Cost Reductions and Product Consistency Improvements Using Unannealed High- Frequency/Solid State (UHFSS) Welded 409 Stainless Steel Heat Exchanger Tubing BY Edward D. Basta, Senior Metallurgical Engineer Atlas Steel Products Co. 7990 Bavaria Road Twinsburg, OH 44087 USA (330) 425-1600 (330) 425-1611 E-Mail: [email protected] Web Site: http://www.atlassteel.com Edward D. Basta Atlas Steel Products Co. Page 2 ABSTRACT BACKGROUND Competitive markets for Commercial and residential commercial and residential HVAC HVAC systems have evolved over the systems are demanding higher quality years from using predominantly and safer, longer lasting products. Due "clamshell" heat exchanger components to increased global competition, HVAC to the more efficient and cost effective system dealers and end users also tubular heat exchanger. Figure 1 shows demand competitive prices and longer a typical tubular heat exchanger unit. warranties. The common experience of coated carbon steel heat exchangers that suffer corrosion failures in less than ten years will be unacceptable. HVAC manufacturers must find ways to meet both performance and pricing expectations. The progressive HVAC manufacturer is transitioning to stainless steel tubular heat exchangers for longer service life. However, the higher costs of annealed tungsten-inert gas (TIG) or laser-welded ferritic stainless steel heat exchanger tubing have forced many manufacturers out of this price-sensitive market. These less productive (lower Figure 1: Tubular Heat Exchanger fpm) tube welding processes and an additional post-weld annealing step The materials used for both contribute to the higher costs of clamshell and tubular heat exchangers stainless steel tubing. have progressed from basic uncoated or This presentation will introduce a epoxy-painted plain carbon steels to lower cost, high quality alternative to hot-dip aluminized coated steels. annealed TIG and laser-welded 409 Although the service life of the ferritic stainless steel tubing. It will be aluminized steel tubular heat exchanger shown how state-of-the-art, high exceeds that of uncoated steels, frequency/solid state welded tubing is increasing heat exchanger firing manufactured at higher speeds, and temperatures for optimum energy why the costly post-weld annealing step efficiencies have pushed aluminized can usually be eliminated. In many steel past its maximum sustainable cases, the HVAC manufacturer can surface temperature limits. Once realize significant reductions in heat operated over these limit temperatures, exchanger tubing costs, and often the aluminized steel heat exchanger experience less variation in subsequent tube suffers catastrophic corrosive tube-bending operations. attack in a short period of time. Edward D. Basta Atlas Steel Products Co. Page 3 To sustain higher firing to normalize the laser welded heat temperatures with fewer corrosion affected zone (HAZ). This annealing issues, the progressive commercial and step adds significant costs and residential heat exchanger manufacturer production lead times to laser-welded has chosen to use an economical ferritic tubing compared to high frequency/solid grade of stainless steel that can state welded tubing. It can also withstand higher heat without suffering introduce mechanical property variations coating degradation problems. Type that have been shown to adversely 409 stainless steel is now being widely affect the tube bending fabrication used in the tubular heat exchanger required for both mandrel-bent and market. wrinkle-bent heat exchanger tubing While the cost of 409 stainless components. steel raw material for heat exchanger tube manufacturing is somewhat higher than aluminized carbon steel, the tube TUBE CORROSION ISSUES welding process can make this heat exchanger component significantly more The primary reason for utilizing costly. The inherent challenges of the 409 ferritic stainless steel heat tube welding process when dealing with exchanger tubing instead of hot-dipped high chromium content alloys like 409 aluminized low carbon steel welded stainless steel have led many tube tubing is to extend the service life of the manufacturers to avoid the more heat exchanger. In welded aluminized productive high frequency/solid state steel tubing, the aluminum/silicon alloy welding process. coating can be exposed to temperatures 0 The more traditional stainless up to 950 F. and still maintain its steel welding technologies of tungsten- integrity as a separate, noble corrosion- inert gas (TIG) welding and laser resistant coating over the substrate low welding avoid some of these challenges, carbon steel. 0 but typically require slower tube mill At temperatures above 950 F. speeds for proper seam welds. The for any length of time over a few laser welding process can be minutes, the coating starts to accomplished somewhat faster than TIG metallurgically diffuse into the steel welding, and is the most common substrate. This irreversible diffusion method used for 409 stainless steel heat process results in a corrosion-resistant exchanger tube welding today. but brittle layer on the substrate steel Recent developments and made of iron and aluminum in almost improvements in high frequency/solid equal parts. As the heat exchanger state welding on tube mills have allowed tube expands and contracts during the innovative heat exchanger tube subsequent heating cycles, this brittle manufacturer to effectively produce 409 layer forms small surface cracks, and stainless steel welded tubing. This the unprotected substrate low carbon tubing can meet and exceed the weld steel becomes subject to qualities of laser welding without the oxidation/corrosion through the slower welding speeds and a post-weld impingement of surface moisture annealing process that is required caused by ambient condensation. Edward D. Basta Atlas Steel Products Co. Page 4 Figure 2 shows a view of a commercial rooftop heat exchanger unit that had been replaced after only six years of service. The severe surface oxidation is evident on the weld seams and in other areas that have suffered coating diffusion due to heating above the diffusion temperature. The coating diffusion is evidenced by the lighter/whitish aluminum oxidation and a darker gray surface coloring. Figure 3: Oxidized Aluminized Steel Tube I.D. Section Aluminized steel heat exchanger tubing also has a remetallized thermal spray aluminum coating applied over the O.D. weld seam after the welding process. It is not commercially feasible to apply such a coating on the I.D. weld seam, however. In addition to the Figure 2: Corroded Aluminized Steel typical surface corrosion formed as a Heat Exchanger result of aluminum alloy coating diffusion, it is also possible for the heat exchanger tube's I.D. weld seam to be Figure 3 shows an I.D. view of a preferentially attacked in the presence section of this aluminized steel heat of condensation moisture over time. exchanger tube section that was cut Figure 4 shows a photomicrographic from the heat exchanger unit. A heavy example of an aluminized steel heat layer of corrosion is visible on the inside exchanger tube's I.D. weld seam cross- surface. The tube's I.D. weld seam is section that has suffered catastrophic noted below the corroded area. corrosion and subsequent weld fracture. Edward D. Basta Atlas Steel Products Co. Page 5 WELDING METHODS The most challenging aspect of welding 409 stainless steel on a continuous-seam welded tube mill is the formation of chromium oxides during the welding process. The avoidance of such oxides is critical to making a high quality tube weld. Additionally, the tube must be formed, welded, cooled and sized to the final close tolerances of heat exchanger tubing on a continuous, highly efficient basis to be marketed as a competitive product in the HVAC arena. There are three basic welding Figure 4: Oxidized Aluminized Steel I.D. methods used to accomplish this task. Tube Weld Cross-section The TIG Welding Process It is clear from these examples that for tubular heat exchanger The TIG welding process utilizes installations that will be exposed to firing a non-consumable tungsten electrode temperatures over 9500 F., and/or with an argon or argon-hydrogen mix installed in locations where abundant shielding gas to prevent chromium oxide condensation moisture will be allowed to formation. TIG tube welding is usually accumulate inside the heat exchanger done in one bead, and can use from one tubes, that a more heat and corrosion- to four electrodes combined in a multiple resistant tube material is required for torch arrangement. longer service life. It has been proven TIG welding speeds are generally that Type 409 ferritic stainless steel in the 10/15 feet per minute (fpm) range meets the requirements of such a for 409 stainless steel sheet thicknesses material. up to 0.060". This slow tube welding Early prototype work with 409 speed required to establish the wider stainless steel tubing for HVAC heat weld bead necessary for a high quality exchanger applications was typically TIG welded tube seam will often make done with tubing that was commercially the TIG welding method the least available. In most cases, this would desirable choice for competitive 409 have been either TIG welded, or more stainless steel heat exchanger tube commonly laser welded and annealed manufacturing. tubing. In many cases, this type of Figure 5 shows a typical stainless tubing became the expected norm, steel tube TIG weld pattern (1.750" O.D. along with its high price. It was during X 0.42" wall SS heat exchanger tube). this period that the high frequency/solid Note the typical wide hourglass shape of state 409 stainless steel welded tube the HAZ, approximately 0.035" wide at option was being developed and the top/widest area of the weld zone. perfected. Edward D. Basta Atlas Steel Products Co. Page 6 Figure 6: Laser Welded Stainless Steel Figure 5: TIG Stainless Steel Tube Tube Weld Pattern Weld Pattern Laser tube welding does allow a The Laser Tube Welding Process considerably higher tube production speed compared to the TIG welding The laser tube welding process process, in the neighborhood of utilizes a highly concentrated beam of approximately 50/60 fpm.