Museum's Shop Forces Accomplish Major Step. (3/10/06)
|A major step in the restoration of WC&C #1’s boiler shell was completed on January 26, 2006 when the longitudinal welds in the three boiler courses were post weld heat treated (PWHT) or stress relived. The purposes of the PWHT is to relieve the residual and thermal stresses that are developed in the welding process when new plates are mated to old plates, temper (soften) the steel, and remove diffusible hydrogen. This heat treatment was called for in the Repair Procedure applied by Becker Boiler and approved by the National Board of Boiler and Pressure Vessel Inspectors. Becker’s Repair Procedures were then accepted by the Federal Railroad Administration thereby simplifying the repair and inspection process for these boiler repairs.
To better understand the how and why we need to begin with the welding process itself. The arc welding process used on the Montreal is actually a very tiny, controlled continuous lightning bolt between the boiler plate and the filler rod clamped in the holder (called a stinger) in the welder’s hand. The arc generates enough heat to melt the steel in the filler rod as well as a small portion of the pieces being welded together. The heat is dissipated fairly quickly by the mass of steel around the weld but the area in the immediate vicinity of the weld is known as the heat-affected zone.
When heated above a certain temperature, steel commonly attempts to return to its original shape. In our case we have used flat plates of steel that were rolled into a round shape for use in the boiler courses. We then weld the ends together to form a cylinder. Heat-affected zones reach a high enough temperature that the steel tries to return to its original flat shape. This creates a high degree of stress in these areas which can lead to future cracking.
If a human encounters too much stress, neck and shoulder muscles tend to tense up and pull against themselves. What’s a common remedy? Apply heat through a heating pad. That is essentially what we are doing with WC&C #1’s boiler today.
The stress relieving process can be accomplished via several processes such as inside a furnace, gas burner heating with an enclosure, or with locally applied electric heat resistance coil heating units (sometimes called pink fingers).
During this process, a real concern was the loosening up or breaking of rivets in the boiler course seams. Stress relieving in a furnace or with a gas burner would have subjected the entire boiler and firebox to the stress relieving temperature and would have been very difficult to bring up at a uniform rate (temperature) due to varying plate thicknesses and reinforcements. With significant temperature differentials, there would have been a high risk of damage to the rivets and the riveted connections. For the WC&C #1 project, the electric coil procedure was chosen as it could be done on-site, cost several thousands of dollars less, and would concentrate on the welds themselves rather than the entire shell. Cost for the process was $9,500, which was entirely paid for with the Steam Fund.
The PWHT work was performed by Team Industrial Services, Inc., a sub-contractor for Becker Boiler who has done all prior welding on WC&C #1. Team is the largest leak sealing, emissions control, heat treating, and NDT inspection company in the U.S. with over 1,200 employees and facilities in over 40 states and several countries. Their heat treating department was originally formed by Peter Cooper in 1950 (formerly known as Cooperheat) and merged into Team Industrial in August 2004. Team’s Heat Treatment Services meets ASME codes and ISO 9000 standards. The unit that came to North Freedom is based at Milwaukee.
Initially the ceramic coil heating units (pink fingers) are attached to the boiler shell by securing them to small diameter pins which have been resistance welded to the shell. The coils provide the heat as well as thermocouples to monitor and control the temperature. After installation, the coils and the immediate surrounding area of the boiler shell are covered with insulating blankets. The inside of the boiler shell plates in the weld area are also insulated. Team provided their own 480 volt DC portable generator for electricity to heat the coils.
In the course of stress relieving, the weld area is brought from ambient temperature to 600°F. From that point up, the temperature is raised at a controlled rate of 400°F per hour until it reaches the target temperature of 1,120°F. During the crest in temperature, the heat is high enough that the steel becomes slightly plastic or easier to bend. While in this state, the stress concentrated in the heat-affected zone is allowed to dissipate over a much larger area and fall below a level that would cause problems.
After holding 1,120°F for an hour, the temperature is reduced, again at a controlled rate of 400°F per hour until reaching 600°F. The system is then shut down and the boiler allowed to cool overnight to ambient. The entire process took a full day at the museum. Several hours were required to apply the equipment to the boiler shell. The controlled process was complete by 9:00 pm. Team returned the following morning to tear down and clean up the site after the boiler had cooled.
From this point forward, no burning or welding can occur in the stress relieved area of the boiler. Crews now plan to begin riveting the course patches together. Installation of flue sheets and smokebox can follow. Just one more way the old Iron Horse is so much like a living being. A little too much stress? Apply a heating pad and it all goes away.
Information provided by Pete Deets, Bill Buhrmaster, Don Meyer, Jim Connor, and Team Industrial’s website.