Precipitation of Laves phase Fe 2 Mo type in HSLA steel with copper addition and high content of molybdenum

Transcription

1 ARCHIVES of FOUNDRY ENGINEERING Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 10 Special Issue 3/ /3 Precipitation of Laves phase Fe 2 Mo type in HSLA steel with copper addition and high content of molybdenum R. Bogucki*, K. Miernik, S. Pytel Institute of Materials Science, Cracow University of Technology, Al. Jana Pawła II 37, Kraków, Poland *Corresponding author: address: rbogucki@mech.pk.edu.pl Received ; accepted in revised form Abstract The results of mechanical properties and microstructure of low-carbon copper bearing steel after quenching and tempering at temperature range of processing precipitation of particles rich in copper and particles intermetallic phase are presented in this paper. When content molybdenum increases in tempering temperature range from 550 C to 600 C that decrease of the impact energy measured at room temperature was observed. Microstructure analysis was conducted by transmission electron microscope (TEM) and was disclosed the occurrence of Fe 2 Mo Laves phase on crystallites boundaries of retained austenite. Observed sudden drop of ductility in higher-molybdenum content steels (1,88 % and 2,94 %) should be connected with occurrence precipitation processes of the hard and brittle Laves phase in range of discussion tempering temperatures. Keywords: heat treatment, impact strength, HSLA, Laves phase 1. Introduction Materials used for heavily loaded marine constructions such as oil rigs, sheathing for submarines, destroyers and aircraft carriers should be characterized by high mechanical properties and high resistance to brittle fracture at low temperature and good weldability. These specifications meet HSLA (High Strength Low Alloy) steels with copper addition which since the 80s are the basic materials used in shipbuilding. As first was prepared HSLA 80 steel. Accordingly to ASTM A710 standard this steel was characterized by yield point from 552 MPa (80 ksi) to 690 MPa (100 ksi) and the impact resistance at temperature of -84 C (-120 F) equal to 81J [1]. In the 90s carried out further development of this grade steel. Results of this research was implementation of HSLA 100 steel which characterized minimum yield strength of 690 MPa (100 ksi) with the ductility unchanged. At the moment are conducted research on modifying the chemical composition of this group of steel in order to achieve a minimum yield point of 890 MPa (130 ksi). In both grades of these steels copper content was increased, while improving metallurgical quality by reduction of sulfur and phosphorus content. Addition of copper in amount to 1,5% in such grade of steels permits to decrease carbon content while maintaining high strength. After heat treatment consisting in hardening and tempering, the precipitation process of copper forms very small particles εcu phase which caused increase of strength [2-6]. In this paper show results of modifications the chemical composition of HSLA 100 steel by increasing the molybdenum addition. 213

2 2. Experiment The three laboratory heats of about 50 kg weight were melted in air-induction furnace. Chemical composition of steels is shown in table 1. In heat W2 and W3 increased addition of molybdenum to a level of 1,88% and 2,94%, respectively. The thermomechanical processing was performed as shown in Fig. 1. As a result of rolling process plate with 25 mm thickness was obtained. The heat treatment of the steels consisted in austenitizing at 900 C for one hour and then quenching in water. Subsequently samples were tempered in the temperature range from 500 C to 800 C for one hour (with step 25 C within), followed by quenching in water. Table 1. Chemical composition of the steels (% mass.) Grade of steels C Mn Si Ni Mo Cu W1 0,015 1,00 0,29 3,53 1,02 1,50 W2 0,017 0,89 0,27 3,55 1,88 1,51 W3 0,017 1,00 0,29 3,53 2,94 1,50 P=0,005%, S=0,003%, Nb=0,05%, Ti=0,02%, Al=0,005% transmission electron microscopy (TEM) Philips CM20. Thin foils were prepared in a jet-polishing Struers TenuPol-5 using a 30% solution of nitrous acid in ethanol at -50 C. Identification of observed phases was carried out by indexed selected area diffraction (SAD) pattern. Interplanar distances were determined by process diffraction program and next crystallographic plane type was defined. 3. Results 3.1. Mechanical properties The course of the impact energy at room temperature of the analyzing steels as a function of the tempered temperature is shown in Fig. 2. In the case of heats W2 and W3 after an initial increase in toughness of tempering temperatures from 450 C to 500 C was observed sudden decrease of impact resistance at temperatures ranging from 575 C to 600 C. In heat W1 the toughness was had tendency growing. The lowest impact strength equal to 37 J were measured for the heat W3 after tempering at 575 C. Minimum toughness for heat W2 was obtained after tempering at 600 C. That suggests a strong influence of molybdenum on the change of impact energy. Impact energy [J] W1 W2 W Tempering temperature [ o C] Fig. 1. Scheme of thermomechanical processing Impact test was conducted by means of standard Charpy V- notch specimens using impact testing hammer with initial energy of 300 J. The impact energy was determined at an ambient (25 C) temperature. Microstructure examinations and fractography research of impact resistance samples after tempering were conducted using scanning electron microscope (SEM) JOEL JSM5510LV. Observations were carried out on longitudinal sections of metallographic specimens etched in 4% nital. A detailed assessment of microstructure for selected specimens characterized by the highest mechanical properties was carried out using Fig. 2. Course of impact energy at 20 C as a function of tempering temperature 3.2. Microstructure Typical microstructure of the W1 and W3 heats after tempering in the temperature range C is shown in Fig Microstructure analysis made by scanning electron microscope didn t reveal significant differences in the phase structure alloys investigated. The only observed difference was a tendency for stronger etching grain boundaries of retained austenite with increasing concentration of molybdenum. 214

3 Fig. 3. Microstructure of the steel W1 after tempering at 550 C Fig. 6. Microstructure of the steel W3 after tempering at 550 C Fig. 4. Microstructure of the steel W1 after tempering at 575 C Fig. 7. Microstructure of the steel W3 after tempering at 575 C Fig. 5. Microstructure of the steel W1 after tempering at 600 C Fig. 8. Microstructure of the steel W3 after tempering at 600 C 215

4 Fig. 9. TEM micrographs of the W3 steel after tempering at 575 C tempered martensite laths Fig. 12. TEM micrographs of the W3 steel after tempering at 575 C polygonal ferrite with of Fe 2 Mo precipitate Fig. 10. TEM micrographs of the W3 steel after tempering at 575 C polygonal ferrite with of εcu precipitates Fig. 13. TEM micrographs of the W3 steel after tempering at 575 C polygonal ferrite with of Fe 2 Mo precipitates, dark field 222 ε Cu [220] εcu εcu 113 εcu Fe 2 Mo 310 Fe α Fe 2 Mo [132] εcu [132] Fe α 134 Fe α 313 εcu 224 Fe α 135 εcu Fig. 11. SAD pattern and index of SAD pattern identifying precipitates of εcu Fig. 14. SAD pattern and index of SAD pattern identifying precipitates of εcu and Fe 2 Mo 216

5 Fig. 16. SEM fractographs of Charpy impact tested sample of steel W3 broken at room temperature after tempering at 550 C Fig. 15. EDX analysis of chemical composition of Fe 2 Mo precipitates Detailed information about the phase structure was obtained on the basis studies thin foils in TEM. Heat W3 after tempering at 575 C was investigated. Analyzed microstructure composed with tempered martensite laths, Fig. 9, and polygonal ferrite, within which coagulated precipitate was observed, Fig. 10. The particles sizes from nm were identified as εcu phase on the basis of electron diffraction Fig. 11. Moreover, it was found the occurrence of precipitation in Mo-rich particles at grain boundaries of retained austenite, Fig Diffraction analysis didn t give an unequivocal answer as to the type of precipitates. The works [7, 8] indicate that the stable carbides of molybdenum occurring at grain boundaries in low carbon steels with the addition of Nb and Mo are isomorphic Mo 2 C carbides. However, the observed precipitates of heat W3 have a spheroid shape, which tends to imply that they are Laves Fe 2 Mo phase particles. Analysis of phase equilibrium diagram of Fe-Mo performed in [9], indicate possibility of release of the Laves phase Fe 2 Mo type. Identification of precipitates in steel 13Cr6Ni2,5MoTi a similar carbon and molybdenum content to the heat W3 carried out in [10] revealed the presence of an intensive precipitate process of Laves phase Fe 2 Mo type during tempering at 590 C. The occurrence of such phases in the steels with the addition of Mo is also confirmed by other research papers [11]. Fig. 17. SEM fractographs of Charpy impact tested sample of steel W3 broken at room temperature after tempering at 575 C 3.3. Fractography Fig shows the topography of fracture notched bar test piece broken at room temperature after tempering in temperature range from 500 C to 625 C for W3 steel. Fig. 18. SEM fractographs of Charpy impact tested sample of steel W3 broken at room temperature after tempering at 625 C 217

6 At a temperature of 550 C the fracture exhibits the characteristic of transcrystalline cracking with small amounts of ductility fracture Fig. 16. The increase of tempering temperature to 575 C causes the change the character of fracture on intercrystalline cracking Fig. 17. This method of cracking is typical for materials that have second phases at the grain boundaries which lead to a reduction of coherency. The further increase of tempering temperature caused a next change in the mechanism of fracture. In the case of W3 steel after tempering at 625 C the sample indicates on ductility and brittle cracking, Fig Conclusions Observed a strong decrease of impact strength after tempering in the temperature range from 575 C to 600 C is associated with the precipitation processes of the molybdenum-rich phase, which was observed in TEM in the form of spheroidal particles on grain boundaries of retained austenite, Fig Analysis of phase equilibrium diagram Fe-Mo, Fig. 19, indicates that these are particles of Laves phase of the Fe 2 Mo type. Very low carbon content non-exceeding 0,02% practically precludes precipitation the carbides of molybdenum, especially in the presence of elements such as Nb and Ti. Fig. 19. Phase equilibrium diagram Fe-Mo The occurrence of the precipitation effect of the molybdenumrich phase was confirmed in the steels of similar chemical composition [10], where during the tempering at 590 C was observed precipitation of the Laves phase Fe 2 Mo. The high concentration of molybdenum in combination with very low carbon content fosters precipitation processes of intermetallic phase Fe 2 Mo in the temperature range of tempering from 575 C to 600 ºC. This effect must be recognized as unfortunate because formation of precipitates at the grain boundaries of retained austenite conducts to change in the nature of the fracture from transcrystalline brittle to intercrystalline cracking and consequently to a sudden drop of impact resistance. Literature [1] MIL-S-24645A(SH), Military Specyfication, Steel Plate, Sheet, or Coil, Age-Hardening Alloy, Structural, High Yield Strength (HSLA-80 and HSLA 100), January [2] M. Blicharski, C. I. Garcia, S. M. Pytel: Structure and Properties of ULCB Steels for Heavy Section Applications, Proceedings of International Symposium on Processing, Microstructure and Properties of HSLA Steels, October 1987, Pittsburgh, PA, USA. [3] S. J. Mikalac and M. G. Vassilaros: Strength and toughness response to aging in a high copper HSLA-100 steel Proceeding of the International Conference on Processing, Microstructure and Properties of Microalloyed and oder Modern High Strength Low Alloy Steels, June , Pittsburgh, Pa, USA, pp [4] A. Ghosh, S. Chatterjee: Characterization of precipitates in an ultra low carbon Cu bearing high strength steel: A TEM study, Materials Characterization 55 (2005) [5] A. Ghosh, S. Chatterjee: Ageing behavior of a Cu-bearing ultrahigh strength steel; Materials Science and Engineering A 486 (2008) [6] S.K. Ghosh, A. Haldar, P.P. Chattopadhyay: Effect of ageing on the mechanical properties of directly quenched copper bearing microalloyed steels; Materials Chemistry and Physics 119 (2010) [7] W.B. Lee, S.G. Honh, C.G. Park, K.H.Kim, S.H. Park: Influence of Mo on precipitation hardening in hot rolled HSLA steels containing Nb; Scripta mater. 43 (2000) [8] A. K. Lis, P. Wieczorek: TEM study of the precipitation process in HSLA-100 copper bearing steel, Inżynieria Materiałowa nr.3(140) XXIV [9] M. Zinkevich, N. Mattern: Thermodynamic modeling of the Fe-Mo-Zr system, Acta Materialia vol. 50, 2002, p [10] R. Rožnovskă, V. Vodărek, A. Korčăk, M. Tvrdŷ: The effect of heat treatment on microstructure and properties of a 13Cr6Ni2,5Mo supermartensitic steel, Rada Hutnika vol. 1, 2005, p [11] E. H. Lee, L. K Mansur: Fe-15Ni-13Cr austenitic stainless steels for fission and fusion reactor applications. II. Effects of minor elements on precipitate phase stability during thermal aging, Journal of Nuclear materials 278 (2000)