Heat Treating: Decision To Become a Captive Heat Treater Pays Dividends for Tube Manufacturer
Atlas Tube Inc., founded in 1984 in Harrow, Ontario, has grown to become the largest supplier of hollow structural sections (HSS) in North America, with manufacturing facilities in Canada and Plymouth, Mich. During the past decade, the company has invested in excess of $100 million in state-of-the-art tube making equipment. With a current capacity of more than 850,000 tons of HSS, Atlas Tube is constantly looking for ways to reduce work in process inventory and costs, as well as to improve lead times and quality.
HSS productionHSS produced at Atlas is made in accordance with CSA (Canadian Standards Association) G40.21 50W or Metric 350W Class H Specification for HSS. The product is made using the electric resistance welded process, in which hot-rolled black wide band coil is slit to specific widths and formed continuously around its longitudinal axis to produce an open seam round section. Strip edges are then heated by means of high frequency induction welding units and forged together by weld rolls to create a continuous weld seam without the addition of weld filler metal. Rounds may be further processed downstream by cold forming to produce round, square and rectangular sections in a range of sizes with wall thicknesses ranging from 0.125 to 0.625 in. (~3 to 16 mm).
During cold forming and welding process, residual stresses are induced into the final product, primarily concentrated in the corners of square and rectangular product and the weld seam. Reheating the cold formed sections to a subcritical temperature of 890°F (475°C) relieves stresses while leaving mechanical properties essentially unchanged. An article published by Stelpipe Division of STELCO in 1989 discussed the increased axial compressive stress of Class H material over Class C HSS (27% increase). The implications of this are that savings of up to 20% in mass are possible when using Class H HSS.
Class H material is part of the Canadian specification only and has not been approved for ASTM 500 in the United States. However, Class H material could be substituted for CSA G40.21 Class C and ASTM 500 providing the size, gage and physical properties are identical.
Heat treating facilityBecause Atlas Tube historically had no on-site thermal processing facility for reheating HSS to comply with the Class H specification, all Class H material was sent outside to a contract heat treatment facility located some distance from the Harrow plant to meet specification requirements. In addition to the increased material handling, transportation costs and logistics, the need to subcontract the heat treatment potentially added two weeks to the overall lead time. Because offering the shortest lead times in the industry to customers was of primary importance, the company started to investigate in late 2002 the possibility of building a heat treatment complex within the Harrow facility. The Atlas Tube management team decided that heat treatment would be a core competency and approved the capital investment for an in-house heat treat facility. The requirements were for a system that could heat treat 70-ton loads of a wide variety of HSS tubing including square, rectangular and rounds.
In August 2003, the company selected Can-Eng Furnaces to engineer, design, install and commission the unit. The design called for a car bottom furnace essentially consisting of a rail-bound car lined with insulating firebrick and a fixed furnace shell with multiple burners to achieve the best possible temperature uniformity. The furnace was ready for hot testing in April 2004.
Furnace constructionHSS was used in the furnace construction, with all of the HSS square and rectangular sections supplied by Atlas Tube. Can-Eng historically has used hot-rolled channels and wide flange beams in the shell reinforcement and car structure system, but the opportunity to manufacture the structure from HSS reduced the overall manufacturing time. The nature of the HSS square and rectangular tubing allowed joints to be readily coped resulting in ease of fabrication.
The entire furnace car and shell were fabricated in the Can-Eng shops. However, the overall size of the furnace (18 ft high ¥ 18 ft wide ¥ 70 ft long, or ~5.5 ¥ 5.5 ¥ 21.5 m) made it necessary to use a modular construction with numerous split lines. All combustion piping and wiring was done in-house, but the insulation was not installed until the equipment was delivered to the field. The furnace system can handle tubular product up to 70 ft (~21.5 m) long.
Environmental considerationsThe Province of Ontario has implemented some of the most stringent NOx requirements in the world, requiring new equipment having a heating capacity of over 10 MMBtu/hr to meet 49.6 ppm of NOx throughout their firing range. The combustion system as designed uses high-velocity burners from Eclipse Combustion (Rockford, Ill.; www.eclipsenet.com) and meets the new Ontario regulations. In addition, Atlas Tube did considerable sound testing to ensure that the new furnace would not negatively impact the surrounding community and neighborhood.
Control systemThe control system for the furnace incorporates the latest multifunctional PLC, temperature controller and operator interface combined in one unit. The furnace is controlled off of load thermocouples embedded in the tubing in the furnace charge. By using this technique as opposed to controlling off of thermocouples measuring the surrounding furnace environment, Atlas can monitor and control the temperature uniformity along the length of the tubing.
The furnace is divided into three zones of burner control using an over-under firing arrangement, which has proven to produce the best temperature uniformity in the absence of fans. The furnace is capable of maintaining a temperature uniformity of ±25°F (±15°C), and the over-under firing pattern produces an excellent thermal profile.
The furnace is designed for a maximum operating temperature of 1200°F (650°C), and the majority of cycles run in the furnace are low temperature stress relieving cycles in the range of 890°F (480°C). The production rate amounts to about two 70-ton loads per day.
The company is capable of capturing, logging, storing and analyzing all temperature data for every load. The control system has the added functionality that allows program modifications to be made remotely via a modem in the panel.