Question

(a)State the laboratory tests that may be carried out on specimens of steel for ships' plate giving reasons for the tests.

(b)The basic compositions of two ships' plates are given in Table below One is an example of modem practice whilst the other is a specification of a 1940's tanker that split in two due to brittle fracture. Compare these two specifications critically and explain which of these two steels would be most resistant to brittle fracture.

C M SI S P AI N

Steel 'A' 0.18% 0.70% 0.30% 0.04% 0.04% 0,015% 0,005%

Steal 'B' 0. 19% 0.57% 0.03% 0.029% 0.042% 0.005% 0.008%

Answer:

Steel may be prone to or more resistant to the phenomena of brittle fracture, depending upon the content of alloying elements or impurities.

Steel 'A' is typical of Lloyds Grade 'E' steel which has a high resistance to brittle fracture whilst steel 'B' would be more prone to brittle fracture due to its compositions which are critically analysed as under :

Carbon content –

Carbon increases strength, elasticity and hardness and lowers the ductility and impact strength.

There is a limit of 0.18% carbon for a brittle fracture resistant steel such as Grade 'E' in steel 'A'.

Since steel 'B' exceeds this limit, it will be more prone to brittle fracture,) even though it is only a small excess.

Manganese Content - Manganese serves as a valuable deoxidising and purifying agent. It combines with sulphur and thereby decreases the harmful effect of sulphur. When used in low carbon steel, it increases strength & toughness and makes the steel ductile and of good bend qualities. There is a minimum content of 0.7% for brittle fracture resistant steel such as Grade 'E'. Steel 'A' meets this requirement, while steel 'B' is deficient.

Silicon Content - Silicon in the finished steel prevents them from becoming porous. It acts as a very good deoxidiser and removes the gases and oxides, prevents blowholes and thereby makes the steel tougher and harder. Silicon percentage upto 0.3% is beneficial, which is correctly met by steel 'A' but steel 'B' falls well within the limit, it is too low to derive any benefits and possibility of porosity remains.

Sulphur Content - Sulphur is an impurity and occurs in steel as iron sulphide or as manganese sulphide. Iron sulphide, because of its low melting points produces red-shortness, i.e., increased brittleness at elevated temperature, while manganese sulphide does not affect so much. Presence of substantial amounts of sulphur unfavourably affects tensile strength, yield point, fatigue limit and decreases its corrosion resistance. Sulphur content should, not exceed 0.05%, here both metals are within the limit but there can be a variation in properties with the distribution of these impurities in the steel, however there is no information regarding distribution.

Phosphorus Content - Phosphorus is also an impurity and imparts cold-shortness i.e., high, brittleness at normal and especially low temperature when exists over 0.05%. It increases the tensile strength but at the same time reduces , the impact strength and ductility. Here both metals are within this limit thus not the cause of brittle fracture.

Aluminium Content - It may be considered as an impurity and should be within 0.05% limit as sulphur and phosphorus,, which both steels 'A' & 'B' do comply, but a small percentage is beneficial in the steel making process to scavenge oxygen i.e.,) so that it do not have porous blow holes. (Mn & Si also do this). Steel 'A' having slightly higher Al content is in advantageous position than steel 'B'.

Nitrogen Content - Nitrogen causes embrittlement and overage composition should generally not exceed 0.005% (the norm for high grade steel produced by the open hearth process). Steel 'A' again falls within the limit but steel 'B' exceeds it and is thus more prone to brittle fracture.

Although the differences are generally small when taken individually, the combined effect can be much more significant^ especially when limits are exceeded or minimums are not achieved.