3.1 Microstructural investigation
Fig. 1 shows the Optical Microscopy and SEM images of the microstructure of martensitic and austenitic substrate.
Fig. 1
SEM and OM images of the (a) martensitic and (b) austenitic substrates
Fig. 2 shows SEM and OM images of the interface between substrates (martensitic and Austenitic stainless steel) and Coating (Stellite cladding).
Fig. 2
The interface between coating and (a) martensitic and (b) austenitic substrates
It is clear that cellular growth initiates at the interface. With increasing the distance from the interface and changing the thermal gradient (G) as same as the solidification rate (R), constitutional super-cooling will be increased. It results in conversion of cellular growth to dendritic growth. It is worth to be mentioned that planar growth cannot be observed in this figure which can be ascribed to its low extension and limitation in magnification of optical microscope.
Fig. 3 (a) presents the SEM image of the coating. The higher magnification SEM image of the area A is also depicted in
Fig. 3 (a). With the aid of EDS analysis (
Fig. 3b), it was found that the dendritic region consisted of Cr, C and Co elements.
Fig. 3
(a) SEM image of the stellite coating structure and (b) The EDS result of the interdendritic region
Stellite’s microstructure is composed of a Co-rich, face- centered cubic α phase and inter-dendritic carbides
11), the Co-rich matrix with carbide particles are seen at grain boundaries.
According to the phase diagram of Stellite
12), the first stage of solidification is formation of a Co solid solution with a cellular and/or dendritic form. After that, the melt chemical composition located in the dendrite spacing becomes carbon and chromium rich and approaches eutectic composition with a laminated structure. Finally, with simultaneous solidification of the eutectic solid solution and eutectic carbides, the remained melt converts to solid.
The first stage of solidification is formation of a Co solid solution with a cellular and/or dendritic. After that, the melt chemical composition located in the dendrite spacing becomes carbon and chromium rich and approaches eutectic composition with a laminated structure. Finally, with simultaneous solidification of the eutectic solid solution and eutectic carbides, the remained melt converts to solid.
The EDS analysis was used to determine the chemical composition of the present carbides in the inter-dendritic region.
Fig. 3(b) depicts the EDS analysis results from the inter-dendritic region of the coating. Observing a significant amount of chromium in the results indicates that most of the carbides are chromium carbides. Due to using two substrate metals with different chemical compositions, distributions of nickel, cobalt, iron and chromium in the metal substrate and the cladding were investigated through line scan analysis.
Fig. 4 shows backscatter image of Stellite coating. Chromium rich carbides could be observable between dendritic arms (light gray).
Fig. 4
Backscatter image of coating
Results of metallographic micrographs show that the microstructure consists of Cr-rich matrix (Co-γ phase) and chromium carbide-rich eutectic. As it is obvious, presence of Cr and Co in the matrix is clear. The composition of the dark phases consisted of chromium carbides.
Fig. 5 shows SEM image of Stellite, including primary dendrites with dendritic structure.
Fig. 5
For more investigation, The EDS analysis of A, B and C points of
Fig. 5 were carried out. Results of The EDS analysis have been shown in
Fig. 6.
Fig. 6
(a), (b) and (c) spot analysis of A, B, C in
Fig. 5
Since the sensitivity of EDS is too poor to allow the detection of light elements like carbon, so, results are presented regardless of percentage of carbon.
According to
Fig. 6, Spot A is the Cobalt-based matrix containing Cr and Fe, Moreover Spot B is the black dendritic phase containing Cr and C which is almost the composition of chromium carbide (Cr23C6 or Cr7C3). Furthermore there are considerable amounts of carbon and chromium in white precipitations (spot C). Considering results, it can be concluded that specific phases in this figures are carbides type of Cr
mC n and CoC
m that correspond to results of other researchers
2,10,13).
In
Fig. 7 with the aid of line scan EDS analysis the distributions of the elements in the (a) austenitic and (c) martensitic substrates is shown in the
Fig. 7(b) and
7(d), respectively.
Fig. 7
The line scan region of the (a) austenitic and (c) martensitic substrates, (b) and (d) the EDS analysis of austenitic and martensitic substrates
It is found that in both substrates, from the interface to the coating, the concentrations of chromium and cobalt increase but the concentration of iron decreases. Also, the iron content in the coating for both substrates increases. According to the literature, the increased amount of iron in the coating increases the stacking fault energy in the cobalt lattice leading to decreased hardness, corrosion and erosion resistance of the coating
7).
Fig. 8 illustrates XRD patterns of the Stellite cladding for both substrates. In both investigated samples, the main constituent phase of the coating is face centered cubic (FCC) cobalt and the metastable phases such as Cr
7C
3 and Cr
23C
6 are t visible in the analysis result.
Fig. 8
The XRD patterns of the stellite cladding on the (a) austenitic substrate and (b) martensitic substrates
Fig. 9 depicts micro-hardness vs. the distance from the coating surface curves for (a) martensitic steel and (b) austenitic steel.
Fig. 9
The variation of the microhardness vs. the distance from the surface to the base metal: (a) martensitic and (b) austenitic
In both samples, the coating hardness is higher than the base metal hardness. Amounts of hardness changes from 200 vickers for substrates up to 600 vickers for coating, as it is illustrated in
Fig. 9. Also, the hardness increases from the interface to the coating. As the EDS line scan analysis results show, the iron concentration in the close vicinity of the interface increases. With raising the iron content, the concentrations of chromium and cobalt decrease. Therefore, reduction of chromium carbide and accordingly reduction of solid solution hardness are expectable
7). In other words, since the grains in the surface are finer than those in the interface, higher hardness in the surface grains is expectable. Because the substrate-coating in the interface is subjected to higher temperature during welding, the cooling rate is slower and the grain growth conditions in the interface are more appropriate compared with the surface
6).
3.2 Erosion and combined erosion and corrosion tests
Fig. 10 shows the results of the erosion and combined erosion and corrosion tests. In this figure the total weight loss (TWL) measured after exposure to the impinging jet at 12 m/s is shown for Coated Specimens. For comparison, stainless steels substrates 410 and 316 are also included. As can be seen, in both investigated samples, formation of the Stellite cladding has resulted in enhanced erosion resistance compared with that of the substrate specimens. The erosion resistance of coated specimens are 3 times better than Raw materials.
Fig. 10
The results of erosion and combined erosion-corrosion tests
This enhancement can be ascribed to the presence of the hard carbide phases in the matrix as well as proper bonding of coating to the substrate metal. Comparing the results of the erosion and erosion- corrosion tests, it can be conclude that the effect of the corrosive medium on the increased rate of erosion in all the samples is significant. For example, the weight loss amount of the Stellite cladding on the austenitic stainless and martensitic steel in the corrosive medium increased from 3.6mg/h to 9.1mg/h and 4.4 to 10.2 mg/h respectively. Although the martensitic stainless steel is of higher erosion resistance than austenitic stainless steel due to its higher hardness, under combined conditions the extent of erosion in the martensitic substrate is greater owing to inherent higher corrosion resistance of austenitic stainless steel. These results are in good agreement with those reported by Lopez et al.
14). They showed that AISI304 stainless steel is of higher corrosion/erosion resistance than AISI420 stainless steel and concluded that inherent corrosion resistance is of a more important role in investigation of corrosion/erosion behavior than hardness. Comparing the erosion extent on both substrates shows that the formed Stellite cladding on AISI stainless steel has a higher erosion behavior than austenitic and martensitic stainless steel. This can be attributed to the distribution of the alloying elements in the coating. Owing to the presence of more chromium in the formed coating on the austenitic substrate more amounts of chromium carbide in the coating is expected. Nonetheless, due to more iron content in the martensitic steel than that in the austenitic steel, the iron content of the coating in the martensitic substrate is greater. Therefore, lower erosion resistance of the coating on the martensitic substrate is expectable.
Fig. 11 shows SEM images of erosion in the austenitic and the martensitic substrates.
Fig. 11
SEM images of erosion in the (a) austenitic and (b) martensitic substrates
In both surfaces, the plastic deformations are observed in the form of parallel lines, from which, one can deduce that the martensitic steel exhibit ductile behavior despite higher hardness than austenitic steel. Lopez et al.
14) reported a ductile behavior against erosion in AISI420 stainless steel by conducting corrosion/erosion test. The relevant images will be presented in the next section. Also B.K. Sreedharinet al.
15) showed that in 316L stainless steel the accumulation of slip bands results in initiation of micro cracks. Plastic deformation then results in enlargement of the micro crack s and void formation. The adjacent voids coalesce leading to material removal
3.3 The effect of angle of impingement and deter- mining the erosion mechanism
To investigate the effect of angle of impingement, the erosion test was carried out at 30 degree, 60 degree and 90degree on the substrates and Stellite cladding in which the results are presented in
Fig. 12.
Fig. 12
The results of erosion test on the substrates and on Stellite coating at different angle of impingements
It is found that in uncoated substrates, the highest weight losses observed at 60 degree, also in both coated samples, the highest erosion rate is observed at 90 degree. Hence, it can be deduced that the Stellite cladding behavior is in agreement with the erosion theory of materials
6).This feature may be associated with the relatively complex microstructures of Stellite in that cast Stellite 6 has a ductile matrix but an extensive network of brittle chromium carbides, which may be exerting a significant influence on the erosion-corrosion performance
2).
Fig. 13 shows SEM images of erosion of the coating surface at the angle of impingements of (a) 30, (b) 60 and (c) 90 degrees.
Fig. 13
SEM images of the erosion of the stellite coating at various angle of impingements: (a) 30, (b) 60 and (c) 90 degree
In the Stellite’s coating damage is initiated at the inter-face between the matrix and hard carbide locations
15). At low angle of impingements, due to impact of abrasive particles with the surface, chip is formed which results in the formation of a lip with a plastic deformation. In the following, owing to impact of abrasive particles, the chips are broken and unbound from the surface. At high angle of impingements, first, lip is formed owing to impact of abrasive particles and in the following; crater is formed due to impact with the other abrasive particles. The formed lip on the surface is extended due to the next impacts resulting in thicker craters. Then, the extended lips are removed from the surface in the form of small planes
2,6).