Influence of Vanadium-Chromium Carbide on the Microstructure of Reinforced FeCrV15 Hardfacing during Laser Cladding Deposit

The increasing manufacturing technologies are a crucial aspect of industrialization. Laser additive manufacturing is the process of manufacturing using laser (heat) technology to manufacture components from scratch and or strengthening and repairing components with the aid of functionally graded material to upgrade the properties of the components. The combination of Chromium-rich and Vanadium-rich Carbide reinforced iron-based hard facings have gotten progressively significant in enhancing the corrosion and wear resistance of tools subject to adverse abrasive and impact conditions. This study investigates the effect of vanadium-chromium carbide on the microstructure of the clad with respect to its laser processing parameters.


Introduction
The increasing manufacturing technologies are a crucial aspect of industrialization. This is because the production of components and spare parts for industries such as aerospace, agriculture, nuclear, energy, and automobile are achieved through them. These technologies may be classified into conventional subtractive or shaping manufacturing and the more recent ones like additive manufacturing (AM). AM is the creation of parts by adding endless layers of the material segment through cutting-edge fabricating innovation with a 3D model design, which is sectioned into numerous layers this enables the creation of complex components without momentary advances. This technology is at present a preferred option in contrast to existing subtractive manufacturing processes. This is attributed to its numerous potential benefits, for example, its capacity to produce complex components utilizing materials that are difficult to machine ( Ref 7). Laser additive manufacturing is the process of manufacturing using laser (heat) technology to manufacture components from scratch (i.e., LENS, SLM, etc.), and, or repairing and strengthening of surfaces that are subject to adverse abrasive and impact conditions by the addition of functionally graded material(mostly in powder form) to upgrade the properties of the components(i.e., laser cladding, etc.) ( Ref 1).
Laser cladding, as surface strengthening, modification, and repairing technology, has benefits like low heat-affected zone (HAZ), minor stress deformation, very low and minimal dilution ratio, and improved metallurgical adhesion of the clad to the substrate. This well-controlled heat-affected zone is a result of a tense and focused high-power laser light creating rapid heating and cooling process which has a minimal effect on the unique property of the material. Moreover, the heataffected zone could be monitored and improved during this process, which could generally increase the strength of the material. Laser cladding also produces energy that could be regulated over the outer part of the material controlling the rate of solidification, this is the fundamental parameter in the formation of microstructure and mechanical properties (Ref 16). The existing tool repair innovation depends on material unfriendly, high-temperature welding measures. This gives laser cladding an advantage over other methods such as gas metal arc welding (GMAW), submerged arc welding (SAW), etc. With the addition (cladding) of functionally graded material as an appropriate reinforcement, laser cladding has application possibilities in enhancing the corrosion resistance and wear resistance of materials ( Ref 1,2). This makes it to be increasingly applied in surface and manufacturing engineering. Numerous variations of materials can be deposited on a substrate through laser cladding by powder injection to form a layer with thicknesses varying between 0.05 to 2 mm and widths as thin as 0.4 mm ( Ref 15).
Iron-based alloy (mild steel and carbon steel) is commonly used for the manufacturing of ground engaging components of agricultural and mining tools, these materials are hot-formed into shapes of tools followed by appropriate heat treatment (i.e., case hardening). Nevertheless, its limitation incorporates its susceptibility to wear and corrosion attacks and its poor tribological properties when exposed to adverse working conditions common to mining and tillage operations (Ref 1,12). Hence the incessant replacement of such tools. There is a need to improve the wear and corrosion resistance of steel used for such implements, and it is the key to expanding their durability in applications. The combination of both chromium carbide and vanadium carbide reinforced iron-based hard facings have gotten progressively significant in enhancing the corrosion and wear resistance of tools subject to adverse abrasive and impact conditions. The precipitate of primary VCs, eutectic VCs, and chromium-rich eutectic carbides stand as resistance against the infiltrating grating medium (Ref 6). VCs and Cr-rich carbide as reinforcements improve grain refinement of iron-based alloy, resulting in increasing the toughness, which has an improvement in wear resistance (Ref 10,14). This work aims to investigate the impact the precipitation of vanadium-chromium carbide has on the microstructure and hardness of reinforced clad on mild steel.

Laser Processing Equipment and Materials
The cladding of the FeCrV15 high carbon ferrochrome powder with a particle size distribution of À150 +50 lm was methodically done layer by layer on a steel baseplate at the Council for Scientific Innovation and Research, Pretoria, South Africa. Five tracts and three layers of the clad with 50 % overlap were deposited on the base plate. Tables 1 and 2 show the composition by weight of the high carbon ferrochrome powder FeCrV15 and the steel base plate, respectively, that was used in the experiment. The powder was supplied commercially by WearTech of South Africa, with purity well above 99%, which was utilized without modification as received. Table 3 displays the samples and their processing parameters, the combinations of these parameters utilized for the cladding were determined through optimization (Ref 4) carried out in our previous experimentation. And the best three were selected for this study.
The deposition was made on the substrate after it was sandblasted to enhance the absorptivity of the laser light to reduce the reflection of the laser beam; this also increases the adhesion between the alloy and the baseplate. The surface was    Fig. 2 The definition of dilution ratio (9) later cleaned and air dried to remove contamination before the laser cladding deposition process. The experiment was completed utilizing a 3 kW continuous wave (CW) Rofin Sinar Nd: YAG laser system represented in Fig. 1.

Metallographic Investigation and Image Processing
The samples were sectioned, grounded, and polished for metallographic examination. Both dilution rate A (see Fig. 2) and microstructure were of interest. The carbide precipitation in the microstructure was observed by optical microscope (OPM) and scanning electron microscope (SEM). The samples were etched with modified FryÕs Reagent (150 ml H 2 O, 50 ml HCl, 25 ml HNO 3 , and 1g CuCl 2 ) solution.
Scanning electron microscopic equipped with electron dispersive Spectron (SEM/EDS) was used to analyze the samples. The elemental compositions were determined by the EDS at a voltage of 20kV. The diffractograms of the coatings were obtained with the aid of an X-ray diffractometer of a parallel beam (1mm radius) and Cu radiation.
Vickers microhardness tester, with a load of 2.94N (300gram force) and a dwelling time of 10s, was used to take the hardness of the coatings beginning at the top surface. The reading was taken every 0.2mm until the core of the substrate is reached.

The General Characteristic of the Clad and its Microstructure
The effect of the laser parameters can be observed in the clad characteristics and the dilution ratio, as represented in Fig. 3. Using Sample A as a reference point with a dilution ratio of $ 18%, the dilution ratio reduces to $ 16% as the scanning speed increases from 8 to 10m/s for sample B. Moreover, the reduction in laser beam power from 1200W to 800W results in a drastic reduction of the dilution ratio from $ 18 to $ 9%. However, this does not have a significant effect on the deposition rate.
From the SEM micrograph, see Fig. 4 Primarily, the vanadium carbide is indicated by the dark star-flower-like structure (Ref 13) and fishbone or rod-like solidification ( Fig. 4c and d) in the micrograph, positions 2 and 3. Furthermore, a precipitate of dark, grayish, and round shapes dispersed all over the microstructure is obvious, these indicate eutectic vanadium carbides, position 1. While the combination of eutectic vanadium-chromium carbide stands out as whitish spots (in Fig. 4d but grayish dark sport in 4c) indicating martensite all aver the microstructure. The grayish portion not affected by the etchant (position 4) is regarded as retained austenite. A moderately darker phase could be identified, separate from the grayish retained austenite with a portrait of fine eutectic lamellar microstructure at position 5. This is presumed to be chromium-rich carbide.
Additional investigations were carried out by the means of SEM-EDS analysis for further confirmation of the above assertion. The elemental composition as detect in the phases is listed in Table 4. Positions 2 and 3 have the highest vanadium content of $53 and $60 wt.%, respectively. This further corroborates the precipitation of vanadium carbide. The coloration of the eutectic vanadium carbide is relatively close to that of the primary VCs; however, the elements stand out like Fig. 4 The image of the microstructure engraved balls on the microstructure with a V-content of $14 wt.%, see Table 4 (Position 1). The Eds analysis for position 5 confirmed the assumption of chromium-rich carbide with a Crcontent of $21 wt.%.

Effect of Clad Depth on Microstructure.
The pattern of the phase arrangement changes as one moves away from the substrate into the clad layer. Close to the substrate, $200 to 400 lm into the clad, there is the formation of the eutectic carbides (VCs & Cr-rich) were observed with dark martensitic phases scatted all over the microstructure, see Fig. 5(a). At about 500 lm into the clad, the precipitation and agglomeration of primary VCs began with an average diameter of about 0.69 lm with dark martensitic phase encapsulating each one(these were identified as eutectic VCs by (Ref 9) and eutectic VCs and Cr-rich carbide were observed, see Fig. 5(b). Eutectic VCs continue to agglomerate to form star-like precipitates of primary VCs with an average diameter of 2.55 lm between 650 to 900 lm into the clad, see Fig. 5(c) and (d), respectively. From the OPM images, it was difficult to distinguish between the Cr-rich carbides and retained austenite phases up to about 750 lm away from the substrate, see Fig. 5(a), (b), and (c). However, as you move farther into the clad, they were distinguishable, see Fig. 5(d), (e), and (f). Furthermore, the eutectic VCs were observed to reduce as precipitation of primary VCs increases, this is proportionate to the height of the clad (distance away from the substrate). The effect of the laser beam power on the clad microstructure was also investigated, see Fig. 7. Figure 7 comprise two samples, A (Fig. 7b, d, and f) and C (Fig. 7a, c, and e). Sample A is fully saturated with carbide of bigger grains compare to sample C. It is observed that lowering laser beam power from 1200W to 800W results in the precipitation of primary VCs of smaller grain diameters. The hardness of sample C is observed to be 778 HV. The hardness profiles of the samples are presented in Fig. 8. The hardness is observed to be relatively higher in the clads for all the samples as compared to the substrate. This emphasizes the upgraded effect of the precipitation of vanadium and chromium-rich carbides on the microstructure of the substrate. This confirms that the behavior of the material springs from the evolution of its microstructures.

Effect of Laser
Moreover, the result of the X-ray diffractometric analysis using Cu radiation confirms and corroborates the presence of the phases as discussed above. Vanadium carbide VC, chromium carbide, and manganese silicide carbide were the carbide phases identified to be present in all the samples, however, VCs were highest in sample C as its peaks can be seen to be highest in the graph of sample C as compare to A and B, see Fig. 9.

Microstructural Characteristics of FeCrV15
From the micrograph, a round-molded VCs was observed, (Fig. 10. position 2), this begins its precipitation at 2656°C The matrix of the clad comprises of retained austenite and martensite, Fig. 10 (Position 3 & 4) because a segment in the matrix retained metastable austenite. Ref 8 explained that this situation is because of the high carbon content of FeCrV15, which reduces the martensitic inception temperature, M s to a temperature lower than room temperature. The martensitic zone encapsulating the formed carbides, lowering C-content adjacent to the carbide in the iron-based matrix. Consequently, reduction in C-content leads to an increase of martensitic inception temperature, M s. This is the justification for the special arrangement of martensite encapsulating the precipitated carbide, see Figs. 5(b) and 6(c).

Effect of Laser Power and Scanning Speed on Microstructural
Borle et al., Günther, and Bergmann stressed that the dilution rate of the melt during weld affects the formation of carbides in the microstructure of the samples ( Ref 5,9). From this experiment, it was observed that close to the substrate and at an appropriate temperature (scanning speed) (Ref 11,3), there was enough melt pool from the substrate sufficient enough for  Fig. 6(a). However, as the distance into the clad increases, there was a remelt of the eutectic carbide-rich layers in addition to the melt of the additional powder, resulting in a higher concentration of eutectic carbide which began to agglomerate into star-like primary carbide, this formation increases in size as you move further into the clad, see sample A, Fig. 6(c), (e), and (g). However, at increased scanning speed, there was a reduction in the temperature-time regime, which leads to a reduction in the volume of melt pool from the substrate (i.e., Ôreduced dilution rateÕ). This results in a higher concentration of eutectic VCs leading to the formation of star-like primary VCs precipitate close to the substrate, sample B, Fig. 6(b). More-

Conclusion
This experiment reaffirms the need for optimization of laser parameters to increase the concentration of carbide formation in the microstructure of the clad. Although the dilution rate is minimal in laser cladding, the temperature-time regime has a great impact on the size and formation of primary carbide, its grain refinement, and the modification of the microstructural features due to the reduced or increased thermal impact of alloying elements promoting heterogeneous nucleation. This has a high improvement on the hardness behavior of the samples as compares to the substrate.
These observed differences would be further study to see the impact of changes in the macrostructure of the clad relative to its height on the abrasive wear.