Construction and metallurgy
Originally, historians thought the iceberg had cut a gash into
Titanic's hull. Since the part of the ship that the iceberg damaged is now buried, scientists used
sonar to examine the area and discovered the iceberg had caused the hull to buckle, allowing water to enter
Titanic between her steel plates.
A detailed analysis of small pieces of the steel plating from the
Titanic's wreck hull found that it was of a metallurgy that loses its elasticity and becomes
brittle in cold or icy water, leaving it vulnerable to dent-induced ruptures. The pieces of steel were found to have very high content of
phosphorus and
sulphur (4× and 2× respectively, compared with modern steel), with
manganese-sulphur ratio of 6.8:1 (compared with over 200:1 ratio for modern steels). High content of phosphorus initiates fractures, sulphur forms grains of iron sulphide that facilitate propagation of cracks, and lack of manganese makes the steel less ductile. The recovered samples were found to be undergoing
ductile-brittle transition in temperatures of 90 °F (32 °C) for longitudinal samples and 133 °F (56 °C) for transversal samples, compared with transition temperature of −17 °F (−27 °C) common for modern steels: modern steel would only become so brittle in between −76 °F and −94 °F (−60 °C and −70 °C). The
Titanic's steel, although "probably the best plain carbon ship plate available at the time", was thus unsuitable for use at low temperatures.
[81] The
anisotropy was probably caused by
hot rolling influencing the orientation of the sulphide
stringer inclusions. The steel was probably produced in the acid-lined, open-hearth furnaces in
Glasgow, which would explain the high content of phosphorus and sulphur, even for the time.
[81][82]
Another factor was the rivets holding the hull together, which were much more fragile than once thought.
[82][83] From 48 rivets recovered from the hull of the
Titanic, scientists found many to be riddled with high concentrations of
slag. A glassy residue of smelting, slag can make rivets brittle and prone to fracture. Records from the archive of the builder show that the ship's builder ordered No. 3 iron bar, known as "best"—not No. 4, known as "best-best", for its rivets, although shipbuilders at that time typically used No. 4 iron for rivets. The company also had shortages of skilled riveters, particularly important for hand riveting, which took great skill: the iron had to be heated to a precise colour and shaped by the right combination of hammer blows. The company used steel rivets, which were stronger and could be installed by machine, on the central hull, where stresses were expected to be greatest, using iron rivets for the stern and bow.
[82] Rivets of "best best" iron had a
tensile strength approximately 80% of that of steel, "best" iron some 73%.
[84] Despite this, the most extensive and finally fatal damage
Titanic sustained at boiler rooms No. 5 and 6 was done in an area where steel rivets were used.
