By the 1970s, energy savings was a priority for most thermal-processing industries. Lightweight ceramic fiber was establishing itself as the leading refractory material that could fulfill the pyro-processing industries’ energy-management goals. Multi-layered linings using monolithic (medium- and high-density castable) coupled with lightweight backup materials were also being developed in the quest for energy efficiency up to this decade.
Both technologies provide savings and reduced installation labor costs compared with traditional brick installations. The push for increased energy efficiency, coupled with a desire for faster turnarounds, pressured the refractory industry to think out-of-the-box to accommodate customer needs.
Unlike monolithic linings, ceramic fiber installs without time-consuming refractory dry-out. The layered ceramic-fiber blanket systems did not require dry-out but were still labor-intensive due to the pre-welded anchors pins and multiple layers (Fig. 1).
It was during this decade that ceramic-fiber module anchored blocks (modules) were developed and patented by J. Mase, R. Sauder and G. Kendrick from Sauder Industries (Fig. 2) and Carlisle Byrd from the J.T. Thorpe Company. These designs incorporate metallic anchoring into the pre-assembled modules, allowing for brick-mason-friendly installation that is popular to this day. Wallpaper ceramic-fiber linings are still a common construction practice for applications operating up to 2000°F (1100°C) or lower.
The new module designs protected the vulnerable stainless steel attachment hardware from maximum service temperatures that, at the time, could exceed 2400°F (1300°C). In addition, these new modular designs centered the metallic attachment at the cold face of the module block. These designs also reduced the risk of metal hardware exposure to full furnace operating temperatures due to ceramic-fiber shrinkage at the module joints.
By the 1980s, the world energy crisis brought ceramic fiber into the spotlight because of its recognized energy savings and reliability. Improvements in ceramic-fiber chemistry enhanced continuous-temperature use limits. Ceramic-fiber manufacturers replaced older pneumatic blown fiber production lines with state-of-the-art centrifugal spinning. Shot (waste) content was lowered.
Increased fiber diameters and length resulted in improved blanket tensile strength and lowered shrinkage caused by sintering. The industry developed large-format stack-bonded fiber systems with the goal of minimizing the overall number of joints in linings. Metallic anchoring required further design improvements to accommodate higher operating temperatures and a multitude of geometric configurations necessary to line complete vessels.
By the 1990s, large ceramic-fiber modules became popular with consumers. Ceramic-fiber linings were reaching the 3000°F (1650°C) threshold, which applied further stress on the fixed-temperature use limits of metallic module hardware.
Since the 1970s, the trend has been to reduce the operational temperatures of internal 304 SS, 310 SS and 316 SS hardware in order to maximize the life of the module linings. Stainless steel refractory hardware oxidizes to failure based on linear relationships between thickness/diameter of hardware, nickel/chrome ratio of the stainless steel, temperature and duration of exposure (Fig. 3).
The effects of higher-temperature applications coupled with longer hardware life expectancy are overcome in three ways: bury the hardware deeper into the ceramic-fiber lining to reduce the continuous exposure temperature at the anchoring interface; use higher-nickel alloys such as Inconel 601; and increase the pathway for heat loss out of the anchoring system into the vessel casing.
Positioning the metallic hardware toward the cold face within the module is preferred for high-temperature applications. Use of safety backup linings behind modules can inhibit this goal of reducing static interface temperatures. This practice places the embedded metallic hardware closer to the furnace hot face, reducing its creep strength and oxidation resistance.
Increasing the nickel-chrome ratio or switching to a specialty nickel alloy drives hardware costs up, which can render them economically impractical. The most often-overlooked means to enhance metallic hardware life expectancy is to eliminate discontinuity in heat-loss pathways. This can be done by eliminating bottlenecks for heat to conduct through the anchoring system to the cold face of the furnace steel shells.
This discussion focuses on thermal conductivity of the overall metallic anchoring “system” incorporated within modules. Except for module anchors that are fillet welded or stud welded to the vessel, most ceramic-fiber attachment systems use threaded components as an interface between the vessel casing and the internal hardware of the module.
Eliminating heat-loss restrictions due to intermediate fasteners is a logical goal since heat-conduction pathways in threaded connections are minimal at best (Fig. 4). A new patented welding method named the TiMig™ Fusion process can weld plate and other thin-gauge metals to steel vessels without the use of an intermediate fastener such as threaded components or mechanical clips (Fig. 5). TiMig, unlike standard MIG welding, is a controlled-penetration weld that provides a direct path for heat to conduct to the steel casing through the weldment of a thin-gauge alloy yoke directly to the steel shell (Fig. 6).
Installing Ceramic-Fiber Modules
Since the 1970s and 80s, ceramic-fiber module designs evolved utilizing a variety of anchoring methods. Outside of the many internal hardware variations, there are two fundamental concepts that have remained central to ceramic-fiber module installation. Regardless of chemistry, module dimension and density, all these configurations use one of two fundamental philosophies for metallic anchoring:
- Attachment to the vessel casing at the center or other location away from module joints requiring a preinstalled anchor attachment
- Side anchoring that holds the modules or stack-bonded packs to the casing where modules do not require any preinstalled anchors
Either of these anchoring philosophies rely upon one of three forms of weldment for adherence to the vessel casing:
- Fillet welding an independent anchor or threaded attachment
- Stud welding an independent anchor or threaded attachment
- The Sauder System, which uses a threaded stud, ferrule, aluminum torque tube and nut stud welded with a drill motor weld-gun system
Closer examination of center-mounted ceramic-fiber module attachment points to a common feature. All methods of metallic anchoring require an intermediate mechanical attachment that interfaces the threaded stud/weldment to the module’s internal hardware. The exception to this is when side-mounted external refractory anchors are fillet welded or stud welded between the ceramic-fiber modules. This installation practice allows the hardware to be welded directly to the vessel, enabling maximum heat transfer to the vessel casing, but it also exposes the anchor to increased oxidation and creep due to possible shrinkage between modules.
Aside from offering a clear pathway for heat transfer, TiMig provides cost savings through simplification. The elimination of intermediate mechanical attachment systems, such as threaded products, reduces cost and difficulties associated with sourcing specialty-alloy fasteners. This is not a minor problem.
Since the 1980s, the 80/20 ratio of 304 SS versus 310 SS usage has flipped to 80% 310 SS versus 20% 304 SS. Furnace linings last longer than they did two decades ago, and the use of alternative fuels has added more complexity to alloy selection. TiMig eliminates the fastener sourcing problem by using standard MIG wire, which can be procured from local weld suppliers in whatever alloy is required.
For 50 years, the Sauder invention using a drill motor/aluminum torque tube/threaded stud and ferrule has been an industry standard for module attachment directly to vessels. Several technology advancements are crippling this aging design. The availability of 110V corded drills is succumbing to battery-operated DC drill motors, eliminating the unit’s ability to conduct the 300-amp welding current through the drill motor to the fastener.
Modern welding power supplies, with few exceptions, incorporate internal software designed to compensate for human error in the process of stick welding. The Sauder system presents this stick-welding performance software with an unrecognizable profile, resulting in random power fluctuations initiated by this software as it attempts to stabilize an unidentifiable electrical load.
This variation in amperage/voltage can be witnessed by snapping a photograph of current output during the weld cycle. This anomaly can contribute to a significant number of misfires. Also, the multiple number of components in this attachment system create a challenge for module manufacturers and installers. If any part of this system is compromised during the sourcing, assembly, shipping or installation, the variables contributing to misfire are increased.
Ceramic-fiber module installers are all too familiar with the problems associated with blind-weld misfires in center-anchored module designs. For this reason, ceramic-fiber modules are internally manufactured in a 12-inch x 12-inch format. The misfire problems often require installers to dismantle the module to replace the failed weld components, which is very time-consuming and costly.
The problems associated with correcting and re-welding a misfired anchor dramatically adds installation labor costs. Anyone who has experienced this frustration would agree it has been a detrimental aspect of this system since its inception in the 1970s. This high probability of misfires also makes it very risky to manufacture larger-sized modules with this type of center-mounted hardware, where multiple welds are required, usually on 12-inch centers.
With the TiMig process, a visible continuity sensor prevents the unit from firing until the hardware embedded within the module contacts the vessel. The TiMig center-mounted hardware can be torque tested individually if there is any concern about the integrity of the weld using a standard long-neck torque wrench. Due to technology improvements now available in the welding industry, TiMig can also monitor voltage, amperage and wire speed on each trigger pull, effectively telling the operator if the weld is within acceptable AWS (American Welding Society) parameters.
The TiMig™ Fusion process represents a paradigm shift in the attachment method for heat-containment linings utilizing center-mounted hardware for ceramic-fiber modules. It simplifies the procurement, assembly, design and installation process for contractors and OEMs by reducing the number of components required to anchor ceramic-fiber modules.
The elimination of the internal stud assemblies decreases installation time since reassembly is never required due to misfires. Also, welds can be checked to ensure they fall within acceptable torque test limits. These components are now replaced by standard MIG wire.
An additional benefit to end users is that the TiMig welder can revert to a standard hand-controlled wire-feed MIG-welding machine with the flip of a switch. The TiMig unit can now serve as a wire-feed weld machine on structural steel, wall gussets and C-clips. A nozzle change adapts TiMig to weld any style of refractory anchor from side-mounted module anchors to wire and plated-style monolithic anchors.