More demanding emission standards have resulted in fire box atmospheric chemistries that are extremely corrosive to conventional iron based low carbon tubing.  This has forced the utilities and the boiler OEMs to clad the tubing with alloys that can with stand these corrosive atmospheres.  Current cladding methods produce thick welded clad layers with high dilution, low quench rates, and rough surfaces. These thicker clad layers present a high thermal resistance due to the low thermal conductivity of the clad materials.  The resulting increase in temperature make these thicker clad more susceptible to age cracking and the rough surface leads to adhesion of slag with molten alkali sulfates that cause accelerated corrosion and further decrease heat transfer efficiency. 

For increased boiler performance efficiencies future cladding must be a thin layer that does not degrade the boiler performance, does not reduce the clad material’s corrosion performance due to dilution, and must have low surface roughness to reduce slag adhesion.  Simultaneously, the weld overlay technology has to be cost effective to reduce the fabrication, and sustainability costs.   

Titanova Inc is an American Society of Mechanical Engineers [ASME] S stamp certified shop. In addition, Titanova Inc is a National board of Boiler and Pressure Vessel Inspectors {NBIC} R stamp certified for metallic repairs and alterations. Titanova’s direct diode laser cladding system has the ability to weld, at high deposition rates, a very thin and smooth single pass layer of metal onto another metal substrate, with little or no dilution.

Figure 1: Diode Laser Clad on Supercritical tubing – P3 type 1.25 OD 0.25 wall

The direct diode laser cladding system has the ability to weld, at high deposition rates, a very thin and smooth single pass layer of metal onto another metal substrate, with little dilution.  See Figure 1.  Laser cladding makes use of the laser as a heat source to melt and fuse [weld] onto the substrate of a component a material that has different and beneficial metallurgical properties.  This allows one to cost effectively customize the surface properties of less expensive substrate to provide greater resistance to corrosion, oxidation, wear and high temperature fatigue strength.  No other welding process comes close to the quality that diode laser cladding offers. This process permits the modification of the surface metal chemistry of functional parts without a significant amount of weld distortion or heat-affected zone.  For the coal fired boiler plants, surface modification benefits are for corrosion or erosion or both.  For surface corrosion/erosion for the fire side of coal boiler laser cladding of corrosion resistant alloys such as all varieties of stainless steels [300 series], nickel and chrome based alloys [Inconel™ and Hastalloys™.   For wear or erosion resistant protection diode laser can clad a variety of cobalt alloys [Stellite™] and ceramic metal matrix materials [Carbides – FeC, CrC, Tungsten Carbide (WC, T-Carbide) ].  See Figures 2 and 3.

Figure 2: Sub-critical 2.25 X 0.3”MWT SA-210 tube with Laser Clad Tungsten- Carbide [top] and 309LSS [Bot].	Figure 3: Laser Hardfacing micrographs of T-Carbide, [Left] Crushed WC particles in 30 HRC matrix, [right] Spherical WC particle in a 40 HRC matrix
Figure 4: Deoxization Zone within a supercritical boiler	Depending on the coal fired boiler all surfaces may require some kind of fireside overlay protection.  These areas such as superheater and reheater tubes and panels, lower and upper waterwalls are all potentially candidates for laser diode cladding for corrosion and erosion  protection, but the area that is of most concern is the deoxygenated zone,  and in the upper reaches of the boiler were soot erosion can be problematic. See Figure 4. These areas have shown that only a thin chemical pure layer of clad material, such as Alloy C22, Inconel 72, Inconel 52,is required for long life. The problem is that standard GMAW or GTAW cladding processes cannot deliver thin and flat clads with low chemical dilution.  This has led to age cracking and subsequent crevice corrosion. They are limited to a minimum of 0.80” or more. See Figure 5. The diode laser allows the user to achieve the most efficient material process for those applications only requiring clads as thin a 0.030”.   See Figure 6.
Figure 5: C22 clad morphology [top] and thickness [bot. 0.080”] with GMAW with GTAW wash on 347 SS 2.5” OD x 0.3” MWT tube.	Figure 6: Diode laser “AS” clad morphology [top] and thickness [bot.  0.030”] 309 SS on SA-210 P1 2.25” OD x 0.3” MWT wall
Diode Laser Cladding DOM tubing Benefit Summary Thin clads [0.030”] that have very low dilution and thus chemically pure and functionally the same as the thicker clads produced by arc-based overlay deposition technologies; Reduces weld induced distortions. Reduces the risk of age cracking This saves tremendous amount on cost of nickel-chrome based cladding alloys. The diode laser cladding has high quench rates that result in very fine grain structure, which has enhanced high temperature creep, erosion and corrosion resistance.