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Procedia Materials Science 3 (2014) 912 - 917
20th European Conference on Fracture (ECF20)
Fractographic aspects of small punch test results
Jan Siegla*, Petr Hausilda, Adam Jancaa, Radim Kopfivab
aCzech Technical University in Prague, FNSPE, Department of Materials. Trojanova 13, Prague 120 00, Czech Republic bUJVRez, a. s., Integrity and Technical Engineering Division. Hlavni 130, Husinec-Rez 250 68, Czech Republic
Abstract
The specific desired properties for structures and components working in critical environments (e.g. pressure vessel of nuclear power plant) require current information about degradation processes coming out in materials. This information is essential and inevitable input for residual lifetime assessment. Possibility of the classical (standard) mechanical testing (tensile test, Charpy test, fracture toughness test, creep test etc.) is very limited, because there is not sufficiently of material to be non-invasively taken from the component. Sampling material from in-service structures and components is very complicated and time consuming, and in addition to that the component has to be repaired after this extraction. Hence, the new innovative techniques based on miniaturized specimens have been developed for evaluation of mechanical properties and their changes. One of very promising techniques is Small Punch Test. This test can be considered as non-destructive method - test sample is disc 0.5 mm thick and 8 mm diameter. Even if this test evaluates relatively small volume of material we can use it for investigation and description of degradation processes of the whole components. Present paper summarizes the results obtained for specimens of steel 15Ch2MFA with different heat treatment and for specimens of steel O8Ch18NT10 with various degree of deformation. Fractographic analysis was carried out in order to describe failure processes taking place in material during Small Punch Test. Obtained information was used to justify the correct test procedure.
© 2014 Elsevier Ltd. Open access under CC BY-NC-ND license.
Selection andpeer-reviewunder responsibilityof theNorwegianUniversity of Science andTechnology(NTNU),Department of StructuralEngineering
Keywords: small punch test, nuclear reactor, stainless steel, fractographic analysis
* Corresponding author. Tel. +420 224 358 508; fax +420 224 358 523. E-mail address: jan.siegl@fjfi.cvut.cz
2211-8128 © 2014 Elsevier Ltd. Open access under CC BY-NC-ND license.
Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering doi:10.1016/j.mspro.2014.06.148
1. Introduction
The reliable and safe service of nuclear power plants requires regular information about degradation processes in materials. This information is an essential input for residual lifetime assessment and for identifying the probability of in-service components failure. The scope for application of classical (standard) mechanical tests (tensile test, Charpy test, fracture toughness test, creep test etc.) is very limited, because there is insufficient quantity of material to be sampled non-invasively from the components. Invasive sampling from in-service structures and components is very complicated and time consuming, and in addition the components have to be repaired after sample extraction, see e.g. Hurst and Matocha (2012). Therefore, new innovative techniques for evaluation and tracking mechanical properties based on miniaturized specimens have been developed. One of the most promising methods is the Small Punch Test (SPT). This test can be considered as non-invasive, since the test sample is a disc 0.5 mm thick and 8 mm in diameter (for detailed description, see CWA Standard (2006). Since this test evaluates only a very small volume of material we can use it for investigation and description of degradation processes in critical points of components, see Foulds and Viswanathan (2001).
Present paper summarizes the results obtained for specimens of steel 15Ch2MFA and O8Ch18NT10. These results complemented by fractographic analysis of failed SPT specimens, are compared with our previous results for steels 10GN2MFA and A533JRQ.
2. Experimentals
The specimens for SPT were prepared from steels 10GN2MFA with different heat treatment and for specimens of steel O8Ch18NT10 with various degree of deformation. The mechanical properties of studied materials are described in Table 1 (in comparison with previous observed steels 15Ch2MFA and A533JRQ).
Table 1 Mechanical properties of studied steels.
Tested materials Rp0.2 Rm Rp0.2/Rm
Steel Treatment [Mpa] [Mpa] [1]
as received 545 637 0.856
heat treatment HT1 630 697 0.904
10GN2MFA heat treatment HT2 584 664 0.880
heat treatment HT3 514 608 0.845
as received 237 564 0.420
deformation 5% 349 588 0.594
O8Ch18N10T deformation 10% 417 617 0.676
deformation 15% 474 635 0.764
15Ch2MFA as received 499 615 0.811
A533 JRQ as received 477 615 0.775
All specimens of disc shape (0.5 mm thick and 8 mm in diameter) were metallographically polished (according to CWA Standard (2006)). SPTs were carried out using a special jig (made by MMR Ltd., Ostrava) in combination with the tensile testing machine Inspekt 20kN. Previous experiments showed that the clamping force imposed by jig tightening can significantly influence the test results, see Siegl et al. (2014). Based on these results the optimal value of clamping force 15 kN was used for all specimens in presented study. A constant punch displacement velocity of 0.4 mm/min was held throughout the test. The applied load, punch tip displacement and specimen deflection were recorded during the tests; i.e. Load Punch Displacement (LPD) curves and Load Specimen Deflection (LSD) curves were obtained. All failed specimens were observed using a stereomicroscope and the scanning electron microscope JSM 840A. The main goal of this fractographic analysis was to validate the SPT.
3. Results and Discussion
3.1. Small Punch test results
SPT results for specimens prepared from 10GN2MFA steels with different heat treatment are plotted in Fig. 1. The highest values of maximal force were measured for heat treatment HT1 and HT2, while the results for as received state and heat treatment HT3 are practically the same. It is clear that they are in good agreement with the mechanical properties established for this material (see Table 1), i.e. the load characterizing the transition from linear to plastic bending stage is correlated to Rp0.2, an increase in the Rp0.2/Rm ratio corresponds to an increase in the maximum force and a decrease in the punch displacement at onset of fracture defined as the punch displacement at 20% load drop after maximum force.
SPT results for specimens prepared from O8Ch18NT10T steels with various degree of deformation are plotted in Fig. 2. In this case, the results show that an increase of cold working degree led to an increase of the maximum punch force, but the punch displacement at onset of fracture remained practically the same.
Comparison of Fig. 1 and Fig. 2 shows that the responses of 10GN2MFA and O8Ch18N10T steels during the small punch test are completely different. This effect is also documented in Fig. 3, where the SPT results for 15Ch2MFA and A533 (JRQ) reactor steels are also included. Austenitic steel O8Ch18N10T shows other behavior during the test, i.e. mechanisms of hardening and following damage processes differ from those taking place in steels with bainitic or ferritic/martensitic microstructure.
2000 1600
<u 1200
800 400
10GN2MFA
influence of heat treatment
0.5 1 1.5
displacement [mm]
Fig. 1. Influence of heat treatment (10GN2MFA)
„ 1600 Oi
Si 1200 o
800 400 0
O8Ch18N10T influence of cold working
—as received
......5% def
— •10% def —15% def
0.5 1 1.5 2
displacement [mm]
Fig. 2 Influence of cold working (Q8Ch18N10T)
— 1600 2Z
u 1200 42
800 400 0
Fig. 3. Results of SPT for different type of steels.
influence of steel type (as recieved state)
,._u___■. '
/ i -O8Ch18N10T .....10Gn2MFA — 15Ch2MFA
- • A533 JRQ
0 0,5 1 1,5 2
displacement [mm]
3.2. Results of fractographic analysis
Steel 10GN2MFA: The main fractographic features for all 10GN2MFA specimens were elongated ductile dimples and clearly evident slip lines (traces of plastic deformation) on specimen surface, see micrographs in Fig. 4. It means that changes caused by the used thermal treatment did not influence failure processes taking place during the small punch test.
(c) 10GN2MFA TT2
Fig. 4. Steel 10GN2MFA - elongated ductile dimples on fracture surfaces and traces of plastic deformation (slip lines) on specimen surfaces.
Steel O8Ch18N10T: Fracture micromorphology of O8Ch18N10T specimens is strongly influenced by occurrence of conglomerated particles in steel micro structure. All (transversal and circumferential) cracks initiated by decohesion of these particles and propagated by coalescence of so formed voids. The main fractographic features for all failed specimens were ductile dimples created around these particles, see micrographs in Fig. 5. Also clearly evident slip lines on specimen surface were observed, see micrographs in Fig. 5. Fractographic findings proved that various degrees of deformation did not influence failure processes taking place during small punch test.
b) Q8Ch18N10 10% deformation
c) O8Ch18N10 15% deformation Fig. 5. Steel O8Ch18N10T - conglomerated particles on fracture surfaces and slip lines on specimen surface.
4. Conclusions
Based on presented research we can summarize conclusions as follows:
• Small punch test results for both investigated steels show that SPT can distinguish even between relatively very small changes in mechanical properties.
• The small punch test can be used not only for mechanical properties evaluation, but also for verification of thermal and/or mechanical treatment of materials.
• Fractographic analysis of failed SPT specimens offers very important information about failure mechanisms taking place during the test. This information can significantly extend interpretation of SPT results.
The results obtained will be used as reference values for evaluating the influence of service conditions (namely neutron irradiation) on their mechanical properties. These experiments will offer information about the degredation of materials properties during the operation of nuclear power plants.
Acknowledgements
The authors gratefully acknowledge the financial support provided by Technology Agency of the Czech Republic in the frame of research project TA02020811.
References
Hurst, R., Matocha, K. 2012. Where are we now with the European Code of Practice for Small Punch Testing? in: K. Matocha, R. Hurst, W. Sun, Determination of Mechanical Properties of Materials by Small Punch and other Miniature Testing Techniques. Ostrava, QCELQT s. r. o., 2012, pp. 4-18.
CEN Workshop agreement CWA 15627:2006 E, 2006. "Small Punch Test Method for Metallic Materials." Brussels, CEN, pp. 70.
Foulds, J.R., Viswanathan, R., 2001. Determination of the Toughness of In-Service Steam Turbine Disks Using Small Punch Testing, Journal of
Materials Engineering and Performance, 10, pp. 614-619. Siegl, J., Hausild, P., Janca, A., Kopriva, R., Kytka, M., 2014. Characterisation of Mechanical Properties by Small Punch Test, Key Engineering Materials, in press.
