Mechanical properties and chemical composition
The mechanical properties of steel are primarily determined by the chemical composition of the steel. As steel has a number of useful mechanical properties in relation to other metals and their alloys, steel has a very wide range of applications.
Mechanical properties
The most important mechanical properties of steel are:
- Strength: the resistance that steel offers to fracture tension.
- Ductility versus brittleness. Ductility is the extent of deformation that steel can withstand before fracturing. Brittleness is the extent to which the steel fractures when force is exerted on it without stretching it.
- Hardness: the resistance that steel offers to fracture tension.
These properties of steel are determined by means of the tension test, the notched-bar impact test and measurement of hardness.
Tension test
In the tension test tensile force is applied to the steel and the behaviour of the steel under gradually increasing tensile strain is measured. When the limit of elasticity is reached, the steel begins to deform permanently.
Notched-bar impact test
The notched-bar impact test measures the amount of energy required to fracture a work piece with a V-shaped notch along the centre line with a sudden impact. The energy required to break the tested bar, the absorbed energy, is expressed in Joules. Ductile materials can absorb lots of energy, whereas in brittle materials, energy absorption is much lower.
It is of great importance to the shipbuilding and offshore industries in particular to know that the materials used are sufficiently ductile. In the offshore and shipping industries, structures are, after all, subjected to substantial forces. These structures must therefore not be fragile or brittle: it is important that they are tough and ductile.
Measurement of hardness
In the measurement of hardness a hard object (ball or diamond stylus) is pushed into the piece of steel being tested. The object leaves an impression in the steel. Depending on the measuring method, either the surface indentation or the depth of the impression is measured. The Brinell indentation hardness scale measures the scale of penetration of an indenter (typically a steel ball) in the test piece. The extent of penetration is a measure of the hardness of the steel and is expressed in terms of hardness on the Brinell scale (HB).
Chemical composition
The chemical composition of steel is extremely important. It is a fundamental factor that determines the mechanical properties of the steel, hence also the use to which the steel can be put. Due to this fundamental role, controls on the production of steel according to applicable European standards are applied strictly. Where steel is used, the engineer or draughtsman must always take into account the exact chemical composition of the steel to be used.
