Thermal treatments in controlled atmospheres

Trattamenti termici e Termochimici Acciai
  • Quenching and tempering: Quenching and tempering is the entire heat treatment cycle of austenitization.

  •  Normalizing: Full austenitization heat treatment followed by cooling in air until room temperature.

  • Annealing: Annealing is the heat treatment of austenitization followed by slow cooling in the furnace to obtain the transformation of the austenite under conditions closer to equilibrium.

  • Distension/ Stabilization: heating to a temperature between 150 and 250°C to eliminate the structural tensions of tetragonal martensite obtained by quenching.

  • Subzero treatment: Under-cooling treatment carried out after carburizing, before tempering, with the task of transforming a possible excess of residual austenite of the cemented layer.

THERMOCHEMICAL TREATMENTS ON STEEL IN CONTROLLED ATMOSPHERE:

 

  • Carburizing: Thermochemical treatment carried out in austenitic field that aims to enrich the surface layer of the steel with carbon, through the phenomenon of diffusion in solid phase.
  • Recarburization: restoration of the amount of carbon in steels in the case of previous decarburization processes.
  • Carbonitriding: Thermochemical treatment carried out in austenitic field in a medium capable of simultaneously releasing carbon and nitrogen.

Our thermochemical treatments on steel are carried out in appropriate atmospheres generated by the use of “technical gases”.

The SOLO Swiss plant, thanks to their useful dimensions, allow the vertical hardening of parts up to 1050 mm in height. In addition, multi-sector electric heating, together with the type of distribution of the charge, allows an excellent temperature uniformity within the chamber allowing better control over deformations.

Deformations during thermal treatments

One of the main problems during this process are deformations after the thermal treatment.

 

Practical cases are very often not reflected in the theory, simply because of the difficulty of identifying the different physical variables that critically affect the final result.

  • As a starting point, we must distinguish the deformations between two types:
    Dimensional variations: contractions and expansions due to structural changes during the treatment, which can therefore increase or decrease the volume of the piece without modifying its geometry (isotropic variation). This type of variation occurs very rarely, due to the fact that the materials are generally heterogeneous and their behavior is anisotropic.
  • Shape variations or distortions: these depend on the variations in anisotropic volumes, but above all on the elastic / plastic deformations generated by internal stresses. To a large extent they depend on the thermal gradients between the different parts in the heating and cooling phases.

The different theories about deformations cannot predict any increase in volume or variation of form, but they can explain their nature. To be able to minimize them it is therefore necessary to be able to control and define different process parameters, depending on the different types of pieces to be treated.

 

The difference in temperature between the heart and the surface of the pieces in the heating and cooling phases causes internal tensions that, in most cases, exceed the yield strength of the material at high temperatures. When this happens, the material undergoes shape variations in addition to inherent dimensional variations caused by the structural change in the austenitization and hardening phase.

In the first instance the allowance necessary for mechanical machining must be greater than the minimum of the dimensional variations caused by the structural change, which for the theoretical case of an isotropic variation, is around 1% of the volume and 0.3% of the linear measures.

In cases where there is a percentage of residual austenite, the volumetric deformation decreases directly proportional to the amount of austenite, and in some cases where the residual austenite is very high, as can happen in tool steels with high carbon content, it can even lead to a contraction of volume.

 

The cause-effect diagram above, helps to understand how many other parameters influence the success of the whole process, where the heat treatment, although the process that highlights most deformations, in reality is not the only one responsible.

 

Bibliography and references:
[1] ASM Metals HandBook Volume 04 – Heat Treating
[2] The criteria for the selection and treatment of structural and tool steels Volume 1 – basic metallurgy – Cibaldi Dr. Cesare
[3] Analysis of the problems related to the deformation of mechanical parts made of 18NiCrMo5 steel subjected to thermochemical treatment of carburizing – OMME snc
29/08/16 Elaborated by Ing. Juan Carlos Inés Vilches

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