The thermomechanical processing of NiMoCr solid solution nickel base superalloy is the way to considerably influence the grain size. As uniform coarse grain size increases the creep strength and crack growth resistance. In the work, the processing to achieve uniform recrystallized grain structure with variation of thermomechanical parameters is investigated. The creep behavior of the alloy after various hot working conditions is determined. The results of creep tests showed that creep characteristics such as strain rate and lifetime were greatly dependent on the initial hot working conditions and annealing parameters

J. Zrník

L. Kraus

P. Wangyao

S. Němeček

English

HRČAK - Portal of Croatian Scientific and Professional Journals

METALURGIJA 46 (2007) 3, 169-172 169L. KRAUS et al.: MATERIALS PROPERTIES OF MODIFEIED Ni-BASED ALLOYReceived - Primljeno: 2006-04-11ISSN 0543-5846METABK 46 (3) 169-172 (2007)UDC - UDK 669.245–153.74:539.388=111Republic, P. Wangyao, Institute of MMSR, Chulalongkorn University, Bangkok, ThailandMATERIALS PROPERTIES OF MODIFEIED Ni-BASED ALLOYINTRODUCTIONSolid solution strengthened nickel base high tempera-ture alloys have a wide use in sheet, in gas turbine engine combustion chambers, transition liners, and in petroleum and chemical processing industry. In these applications, the components are often subjected to creep and low cycle fatigue.The development of reactors that incorporate a circulat-construction material that will contain the molten salt over long time periods and, also ensure the required mechanical properties [1, 2]. The container must be metallurgically stable and to show enhanced resisting to oxidation and grain boundary embrittlement [3].The reference alloy system was based on nickel with as primary strengthening the addition of 15 - 20 % Mo for the increase of oxidation resistance. Because the poor mechanical properties, additions of various solid solution alloying elements were evaluated. An optimum alloy com-position was selected which ensures satisfying mechanical The thermomechanical processing of NiMoCr solid solution nickel base superalloy is the way to considerably The results of creep tests showed that creep characteristics such as strain rate and lifetime were greatly de-Key words: Ni-based alloy, hot working, annealing, recrystallization, structure, creep Ni-superlegura, topla obradba, rekristalizacija, struktura, puzanjeproperties and corrosion properties with Cr, Al, Ti, V, Fe, Nb, and W additions [4].It is known that the mechanical and physical properties altered through variation in thermomechanical (TM) pro-cessing. The role of TM processing has to be considered To verify the hot working characteristics of solid solution strengthened nickel base superalloys and enable the de-velopment of suitable hot working practises a laboratory test to assess hot workability is essential.Several microstructural features were developed to in-crease creep strength in solid solution strengthened materials with the purpose to lower the diffusion rate and to improve the deformability at elevated temperatures while preserving the creep compatibility of the material. Grain boundary and phase boundary sliding can be controlled by segregation and particles dispersions at the grain boundaries. This sliding should take place at the same rate as crystal lattice deformation to avoid the voids formation [6]. Structurefor improving creep strength by utilised the ideas above. To ensure a higher creep strength and crack growth resis-tance the designed coarse grained microstructure should be maintained with also uniform the particles dispersions. METALURGIJA 46 (2007) 3, 169-172170L. KRAUS et al.: MATERIALS PROPERTIES OF MODIFEIED Ni-BASED ALLOYThis type of structure can be achieved with thermome-chanical processing such as hot rolling combined with The purpose of the presented investigation was to exam-post-deformation annealing on microstructure evolution and creep properties of the nickel base NiMoCr alloy.EXPERIMENTAL PROCEDUREThe investigated material was wrought nickel base NiMoCr alloy. The chemical composition of the alloy in wt. % was as follow: 72,7 % Ni; 17,8 % Mo; 6,3 %  Cr; 2,8 % Fe; 0,16 % Al; 0,06 % Ti; 0,06 % W; 0,06 % Co; 0,05 % Si; 0,01 % Cu; 0,01 % B; 0,001 % S and 0,02 % C. The initial alloy was obtained with casting and multi steps press forging and annealing.However, the obtained alloy structure was very hetero-geneous. The various testing hot working conditions were carried out as shown in Table 1. The grain morphology -tigated with metallographical examinations. Specimens specimens were machined for creep testing. The creep tests were performed in air with constant applied load. All creep tests were carried out at stress level of 160 MPa and temperature of 710 °C. The elongation was recorded by two extensometers. The testing temperature was controlled by two thermocouples and it was maintained within the range of  5 °C.RESULTS AND DISCUSSIONMicrostructural analysisIn the initial microstructure obtained after casting, multisteps hot pressing and annealing process it was found that it was very heterogeneous and of bimodal press forging-annealing process did not provide uniform -tion process did not occur uniformly. Generally, in the might be generated preferentially at grain boundaries by -curring during the multisteps hot forging.After hot working according to both programs A and B (specimen no. A1, A2 and B1) immediate quenching or air cooling the microstructures were still heterogeneous with a large number of twins and coarse grains in the middle of samples. However, that the deformed grains were slightly elongated in the rolling direction. This sug-gested a non-uniform displacement of deformation and of the rolled bands.After additional hot working consisting of two steps of reductions in specimens No. A3-A8 and B2-B6, the hetero-geneity in microstructure was smaller if the annealing time according to the A programme produced more uniform grain structure comparing to the annealed microstructures -ogy depends greatly on the degree of deformation before annealing. The greater was the degree of deformation the Table 2., 3. The relationship between the number of grains per area and annealing time is shown in Figure 1.METALURGIJA 46 (2007) 3, 169-172 171L. KRAUS et al.: MATERIALS PROPERTIES OF MODIFEIED Ni-BASED ALLOYCreep behaviourThe creep results for TM processed speci-mens according to the A and B programmes are presented in Figure 2. and 3. evidence that various annealing times did not effect greatly the creep lifetime (2000 min). The speci-mens only quenched or air cooled immediately after hot working had much longer lifetime and correspondingly lower creep rate than those of all annealed specimens. Considering this fact the creep behaviour was explained in terms of the effect strain hardening on creep behaviour. Furthermore, it was concluded that the higher strength resulted also from the initial non-uniform coarse grain structure of the alloy. The specimens with the highest creep strength could not be used further because of their poor uniform formability made than unsuited for further forming including extrusion and tube drawing. For this reason, for application, only investigated further, since alloy formability, creep and fatigue properties are required.The creep behaviour of the alloy processed accord-ing to and A and B programmes was very similar. The diagrams in Figure 4. and 5. summarise the relationship between creep lifetime, minimum strain rate and anneal-ing time. The result shows that the creep strength of water quenched specimens from A programme was better than those from B programme. The ex-planation is again the higher work hardening in specimen after hot working. However, as creep results show, in both programmes creep lifetime decreased very sharply when speci-mens were annealed in range of 3-25 minutes where deformed grain structure was replaced structure. The effect of work hardening was also the creep lifetime difference between A and B pro-METALURGIJA 46 (2007) 3, 169-172172L. KRAUS et al.: MATERIALS PROPERTIES OF MODIFEIED Ni-BASED ALLOYincreased gradually as the annealing time was prolonged over 25 minutes and the creep lifetime began to increase slightly, due to the grain coarsening. The prolonging of the annealing time over 25 minutes produced a slightly coarse and uniform grain structure, as shown  in Table 2. and Table 3., and a small increase of creep strength.REFERENCESTrans. A, 26A (1995), 673.[2] W. D. Manly et al.: Aircraft Reactor Experiment-Metallurgical As-pects, ORNL-2349, Oak Ridge National Laboratory, USA 1957.[3] J. H. deVan, J. R. DiStefano, W. P. Eatherly, J. R. Keiser, R. L. Klueh, In Proc. The 1st International Conference on ADTT, American Institute of Physics, Las Vegas, USA 1994, p. 476.[4] X. Xie, S. Zhao, J. Dong, G. D. Smith, S. J. Patel: An Investiga-tion of Structure Stability and its Improvement on New DevelopedNi/Cr/Mo/Nb/Ti/Al superalloy, Mater Sci Forum 475 (2005), 613.[5] G. D. Smith, S. J. Patel, N. C. Farr, M. Hoffmann: The Corrosion Resistance of Nickel Containing Alloys, Corrosion 99, NACE In-ternational, Houston 1999, p. 12. [6] S. Zhao, J. Dong, X. Xie, G. D. Smith, S. J. Patel: Thermal stability Study on a new Ni/Cr/Co/Mo/Ti/Al Alloy, Superalloys 2004, TMS(2004), 63. [7] R. M. F. Jones, L. A. Jackman, JOM, January (1999), 27. [8] S. L. Semiatin, T. R. Bieler: Microstructural evolution during the hot working of superalloys, JOM, January (1999), 13.CONCLUSIONSThe results obtained for TM processing of NiMoCr alloy allow to conclude that the best creep strength was surprisingly obtained in case of applied two steps hot working without following annealing, and it was probably due to the effect of residual strain hardening resulting from hot working process. The annealing after hot working independently of the hold time, decreased the life time in comparison to the quenched or air cooled specimens with -behaviour of the alloy was changed with recovery and grain structure did ensure only a low creep strength. In order to improve the creep strength of the alloy the more uniform and coarse structure should be maintained by annealing time prolongation. The higher deformation during two steps hot working in A programme produced more uniform and lower creep strain rate and lifetime. After annealing, a fully the deformation procedure before the annealing. To improve the creep properties of the alloy it is necessary to perform and coarse grained microstructure.

Materials Properties of Modifeied Ni-Based Alloy

Abstract

Similar works

Full text

Available Versions

HRČAK - Portal of Croatian Scientific and Professional Journals