Karel MATOCHA, Miroslav FILIP, Šárka STEJSKALOVÁ

Transcription

1 DETERMINATION OF CRITICAL TEMPERATURE OF BRITTLENESS T K0 BY SMALL PUNCH TESTS Karel MATOCHA, Miroslav FILIP, Šárka STEJSKALOVÁ MATERIAL & METALLURGICAL RESEARCH, Ltd. Ostrava, Czech Republic, EU, karel.matocha@mmvyzkum.cz Abstract CWA Small Punch Test Method for Metallic Materials Part B: A Code of Practice for Small Punch Testing for Tensile and Fracture Behaviour describes the determination of yield and tensile strength, Ductile Brittle Transition Temperature (DBTT), measured by Fracture Appearance Transition Temperature (FATT) and/or 41J absorbed energy TT, and fracture toughness of the metallic materials. The present paper describes the procedure used for determination of yield strength, tensile strength, FATT and critical temperature of brittleness t k0 of carbon steel of type 22K by Small Punch tests using empirical correlations. Keywords: Small Punch test, mechanical properties, fracture energy, critical temperature of brittleness t k0, Fracture Appearance Transition Temperature. 1. INTRODUCTION The need for evaluating the actual mechanical properties of structural components by direct testing method has led to development of innovative techniques based on miniaturized specimens. Among these, a technique called the Small Punch (SP) test has emerged as a promising candidate [1,2]. It can be used to obtain tensile, fracture and creep data from very small quantities of experimental material. Major benefits of this method are [3]: 1. It can be applied as a virtually non-destructive tool to monitor in service components in power plants. 2. It enables determination of the tensile, fracture and creep characteristics of materials at critical locations of the components. 3. Test itself is rather simple to perform and inexpensive. 4. It gives the possibility to study locations such as interfaces, coatings, welded joints (BM, HAZ, WM) and exotic materials, e.g. anisotropic. The use of this method was accelerated by the development of sampling devices for obtaining and retrieving a sample of material for analysis in virtually non-destructive manner not requiring post sampling repair. On the base of technical consensus of 32 participated organizations (without geographic restriction) CWA Small Punch Test Method for Metallic Materials was issued in December 2007 [4]. It comprises two parts, Part A: A Code of Practice for Small Punch Creep Testing, and Part B: A Code of Practice for Small Punch Testing for Tensile and Fracture Behaviour. ANNEX B1: Derivation of tensile and Fracture material properties describes the methods for estimation of yield and tensile strength, DBTT and fracture toughness of the metallic materials from SP test records. ANNEX B2 Guidance on Relevant Technological Issues: specimens sampling from components describes potential applications of the SP test; some available miniature sample removal technologies that have been used to generate material for SP testing; considerations in sample removal (selection of sample locations, amount of material, etc.); and preliminary guidance on the use of SP test data.

2 The objective of the test is to produce a load-punch displacement (load- specimen deflection) record under cross head control, which contains information about the elastic-plastic deformation and strength properties of the material. A displacement rate of the punch in the range between 0, 2 to 2 mm/min. is recommended. The load-punch displacement record obtained for ductile materials can be divided into four regions (see Fig. 1) [5]. The initial linear region corresponds to elastic deformation and micro-yield under the penetrator. This is followed by deviation from linearity associated with the spread of the yield zone through the specimen thickness and then outward radially. The subsequent inflection point is associated with transition from bending to membrane stretching. The maximum load is associated with local thinning and crack initiation. Fig. 1 Load punch displacement curve recorded during a small punch test of a ductile material The following SP related parameters are used for determination of YS, UTS, FATT and J IC from such a load/punch displacement curve. F m [N] F e [N] maximum load recorded during SP test, load characterising the transition from linearity to the stage associated with the spread of the yield zone through the specimen thickness, u m [mm] displacement corresponding to the maximum load F m u f [mm] displacement corresponding to load F f = 0,8. F m E SP [J] SP fracture energy obtained from the area under the load- punch displacement curve up to fracture Post-test examination covers the determination of the elastic-plastic transition load F e, the determination of the SP fracture energy E SP and the determination of the effective fracture strain ε f = ln(h 0 /h f ) where h f is the final thickness of the test specimen adjacent to the area of failure. The load-punch displacement curves obtained can be analysed either in terms of elastic-plastic finite element method or can be utilised to derive empirical correlations between SP and standardised test results. Both of the above mentioned approaches are used at the present time for determination of YS, UTS and J IC. However ductile brittle transition temperature (DBTT) is determined only using empirical correlations.

3 The present paper describes the procedure used for determination of yield strength, tensile strength, FATT and critical temperature of brittleness t k0 of carbon steel of type 22K by Small Punch tests using empirical correlations. 2. TESTING MATERIAL Tube plate of WWER 1000 MW feed water high temperature heater, 350 mm in thickness, made of carbon steel of type 22K was used as a testing material for both standardized and SP tests. Testing material for determination of the empirical correlations were cut out of the near-surface area, 1/4, 1/2 and 3/4 of the tube plate thickness (see Fig.2). Fig. 2 Photo of testing material sampling out of tube plate In order to obtain the empirical correlations in sufficiently wide extent of mechanical properties desks, 170 x 200 x 15 mm in dimensions, cut out of the tube plate, were heat treated by the following heat treatment procedures: 920 C/45 min./air C/2 hours/air (heat treatment 1) and 920 C/45 min./air C/2 hours/air (heat treatment 2). Tensile test specimens and Charpy V notch specimens were oriented in tangential direction. Disc test specimens 8 mm in diameter and 0,5 mm in thickness used for penetration tests were oriented in accord with CWA RESULTS OF STANDARDIZED AND SMALL PUNCH TESTS Results of standardized tensile tests and Small Punch tests at laboratory temperature are summarized in Tab.I. The values mentioned in this table are the arithmetic means of three tests. The offset method (offset displacement of 0,1 mm) was used for determination of load F e instead of the procedure described in the Code. The use of this method resulted in significant lowering of the scatter band of the empirical correlation [6].

4 Tab. 1 Results of tensile and Small Punch tests of carbon steel of type 22K at laboratory temperature 2 F u=0,1 /h 0 Yield strength F m /(u m.h 0 ) Tensile strength Near the surface ¼ of the thickness ½ of the thickness ¾ of the thickness Heat treatment Heat treatment Fig. 3 and Fig. 4 show the empirical correlations for determination of yield strength and tensile strength at laboratory temperature from the results of Small Punch tests for carbon steel of type 22K. Fig.3 Empirical correlation for determination of yield strength at laboratory temperature from the results of Small Punch tests for carbon steel of type 22K Fig.4 Empirical correlation for determination of tensile strength at laboratory temperature from the results of Small Punch tests for carbon steel of type 22K

5 Transition temperature FATT and critical temperature of brittleness t k0 of the tube plate material determined using standardized Charpy test specimens together with the Small Punch transition temperature T SP determined from the temperature dependence of fracture energy E SP in accord with the Code are summarized in Tab. 2 Tab. 2 Results of fracture behaviour of 22K steel evaluated by standardized Charpy tests and Small Punch tests Sampling place FATT [ C] FATT [K] Near the surface ¼ of the thickness ±0 ½ of the thickness ¾ of the thickness ,5 +20 The ratio of T SP /FATT calculated from the results obtained was found to be T SP /FATT = 0,38 for the steel investigated. Fig. 5 shows the empirical correlation between critical temperature of brittleness t k0 and Small Punch transition temperature T SP. T SP [K] t k0 [ C] Fig.5 Empirical correlation between t k0 and T SP for carbon steel of type 22K 4. CONCLUSIONS The following conclusion can be drawn from the results of standardized tensile and Charpy Impact tests and Small Punch tests carried out at temperatures ranging from -193 C up to +23 C on tube plate 350 mm in thickness made of carbon steel of type 22K: 1) Both FATT and critical temperature of brittleness t k0 has changed throughout the thickness of the tube plate made of the steel of type 22K. 2) The ratio of T SP /FATT for the investigated steel is equal to T SP /FATT = 0,38. 3) There is a very good correlation between critical temperature of brittleness t k0 and Small Punch transition temperature T SP.

6 ACKNOWLEDGEMENT This paper was created in the frame of the project TA Development of procedure for evaluation of irradiated materials properties degradation of hardly replaced components of nuclear power plants by the use of punch tests financed by Technology Agency of CR in the programme ALFA. LITERATURE [1] HURST, R.-MATOCHA,K.: The European Code of Practice for Small Punch Testing-where do we go from here? In: Proc. Of 1 st International Conference Determination of Mechanical Properties of Materials by Small Punch and other Miniature Testing Techniques. Ostrava, Czech Rep., August 31 to September 2, 2010, p.5-11, ISBN [2] MATOCHA,K.: The evaluation of mechanical properties of structural steels by penetration tests. Monograph. Published by Technical University Ostrava & MATERIAL AND METALLURGICAL RESEARCH Ltd., 20010, ISBN (in Czech). [3] MATOCHA, K. Determination of Actual Tensile and Fracture Characteristics of Critical Components of Industrial Plants under Long term Operation by SPT. Proceedings of the ASME 2012 Pressure & Piping Division Conference PVP 2012, July 15-19, 2012, Toronto, Ontario, Canada (CD-ROMM). [4] CEN WORKSHOP AGREEMENT Small Punch Test method for Metallic Materials CWA 15627:2007 D/E/F, December [5] LUCAS, G.E. Review of Small Specimen Test Techniques for Irradiation Testing. METALLURGICAL TRANSACTIONS A, Vol. 21A, May 1990, p [6] HURST, R.-MATOCHA, K.: Where are we now with the European Code of Practice for Small Punch Testing? In: Proc. of 2 nd International Conference Determination of Mechanical Properties of Materials by Small Punch and other Miniature Testing Techniques. Ostrava, Czech Rep., October 2 to 4, 2012, p.4-18, ISBN