Composition
During an expedition to the wreckage in the North Atlantic on August 15, 1996, researchers brought back steel from the hull of the ship for metallurgical analysis. After the steel was received at the University of Missouri-Rolla, the first step was to determine its composition. The chemical analysis of the steel from the hull is given in Table II. The first item noted is the very low nitrogen content. This indicates that the steel was not made by the Bessemer process; such steel would have a high nitrogen content that would have made it very brittle, particularly at low temperatures. In the early 20th century, the only other method for making structural steel was the open-hearth process. The fairly high oxygen and low silicon content means that the steel has only been partially deoxidized, yielding a semikilled steel. The phosphorus content is slightly higher than normal, while the sulfur content is quite high, accompanied by a low manganese content. This yielded a Mn:S ratio of 6.8:1—a very low ratio by modern standards. The presence of relatively high amounts of phosphorous, oxygen, and sulfur has a tendency to embrittle the steel at low temperatures.
Davies7 has shown that at the time the Titanic was constructed about two-thirds of the open-hearth steel produced in the United Kingdom was done in furnaces having acid linings. There is a high probability that the steel used in the Titanic was made in an acid-lined open-hearth furnace, which accounts for the fairly high phosphorus and high sulfur content. The lining of the basic open-hearth furnace will react with phosphorus and sulfur to help remove these two impurities from the steel. It is likely that all or most of the steel came from Glasgow, Scotland.
Included in Table II are the compositions of two other steels: steel used to construct lock gates at the Chittenden Ship Lock between Lake Washington and Puget Sound at Seattle, Washington,8 and the composition of a modern steel, ASTM A36. The ship lock was built around 1912, making the steel about the same age as the steel from the Titanic.
Metallography
Standard metallographic techniques were used to prepare specimens taken from the hull plate of the Titanic for optical microscopic examination. After grinding and polishing, etching was done with 2% Nital. Because earlier work by Brigham and Lafrenière9 showed severe banding in a specimen of the steel, specimens were cut from the hull plate in both the transverse and longitudinal directions. Figure 2 shows the microstructure of the steel. In both micrographs, it is apparent that the steel is banded, although the banding is more severe in the longitudinal section. In this section, there are large masses of MnS particles elongated in the direction of the banding. The average grain diameter is 60.40 µm for the longitudinal microstructure and 41.92 µm for the microstructure in the transverse direction. In neither micrograph can the pearlite be resolved. For comparison, Figure 3 is a micrograph of ASTM A36 steel, which has a mean grain diameter of 26.173 µm.
Figure 4 is a scanning electron microscopy (SEM) micrograph of the polished and etched surface of steel from the Titanic. The pearlite can be resolved in this micrograph. The dark gray areas are ferrite. The very dark elliptically shaped structure is a particle of MnS identified by energy-dispersive x-ray analysis (EDAX). It is elongated in the direction of the banding, suggesting that banding is the result of the hot rolling of the steel. There is some evidence of small nonmetallic inclusions and some of the ferrite grain boundaries are visible.
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Comparing the composition of the Titanic steel and ASTM A36 steel shows that the modern steel has a higher manganese content and lower sulfur content, yielding a higher Mn:S ratio that reduced the ductile-brittle transition temperature substantially. In addition, ASTM A36 steel has a substantially lower phosphorus content, which will also lower the ductile-brittle transition temperature. Jankovic8 found that the ductile-brittle transition temperature for the Chittenden lock gate steel was 33°C. The longitudinal specimens of the Titanic hull steel made in the United Kingdom and those specimens from the Chittenden lock steel made in the United States have nearly the same ductile-brittle transition temperature.