture that can also be The fractographic analysis does not show any significant differences between the standard and in Figures 11specimen and 12. There is the presence of dimples and quasi-cleavage fracture that can as also be miniature creep fracture. Fracture morphology is mainly ductile transgranular, shown in found in other austenitic steels (e.g., 316L) [15]. However, the presence of dimples is significantly miniature creep specimen fracture. Fracture morphology is mainly ductile transgranular, as shown found in other austenitic steels (e.g., 316L) [15]. However, the presence of dimples is significantly Figures 11 and 12. There is the presence of dimples and quasi-cleavage fracture that can also be found decreased fracture (Figures 11b and in comparison with tensile test fracture high in Figures in 11 creep and 12. There is the presence of 12c) dimples and quasi-cleavage fracture that can at also be decreased in creep (Figures 11b and 12c) in comparison tensile fracture at high in other austenitic steelsfracture (e.g., 316L) [15]. However, the presence ofwith dimples isintest significantly decreased temperature (Figure 5b). The miniature specimen head visibly did not deform the creep test as in found in other austenitic steels (e.g., 316L) [15]. However, thedid presence of dimples is significantly temperature (Figure 5b). The miniature specimen head visibly not deform in the creep test as in in creep fracture (Figures 11b and 12c) in comparison with tensile test fracture at high temperature the case of the tensile test at room temperature (Figure 12a compared to Figure 4a). decreased fracture (Figures 11b and 12c) in comparison with tensile 4a). test fracture at high the case of in thecreep tensile test at room temperature (Figure 12a compared to Figure (Figure 5b). The miniature specimen head visibly did not deform in the creep test as in the case of the temperature (Figure 5b). The miniature specimen head visibly did not deform in the creep test as in tensile temperature (Figure 12a compared to Figure 4a).to Figure 4a). thetest caseatofroom the tensile test at room temperature (Figure 12a compared

(a) (a)

(b) (b)

(a)

(b)

Figure 11. SEM fractographs of standard creep specimen (σ = 300 MPa, T = 700 ◦ C, tr = 85.2 h): (a) fracture surface 40×; (b) fracture surface 500×.

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Figure Materials 2018,11. 11, SEM 142 fractographs of standard creep specimen (σ = 300 MPa, T = 700 °C, tr = 85.2 h):8 of 10 (a) fracture surface 40×; (b) fracture surface 500×.

(a)

(b)

(c)

Figure 12. SEM = 300 = 700 ◦ C, ttr == 60.2 Figure 12. SEM fractographs fractographs of of miniature miniature creep creep specimen specimen (σ (σ = 300 MPa, MPa, T T= 700 °C, 60.2 h): h): r (a) 25×; (a) magnification magnification 25 ×;(b) (b)fracture fracturesurface surface40×; 40×(c) ; (c)fracture fracturesurface surface500×. 500×.

3.3. 3.3. Perspective Perspective of of Miniature Miniature Specimen Specimen Testing Testing The results presented presented herein herein are are promising. promising. Based The authors authors believe believe that that the the results Based on on these these results, results, research should further proceed to broader studies that concentrate on: (i) optimization of specimen research should further proceed to broader studies that concentrate on: (i) optimization of specimen dimensions; (ii) testing various materials, such as ferritic-martensitic steels, austenitic steels, light dimensions; (ii) testing various materials, such as ferritic-martensitic steels, austenitic steels, light alloys, and others in a similar way as SPT and SPC [8–20]; and (iii) application to practical tasks for alloys, and others in a similar way as SPT and SPC [8–20]; and (iii) application to practical tasks for the industry (e.g., determination of weld properties or service life extension in a similar manner as the industry (e.g., determination of weld properties or service life extension in a similar manner as presented in [27,30,31]). It is necessary to define the size effect related to the particular microstructures presented in [27,30,31]). It is necessary to define the size effect related to the particular microstructures and specimen geometry. Additionally, it is key to prove the repeatability and reproducibility of the and specimen geometry. Additionally, it is key to prove the repeatability and reproducibility of test results on different testing machines. After successful resolution of these challenging tasks, the the test results on different testing machines. After successful resolution of these challenging tasks, standardization of the miniature tensile and creep test should be feasible. the standardization of the miniature tensile and creep test should be feasible. 4. 4. Conclusions Conclusions Conventional applied to identify the the mechanical mechanical Conventional and and miniature miniature tensile tensile and and creep creep tests tests were were applied to identify properties of the new austenitic steel Sanicro 25 at room and elevated temperatures. Based properties of the new austenitic steel Sanicro 25 at room and elevated temperatures. Based on on the the comparison of both types of test results, it is possible to draw following conclusions: comparison of both types of test results, it is possible to draw following conclusions: •  •  •  •

Miniature specimens can can give Miniature specimens give very very precise precise estimation estimation of of mechanical mechanical properties properties from from aa very very small small volume of material volume of material The creep life of miniature specimens was about 30–40% lower than that of standard ones, and The creep life of miniature specimens was about 30–40% lower than that of standard ones, and the the difference decreased with lower stress, mainly due to lower ductility difference decreased with lower stress, mainly due to lower ductility Comparable minimum creep rates at the same stress were obtained from both types of tests Comparable minimum creep rates at the same stress were obtained from both types of tests Special care must be paid if the miniature specimens are prepared by different technology than Special care must be paid if the miniature specimens are prepared by different technology than the standard specimens (here machining and grinding vs. grinding and EDM), since changes in the standard specimens (here machining and grinding vs. grinding and EDM), since changes in the fracture behavior of the steel at elevated temperature were demonstrated in a small local the fracture behavior of the steel at elevated temperature were demonstrated in a small local area. area.

Acknowledgments: Authors wish to thank Peter Barnard and Gerry McMillan from Doosan Babcock Energy Ltd. for Acknowledgments: Authors wish toAuthors thank Peter Barnard and Gerry McMillan from Doosan Babcock Energy supply of the experimental material. greatly acknowledge Czech Science Foundation financial support of Ltd. for supply of the P.D. experimental material. Authors greatly acknowledge Foundation financial the project 15-21394S. acknowledges the use of infrastructure within theCzech projectScience CEITEC 2020 (LQ1601) with support the project P.D. the and use of infrastructure within the project 2020 financialof support from15-21394S. the Ministry of acknowledges Education, Youth Sports of the Czech Republic underCEITEC the National Sustainability II. (LQ1601) withProgramme financial support from the Ministry of Education, Youth and Sports of the Czech Republic under the National Sustainability Programme II. conceived and designed the experiments; M.J. and P.D. performed Author Contributions: F.D., M.J. and L.K. the experiments; M.J. and P.D. analyzed the data; P.D. performed the fractographic analysis on SEM; P.D. wrote Author Contributions: F.D., M.J. and L.K. conceived and designed the experiments; M.J. and P.D. performed the paper. the experiments; M.J. and P.D. analyzed the data; P.D. performed the fractographic analysis on SEM; P.D. wrote Conflicts the paper.of Interest: The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

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