EFFECT OF THE AGING TREATMENT IN THE FRACTURES MECHANICS OF WELDED JOINTS OF STEEL 316L

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1 2013 International Nuclear Atlantic Conference - INAC 2013 Recife, PE, Brazil, November 24-29, 2013 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: EFFECT OF THE AGING TREATMENT IN THE FRACTURES MECHANICS OF WELDED JOINTS OF STEEL 316L Luísa Lima Silveira Dias 1, João Pedro Santiago Carneiro 2 and Jefferson José Vilela 3 1 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN) Av. Avenida Antônio Carlos, Belo Horizonte, MG luisa_lima11@yahoo.com.br 2 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN) Av. Avenida Antônio Carlos, Belo Horizonte, MG jsantiagocarneiro@gmail.com 3 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN) Av. Avenida Antônio Carlos, Belo Horizonte, MG jjv@cdtn.br ABSTRACT The austenitic stainless steel 316L is widely used in nuclear industry because of its excellent mechanical properties and corrosion resistance. These properties must be evaluated in order to prevent failure and extend the life of equipment. The microstructure in the weld fusion zone consists on an austenite matrix with 5 12% of delta ferrite met stable at room temperature. However the pressurized water reactors operate at temperatures in the range C, thus welds may be susceptible to thermal aging embrittlement after long service life. According to the literature, this occurs due to the spinodal decomposition. Therefore, the purpose of this study was to evaluate the mechanical properties of 316L stainless steel welds by hardness and tensile tests before and after heat treatment. In this regard, two steel plates were welded and part of the material was heat treated at 335 C for 1000 hours. The tests after heat treatment showed an increase of only 4% in ultimate tensile strength and an increase of 28% in hardness. No changes were observed in the material microstructure, however according to literature changes can be identified by transmission electron microscopy. The curves of impact energy vs. temperature showed little change but, it was not able to observe a ductile-brittle transition and images of microstructure from scanning electronic microscopy (SEM) did not show fragile behavior. 1. INTRODUCTION Austenitic stainless steel 316L is used in Pressurized Water Reactor (PWR) due to its suitable mechanical properties and corrosion resistance [1]. Welds in this steel present, at melted zone, a microstructure composed of austenitic matrix, which has to 5% from 12% of metastable delta ferrite at room temperature [1, 2]. However the pipe components of reactor PWR operate at temperatures between 290 C and 325 C and welds can suffer the embrittlement by thermal aging after long service time. Studies indicate that this embrittlement occurs due to formation of alpha line phase in a BCC structure with lots of Chrome [1, 2]. The generation mechanism of alpha line phase is controversial and it can vary, but depending on chrome s quantity and heat treatment s

2 temperature, this phase could be formed by nucleation and growing or spinodal decomposition. Austenitic stainless steel 316L welds studies from Abe e Watanabe [2] and Chandra et al [1], show spinodal decomposition as generation mechanism of alpha line phase at temperatures below 400 C. Since there is no a set of data for spinodal decomposition rate of delta ferrite in 316L welds, it is possible to use aging data for duplex cast stainless steels CF-8 e CF-8M (Fig. 1). Figure 1: Aging data for duplex cast stainless steels CF-8 e CF-8M. Source: Abe and Watanabe, (modified) (2) The alpha line phase is finely dispersed in ferrite, reduced in size, in Å zone, with structural similarity and little contrast to the ferritic matrix [3]. Thus, the alpha phase line is hard to be observed by electron and optical microscopy [1, 2]. Studies indicate that it is possible to observe the alpha phase line by transmission electron microscopy [1, 2]. This phase is responsible for the phenomenon of embrittlement that occurs in ferritic and duplex stainless steel, in the range C [4]. When the phase is formed, hardness, yield strength and tensile strength of steel are increased while the elongation, impact resistance and corrosion resistance are decreased. The changes in mechanical properties happen because the mobility of dislocations decreased due to there is alpha phase line s chromium rich

3 precipitates in ferritic matrix. The decrease in corrosion resistance is due to the creation of chromium depleted regions around the precipitates, making the material more susceptible to localized corrosion. [3]. The reliability of nuclear plant components has become an important factor to be investigated in nuclear power plants due to aging of Brazilian industry. Therefore, investments to monitoring the operating conditions of the vulnerable components, in order to extend the life of equipment, are justified by economic and environment issues [5]. In the evaluation of structural integrity, it is necessary to know the mechanical behavior of material. Thus, this paper aims to analyze the mechanical properties of welds of 316L austenitic stainless steel before and after heat treatment simulating conditions of temperature and extension on PWR power reactor. 2. METHODOLOGY The methodology applied in this work is described in this section Material This study has used two rolled plates of AISI 316L austenitic stainless steel with 300x150x12,7mm as dimensions, the weld metal was AWS 316L steel and the chemical compositions of both were indicated in Tables 1and 2. Table 1: Chemical composition of AISI 316L austenitic stainless steel (Base Metal). Material Cr Mo Si Mn C S P Ni AISI 316L , Table 2: Chemical composition of AWS 316L austenitic stainless steel (Weld Metal). Material Cr Mo Si Mn C S P Ni AWS 316L < Welding The steel plates AISI 316L were beveled on one side with 30 angle and welded in multiple passes using GTAW (Gas Tungsten Arc Welding). As a weld metal was used wires of AWS 316L austenitic stainless steel with 1.6 mm diameter. The flow of argon was 15 L / min, the voltage was V and the current was 100 A in the root pass and 130 A in subsequent passes. The welding speed was approximately 66 mm / min for the root pass and varied from 111 mm / min to 176 mm / min for ten subsequent passes. Packing was used on the reverse of the weld to prevent contamination, oxidation and warping on material Heat treatment

4 The heat treatment was done in half of the welded plate at 335 C per 1000 hours on SFL Instron oven. The natural cooling was at room temperature Ferritoscope Percentage of delta ferrite present inside of melted zone in untreated and treated was measured with the aid of Fischer MP3 ferritoscope. 10 measures in the center and 10 measures on the side of weld were carried out Sample preparation for metalography Treated and untreated sample were cut, embedded and sanded with sandpaper grain size 80, 180, 280, 400, 600, 1200 e 2000 mesh. The polishing was carried out with polishing cloths with diamond paste (3 µm and 1 µm). The electrolytic etching was performed with nitric acid 40% 1,5V per 30 seconds, after etching, samples were cleaned with alcohol and dried. Follow they were observed at optical microscope Specimen The round specimens of tensile testing were machined to 4 mm in diameter and 50 mm long, centered within the weld region (Fig. 2). Specimens were oriented transversely to the direction of welding (Fig. 3). Figure 2: Round specimen for tensile test. Source: ASTM E8, 2009 (modified) [7]. The specimens for Charpy impact testing were machined with dimensions of 10 mm wide, 10 mm thick and 55 mm in length (Fig. 4). They were machined in broaching machine Blacks Equipment Limited, notch type V, 2 mm deep, with guidance TS and in the center of the weld. The first letter designates the normal direction to notch plane and the second, the direction of crack growth (Fig. 4). Six Charpy-type specimens and 3 specimens for tensile test for both, treated as well as untreated materials were machined. The samples from treated material were machined after heat treatment of half of welded plate.

5 Figure 3: Schematic diagram showing the orientations of the specimens in relation to the weld. Source: Chandra et al, 2012 (modified) [1]. Figure 4: Specimen Charpy dimensions for impact test. Source: ASTM E23, 2007 [8] Hardness test The test was carried out at room temperature in Wolpert Wilson Instruments 930 N equipment. Twelve measures of Vickers hardness (six in center and six in the sides) for each sample (treated and untreated) were performed with a load of 100 kn Tensile test The test was performed at room temperature with a speed of 2 mm/min on an Instron 5882 machine and load cell capacity is 100kN. Three tests were made for each condition (treated and untreated).

6 2.9. Charpy impact test The impact test was performed in an Instron Wolpert PW30 with 300J-capacity. The impact energy versus temperature curve was generated by Impact software. The test was conducted according to ASTM E23 (2007). One specimen was used at each temperature (-196 o C, o C, -50 o C.0 o C, 23 o C e 100 o C) and for each condition (treated and untreated). Fracture surface was analyzed in a Scanning Electronic Microscope brand Jeol and model JSM5310 to characterize the fracture mechanism 3. RESULTS AND DISCUSSIONS The results and discussions are showed in this section Ferritoscope Ferritoscope test were performed on heat treated and untreated materials for a quantitative characterization of volume fraction of delta ferrite, which is a magnetic component (Table 3). Table 3: Delta ferrite volume fraction on treated and untreated materials. Material Fraction of Delta ferrite (%) Standard deviation Untreated Sides Center Treated Sides Center Despite the small variation of the volume fraction of ferrite, this variation may be within the standard deviation, there is evidence, which this fraction decreased with the heat treatment. Thus, the test suggests that there is alpha line phase, a paramagnetic phase [3] and it was not identified by ferristoscope from the original ferrite 3.2. Metallography The microstructure of the 316L steel weld is shown before and after heat treatment in Figure 5. The micrographs of untreated and thermally aged samples do not show significant differences in the apparent microstructure observed by optical microscopy. This fact confirms that alpha line phase cannot be observed by microscopy due to its small size and structural similarity with the ferritic matrix.

7 a. b. c. d. Figure 5: Microstructure of 316L steel weld for untreated sample a) magnification of 200x b) magnification of 800x, and heat treated sample c) magnification of 200x d) magnification of 800x Tensile test The stress-strain curves obtained during the tensile test are shown in Figure 6. And the properties obtained after tensile test are shown in Table 4. After the heat treatment, it can be observed that there was a small increase in tensile strength and yield strength, while a slight decrease in elongation and necking, representing reduction in ductility. These variations in the properties were small, but it indicates a change in microstructure and suggests the occurrence of spinodal decomposition.

8 Figure 6: Stress-strain curve obtained for 316L steel before and after heat treatment. Table 4: Properties obtained after tensile tests for 316L steel before and after heat treatment Material Tensile Strength (MPa) Yield Strength (MPa) Elongation (%) Reduction area (%) Untreated Treated Variation Increased 4% Increased 1% Decreased 13% Decreased 7% 3.4. Hardness test Mean values, standard deviations and hardness variations values were shown in Table 5. It is possible to observe a significant increase in hardness by 28% after heat treatment. In micro hardness test on each individual phase, made by Abe and Watanabe [2] and Chandra [1], it was found that only the hardness of the ferrite phase is increased with aging time while the hardness of the austenite phase does not change. So this increase in hardness is probably due to changes in the ferrite phase, with formation of alpha line phase. Table 5: Measures of hardness test Material Hardness (HV10) Standard Deviations Variation (%) Untreated Treated Charpy Impact Test The impact energy-temperature curves were obtained for a heat treated and an untreated material (Fig. 7). According to figure 7, the absorbed energy during this test for treated material is smaller than the untreated one for each temperature, it demonstrates that a material

9 embrittlement occurred after heat treatment at 335 o C. However at 23 o C, the opposite situation happened, the absorbed energy for treated material is bigger than the untreated one. It is being considered a dispersion point, due to the small number of specimens available for tests. The embrittlement, which was identified by reduced impact toughness after heat treatment, can be explained as a result of a facilitated fracture in ferrite region of 316L steel weld due to the increased hardness of this phase. It suggests that the resistance to movement of dislocations increases, probably due to formation of alpha line phase, reducing the toughness of aged material. Figure 7: Impact energy vs. Temperature curve for specimens heat treated and untreated at 335 o C The curve does not have a temperature range with a sudden fall of absorbed energy. This indicates that the material does not experience a ductile-brittle transition. However Chandra et al. (2012) found a significant effect on the impact properties and a ductile-brittle transition temperature in 316L steel welds. For thermal aging at 335 o C after 1000h, Chandra et al. (2012) found a ductile-brittle transition temperature at -113 o C. However Chandra et al. (2012) did not show any images in scanning electronic microscopy of material s fracture surface at this temperature and treatment time, which could prove that transition. Comparing the impact energy vs. temperature curves obtained by Chandra et al (2012) and in this study, when the reduction of temperature is the same considering these 2 works, the decrease in impact energy in Chandra s work is much bigger than in this work.

10 Figure 8: Charpy impact energy x temperature curve for specimens heat treated at 335ºC and untreated. Results were obtained by Chandra el al, 2012 and in this work. Fracture surface was analyzed in scanning electronic microscopy (SEM) to characterize the fracture mechanism. Dimples in fracture surface of all specimens were able to be observed by images obtained in SEM. It characterizes ductile fracture. It was not able to observe cleavage surface, typically present in brittle fracture. It indicates that the material suffered ductile fracture in all testing conditions, what is strong evidence; there is not transition temperature for this material. This means that the fraction of ferrite in BCC structure was not sufficient to change one characteristic of 316L steel, it does not suffer ductile- brittle transition.

11 a) b) c) d) Figure 9: SEM images of specimens fracture surfaces after Charpy Impact test a) Untreated specimen, test at 100ºC, b) Untreated specimen, test at -196ºC, c) Heat treated specimen, test at 100ºC, d) Heat treated specimen, test at -196ºC (magnification of 1000x). 3. CONCLUSIONS The conclusions are showed bellow. -Ferritoscope was not a good tool to determinate the presence or absence of alpha line phase. -Heat treatment has had a significant effect on material hardness, suggesting the formation of alpha line phase. -Optical microscopy was not sufficient to identify the formation of alpha line phase after heat treatment, Transmission Electronic Microscopy (TEM) must be used to see it. -The heat treatment simulating operating conditions of a PWR nuclear reactor had little effect on the mechanical properties of 316L weld determined by the tensile test. Despite the variations are small, the increase in tensile strength and yield strength and decrease elongation and reduction area suggested a change in microstructure due to it should be the formation of the alpha prime phase. -Charpy impact test demonstrates brittleness of the material after heat treatment, but it doesn t suffer a ductile-brittle transition. Fracture surface s analysis indicated ductile fracture mechanism.

12 ACKNOWLEDGMENTS Fundação de Amparo à Pesquisa de Minas Gerais (FAPEMIG) Centro de Desenvolvimento de Tecnologia Nuclear (CDTN/CNEN). Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) The professors Geralda Cristina Godoy and Paulo Modenesi of Universidade Federal de Minas Gerais (UFMG). REFERENCES 1. CHANDRA, K. et al. Low temperature thermal aging of austenitic stainless steel welds: Kinetics and effects on mechanical properties. Materials Science and Engineering A. v. 534, p , fevereiro ABE, H., WATANABE, Y., Low-Temperature Aging Characteristics of Type 316L Stainless Steel Welds: Dependence on Solidification Mode. Metallurgical and Materials Transactions A. v. 39, p , junho FONTES, T. F. Efeito da Fase Alfa Linha nas Propriedades Mecânicas e de Resistência à Corrosão do Aço Inoxidável Duplex Ur 52n+. Dissertação de Mestrado. Instituto de Pesquisas Energéticas e Nucleares (IPEN). São Paulo, MAGNABOSCO, R. Cinética das Transformações de Fase em Aço Inoxidável Superdúplex. Relatório final submetido ao CNPq. Departamento de Engenharia Mecânica Centro Universitário da FEI, CARVALHO, J. D. Avaliação da Tenacidade à Fratura Dinâmica do Aço Inoxidável ABNT 316L Sensitizado. Trabalho de Graduação. Escola de Engenharia da UFMG. Belo Horizonte, RODRIGUES, B.J.F. Avaliação da Fratura Dinâmica no Aço Inoxidável Austenítico 316 através do Ensaio de Impacto Instrumentado Charpy. Trabalho de Graduação. Escola de Engenharia da UFMG. Belo Horizonte, AMERICAN SOCIETY FOR TESTING AND MATERIALS, E8; Standard Test Methods for Tension Testing of Metallic Materials. West Conshohocken, dezembro AMERICAN SOCIETY FOR TESTING AND MATERIALS, E23; Standard Test Methods for Notched Bar Impact Testing of Metallic Materials. West Conshohocken, julho 2007.