Steel is an alloy consisting mainly of iron and carbon, to which other elements can be added. In typical steel alloys, carbon contributes up to about 2% of its weight: over this percentage, the alloy is defined as cast iron.
Because of its high tensile strength and low cost, steel is a material widely used in various sectors: from construction, aviation, automotive, instrumentation, etc..
By varying the percentages of carbon and other elements, we obtain types of steel with very different characteristics and hardnesses.
Hardness test on steels
For steels, the Brinell test is fundamental for measuring not only hardness but also tensile strength: these are two variables linked together by a constant and specific ratio for each type of steel. The Brinell test (and in particular the HB/30 set of scales) is the only non-destructive method for knowing the tensile strength of steel.
The surface treatment
Very often steel is heat treated with the aim of modulating the surface hardness, according to specific needs; the response of steel to heat treatment depends on its chemical composition.
The surface hardness resulting from the heat treatment is generally very high and no longer measurable with the Brinell method: in these cases the measurement is made with the Rockwell method or Vickers, which use diamond penetrators. A relevant parameter of heat treatments concerns the depth reached by the induced alteration, as the performance of hardness and strength of the steel depends on it.
Non-destructive test on heat-treated steel
Traditionally, depth measurement is done destructively, because of the need to work on the section of the pieces, but there is now an alternative non-destructive method (HTD 1500/4000 brev. ERNST) based on the sequence of hardness measurements made on the surface (and not on the section).