Refractories: Ceramic Welding—Performance by Design

Ceramic welding is used to repair refractories in industrial furnaces at operating temperature, minimizing operational costs due to shutdowns and produciton delays.

The ceramic welding process is a method used to repair refractories in industrial furnaces at operating temperature. As a result, operational costs due to shutdowns and production are minimized and sometimes eliminated.

Ceramic welding uses a mixture of ceramic and metallic particles, which are conveyed in an oxygen or oxygen/air mixture through a system of hoses and specialized cooling pipes to the repair area. The heat from the furnace atmosphere and base refractory initiates an exothermic reaction between the metals and the oxygen. The heat generated from this exothermic reaction is substantial enough to melt or soften the ceramic particles as they pass through the reaction zone before they are deposited onto the surface of the base refractory. The heat also softens or melts the surface of the base refractory being repaired. Thus, upon cooling, a ceramic or crystalline bond is created between the base refractory and the ceramic weld repair mass. Subsequent layers of repair material are, in turn, deposited onto the area until the desired thickness is achieved. (See sidebar: Example of Exothermic Reaction.)

The repair material used in the ceramic welding process varies with the type of substrate that is being repaired. These include silica, fused silica, magnesia, fireclay, AZS and high alumina, as well as other materials. Depending on the repair criteria, the selection of equipment can also affect the weld quality.

Figure 1. A closed pressurized system.

Ceramic Welding System Types

The machines that are used to meter the material and oxygen and transport them to the repair site can be categorized into two principle types: closed pressurized machines and open venturi-style machines. The closed pressurized system can reach material flow rates of 0.5 to 2.0 lbs/minute (0.2 to 0.9 kg/minute), while the open venturi systems can reach 1.5 to 25 lbs/minute (0.7 to 11.3 kg/minute), depending on machine design.

The closed pressurized machine uses a hopper pressurized with pure oxygen at a constant pressure to convey the material to the welding zone (see Figure 1). This type of system allows no direct opening to the atmosphere, so no air is entrained into the material stream. Since this system is pressurized, the backpressure resistance that results from excessive hose and pipe length and elevation are negligible, and therefore should not be considered a limiting factor in its use.

Figure 2. An open venturi-style system.
The open venturi system uses a high-pressure/velocity oxygen source to convey the welding material through a venturi to the welding zone (see Figure 2). This type of system also entrains atmospheric air along with the welding material as a part of the venturi effect. Because all venturi systems are sensitive to backpressure, the resistance seen from hose and pipe length and elevation gain can become factors with this type of system.

The effects of the entrained air become apparent when particles that are conveyed in the fluid stream—either oxygen or an oxygen/air mixture—pass through the combustion zone. The velocity of the fluid stream in the open venturi-style system is increased by the addition of the entrained air. This increase can be determined with the following equation:

V = Q/A

where V = velocity, Q = volumetric flow rate and A = cross sectional area. The addition of the entrained air has been found to increase the fluid stream velocity by 50 to 75%. This increase in velocity interferes with the complete combustion of the metallic particles at the reaction site, which affects the resulting repair in two ways: a deterioration of the physical properties of the finished weld mass, and increased rebound of excess weld material during the repair process.

The percentage of melting/softening occurring at the site of the exothermic reaction is reduced due to the increased speed of the ceramic particles through the combustion zone. The fluid velocity in an open venturi system allows the material to pass through the combustion zone in 0.03 to 0.05 seconds versus the 0.05 to 0.08 seconds required in a closed system. This decreased dwell time in the combustion zone results in unreacted or partially reacted metals in the finished repair mass, which can contribute to increased rebound and decreased weld mass properties.

Additionally, the presence of nitrogen in the air interferes with the complete combustion of the metallic particles. Since the nitrogen in the entrained air does not contribute to the combustion, it acts as a heat sink by absorbing thermal energy that would normally be used in the heating of the ceramic particles. The amount of heat absorbed by the inspired air can be seen with the following equation:

Q = M (hf - hi)

where Q = heat transfer rate, M = mass flow rate, hf = final enthalpy, and hi = initial enthalpy. (See sidebar: Example of Absorbed Heat.)

Table 1. Improvement in weld mass physical properties of a closed pressurized system vs. an open venturi-style system.
The common factors used to compare two similar weld masses are cold crushing strength, gas permeability, density, apparent porosity and weight increase from unreacted metals. Table 1 shows the relative difference between the weld mass of the two systems.

Choosing the Right Machine

Several factors need to be taken into consideration when deciding which style of machine is best suited for a ceramic welding repair. The most typical considerations are time, quality and any possible contamination risks that the rebound may have in the production unit.

The open venturi-style machine allows for a weld mass to be applied more quickly. The tradeoff of the higher welding flow rate is increased rebound (pollution risks) and decreased physical properties of the repair mass. This style of machine works best in areas such as coke ovens and ferrous and nonferrous vessels, where time restriction is a major factor, and glass tank regenerators and ports, where rebound is usually not an issue and temperatures are lower.

The closed pressure-style machine allows the material to be applied without the negative influences of entrained air. Although the application can be much slower, the resulting weld properties are often superior to the resulting weld mass of venturi-style machines. Additionally, the lower rebound rates mean less contamination risks. This style of machine is best used when the highest quality repair is needed, as in the main melter of glass tanks.

Although the two styles of systems perform the same task, choosing which machine will best achieve the optimal end result requires weighing the advantages and disadvantages of each type of system against the operational constraints of the industrial furnace or vessel.

SIDEBAR: Example of Absorbed Heat

Given the following situation:
M = 0.27 kg/min air entrainment
h2500K = 2,882.3 kJ/kg
h300K = 300.4 kJ/kg

The total heat lost to the air entrainment is:
Q = 0.27 kg/min (2,882.3 - 300.4) kJ/kg
Q = ~700 kJ/min (663 BTU/min)*

*~15% of the total heat input

SIDEBAR: Example of Exothermic Reaction

Metal + Oxygen = Metal Oxide + Heat Si + O2 = SiO2 + ~910 kJ/mole (862 BTU/mole)
4Al + 302 = 2Al2O3 + ~1675 kJ/mole (1590 BTU/mole)

SIDEBAR: Welding System Comparison

Closed Pressurized Systems
  • Line length and elevation not a factor
  • No entrained air
  • Flow rates from 0.5-2.0 lbs/min (0.2-0.9 kg/min)
  • Lower fluid velocity, resulting in lower rebound
  • Better combustion of metals
  • Best physical properties for repair mass

Open Venturi-Style Systems

  • Line length and elevation gain become factors due to back pressure
  • Entrained air increases material speed thus increasing rebound
  • Entrained air absorbs heat away from the welding process leaving more unreacted metal in weld mass
  • Increased flow rates: 1.5-25.0 lbs/min (0.7-11.3 kg/min)
  • Good physical properties for repair mass

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