COMPARISON OF HARDNESS, TENSILE STRESS AND YIELD STRESS, DEPENDING ON TEMPERATURE AND ANNEALING TIME OF DEGRADATION OF P91 STEEL

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1 COMPARISON OF HARDNESS, TENSILE STRESS AND YIELD STRESS, DEPENDING ON TEMPERATURE AND ANNEALING TIME OF DEGRADATION OF P91 STEEL DANIELA POLÁCHOVÁ 1, PAVLÍNA HÁJKOVÁ 2, JOSEF UZEL 2 1 UJP PRAHA a.s., Nad Kamínkou 1345, Prague Zbraslav, CZ, polachova@ujp.cz 2 Department of Materials Engineering, FS, CTU in Prague, Karlovo náměstí 13, Prague, CZ Abstract Presented paper is devoted to checking the properties of weld joints of P91 steel considering the heat affected zone. Similar welds of P91 steel were subjected to isothermal heat exposure at 650 C for 650, 1000, 7, 5000 and hrs. The main focus was to examine the mechanical properties of weld joins using hardness and tensile tests. Tensile tests were carried out at temperatures of 20, 600 and of welded joints after PWHT and as-exposed to isothermal endurance during hrs. Also, hardness was measured at three levels, i.e. in the centre and near both surfaces. The aim was to determine the relationship between hardness and strength of the welds. Key words: P91 steel, ageing, hardness, tensile stress, yield stress 1. INTRODUCTION Degraded materials were achieved by prior isothermal heat exposure at for hrs in laboratory. This isothermal heat exposure at higher temperatures in comparison to anticipated operating temperatures (for T23 steel to about 580 C, for steels P91 and P92 to about 610 C) should accelerate the degradation processes occurring in these steels at operating temperatures. Creep tests of degraded states should highlight the creep behaviour of the studied steels near the end of their planned service life (normally considered as hrs). An effect of prior isothermal heat exposure on creep properties is shown in Fig. 1. This chart compares the value of the Larson - Miller parameter calculated for the initial states (virgin steels), marked LMP c VS, and degraded states (exposed steels), marked LMP c DS (index c here means that the Larson - Miller parameter is determined from creep behaviour). Fig. 1. Comparison of the Larson - Miller parameter of crept steels T23, P91 and P92 in as-treated and degraded condition

2 s, MPa The graph in Fig. 1 shows that the displayed data for steels T23, P91 and P92 are in a straight line with the regression line expressed by the equation:, where b is the slope of the line with a value of 0.88±0.01 and a is the absolute value of the line with a value of (4.1 ± 0.3).10 3 (precision fit of the regression line is given by the coefficient of determination R = ). Ideally, the slope of the line should have a value of 1, so the absolute value would completely describe the effect of previous isothermal ageing. In this case, there would be no change in a creep mechanism, just only a movement of the time to fracture. Thus, doing more detailed analysis of data for different types of steels we can see that the regression line fitted data for 9Cr steels has the slope value actually very close to 1 (with regard to the accuracy directive), see Table 1. However, this is not true for the regression line for the T23 steel. Tab. 1. Parameters of regression analysis data from the Fig.1. Steel slope b Parameters of regression absolute value a coefficient of determination R T ± ± P ± ± P ± ± Therefore, considering the determination of influence of previous isothermal ageing, based on the values in Tab. 1, we can say that the 9Cr steels does not change the creep mechanism even in degraded conditions due to the previous isothermal ageing. Thus, the absolute value completely describes the effect of previous ageing. 2. TENSILE TEST Mechanical tests performed on welded P91 steel in as-treated state (after PWHT) and after isothermal heat exposure at / hrs showed that the decrease in ultimate strength and yield strength was in an advance of expected values and the order of several percent. Tensile strength should not fall below 585 MPa at 20 C, below 290 MPa at, and below 215 MPa at 650 C according to the ASTM Code. In all measurement, the tensile strength values were above the limit values. Moreover, the results did not approach even those values. s, MPa Initial state C C e, % hrs C e, % Fig. 2. Comparison of tensile tests curves at various temperatures for as-treated (left) and as-exposed welded P91 steel (right)

3 Hardness HV10 Hardness HV10 3. HARDNESS The Vickers hardness (HV10) was measured on welded samples at three levels, i.e. close to the root pass, close to the cap pass and in the centre of the weld. In the case of first two levels, the hardness was measured about 2 mm from the surface of the material. The aim was to determine the relationship between hardness and strength of the weld. The following graphs (Fig. 3-4) show the hardness values measured on specimens in different states. Initial state cap line root line middle line Position of indentation [mm] Fig. 3. The hardness behaviour of the P91 weld in an initial state hrs cap line root line middle line Position of indentation [mm] Fig. 4. The hardness behaviour of the P91 weld after isothermal heat exposure at / hrs The graphs in Fig. 3-4 show a strong variation of hardness in the heat affected zone, so, there is very difficult to trace a clear trend. But it is possible to observe it in the weld metal in the weld axis (0 mm position of indentation) on the cap of the weld. With increasing annealing time, there is a decrease in hardness. This phenomenon is documented in the following graph (Fig. 5). This graph shows a steep decrease in hardness during the first 1000 hours of annealing. After that the hardness fall starts to slow down and then further decreases linearly. To monitor the properties of the weld in the cap pass of the weld, it can be used with an advantage, because the hardness is measured in this very place in practice.

4 R p0.2 /R m [-] Hardness HV10 Hardness in the cap line Fig. 5. The dependence of hardness in the cap of the weld on annealing time 4. COMPARISON OF HARDNESS, TENSILE STRESS AND YIELD STRESS The hardness values were related to tensile and yield strength measured during the tensile tests. Because it is not possible to measure the hardness at higher temperatures, all results of tensile tests are related to the hardness values measured at room temperature. We have assumed that the yield stress and strength are a linear function of hardness. Based on this assumption, a regression was performed between an initial state (after PWHT) and the state after hours of annealing. According to the equations of individual lines (for tree temperature levels of tensile tests) in Tab. 2, values of tensile and yield strength for each hardness measured at different exposed samples were calculated and assigned to the specific annealing times. Tab. 2. Equations of strength dependencies on hardness at various temperatures (R m, R p0.2 as a function of HV) Equations of lines R m R p C R m = HV R p0.2 = 0.61 HV R m = HV R p0.2 = HV R m = HV R p0.2 = HV Dependency of ratio R p0.2 /R m on the test temperature and annealing time 1 0,95 0,9 20 C 0,85 0,8 0, Fig.6. Dependency of ratio R p0.2 /R m on the testing temperature and annealing time

5 R m [MPa] Hardness HV10 Plotting this dependence (Fig. 6.) demonstrates how the yield stress with increasing temperature gets closer to the tensile strength, which limits the ability of plastic deformation of material, i.e. after exceeding the yield point of the material fracture occurs. On the other hand, nearly horizontal character of curves indicates only a very limited dependence on the time of exposure of the sample. It is necessary to recalculate achieved data of measured mechanical properties on the samples to the operating conditions which are in the range of 580 C to. For this we use Larson-Miller parameter [1]. Converted hardness values depending on the annealing time are plotted in Fig. 7 with a logarithmic time axis C logarithmically Fig. 7. Dependence of hardness on the temperature and annealing time (logarithmic time axis) The values of yield and tensile strength at different temperatures depending on the exposure time are plotted in the following graph (Fig. 8). Our goal was to predict R m and R p0.2 after hours in the temperature range from 580 C to 600 C (designed service conditions). The curve for 600 C, unfortunately, does not reach the required border, so it is necessary to estimate the trend curve according to this value. Tensile stress R m C hrs Fig. 8. The dependence of tensile stress on annealing time at different temperatures

6 Rp 0.2 [MPa] Yield stress R p C hrs Fig. 9. The dependence of yield stress on annealing time at different temperatures Tab. 4. Estimate of R m and R p0.2 values for operating conditions Operating conditions R m, MPa R p0,2, MPa CONCLUSION According to the above, we can predict that if only the thermal load at 580 to 600 C is considered, the values of tensile and yield strength would stay above the minimum limits specified in ASTM Code even after hours of operation. But, in real conditions, the change of mechanical properties is accelerating of many causes, mainly due to degradation processes like creep, cyclic thermal stress (start-ups and going down) and many other influences. Therefore, these theoretical calculated values are not fully representative to the service conditions, but they serve us as a very good base for further estimation of material behaviour under operating conditions. ACKNOWLEDGEMENTS This work was financially supported within projects 2A-1TP1/057 program MPO TRVALÁ PROSPERITA and SGS 10/258/OHK2/3T/12. LITERATURE [1] Svobodová, M.: Larsonův- Millerův parametr pro modifikované žáropevné oceli T/P23, 91 a 92. [Technická zpráva T 522] UJP PRAHA a.s., Praha, listopad 2011, 9 s. [2] Čmakal, J et al: Řešení materiálových a technologických inovací pro energetická a chemická zařízení nové generace pracující za vysokých teplot. [Zpráva UJP 1415] UJP Praha a.s.: Praha, prosinec 2010 [3] Abe, F; Kern, T.U. Creep-resistant steels.(8) ISBN URL:< [4] Svobodová, M. Čmakal, J. Douda, J. Kudrman, J.: Degradation processes in creep-resisting steels. In: Archives of Materials Science, Vol. 28, No. 1-4 (7), pp ISSN [5] Svobodová, M. Douda, J. Čmakal, J. Sopoušek, J. Dubský, J.: Homogenní a heterogenní svarové spoje žáropevných ocelí, In: METAL 9 Proceedings, , Hradec nad Moravicí, CD, 039.pdf, s. 8.