THERMOCHEMICAL TREATMENTS – DEVELOPMENTS TRENDS

Take into consideration that the heat treatment and surface engineering field provides an important contribution to the increasing of the exploiting performances of the metallic parts, the international community (research institutes, professional associations, industrial companies with research centers) established the research trends for the future.

For the technologies listed below are foreseen important evolutions for the next years:

  • Diffusion processes with C, N, O at temperatures below 700°
  • Diffusion processes with C, N at temperatures above 850°

For both processes, the currently researches are dedicated to the improving of the available control methods, reducing of the process times, developing of new automation or improving the available automation.

The diffusion processes below 700° – the nitriding, nitrocarburising, and oxinitrocarburising were highly developed in the past 10 years due to advantages provided by these as:

  • minimal distortion;
  • short process time ;
  • low energy consumptions;
  • very good exploiting performances (wear resistance, corrosion resistance)

Being a permanently competition between salt bath nitrocarburising (Tenifer, Melonite), gaseous nitrocarburising and plasma nitriding, the gaseous nitrocarburising become more and more used, especially due to much easier application and environmental respecting in comparison with the other processes.

Recent researches show the possibility of diffusion processes acceleration by means of pre or post oxidation and respecting of an optimal rapport between the technological gases. This kind of process – oxinitrocarburising led to very good treated layers by point of view of performances in exploiting.

In this manner, the realising of oxidation in the cooling stage combined with cooling in aditivated oil led to an important increasing of the corrosion resistance, being possible the replacement of some anticorrosive coatings (painting, electrochemical coatings, etc)

The diffusion with C, N above 850°C was well studied in the latest years. The diffusion processes with C, N above 850°C represented the object of study by point of view of fabrication costs minimizing through the reducing of the energy and technological consumptions costs.

For the parts that requested layers with depth below 0,8mm the recommended process is nitrocarburising.

In the latest years, the low-pressure carburising has constituted a research object for different companies and research laboratories in this field of activity.

The developing of the micro-alloyed steels with rare earths allows the increasing of the carburising temperature in the range of 1000-1050 °C, the diffusion being accelerated.

Also, the using of the acetylene (C2H2) led both to the gas flow rate reducing and to a greater carbon input during diffusion. A disadvantage in the developing of the vacuum carburising is the equipment cost.

The main advantages and disadvantages of vacuum carburising in comparsion with controlled atmosphere carburising are shown in the table listed below:

Parameters Controlled atmosphere carburising Vacuum carburising
Temperature range (°C) 880-980 880-1100
Carbon potential control Yes No
Carburising time Normal < 30%
Internal oxidation Possible No
Decarburising Possible No
Energy consumption Normal < 30%
Gas consumption Normal < 60 – 70 %
Integration in automatic lines Difficult Very easy
Investments Normal Great

With all these disadvantages as the investments volume, can be appreciated that in the next 10 years the vacuum carburising will be more and more applied until a percentage of 10-15%.

Also, researches regarding the furnaces performances improving are made as it follows:

  • reducing of the energy consumptions with 10-15%;
  • noxes minimizing;
  • productivity increasing and production costs reducing with 50%;
  • the increasing of life time;
  • the reducing of the deformations through changing of the cooling systems;
  • furnaces prices decreasing with 20-40%.

Another research range is the field of real time process sensors, sensors used both for heating and cooling control and for layer depth measuring.

On basis of technologies and sensors developments, the modelling will led to the possibility of simulation and predicting of the results of the heating, diffusion and solid state transformation processes.

The modelling will allow the obtaining of some data bases regarding the correlation tensions-deformations, the nfluence of the material and process parameters on the exploiting properties of the treated parts and finally will provide the design of the heat treatment processes depending on requested properties of the treated parts during exploiting.

Some of the issues listed above are also researched in the Romanian research centers in view of developing of industrial applications in the future.

Also, in many Romanian commercial societies operate different nitrocarburising installations, carburising and carbonitriding installations endowed with process computers and automat control of the processes.

The Romanian Heat Treatment and Surface Engineering Association – ATTIS has the task to inform the ATTIS members about the developments concepts and also to recommend the introduction of the new and efficient technologies in the Romanian industry.

Certainly, one of the ATTIS tasks is to stimulate the development of commercial societies with heat treatment and surface engineering field of activity – the only way to keep and develop these specific activities.

Leontin Druga, Professor, Ph.D
President of ATTIS