The direct laser deposition of AISI316 stainless steel and Cr3C2 powder
Introduction
The melting point of chromium carbide is relatively low (1850 °C), and its survival without melting during laser deposition is less certain than that of higher melting point materials such as silicon carbide or tungsten carbide. As a consequence there have been relatively few reports on the use of this material as a reinforcement included in metal coatings deposited by laser. Its application to coatings is more popular in High Velocity Oxygen Fuel (HVOF) spray, plasma spray and Detonation Gun processes. These processes are described by several authors including Staia et al. (2001), Davis (2001), Kunioshi et al. (2006), Morimoto et al. (2006), Liu and Gu (2006), Murthy and Venkataraman (2006), Murthy et al. (2007), Zhang et al. (2008), Matthews et al. (2008) and Xie et al. (2008).
In the course of research carried out on the application of laser deposition techniques to component repair and modification, chromium carbide and AISI316 stainless steel powder blends were co-deposited to improve wear and erosion resistance on stainless steel components. The results obtained from the direct deposition process and the investigation of the tracks and surfaces produced were characterized by means of SEM/EDX and XRD analysis, and by microhardness testing. Based on parameters which resulted in uniform and continuous deposition of material, continuous surfaces were deposited by overlapping tracks and subsequently surface grinding them. Sliding wear and corrosion testing were carried out on these to verify the effect of the inclusion of chromium carbide.
Section snippets
Laser deposition of chromium carbide/metal blends in the literature
Kathuria, 1998, Kathuria, 2001 deposited Cr3C2–NiCr over SU304 mild steel with 50% or 70% Cr3C2 using an Nd:YAG laser to produce surfaces by depositing tracks with an overlap of 50–60%. The higher Cr3C2 content mixture (70%) produced a track with a higher hardness; the lower carbide content (50%) tracks presented fewer thermally-induced cracks. A higher dissolution of carbides was observed in the 50% carbide tracks. The clad hardness measured was 992HV for the 70% Cr3C2 content and 697HV for
Method
The chromium carbide powder chosen was a sintered Cr3C2 powder of −325 mesh/+5 μm, −45 μm/+5 μm distribution designed for thermal spray, metal injection moulding, laser rapid prototyping and laser cladding. Blends of 33, 50 and 67 vol% Cr3C2 were co-deposited with AISI316 austenitic stainless steel powder of similar particle size; the powder and substrate material composition is given in Table 1. A Praxair Twin 10P powder feeder was used with a coaxial deposition head developed at the Department of
Visual appearance and optical metallography
Single tracks and track surfaces deposited with 33–67 vol% Cr3C2/AISI316 using CW or pulsed beam deposition presented a uniform dull grey metallic appearance for all material parameter sets used. Cracking was not observed in single tracks, but was evident in most surfaces deposited with 50 or 67 vol% chromium carbide. Low magnification optical microscopy revealed the presence of powder particles adhering to the track surfaces: these had been partially melted on traversing the laser beam, and had
Discussion
The deposition process produced uniform, continuous tracks and surfaces with excellent repeatability. No crust or slag was produced in any track or surface. The texture of the surface was granular due to the adhesion of stray particles passing through the laser beam to the solidifying surface. Isolated tracks did not present any cracks resulting from thermal stresses or other causes. Although 33 vol% chromium carbide powder content surfaces presented no cracks, surfaces deposited with 50 or 67
Conclusion
The deposition of Cr3C2 carbides was successful, with minimal porosity encountered and an excellent consistency in results. The dissolution of the original carbides was a consequence of their relatively low melting point and their high solubility in the AISI316 melt pool. The high affinity of the two co-deposited materials made successful deposition easier, and the reprecipitation of the M7C3 phase, which has a high hardness and contributes to wear resistance, ensured a good ceramic–metal bond.
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