Heat-treatment of High Carbon Steel Wire - Patenting

Patenting consists of passing the wire through tubes in a furnace at about 970oC. This high temperature treatment produces uniform austenite of rather large grain size. The subsequent cooling – in air or molten lead – is rapid since the sections treated are generally small (e.g. wire rods), so that the resulting structure consists of very fine pearlite preferably with no separation of primary ferrite

Wire ropes are usually made from carbon steel wires ranging from 0,35 to 0,5% carbon, and before drawing the material is subject to a heat-treatment known as patenting.

The large crystals would give rise to brittleness if the material was left in the heat-treated condition, but this effect is not noticed after a few drawing passes. Variation in hardness - either softer or harder - can be produced by tempering martensite, but such material does not draw so well as patented wire, which is able to withstand reductions of area up to 90%. The strength is explained on the basis of the reduced ferrite cells and the alignment of cementite in fibres.

Hardenability, mass effect, ruling section

Hardenability is the measure of the depth to which a steel will harden on quenching.

The maximum hardness is mainly a function of the carbon content. The hardenability of steel depends on:

  1. The quenching medium and method of quenching.
  2. Composition of the steel and method of manufacture.
  3. Section of the steel.
The so-called "Mass effect" arises from the fact that even with the most severe quench the cooling of the wire rod is progressively slower, from the outside to the centre due to the low thermal conductivity of the steel. It must be appreciated, therefore, that it is the rate of cooling of a piece of steel which determines the properties resulting from a quenching process, and not mass or weight.