Martensite transformation of quenched and aged austenite in N26MT2Nb steel

E L S E V I E R Journal of Materials Processing Technology 53 (1995) 195-202

Journal of Materials Processing Technology

M a r t e n s i t e t r a n s f o r m a t i o n o f q u e n c h e d and a g e d austenite in N 2 6 M T 2 N b

steel

J.Jelefikowski Warsaw University of Technology, Department of Materials Science and Engineering, Narbutta 85, 02-524 Warszawa, Poland

The effect of quenching and aging on the formation of martensite from austenite of various degrees of stability was investigated. After quenching in water from 1273K the specimens were aged under calorimetric conditions and, after cooling to room temperature, subjected to microscopic examination. After quenching, austenite in the surface layer transformed isothermically. Aging at 823 K for 7 hours did not change the stability of austenite. After aging at 923 K for 4 and 7 hours an increase in the martensite content was found amounting to 2 and 28%, respectively. The absence of heat effects on the calorimetric curves recorded during cooling indicates that the transformation in the calorimetric specimens (i. e. specimens of a small volume) proceeded isothermically. It is also supposed that in specimens with a high martensite content a considerable proportion of this phase was formed in a later period of the transformation which proceeded athermally.

1. I N T R O D U C T I O N Steels and alloys with metastable austenite undergo abrupt changes of properties during the strain-induced martensitic transformation. This refers to physical, chemical and, primarily, mechanical properties. As the result of the TRIP effect, fracture toughness, wear resistance and fatigue strength can increase considerably [1,2]. As an example, austenitic-martensitic alloys undergoing precipitation strengthening or strain-induced aging can be regarded [3-5]. If the chemical composition and conditions of heat treatment allow metastable austenite to persist, the alloys can be used for pressure vessels, elements of wear-resisting chemical equipment and tools [5,7,8]. During the deformation of metastable austenite some disadvantageous results of the strain-induced transformation of austenite into martensite can appear [8-11]. To explain the reasons involved, a lot of investigations of the strain-induced transformation of austenite into martensite were carried out [8-14]. |t has been found that the M temperature changes under the influence of elastic ( M -M s ~) and plastic (M~ -M d range) strains (Fig. 1.). The main aspect of martensitic transformations are discussed from this point of view in [12-16]. The martensitic transformation starts spontaneously at the M s temperature. Above M S (Fig. l.) the transformation can be initiated by the application of tangential stress Zam in the direction of shear which would appear below M~ if no external load were involved. The value of L,m in martensite is equal to zero at the M s temperature and increases with decreasing undercooling AT [6]. During deformation at a temperature near to M~, the nucleation of Elsevier Science S.A. SSDI 0924-0136(95)01976-L

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martensite occurs in the regions of austenite in which it would take place during cooling [17]. During deformation in the temperature range M s -M d the nucleation of martensite is favoured by the formation of stacking faults [18]. ¥

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Figure I. Schematic representation of thermodynamic and mechanical properties of undercooled austenite (y): a) Temperature dependence of free energy of the phases y and c~ (martensite). b) At Tc~' transformation occurs on cooling. External shear stress is not necessary Range 2. y->c(' transformation is induced by stress. Range 3. plastically deformed metastable austenite, strain- induced y->c~' transformation. Range 4. stable austenite, no martensite is formed. Friction or stress relaxation occuring in the deformation below the M d temperature can -with the cooperation of tangential stress cause the formation of surface martensite [15]. Compressive and tangential stresses lead to the appearance of triaxial state of stress in which a variable component is the martensitic shearing stress. If the value of this component surpasses "c, nuclei of martensite are formed, most easily at free surfaces [6,7,19,20]. The thickness of the martensitic layer is a function of the strength of austenite, its thermodynamic stability and the extent of the external stress field. Hence, the controllable destabilization of austenite in the

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surface layer can be utilized for increasing the wear resistance o f the surface with a regenerating martensitic layer. The aim o f the present work was to determine aging conditions assuring a controlled destabilization o f austenite in the surface layer o f N26MT2Nb steel and to establish the characteristics of the martensite formed. It was undertaken with the view to endow constructional elements, made o f this steel, with high wear resistance by spontaneous formation of martensite in the surface layer.

2. MATERIALS FOR INVESTIGATIONS AND HEAT TREATMENT APPLIED The chemical composition o f the alloy under investigation is given, in weight per cent, in Table 1. Table 1. N26MT2Nb

C 0.02

P 0.007

S Mn 0.009 0.17

Si 0.11

Ni 26.0

Ti 2.15

A1 0.04

Mo 1.15

Nb 0.11

Calorimetric specimens were made from bars subjected to homogenizing during 10 hours at 1373 K in a vacuum furnace and cooling in oil. The specimens o f the dimensions 03 x 2 mm were put into quartz phials and austenitized for 1 hour at 1273 K in a vacuum furnace. Thereupon they were quenched in water and aged in a Parker- Elmer DSC-2 calorimeter in argon atmosphere. Heating to the aging temperature and cooling to room temperature was performed at a rate o f 40 K/min. The accurasy oftemperarure measurement was ± K.

2.1. Microscopic examination Microsections for the examination on a Neophot light microscope were prepared by mechanical and electrochemical polishing, and etching with nital. Specimens with revealed microstructure were subjected to microhardness measurements by means of a Hanemann tester at a load of 20 g. Foils to be examined on a JEOL 100B microscope were obtained from discs cut out from calorimetric specimens thinned mechanically and, then, electrochemically using the Struers device. The observation was conducted in bright and dark field. Electron diffraction was also taken advantage of. 2.2. Magnetometric and X-ray examinations The measurement o f the martensite content was performed using the F6rster device. X-ray microanalysis was made using a JXA 3A JEOL electron probe analyzer.

3. R E S U L T S O F EXPERIMENTS

3.1. Microstructure In the supersaturated austenite annealing twins were observed with a proportion of twin boundaries amounting to 15%. The grain size corresponded to No.4 of the A S T M scale.

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The specimens of this steel had after quenching an austenitic structure, irrespective of the mode of preparation. However, after a lapse of one week surface martensite appeared, initially at twin boundaries (Fig. 2a). After a month the specimens were completely covered with a martensitic relief. Etched with nital they revealed the microstructure shown in Fig. 2b, the proportion of martensite, as determined by means of the FOrster device, being equal to ! %.

a)

b)

Figure 2. Microstructure in the as-quenched condition. Relief on the surface of a polished specimen with crystals of surface martensite two weeks after quenching (a) and microstructure revealed by etching after a month (b). After aging for 4 hours at 823 K the microstructure did not show any changes (Fig. 3a) The presence of martensite crystals in the vicinity of extinction lines indicates that an "in situ" transformation was involved (Fig. 3b). After aging 7 hours at 823 K a greater proportion of isothermal martensite was revealed and the precipitates in austenite were more distinctly visible (Figs. 4a and 4b).

a)

b)

Figure 3. Microstructure after aging for 4 hours at 823 K. a) martensitic relief on the surface of a polished specimen; b) martensitic plates in the vicinity of extinction lines.

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a) b) Figure 4. Microstructure after aging for 7 hoursat 823 K. a) martens±tic relief on the surface of the polished specimen; b) precipitates of the 7' phase in austenite. After aging for 4 hours at 923 K isothermal martensite was visible in light microscope, namely lenticular martensite with midribs, colonies of laths and butterflies (Fig. 5a). In TEM examinations, numerous precipitated particles in austenite, surrounded by stresses and colonies of 7 and q plates resulting from the cellular decomposition, were revealed (Figs. 5b and 5c). Observation in dark field (Fig. 5d) established the presence of stresses around the precipitated particles of the 7' phase in martensite, inherited from austenite. After aging for 7 hours at 923 K the microstructure did not show any essential changes. Only the proportion of martensite in the structure increased as compared with the previous state. 3.2. Surface microhardness and proportion of martensite in the structure. Hardness values were determined from a dozen or so of indentations made for a single phase or a phase mixture. The level of confidence was adopted as equal to 0.95. The results obtained are tabulated in Table 2. The numbers contained in brackets denote the proportion of martensite determined by the magnetometric method. Table 2 Condition of the specimen

Type ofmicrostructure

HV0.02 hardness

Quenching from 273 K

Austenite Surface martensite

238+8 198+5 (1.0)

After quenching and aging for 4h at 823 K

Austenite Surface martensite

304±8 320±8 (0.1)

As above, but aging for 7 hours

Austenite

330±10

After quenching and aging for 4h at 923 K

Retained austenite + + thin-plate martensite

330±10 (2)

As above, but aging for 7 hours

Lenticular martens±re Retained austenite + + thin-plate martensite Plate martensite

315+ 10 483±20 (28) 536±25

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a)

b)

c)

d) Figure 5. Microstructure after aging for 4 hours at 923 K. a) martensitic relief on the surface of the specimen; b) stress field around precipitates in austenite; c) martensite plates in bright and dark field; in dark field stress fields around the precipitates are visible; d) cellular decomposition in the region of grain boundaries of austenite

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3.3. Calorimetric analysis Calorimetric curves recorded during cooling from aging temperatures to room temperature at a rate of 40 K/min did not show any measurable heat effects.

4. DISCUSSION OF T H E R E S U L T S OF I N V E S T I G A T I O N After quenching, the specimens had an austenitic structure, irrespective of the mode of preparation of microsections. However, already after several days the polished surfaces became tarnished and a martensitic relief appeared. Aging for 7 hours at 823 K slowed down this transformation so that only after a month the surface of the specimen was almost entirely covered with a martensite layer. In specimen aged for 4 hours at 923 K the progress of structural changes was similar, but a greater variety of structures of the isothermal martensite appeared (plate martensite with lenticular, butterfly-like and thin-plate morphology). After aging for 7 hours the appearance of the microstructure, as seen in light microscope, did not undergo any essential changes. In TEM examinations, stress contrast and the initial stage of cellular decomposition of austenite was observed around the 7' precipitates in austenite and q precipitates in martensite. Calorimetric records made on cooling did not reveal the existence of M temperature in spite of a marked, namely equal to 28%, proportion of martensite. This means that after aging for 7 hours at 923 K martensite was formed only after cooling down to room temperature, probably with the participation of an athermal transformation. Linear X-ray analysis for alloying elements did not reveal, within the limits of sensitivity of the device, any inhomogeneity of chemical composition on the cross-section of the grains.

5. C O N C L U S I O N S 1. 2. 3.

4.

In the as-quenched condition austenite was metastable and surface martensite, initially nucleating at twin boundaries, was formed. Aging at 823 K slows down slightly the isothermal transformation and causes an increases in hardness which rose with the aging time. Aging at 923 K markedly destabilizes austenite with respect to the isothermal transformation which, probably, precedes the athermal transformation. After this aging, considerable quantities of plate martensite in the whole volume of the specimen are formed and a marked increase in hardness of martensite as well as of the two-phase structure takes place. Aging at 923 K favours the cellular decomposition of austenite.

REFERENCES 1. 2. 3.

V.F. Zackay, E.R. Parker, D. Fahr and R. Busch, Trans. ASM, v.60 (1967) 252. W.W. Gerberich, P . L Hemmings, M.D. Merz and VF. Zackay, Trans. ASM, v.61 (1968) 843. R.B.G. Yeo, Trans. AIME, 224 (1962) 513.

202 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

J. Jelehkowski / Journal of Materials Processing Technology 53 (1995) 195-202 R.F. Decker, Structure and Properties of Maraging Steels. Proc. Cone National Physical Laboratory, Jan., (1963). G.R. Speich and S. Floreen, Trans. ASM v.57 (1963) 15. J. Schmidt, Metastable Ferrous Austenite-Consequence on Fracture and Tribology. Eds. E. Hornhogen and N. Jost, Bohum, 1989, 361. V.V. Sagaradze and A.U. Uvarov, Uprochnenye austenitnykh staley Ed. Nauka, Moskva, 1989. J.P. Brasneli and A. Moskowitz, Trans ASM, 59 (1966) 223. D. Bhandarkhar, V.F. Zackay and E.R. Parker, Met. Trans. 3 (1972) 2619. ER. Parker and V.F. Zackay, Eng. Frac. Mech., 5 (1973) 147. GB. Olson, D.M. Parks and Chen I-Wei, Mechanism of Transformation Toughening, Ed. MIT, Cambrige MASS 1987. L. Kaufman and M. Cohere Prog. Met. Phys., 7 (1958) 165. M.A. Filipov, V.S. Litvinov and Yu.R. Niemirovskiy, Stali s metastabilnym austenitom. Ed. Metallurgiya, Moskva 1988. E. Hombogen, Acta Metall., 33 (1985) 595. Y.N. Dastur and W.C Leslie, Met. Trans. 12A (1981) 749. Yu. N. Petrov, Defekty i besdiffusyonnoe prevrashchenye v stali. Ed. Naukova Dumka, Kiev 1978. G.B. Olson and M Cohen, Less-Common Met. 28 (1972) 107. Ya.D Vishnyakov and G.S. Faynshtayn, Prevrahschenya v metallakh s raznoy energey defektov upakovki. Ed. Metallurgya Moskva 1984. J.A. Klostermann and W.E. Burgers, Acta Metall., 12 (1964) 355. V.I Izotov and P.A. Khandarov, Fiz. Met. Metallov. 35 (1972) 332.