CREEP PROPERTIES OF STEEL P92

Transcription

1 CREEP PROPERTIES OF STEEL P92 Tomáš VLASÁK 1, Jan HAKL 1, Josef KASL 2 a Josef ČMAKAL 3 1 SVÚM a.s., Podnikatelská 595, Praha 9 <vlasak@svum.cz> 2 Škoda výzkum, Tylova 57, Plzeň <josef.kasl@skodavyzkum.cz> 3 UJP a.s., Nad Kamínkou 1343, Praha 5 <cmakal@ujp.cz> Abstract Within the previous few years it is possible to observe an application of steel P91 and P92 in energy. Steel P92 was developed in Japan. It is alloyed by 9%Cr-1,8%W and further by little additions of Mo, V, Nb, B and N. Our contribution deals with the creep properties of this material in dependence on way of taking samples. 1. INTRODUCTION Martensitic steels P91 and P92 are beginning to play a steadily more important role in power engineering in the last years. The steel P91 originates from the USA and contains 9Cr-1Mo (V,Nb,N). It was developed at the end of the seventies in the past century. The steel P92 with a higher resistance to heat was developed later in Japan and its alloying consists of 9Cr-1,8W (Mo,V,B,Nb,N). Our contribution deals with a thick wall tube from the steel P92 and concerns creep properties in dependence on the manner of taking samples. 2. EXPERIMENTAL MATERIAL A seamless tube from the P92 steal with the tube outside diameter of 350 mm and the wall thickness of 39 mm was used for studying creep properties. The length of the tube that was available was 139 mm. The tube was made in Productos Tubulares, s.a.u., in Spain under the melt designation The chemical composition of the material according to [1] and the Spain certificate is indicated in Table I. The material was processed thermally using the 1050 C/60min+780 C/140min procedure. Its properties given in the acceptance report are in Table II. It is obvious from Table I and II that the material complies. Tab.I. Chemical composition (% wt). C Mn Si P S Cr Ni Mo V B Nb Al N W min max Heat For understanding the creep properties, the tube was divided so that it was possible to observe the creep resistance on the surface, in the middle and on the inner tube part. The intake of samples whose specific sizes were Ø5x25 mm is illustrated in Fig.1. Tab.II. Mechanical properties Mechanical properties R P 0,2 (MPa) R m (MPa) A (%) HB min max 250 Heat A sample was also taken from the tube on which it was possible to measure the HV10 hardness in the direction from the outer surface toward the inner surface.

2 3. THE EFFECT OF THICKNESS ON THE MATERIAL PROPERTIES The creep properties were determined at temperatures 575 to 650 C and in the range of stress from 120 to 220 MPa. The duration of the longest finished tests was approximately 6,500 hours. Tests are continuing further. The results of the creep tests are illustrated graphically in Fig.2. It is obvious that the area near the tube surface has the highest resistance against creep, the area at the inner surface has the lowest resistance. The properties in the centre of the tube bearing cross section are definitely minimal. This is reviewed in Fig.3. r175 r141 r155,5 r Fig.1 Scheme of tube sampling C 625 C 600 C 575 C 2,4 2,3 Time to rupture [h] Outer Central Inner Stress [MPa] Fig.2 Creep strength in different places of wall log stress ([MPa]) outer 2,2 inner central 2, P LM=T.(logt r+40), T[K], t r[h] Fig.3 Creep strength in different places of wall The course of hardness is analogous. Although the HV10 results are affected by the errors of measurements, we can conclude by fitting them mathematically that the first maximum of the curve is near the outer surface. This is obvious from Fig 4. The minimum is approximately in the middle of the bearing part. The second maximum, a little bit lower than the first one, is at the inner rim of the tube. The course of HV10 thus corresponds to the results of heat resistance. The minimum of creep resistance is at the hardness minimum. Hardness HV10 [-] y = 1E-05x 5-0,0014x 4 + 0,0575x 3-0,9702x 2 + 5,7481x + 226, Distance from outer surface [mm] Fig.4 Hardness profile of tube wall

3 4. METALLOGRAPHY In relation to these creep results and hardness measurements, metallographic investigation was carried in the areas with maximum and minimum properties. Samples represented the material in the initial state, i.e. before creep. Analyses were carried out with a magnification of 100 to 1 000, after etching the surface with agent Kalling No.2. However, optical metallography did not bring any result. Structures representing surfaces at the outer and inner rim and the central part are illustrated in Fig. 5 with a magnification of 400. Structures are formed by tempered martensite and do not exhibit observable differences. a b Magnitude 400x Fig.5 Structure of outer (a), central (b) and inner part of wall thickness c For this reason a scanning electron microscope was used making it possible to observe structures with a higher magnification. Samples were observed in a state etched by agent Villela-Bain with a magnification of to The structure of the tube is displayed in Fig. 6 in the same manner as in Fig. 5 at a magnification of to The occurrence of an increased number of coarse particles was found in the sample from the central area in comparison with the samples from the tube rim. The M 23 C 6 carbides are coarser, but according to the morphology of the coarsest particles, one can also consider the presence of a Laves phase. This was not possible to prove by ED microanalysis due to the subtle character of the particles, but the study continues by observing the carbon extraction replicas.

4 REM Ph.No magnitude 3 000x REM Ph.No magnitude x a) outer part of wall thickness REM Ph.No magnitude 3 000x REM Ph.No magnitude x SEI b) central part of wall thickness REM Ph.No magnitude 3 000x REM Ph.No magnitude x SEI c) inner part of wall thickness Fig.6 Microstructures of outer (a), central (b) and inner part of wall thickness in magnitudes 3000x and 10000x

5 5. DISCUSSION In our opinion, the differences in the properties of steel are caused by imperfect thermal processing. The reason may be that the material was heated during homogenization for a shorter time than was necessary for heating the inner part of the tube. However, this finding is surprising, because no such effect has been found so far in any (8-12)%Cr steel. The common fact is that the thicknesses of the walls of products may also be significantly larger. We have not even found in the available literature that the dependence of the properties on the wall thickness differences would be mentioned. In order to be able to examine the properties of the material made under the melt designation 60074, we decided to take samples for creep tests from the central part of the tube. The present results are plotted in Fig. 7. The curve corresponding to the mean creep properties of the steel P92 [3] is plotted in the same figure for comparison; this curve is the result of processing the results of heat resistance exceeding 10 5 hours. It is obvious that the creep properties of the central part of the tube are in the ±20% range around the average value. The heat resistance of the material of the tube centre is therefore acceptable. The coarsening of carbides and the occurrence of the Laves phase in the central parts of the tube caused the depletion of the solid solution that leads to the reduction of hardness and the drop of heat resistance P % [3] Stress [MPa] P92 mean [3] Experiments P92-20% [3] P LM =T.(log(t r )+40), T[K], t r [h] Fig.7 Comparison of experimental and literary creep strength values of P92 steel 6. CONCLUSION The seamless tube with the outer diameter 350 mm and the wall thickness of 39 mm was subjected to the tests of heat resistance, hardness and the metallographic study. It was found that the properties of the material depended on the manner of sample intake. The lowest heat resistance and hardness were in the middle of the cross section. ACKNOWLEDGEMENTS This work was supported by Ministry of Education, Youth and Sports of Czech Republic in frame of project VZ REFERENCES [1] RICHARDOT,D.-VAILLANT,J.C.-ARBAB,A.-BENDICK,W.: The T92/P92Book. Vallourec and Mannesmann Tubes, [2] Attest No by Productos Tubulares, s.a.u., Espaňa. [3] Steel ASTM Grade 92. ECCC Data Sheets Published by ETD.