Laser cladding and erosive wear of Co–Mo–Cr–Si coatings
Introduction
In modern technology hardfacing is often used to produce surface coatings protecting parts against different kinds of wear, including sliding, abrasion or erosion. In addition, depending on coating material selection, chemical or high temperature corrosion protection can be provided to the surface. Hardfacing is a method to obtain desired surface properties and allow for economical consumption of expensive wear resistant alloys.
Materials particularly useful in wear applications and often used in as-cast condition are Tribaloys (trademark of Delloro Stellite Ltd.). The alloy Tribaloy T-400 contains about 50 vol.% of CoMoSi Laves phase of hardness up to 1300 HV providing adhesive wear resistance. The Mo addition gives high temperature strength to the Co based matrix. The Cr content in both matrix and Laves phase contributes to their corrosion resistance [1]. The matrix fcc crystal structure is stabilized by Ni and Fe addition [2]. This lattice type is desirable for better strength and ductility due to more active slip systems over the hcp type [3]. On the other hand, Mo and Cr, by reducing the stacking fault energy, enhance the plastic deformation induced fcc→hcp crystal structure transformation in the Co based matrix [2], [4]. This increases the alloy work hardening rate reported to be beneficial for abrasive wear resistance of other Co based alloys [5]. In the work by Halstead and Rawlings T-400 alloy in a cast and heat treated form is said to be erosion resistant owing to the combination of brittle intermetallic embedded in the softer Co solid solution matrix [6].
The solid particle erosion (SPE) effect occurs whenever solid particles carried in a gas or liquid jet at a speed exceeding 1 m/s impinge on the component surface. Particularly susceptible for SPE are components of steam and gas turbines (vanes, disks, rivets, valves), helicopter rotor blades, fluidized bed heat exchanger elements and others. There are three basic groups of erosion parameters: referring to particle flow, abrasive material properties and eroded material properties. The third group comprises primarily: material microstructure, hardness, toughness, plasticity, work hardenability, residual stresses and others. Numerous parameters affecting the SPE process make it difficult to establish a uniform model quantifying the erosion effects. In practice most cases have to be evaluated individually based on empirical data [7]. All the properties can be widely affected by the deposition technology selection of protective coatings.
Laser cladding is a modern technology oriented on producing, among others, wear resistant coatings. The process objective is to use a laser beam to melt the coating material in the form of continuously fed powder or wire or a pre-deposited paste together with a substrate layer of controlled thickness [8]. As a result of using the high power density accomplished by the laser beam, a high temperature gradient at the coating–substrate interface and rapid cooling of the coating material can be achieved [7]. The result is a strong microstructure refinement, solid solution supersaturation in alloying elements and high residual stresses in the coating. An important feature is the ability to control the coating from the substrate dilution in laser cladding. Numerous tests indicated superior sliding and abrasive wear resistance of Co based alloys deposited by laser cladding, compared with other welding techniques [5], [7], [9]. On the other hand, there is little data available regarding the erosion resistance of laser deposited coatings.
This paper shows the results of an investigation carried out to study the microstructure and properties of laser cladded Tribaloy T-400 coatings as well as their solid particle erosion behaviour.
Section snippets
Experimental procedure
The alloy T-400 of chemical composition listed in Table 1 was deposited as coatings on 0.45% C structural steel and IN 718 substrates by means of laser cladding.
The laser treatment was performed on 25×25×6 mm3 steel coupons using a 1500 W continuous wave CO2 laser with generated beam power of 1000 W. The scanning speed of 8 mm/s was applied during deposition. The laser beam in TEM00 mode (with Gaussian energy density distribution) was defocused on the surface to a spot about 2.6 mm in diameter. The
Results and discussion
The cross-section of Tribaloy T-400 coating laser deposited on structural carbon steel substrate is presented in Fig. 1. The layer of 1.15 mm average thickness was crack and porosity free. The structure was not uniform on the coating cross-section with heat-affected zones due to laser track overlapping effect. The microstructure of the cladded track center consisted of a fine lamellar eutectic mixture of Laves and Co based phases forming colonies of 7–15 μm in diameter, elongated in the direction
Concluding remarks
Proper selection of laser cladding parameters allows us to obtain high quality and low dilution Tribaloy T-400 coatings on different substrates. Such a material is characterized by high hardness due to microstructure refinement, supersaturation of phases by alloying elements and presence of highly dispersed and brittle Laves phase. At the same time, however, the plastic properties of the alloy deteriorates, which may have a negative effect on the erosion behavior of T-400 alloy by promoting the
Acknowledgments
This project was supported by KBN under Grant No. 7 T08C 005 12.
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