Shop Improvements Meet Challenges of Heat Treating Very Large Parts
FRISA Forjados S.A. de C.V. has been producing seamless rolled carbon steel, alloy, and stainless steel rings for more than 30 years. Founded in 1971 as a small forge shop, FRISA has grown into one of the world's leading manufacturers of these components. The rings, with up 20 ft (6 m) diameter and weighing up to 15 metric tons, are used as bearings, valve pieces, gears, flanges, fittings and valves in oil rigs, cement processing plants, mining, energy production, construction and transportation equipment. For example, 5-ft (1.5 m) diameter rings might be used as wheel rims in mining trucks, while 12-ft (3.6 m) diameter rings might be used in steam turbines in power generation. They are also used in deep-water equipment such as connectors, blowout preventers, buckle arrestors, and risers, as well as in steam and gas turbines, eolic generators, flanges for eolic towers, pelletizers and locomotive parts (Fig. 1).
Parts must be manufactured to exacting customer specifications with specific mechanical and physical properties such as hardness, ductility, toughness and machinability. To accomplish this, FRISA has five ring rolling machines, four hydraulic presses, and an extensive heat treating operation. Thanks to recent investments in the latest heat-treating equipment and technology, FRISA is now able to manufacture another 120 metric tons per day with few reworks. This case study details how FRISA evaluated its needs, and implemented a heat-treating operation that maximizes the performance of existing equipment along with a custom, state-of-the-art solution.
Large ring heat treatment
The equipment that FRISA uses to handle the heat treatment of large parts weighing 15 metric tons includes 16 furnaces, six quench tanks and custom handling and racking equipment. A team of highly skilled engineers, metallurgists, and heat treatment specialists at the company have successfully addressed many issues related to heat treating such large products (such as hard spots during the quench to distortion and cracking of the finished parts), and have created an operation that achieves maximum yield and product quality.
For example, the company faced several challenges during the quenching process. The line consisted of eight austenitizing and tempering furnaces, two 90 m3 (~24,000 gal) quench tanks (one with water and one with a PAG polymer quenchant) and an overhead crane to transport the charge. A network of piping inside the tank provided agitation to the quenchant (Fig. 2). The company was finding it difficult to achieve optimum cooling curves for the different materials using a 6% concentration of quenchant. According to FRISA production manager Bernardo Servin, they were looking for a lower heat-extraction rate in the convection phase to help achieve less distortion.
The company contacted Houghton International (Valley Forge, Pa.; www.houghtonintl.com), a leading global supplier of heat-treating fluids and products, to consult on improving the heat treating operation. Houghton personnel initiated a plant survey in October 1997 to understand FRISA's overall heat treatment process-from austenitizing, to racking, to quenching, to tempering.
The plant survey included all technical specifications including the various products to be treated (size and shape, material, weight, etc.), plant production demands, properties required after heat treatment and available plant space. In addition, an engineering study was conducted to gain a thorough understanding of the quench tank dynamics with and without the 12-ton charge (Fig. 3). Quench tank measurements included flow/agitation rate (m/s) at various zones within the tank at various depths. This data identified areas where the cooling rate was above or below optimum levels, and the results varied widely (Fig. 4a).
The engineering study demonstrated that the original agitation system provided nonuniform agitation. Agitation was high in the nozzle, but weakened as it moved away from the nozzle (0.03, 0.015, 0.04 m/s), and was practically nonexistent in the working zone. Figure 5 illustrates the ISO speed flow of one unit of the agitator. The working zone is between 0 and 0.04 m/s. In addition, the study showed that agitation was not sufficient to break the vapor blanket at the initial stage of the quench. Houghton determined that a minimum speed of 0.30 m/s was necessary.
The parts were cooling too quickly in the convection phase of the quench. Houghton made two recommendations to provide slower quenching in the convection phase and to achieve uniform agitation throughout the charge with less variation from zone to zone within the tank: 1) Use a PAG polymer quenchant having a higher molecular weight; and 2) Change from the pipe agitation to a propeller technology. Four propellers (one in each corner) were installed, and a second engineering study showed there was a marked improvement, with increased agitation speed and more homogeneous quenching (Fig. 4b).
Following these improvements, FRISA installed an additional two propellers at the bottom of the tank, resulting in further improvements including an overall speed increase from the nozzles to the first four propellers ranging from an average of 0.13 to 0.35 m/s (Fig. 4c).
A cooling curve at 350°C (660°F) showed a slower cooling rate using the higher molecular rate PAG quenchant (Fig. 6). The new quenchant demonstrated faster cooling rates at the beginning of the quench and reduced cooling at the finish, which meant that they were able to achieve the desired metallurgical properties with less warpage and cracking. Hard spots were eliminated and distortion was minimized. After experimenting with different quenchant concentrations, FRISA fixed the concentration range at 11 to 12% for minimum warpage and best hardness results. The CCT curve (Fig. 6) illustrates the optimum cooling rate to achieve the least amount of deformation and warpage. The company reported less warping of the thin rings, less egg-shaped product due to less distortion and a more homogenous hardness along the pieces.
After the quench tank was optimized, Houghton brought in its partner company Tecnoalloy (Italy), a world leader in designing and manufacturing custom and specialized racking systems for heat treatment operations. In the past, FRISA had to replace its racks every few months as they became worn and distorted. Tecnoalloy recommended that the company convert from 3,000-kg (6,600 lb) steel racks to racks made of a lighter alloy material weighing only 1,000 kg (2,200 lb). The new racks, called serpentine trays, are designed to resist high temperatures and the thermal shock produced in polymer and water quench applications. The trays also require less maintenance, and have a much longer life-up to three times longer. Since FRISA began using the new racks in 2003, they have not yet had to be replaced. Figure 7 shows the old and new racks after 5 and 14 months of service, respectively.
The improvements in the line reduced the rework costs significantly. Figure 8 demonstrates a 22% improvement in rework. With an average heat treatment cost/kg of $0.04 and an average of 18,000 metric tons of production per year, FRISA realized a $720,000 savings in rework with the existing line improvements (polymer, agitation and racking).
Custom heat treatment line
FRISA was able to maximize production results using its existing equipment, but in 2003 the company experienced a sudden surge in production demand, which required an expansion of its heat-treating operation to handle the increased capacity. The company decided to invest in a custom solution, and began evaluating options for a new, custom heat-treating line in October 2003. FRISA contracted Houghton to design and install the quench-tank portion of the new line. Houghton also recommended using its furnace partner, Hi Tech Engineering (Torino, Italy) a company that designs and manufactures advanced furnace and materials handling systems. Hi Tech already developed and implemented similar technology for forging companies in Europe with excellent results.
FRISA looked at HiTech's technology in action at several customer locations in Europe that were using the concept, and was impressed with the speed of the manipulator and the excellent condition of the furnaces after more than eight years of operation without any significant maintenance on them; the fiber was intact and the floor was like new. Based on the positive visit, FRISA was interested in installing a custom line that would include a furnace, quench tank, racking and quenchant expertise.
The design process
Houghton reviewed all specifications of the various products and required properties to be achieved via heat treating, as well as the plant's production demands and the available plant space to achieve that production. FRISA's goal was to move the charge from the furnace to quench tank in less than 60 seconds. Based on the results of the previous heat treatment line installation, Houghton determined that the various steel grades needed to be heated to a temperature of 1050°C (1920°F) and cooled to 50°C (120°F) to achieve the optimum CCT curves.
A turnkey, three-stage system was recommended to achieve the objectives. Furnace charges would be transported on custom designed manipulators mounted on floor rails from the austenitizing furnace to the quench tank to the tempering furnace. The system had to be able to heat treat pieces ranging from 60 to 240 in. OD (1,524 to 6,096 mm) with and without racking, at charge weights up to 30 metric tons (66,000 lb). Because the proposal included only one quench tank, transfer from the pit to the quench tank needed to be as short as possible. The 65,000 gal (246,000 liter) volume transfers from pit to tank in 40 minutes.
The quench tank was required to quench a variety of materials, so two quenching zones were designed with variable speed for intensive agitation for low allow steels and slower agitation for higher alloy steels. The furnaces should be able to have a homogeneity of ±8°C (±14°F) at tempering temperatures and ±14°C (±25°F) within the set point on a survey on temper temperature range and be as efficient as possible for fuel savings. The new austenitizing furnace incorporates nickel piers instead of cement piers, so heating and cooling rates are faster and it consumes less gas. The furnace also uses modulated flame instead of pulse firing.
Building the new line
The furnaces were designed at the HiTech plant in Italy beginning in March 2004, and the furnace, quench tank and auxiliary equipment were constructed and assembled on site by Houghton's technology partner DEPPSA. The quench tank was equipped with agitation technology, filtration and heat exchangers. The manipulator floor rails were installed and the manipulator was put into place. The same high molecular weight PAG quenchant technology as the existing heat treatment line was implemented at an 11-12% range.
FRISA's new heat-treating line became fully operational in March 2005 (Fig. 9) and can heat treat 120 ton/day, and is claimed to be one of the most advanced turnkey heat treating lines in the world. The speed from the furnace to full submersion in the tank is 53 seconds versus 90 seconds with the existing line. The maximum weight per load increased from 11 to 30 metric tons, and the maximum ring size that can be quenched increased from 3,200 to 6,096 mm (126 to 240 in.) in diameter. Monthly production capacity increased from 3,000 to 6,000 metric tons.