Then the boiler was flipped on its back and the stays welded in on the front. This might be a good time to explain the other things we're seeing in the photos. The lifting chain in both photos is threaded through the tube sheets. The tubes will be rolled into these holes after stress-relieving the boiler. The big nozzle at the bottom of both pictures is one of two hand holes for inspection, washing, etc. Think of it as a window into the boiler's soul. The smaller nozzle in the bottom of the picture, below, is the fitting for the boiler's blowdown. The boiler is drained through the blowdown. Crud can also be blown out the blowdown during operation. This is done to the greatest visual effect by placing the blowdown through-hull fitting on the hull just above the water line. After this second photo, I give a brief description of stress-relieving.
Welding is a fusion and metal deposition process. As such, uneven stresses can build up in the pieces being welded and in the weld itself. With a small boiler such as this one, we have the luxury of placing the whole assembly into an oven and raising the temperature for an extended period of time until about 90% of the stress is relieved. Think of it as a nice soak in the tub after working hard in your garden or shop. Not all of life may be mended, but emerging sanguine in complexion and disposition leaves one more fit to be around. The boiler feels the same way. For those who need to know, this fuller explanation from Houston Heat Treat, below:
Stress-relief operations are typically done by subjecting the parts to a temperature approximately 40- 75ºC (105-165ºF) below the Ac1 transformation temperature — about 727ºC (1340ºF) for steel. Stress relief is typically performed for carbon steel at approximately 500-650ºC (930-1200ºF). The elimination of stress is not instantaneous (that is, the process is a function of both temperature and time). To achieve the maximum benefit, some time at temperature (typically one hour per 25 mm of cross-sectional area once the part has reached temperature) is required. This removes more than 90% of the internal stresses.
And a handy diagram lifted from Wikipedia, below, shows the A1 line. Aren't steam launches an education?