Steel chemistry

Rehabilitation of former the Northern Pacific Railway steam locomotive 924 has been underway in the Conservation and Restoration Center for the past year and a half. One of the remaining boiler vessel requirements is completing the package of documentation used to obtain regulatory approval. And one of the remaining details is confirming the chemical composition of the boiler plate, which is a factor used in determining the pressure vessel's maximum operating pressure.

The preliminary calculations used data from the manufacturer's marketing literature.  This is useful for a theoretical exercise, but unreliable for the final document because every batch of steel is different. Furthermore, steel used for patches and other repairs conducted over the last 118 years may greatly differ from what the locomotive was constructed with in 1899. So 23 test sites were identified and prepared with a flapper wheel to remove all paint, rust and other contaminants.

The Mistras Group was engaged to test each of the plates used in the boiler. In all, 23 individual plates were identified and received an arc-flash optical emission spectrometer test. Carbon is one of the components that the Museum is concerned about and this method is an efficient way to determine if the sample is OK, or if further investigation is required. Rogers Locomotive Works claimed that their boiler plate was manufactured with not more than .18% carbon content. More carbon means stronger plates, but it also makes them brittle and prone to cracking, particularly if welded.   

The Mistras technician Haley Lowe conducted optical emissions spectroscopy using a Metorex Arc-Met 930 Metals Analyzer.  The portable device (or at least when compared to the small refrigerator-sized mass spectrometer) was introduced more than 25 years ago and is popular in laboratories and foundries where chemical analysis of metals is required.

The device creates a high voltage electric arc on the surface of the metal.  The device analyzes the light created by the arc and produces a print out indicating material composition by percent. The report includes sulfur, silicone, and carbon, but also other common components including chromium, manganese, copper, tungsten, and phosphorus.

The testing provided lots of good news and reason for optimism, but also revealed three plates that will have to receive further testing, and in the absence of better news may have to be replaced due to abnormally high carbon content. Two sheets are in the firebox and are most likely replacements installed after the locomotive was built.  The third sheet appears to be a patch that will most likely have to be replaced.  Is this bad news?  Not really.  The locomotive is 118 years old and defects were expected.  And it may be good news because these potential issues have been discovered before the locomotive was reassembled!