For the purposes of this article, we define an IQ as a controlled atmosphere batch furnace with an integral quench. It is equipped with one heating chamber, one liquid quench tank, and one or more atmosphere transfer or loading vestibules. Loading volumes run from as much as 80 cubic feet to as little as six cubic feet. Loading capacities run from 7,000 pounds to 1,500 pounds. The furnace can stand-alone or be integrated into a modular system. Basic designs are frequently modified to meet more specific customer requirements. The IQ was first introduced in the mid 1950's and quickly gained in popularity.
Why has the integral quench batch furnace become a workhorse in the heat treating industry? We posed this question to a number of leading manufacturers and surveyed our readers. While the responses differed somewhat, when the feedback was tallied, a strong consensus emerged. The IQ's flexibility is the reason for its popularity.
The IQ provides process flexibility. Neutral hardening, carburizing, annealing, carbo-nitriding, normalizing, etc. can all be accomplished in the same furnace. The IQ can accommodate wide changes in processing temperatures. Heating rates can be controlled to minimize distortion. When a load is transferred from the heating section to the cooling or quench section(s), the parts are kept under a protective atmosphere to control oxidation and/or decarburization.
The IQ provides loading flexibility. When equipped with the appropriate trays and fixtures, the same furnace can process many different part types and geometries. Items that can effectively be processed include, but are not limited to, weldments, castings, forgings, machined and powdered metal parts. These range in size from large parts weighing several hundred pounds, requiring special fixtures to control distortion, to small parts weighing a fraction of an ounce that are bulk loaded in baskets.
Quenching or Cooling Flexibility
The IQ provides quenching or cooling flexibility. The IQ furnace is most commonly equipped with a liquid quench that uses oil, water, polymer, or molten salt. The quench is generally fitted with devices to vary the severity of the quench. These furnaces are frequently equipped with an atmosphere cooling section when the process calls for gradual cooling parts in a protective atmosphere.
Batch or Lot Size Flexibility
The IQ provides batch or lot size flexibility. Properly applied, the integral quench furnace complements the efficiency and effectiveness of a heat treat operation with large continuous furnaces because it can handle the small or urgently needed lots of parts without interrupting previously scheduled production. Furthermore, in order to reduce costs and compete in the world market, manufacturers continually seek to reduce in-process inventory. This invariably results in the need to process ever-smaller lot sizes. An IQ provides an efficient and effective means to process these small lots.
In the interest of learning more from our readers who operate integral quench batch furnaces, we surveyed readers and received responses from 51 users. We also posed similar questions to major manufacturers.
Questions included "What do you like most about your integral quench furnaces?" and "If there was one thing you could improve on your integral quench furnace, what would it be?"
On the positive side, the most common response, as stated earlier, was flexibility. Other repeated positive responses included reliability, ease of operation, uniformity, repeatable control and compactness. Ease of maintenance was listed as both a positive and a problem, indicating the soundness of the general configuration but pointing out areas for improvement.
As for areas of needing improvement, the end user suggestions clustered in several areas: the quench system, heating section, controls and maintainability. In most cases, manufacturers were aware of end users concerns and felt their current IQ furnace offerings addressed these concerns.
QuenchQuench Uniformity and Control
Quench system problems was the most common end user response. These complaints centered on quench uniformity and the control of quench severity. On the other hand, when asked how they differentiate their furnaces from those of their competitors, manufacturers frequently referenced such features as the volume, adjustability and uniformity of quenchant flows. A quench tank must provide adequate volume to properly extract the heat from the work being quenched without an excessive rise in temperature. One reader requests "quantitative in-process measurement / control of quench flow." The ability to vary quench flows enhances the IQ's flexibility because different steel chemistries and part geometries require significantly different cooling rates to achieve the desired hardness. Superior quench design provides a uniform "as quenched" hardness profile as well as the ability to tailor the severity of the quench to control distortion.
Cleaning the Quench
Cleaning the quench is another problem. No matter how well it is designed, heavy sludge buildup will reduce the quantity and uniformity of the quench media flow. Our readers are looking for designs that not only work when new, but also are easy to maintain. Another reader suggests the furnace's quench be equipped with continuous filtration to keep both the quenchant and the tank clean. Magnetic scavenging arrays in the quench flow pattern can effectively remove metal fines and extend oil life and cleanliness.
Overflowing Quench Tanks
Overflowing quench tanks remain a significant concern of both users and manufacturers. There are several potential causes for overflows. They include, but are not limited to, water in an oil tank, excessive work weight or surface area, or improper quench levels.
Water is heavier than oil and naturally settles to the bottom of the tank. When a hot load is quenched, the water flashes to steam, rapidly pushing the oil up into the vestibule and out the front of the furnace. More than a few major fires have been ignited when a large stream of flaming oil exits the furnace. The source of water can be a leaking water-to-oil heat exchanger or be introduced in makeup or reprocessed oil. Air-cooling of quench oil has become common for this reason.
If the work being quenched exceeds the system's cooling capacity, oil vapor can cause an eruption of oil similar to that caused by water. Caution must be exercised when processing very thin parts. Parts with very high surface areas liberate their heat almost instantly. If this is the case, the oil can flash to vapor even when the furnace is very lightly loaded.
Proper Quench Levels
Proper quench levels are critical to the safe operation of a quench system. Too much quenchant, the load displaces the quenchant resulting in an overflow. Too little quenchant and the load is not completely covered and cannot properly liberate its heat to the quenchant. It is a critical end user responsibility to periodically check the operation of all furnace safeties.
One reader suggested the quench be equipped with fire safeties and a system that will control the fire when a load fails to drop completely in the quench media.
Heating ChamberNot surprisingly, another area of both manufacturer and end user concern is the furnace heating system. Concerns fall into three categories: Performance (heating rate and uniformity), Maintenance (burner adjustment and heating component life) and Efficiency (reducing the quantity of energy consumed per pound of work produced).Performance And Maintenance
IQ users want heating uniformity and rapid heating and a ready means to measure both. Superior temperature uniformity ensures consistent and repeatable case depth and quenching. Improved heat rates will result in faster furnace recovery time and, depending on the process, a significant increase in furnace throughput. Furnaces should have a means to run inspection thermocouples from the load outside the furnace to a data logger. Not all users are satisfied with the uniformity of their present system.
Fans should be designed for uniform atmosphere flow over the heating elements and through the work. This circulation improves both heating and atmosphere uniformity. Cast fans and fabricated fans are used, some driven with variable speed drives to slow fans as the temperature increases for optimum performance without excessively shortening the fan's life.
If the furnace is heated with natural gas, propane or oil, the frequency of end user burner adjustment is a common concern of furnace manufacturers. Improper air/fuel ratio adjustment results in not only increased fuel consumption and a slower heating furnace, but it also will reduce the life of heating components. Periodic combustion adjustment is a preventative maintenance initiative guaranteed to pay for itself many times over.
End users expect heating systems that are easy to adjust. Systems must have a ready means to measure pressures, flows and products of combustion composition. Furthermore, all components should be installed so they can be removed, if required, for maintenance.
Heating element, either electric elements or fuel fired radiant tubes, should be easy to change.
The unprecedented increase in the cost of natural gas has made every IQ user acutely desirous of methods to improve the efficiency of their furnace systems. There are several means identified by both end users and manufacturers to reduce en-ergy consumption. They include improved burner system performance, more effective insulation systems, and equipment to continuously monitor energy consumption.
Modern burner systems provide efficient operation though improved radiant tube uniformity, superior mixing proving complete combustion and by recovering waste heat using recuperation. A recuperator is a device that uses the waste heat in the products of combustion to preheat combustion air entering the burner.
An IQ loses a significant portion of the net heat provided by the heating system through wall loss, i.e. heat that passes through the furnace refractory walls and through hole losses, heat that exits the furnace through openings under or around doors. Many older furnaces were compromised designs that sought to balance cheap fuel with expensive insulation. Thicker insulation can reduce fuel consumption. It provides a furnace with better heating rates, improved efficiency, but incremental in-creases in the thickness of a furnace's insulation results in diminishing improvements in thermal efficiency.
One user suggests IQ should be equipped with a means to continuously monitor the energy and atmosphere used to process a given load. The information gathered from such an approach can be used to identify any number of problems with the furnace's heating system, burner being out of adjustment, damaged refractory, etc.
ControlsOne critical area identified by both our readers and furnace manufacturers is the need for state-of-the-art atmosphere/process control. Modern controls provide real time monitoring and control of the furnace atmosphere, temperatures and process times. Programmable systems provide consistent process execution to ensure repeatable results. The emergence of graphic user interfaces provides the furnace operator significantly more information in an easy-to-read format. These enhanced interfaces provide the operator with the information required to continuously understand the furnace's status and to rapidly diagnose problems. If a problem does occur, enhanced data logging helps the metallurgist determine what adjustments, if any, are re-quired for the next load.
More extensive use of sensors can help operators and maintenance personnel anticipate component failures before they occur. Controls can be equipped to alert operators when a component is not operating as expected but long before it fails. This will allow replacement parts to be purchased and maintenance scheduled ahead of time, in a way that does not adversely affect production.
Another emerging opportunity to improve the long-term performance of IQ's is remote monitoring. Using modern controls, manufacturers can, via a phone line or the internet, remotely access the furnace controller to update software and / or identify the cause of a malfunction. This allows the author of the system software to diagnose problems without the costs and delays associated with travel.
Maintainability and Ease of OperationSeveral end users mentioned the need to improve the maintainability of their integral quench furnaces. Our readers are concerned that once a faulty component has been identified, how easy is it to replace? Did the manufacturer "think through" the steps required to repair a door, change a radiant tube, replace a flow meter? Are there access plates, unions in the pipe, adequate clearances? This is a common concern for everyone, user and manufacturer alike.
Manufacturers remain concerned about the effectiveness of end user maintenance programs. Does the end user operate the furnace within its designed weight limits? Is an adequate spare parts inventory kept on hand to ensure rapid repairs? Are thermocouples changed regularly? Is excess carbon cleaned or burned out on a regular basis? Are alloy trays replaced when they become warped?
Operational ease is important in IQ furnace design. Since many fluids are involved, it is necessary for the operator to be able to easily access a number of valves and flow control devices. Controlling and monitoring pressures and flows are very important so devices must appropriately located. A logical layout of all control devices will pay real dividends in enhanced safety and error-free operation.