Answer EKG Question 47

Q47. 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.

Answer: Tests carried out on specimens of steel for ships plate are destructive tests, which are done on specimens obtained from the same product as the finished material.

The tests that are carried out are;

1. Tensile test

2. Impact test

Tensile test, this is where a specially shaped specimen of standard size is gripped between the jaws of a testing machine, load is then gradually applied to the specimen to draw it apart, so it is subject to tensile stress. A graph of strain against stress is plotted for the specimen.

The significance of the tensile test is to work out the yield stress, ultimate tensile stress or loading.

Impact test; this test measures the energy absorbed by a material when it is fractured. This is normally performed using the Charpy V notch test.

The significance of the impact test is to obtain the energy absorbed by and maintained by the material when fractured and also toughness of material, this can be useful to know for safety reasons, i.e. how plating will with stand collision and knocks.

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

B. 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 he more prone to brittle fracture.

Carbon increases strength and hardness but reduces ductility and toughness. Thus, there is a limit of 0.18% C for brittle fracture resistant steel such as Grade 'E'. Since 'B' exceeds this limit it will be more prone to brittle fracture, even though it is only a little excess.

The addition of manganese will increase strength, but it also increases toughness, hence there is a minimum content of 0.7% for brittle fracture resistant steel such as Grade 'E'. Again, steel 'A' meets this requirement whilst steel 'B' is deficient.

Sulphur and phosphorus are both impurities that reduce toughness and are limited to 0.05%. Both materials are within these limits, but there can be a variation in properties with the distribution of these impurities in the steel, however there is no information regarding distribution.

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

Silicon has beneficial properties to toughness to a fairly high maximum of 0.3% which is not exceeds by 'A' whilst 'B' falls well within the limit. It will be too low to drive any benefits.

Aluminium may be considered as an impurity and should be within the same 0.05% Limit as S and P, which both steels 'A' and 'B' do comply, but a small percentage is beneficial in the steel making process to scavenge oxygen and ‘kill the steel. i.e. so that it do not have porous blow holes.

(Mn and Si also do this)

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.