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Formation of solid-state dendrites in an alloy steel
Surface and Coatings Technology 179 (2004) 33–38
Formation of solid-state dendrites in an alloy steel A. Nusair Khana,*, I. Salamb, A. Tauqirb a
LASMIS, Universite´ de Technologie de Troyes, BP 2060, 12 Rue de Marie Curie, Troyes, France b Metallurgy Division, G.P.O. Box No. 502, Rawalpindi, Pakistan Received 21 December 2002; accepted in revised form 9 May 2003
Abstract An interesting phenomenon of solid-state dendrite formation was observed during elevated temperature treatment of maraging steel in air. The steel was subjected to heat treatment at temperatures ranging from 900 to 1350 8C for 1–8 h. The study revealed different phases formed into the depth. A band containing dendrites was observed below the oxidized surface layer. Energy dispersive spectrometry and X-ray phase analysis confirmed that these dendrites were enriched in titanium and are of type Ti1yx(Mo, Fe, Co)x(N, O) where 0.2-x-0.3. The formation of dendrites during solid-state transformation is a rare phenomenon and has been reported to take place in specific cases. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Solid-state dendrites; Oxidation; Maraging steel; Diffusion of elements
1. Introduction Maraging steels are high strength low carbon steels where the strength is achieved by the precipitation of intermetallics. Usually, the steels are solution annealed at temperatures between 800 and 850 8C. The austenite transforms to martensite during cooling. It is convenient to mechanically process the material in annealed condition, where the hardness is 30 HRC–35 HRC. Hardening of these steels is achieved by aging at temperatures between 480 and 510 8C w1x. The alloying elements present in maraging steels generally lower the Ms temperature. However, cobalt up to ;8% has an opposite effect, which is useful in ensuring the formation of lath martensite in the presence of higher concentration of the other alloying elements i.e. Ni, Mo and Ti w1x. Maraging steel has a complex chemistry. Its production is quite involved due to the severe segregation of alloying elements. High temperature treatments are imperative for both forging operation and for subsequent homogenization. At elevated temperatures the formation of solid-state dendrites is observed which is the subject *Corresponding author. Tel.: q33-3-25-715831; fax: q33-3-25715676. E-mail address: [email protected] (A.N. Khan).
of this study. The formation of solid-state dendrites is a rare phenomenon, reported in few systems w2–5x. The article discusses formation of such dendrites in system of maraging steel. 2. Experimental Samples of three different maraging steels were subjected to study. The chemical composition was determined using energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM) and carbonysulfur analyzer. The results are given in Table 1. Samples of size 15=15=10 mm3 were prepared and were subjected to heat treatment process at temperatures ranging from 900 to 1350 8C in air for 1–8 h. The same treatment was carried out in a vacuum of 10y3 Torr and also in nitrogen atmosphere. X-ray diffraction (XRD) was carried out to characterize the products of reaction. After heat treatment, samples were sectioned and prepared for optical microscopy. The same samples were also analyzed using EDS to determine the chemical composition of different features formed during the exposure to elevated temperature. The solid-state dendrites were extracted chemically by the dissolution of samples in an aqua regia and then this extract was subjected to EDS and phases were confirmed by Debye–Sherrer technique.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00787-4
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
34 Table 1 Composition of the steel Steel grade
M-350 M-300 M-250
Element (wt.%) Ni
Co
Mo
Ti
Al
C
S
Fe
17.4"0.4 18.9"0.3 18.1"0.1
12.6"0.1 9.6"0.4 8.3"0.1
4.8"0.1 4.4"0.4 4.6"0.1
1.37"0.1 0.76"0.01 0.36"0.02
-0.1 -0.1 -0.1
0.02"0.01 0.02"0.1 0.02"0.01
0.005"0.002 0.005"0.002 0.005"0.002
Bal. Bal. Bal.
Fig. 2. Depth profile from surface towards matrix. Table 2 EDS analyses of different layers of M-350 steel, wt.% Region
Fe
Mo
Ti
Co
Ni
Layer-1 Layer-2 Layer-3 Layer-4 Layer-5a
78"2 74"2 44"0.5 54"5 63"3
0 1"0.5 9"0.8 6"0.8 4"0.5
0 8"1 3"0.2 2"0.5 4"0.5
20"1 7"0.5 16"0.5 15"2 11"1
2"0.5 10"1 28"0.8 23"2 18"3
a
See Table 5 for the composition of precipitates in layer-5.
3. Results and discussion
Fig. 1. (a) Oxidized layer on the surface of the specimen. (b) Dendritic precipitates. (c) Schematic view of different features in the oxidized layer.
Samples were examined under optical electron microscope and SEM. The results of metallography and phase analysis of a sample, heat treated in air, are discussed in detail and then supplemented with the findings of heat treatment in vacuum and nitrogen. Samples of maraging 350-grade steel heat-treated in air showed a thick oxide layer on the surface; see Fig. 1a. Below the oxidized surface layer, the oxygen attack was predominant at and near the grain boundary regions. Layers of varying microstructural features were observed from the surface towards the bulk. The microstructural features in different layers are more clearly visible at higher magnification in Fig. 1b and are summarized in the schematic in Fig. 1c. The variation in microstructural features accompanied the changes in composition. The change in the concentration of the alloying elements with depth, as determined by EDS (point analysis), is shown in Fig. 2. Based on microstructural observations and EDS results, the oxidized region can roughly be
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
35
Table 3 Phases in different layers Layer-1
Layer-2
Layer-3
Layer-4
Layer-5
dobs
Phase
dobs
Phase
dobs
Phase
dobs
Phase
2.92 2.68 2.52 2.07 1.67 1.60 1.47 1.45
Fe2O3 and CoFe2O4
2.94 2.70 2.52 2.09 2.04 1.84 1.70 1.61 1.48 1.45 1.09
M3O4ØTiO where MsFe,Co, Ni and Fe2O3
2.95 2.53 2.10 2.04 1.77 1.62 1.49 1.26
M3O4ØTiO where MsFe, Co, Ni and g (Fe, Ni)
3.00 2.55 2.05 1.78 1.63 1.50 1.26 1.07
g (Fe, Ni) and M3O4ØTiO where MsFe, Co, Ni
divided into five different layers. The bulk chemical composition of these layers is shown in Table 2 where deviation from the general composition of the material was observed. Although, deviation from bulk is not quite significant in the fifth layer, a Ti-rich phase having dendritic morphology is observed. The phase analyses Table 4 Results of Debye–Sherrer powder patterns of extracted precipitates Information from Ref. w6xa
Observed d-values
IyI0*
4.70 3.70 3.48 2.97 2.87 2.53 2.49 2.34 2.23 2.15 1.89 1.60 1.54 1.46 1.36 1.30 1.29 1.27 1.24 1.21 1.19 1.18 1.17 1.05
St. W W St. St. St. W M W Mq M W W W W W W W W W W W W W
Ti(1yx)CoxN xs0.3
Ti(1yx)NixN xs0.3
FeMoO4 4.704 3.712
2.903 2.55x
2.893 2.55x
2.989 2.89x 2.484 2.353 2.152
1.91x
1.91x
1.472, 1.451
1.472, 1.441
1.311
1.311
1.271, 1.261
1.271, 1.261
Phases discussed in Table 4
and EDS of the dendritic phase are given in Table 4 and Table 5, respectively. The outer most region of the oxidized sample is a delaminated scale. The outer and inner surfaces (layers1 and 2 in Table 2) of the scale are not similar. Exposed to atmosphere, layer-1 is bright and grainy. It is depleted in Mo, Ti and Ni but retained cobalt in addition to iron, which explains the presence of Fe2O3 and CoFe2O4 as the major phases. The d-values and the phases present in different layers, determined by XRD are summarized in Table 3. The underside of the scale, layer-2, has a black sooty appearance. In this region Ni and Mo, although higher than layer-1, remains depleted. Co content is significantly lower while Ti is markedly higher. The phases determined by XRD are M3O4ØTiO (MsFe, Co, Ni) and Fe2O3. The oxidation process also affects layer-3; the layer is revealed after polishing the sooty surface. It is highly enriched in Ni and Mo. The layer has bulky oxide precipitates, which are of the type M3O4ØTiO as in layer2. The matrix, however, is g(Fe, Ni). All the phases are confirmed by XRD. On polishing the layer-3, the other surface appears which have different features when observed under the optical microscope. We called this oxidized region as layer-4. This layer continues to be enriched in Ni and Mo but as a general trend, the composition is getting nearer to that of the unaffected matrix. The phase Table 5 Chemical composition of extracted precipitate mixture, wt.%
1.171 1.031
1.161 1.031
St.-strong; M-medium; W-weak. a Subscript following d-values represents peak intensity. * Fraction of peak density.
S. No.
Ni
Co
Mo
Ti
Fe
1 2 3 4
3 11 4 1
2 7 3 0
54 5 14 42
15 9 12 9
26 67 67 48
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
36
Fig. 3. Orientation of dendrites in two neighboring grains.
Fig. 4. Dendrites along the boundaries of the martensite plates.
analysis reveals gamma phase as in layer-3 and oxide particles are fewer and smaller. This layer (layer-4) is present in front of the dendritic colony (layer-5). The latter is the deepest portion of the sample that shows features affected by the oxidation process. The general EDS analysis of this region reveals a hump in Ti concentration, see Fig. 2. An intriguing feature of this layer is the presence of Ti-rich dendrites. As we approach this region from the surface, we encounter a region containing coarsened dendrites (upper portion of layer-5) followed by finer ones, see Fig. 1b. The color of coarser dendrites is gray while the finer ones are yellow. A consistent observation is a definite pattern and orientation of dendrites with in a colony in a grain, which is different from the neighboring one, Fig. 3. The dendrites grow preferentially along the straight parallel lines. The growth orientation is approximately 458 to the lath of the martensite, Fig. 4. The well-developed dendrite formed at 1250 8C is shown in Fig. 5.
plexes and are eliminated from the surface w8x but it is also observed that Ti is concentrated in layer-2 where it is engaged by the O2y ions in the form of M3O4 –TiO type oxide. Layer-5 is formed due to the interaction of the incoming oxygen and nitrogen ions with less noble metals—Ti and Mo. The pattern of nitride formation along the grain boundaries suggests that the source of inward diffusing nitrogen anions are the grain boundaries.
3.1. Oxidation process Growth of the oxidized layer proceeds with a typical pattern involving external and internal oxidation. Oxygen anions diffuse into the metal while metallic cations diffuse outwards. The faster outward diffusion creates vacancies at the interface of layers-2 and -3. Their coalescence forms voids resulting in the delamination of the layer. The scatter in the alloying elements in layers 1–4 is related to the differing diffusion rates of the alloying elements in the matrix on one hand and the variation in the affinity of cations with the incoming anions on the other hand. In layer-1, the high concentration of Co could be due to the high mobility of Co2q ions as compared to Ni2q ions, possibly through the preferred diffusion paths w7x. It is quite possible that Mo and Ti cations reaching the exposed surface form volatile com-
3.2. Dendrites of (Ti, M)N phase The dendrites (present in layer-5) were extracted by chemically dissolving the matrix in an acid solution. The extracted particles were subjected to phase analysis using Debye–Sherrer powder pattern technique and the results are summarized in Table 4. EDS of the mixture of extracted phases confirms that it contains Mo, Ti and Fe rich phases. The results confirm that mixture contains FeMoO4 and complex Ti(1yx)MxN, where xs0.3 and MsNi, Co, phases.
Fig. 5. Dendrites formed at 1250 8C=6 h in air.
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38 Table 6a Chemical composition of individual dendrites of layer-5 in polished sample, wt.%
Table 8 Width of the dendritic band in two steels Environment
Ni
Co
Mo
Ti
Fe
6 8 7
5 6 6
17 13 8
43 44 49
30 29 30
Table 6b Calculated composition of dendrites eliminating matrix effect, at.% Ni
Co
Mo
Ti
Fe
Stochiometric formula
Eliminated
2 1 2
13 11 6
73 84 84
12 4 8
Ti0.7(Mo,Fe,Co)0.3(N,O) Ti0.8(Mo,Fe,Co)0.2(N,O) Ti0.8(Mo,Fe,Co)0.2(N,O)
37
Air Nitrogen Vacuum
Width (mm) M-300
M-350
106"7 165"5 Nil
67"8 100"10 Nil
type precipitates along grain boundaries has been reported when maraging steels are heated to 1000 8C w10x. Oxidation experiments conducted on three 18 wt.% Ni maraging steels (i.e. maraging-250, maraging-300 and maraging-350) show noticeable decrease in the concentration of the dendritic phase in layer-5 with a decrease in Ti content of the steel, see Table 7.
Ti1yx(Mo,Fe,Co)x(N,O) where 0.2-x-0.3.
3.3. Heat treatment in vacuum and nitrogen Analysis of individual particles (dendrites) in layer-5 was conducted on the polished samples and the results are summarized in Table 6a. It can be seen from the table that dendrites are rich in titanium. An intrinsic problem with EDS is that the X-ray signals coming from an excited region is couple of times larger than the beam size and there is a possibility of some matrix effect. Assuming that the signal for Ni is coming only from the matrix, the corresponding concentrations of all the alloying elements are deducted and the calculated composition of the dendrites is summarized in Table 6b. The results confirm that the dendrites are rich in Ti and contain Mo, Fe and small amounts of Co. If we look at the results of Tables 4 and 6b together, we conclude that the phase of the type Ti1yx(Mo, Fe, Co)x(N, O) where 0.2-x-0.3 formed. The results are in conformity with the published information. It has been reported that nitrogen and oxygen are very stable in HCP a-titanium, occupying the octahedral interstices: the maximum solubility of these two elements are, respectively, 34 and 25 at.%. Higher concentrations of interstitials result in the formation of TiN and TiO, both golden yellow in color having melting points 2950 and 1737 8C, respectively. TiN has been reported to form in several steels containing titanium. It forms as idiomorphic, regular, yellowish crystals usually at the grain boundaries. The TiN precipitates often hold atoms like carbon and oxygen in solid solution w9x. Formation of M(C, N) Table 7 Concentration of dendrites in different steels (oxidized in air at 1300 8C for 6 h) Steel
wt.% Ti in steel
Area, percent dendrites
M-350 M-300 M-250
1.37 0.76 0.36
4.4"0.8 1.4"0.3 0.7"0.1
In order to confirm that the dendrites are basically oxy-nitrides or nitrides, experiments on steel grades M350, M-300 and M-250 (Table 1) were repeated in vacuum. All the three steels showed no oxide layer and no dendritic band. It was concluded that the diffusion of species in air (oxygen and nitrogen) played the key role in the formation of dendrites. Experiments in nitrogen atmosphere confirmed the formation of nitrides. The width of layer-5 is significantly broader in nitrogen as compared to air in both the steels having high concentration of titanium comparatively (Table 8). 4. Conclusions Maraging steel was heat treated at elevated temperatures in air, vacuum and nitrogen atmospheres. The oxidized surface consisted of layers of different oxide and nitride phases. The outer layers are oxides formed due to the interaction of outward diffusion of metallic cations while the formation of subsurface nitride phase was due to the inward diffusion of nitrogen and oxygen anions. The oxy-nitride phase has dendritic morphology and is of the type Ti1yx(Mo, Fe, Co)x(N, O) where 0.2-x-0.3. It forms as a result of solid-state transformation. The phase exhibits specific orientation relationship with the martensitic matrix. References w1x N.R. Comins, J.B. Clark (Eds.), Specialty Steels and Hard Materials, Pergamon Press, 1983, p. 35. w2x J.A. Malcolm, G.R. Purdy, Trans. Metall. Soc. AIME (1976) 1391. w3x S. Wilayat Husain, I. Qamar, M. Saeed Ahmed, Proceeding of the First International Conference on Phase Transformations, in: A. ul Haq, A. Tauqir, A.Q. Khan (Eds.), Dr A.Q. Khan Research Labs., Pakistan, 1996, p. 19. w4x M.S. Ahmed, S.W. Husain, I. Qamar, Proceeding of the Fifth International Symposium on Advanced Materials, in: M. Afzal
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
38
w5 x w6 x w7 x
Khan, A. ul Haq, K. Hussain, A.Q. Khan (Eds.), Dr A.Q. Khan Research Labs., Pakistan, 1997, p. 414. P.G. Shewmon, Trans. Metall. Soc. AIME (1965) 736. Powder Diffraction File—Inorganic Phases, compiled by the JCPDS, International Phases for diffraction data, USA, 1981. G.C. Wood, Oxidation of Metals and Alloys, Seminar of American Society for Metals, ASM, Metals Park, Ohio, October 17–18, 1970, pp. 194, 201–206.
w8x J.M. West (Ed.), Basic Corrosion and Oxidation, Wiley, NY, 1980, p. 185. w9x R. Kiessling, N. Lange (Eds.), Non-metallic Inclusions in Steel, Part II, The Institute of Materials, London, 1997, p. 88. w10x Maraging Steels Recent Developments and Applications, in: R.K. Wilson (Ed.), Proc. Symp. Held at TMS Annual Meeting, Arizona, January 25–26, 1988, p. 9.
Formation of solid-state dendrites in an alloy steel A. Nusair Khana,*, I. Salamb, A. Tauqirb a
LASMIS, Universite´ de Technologie de Troyes, BP 2060, 12 Rue de Marie Curie, Troyes, France b Metallurgy Division, G.P.O. Box No. 502, Rawalpindi, Pakistan Received 21 December 2002; accepted in revised form 9 May 2003
Abstract An interesting phenomenon of solid-state dendrite formation was observed during elevated temperature treatment of maraging steel in air. The steel was subjected to heat treatment at temperatures ranging from 900 to 1350 8C for 1–8 h. The study revealed different phases formed into the depth. A band containing dendrites was observed below the oxidized surface layer. Energy dispersive spectrometry and X-ray phase analysis confirmed that these dendrites were enriched in titanium and are of type Ti1yx(Mo, Fe, Co)x(N, O) where 0.2-x-0.3. The formation of dendrites during solid-state transformation is a rare phenomenon and has been reported to take place in specific cases. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Solid-state dendrites; Oxidation; Maraging steel; Diffusion of elements
1. Introduction Maraging steels are high strength low carbon steels where the strength is achieved by the precipitation of intermetallics. Usually, the steels are solution annealed at temperatures between 800 and 850 8C. The austenite transforms to martensite during cooling. It is convenient to mechanically process the material in annealed condition, where the hardness is 30 HRC–35 HRC. Hardening of these steels is achieved by aging at temperatures between 480 and 510 8C w1x. The alloying elements present in maraging steels generally lower the Ms temperature. However, cobalt up to ;8% has an opposite effect, which is useful in ensuring the formation of lath martensite in the presence of higher concentration of the other alloying elements i.e. Ni, Mo and Ti w1x. Maraging steel has a complex chemistry. Its production is quite involved due to the severe segregation of alloying elements. High temperature treatments are imperative for both forging operation and for subsequent homogenization. At elevated temperatures the formation of solid-state dendrites is observed which is the subject *Corresponding author. Tel.: q33-3-25-715831; fax: q33-3-25715676. E-mail address: [email protected] (A.N. Khan).
of this study. The formation of solid-state dendrites is a rare phenomenon, reported in few systems w2–5x. The article discusses formation of such dendrites in system of maraging steel. 2. Experimental Samples of three different maraging steels were subjected to study. The chemical composition was determined using energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM) and carbonysulfur analyzer. The results are given in Table 1. Samples of size 15=15=10 mm3 were prepared and were subjected to heat treatment process at temperatures ranging from 900 to 1350 8C in air for 1–8 h. The same treatment was carried out in a vacuum of 10y3 Torr and also in nitrogen atmosphere. X-ray diffraction (XRD) was carried out to characterize the products of reaction. After heat treatment, samples were sectioned and prepared for optical microscopy. The same samples were also analyzed using EDS to determine the chemical composition of different features formed during the exposure to elevated temperature. The solid-state dendrites were extracted chemically by the dissolution of samples in an aqua regia and then this extract was subjected to EDS and phases were confirmed by Debye–Sherrer technique.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00787-4
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
34 Table 1 Composition of the steel Steel grade
M-350 M-300 M-250
Element (wt.%) Ni
Co
Mo
Ti
Al
C
S
Fe
17.4"0.4 18.9"0.3 18.1"0.1
12.6"0.1 9.6"0.4 8.3"0.1
4.8"0.1 4.4"0.4 4.6"0.1
1.37"0.1 0.76"0.01 0.36"0.02
-0.1 -0.1 -0.1
0.02"0.01 0.02"0.1 0.02"0.01
0.005"0.002 0.005"0.002 0.005"0.002
Bal. Bal. Bal.
Fig. 2. Depth profile from surface towards matrix. Table 2 EDS analyses of different layers of M-350 steel, wt.% Region
Fe
Mo
Ti
Co
Ni
Layer-1 Layer-2 Layer-3 Layer-4 Layer-5a
78"2 74"2 44"0.5 54"5 63"3
0 1"0.5 9"0.8 6"0.8 4"0.5
0 8"1 3"0.2 2"0.5 4"0.5
20"1 7"0.5 16"0.5 15"2 11"1
2"0.5 10"1 28"0.8 23"2 18"3
a
See Table 5 for the composition of precipitates in layer-5.
3. Results and discussion
Fig. 1. (a) Oxidized layer on the surface of the specimen. (b) Dendritic precipitates. (c) Schematic view of different features in the oxidized layer.
Samples were examined under optical electron microscope and SEM. The results of metallography and phase analysis of a sample, heat treated in air, are discussed in detail and then supplemented with the findings of heat treatment in vacuum and nitrogen. Samples of maraging 350-grade steel heat-treated in air showed a thick oxide layer on the surface; see Fig. 1a. Below the oxidized surface layer, the oxygen attack was predominant at and near the grain boundary regions. Layers of varying microstructural features were observed from the surface towards the bulk. The microstructural features in different layers are more clearly visible at higher magnification in Fig. 1b and are summarized in the schematic in Fig. 1c. The variation in microstructural features accompanied the changes in composition. The change in the concentration of the alloying elements with depth, as determined by EDS (point analysis), is shown in Fig. 2. Based on microstructural observations and EDS results, the oxidized region can roughly be
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
35
Table 3 Phases in different layers Layer-1
Layer-2
Layer-3
Layer-4
Layer-5
dobs
Phase
dobs
Phase
dobs
Phase
dobs
Phase
2.92 2.68 2.52 2.07 1.67 1.60 1.47 1.45
Fe2O3 and CoFe2O4
2.94 2.70 2.52 2.09 2.04 1.84 1.70 1.61 1.48 1.45 1.09
M3O4ØTiO where MsFe,Co, Ni and Fe2O3
2.95 2.53 2.10 2.04 1.77 1.62 1.49 1.26
M3O4ØTiO where MsFe, Co, Ni and g (Fe, Ni)
3.00 2.55 2.05 1.78 1.63 1.50 1.26 1.07
g (Fe, Ni) and M3O4ØTiO where MsFe, Co, Ni
divided into five different layers. The bulk chemical composition of these layers is shown in Table 2 where deviation from the general composition of the material was observed. Although, deviation from bulk is not quite significant in the fifth layer, a Ti-rich phase having dendritic morphology is observed. The phase analyses Table 4 Results of Debye–Sherrer powder patterns of extracted precipitates Information from Ref. w6xa
Observed d-values
IyI0*
4.70 3.70 3.48 2.97 2.87 2.53 2.49 2.34 2.23 2.15 1.89 1.60 1.54 1.46 1.36 1.30 1.29 1.27 1.24 1.21 1.19 1.18 1.17 1.05
St. W W St. St. St. W M W Mq M W W W W W W W W W W W W W
Ti(1yx)CoxN xs0.3
Ti(1yx)NixN xs0.3
FeMoO4 4.704 3.712
2.903 2.55x
2.893 2.55x
2.989 2.89x 2.484 2.353 2.152
1.91x
1.91x
1.472, 1.451
1.472, 1.441
1.311
1.311
1.271, 1.261
1.271, 1.261
Phases discussed in Table 4
and EDS of the dendritic phase are given in Table 4 and Table 5, respectively. The outer most region of the oxidized sample is a delaminated scale. The outer and inner surfaces (layers1 and 2 in Table 2) of the scale are not similar. Exposed to atmosphere, layer-1 is bright and grainy. It is depleted in Mo, Ti and Ni but retained cobalt in addition to iron, which explains the presence of Fe2O3 and CoFe2O4 as the major phases. The d-values and the phases present in different layers, determined by XRD are summarized in Table 3. The underside of the scale, layer-2, has a black sooty appearance. In this region Ni and Mo, although higher than layer-1, remains depleted. Co content is significantly lower while Ti is markedly higher. The phases determined by XRD are M3O4ØTiO (MsFe, Co, Ni) and Fe2O3. The oxidation process also affects layer-3; the layer is revealed after polishing the sooty surface. It is highly enriched in Ni and Mo. The layer has bulky oxide precipitates, which are of the type M3O4ØTiO as in layer2. The matrix, however, is g(Fe, Ni). All the phases are confirmed by XRD. On polishing the layer-3, the other surface appears which have different features when observed under the optical microscope. We called this oxidized region as layer-4. This layer continues to be enriched in Ni and Mo but as a general trend, the composition is getting nearer to that of the unaffected matrix. The phase Table 5 Chemical composition of extracted precipitate mixture, wt.%
1.171 1.031
1.161 1.031
St.-strong; M-medium; W-weak. a Subscript following d-values represents peak intensity. * Fraction of peak density.
S. No.
Ni
Co
Mo
Ti
Fe
1 2 3 4
3 11 4 1
2 7 3 0
54 5 14 42
15 9 12 9
26 67 67 48
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38
36
Fig. 3. Orientation of dendrites in two neighboring grains.
Fig. 4. Dendrites along the boundaries of the martensite plates.
analysis reveals gamma phase as in layer-3 and oxide particles are fewer and smaller. This layer (layer-4) is present in front of the dendritic colony (layer-5). The latter is the deepest portion of the sample that shows features affected by the oxidation process. The general EDS analysis of this region reveals a hump in Ti concentration, see Fig. 2. An intriguing feature of this layer is the presence of Ti-rich dendrites. As we approach this region from the surface, we encounter a region containing coarsened dendrites (upper portion of layer-5) followed by finer ones, see Fig. 1b. The color of coarser dendrites is gray while the finer ones are yellow. A consistent observation is a definite pattern and orientation of dendrites with in a colony in a grain, which is different from the neighboring one, Fig. 3. The dendrites grow preferentially along the straight parallel lines. The growth orientation is approximately 458 to the lath of the martensite, Fig. 4. The well-developed dendrite formed at 1250 8C is shown in Fig. 5.
plexes and are eliminated from the surface w8x but it is also observed that Ti is concentrated in layer-2 where it is engaged by the O2y ions in the form of M3O4 –TiO type oxide. Layer-5 is formed due to the interaction of the incoming oxygen and nitrogen ions with less noble metals—Ti and Mo. The pattern of nitride formation along the grain boundaries suggests that the source of inward diffusing nitrogen anions are the grain boundaries.
3.1. Oxidation process Growth of the oxidized layer proceeds with a typical pattern involving external and internal oxidation. Oxygen anions diffuse into the metal while metallic cations diffuse outwards. The faster outward diffusion creates vacancies at the interface of layers-2 and -3. Their coalescence forms voids resulting in the delamination of the layer. The scatter in the alloying elements in layers 1–4 is related to the differing diffusion rates of the alloying elements in the matrix on one hand and the variation in the affinity of cations with the incoming anions on the other hand. In layer-1, the high concentration of Co could be due to the high mobility of Co2q ions as compared to Ni2q ions, possibly through the preferred diffusion paths w7x. It is quite possible that Mo and Ti cations reaching the exposed surface form volatile com-
3.2. Dendrites of (Ti, M)N phase The dendrites (present in layer-5) were extracted by chemically dissolving the matrix in an acid solution. The extracted particles were subjected to phase analysis using Debye–Sherrer powder pattern technique and the results are summarized in Table 4. EDS of the mixture of extracted phases confirms that it contains Mo, Ti and Fe rich phases. The results confirm that mixture contains FeMoO4 and complex Ti(1yx)MxN, where xs0.3 and MsNi, Co, phases.
Fig. 5. Dendrites formed at 1250 8C=6 h in air.
A.N. Khan et al. / Surface and Coatings Technology 179 (2004) 33–38 Table 6a Chemical composition of individual dendrites of layer-5 in polished sample, wt.%
Table 8 Width of the dendritic band in two steels Environment
Ni
Co
Mo
Ti
Fe
6 8 7
5 6 6
17 13 8
43 44 49
30 29 30
Table 6b Calculated composition of dendrites eliminating matrix effect, at.% Ni
Co
Mo
Ti
Fe
Stochiometric formula
Eliminated
2 1 2
13 11 6
73 84 84
12 4 8
Ti0.7(Mo,Fe,Co)0.3(N,O) Ti0.8(Mo,Fe,Co)0.2(N,O) Ti0.8(Mo,Fe,Co)0.2(N,O)
37
Air Nitrogen Vacuum
Width (mm) M-300
M-350
106"7 165"5 Nil
67"8 100"10 Nil
type precipitates along grain boundaries has been reported when maraging steels are heated to 1000 8C w10x. Oxidation experiments conducted on three 18 wt.% Ni maraging steels (i.e. maraging-250, maraging-300 and maraging-350) show noticeable decrease in the concentration of the dendritic phase in layer-5 with a decrease in Ti content of the steel, see Table 7.
Ti1yx(Mo,Fe,Co)x(N,O) where 0.2-x-0.3.
3.3. Heat treatment in vacuum and nitrogen Analysis of individual particles (dendrites) in layer-5 was conducted on the polished samples and the results are summarized in Table 6a. It can be seen from the table that dendrites are rich in titanium. An intrinsic problem with EDS is that the X-ray signals coming from an excited region is couple of times larger than the beam size and there is a possibility of some matrix effect. Assuming that the signal for Ni is coming only from the matrix, the corresponding concentrations of all the alloying elements are deducted and the calculated composition of the dendrites is summarized in Table 6b. The results confirm that the dendrites are rich in Ti and contain Mo, Fe and small amounts of Co. If we look at the results of Tables 4 and 6b together, we conclude that the phase of the type Ti1yx(Mo, Fe, Co)x(N, O) where 0.2-x-0.3 formed. The results are in conformity with the published information. It has been reported that nitrogen and oxygen are very stable in HCP a-titanium, occupying the octahedral interstices: the maximum solubility of these two elements are, respectively, 34 and 25 at.%. Higher concentrations of interstitials result in the formation of TiN and TiO, both golden yellow in color having melting points 2950 and 1737 8C, respectively. TiN has been reported to form in several steels containing titanium. It forms as idiomorphic, regular, yellowish crystals usually at the grain boundaries. The TiN precipitates often hold atoms like carbon and oxygen in solid solution w9x. Formation of M(C, N) Table 7 Concentration of dendrites in different steels (oxidized in air at 1300 8C for 6 h) Steel
wt.% Ti in steel
Area, percent dendrites
M-350 M-300 M-250
1.37 0.76 0.36
4.4"0.8 1.4"0.3 0.7"0.1
In order to confirm that the dendrites are basically oxy-nitrides or nitrides, experiments on steel grades M350, M-300 and M-250 (Table 1) were repeated in vacuum. All the three steels showed no oxide layer and no dendritic band. It was concluded that the diffusion of species in air (oxygen and nitrogen) played the key role in the formation of dendrites. Experiments in nitrogen atmosphere confirmed the formation of nitrides. The width of layer-5 is significantly broader in nitrogen as compared to air in both the steels having high concentration of titanium comparatively (Table 8). 4. Conclusions Maraging steel was heat treated at elevated temperatures in air, vacuum and nitrogen atmospheres. The oxidized surface consisted of layers of different oxide and nitride phases. The outer layers are oxides formed due to the interaction of outward diffusion of metallic cations while the formation of subsurface nitride phase was due to the inward diffusion of nitrogen and oxygen anions. The oxy-nitride phase has dendritic morphology and is of the type Ti1yx(Mo, Fe, Co)x(N, O) where 0.2-x-0.3. It forms as a result of solid-state transformation. The phase exhibits specific orientation relationship with the martensitic matrix. References w1x N.R. Comins, J.B. Clark (Eds.), Specialty Steels and Hard Materials, Pergamon Press, 1983, p. 35. w2x J.A. Malcolm, G.R. Purdy, Trans. Metall. Soc. AIME (1976) 1391. w3x S. Wilayat Husain, I. Qamar, M. Saeed Ahmed, Proceeding of the First International Conference on Phase Transformations, in: A. ul Haq, A. Tauqir, A.Q. Khan (Eds.), Dr A.Q. Khan Research Labs., Pakistan, 1996, p. 19. w4x M.S. Ahmed, S.W. Husain, I. Qamar, Proceeding of the Fifth International Symposium on Advanced Materials, in: M. Afzal
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38
w5 x w6 x w7 x
Khan, A. ul Haq, K. Hussain, A.Q. Khan (Eds.), Dr A.Q. Khan Research Labs., Pakistan, 1997, p. 414. P.G. Shewmon, Trans. Metall. Soc. AIME (1965) 736. Powder Diffraction File—Inorganic Phases, compiled by the JCPDS, International Phases for diffraction data, USA, 1981. G.C. Wood, Oxidation of Metals and Alloys, Seminar of American Society for Metals, ASM, Metals Park, Ohio, October 17–18, 1970, pp. 194, 201–206.
w8x J.M. West (Ed.), Basic Corrosion and Oxidation, Wiley, NY, 1980, p. 185. w9x R. Kiessling, N. Lange (Eds.), Non-metallic Inclusions in Steel, Part II, The Institute of Materials, London, 1997, p. 88. w10x Maraging Steels Recent Developments and Applications, in: R.K. Wilson (Ed.), Proc. Symp. Held at TMS Annual Meeting, Arizona, January 25–26, 1988, p. 9.