May 2021 Volume 3

MATERIALS

Deep Dive on Roof Failures The most common type of refractory in forge and heat treat applications is a ceramic fiber blanket material that is applied on walls, roofs, and sometimes on the lintel of the furnace. These highly versatile ceramic fiber blanket products are lightweight, have low thermal conductivity, low heat storage and excellent resistance to thermal shock. Ceramic fiber blankets are available in a variety of densities, thicknesses and temperature capabilities. Let’s take a look at the three most commonly used in the forging industry. Refractory Ceramic Fiber (2300 o F, 8# density): Refractory ceramic fiber is predominantly made of alumina and silica. It is a spun or blown into bulk fiber. Then, the bulk fiber is air-laid into a continuous mat and needled to produce a blanket. The blanket can be folded and compressed to produce modules. The trick is this fiber is only rated for continuous use at 2200oF, so it is not a great choice for forge furnaces operating continuously at 2250oF. The fiber will experience extreme shrinkage at around 5.5% after 24 hours of isothermal heating or possibly melt. Refractory Ceramic Fiber with Zirconia (2600 o F, 8# density): In this case, the 2600 o F blanket is produced in the same way as the 2300 o F ceramic fiber blanket. However, zirconia is added to the alumina and silica mix in order to increase the continuous operating temperature. While 2300 o F fiber will shrink at 5.5%, the addition of zirconia means the 2600 o F blanket will only shrink linearly at a rate of 2% when continuously operating at forging temperatures. Polycrystalline Wool: Polycrystalline fiber is produced by a sol-gel method from aqueous spinning solutions and is suitable for use at application temperatures up to 2372 o F. However, this fiber is very expensive to purchase with a limited number of manufacturers. So, what is the best choice for roofs that are approaching 2300 o F in continuous operating temperature? An engineered combination module that combines the standard 2600 o F alumina, silica and zirconia ceramic fiber with a polycrystalline hot face.This increases the temperature rating of the system to withstand higher forging temperatures while allowing the customer to benefit from cost savings by not using a full polycrystalline module. A combination module will outlast a 2600 o F ceramic fiber module because there will be little to no shrinkage of the combination module allowing heat to attack the furnace shell and anchoring system. If the anchoring system is compromised, the modules will fall out of the roof. Exhibit 3 is a photo of a successful install of a combination module engineered roof system.

of the refractory lining that experience impact, abrasion or thermal shock? Does the furnace pass uniformity testing? Common ProblemAreas in a Forge Furnace Several areas of a forge furnace tend to be more problematic than others.The roof, door seal, hearth, and flue are all areas where frequent problems occur. Let’s take a quick look at each problem area. Roof: Ceramic fiber roof modules have a tendency to shrink, allowing heat to escape through the crack between modules. Once the heat is behind the modules, the anchoring system begins to fail and modules begin to fall out. The heat oxidizes the shell causing a cancer in the roof which becomes a maintenance nightmare. Door Seal: A proper door seal will help maintain the furnace operating pressure and uniformity. A soft, ceramic fiber door lining should be used against a hard, dense refractory furnace perimeter. This design will allow for compression, eliminating stingers. Also, an engineered, refractory brick lintel system will ensure a low maintenance door seal.

Exhibit 2: Box Furnace Refractory Isometric View Hearth: Hearths see a variety of failure mechanisms. The most common are mechanical abuse such as impact, abrasion or thermal shock. However, scale from the steel can cause a chemical attack. In addition, scale can penetrate refractory cracks and crevices causing the hearth to be jacked or pushed apart. Flue: A proper fluing furnace is important for many reasons. If the furnace is under pressurized, cold air will enter the chamber affecting uniformity and fuel efficiency. If the furnace is over- pressurized, stingers will appear around the doors and other furnace penetrations.

FIA MAGAZINE | MAY 2021 37