Whenever an industrial process requires the use of large quantities of heat for the manufacturing of new products or semi-finished goods, it is necessary to use refractory materials to protect the structure.
For this purpose, these materials are applied as a coating to the structures they are designed to protect.
Refractory concrete, in terms of benefits in relation to fast-lay, ensures high performances, but it also has some limits linked to fragility and shrinkage. During drying and firing, refractory concrete undergoes a negative deformation (i.e. contraction) which, together with the stresses induced by abrupt temperature differences, may cause cracks in the material which trigger off a rather rapid deterioration due to heat-induced fractures. To overcome this drawback, since the 1970s this problem could be avoided by adding Steel fibres to the concrete design mix.The presence of Steel fibres makes the mix tougher and more compact, reducing its fragility; they also act as a reinforcement against the spread of cracks induced by shrinkage and thermal stresses.
The market consists in a huge number of fibres; among which, sheet fibres, unlike the needle fibres obtained from drawn wire, have the advantage of being free of lubricants. Moreover, their irregular surface and higher Surface/Volume ratio ensures a more effective bond with the mix.
The type of steel is chosen according to the operating temperature which the refractory concrete will be exposed to, the mix design and characteristics of the atmosphere of the furnace. The choice must be made taking into account the fact that the effectiveness of the fibres is conditioned by the resistance to heat corrosion and the residual tensile strength at high temperatures of the steel employed.
When choosing which steel alloy to use, it is important to remember that chromium and nickel, respectively, are the elements capable of ensuring such features.