The Titanic - A Study in Metal Failure
Why did the Titanic’s hull fail?
The story of the Titanic is a tragic tale of life lost. We have long been intrigued with the reasons why this catastrophe occurred. Design certainly played a role, but other similarly designed ships had useful service lives. At least one “cause” can be attributed to the metal used to make the hull of the ship. The melting and forming of this material certainly contributed. A Journal of Metals article from 1998 explains why.
The Titanic was one of three ships that was built to compete with two of the largest and fastest steamers in the North Atlantic – the Lusitania and the Mauritania. The Titanic and its sister ships – the Olympic and the Britannic – were to be designed to provide superior accommodations but not to be faster. In the early 20th century, the only means of transportation for travelers and mail between Europe and North America was by passenger steamship. The Titanic would transport passengers in style, having the first onboard swimming pool and gymnasium.
The power plant for a ship this massive was rated at 51,000 I.H.P. To provide the necessary steam, 159 furnaces fired 29 boilers. Coal was burned as fuel at a rate of 650 tons per day. The construction used wrought-iron rivets to attach steel plates to each other or to a steel frame. The frame was held together with similar rivets. Each rivet was heated well into the austenite temperature region, inserted into the mated holes and hydraulically squeezed to fill the holes and form a head. Three million rivets were used in the construction of the Titanic.
Much has been documented about the time between the launch of the Titanic on April 10, 1912, and the early morning of April 15 when it made contact with an iceberg three to six times its size. But what have we learned about the sinking of this great ship in the years that followed? Impact with the iceberg caused discontinuous damage along a 100 meter length of the hull, which created openings between 1.1 and 1.2 m2. In 1985, Robert Ballard found the Titanic in over 12,000 feet of water. The ship had broken into two major sections that were about 2,000 feet apart.
During an expedition in August 1996, researchers obtained steel from the hull for metallurgical analysis. Chemical analysis showed a very low nitrogen content, which meant that the steel was not made by the Bessemer process. It was instead a product of the open-hearth process, and it was likely manufactured in Glasgow, Scotland. Low silicon, high oxygen and high sulfur in the steel indicate that it was only partially deoxidized, or semikilled. This, in addition to the low manganese content – creating a low Mn:S ratio – has a tendency to embrittle steel at low temperatures.
A micrograph of the hull material showed an average grain diameter of 60.4 µm, which is quite large. Comparison of other mechanical properties indicates that the yield strength is lower than typical for this material, probably due to the large grain size. The most telling property was the Charpy Impact tested over a range of temperatures. It indicated a ductile-to-brittle transition temperature of 133°F, and the seawater at the time of collision was 30°F. Clearly, the hull material would have been quite brittle at this water temperature.
So, was it the brittle metal that caused this tragic accident that killed more than 1,500 people? Clearly, it was contact with a 300,000-ton iceberg that did it. The sister ship the Olympic was made from similar steel in the same shipyard from the same design, and it enjoyed a career of more than 20 years. Had the steel not been so brittle at the operating temperatures, however, the fate of many of its passengers might have been different. We will never know. What we do know is that thermal processing of material affects us in ways we may not be aware. IH