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Journal of Malerials Processing Technology 69 (1997) 257 -263
Microstructure and phase analyses of Stellite 6 plus 6 wt.% Mo alloy V. Kuzucu a, M. Ceylan
H. Celik b [. Aksoy
" Universio' o] FIrat, f,.culty o.[ Science and Arts, Department of Physics, Elaztg, Turkey b Unilersity o! Ftr,~t, Faculty o/Techn&zd Education, Department ~[ Metalh~rg.r, Elaztg, Turkey Unirersitv of bff~nii, Faculty of Science and Arts, D~Tartment t~f Physics, Malatya, Turkey Received 13 February 1996
Abstract
The alloy Stellite-6 was cast with 6% Mo. Then, some pieces of this alloy were cooled in water alter annealing at 950°C for 2 h. A piece of the water-cooled samples was cooled in liquid nitrogen again. Later, all of the samples were investigated using metallographic and x-ray diffraction techniques and thermal (DSC and DTA) analysis methods. The hardnesses were also measured as 40, 46 and 49 HRC for samples cast, cooled in water and cooled in liquid nitrogen, respectively. In addition to IL, R and a phases, it was determined that various metal carbides such as MC, M7C3, M23C6, M6C and some other inter-metallic phases may exist in these samples. It was identilied by x-ray diffraction that a phase transfonriation from ~ phase with fcc (face centered cubic) structure to e phase with hcp (hexagonal close-packed) structure occurred in the sample cooled in water, the amount of the e, phase (martensite phase) increasing in the sample cooled in the liquid nitrogen, and the hardnesses of these samples also increasing. In addition, from the DTA curves it was seen that a second allotropic transformation of cobalt from :~ phase to c phase had taken place in the temperature range between the Curie temperature (1121°C) and 1223°C for all of the samples. © 1997 Elsevier Science S.A. Keywords: Steilite 6; Molybdenum: Phase analysis; Differential scanning calorimetry: Differential thermal analysis: X-ray diffraction
1. Introduction Various kinds of cobalt-based alloys called 'Stellite' have been used in fields requiring high heat and corrosion resistance and high wear strength, such as the nuclear, aerospace and gas-turbine industries [1]. Because of their good quality, studies on the production of new kinds of cobalt-based alloys are still being carried out extensively. At the same time, some other products such as wires, plates and welding electrodes made from these alloys have been used successfully in different fields. Recently, C o - C r - W / M o - N i / F e - C + S i type new stellite alloys had been developed from the C o - C r - W C type alloys, which were developed originally circa 1900 by Elwood Haynes, adding alloying elements such as Mo, Ni, Fe and Si [2]. Pure cobalt, the main element of stellite alloys, exists in two allotropes, a high-temper* Corresponding author. Fax: + 424 2330062. 0924-0136/97/$17.00 © ! 997 Elsevier Science S.A. All rights reserved.
PI! S0924-01 36(97)00027-7
ature aUotrope ~ with a fcc (face centered cubic) crystal structure, stable at high temperatures up to the melting point (1495°C), and a low-temperature allotrope s with a hcp (hexagonal close packed) crystal structure, stable at temperatures below 417°C [1-3]. However, a longstanding controversy exists as to the stability of the fcc allotrope. Several investigators believe that there is a second allotropic transformation of cobalt (fcc to hcp) at or near to the Curie temperature (ll21°C) [4,5], whilst ethers [6,7] have presented evidence that the fcc phase is stable from 450°C to at least 1223°C [3]. The 0c--. ~ allotropic transformation in cobalt below 417°C is often classified as martensitic [2]. The transformation essentially is athermal in nature and occurs by shear. During cooling and reheating the transformation is seen to be reversible: on cooling 0~~ e, occurs at 390°C and is referred to as the Ms (martensite start) temperature, reverse reaction e--*~ occurring at 417°C and being referred to as the As (austenite start) temperature [8,9]. During transformation surface tilts and upheavals
V. Kuzucu et al. IJournql of Materials Processing Technolog)' 69 (i 997) 257-263
258
are observed. The crystallographic relatio~ships between the ~ and the t: phases are: {lll}J/{0001}~
and
(112)~//(10]0)~.
The amount of e depends on the purity and the grain size [8,9], a fine grain size in pure cobalt preventing the a--,e reaction. The martensitic reaction tends to be pa~icularly sluggish in alloyed cobalt [2]. The addition of alloying elements alters the thermodynamic stability of the fee and hcp phases by either enlarging or constricting their fields. These alloying elements will also effect the martensitic transformation by influencing the M~ and A~ temperatures [8]. In Stellite alloys, chromium improves oxidation and heat-corrosion resistance, and produces strengthening by the formation of M7C3 and M23C 6 carbides. Molybdenum and tungsten are solid-solution strengtheners, producing strengthening by the formation of intermetallic compound Co3M and MC carbide, and forming M 6 C carbide. Carbon produces strengthening by the formation of carbides MC, M7C3, M,3C~ and possibly M~C. In addition, alloying additions of nickel, carbon, and iron tend to stabilize the fcc structure, whilst chromium, molybdenum and tungsten tend to stabilize the hcp structure [10]. The original composition of the stellite 6 alloy used in this study is given as C o - 2 8 C r - 4 W - I . I C (wt.%). The first phase to form during cooling from the liquid state consists of unfaceted cobalt-rich dendrites with a face-centered cubic (fcc) crystal structure [11]. The remaining liquid eventually solidifies by a eutectic reaction into an inter-dendritic, intimate lameilar mixture of the fee phase and (Cr, Co,W)TC~ eutectic carbides. The changes in the microstructure of the alloy, with the nominal composition C o - 2 8 C r - 6 M o - 4 W - I . 1 C (wt.%) obtained by adding molybdenum to stellite 6 alloy, due to various applied heat treatments and the phases in this alloy were investigated by optical microscopy, x-ray diffraction and thermal analysis, DTA (differential thermal analysis) and DSC (differential scanning calorimetry). In addition, the hardnesses of the samples obtained under various heat treatments were measured.
m~croscope after having been etched with the etchant HNO~ + C 2 H 4 0 2 4- 4HCI + H_~O. Powder samples were taken from the specimens in Table 1 for X-ray diffraction and thermal analysis. X-ray diffractograr,as for each specimen were taken using a Rigaku-Geigerflex diffractometer. DTA and DSC results were obtained by using a Shimadzu 50 instrument employing powders of the same samples. DTA measurements were taken in the heating regime 40°C min ....t up to 700°C, and then in the heating regime 10°C min t after 700°C, whilst DSC measurements were taken in the heating regime Table 1 Heat treatments applied to specimes~s and the symbols of the specimens Sample symbols
Heat treatment
SI $2
Cast specimen Si cooled in water after annealing at 950°C for 2 h $2 cooled in liquid nitrogen at room temperature for 5 min
53
,
!!;i,~ ~ : X ' , ~~1~
2. Experimental
Test samples of composition C o - 2 8 C r - 6 M o - 4 . 0 W I.IC (wt.%), dimensions of which are ~ 2 5 × 50 ram, were made by adding molybdenum to stellite 6 alloy. The melted sample in the mould was left for cooling at room temperature. Three distinct samples were obtained by applying the heat treatments given in Table 1 to the cast sample. These samples were polished mechanically to reveal their microstructures and their surface photographs were taken using an Olympus metal
Fig. I. Surface photographs a! two differe,~t magnifications ( x 100 and x 1000) of sample Sl.
V. Kuzut'u el al. Jourmd oJ M , ferhds Pr,,~ v.~.~ing 7"echm~h,gy 69 (1997) 257- 263
259
Fig. 2. As for Fig. I, but for sample $2.
Fig. 3. As for Fig. I, but for sample S3.
IO°C rain ~ In addition, the hardnesses of the specimens. given in Table 1, were measured on the HRC scale.
dendritic regions. This situation is a result of the addition of molybdenum to stellite 6 alloy. The microstructure of sample $2, produced as a result of cooling in water after annealing at 950°C for 2 h, is seen in Fig. 2. This microstructure is in the form of an oriented dendritic structure. In addition, the carbide
3. Results and discussion
3.1. Microstructural evohttion
Fig. 1 shows surface photographs at two different magnifications of the cast sample (S l). The cast sample has a dendritic microstructure consisting of carbide phases disr~ersed in inter-dendritic regions in the cobaltrich matrix. After the first phase, consisting of cobaltrich dendrites with an fcc crystal structt~re, forms during cooling from the liquid state in the original stellite 6 alloy, the lamellar eutectic mixture of fcc phase and M7C3 eutectic carbides--reported to eventually solidify by a eutectic reaction into an inter-dendritic region from the remaining liquid [1 I]--did not form in this alloy. The carbide phases are not iamellar forms, but are precipitated as if a mass of sand in the inter-
$3 ! 28.I~e
3S. 88
4 e . 88
Fig. 4. X-ray diffractograms of samples SI, $2, and $3 (target: Mo).
V. Kuzucu et al./Journal of Materials Process#:g 72,chnology 69 (1997) 257-263
260
Table 2 20 angles of the x-ray diffraction peaks Peak No.
1
2
3
4
5
6
7
8
9
10
20 angle (*)
17.77
18.84
20.02
21.02
21.64
22.69
27.85
32.71
35.62
38.39
phases are dispersed more homogeneously in the cobalt-rich matrix, and the dendrite branches become thinner, compared with the cast sample S I. As shown in Fig. 3, except that the carbide phases are dispersed more homogeneously in the cobalt-rich matrix, and the dendrite branches become much thinner compared with the sample $2, it seems that change did not occur in the microstructure of sample $3, due to the cooling in the liquid nitrogen of the annealed sample.
3.2. X-ray investigation Fig. 4 shows the x-ray diffractograms obtained from the SI-$3 samples. The 20 values of the x-ray peaks for these samples are given in Table 2. The indexes of the crystal planes that contribute to the x-ray peaks of phases occurring or expected to occur in the samples, the crystal structure of the phases, and the lattice parameters, are listed in Table 3. Some differences have occurred in the diffractograms of the samples, as shown in Fig. 4. In the; diffractograms obtained from samples $2 and $3, peaks numbered 1 and 3 have a lower intensity, according to the diffractogram obtained from sample SI, and peak number 4 is annihilated. Contrastingly, the inten.sities of peak numbers 2, 5 and 7 increased in the diffractograms obtained for samples $2 and $3. As regards the increase and decrease in the peak intensities, the following conclusions can be derived from Table 3: (i) the CoaC phase in specimens $2 and $3 has been decreased considerably or transformed to another phase; (ii) in sample S1, the cobaltrich matrix has an unstable fcc structure at room temperature; in this sample, the diffraction peaks numbered 2, 3 and 9 represent the contributions from some of the other phases given in Table 3, and not the contribution from the e phase with fcc structure; (iii) an 0t~ e transformation known as martensitic transformation has occurred in samp!le $2 as a result of cooling in the water, the amount of this transformation having increased in sample $3 owing to cooling in the liquid nitrogen, the increase of the intensities of the peaks numbered 2, 5 and 9 verifying this result; (iv) the appearance of the peak numbered 7 in samples $2 and $3 shows that the heat treatments applied to these samples assist the formation of the Co3Mo, 7MoC and W2C phases; and (v) the types and percentages of the other carbides, compounds and phases have not changed considerably.
3.3. Thermal analysis In DSC and DTA, chemical reactions and structural changes within a crystalline substance are accompanied by the evolution or absorption of energy in the form of heat [15]. When a substance crystallizes, for example, an exothermic effect occurs, since the free energy of the regular crystal lattice is less than that of the disordered liquid state. Conversely, the melting of a crystal gives rise to an endothermic effect. Differential thermal analysis is a technique which enables reactions or phase transformations to be studied for substances at high temperature, whilst differential scanning calorimetry is a technique to be used for substances at low temperatures. In both of the techniques, exothermic effects are indicated as peaks on the curve obtained and endothermic effects as dips in the curve. Fig. 5(a)-(c) show the DSC curves obtained for samples S l, $2 and $3 at the same heating regime of 10°C min- ~, respectively. The exothermic peaks having athermal character have occurred at the temperatures of 289.9, 237 and 242.8°C, for samples SI, $2 and $3, respectively. These peaks indicate that some new crystalline phases have formed at these temperatures at the heating regime in these samples. The crystal structures of these phases have not been determined in this study. During the crystallization of these phases, samples S1, $2 and $3 released energies of 1.55, 1.04 and 1.05 J g-~, respectively. Fig. 6(a)-(c) show the DTA curves obtained for samples Sl, $2 and $3 at the 10°C min -~ heating regime. Exothermic peaks having athermal character have occurred at the temperatures of 1201.7, 1222.3 and 1222.3°C for samples S1, $2 and $3, respectively, the energies released during the formation of these peaks being 1974.81, 1047.22 and 1029.85 J g - l , respectively. These peaks correspond to a second allotropic transformation from ~ to e. phase of the pure cobalt. As shown in Fig. 6(a)-(c), the formation of these peaks is in the temperature region between approximately the Curie temperature (1121°C) and 1223°C, as was mentioned in the introduction, which both explains the doubts about a second allotropic transformation of cobalt [3] and clarifies the discussions about the temperature of this allotropic transformation that have taken place for a long time [4-7]. Fig. 7 shows DTA curve obtained at the heating regime of 500C m i n - ~ for sample $3. The endothermic peak shown in this figure corresponds to the transformation of the martensite phase (e) formed as a result of
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V. Kuzucu et al./Journal of Materials Processing Technology 69 (1997) 257-263
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cooling in liquid nitrogen to • phase in the heating regime. It is understood that this transformation, reported to occur at 417"C [2,8], is very complex. As shown in Fig. 7, this transformation occurring in the temperature range between 385.2°C and 499.6°C, has formed in three stages at the temperatures of 419, 453 and 461"C, the energy absorbed during the transformation being 58,95 J g - - 1
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3.4. Hardness measurement
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In Table 4, the hardnesses of samples SI, $2 and $3 are given. The hardness of the samples did not change very much with the applied heat treatment, but the process of cooling in water after annealing increased some the hardnesses and the process of cooling in liquid nitrogen after annealing increased the hardness even more. The increment in the hardnesses may have origi
263
V. Kuzucu et al./ Journal o/" Materials Processing Tedmology 69 (1997) 257-263
Table 4 Hardness measurements of the samples
" w n v O v n v r ~ p r ~ r ~ ~ ~ ~
Sample Hardness (HRC}
lahlltllultllialltl/tladl~a~aa,lLal~l111
! J ~ l * I ~ I I I I | H ~ RH H ~ b H
H ~#j d_j
Fig. 7. DTA curve obtained for 40°C rain- ~ heating regime for sample $3.
nated from the martensitic transformation occurring as a result of rapid and over cooling, or the distribution of the carbides in the matrix being more homogeneous.
4. Conclusions
1. The cobalt-rich main matrix in the cast sample (S1) has an unstable fcc structure at and around room temperature. In this sample, different kinds of carbides have been distributed over the inter-dendritic regions as granular shapes, not lamellar eutectic as seen in the original stellite 6 alloy. 2. In samples $2 and $3 cooled in water or in liquid nitrogen after annealing, a large part of the cobalt-rich main matrix has transformed to e phase with hcp structure. The microstructure of these samples has the appearance of an oriented dendritic structure. The carbide phases are distributed more homogeneously in the inter-dendritic regions. The cooling in the liquid nitrogen has caused an increase in the amount of the e phase, a more homogeneous distribution of carbides in the inter-dendritic regions, and thinner branches of dendrites. At the same time, the fast cooling of the samples has also assisted the occurrence of the inter-
Sl 40
$2 46
S3 49
metallic compound C o s M o and some carbides such as C%C, ),MoC and W2C. 3. The heat treatments applied to the samples have not made considerable changes in the amounts of the cal::!des MvC3, M2sC6, M6C, M e and M3C2. 4. A second allotropic transformation of cobalt from fcc structure to hcp structure is seen at the temperature range between the Curie temperature (i121°C) and 1223°C, as has been mentioned by several researchers earlier. 5. The rapid cooling after annealing at 950°C for 2 h has increased the hardness of the samples.
References [1] P. Crook, in: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, vol. 2, edn. 10, Metals Handbook, ASM International, 1993, 446 p. [2] K.C. Antony, J. Met. 35 (1983) 52. [3] Cobalt Monograph, Ed. Centre d'information du Cobalt, 35, Rue des colonies, Brussels, Belgium, 1960. [4] A.G. Metcalfe, Am. Soc. Met. (1952) 717. [5] A.G. Metcalfe, Acta Metall. 1 (1953) 609. [6] J.B. Newkirk, A.M. Geissler, Acta Metail. ! (1953) 456. [7] W.F. Meyer, Z. Krist. 97 (1937) 145. [8] S. Atamert, Ph.D. Thesis, University of Cambridge, England, 1989. [9] J. Singh, S. Ranganathan, Phys. Status Solidi 73 (1981)243. [10] C.P. Sullivan, M.J. Donachie, F.R. Morrak Cobalt-base Super Alloys-1970, Centre d'lnformation du Cobalt, Brussels, 1970, p. 4. [11] S. Atamert, H.K.D.H. Bhadeshia, Metall. Trans. 20A (1989) 1037. [12] C,S. Barrett, T.B. Massalski, Structure of Metals, Pergamon Press, Oxford, 1982. [13] K. Mills et al. (eds.), Metallography and Microstructures, vol. 9, edn. 9, Metals Handbook, ASM international, 1992. [14] E.A. Brandes (ed.), Smithells Metals Reference Book, edn 6, Butterworth and Co. L~d., 1983. [15] W.W. Wendlandt, Thermal Methods of Analysis, 2nd edn., Wiley, 1974r