EFFECT OF COLD DEFORMATION ON THE PROPERTIES OF NEW AUSTENITIC STAINLESS STEEL FOR BOILER SUPERHEATER TUBES

Transcription

1 EFFECT OF COLD DEFORMATION ON THE PROPERTIES OF NEW AUSTENITIC STAINLESS STEEL FOR BOILER SUPERHEATER TUBES Šárka HERMANOVÁ a, Lenka DOBROVODSKÁ a, Ladislav KANDER b a VÍTKOVICE POWER ENGINEERING a.s., Ruská 24/83, Ostrava Vítkovice, Czech Republic, EU, sarka.hermanova@vitkovice.cz, lenka.dobrovodska@vitkovice.cz b Materiálový a metalurgický výzkum s.r.o., Pohraniční 693/31, Ostrava Vítkovice, Czech Republic ladislav.kander@mmvyzkum.cz Abstract This paper focuses on evaluation of the plastic deformation effect after the cold bending process for new austenitic materials Super304H, HR3C and Tp347HFG designed for supercritical conditions. The qualification tests were carried out on bends in two dimensions of tubes which are the most common sizes used in superheaters for supercritical power plant boiler. The bends were produced by the method of cold bending with mandrel bending technology and without mandrel on several bending radii in order to obtain information about the structure and behavior of materials at various stages of deformation. To obtain reliable information, tensile test, hardness test and metallographic examination were used on material from straight tubes of the bends, drawn and compressed parts which are presented in this article. We acquire new information about the stress-strain behavior of advanced structural materials thanks to the realization this experimental program. We also obtained the input data for subsequent long-term corrosion tests and thermal degradation tests and correlation relations for the determination of the strength properties of steels studied by SPT. The tests are carried out under the grand project of the Technology Agency TA with the name Bending-tubes-technology for superheaters and inter-superheaterstubes for the progressive boiler construction which VÍTKOVICE POWER ENGINEERING a.s. researches in cooperation with UJP a.s. and SVÚM a.s. Keywords: Austenitic steels, Tube bending, Small punch test, Cold deformation, power plant. 1. INTRODUCTION Our project, whose first results we hereby present, deals with the technology of bending of tubes with dimensions of 38x6.3 mm 57x4.5 mm, and the influence of plastic deformation caused by cold tube bending on steel structure, its mechanical and corrosion properties. As follows from the enquiries received by VÍTKOVICE POWER ENGINEERING a.s., the supercritical boiler superheater tubes are proposed for manufacturing increasingly from new austenitic materials SUPER304H, HR3C and Tp347HFG. Production of these devices consists of bending and welding. These are technological operations that affect the quality of materials, and in contrast to the information of the characteristics of steel and straight pipes, see literature [1], [2], [3], [4], [5], [6], it is almost impossible to obtain information from the literature on the effect of these operations on the properties of these new steels (e.g. creep and corrosion in bend). None of these materials is currently included within European standards (for example EN or EN 12952). In Europe, they are provided only with German material data sheets. In order to be able to qualify them and obtain the certificates, we proceeded as required by EN and German material data sheets of individual steels. According to EN , cold-bent austenitic materials do not have to be heat treated.

2 But the material data sheets require solution annealing in the case of use these steels in the creep area, and for some steels also in the case of use them in corrosive conditions when the deformation exceeds some defined level. Therefore, we compared the influence of deformation on the properties without heat treatment and after heat treatment. Here we present only the results of tubes with dimensions of 38x6.3 mm, mainly for the reason of a wider range of testing due to the greater wall thickness and better placement of test specimens in drawn or compress side at the top of the bend. As required by EN and German material data sheets only hardness and geometry of the bend is required after cold bending. There is, however, no limit of hardness after cold bending anywhere and hardness appears neither in the data sheets for the initial tubes. To study the influence of plastic deformation it was necessary to extend the works beyond the required scope of the standards with the tensile tests from the deformed areas (drawn and compressed), because this is the only variable precisely defined by the standard. Because it is not possible to located conventional tensile tests specimen into pipe bends and it is not even possible to accommodate miniaturized specimen in the pressure side, we used only small punch tests to verify the properties on the compress side. To determine the yield strength and ultimate strength of these tests it is necessary to know the empirical relationship between the measured variable P e and yield strength and P m and tensile strength. The current database is based on the values of steels with KPC unit cell, it was necessary to extend the works with the tensile tests at room temperature on miniaturized specimens and extend the current database with the results on the "new materials". 2. CHARACTERISTICS OF THE STUDIED STEELS Super 304H (X10CrNiCuNb ) is an austenitic stainless steel alloyed with copper, niobium and boron to increase the creep strength. Special process of forming, niobium alloying and heat treatment provide this steel with fine-grained structure, which ensures superior resistance to corrosion, both flue gas and oxide corrosion on the steam part. It has excellent creep resistance in the temperature range from 580 C to 640 C. HR3C (X6CrNiNbN25-20) is a highly alloyed austenitic steel with 25% Cr and 20% Ni. Grain size is not defined in the literature. The final structure is achieved by solution annealing. It has very good corrosion resistance at high temperatures and creep resistance especially in the temperature range from 600 C to 670 C. Tp347HFG (X8CrNi19-11) is a Cr-Ni austenitic steel with a fine grain structure, which is achieved by thermomechanical rolling involving soft annealing by 50 C higher than the final solution annealing introduced before cold drawing. This is followed by the final solution annealing. Achievement of a high creep strength is conditioned by the precipitation of carbides and their distribution in the structure. 2.1 Chemical composition and mechanical properties This part sets out the requirements of individual material data sheets (highlighted in grey in the table) and the values of tubes provided by the Japanese manufacturer SUMITOMO, as we will further present only the results for TR 38x6.3 mm, even here we specify only the values of supplied tubes of this dimension. These are average values from several heats. The tubes were delivered in the state after solution annealing, cold rolled according to requirements of German material data sheets.

3 Tab. 1 Chemical composition and mechanical values of Super304H (X10CrNiCuNb18-9-3) VdTUV Werkstoffblatt 550 C Cr Cu Ni N B Nb Al R p 0.2 R m R p 0.2* A Type and dimension KV [J] KV 2 10x , x Tab. 2 Chemical composition and mechanical values of HR3C (X6CrNiNbN25-20) Vd TUV Werkstoffblatt 546 Typ a C Mn Si Cr Ni N Nb Rp0.2 Rm Rp0.2* A KV rozměr [J] zk KV 2 10x x Tab. 3 Chemical composition and mechanical values of Tp347HFG (X8CrNi19-11) VdTUV Werkstoffblatt 547 C Mn Si Cr Ni Nb+Ta Rp0.2 Rm Rp0.2* A KV2 10x10 Aver. KV [J] xC x Note: R p 0,2* = R p 0,2 at temperature 600 C 2.2 Scope of testing Bend testing was carried out in two testing houses, VTC (qualifying tests - exactly as required by the standards) and MMV s.r.o., where properties of the individual parts of the bends were verified - "Study of the properties of materials after cold deformation". The tests on tubes 38x6, 3 mm were performed with three different bends radii. (R1<R2<R3(or R4), R3, R4 slightly above the maximum value without heat treatment acc. to the material data sheets). The bends were tested after the cold bending and the solution annealing condition. The figure below shows the scope of the work performed Fig. 1 Scope of works performed

4 3. TESTING Small punch tests were used to determine the mechanical values in the drawn, compress and the neutral axis. To determine the correlation coefficients tensile tests were used (round threaded specimen with a diameter of 5 mm and a length 38 mm) from the straight part of the bend and the drawn side at each bend radius after heat treatment and without it. These tests were supplemented by metallographic examination, the aim of which was to identify differences in the structure of the individual parts of the bend and the influence of heat treatment on the distribution and size of carbides and austenitic grain size. HV 10 hardness tests and metallographic examination was carried out according to applicable ČSN EN standards. 3.1 Principle of small punch tests The exact description and procedure for the preparation, testing and information derived therefrom is given in [7]. The actual small punch test was carried out on a testing device TSM INOVA 10 with the rate of 1.5 mm/min, the same as in test equipment for standardized tensile test. 3.2 Methodology for determination of yield strength and tensile strength with small punch test at room temperature The resulting value of the yield strength, or respectively the tensile strength is determined from the values obtained from the graphical record of the small punch test (see figure 2). We require P e for determination the yield strength and by its substitution in the correlation equation we obtain the value of the yield strength, which corresponds to the value specified by classical tensile test. The value of tensile strength is determined as P m of the penetration test and also the value of deflection (dm) and its substitution in the correlation equation. Fig. 2 Characteristic record of small punch test with evaluated values 3.3 Results of small punch tests and hardness tests In this section we provide the results of small punch tests (already converted to R p 0,2 a R m ) from individual parts of the bend - neutral, drawn or compress part and on the given radii of the bend always with heat treatment and without it. Fig. 3 Results of small punch tests in on the drawn, pushed and straight part of the tube - Super 304H

5 Fig. 4 Results of small punch tests in on the drawn, pushed and straight part of the tube - HR3C Fig. 5 Results of small punch tests in on the drawn, pushed and straight part of the tube - Tp347HFG The below charts show the results summarizing the results of yield strength, tensile strength and hardness, depending on the size of deformation. They also indicate the limit values according to material data sheets. Because the values after heat treatment are almost identical in different parts of bend as on the initial tube, they are not included in the graph below (fig. 6). Fig. 6 Results of yield strength, tensile and hardness depending on the size of the deformation 3.4 Results of metallographic investigation Various parts of the bend were also subjected to metallographic investigation. There was no significant difference found in the distribution of carbides (or precipitates) or their size, as well as in grain size between the individual parts of the bend. A significant difference was found between the untreated and heat treated bend. Generally, there was a slight increase in the grain, which, except for the subsurface area was within the limits of the material data sheet requirements. In the straight part of the bend in subsurface area there was a significant increase of grain size, in fine-grained steels already outside the scope of the standard. This growth is being further investigated.

6 4. DISCUSSION OF THE RESULTS Strain hardening of all tested materials was also confirmed. The Super 304H material was subject to hardening manifested by an increased yield strength near the level of tensile strength, but the tensile strength did not exceed the maximum value specified in the material data sheet for straight tube even with the smallest bend radius. In contrast, the materials HR3C and Tp347HFG, even with less deformation, exceeded the maximum tensile strength given by the material data sheet. The material Tp347HFG exceeded the upper limit of the tensile strength also with the biggest bend radius R4. There was no significant effect on the structure of steels in the individual deformed parts of the bend. We discovered the influence of heat treatment on the size of precipitates, their distribution and the grain size especially in the subsurface area. It will be the subject of further investigation. As the same technology is used for manufacturing of samples for long-term tests, it will be interesting to monitor the effect of deformation on corrosion and thermal degradation. Only after the evaluation of longterm of tests it will be possible to recommend maximum deformation without heat treatment. CONCLUSION This work discovered influence of the size of plastic deformation on the mechanical properties of materials, the microstructure of steels, verified the influence of heat treatment on material properties. These results are input data for the long-term tests such as corrosion. We also obtained correlation coefficients for materials with KSC unit cell, with which it will be possible to extend the database for use of small punch test method as a substitute for tensile tests in areas where, they can not be used for dimensional reasons. ACKNOWLEDGMENT This project was supported by the Technological Agency of the Czech Republic within the framework of a research and development (Alpha program). Acknowledgement also belongs to the principal investigator UJP PRAHA a.s. and other co-investigators SVÚM Praha a.s. as well as the organizations that participated in the acquisition of material data: MATERIÁLOVÝ A METALURGICKÝ VÝZKUM s.r.o., VÍTKOVICE TESTING CENTRUM s.r.o., TÜV NORD Czech, s.r.o. LITERATURE [1] Viswanathan R. and Bakker W. Materials for Ultrasupercritical Coal Power Plants Boiler Materials: Part 1-. July [2] Okada. Igarashi. Long-term service experience with advanced austenitic alloys in Eddystone power station- Creep [3] Properties after long time service exposure of Super 304H steel tubes in power plant. Sumitomo Metal Inc. September [4] Bystrianský. Kučera. Processes affecting formation of oxide layers with good protective properties on steels in steam-ducting environment of energy circuits, Metal [5] Iseda. Okada. Semba. Igarashi. Long-term creep properties and microstructure of Super304H. Tp347HFG a HR3C. [6] Husemann. Advanced (700 C) PF power plant: A clean European technology. Babcock-Hitachi. October [7] Matocha K. Evaluation of mechanical properties of structural steels using small punch tests, Technical University of Ostrava, 2010, ISBN