Heat treatments

13 April 2017 Author : 

Without going into detail, because it is a complex subject that requires knowledge of chemistry and physical-chemistry, it is necessary to know that crystalline grains structure of the steel is closely related to the temperature at which the material is treated and with its cooling speed.


Heat treatments consist of three main steps:

  • Heating of the material
  • Maintaining it at a defined temperature
  • Cooling with a rate that following appropriate laws


The dependence of the structure obtained by the temperature and cooling rate can be studied through the transformation diagrams (whose lines depends from the carbon content present in steel, also by the presence of alloying elements in solid austenitic solution):

  • TTT (Temperature, Time, Transformation), made rapidly cooling the steel at various temperatures and then keeping them constant and evaluating the beginning and end of the various transformations over time;
  • CCC (Continuous cooling curves) constructed by varying the beginning and the end of the various transformations over time according to various trajectories cooling.

A steel during cooling undergoes these phase transformations :

  • Pearlitic transformation : austenite isothermal transformation of a steel with eutectoid composition that leads to the formation of pearlite ( constituted by aggregates of alternate laminae of ferrite and cementite);
  • Bainitic transformation : isothermal transformation of austenite of a eutectoid steel that leads to the formation of bainite ( upper or lower depending on the temperature ), constituted by an aggregate of ferrite and iron carbide .
  • Martensitic transformation : the transformation that occurs in very short time at a certain temperature (start temperature of austenite - martensite transformation).

This transformation is different from the previous ones because it has no time for nucleation and growth. This is because the transformation is done through a coordinated movement of atoms and without diffusion processes, accordingly the chemical composition of martensite is identical to that of austenite from which it is formed.

The main steel heat treatments can be divided into:

  • heat treatment in which the steel are heating to a temperature greater than critical temperature: ANNEALING, NORMALIZING, HARDENING;
  • heat treatment in which the steel is heated at a temperature below the critical temperature: annealing softening or improvement of workability, etc.
  • heat treatments in order to achieve particular results: surface hardening, solubilization hardening of austenitic steels, etc.

Let us examine below only the general aspects of some of these heat treatments.

ANNEALING

It is a heat treatment characterized by a high temperature heating, by the prolonged stay at that temperature, and a subsequent slow cooling .
The aim of treatment is to make the material more easily workable with machine tools, or to allow the further cold deformation if it has been work hardened by the previous deformation.
The purpose of annealing is also to homogenize the composition of the raw materials, to obtain a specific microstructure with specific physical and mechanical properties and, in the case of quenched steels, to cancel the effects of the martensitic hardening .
The complete annealing is almost never performed because, in addition to being uneconomical as it reaches temperatures higher than the critical one, the annealing usually leads to a coarse grain structure. High grain size leads to a drastic decrease of the toughness of the material ( with increased fragility ) , therefore, it is better to avoid it.

NORMALIZING

This process is performed by heating the steel to a temperature of about 70 ° C above the critical temperature, maintaining at this temperature for a time sufficient to complete austenitization and letting it cool freely in air.
In this way you get a fine grain steel with smooth and homogeneous structure regardless of the initial situation.

HARDENING

Consists in steel heating above the critical point, in maintaining it at that temperature for a sufficient time to obtain an austenitic structure in the heart, then in cooling with higher speed than the critical hardening speed, to obtain at room temperature a martensitic structure characterized by great hardness.
The fundamental conditions because a steel can take a fully martensitic structure are:

  • the temperature at which the steel is heated and the residence time at this temperature should be such as to allow the starting structure to become fully austenitic;
  • the cooling rate must be so high to prevent the transformations at high temperature;
  • the austenite-martensite transformation temperature must be higher than room temperature;

To avoid overheating, then the grain enlargement, the temperature at which the steel must be brought before being hardened (hardening temperature) must be indicatively of about 50-70°C above the critical temperature.
The rapid cooling necessary to harden the steel is achieved by immersing the piece in a quenching bath which can be, in order of increasing severity, air, oil, water, brine (concentrated solution of salts in water), and molten salts when it is necessary a quenching bath at high temperature.

Since the austenite-martensite transformation occurs with an almost instantaneous volume increase, the hardening induces internal stresses in steels also relevant that can lead to deformation or even breakage inside steel (with the formation of quenching cracks).
To avoid the risk of quenching crack formation it is necessary to carefully choose the less drastic quenching bath. There were also carried out some special heat treatments (such as bainitic hardening).
The optimal choice (which depends on the type of steel that the size of the piece) is not simple, and it is generally necessary to consult the steel manufacturer indications or rely on experience.

In this regard, we attach a pdf file created by the knife maker DENIS MURA, where there are indications, according to his experience, to carry out the heat treatment of some steels.

TEMPERING


The martensite is tough and durable, but it is fragile. The heating of martensite, said tempering, allows to obtain structures with an advantageous combination of hardness and toughness.
The tempering consists in heating a steel hardened to a temperature below to the critical (maximum at 600-650°C), maintaining it at that temperature for an appropriate time, and then to the subsequent cooling generally in air.
The magnitude of the effects obtained (decrease hardness but simultaneous increase of the toughness and ductility of the steel) is a function of tempering temperature and its duration, and the results are more marked at higher temperature and with longer treatment time.

 

Bibliography

Structure and properties of metallic materials - Alberto Cigada - Città studi Edizioni

 

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