|Fig. 1. Carbon-rich subsurface layer inside a refractory wall|
Soot ... the very word sends shivers down the spines of even the bravest of heat treaters. The problems this seemingly innocuous black powder creates cause management to bemoan lost productivity, maintenance men to vow to seek out those responsible and, until a permanent solution is found, makes all our lives miserable. So, how can we avoid creating soot in the first place and what can we do if/when it forms? Let’s learn more.
|Fig. 2. Oxygen probe and its alloy protection tube destroyed by melting at the exact location of the carbon-rich subsurface layer|
The presence of soot raises many red flags, one of them being that carbon from unstable furnace atmospheres is rapidly absorbed into refractory linings, which are incredibly porous. Infrequent or improper burnouts (so-called flash burnouts) give a false sense of security, as they address only the refractories near surface layer. Once absorbed, carbon will continue to diffuse deeper and deeper into the refractory (Fig. 1). The penetration halts in the temperature range of approximately 900-1300°F (480-705°C).
As more and more carbon is absorbed, the refractory loses its thermal properties and becomes conductive. The result can be damage or melting of alloy components that extend into or through these conductive layers. Heating elements, terminal ends and support hangers in electric furnaces are particularly vulnerable as are atmosphere inlets, sample tubes and even oxygen probes (Fig. 2). Gas-fired furnaces are not immune because radiant tubes are also susceptible to carbon attack and metal dusting (Fig. 3).
Many heat treaters believe box and integral-quench furnaces are only vulnerable to the effects of soot if they are performing deep-case carburizing cycles. This is not true. Furnaces running shallow case depths or even atmospheres neutral to medium and high-carbon steels (e.g., 0.60-0.80%C) are at risk.
The use of low-iron brick (<0.5% Fe) helps to retard carbon absorption. In all cases, a robust and frequent burnout procedure in which air reacts with the soot (carbon) to form carbon dioxide and carbon monoxide gas must be established to return the furnace to good working order.
There are several ways in which this furnace burnout can be accomplished. Each have advantages as well as limitations, and it is important to recognize that, done improperly, air burnouts may cause extensive damage to equipment. Extreme care must be exercised so as to avoid this situation. For integral-quench or pusher-style carburizing furnaces running endothermic gas, the following methods have been found to be effective.
|Fig. 3. Alloy radiant tube destroyed by soot accumulation in the insulation|
Method 1: Raise the dew point of the furnace atmosphere to around +70°F (20°C) with air additions and maintain this condition over a long period of time (typically 24-72 hours).
Method 2: Remove the furnace atmosphere and add a fixed volume of air, entering the furnace through a flowmeter, for a prolonged period of time (typically 12-36 hours). This is often combined with opening and closing of the furnace door(s).
Method 3: Remove the furnace atmosphere and use an air lance or wand (under highly reduced pressure) directed at locations in the furnace interior with heavy soot deposits. This is normally followed by the introduction of air through a flowmeter for a period of time (typically 4-12 hours). Special note: This is the riskiest of the air burnout methods given the potential to significantly damage the furnace interior, including an alloy fan if present. Only highly experienced personnel should be allowed to attempt this procedure.
Care must be taken to prevent overheating of the furnace during an air burnout. For this reason, the furnace temperature is usually lowered to around 1550-1600°F (845-870°C), and the process is stopped if the furnace temperature rises by 100°F (38°C) or more. Most people are not aware that carbon (soot) burns at over 4500°F (2480°C), which is high enough to melt through any of the materials in the furnace!
Furnace burnouts must be done before carbon has had a chance to build up in the refractory. They must also be frequent enough to ensure carbon diffusion is not occurring. One way to measure the effectiveness of the current burnout procedure is to extract a small section of insulation and inspect it on a frequent basis. If at any point it is determined that the insulation is absorbing an unacceptable amount of carbon, either increase the time of the burnout or the number and/or duration of burnouts or use some combination of the methods explained above.
Soot not only affects alloy life, but it deposits on the work and is then carried into the quench tank, where it negatively influences the performance and life of the quenchant.
Finally, it is a good idea to monitor the carbon potential of the furnace atmosphere during operation (via shim stock, turnbars or other methods). Atmosphere systems must be properly calibrated, and control devices such as oxygen probes should be complemented by three-gas analyzers and/or dew-point meters. Also, an automatic burnout of the oxygen probe during processing is mandatory to make sure the probe does not soot up and give false readings. In addition, the reference-air system should be checked to be sure it is operating properly. Oxygen probes should be changed based on manufacturer’s recommendations.
|Fig. 4. Soot depositing on the front load table of a mesh-belt furnace after one hour with the furnace atmosphere out of control|
Furnace Atmospheres Gone Wild
Simply stated, an out-of-control gas generator or furnace atmosphere is a nightmare scenario in which soot can form in seconds and build up significantly in just a few minutes (Fig. 4). Fluctuating dew points, either at the generator or the furnace, are a clear indication of an unstable atmosphere condition.
Today, many endothermic generators are operated in a dew-point range of +40°F to +45°F. Lowering the endothermic-gas generator dew point aids in reducing the amount of hydrocarbon enrichment gas required at the furnace but increases the frequency of maintenance on the generator itself.
|Fig. 5. Typical vacuum-furnace hot zone (Photograph courtesy of ALD-Holcroft)|
Soot and Tar Formation in Vacuum Carburizing
Two significant technological advances, low-pressure techniques – at or below 20 torr (27 mbar) – and the use of acetylene (C2H2), have so significantly reduced soot and tar formation in vacuum-carburizing furnaces that only routine maintenance is now required (Fig. 5). Today, typical maintenance of these systems consists of vacuuming out the chamber every 3-8 weeks, and the amount of soot produced is in the order of less than a handful.
The reasons for this are the reduced volume of gas needed, the lower pressure and the fact that acetylene catalytically decomposes. In other words, as acetylene enters the hot zone, it remains thermally stable until coming in contact with the load surface. In addition, the pyrolysis reaction is virtually instantaneous, which allows for carbon to quickly begin diffusing into the steel without the formation of undesirable intermediate compounds unlike so many of the other hydrocarbon gases previously used.
Soot formation inside heat-treating furnaces should be taken very seriously. Methods must be devised to prevent its formation and/or limit its deleterious effects. This is one New Year’s resolution worth keeping! IH