Effect of Post-heat Treatment on Microstructure and Mechanical Properties of Laser Cladded High Co-Ni Secondary Hardening Steel Coating

The effect of post-heat treatment on the microstructure and microhardness of laser cladded high Co-Ni secondary hardening steel coating was investigated. The microstructures were analyzed using a SEM equipped with an EDS, and the microhardness was measured with a Vickers indenter. Decomposition of the retained austenite in the coating occurred during the post-heat treatment. As the temperature increased from 200 °C to 600 °C, the quantity of the retained austenite at the boundaries decreased significantly, while that of the needle-shaped M3C cementite and M2C carbides increased. The M2C carbides evidently coarsened when the temperature was higher than 500 °C. The microhardness of high Co-Ni steel coating increased as the temperature of post-heat treatment increased from 200 °C to 400 °C because the fine-scale M2C carbides were coherent with the matrix and increased distinctly in this temperature range. It decreased sharply when the temperature further increased from 500 °C to 600 °C due to both the incoherency of the coarsened M2C carbides and the recovery of dislocations in the carbon-supersaturated matrix.


Introduction
Wear is a frequently encountered surface failure phenomenon for the structural steel components of aviation aircrafts after long-term service.Since this failure always occurs at the surface, it is effective to prolong the service lives and reduce the maintenance cost of these components by selecting an appropriate surface modification method to deposit a high wear-resistance coating on component surfaces [1].Laser cladding is one of the most effective and nontraditional surface coating technologies, which utilizes a high-power laser beam as the heat source to deposit a desired thin metallic layer on substrate with the metal powders or wires [2,3].Compared with many conventional coating processes such as micro-arc welding, physical vapor deposition and thermal spraying, the laser cladding process has many advantages, such as almost pore free, fine grains, strong bonding between coating and substrate, low heat input and less distortion of the substrate [3,4].Therefore, laser cladding has become a very important technique for the surface modification and repair of metallic components [2,4], and plays a vital role in aeronautical manufacture and maintenance.In order to improve the surface wear resistance of structural steel components, ultra-high-strength steels are suggested to be the most appropriate protective coating for these components due to the similar thermal expansion coefficient and higher strength and hardness [5,6].High Co-Ni secondary hardening steels are one of ultra-high-strength high-alloy steels, which are mainly strengthened by precipitation hardening (i.e. the precipitation of fine-scale M 2 C carbides) and solid solution of Co, Ni and C additions [7,8].Compared with many commercially ultra-high-strength low-alloy martensitic steels, high Co-Ni secondary hardening steels used as the filler material are more suitable for surface modification using laser cladding because of their superior toughness and weldability [9,10].High residual stress commonly occurs in laser cladded coating due to the rapid local thermal shrinkage during the process, which increases the cracking tendency and deteriorates its mechanical properties (such as ductility and fatigue) [4,11,12].Heat treatment is a conventional and effective method for material modification by means of heating, heat preservation and cooling in the solid state.Therefore, it is frequently applied to relieve residual stress and/or modify the microstructure and properties of the coating after laser cladding [13,14].In the past several decades, extensive research has been conducted on the heat treatment of the high Co-Ni secondary hardening steels produced by conventional methods, such as casting and forging.The usual sequential steps of their produced heat treatment are austenitizing, quenching and tempering [15][16][17].Research results show that the microstructure mainly consists of martensite (i.e.matrix) and few retained austenite at the grain boundaries after quenching.A mass of uniform fine M 2 C carbides can be precipitated from the carbon-supersaturated matrix during the final tempering, resulting in an excellent combination of ultra-high strength, toughness and hardness [16,18].However, the laser cladded high Co-Ni steels and their coatings exhibit different microstructural characteristics [10,19].In order to avoid deterioration of the microstructure and mechanical properties of the substrate, the temperature of post-heat treatment used for surface repair and/or modification is usually below 650 o C [13,14].So far, there has been insufficient study on the microstructural evolution of laser cladded high Co-Ni secondary hardening steels during post-heat treatment and its effect on the mechanical properties.In this study, the high wear-resistance coating was fabricated by the laser cladding using high Co-Ni secondary hardening steel powders.Six different temperatures were chosen for the post-heat treatment on the laser cladded coating.Its effect on the microstructure and microhardness evolution was investigated, and the corresponding mechanism was also discussed.

Material
The material used for the laser cladding process was high Co-Ni secondary hardening steel powders with particle size of 45 μm-150 μm.The metal powder particles were prepared in a spherical shape by an argon atomized process.Figure 1 shows the morphology of the high Co-Ni steel powder particles, the chemical composition is listed in Table 1.

Laser cladding apparatus and parameters
The high Co-Ni secondary hardening steel coating was deposited by a 6 KW optical fiber laser system equipped with a coaxial powder delivery system and an argon shielding gas feed device.The laser beam at the focal length (255 mm) was a circular spot with the diameter of 0.8 mm.The laser cladding parameters are summarized in Table 2.The schematic of the experiment is depicted in Figure 2(a), and the laser cladded single-layer specimen is displayed in Figure 2(b).From the cladding surface, no evident oxidation, crack and porosity can be observed on the appearance of coating, and the span between cladding passes is very uniform.
Table 2. Laser cladding parameters used in the experiment.C respectively, and then cooled to the ambient temperature in air.

Microstructural characterization and phase analysis
Before evaluating the microstructures, the as-deposited and post-heat-treated specimens were prepared by sectioning, mounting, lightly grinding, polishing and etching.The final stage of polishing was performed with 0.5μm colloidal silica, and then the specimens were etched for 30s with a 4 % natal solution.Their microstructures were then observed and analyzed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy X-ray spectrometer (EDS).The chemical compositions of the subgrain structure in the high Co-Ni secondary hardening steel coatings were analyzed by the EDS.

Post-heat treatment
The microhardness of the above specimens was measured by a FM-800 Vickers microhardness tester at room temperature under a load of 500 g and a dwell time of 10 s.The final microhardness value of each sample was calculated by averaging data from four indentations.

The as-deposited coating.
After the laser cladding, the coating of high Co-Ni secondary hardening steel was virtually of 0.4± 0.02 mm in thickness and metallurgically bonded with the substrate, as shown in the cross-sectional micrograph (Figure 3(a)).In addition, it exhibited a fine and uniform microstructure consisting of the typical columnar morphology, as shown in Figure 3(b).The columnar grains were relatively fine and formed with evident preferred orientations.This is attributed to the rapid solidification in the laser cladding process, during which the solidified grains always grew along the direction of maximum temperature gradient.The columnar grain spacing was markedly less than 10 µm.   4 displays the microstructure of the as-deposited high Co-Ni steel coating that was observed under the secondary electrons of SEM.It can be clearly seen that the microstructure of the asdeposited coating was mainly composed of four phases with different contrasts and morphologies.Three types of phases were annotated in Figure 4(a), which were dark gray columnar grains (i.e.matrix), gray lath grain boundary phases with the width of about 500 nm-1.0μm and white needleshaped phases with the length of approximately 500 nm-3.0μm precipitated in the columnar grains, respectively.Their chemical compositions were analyzed by EDS on SEM and given in Table 3.The fourth phase was the adequately fine-scale (less than 200 nm in length) and rod-shaped precipitates that always dispersed in the dark gray columnar grains, as indicated by the yellow arrows in    This indicates that the above post-heat treatments have no apparent effect on the grain morphology of the laser cladded high Co-Ni steel coating.However, some microstructure evolutions occurred in the high Co-Ni steel coating after these post-heat treatments.When carefully comparing these SEM micrographs shown in Figure 5, it was found that the quantity of the retained austenite at the boundaries decreased significantly as the temperature of post-heat treatment increased from 200 o C to 600 o C, whereas that of the needle-shaped M 3 C cementite tended to increase.It is obvious that the increased M 3 C cementite should be a decomposition product of the retained austenite, which results in many M 3 C cementite precipitates at the boundaries, accompanied by reduction of the retained austenite (Figure 5(d) to 5(f)).
On the further magnified SEM micrographs (Figure 6 and 7), in addition to the coating heat-treated at 200 o C, there were more precipitates of the M 2 C-type carbide in the post-heat-treated coatings than in the as-deposited coating.Meanwhile, the quantity of fine M 2 C carbides increased with an increase in the post-heat-treatment temperature from 200 o C to 600 o C. Furthermore, the M 2 C carbides showed an obvious coarsening at both 550 o C and 600 o C, as shown in Figure 6(e), 6(f) and 7(b).This is in accordance with the findings of Lee [21], Kwon [22] and Zhong [23].It can be seen that the diameters of the coarse M 2 C carbides (Figure 7) post-heat-treated at 600 o C were more than twice that of the fine carbides when post-heat-treated at 400 o C.

Microhardness of the as-deposited and post-heat-treated high Co-Ni steel coatings
The

Effect of post-heat-treatment temperature on the coating microstructure
The tempering and aging of the as-quenched high Co-Ni secondary hardening steels have been widely studied in the past twenty years [15,16].It is well known that heat-treatment temperature is a significant factor influencing the microstructure of these secondary hardening steels.In fact, the microstructure of the laser cladded high Co-Ni steel coating exhibited obvious microstructural transformations after the post-heat treatment in the temperature range of 200 o C to 600 o C (Figure 5, 6 and 7).The lath austenite exiting at the boundaries of the coating, called retained austenite, exhibited high sensitivity to the temperature.They reduced markedly with the increase of the heat-treatment temperature and disappeared completely after the post-heat treatment at 550 o C and 600 o C for 2 h (Figure 5(e) and 5(f)).This is different from some observations for as-quenched high Co-Ni steels in literature [10,16,24].It was reported that the retained austenite in the alloy steels also might lose stability and decompose into a mixture consisting of ferrite and carbides (mainly cementite) in the temperature range from around 280 o C to 650 o C [25][26][27].Obviously, the decomposition of retained austenite should depend on the holding time and temperature.A higher heat-treatment temperature would accelerate the austenite decomposition at the same holding time, because the increase of the temperature improves the diffusion rates of the M (Fe, Cr, Co and Ni) and C atoms, promoting the M 3 C cementite and ferrite to be produced from the retained austenite [27,28].Consequently, this also resulted in the quantity of the M 3 C cementite increasing greatly with an increase in the post-heattreatment temperature (Figure 5(b) to 5(f)).and 6).This is because many dislocations exited in the carbon-supersaturated matrix of the laser cladded high Co-Ni steel coating, which offer the heterogeneous nucleation sites for the formation of M 2 C carbides [26,29].Moreover, it is known that the relatively high post-heat-treatment temperature is able to greatly promote the carbon atoms to diffuse interstitially, which provides a sufficient thermodynamic driving force for the nucleation and growth of M 2 C carbides [16,29].Compared carefully with the asdeposited high Co-Ni steel coating (Figure 4(b)), more precipitates of the M 2 C carbide were observed, as expected, in the columnar matrix of the laser cladded coatings after the post-heat treatment in the temperature range from 300 o C to 600 o C for 2 h (Figure 6).This indicated that the M 2 C carbides could begin to precipitate from the unrecovered matrix at a heat-treatment temperature of around 300 o C. As discussed in the previous paragraph, the higher temperature significantly accelerates the diffusion rates of the M (such as Fe, Cr, Co and Ni) and C atoms in the carbon-supersaturated matrix, directly inducing a higher driving force for the formation and growth of M 2 C carbides.Therefore, this explains the increase of the quantity of

Relationship between the microstructure and microhardness
The difference of the microhardness among the as-deposited and six post-heat-treated coatings is mainly attributed to their different microstructures (Figure 5, 6 and 7).It is known that the ultra-high strength and hardness of the high Co-Ni secondary hardening steels are primarily dominated by the precipitation of M 2 C carbides, followed by their carbon-supersaturated matrix.In the literature, it was confirmed that theM 2 C carbides possessing high coherency with the BCC ferrite matrix could provide a reasonably strong secondary hardening for the high Co-Ni steels [10,15,23].However, it was also indicated that due to over-tempering, the M 2 C carbides would lose coherency with the matrix as they coarsened, which invariably decreased the hardness and strength [15,17,21].The onset of the coarsening of M 2 C carbides and the loss of coherency occurred at the temperature of approximately 500 o C in the high Co-Ni steels [7,23,29], which are in accordance with the result of this work (Figure 6d to 6f).In addition, it is noted that the dislocation recovery occurs in the carbonsupersaturated matrix of all the steels during high-temperature tempering and the dislocation density reduces rapidly as the temperature increases [22,30,31], further declining the hardness due to loss of dislocation strengthening.The above three factors resulted in the increase in the microhardness of the laser cladded high Co-Ni steel coatings with an increase of the post-heat-treatment temperature from 200 o C to 400 o C, followed by a drastic decrease between400 o C and 600 o C (Figure 8).High residual stress is frequently present in laser cladded coatings, inducing a slight increase of their hardness, as reported by Sun [12].It is known that heat treatment is very beneficial to relieve the residual stress of the coatings.The post-heat treatment at 200 o C for 2 h could slightly reduce the residual stress of laser cladded coatings and promote some carbides (primarily M 3 C cementite) (Figure 6) to be transformed from the supersaturated matrix, although the low temperature and short holding time may not provide sufficient thermodynamic driving force for the precipitation of M 2 C carbides.Obviously, any decomposition of the carbon-supersaturated ferrite would reduce the density of dislocations and the interstitial carbon concentration in the matrix, thus inducing a loss of dislocation and solid-solution strengthening.This effect was observed by a slight decrease in microhardness of the laser cladded high Co-Ni steel coating after the post-heat treatment at 200 o C (Figure 8).

Figure 1 .
Figure 1.Morphology of the high Co-Ni steel powder particles.

Figure 2 .
Figure 2. Morphology and dimensions (mm) of the laser cladding specimen: (a) experimental schematic, (b) experimental specimen.2.3.Post-heat treatment After laser cladding, part of laser-clad specimens were subjected to a post-heat treatment in an electrical resistance furnace.They were held for 2 h at 200 o C, 300 o C, 400 o C, 500 o C, 550 o C and 600 o

Figure 3 .
Figure 3. Optical micrographs of the as-deposited high Co-Ni steel coating: (a) cross-section morphology, (b) microstructural morphology of the coating.Figure4displays the microstructure of the as-deposited high Co-Ni steel coating that was observed under the secondary electrons of SEM.It can be clearly seen that the microstructure of the asdeposited coating was mainly composed of four phases with different contrasts and morphologies.Three types of phases were annotated in Figure4(a), which were dark gray columnar grains (i.e.matrix), gray lath grain boundary phases with the width of about 500 nm-1.0μm and white needleshaped phases with the length of approximately 500 nm-3.0μm precipitated in the columnar grains, respectively.Their chemical compositions were analyzed by EDS on SEM and given in Table3.The fourth phase was the adequately fine-scale (less than 200 nm in length) and rod-shaped precipitates that always dispersed in the dark gray columnar grains, as indicated by the yellow arrows in Figure 4(b).
Figure 3. Optical micrographs of the as-deposited high Co-Ni steel coating: (a) cross-section morphology, (b) microstructural morphology of the coating.Figure4displays the microstructure of the as-deposited high Co-Ni steel coating that was observed under the secondary electrons of SEM.It can be clearly seen that the microstructure of the asdeposited coating was mainly composed of four phases with different contrasts and morphologies.Three types of phases were annotated in Figure4(a), which were dark gray columnar grains (i.e.matrix), gray lath grain boundary phases with the width of about 500 nm-1.0μm and white needleshaped phases with the length of approximately 500 nm-3.0μm precipitated in the columnar grains, respectively.Their chemical compositions were analyzed by EDS on SEM and given in Table3.The fourth phase was the adequately fine-scale (less than 200 nm in length) and rod-shaped precipitates that always dispersed in the dark gray columnar grains, as indicated by the yellow arrows in Figure 4(b).

Figure 4 (
Figure 3. Optical micrographs of the as-deposited high Co-Ni steel coating: (a) cross-section morphology, (b) microstructural morphology of the coating.Figure4displays the microstructure of the as-deposited high Co-Ni steel coating that was observed under the secondary electrons of SEM.It can be clearly seen that the microstructure of the asdeposited coating was mainly composed of four phases with different contrasts and morphologies.Three types of phases were annotated in Figure4(a), which were dark gray columnar grains (i.e.matrix), gray lath grain boundary phases with the width of about 500 nm-1.0μm and white needleshaped phases with the length of approximately 500 nm-3.0μm precipitated in the columnar grains, respectively.Their chemical compositions were analyzed by EDS on SEM and given in Table3.The fourth phase was the adequately fine-scale (less than 200 nm in length) and rod-shaped precipitates that always dispersed in the dark gray columnar grains, as indicated by the yellow arrows in Figure 4(b).

Figure 4 .
Figure 4. SEM micrographs of the as-deposited high Co-Ni steel coating: (a) microstructure and phases, (b) carbides precipitated in the matrix.According to the chemical compositions in Table3and previous investigation on laser cladded high Co-Ni secondary hardening steel coating by the authors[19], it is readily verified that the columnar grains indicated by 1 in Figure4(a) were carbon-supersaturated ferrite with BCC lattice, which contained remarkably higher Fe content than the other two phases and relatively lower Co and Ni contents.The lath phases (indicated by 2) in the grain boundaries with evidently higher Co and Ni contents were identified as retained austenite.The needle-shaped phases (indicated by 3) containing large percentages of C were undoubtedly considered as the M 3 C cementite.The fine-scale and rodshaped precipitates shown in Figure4(b) were distributed only in the ferrite matrix and EDS was unable to analyze the chemical composition due to their ultra-small size.However, according to the specific distribution, fine size and morphology, it is suggested that they were M 2 C-type carbides,

Figure 5
and 6 show the microstructures of the laser cladded high Co-Ni steel coatings after the postheat treatment at 200 o C, 300 o C, 400 o C, 500 o C, 550 o C and 600 o C for 2h.Although the coatings were heat-treated at different temperatures, uniform columnar morphology was still clearly observed in all of the coatings and similar to the grain morphology of the as-deposited coating (Figure 4(a)).
microhardness variations with post-heat-treatment temperature for the laser cladded high Co-Ni secondary hardening steel coatings are shown in Figure 8.The curve of these microhardness values exhibits a reverse parabola shape in the temperature range of 200 o C to 600 o C.It is obvious that the microhardness of high Co-Ni steel coating increased distinctly as the temperature of post-heat treatment increased from 200 o C to 400 o C, then decreased sharply between 400 o C and 600 o C. In addition, although the microhardness of the coating heat-treated at 200 o C for 2h appeared to decline slightly comparing to that of the as-deposited coating, the microhardness value improved remarkably after the post-heat treatment in the temperature range of 300 o C to 500 o C.This indirectly confirms that post-heat treatment is indeed an effective method for ameliorating the wear resistance of the laser cladded high Co-Ni secondary hardening steel coating.

Figure 5 .
Figure 5. Microstructures of the high Co-Ni steel coatings after the post-heat treatment at 200 o C (a), 300 o C (b), 400 o C (c), 500 o C (d), 550 o C (e), 600 o C(f).

Figure 6 .
Figure 6.Precipitates of the M 2 C carbide in the matrix after the post-heat treatment at 200 o C (a), 300 o C (b), 400 o C (c), 500 o C (d), 550 o C (e), 600 o C (f) Different from the M 3 C cementite, the fine-scale M 2 C carbides precipitated in the columnar matrix were obviously transformed from the carbon-supersaturated ferrite (i.e.matrix) (Figure 4(b)and 6).This is because many dislocations exited in the carbon-supersaturated matrix of the laser cladded high Co-Ni steel coating, which offer the heterogeneous nucleation sites for the formation of M 2 C carbides[26,29].Moreover, it is known that the relatively high post-heat-treatment temperature is able to greatly promote the carbon atoms to diffuse interstitially, which provides a sufficient thermodynamic driving force for the nucleation and growth of M 2 C carbides[16,29].Compared carefully with the asdeposited high Co-Ni steel coating (Figure4(b)), more precipitates of the M 2 C carbide were observed, M 2 C carbides in the laser cladded high Co-Ni steel coatings with an increase of the post-heat-treatment temperature from 300 o C to 600 o C and the coarsening of M 2 C carbides at 550 o C and 600 o C.

Figure 7 .
Figure 7. Morphology of the M 2 C carbides in a high magnification: fine precipitates post-heat-treated at 400 o C(a), coarse precipitates post-heat-treated at 600 o C (b).

Figure 8 .
Figure 8. Microhardness evolution after the post-heat treatment at the studied temperature range for laser cladded high Co-Ni steel coatings 5. Conclusion The effects of post-heat treatment on the microstructure and microhardness of laser cladded high Co-Ni secondary hardening steel coating were studied.The key conclusions are summarized as follows: 1.The microstructure of laser cladded high Co-Ni steel coating changed evidently after the post-heat treatment.The quantity of the retained austenite at grain boundaries decreased significantly when the temperature of post-heat treatment increased from 200 o C to 600 o C, while that of the needle-shaped M 3 C cementite and rod-shaped M 2 C carbides increased.This was also accompanied by the decomposition of some retained austenite.The M 2 C carbides evidently coarsened at 550 o C and 600 o C. 2. The microhardness of the laser cladded high Co-Ni steel coating at each post-heat-treatment temperature correlations with the precipitates of the M 2 C carbide.The value increased with an increase of the fine-scale M 2 C carbides in the temperature range of 200 o C to 400 o C. It then decreased sharply between 400 o C and 600 o C due to the coarsening and incoherency of M 2 C carbides and recovery of dislocations in the carbon-supersaturated matrix.3.An appropriate post-heat treatment is beneficial to modify the microhardness of laser cladded high Co-Ni secondary hardening steel coating.The optimum temperature range is 400 o C to500 o C.

Table 3 .
Chemical compositions of the phases, annotated in Figure4(a), analyzed by EDS