Editing Wrought iron (section)

===Ductility===
For most purposes, ductility rather than tensile strength is a more important measure of the quality of wrought iron. In tensile testing, the best irons are able to undergo considerable elongation before failure. Higher tensile wrought iron is brittle.

Because of the large number of boiler explosions on steamboats in the early 1800s, the U.S. Congress passed legislation in 1830 which approved funds for correcting the problem. The treasury awarded a $1500 contract to the Franklin Institute to conduct a study. As part of the study, Walter R. Johnson and Benjamin Reeves conducted strength tests on boiler iron using a tester they had built in 1832 based on a design by Lagerhjelm in Sweden. Because of misunderstandings about tensile strength and ductility, their work did little to reduce failures.<ref name="Gordon1996" />

The importance of ductility was recognized by some very early in the development of tube boilers, evidenced by Thurston's comment:

{{Blockquote|1=If made of such good iron as the makers claimed to have put into them "which worked like lead," they would, as also claimed, when ruptured, open by tearing, and discharge their contents without producing the usual disastrous consequences of a boiler explosion.<ref>{{cite web |url=http://www.history.rochester.edu/steam/thurston/1878/ |archive-url=https://web.archive.org/web/19970629015211/http://www.history.rochester.edu/steam/thurston/1878/ |archive-date=29 June 1997 |url-status=dead |last=Thurston |first=Robert |date=1878 |title=A history of the growth of the steam engine |page=165}}</ref>}}

Various 19th century investigations of boiler explosions, especially those by insurance companies, found causes to be most commonly the result of operating boilers above the safe pressure range, either to get more power, or due to defective boiler pressure relief valves and difficulties of obtaining reliable indications of pressure and water levels. Poor fabrication was also a common problem.<ref name="Hunter">{{cite book |title=A History of Industrial Power in the United States, 1730–1930 |volume=2: Steam Power |last1=Hunter |first1=Louis C. |year=1985 |publisher =University Press of Virginia |location= Charlottesville |url=https://books.google.com/books?id=upQBtQEACAAJ}}<!-- Possibly the most comprehensive work on steam power --></ref> Also, the thickness of the iron in steam drums was low, by modern standards.

By the late 19th century, when metallurgists were able to better understand what properties and processes made good iron, iron in steam engines was being displaced by steel, whilst the old cylindrical boilers with fire tubes were displaced by inherently safer water tube boilers.<ref name="Hunter" />