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Fracture behavior of laser-clad joint of Stellite 21 on AISI 316L stainless steel
Materials Science and Engineering A 527 (2010) 3748–3756
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Fracture behavior of laser-clad joint of Stellite 21 on AISI 316L stainless steel P. Ganesh a,∗ , A. Moitra c , Pragya Tiwari b , S. Sathyanarayanan c , Harish Kumar a , S.K. Rai b , Rakesh Kaul a , C.P. Paul a , R.C. Prasad d , L.M. Kukreja a a
Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh 452013, India Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh 452013, India c Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, TN, India d Metallurgy and Materials Science Dept., IIT Bombay, Mumbai 400 076, Maharashtra, India b
a r t i c l e
i n f o
Article history: Received 11 December 2009 Received in revised form 2 February 2010 Accepted 4 March 2010
Keywords: Laser cladding Graded cladding Clad interface Stellite Stainless steel Fatigue Impact test
a b s t r a c t Present paper describes microstructural and mechanical characterization of laser-clad composite joint made of Stellite 21 and type 316L stainless steel (SS). The study has been broadly performed on two kinds of laser-clad specimens viz. (i) involving direct deposition of Stellite 21 on SS and (ii) involving gradient in chemical composition across substrate/clad interface. Both kinds of specimens exhibited nearly similar tensile strength, which was higher than that of SS in the interface region. Inter-dendritic carbides provided low energy fracture path in laser-clad deposits of Stellite 21. Both direct and graded interfaces of laser-clad specimens exhibited superior fatigue strength than the SS substrate. Instrumented impact testing brought out distinct difference in the mode of crack propagation across direct and the graded clad specimens. In contrast to initiation-controlled brittle crack propagation across the interface region in “direct clad” specimens, crack propagation across “graded interface” was marked with significant plastic deformation.
1. Introduction Cobalt-base hard-facing alloys (Stellites), due to their outstanding resistance against high temperature oxidation and wear, find wide applications in turbine blades, vanes, hot die applications, components in combustion or exhaust systems, valve trim for petrochemical and power generation [1,2]. Stellite hard facing of austenitic stainless steel is used in fast breeder reactors to obtain high galling resistance between stainless steel surfaces exposed to high temperature (about 823 K) sodium coolant with very low level of oxygen [3,4]. High power lasers, because of their ability to deposit lowdilution clad deposits with controlled heat input and associated distortion, are emerging as attractive tools for hard-facing applications [5,6]. Integrity of substrate/clad interface of laser-clad deposit is very important for satisfactory performance of the hard-faced components. During laser cladding, large mismatch in thermo-physical properties between substrate and clad mate-
∗ Corresponding author at: Department of Atomic Energy, Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Block-B, RRCAT, P.O. CAT, Indore, Madhya Pradesh 452013, India. Tel.: +91 9827631359; fax: +91 7312488380. E-mail addresses: ganesh [email protected], [email protected] (P. Ganesh). 0921-5093/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.msea.2010.03.017
© 2010 Published by Elsevier B.V.
rial often results in the development of residual stresses at substrate/clad interface, causing cracking of clad deposits. Preheating/post-annealing can be used to relieve residual tensile stresses and to prevent cracking of laser hard-faced components [5,7]. Substrate/clad interfaces of laser-clad deposits are usually associated with abrupt transition in chemical composition and associated physical and mechanical properties. However, introduction of a compositionally graded substrate/clad interface, involving gradual transition in thermo-physical/mechanical properties, helps in reducing undesirable stress concentration effect. Kaul et al. have reported superior cracking resistance of graded Stellite 6 deposits on austenitic stainless steel (SS) under thermal cycling conditions [8,9]. When a hard-faced component is subjected to service conditions, involving fatigue or thermal cycling, nature of substrate/clad interface plays a vital role in determining overall performance of the component and hence, characterization of substrate/clad interface is essential. Published literature on laser cladding is largely focused on microstructural analysis and associated tribological behavior [10–16]. There is no published reference of a study on mechanical property characterization (tensile, fatigue and impact) of substrate/clad interface of laser-clad deposits. Present work is undertaken with the objective of investigating metallurgical characteristics of substrate/clad interface of Stellite-clad austenitic SS specimens, with specific emphasis on strength and fatigue performance. The work
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involves study of fracture behavior of direct and graded clad specimens. 2. Experimental 2.1. Laser cladding Laser cladding experiment was carried out with an indigenously developed 3.5 kW CW CO2 laser [17,18]. The experimental set-up consisted of a laser system, integrated with a beam delivery system, co-axial nozzle, powder feeder and a computerized-numerically controlled workstation. Laser beam, emanating out of the laser system, was focused with the help of a 125 mm focal length Zinc Selenide lens, housed in a water-cooled copper nozzle. Laser cladding process involved scanning the surface of the type 316L SS substrate with a defocused laser beam of 3 mm diameter along with simultaneous injection of alloy powder (particle size = 45–105 m) into laser-interaction region through a co-axial nozzle with argon as carrier gas. Large surface area was covered by depositing overlapping clad tracks. On the other hand, deposition of thicker clad layer involved layer-by-layer deposition. For the deposition of graded overlays, chemical composition of clad layers was controlled by mixing powders of Stellite 21 and AISI 316L SS in predetermined ratios. Graded overlay of three layers was formed by cladding with premixed powders of Stellite 21 and type 316L SS in the ratios of 30:70, 70:30 and 100:0, respectively. In the subsequent part of the paper, SS specimens clad with Stellite 21 deposits and graded Stellite deposits are referred as “direct clad” and “graded clad” specimens, respectively. Table 1 presents chemical compositions of Stellite 21 and 316L SS powders used for laser cladding while experimental parameters used for the study are summarized in Table 2. Experimental parameters were selected based on an earlier experimental study carried out in authors’ laboratory [5,8,19]. Laser-clad specimens were characterized by optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), tensile test, rotating bending fatigue (RBF) test and instrumented Charpy impact (CI) test. 2.2. Fabrication of tensile test specimens for interface strength characterization Fabrication methodology adopted for the preparation of tensile test specimens (refer Fig. 1) involved (i) laser cladding of Stellite 21 on the transverse cross-section of 12.5 mm diameter type 316L SS rods (with and without grading) to obtain total deposit thickness of about 12 mm, to form laser-clad SS rod (ii) gas tungsten arc welding (GTAW) of another SS rod to the Stellite end of laser-clad SS rod, followed by (iii) machining of the GTA welded laser-clad SS rod to fabricate tensile test specimens. In the resultant test specimen, Stellite 21/type 316L SS interface fell almost at the center of 10 mm long reduced section of 8 mm diameter. Two types of specimens were prepared viz. “direct clad” and “graded clad”. In addition to specimens with smooth gauge length section, notched ten-
Fig. 1. Fabrication methodology adopted for the preparation of tensile test specimens.
sile test specimens (notch depth = 1 mm; notch angle = 90◦ ) were also prepared with the notch at (i) substrate/clad interface and (ii) Stellite-clad region. Although these were not standard specimens, the intent of present specimen design was to evaluate tensile strength of substrate/clad interface and that of Stellite-clad zone. Tensile tests were conducted on a 100 kN universal testing machine under stroke control mode. Tensile tests were carried out with the objective of evaluating tensile strength of direct and graded substrate/clad interface. Therefore, tensile tests were done with the interface normal to the tensile axis. 2.3. Fabrication of rotating bending (RBF) fatigue test specimens The methodology adopted for the fabrication of RBF test specimen involved (i) machining out central part of 226 mm long type 316L SS rod (in as received condition of cold work) with a diameter of 12.5 mm diameter (refer Fig. 2a), (ii) filling up resultant cavity with Stellite 21 powder by laser cladding, followed by (iii) final machining to obtain fatigue test specimen (refer Fig. 2b). Transverse cross-section of parallel length of the test specimen carried laser-deposited Stellite 21 and type 316L SS in the two halves with substrate/clad interface at the center running along the length of parallel region (refer Fig. 2c). During RBF testing of such composite specimen, each part of the specimen viz. substrate, substrate/clad interface and clad, is subjected to identical sinusoidal loading while stress remains constant along the length of the parallel region. Gauge section of all test specimens were polished to obtain surface roughness (Ra) of 0.2–0.5 m. Out of 24 numbers of RBF test specimens so fabricated, 12 each belonged to “direct clad” and “graded clad” types.
Table 1 Chemical composition (weight %) of the powders used for laser cladding. Material
C
Cr
Ni
Mn
Si
Mo
Fe
Co
P
S
SS 316L St-21
0.025 0.26
18.6 26.3
12.3 2.8
1 0.65
0.5 1.88
2 5.53
Bal 1.4
– Bal
0.03 –
0.02 –
Table 2 Experimental parameters for laser cladding. Laser power
Beam diameter
Powder feed rate
Scan speed
Carrier gas flow rate
Overlap between successive clad tracks
2.8–3.0 kW
3 mm
6–7 g/min
6.7–8.3 mm/s
12 lpm
50%
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Fig. 2. Sequence of stages in the fabrication of rotating bending fatigue specimens. (A) Stainless steel rods with machined central region for laser deposition, (B) specimens after laser deposition and final machining, (C) schematic illustration of the cross-section of gauge length portion.
Fig. 3. Sequence of stages in the fabrication of Charpy impact specimens. (A) 316L stainless steel block with machined central region for laser deposition and (B) stainless steel block after laser cladding and machining with inset showing final machined specimen.
Fatigue endurance limits of Co–Cr–Mo alloys (Stellites) and type 316L SS are 380–450 MPa and 270 MPa, respectively [20]. Since fatigue strength and location of failure of a multi-material specimen is determined by the material of lower endurance limit, the failure of the composite specimen was likely to take place either in wrought type 316L SS or at substrate/clad interface with unknown fatigue endurance limit. Also the residual stresses at the substrate/clad interface or in the clad overlay may contribute to fatigue-damage initiation. Therefore, in order to determine which of these zones is more prone to fatigue failure, composite specimens were tested both below and above the endurance limit of type 316L SS. RBF tests were conducted at room temperature in the stress range of 200–300 MPa with at least three specimens of each type tested at a particular stress level.
imen was machined in such a way that fracture would initiate in the SS region and propagate through the interface into Stellite 21 clad region. These composite impact specimens carried about 3 mm-thick laser-clad deposit of SS (after V groove machining) and about 5 mm-thick Stellite 21 deposit (with and without grading). Instrumented impact testing of laser-clad bimetallic composite specimens was carried out with a Tinus-Olsen impact testing machine with instrumented striker with an energy range of 0–358 J. The objective of impact test was to study crack propagation behavior across direct and compositionally graded substrate/clad interface. Hence the orientation of the crack propagation in the case of impact test samples was normal to the substrate/clad interface. 3. Results
2.4. Fabrication of instrumented Charpy impact test samples 3.1. Microstructural analysis The procedure adopted for the fabrication of test specimens included (i) machining a V groove in the central part of a block of type 316 L SS block (refer Fig. 3a), (ii) filling up of resultant groove with 316L SS and Stellite 21 by laser cladding, followed by (iii) machining to obtain standard Charpy impact specimen, as per ASTM E-23 (refer Fig. 3b). The notch in the impact spec-
Metallographic examination of longitudinal and transverse cross-sections of laser-clad specimens did not reveal any defects. Cross-sections of “direct clad” and graded clad” specimens exhibited distinct difference in etching contrast. “Direct clad” deposits of Stellite were associated with uniform etching contrast across its
Fig. 4. Longitudinal cross-sections of (A) “direct clad” and (B) “graded clad” specimens. Partially melted zone (PMZ) at the interface of “direct clad” specimen is marked with arrows.
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Fig. 6. Microstructure of Stellite 21 deposit. Carbides are marked with arrows.
Fig. 5. SEM-EDS concentration profiles of Iron (Fe), Cobalt (Co), Chromium (Cr) and Molybdenum (Mo) across substrate/clad interface. Top: Direct clad specimen, EDS line scan length: 1.4 mm; bottom: graded clad specimen, EDS line scan length: 5 mm. SS: Stainless steel region; St21: Stellite 21 clad region.
thickness, whereas “graded clad” specimen exhibited gradual transition in etching contrast from light to dark along the direction of build up. Laser-clad deposits were characterized by fine columnar grains growing along the direction of build up. Interfaces between successive layers of laser-clad deposits were marked with clear signs of epitaxiality.
Fig. 4 compares microstructures of substrate/clad interface regions of “direct clad” and “graded clad” specimens. In contrast to sharp substrate/clad interface of “direct clad” specimen, “graded clad” specimens were marked with diffused interface between substrate and clad regions. Sharp substrate/clad interface in “direct clad” specimen was associated with a 50–60 m thick partially melted zone (PMZ) in the substrate shown in Fig. 4A marked with arrows. In contrast, diffused interface of “graded clad” specimen was characterized by a gradual transition in microstructure from wrought equi-axed grain to columnar dendrites (refer Fig. 4B). SEM-EDS analysis, carried out on the transverse cross-sections of laser-clad specimens, revealed sharp transition in chemical composition (particularly in terms of the concentrations of Fe, Co and Cr) across substrate/clad interface of “direct clad” specimen whereas, more gradual transition in chemical composition was recorded in “graded clad”
Fig. 7. SEM-EDS concentration profiles of Cobalt (Co), Chromium (Cr), Molybdenum (Mo) and Carbon across two carbide particles, shown in the SEM photomicrograph given below.
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P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 3748–3756 Table 3 Results of tensile tests.
Fig. 8. Micro-hardness profiles across substrate/clad interface of “direct clad” and “graded clad” specimens.
specimen. Fig. 5 compares EDS concentration profiles of Fe, Co, Cr and Mo across substrate/clad interface of “direct clad” and “graded clad” specimens. Both kinds of specimens exhibited cast microstructure involving fine columnar dendrites with dark carbide particles at inter-dendritic/inter-cellular boundaries as well as on the grain boundaries as shown in Fig. 6. EDS line scans across second phase particles present at inter-cellular boundaries revealed that the particles were relatively depleted of Co and enriched with Mo and Cr with respect to core of the dendrite, as shown in Fig. 7. Due to low atomic number of carbon, its EDS line scan did not show corresponding rise in counts at the particle site. Micro-hardness measurements, carried out on the 316L SS substrate yielded a value in the range of 230–260 VHN, which is higher than the generally expected values for the 316L SS due to the cold worked conditions of the substrate used. The “direct clad” specimen exhibited a sharp transition in micro-hardness to 475–500 VHN in laser-deposited Stellite 21 region. Fig. 8 compares micro-hardness profiles across transverse cross-sections of “direct clad” and “graded clad” specimens. In “direct clad” specimens, complete transition in micro-hardness was realized at a distance of 600 m from substrate/clad interface. On the other hand, “graded clad” specimen exhibited a gradual rise in micro-hardness across substrate/clad interface. X-ray diffraction was carried out to confirm that no new phases are formed as a result of laser cladding. XRD conducted on laserdeposited specimens of different orientations with CuK␣ radiation ( = 1.54 Å) confirms that there are no additional peaks of any new phase and laser-deposited Stellite 21 has face centered cubic (FCC) phase, as exhibited by its powder. XRD of the interface region also revealed that same FCC phase is retained and no new phase is formed. An interesting feature brought out by XRD is strong texturing effect in laser-deposited Stellite 21, with respect to its randomly textured powder. Top surface of laser-clad deposit exhibited preferred (2 0 0) orientation while lateral surface exhibited (1 1 1) texture.
Specimen
Tensile strength (MPa)
Failure location
Direct clad (smooth gauge section) Graded clad (smooth gauge section) Specimen with notch in laser-deposited Stellite 21 Direct clad (notch at substrate/clad interface) Graded clad (notch at substrate/clad interface)
600–630
Wrought SS
600–630
Wrought SS
950, 955, 968
Stellite 21 (clad)
717, 748, 754
Substrate/clad interface Graded substrate/clad interface
679, 706
higher UTS value of 950–968 MPa. Both “direct clad” and “graded clad” specimens with the notch at substrate/clad interface failed at an intermediate stress value. Interface tensile strengths of “direct clad” and “grade clad specimens” were found to be 717–754 MPa and 679–706 MPa, respectively. SEM fractographic examination of “direct clad” and “graded clad” specimens, with smooth gauge length section, exhibited typical dimpled appearance, indicating extensive plastic deformation before final fracture. On the other hand, fracture surface of test specimens with notch in Stellite-clad region did not carry signatures of associated plastic deformation. Fracture surface exhibited columnar dendrites with different orientations, indicating crack propagation along inter-dendritic boundaries as shown in Fig. 9. Fracture surface of notched “direct clad” specimens (notch at substrate/clad interface) exhibited regions associated with plastic deformation (dimpled fracture morphology) and brittle fracture along inter-dendritic boundaries (refer Fig. 10). These two kinds of regions were randomly distributed over the fracture surface, indicating that the fracture path was confined to the vicinity of substrate/clad interface. The part of the fracture surface, exhibiting dimpled appearance was rich in Fe while the other part with dendritic appearance was found to be rich in Co. On the other hand, fracture surface of notched “graded clad” specimen (notch at substrate/clad interface) carried signatures of plastic deformation along with cleavage-type appearance at some locations, as shown in Fig. 11. EDS analysis of the region confirmed that the associated chemical composition is intermediate to those of type 316L SS and Stellite 21.
3.2. Tensile testing The results of tensile tests summarized in Table 3, revealed that both “direct clad” and “graded clad” composite specimens, with smooth gauge length section, failed in wrought SS region with UTS value of 600–630 MPa. All these specimens exhibited typical cup and cone fracture, indicating ductile fracture. On the other hand specimens with the notch in the Stellite-clad region failed at a much
Fig. 9. Fracture surface of “direct clad” composite tensile specimen, which failed in the laser-clad Stellite 21 region. Note distinct evidence of crack propagation along inter-dendritic boundaries.
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Table 4 RBF test data of laser-clad composite specimens at various stress levels. Bending stress
300 MPa 250 MPa 200 MPa
No. of cycles to failure (×105 )/failure location Direct clad
Graded clad
8.6–11.4/Wrought SS 11.6–15.1/Wrought SS 11.2–14/Wrought SS
9–12.3/Wrought SS 11.6–14.1/Wrought SS 11.4–15.74/Wrought SS
Fig. 10. Fracture surface of notched “direct clad” composite tensile specimen, which failed at substrate/clad interface. Figure compares morphological fracture surface features (left) inter-dendritic crack propagation in Co-rich region and (right) ductile crack propagation in Fe-rich region.
Fig. 12. Macro-fracture appearance of RBF-tested “direct clad” specimen. “O” represents fatigue initiation region while beach marks are marked with arrows. Initial smooth region of the fracture surface (marked “O”) is caused by rubbing of mating surfaces as crack propagated into the material. SS: Stainless steel; St21: Stellite 21.
Fig. 11. Fracture surface of “graded clad” composite tensile specimen, which failed at substrate/clad interface.
SEM examination of the fracture surface of the “direct clad” specimen tested at a bending stress of 300 MPa revealed that fatigue fracture initiated from wrought SS region. Fracture surface carried distinct beach marks progressing inward from the point of initiation of the fatigue failure, marked as “O” in Fig. 12. Stainless steel part of the fracture surface was marked with dimpled appearance (refer Fig. 13A) while mode of crack propagation changed from ductile in the wrought SS region to brittle fracture along inter-dendritic regions in Stellite-clad part of the composite specimen, as shown in Fig. 13B.
3.3. Rotating bending fatigue testing 3.4. Instrumented impact testing RBF testing was carried out with the objective of determining location of fatigue-damage initiation in the laser-clad composite specimens. The results of the tests revealed that failure of both “direct clad” and “graded clad” specimens initiated in the wrought SS portion of the gauge section. All specimens, tested in the stress range of 200–300 MPa, failed after experiencing largely similar number of loading cycles, as shown in Table 4.
Instrumented Charpy impact testes were conducted on eight numbers of composite specimens (Stellite 21/316L SS), four each with “direct clad” and “graded clad” interfaces. Fracture of both kinds of specimens consumed largely similar amount of energy. Fracture energies of “direct clad” and “graded clad” specimens were in the range of 32–36 J and 35–37 J, respectively. However,
Fig. 13. Magnified views of regions “A” and “B”, marked in Fig. 12.
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Table 5 Instrumented Charpy impact (Cv ) test results of laser-clad composite specimens. Graded clad Impact energy, J (Cv ) 35 35.6 37 –
Direct clad Initiation energy, J (% Cv ) 29.4 (84) 28.22 (79.3) 29.29 (79.2) –
Propagation energy, J (% Cv )
Impact energy, J (Cv )
Initiation energy, J (% Cv )
Propagation energy, J (% Cv )
5.6 (16) 7.38 (20.7) 7.71 (20.8) –
37.35 35 32 33.23
34.95 (93.6) 33.8 (96.6) 31.25 (97.7) 31.99 (96.3)
2.4 (6.4) 1.2 (3.4) 0.75 (2.3) 1.24 (3.7)
Fig. 14. Load–displacement traces of instrumented impact test of laser-clad composite samples.
load–time traces of two kinds of specimens exhibited distinct difference in the mode of crack propagation after general yield. Failure in “direct clad” specimen exhibited higher yield load followed by drastic drop in load as crack propagated into Stellite region. On the other hand, fracture of “graded clad” specimens involved yielding at lower load followed by relatively gradual drop in load as crack propagated into Stellite region. Load–time data obtained during the test are converted into load–displacement data [21]. Fig. 14 presents load–displacement traces of “direct clad” and “graded clad” specimens. The energy consumed in crack initiation and propagation was derived from the respective graphs. Table 5 presents the results of Charpy impact tests with along with the details of crack initiation and propagation energies. SEM examination of the resultant fracture surfaces revealed that SS part of the fracture surface (ahead of the notch) was associated with equi-axed dimples. In “direct clad” specimens, nature of fracture surface underwent drastic change from dimpled to brittle fracture along inter-dendritic boundaries, as crack propagated across substrate/clad interface. Fig. 15 presents SEM fractographs of different regions of “direct clad” impact specimen. On the other hand, fracture surface of “graded clad” impact specimens, exhibited mixed-mode fracture features in the interface region, as shown in Fig. 16. 4. Synthesis of results and discussion Cobalt exhibits two allotropic forms viz. hexagonal close packed (HCP) phase at temperatures below 690 K while at temperatures above 690 K it exists in face centered cubic (FCC) phase. In order to avoid phase transformation during service, virtually in all Cobase alloys, alloying elements like Ni, C, Mn, and Fe, are added, to stabilize FCC phase from room temperature to melting temper-
ature. In Co-base alloys, no intermetallic phase is formed which can provide strengthening to the same degree, that ␥ provides in Ni-base superalloys. Instead, these alloys are usually strengthened by solid solution strengthening and precipitation of alloy carbides [22]. Stellite alloys contain large amount of carbide forming elements like Cr, W, Mo etc., with carbon content ranging from 0.1 to 3.3%. The microstructure usually consists of Cr-rich M7 C3 carbides in solid solution strengthened FCC matrix (containing Co, Cr, and W/Mo). On the other hand, austenitic SS exists in FCC phase (austenite) from room temperature to melting temperature. However, during melting and re-solidification of these alloys, some amount of ␦-ferrite may form depending upon associated chemical composition and cooling rate. Both FCC phases of Co and Fe (austenite) have almost similar lattice parameters (aCo = 3.5441 Å; aFe = 3.6468 Å, respectively) and they form a single phase FCC solid solution in this composition range [23]. Direct laser cladding of Stellite 21 on type 316L SS produced low-dilution deposits with diluted region of about 600 m. The process of graded cladding was successful in engineering gradual transition in chemical composition and micro-hardness across substrate/clad interface. In contrast to sharp substrate/clad interface of “direct clad” specimen, “graded clad” specimen exhibited a diffused interface. Both kinds of laser-clad deposits exhibited typical cast microstructure involving columnar dendrites/cells with Cr- and Mo-rich fine carbides on inter-dendritic/cellular and grain boundaries. The micro-hardness of laser-deposited Stellite 21 (475–500 VHN) is higher than the reported value of 290–430 VHN [1,2]. The hardness of Stellite deposit is strongly influenced by the associated microstructure and in particular on the size of the dendrites as well as on the size and distribution of carbides in the matrix [2,24–26]. Fine network of dendrites and associated fine carbides, generated due to fast cooling rate imposed by laser cladding [27], is responsible for higher hardness of the clad deposit over the conventionally processed Stellite 21. The micro-hardness of laser-deposited Stellite 21 is in agreement with the results published in the literature [24,25,27]. Laser deposits were associated with largely FCC solid solution with strong texturing effects. It should be noted that BCC and FCC alloys preferentially grow along least close packed 1 0 0 crystallographic direction, as growth in this direction allows faster growth rate than in other close packed crystallographic directions. During rapid solidification associated with laser cladding, grains with their easy growth direction parallel to the direction of maximum temperature gradient grow easily at the cost of those whose easy growth direction deviates significantly from the direction of maximum temperature gradient [28,29]. Hence, during solidification of laser-clad deposits, columnar grains with their 1 0 0 crystallographic direction aligned along the direction of heat extraction (which is normal to solid/liquid interface) grow preferentially at the cost of other unfavorably oriented grains. Accordingly it is found that, Stellite-clad deposits in the present case were predominantly associated with columnar grains oriented along the growth direction, which is normal to substrate/clad interface. Since growth direction of these columnar grains represents 1 0 0 crystallographic direction, (1 0 0) crystallographic plane of these grains should be oriented parallel to the top
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Fig. 15. Fracture surface of Charpy impact tested “direct clad” specimen. (A) Dimpled morphology in SS clad, (B) magnified view of region shown in “A”, (C) transition in fracture surface morphology at stainless steel/Stellite 21 interface (marked with arrows) and (D) magnified view of region shown in “C”. SS: Stainless steel; St21: Stellite 21.
surface of clad deposit. This explains strong (1 0 0) texture present on the top surface of Stellite-clad deposit. The results are in agreement with those reported in literature [27]. The mechanical tests viz. tensile, RBF and impact were performed with different objectives and the corresponding specimens have been designed accordingly. Tensile testing of Stellite-clad type 316L SS specimens demonstrated that substrate/clad interfaces of both “direct clad” and “grade clad” specimens were stronger than austenitic SS substrate and this is the reason for the failure of smooth composite specimens in the wrought SS region. Graded substrate/clad interface, because of greater dilution effect from austenitic SS, exhibited relatively lower tensile strength than substrate/clad interface region of “direct clad” specimens. Due to good ductility of austenitic SS, failure of smooth composite specimens was preceded by extensive plastic deformation. On the other
Fig. 16. Mixed-mode fracture surface morphology in the interface region of “graded clad” impact specimen.
hand failure in Stellite-clad region proceeded along inter-dendritic boundaries (refer Fig. 9) with little sign of plastic deformation. This is indicative of weakness at inter-dendritic boundaries, possibly due to presence of carbide particles on inter-dendritic boundaries, which provided easy path for crack propagation. Hao and Chunxu [30] also attributed brittle inter-dendritic fracture in laser-clad Co-base alloy to hard carbides formed in inter-dendritic boundary region. Crack propagation in “direct clad” specimens with notch at substrate/clad interface exhibited signs of both ductile and brittle failure. Fe-rich regions were associated with ductile crack propagation while Co-rich regions exhibited brittle crack propagation along inter-dendritic/cellular interface (refer Fig. 10). On the other hand, “graded clad” specimens with notch at substrate/clad interface involved crack propagation with relatively larger degree of plastic deformation (with respect to corresponding “direct clad” specimens). RBF testing of Stellite-clad austenitic SS specimens also demonstrated that wrought austenitic SS part of the composite specimen was the weakest region for fatigue crack nucleation. Fatigue failure in all the tested specimens originated in wrought SS portion with largely similar fatigue life in the investigated stress range of 200–300 MPa. Stainless part of the fracture surface was marked with ductile crack propagation while crack propagation during subsequent part of the failure was largely governed by local chemical composition. As observed in the case of tensile testing, fatigue crack propagation in Fe-rich zones involved significant plastic deformation whereas Co-rich zones exhibited brittle crack propagation along inter-dendritic boundaries. Instrumented Charpy impact test brought out significant difference in mode of crack propagation in “direct” and “graded” clad specimens at high strain rate. It can be inferred form results provided in Table 5 that the fraction of total energy consumed for crack initiation in direct clad specimens is about 95%, whereas in the case of graded clad specimens it is about 80%. In “direct clad” specimen, abrupt drop in load–displacement trace, after peak load,
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indicates that the fracture in Stellite-clad region of the specimen is largely initiation-controlled (refer Fig. 14). Due to the presence of brittle inter-dendritic boundaries, fracture in Stellite-clad region proceeded with little/no plastic deformation. On the other hand, due to compositional grading at substrate/clad region of “graded clad” specimen, crack propagation was associated with plastic deformation which was reflected in gradual drop in load after peak load. Lower peak load associated with these specimens is caused by the presence of soft graded zone ahead of the notch tip. 5. Conclusions In the light of the results obtained during the course of the present investigation, it is inferred that laser cladding process successfully engineered grading in chemical composition across substrate/clad interface, which was reflected in similar grading in terms of micro-hardness. Both “direct clad” and “graded clad” specimens displayed sound metallurgical bonding at the interface, with largely similar tensile strength values. Solute segregation at inter-dendritic boundaries of Stellite 21 clad deposits provided low energy fracture path. Both “direct” and “graded” interfaces of laser-clad specimens exhibited superior fatigue strength (in the investigated stress range of 200–300 MPa) than type 316L SS substrate, as fatigue crack nucleation in all the composite specimens took place in the substrate. “Direct clad” and “graded clad” specimens exhibited distinct difference in crack propagation (from SS to Stellite) under high strain rate loading conditions. In contrast to initiation-controlled brittle crack propagation across substrate/clad interface in “direct clad” specimens, crack propagation across “graded interface” was marked with significant plastic deformation, which is translated into slower crack propagation. Acknowledgements Authors sincerely acknowledge the help and support the staff members of Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore, India. References [1] http://www.matweb.com/search/QuickText.aspx?SearchText=stellite%2021.
[2] http://stellite.com/Portals/0/pdf/Datasheets/Stellite%2021%20DS0122208%20(S%20R0808).pdf. [3] IGC’97 Highlights, Indira Gandhi Centre for Atomic Research, Kalpakkam, India, 1997, pp. 87–88. [4] K. Ahmed, J. Krishnan, Trans. Met. Finish. Assoc. India 10 (3) (2001) 163–169. [5] R. Kaul, P. Ganesh, S.K. Albert, A. Jaiswal, N.P. Lalla, A. Gupta, C.P. Paul, A.K. Nath, Surf. Eng. 19 (4) (2003) 269–273. [6] http://www.enise.fr/commun/la recherche/DIPI/recherche2/AxeRecherche/ fabricationdirecte/DMD/ENISE DMD.pdf. [7] C.P. Paul, A. Jain, P. Ganesh, J. Negi, A.K. Nath, Opt. Lasers Eng. 44 (2006) 1096–1109. [8] Rakesh Kaul, P. Ganesh, M.K. Tiwari, A.K. Singh, Pragya Tripathi, Ajay Gupta, A.K. Nath, Lasers Eng. 12 (3) (2002) 207–225. [9] Rakesh Kaul, P. Ganesh, C.P. Paul, S.K. Albert, U.K. Mudali, A.K. Nath, in: W.L. Bohn, V.S. Golubev, A.A. Lonin, V. Ya Panchenko (Eds.), Int. Conf. on Lasers, Applications and Technologies, Proc. of SPIE. Vol. 6053 (2006) pp.60530 O1O10. [10] R. Jendrzejewski, A. Conde, J. de Damborenea, G. Sliwinski, Mater. Des. 23 (2002) 83–88. [11] A. Frenk, M. Vandyoussefi, J.-D. Wagniere, A. Zryd, W. Kurz, Metall. Mater. Trans. B 28 (3) (1997) 501–508. [12] R. Vilar, J. Laser Appl. 11 (2) (1999) 64–79. [13] C.L. Sexton, G. Byrne, K.G. Watkins, J. Laser Appl. 13 (1) (2001) 2–11. [14] J. Rafal, S. Gerard, C. Ana, Navas Carmen, J. de Damborenea Juan, Proc. SPIE 5777 (2005) 921–924, http://adsabs.harvard.edu/abs/2005SPIE.5777.921J. [15] Igor Smurov, Surf. Coat. Technol. 202 (2008) 4496–4502. [16] Minlin Zhong, Wenjin Liu, Kefu Yao, Jean-Claude Goussain, Cecile Mayer, AhimBecker, Surf. Coat. Technol. 157 (2002) 128–137. [17] A.K. Nath, L. Abhinandad, P. Choudhary, Opt. Eng. 33 (6) (1994) 1889–1893. [18] J. Khare, R. Sreedhar, C.P. Paul, T. Raghu, A.K. Nath, Pramana 1 (2003) 99–107. [19] P. Ganesh, R. Kaul, S. Mishra, P. Bhargav, C.P. Paul, Ch. Prem Singh, P. Tiwari, S.M. Oak, R.C. Prasad, Trans. Indian Instit. Met. 62 (2) (2009) 169–174. [20] J.A. Disegi, R.L. Kennedy, R. Pillar (Eds.), Cobalt-Base Alloys for Biomedical Applications, ASTM-STP 1365, ASTM, West Conshohocken, PA, 1999, pp. 62–88. [21] R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, fourth ed., John Wiley & Sons Inc, NY, USA, 1996, pp. 397–400. [22] Metals Hand Book, vol. I, tenth ed., ASM International, Materials Park, OH, 1997, p. 964. [23] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, seventh ed., Reed Educational and Professional Publishing Ltd, Oxford, 2000, pp. 11–193. [24] D.H.E. Persson, S. Jacobson, S. Hogmark, Wear 255 (2003) 498–503, doi:10.1016/S0043-1648(03)00122-4. [25] L. Thivillon, D. Pervushin, Ph. Bertrand, I. Smurov, Industrial technology of laser assisted direct metal deposition, http://www.enise.fr/ commun/la recherche/DIPI/recherche2/AxeRecherche/fabricationdirecte/ DMD/ENISE DMD.pdf. Date last visited: 29-3-2010. [26] A. Frenk, W. Kurz, Wear 174 (1–2) (1994) 81–91. [27] D.H.E. Persson, Laser processed low friction surfaces, PhD thesis, Uppsala University, Sweden, 2003. http://www.teknik.uu.se/tribomaterials/ Research/Daniel%20P%20lic.pdf. [28] S. Kou, Welding Metallurgy, John Wiley & Sons Inc, New York, 1987, p. 146. [29] K. Easterling, Introduction to the Physical Metallurgy of Welding, second ed., Butterwroth–Heinemann Ltd, Oxford, 1993, pp. 63–65. [30] C. Hao, P. Chun-xu Jr., Wuhan University of Technology: Mater. Sci. Ed. 18 (3) (2003) 30–32.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Fracture behavior of laser-clad joint of Stellite 21 on AISI 316L stainless steel P. Ganesh a,∗ , A. Moitra c , Pragya Tiwari b , S. Sathyanarayanan c , Harish Kumar a , S.K. Rai b , Rakesh Kaul a , C.P. Paul a , R.C. Prasad d , L.M. Kukreja a a
Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh 452013, India Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh 452013, India c Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, TN, India d Metallurgy and Materials Science Dept., IIT Bombay, Mumbai 400 076, Maharashtra, India b
a r t i c l e
i n f o
Article history: Received 11 December 2009 Received in revised form 2 February 2010 Accepted 4 March 2010
Keywords: Laser cladding Graded cladding Clad interface Stellite Stainless steel Fatigue Impact test
a b s t r a c t Present paper describes microstructural and mechanical characterization of laser-clad composite joint made of Stellite 21 and type 316L stainless steel (SS). The study has been broadly performed on two kinds of laser-clad specimens viz. (i) involving direct deposition of Stellite 21 on SS and (ii) involving gradient in chemical composition across substrate/clad interface. Both kinds of specimens exhibited nearly similar tensile strength, which was higher than that of SS in the interface region. Inter-dendritic carbides provided low energy fracture path in laser-clad deposits of Stellite 21. Both direct and graded interfaces of laser-clad specimens exhibited superior fatigue strength than the SS substrate. Instrumented impact testing brought out distinct difference in the mode of crack propagation across direct and the graded clad specimens. In contrast to initiation-controlled brittle crack propagation across the interface region in “direct clad” specimens, crack propagation across “graded interface” was marked with significant plastic deformation.
1. Introduction Cobalt-base hard-facing alloys (Stellites), due to their outstanding resistance against high temperature oxidation and wear, find wide applications in turbine blades, vanes, hot die applications, components in combustion or exhaust systems, valve trim for petrochemical and power generation [1,2]. Stellite hard facing of austenitic stainless steel is used in fast breeder reactors to obtain high galling resistance between stainless steel surfaces exposed to high temperature (about 823 K) sodium coolant with very low level of oxygen [3,4]. High power lasers, because of their ability to deposit lowdilution clad deposits with controlled heat input and associated distortion, are emerging as attractive tools for hard-facing applications [5,6]. Integrity of substrate/clad interface of laser-clad deposit is very important for satisfactory performance of the hard-faced components. During laser cladding, large mismatch in thermo-physical properties between substrate and clad mate-
∗ Corresponding author at: Department of Atomic Energy, Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Block-B, RRCAT, P.O. CAT, Indore, Madhya Pradesh 452013, India. Tel.: +91 9827631359; fax: +91 7312488380. E-mail addresses: ganesh [email protected], [email protected] (P. Ganesh). 0921-5093/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.msea.2010.03.017
© 2010 Published by Elsevier B.V.
rial often results in the development of residual stresses at substrate/clad interface, causing cracking of clad deposits. Preheating/post-annealing can be used to relieve residual tensile stresses and to prevent cracking of laser hard-faced components [5,7]. Substrate/clad interfaces of laser-clad deposits are usually associated with abrupt transition in chemical composition and associated physical and mechanical properties. However, introduction of a compositionally graded substrate/clad interface, involving gradual transition in thermo-physical/mechanical properties, helps in reducing undesirable stress concentration effect. Kaul et al. have reported superior cracking resistance of graded Stellite 6 deposits on austenitic stainless steel (SS) under thermal cycling conditions [8,9]. When a hard-faced component is subjected to service conditions, involving fatigue or thermal cycling, nature of substrate/clad interface plays a vital role in determining overall performance of the component and hence, characterization of substrate/clad interface is essential. Published literature on laser cladding is largely focused on microstructural analysis and associated tribological behavior [10–16]. There is no published reference of a study on mechanical property characterization (tensile, fatigue and impact) of substrate/clad interface of laser-clad deposits. Present work is undertaken with the objective of investigating metallurgical characteristics of substrate/clad interface of Stellite-clad austenitic SS specimens, with specific emphasis on strength and fatigue performance. The work
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involves study of fracture behavior of direct and graded clad specimens. 2. Experimental 2.1. Laser cladding Laser cladding experiment was carried out with an indigenously developed 3.5 kW CW CO2 laser [17,18]. The experimental set-up consisted of a laser system, integrated with a beam delivery system, co-axial nozzle, powder feeder and a computerized-numerically controlled workstation. Laser beam, emanating out of the laser system, was focused with the help of a 125 mm focal length Zinc Selenide lens, housed in a water-cooled copper nozzle. Laser cladding process involved scanning the surface of the type 316L SS substrate with a defocused laser beam of 3 mm diameter along with simultaneous injection of alloy powder (particle size = 45–105 m) into laser-interaction region through a co-axial nozzle with argon as carrier gas. Large surface area was covered by depositing overlapping clad tracks. On the other hand, deposition of thicker clad layer involved layer-by-layer deposition. For the deposition of graded overlays, chemical composition of clad layers was controlled by mixing powders of Stellite 21 and AISI 316L SS in predetermined ratios. Graded overlay of three layers was formed by cladding with premixed powders of Stellite 21 and type 316L SS in the ratios of 30:70, 70:30 and 100:0, respectively. In the subsequent part of the paper, SS specimens clad with Stellite 21 deposits and graded Stellite deposits are referred as “direct clad” and “graded clad” specimens, respectively. Table 1 presents chemical compositions of Stellite 21 and 316L SS powders used for laser cladding while experimental parameters used for the study are summarized in Table 2. Experimental parameters were selected based on an earlier experimental study carried out in authors’ laboratory [5,8,19]. Laser-clad specimens were characterized by optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), tensile test, rotating bending fatigue (RBF) test and instrumented Charpy impact (CI) test. 2.2. Fabrication of tensile test specimens for interface strength characterization Fabrication methodology adopted for the preparation of tensile test specimens (refer Fig. 1) involved (i) laser cladding of Stellite 21 on the transverse cross-section of 12.5 mm diameter type 316L SS rods (with and without grading) to obtain total deposit thickness of about 12 mm, to form laser-clad SS rod (ii) gas tungsten arc welding (GTAW) of another SS rod to the Stellite end of laser-clad SS rod, followed by (iii) machining of the GTA welded laser-clad SS rod to fabricate tensile test specimens. In the resultant test specimen, Stellite 21/type 316L SS interface fell almost at the center of 10 mm long reduced section of 8 mm diameter. Two types of specimens were prepared viz. “direct clad” and “graded clad”. In addition to specimens with smooth gauge length section, notched ten-
Fig. 1. Fabrication methodology adopted for the preparation of tensile test specimens.
sile test specimens (notch depth = 1 mm; notch angle = 90◦ ) were also prepared with the notch at (i) substrate/clad interface and (ii) Stellite-clad region. Although these were not standard specimens, the intent of present specimen design was to evaluate tensile strength of substrate/clad interface and that of Stellite-clad zone. Tensile tests were conducted on a 100 kN universal testing machine under stroke control mode. Tensile tests were carried out with the objective of evaluating tensile strength of direct and graded substrate/clad interface. Therefore, tensile tests were done with the interface normal to the tensile axis. 2.3. Fabrication of rotating bending (RBF) fatigue test specimens The methodology adopted for the fabrication of RBF test specimen involved (i) machining out central part of 226 mm long type 316L SS rod (in as received condition of cold work) with a diameter of 12.5 mm diameter (refer Fig. 2a), (ii) filling up resultant cavity with Stellite 21 powder by laser cladding, followed by (iii) final machining to obtain fatigue test specimen (refer Fig. 2b). Transverse cross-section of parallel length of the test specimen carried laser-deposited Stellite 21 and type 316L SS in the two halves with substrate/clad interface at the center running along the length of parallel region (refer Fig. 2c). During RBF testing of such composite specimen, each part of the specimen viz. substrate, substrate/clad interface and clad, is subjected to identical sinusoidal loading while stress remains constant along the length of the parallel region. Gauge section of all test specimens were polished to obtain surface roughness (Ra) of 0.2–0.5 m. Out of 24 numbers of RBF test specimens so fabricated, 12 each belonged to “direct clad” and “graded clad” types.
Table 1 Chemical composition (weight %) of the powders used for laser cladding. Material
C
Cr
Ni
Mn
Si
Mo
Fe
Co
P
S
SS 316L St-21
0.025 0.26
18.6 26.3
12.3 2.8
1 0.65
0.5 1.88
2 5.53
Bal 1.4
– Bal
0.03 –
0.02 –
Table 2 Experimental parameters for laser cladding. Laser power
Beam diameter
Powder feed rate
Scan speed
Carrier gas flow rate
Overlap between successive clad tracks
2.8–3.0 kW
3 mm
6–7 g/min
6.7–8.3 mm/s
12 lpm
50%
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Fig. 2. Sequence of stages in the fabrication of rotating bending fatigue specimens. (A) Stainless steel rods with machined central region for laser deposition, (B) specimens after laser deposition and final machining, (C) schematic illustration of the cross-section of gauge length portion.
Fig. 3. Sequence of stages in the fabrication of Charpy impact specimens. (A) 316L stainless steel block with machined central region for laser deposition and (B) stainless steel block after laser cladding and machining with inset showing final machined specimen.
Fatigue endurance limits of Co–Cr–Mo alloys (Stellites) and type 316L SS are 380–450 MPa and 270 MPa, respectively [20]. Since fatigue strength and location of failure of a multi-material specimen is determined by the material of lower endurance limit, the failure of the composite specimen was likely to take place either in wrought type 316L SS or at substrate/clad interface with unknown fatigue endurance limit. Also the residual stresses at the substrate/clad interface or in the clad overlay may contribute to fatigue-damage initiation. Therefore, in order to determine which of these zones is more prone to fatigue failure, composite specimens were tested both below and above the endurance limit of type 316L SS. RBF tests were conducted at room temperature in the stress range of 200–300 MPa with at least three specimens of each type tested at a particular stress level.
imen was machined in such a way that fracture would initiate in the SS region and propagate through the interface into Stellite 21 clad region. These composite impact specimens carried about 3 mm-thick laser-clad deposit of SS (after V groove machining) and about 5 mm-thick Stellite 21 deposit (with and without grading). Instrumented impact testing of laser-clad bimetallic composite specimens was carried out with a Tinus-Olsen impact testing machine with instrumented striker with an energy range of 0–358 J. The objective of impact test was to study crack propagation behavior across direct and compositionally graded substrate/clad interface. Hence the orientation of the crack propagation in the case of impact test samples was normal to the substrate/clad interface. 3. Results
2.4. Fabrication of instrumented Charpy impact test samples 3.1. Microstructural analysis The procedure adopted for the fabrication of test specimens included (i) machining a V groove in the central part of a block of type 316 L SS block (refer Fig. 3a), (ii) filling up of resultant groove with 316L SS and Stellite 21 by laser cladding, followed by (iii) machining to obtain standard Charpy impact specimen, as per ASTM E-23 (refer Fig. 3b). The notch in the impact spec-
Metallographic examination of longitudinal and transverse cross-sections of laser-clad specimens did not reveal any defects. Cross-sections of “direct clad” and graded clad” specimens exhibited distinct difference in etching contrast. “Direct clad” deposits of Stellite were associated with uniform etching contrast across its
Fig. 4. Longitudinal cross-sections of (A) “direct clad” and (B) “graded clad” specimens. Partially melted zone (PMZ) at the interface of “direct clad” specimen is marked with arrows.
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Fig. 6. Microstructure of Stellite 21 deposit. Carbides are marked with arrows.
Fig. 5. SEM-EDS concentration profiles of Iron (Fe), Cobalt (Co), Chromium (Cr) and Molybdenum (Mo) across substrate/clad interface. Top: Direct clad specimen, EDS line scan length: 1.4 mm; bottom: graded clad specimen, EDS line scan length: 5 mm. SS: Stainless steel region; St21: Stellite 21 clad region.
thickness, whereas “graded clad” specimen exhibited gradual transition in etching contrast from light to dark along the direction of build up. Laser-clad deposits were characterized by fine columnar grains growing along the direction of build up. Interfaces between successive layers of laser-clad deposits were marked with clear signs of epitaxiality.
Fig. 4 compares microstructures of substrate/clad interface regions of “direct clad” and “graded clad” specimens. In contrast to sharp substrate/clad interface of “direct clad” specimen, “graded clad” specimens were marked with diffused interface between substrate and clad regions. Sharp substrate/clad interface in “direct clad” specimen was associated with a 50–60 m thick partially melted zone (PMZ) in the substrate shown in Fig. 4A marked with arrows. In contrast, diffused interface of “graded clad” specimen was characterized by a gradual transition in microstructure from wrought equi-axed grain to columnar dendrites (refer Fig. 4B). SEM-EDS analysis, carried out on the transverse cross-sections of laser-clad specimens, revealed sharp transition in chemical composition (particularly in terms of the concentrations of Fe, Co and Cr) across substrate/clad interface of “direct clad” specimen whereas, more gradual transition in chemical composition was recorded in “graded clad”
Fig. 7. SEM-EDS concentration profiles of Cobalt (Co), Chromium (Cr), Molybdenum (Mo) and Carbon across two carbide particles, shown in the SEM photomicrograph given below.
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P. Ganesh et al. / Materials Science and Engineering A 527 (2010) 3748–3756 Table 3 Results of tensile tests.
Fig. 8. Micro-hardness profiles across substrate/clad interface of “direct clad” and “graded clad” specimens.
specimen. Fig. 5 compares EDS concentration profiles of Fe, Co, Cr and Mo across substrate/clad interface of “direct clad” and “graded clad” specimens. Both kinds of specimens exhibited cast microstructure involving fine columnar dendrites with dark carbide particles at inter-dendritic/inter-cellular boundaries as well as on the grain boundaries as shown in Fig. 6. EDS line scans across second phase particles present at inter-cellular boundaries revealed that the particles were relatively depleted of Co and enriched with Mo and Cr with respect to core of the dendrite, as shown in Fig. 7. Due to low atomic number of carbon, its EDS line scan did not show corresponding rise in counts at the particle site. Micro-hardness measurements, carried out on the 316L SS substrate yielded a value in the range of 230–260 VHN, which is higher than the generally expected values for the 316L SS due to the cold worked conditions of the substrate used. The “direct clad” specimen exhibited a sharp transition in micro-hardness to 475–500 VHN in laser-deposited Stellite 21 region. Fig. 8 compares micro-hardness profiles across transverse cross-sections of “direct clad” and “graded clad” specimens. In “direct clad” specimens, complete transition in micro-hardness was realized at a distance of 600 m from substrate/clad interface. On the other hand, “graded clad” specimen exhibited a gradual rise in micro-hardness across substrate/clad interface. X-ray diffraction was carried out to confirm that no new phases are formed as a result of laser cladding. XRD conducted on laserdeposited specimens of different orientations with CuK␣ radiation ( = 1.54 Å) confirms that there are no additional peaks of any new phase and laser-deposited Stellite 21 has face centered cubic (FCC) phase, as exhibited by its powder. XRD of the interface region also revealed that same FCC phase is retained and no new phase is formed. An interesting feature brought out by XRD is strong texturing effect in laser-deposited Stellite 21, with respect to its randomly textured powder. Top surface of laser-clad deposit exhibited preferred (2 0 0) orientation while lateral surface exhibited (1 1 1) texture.
Specimen
Tensile strength (MPa)
Failure location
Direct clad (smooth gauge section) Graded clad (smooth gauge section) Specimen with notch in laser-deposited Stellite 21 Direct clad (notch at substrate/clad interface) Graded clad (notch at substrate/clad interface)
600–630
Wrought SS
600–630
Wrought SS
950, 955, 968
Stellite 21 (clad)
717, 748, 754
Substrate/clad interface Graded substrate/clad interface
679, 706
higher UTS value of 950–968 MPa. Both “direct clad” and “graded clad” specimens with the notch at substrate/clad interface failed at an intermediate stress value. Interface tensile strengths of “direct clad” and “grade clad specimens” were found to be 717–754 MPa and 679–706 MPa, respectively. SEM fractographic examination of “direct clad” and “graded clad” specimens, with smooth gauge length section, exhibited typical dimpled appearance, indicating extensive plastic deformation before final fracture. On the other hand, fracture surface of test specimens with notch in Stellite-clad region did not carry signatures of associated plastic deformation. Fracture surface exhibited columnar dendrites with different orientations, indicating crack propagation along inter-dendritic boundaries as shown in Fig. 9. Fracture surface of notched “direct clad” specimens (notch at substrate/clad interface) exhibited regions associated with plastic deformation (dimpled fracture morphology) and brittle fracture along inter-dendritic boundaries (refer Fig. 10). These two kinds of regions were randomly distributed over the fracture surface, indicating that the fracture path was confined to the vicinity of substrate/clad interface. The part of the fracture surface, exhibiting dimpled appearance was rich in Fe while the other part with dendritic appearance was found to be rich in Co. On the other hand, fracture surface of notched “graded clad” specimen (notch at substrate/clad interface) carried signatures of plastic deformation along with cleavage-type appearance at some locations, as shown in Fig. 11. EDS analysis of the region confirmed that the associated chemical composition is intermediate to those of type 316L SS and Stellite 21.
3.2. Tensile testing The results of tensile tests summarized in Table 3, revealed that both “direct clad” and “graded clad” composite specimens, with smooth gauge length section, failed in wrought SS region with UTS value of 600–630 MPa. All these specimens exhibited typical cup and cone fracture, indicating ductile fracture. On the other hand specimens with the notch in the Stellite-clad region failed at a much
Fig. 9. Fracture surface of “direct clad” composite tensile specimen, which failed in the laser-clad Stellite 21 region. Note distinct evidence of crack propagation along inter-dendritic boundaries.
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Table 4 RBF test data of laser-clad composite specimens at various stress levels. Bending stress
300 MPa 250 MPa 200 MPa
No. of cycles to failure (×105 )/failure location Direct clad
Graded clad
8.6–11.4/Wrought SS 11.6–15.1/Wrought SS 11.2–14/Wrought SS
9–12.3/Wrought SS 11.6–14.1/Wrought SS 11.4–15.74/Wrought SS
Fig. 10. Fracture surface of notched “direct clad” composite tensile specimen, which failed at substrate/clad interface. Figure compares morphological fracture surface features (left) inter-dendritic crack propagation in Co-rich region and (right) ductile crack propagation in Fe-rich region.
Fig. 12. Macro-fracture appearance of RBF-tested “direct clad” specimen. “O” represents fatigue initiation region while beach marks are marked with arrows. Initial smooth region of the fracture surface (marked “O”) is caused by rubbing of mating surfaces as crack propagated into the material. SS: Stainless steel; St21: Stellite 21.
Fig. 11. Fracture surface of “graded clad” composite tensile specimen, which failed at substrate/clad interface.
SEM examination of the fracture surface of the “direct clad” specimen tested at a bending stress of 300 MPa revealed that fatigue fracture initiated from wrought SS region. Fracture surface carried distinct beach marks progressing inward from the point of initiation of the fatigue failure, marked as “O” in Fig. 12. Stainless steel part of the fracture surface was marked with dimpled appearance (refer Fig. 13A) while mode of crack propagation changed from ductile in the wrought SS region to brittle fracture along inter-dendritic regions in Stellite-clad part of the composite specimen, as shown in Fig. 13B.
3.3. Rotating bending fatigue testing 3.4. Instrumented impact testing RBF testing was carried out with the objective of determining location of fatigue-damage initiation in the laser-clad composite specimens. The results of the tests revealed that failure of both “direct clad” and “graded clad” specimens initiated in the wrought SS portion of the gauge section. All specimens, tested in the stress range of 200–300 MPa, failed after experiencing largely similar number of loading cycles, as shown in Table 4.
Instrumented Charpy impact testes were conducted on eight numbers of composite specimens (Stellite 21/316L SS), four each with “direct clad” and “graded clad” interfaces. Fracture of both kinds of specimens consumed largely similar amount of energy. Fracture energies of “direct clad” and “graded clad” specimens were in the range of 32–36 J and 35–37 J, respectively. However,
Fig. 13. Magnified views of regions “A” and “B”, marked in Fig. 12.
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Table 5 Instrumented Charpy impact (Cv ) test results of laser-clad composite specimens. Graded clad Impact energy, J (Cv ) 35 35.6 37 –
Direct clad Initiation energy, J (% Cv ) 29.4 (84) 28.22 (79.3) 29.29 (79.2) –
Propagation energy, J (% Cv )
Impact energy, J (Cv )
Initiation energy, J (% Cv )
Propagation energy, J (% Cv )
5.6 (16) 7.38 (20.7) 7.71 (20.8) –
37.35 35 32 33.23
34.95 (93.6) 33.8 (96.6) 31.25 (97.7) 31.99 (96.3)
2.4 (6.4) 1.2 (3.4) 0.75 (2.3) 1.24 (3.7)
Fig. 14. Load–displacement traces of instrumented impact test of laser-clad composite samples.
load–time traces of two kinds of specimens exhibited distinct difference in the mode of crack propagation after general yield. Failure in “direct clad” specimen exhibited higher yield load followed by drastic drop in load as crack propagated into Stellite region. On the other hand, fracture of “graded clad” specimens involved yielding at lower load followed by relatively gradual drop in load as crack propagated into Stellite region. Load–time data obtained during the test are converted into load–displacement data [21]. Fig. 14 presents load–displacement traces of “direct clad” and “graded clad” specimens. The energy consumed in crack initiation and propagation was derived from the respective graphs. Table 5 presents the results of Charpy impact tests with along with the details of crack initiation and propagation energies. SEM examination of the resultant fracture surfaces revealed that SS part of the fracture surface (ahead of the notch) was associated with equi-axed dimples. In “direct clad” specimens, nature of fracture surface underwent drastic change from dimpled to brittle fracture along inter-dendritic boundaries, as crack propagated across substrate/clad interface. Fig. 15 presents SEM fractographs of different regions of “direct clad” impact specimen. On the other hand, fracture surface of “graded clad” impact specimens, exhibited mixed-mode fracture features in the interface region, as shown in Fig. 16. 4. Synthesis of results and discussion Cobalt exhibits two allotropic forms viz. hexagonal close packed (HCP) phase at temperatures below 690 K while at temperatures above 690 K it exists in face centered cubic (FCC) phase. In order to avoid phase transformation during service, virtually in all Cobase alloys, alloying elements like Ni, C, Mn, and Fe, are added, to stabilize FCC phase from room temperature to melting temper-
ature. In Co-base alloys, no intermetallic phase is formed which can provide strengthening to the same degree, that ␥ provides in Ni-base superalloys. Instead, these alloys are usually strengthened by solid solution strengthening and precipitation of alloy carbides [22]. Stellite alloys contain large amount of carbide forming elements like Cr, W, Mo etc., with carbon content ranging from 0.1 to 3.3%. The microstructure usually consists of Cr-rich M7 C3 carbides in solid solution strengthened FCC matrix (containing Co, Cr, and W/Mo). On the other hand, austenitic SS exists in FCC phase (austenite) from room temperature to melting temperature. However, during melting and re-solidification of these alloys, some amount of ␦-ferrite may form depending upon associated chemical composition and cooling rate. Both FCC phases of Co and Fe (austenite) have almost similar lattice parameters (aCo = 3.5441 Å; aFe = 3.6468 Å, respectively) and they form a single phase FCC solid solution in this composition range [23]. Direct laser cladding of Stellite 21 on type 316L SS produced low-dilution deposits with diluted region of about 600 m. The process of graded cladding was successful in engineering gradual transition in chemical composition and micro-hardness across substrate/clad interface. In contrast to sharp substrate/clad interface of “direct clad” specimen, “graded clad” specimen exhibited a diffused interface. Both kinds of laser-clad deposits exhibited typical cast microstructure involving columnar dendrites/cells with Cr- and Mo-rich fine carbides on inter-dendritic/cellular and grain boundaries. The micro-hardness of laser-deposited Stellite 21 (475–500 VHN) is higher than the reported value of 290–430 VHN [1,2]. The hardness of Stellite deposit is strongly influenced by the associated microstructure and in particular on the size of the dendrites as well as on the size and distribution of carbides in the matrix [2,24–26]. Fine network of dendrites and associated fine carbides, generated due to fast cooling rate imposed by laser cladding [27], is responsible for higher hardness of the clad deposit over the conventionally processed Stellite 21. The micro-hardness of laser-deposited Stellite 21 is in agreement with the results published in the literature [24,25,27]. Laser deposits were associated with largely FCC solid solution with strong texturing effects. It should be noted that BCC and FCC alloys preferentially grow along least close packed 1 0 0 crystallographic direction, as growth in this direction allows faster growth rate than in other close packed crystallographic directions. During rapid solidification associated with laser cladding, grains with their easy growth direction parallel to the direction of maximum temperature gradient grow easily at the cost of those whose easy growth direction deviates significantly from the direction of maximum temperature gradient [28,29]. Hence, during solidification of laser-clad deposits, columnar grains with their 1 0 0 crystallographic direction aligned along the direction of heat extraction (which is normal to solid/liquid interface) grow preferentially at the cost of other unfavorably oriented grains. Accordingly it is found that, Stellite-clad deposits in the present case were predominantly associated with columnar grains oriented along the growth direction, which is normal to substrate/clad interface. Since growth direction of these columnar grains represents 1 0 0 crystallographic direction, (1 0 0) crystallographic plane of these grains should be oriented parallel to the top
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Fig. 15. Fracture surface of Charpy impact tested “direct clad” specimen. (A) Dimpled morphology in SS clad, (B) magnified view of region shown in “A”, (C) transition in fracture surface morphology at stainless steel/Stellite 21 interface (marked with arrows) and (D) magnified view of region shown in “C”. SS: Stainless steel; St21: Stellite 21.
surface of clad deposit. This explains strong (1 0 0) texture present on the top surface of Stellite-clad deposit. The results are in agreement with those reported in literature [27]. The mechanical tests viz. tensile, RBF and impact were performed with different objectives and the corresponding specimens have been designed accordingly. Tensile testing of Stellite-clad type 316L SS specimens demonstrated that substrate/clad interfaces of both “direct clad” and “grade clad” specimens were stronger than austenitic SS substrate and this is the reason for the failure of smooth composite specimens in the wrought SS region. Graded substrate/clad interface, because of greater dilution effect from austenitic SS, exhibited relatively lower tensile strength than substrate/clad interface region of “direct clad” specimens. Due to good ductility of austenitic SS, failure of smooth composite specimens was preceded by extensive plastic deformation. On the other
Fig. 16. Mixed-mode fracture surface morphology in the interface region of “graded clad” impact specimen.
hand failure in Stellite-clad region proceeded along inter-dendritic boundaries (refer Fig. 9) with little sign of plastic deformation. This is indicative of weakness at inter-dendritic boundaries, possibly due to presence of carbide particles on inter-dendritic boundaries, which provided easy path for crack propagation. Hao and Chunxu [30] also attributed brittle inter-dendritic fracture in laser-clad Co-base alloy to hard carbides formed in inter-dendritic boundary region. Crack propagation in “direct clad” specimens with notch at substrate/clad interface exhibited signs of both ductile and brittle failure. Fe-rich regions were associated with ductile crack propagation while Co-rich regions exhibited brittle crack propagation along inter-dendritic/cellular interface (refer Fig. 10). On the other hand, “graded clad” specimens with notch at substrate/clad interface involved crack propagation with relatively larger degree of plastic deformation (with respect to corresponding “direct clad” specimens). RBF testing of Stellite-clad austenitic SS specimens also demonstrated that wrought austenitic SS part of the composite specimen was the weakest region for fatigue crack nucleation. Fatigue failure in all the tested specimens originated in wrought SS portion with largely similar fatigue life in the investigated stress range of 200–300 MPa. Stainless part of the fracture surface was marked with ductile crack propagation while crack propagation during subsequent part of the failure was largely governed by local chemical composition. As observed in the case of tensile testing, fatigue crack propagation in Fe-rich zones involved significant plastic deformation whereas Co-rich zones exhibited brittle crack propagation along inter-dendritic boundaries. Instrumented Charpy impact test brought out significant difference in mode of crack propagation in “direct” and “graded” clad specimens at high strain rate. It can be inferred form results provided in Table 5 that the fraction of total energy consumed for crack initiation in direct clad specimens is about 95%, whereas in the case of graded clad specimens it is about 80%. In “direct clad” specimen, abrupt drop in load–displacement trace, after peak load,
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indicates that the fracture in Stellite-clad region of the specimen is largely initiation-controlled (refer Fig. 14). Due to the presence of brittle inter-dendritic boundaries, fracture in Stellite-clad region proceeded with little/no plastic deformation. On the other hand, due to compositional grading at substrate/clad region of “graded clad” specimen, crack propagation was associated with plastic deformation which was reflected in gradual drop in load after peak load. Lower peak load associated with these specimens is caused by the presence of soft graded zone ahead of the notch tip. 5. Conclusions In the light of the results obtained during the course of the present investigation, it is inferred that laser cladding process successfully engineered grading in chemical composition across substrate/clad interface, which was reflected in similar grading in terms of micro-hardness. Both “direct clad” and “graded clad” specimens displayed sound metallurgical bonding at the interface, with largely similar tensile strength values. Solute segregation at inter-dendritic boundaries of Stellite 21 clad deposits provided low energy fracture path. Both “direct” and “graded” interfaces of laser-clad specimens exhibited superior fatigue strength (in the investigated stress range of 200–300 MPa) than type 316L SS substrate, as fatigue crack nucleation in all the composite specimens took place in the substrate. “Direct clad” and “graded clad” specimens exhibited distinct difference in crack propagation (from SS to Stellite) under high strain rate loading conditions. In contrast to initiation-controlled brittle crack propagation across substrate/clad interface in “direct clad” specimens, crack propagation across “graded interface” was marked with significant plastic deformation, which is translated into slower crack propagation. Acknowledgements Authors sincerely acknowledge the help and support the staff members of Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore, India. References [1] http://www.matweb.com/search/QuickText.aspx?SearchText=stellite%2021.
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