Intensive Quenching Process Commercialization
In 1999, IQ technology was introduced to the U.S. heat-treating industry. The first production stand-alone 6,000-gallon IQ water tank was built and installed at Akron Steel Treating Co. of Akron, Ohio. Since this time, hundreds of IQ demonstrations have been conducted on test samples and actual parts made of different steel alloys. The IQ method provides the following proven benefits:
• Higher power-density parts – lighter yet stronger steel parts with improved mechanical properties, residual surface compressive stresses and wear and fatigue resistance.
• Stronger and tougher tool products with longer service life
• A clean, environmentally friendly quenching process – no hazardous oils or environmentally unfriendly polymers are used
• Part cost reduction by substitution of plain-carbon steels for more expensive alloy steels
• Reduction of process cost by saving energy – significant reduction of the carburization cycle or complete elimination of carburization with the use of optimal-hardenability steels while eliminating the environmental hazards of traditional oil quenching.
• Heat-treatment operations can be integrated or embedded in the part manufacturing cell for in-line, single-part processing.
This paper summarizes the commercialization status of the IQ process.
What is the IQ method, and how does it differ from conventional quenching?
The IQ process is an interrupted water-quench method. IQ differs from conventional oil, polymer and water quenching by providing a much greater heat-extraction rate from the parts being quenched. Heat fluxes from the part surface (and, as a result, the part cooling rates) are several times greater than that for conventional quenching. Extremely high heat-extraction rates result in a much greater temperature gradient throughout the part cross section, which, in turn, generate very high “current” surface compressive stresses, preventing parts from cracking. These surface stresses remain compressive after the IQ process is completed. This is in contrast to conventional quenching, when residual surface stresses are tensile or neutral.
High heat-extraction rates during the IQ process are achieved by providing very “intensive” and very uniform water agitation throughout the quench bath. High water-flow velocities, as well as an additive of mineral salt to water in optimum concentration, fully eliminate the film-boiling process that takes place at the beginning of the traditional oil or water quench. As known, film boiling is a sporadic, unstable process that causes a delay in quench cooling, spotty hardness and part distortion.
A key element of the IQ process is the interruption of the uniform intensive water quench at the proper time. This interruption time is calculated by proprietary software, and it depends on part shape, its dimensions, type of steel and part specifications. For example, for parts made of through-hardened, medium- and high-alloy steels, the quench is usually interrupted at the moment of time when surface compressive stresses are at their maximum value and the part hardened layer is at an optimum depth. After interruption of the intensive water quench, the cooling then continues in the air. The thermal energy coming from the still very hot part core tempers the martensitic surface layer, making it tougher and preventing possible cracking. For parts made of low- or medium-carbon steel having a low hardenability or for parts made of carburized grades of steel, the interruption criteria is often calculated to provide the hardened case as deep as possible.
The IQ process can be implemented for batch and continuous heat-treatment operations as well as for single-part quenching. Different production IQ systems installed in the field over the last several years are considered in the section below.
Production IQ Equipment Currently Installed
Currently, two types of production IQ equipment are used: water-quench tanks for batch-hardening operations and high-velocity quenching units for single-part hardening processes. Note that an IQ water tank can be a stand-alone unit or it can be a part of an integral-quench furnace. Examples of production batch IQ equipment are shown in Figs. 1 and 2. Figures 3 and 4 present pictures of production single-part quenching systems. Note that austenitizing of the parts prior to processing in single-part quenching systems can be done in an atmosphere furnace or by induction through heating.
The IQ system presented in Fig. 1 consists of a radiant tube atmosphere furnace (on the left) and a stand-alone IQ water tank (on the right). This system is installed at Akron Steel Treating Co. The figure also shows a picture of the load of three steel-mill rollers of 16.5-inch OD, 7.24-inch ID and 12-inch height. The rollers are made of ultrahigh-carbon Dura-Bar material. Note that this material is not considered a “water quench” steel. Due to high carbon content, this steel is prone to cracking when quenched in water. Therefore, these rollers are normally quenched in oil. When quenching the rollers in the IQ water tank, cooling is interrupted when the surface current compressive stresses are at their maximum value. The rollers then continue cooling in the air. The roller surface layer is tempered by the heat coming from the part core.
High residual surface compressive stresses and self-tempering of the roller’s hardened surface layer prevent part cracking after the quench is completed. Note that residual surface stresses are tensile for these parts after conventional quenching in oil. Both the high residual surface compressive stresses and the deeper hardened layer from the IQ process result in longer roller service life than the same rollers quenched in oil.
Fig. 2 presents an integral-quench furnace equipped with an 11,000-gallon IQ water tank. The furnace was built by AFC-Holcroft of Wixom, Mich., and installed at Euclid Heat Treating Co. of Cleveland, Ohio. As an example, Figures 5 and 6 show two typical production loads processed in this furnace. Figure 5 presents a load of 24 transmission shafts made of 1040 steel. The shaft diameter varies from 1.66-2.17 inches, and the shaft total length is 25.7 inches. A conventional oil or polymer quenching method could not provide the required surface-hardness uniformity and the hardened depth for these shafts. Also, the part distortion was excessive (the maximum distortion was 0.245 inches while the average shaft distortion was 0.140 inches).
In contrast to the steel-mill rolls, intensive quenching of the shafts was interrupted when the hardened depth achieved its maximum value. The shafts then continue cooling in the air. The surface martensitic layer was tempered by the heat coming from the core. The surface and core hardness met all customer’s requirements. After intensive quenching, the shaft maximum distortion was 0.081 inches, and the average distortion was 0.040 inches. Thus, the shaft distortion was reduced by more than three times. Note that no special fixture was used for racking the shafts for intensive quenching. The shafts were supported in the basket by standard screens (Fig. 5). The use of a special fixture that minimizes the shaft movement during the quench will further reduce the part distortion.
Figure 6 presents a load of forged rings of 9.5-inch OD, 7.5-inch ID and thickness of 0.5 inches. The rings were made of 1045 steel. Previously, the parts were quenched from a belt-type furnace into a polymer quench. The quality defects exceeded 18% due to an excessive part distortion after quenching in polymer. By using the IQ process, the defects dropped to 6% due to a high uniformity of cooling in the IQ water tank.
Figures 3 and 4 present single-part quenching IQ systems installed at Euclid Heat Treating Co. The IQ unit (Fig. 3) is designed for processing steel parts with a maximum diameter of 8 inches and with lengths of up to 20 inches. The IQ system is paired with a box atmosphere furnace for austenitizing the parts. The IQ system (Fig. 4) is designed for processing long shafts of up to 2-inch diameter and 36-inch length and gun-barrel steels. This system is equipped with a low-frequency induction-heating unit (on the right) for austenitizing the parts prior to quenching.
Figure 7A shows a hot gun-barrel blank ready to be moved from the induction-heating station for intensive quenching. Figure 7B shows the same hot gun-barrel blank placed into the IQ system loading/unloading table before quenching. Figure 7C shows the gun barrel after the quench is completed. Both single-part IQ systems are equipped with a chiller for maintaining the water temperature at a proper level.
Another production IQ system for single-part quenching is installed at Meritor Suspension Systems Co. of Chatham, Canada. The IQ unit is designed for quenching of automotive torsion bars. In contrast to the above single-part quenching systems, the parts are processed in horizontal orientation in this IQ unit. The system is currently running parts under customer evaluation.
Note that single-part IQ systems provide much greater water-flow velocities compared to IQ water tanks. These higher water-flow velocities result in greater heat-extraction rates in single-part quenching IQ systems, which, in turn, result in greater residual surface compressive stresses, a deeper hardened layer, better material microstructure and less part distortion compared to batch IQ units. For example, mechanical properties (tensile yield and impact strength) for gun-barrel steels processed in the IQ system (Fig. 4) were improved by up to 20%. The gun barrels made of intensively quenched steel demonstrated improvements in both fatigue life (reduced muzzle and chamber wear) and shooting accuracy. The distortion for the drive shafts made of alloy carburized steel and having a diameter of about 40 mm with a length of 385 mm was only 50-70 microns, which met the customer‘s specifications.
• The IQ process is a very effective and environmentally friendly method of hardening steel parts for superior mechanical properties and performance characteristics compared to conventional oil or water/polymer quenching.
• The IQ process is a well-proven and mature technology that is being widely commercialized.
• Several production IQ systems for batch and single-part processing operations are installed in the field. IH
For more information: Contact Dr. Michael Aronov, CEO, IQ Technologies, Inc., P.O. Box 1787, Akron Ohio 44309; tel.: (440) 542-0821 or (330) 773-4850. Or click on www.intensivequench.com.