Thermochemical treatment is an important process in metallurgy aimed at improving the performance properties of materials, such as hardness, wear resistance and corrosion resistance. The process involves the diffusion of chemical elements into the metal surface at high temperatures, which changes its structure and properties. Despite its obvious benefits, thermochemical treatment can cause surface defects such as oxidation, cracks, distortion and changes in metal composition. Let’s look at the main causes of these defects and how to prevent them.
1. Correct material selection and surface preparation
The first step in preventing defects is choosing the right materials and preparing them properly before thermochemical treatment. The metal must be cleaned of contaminants, oxide films and grease, which can cause uneven penetration of chemical elements and lead to defects.
Methods such as sandblasting, pickling and polishing help remove any surface contamination and prepare the metal for treatment. It is important to make sure the metal has the required chemical purity and contains no unwanted impurities that could affect the process.
2. Controlling temperature and treatment time
Temperature and process duration are among the main factors affecting surface quality during thermochemical treatment. Excessive temperature or overly long exposure can overheat the metal and cause oxide films to form, resulting in poorer surface quality.
Carefully controlled temperature and time settings are recommended to minimize the risk of overheating and ensure uniform distribution of chemical elements in the metal structure. For example, steel carburizing is usually carried out at 850–950 °C, and the treatment time depends on the required case depth.
3. Using protective atmospheres
To prevent oxidation and decarburization of the metal surface during thermochemical treatment, protective media such as vacuum or inert gases (argon, nitrogen) should be used. These media prevent the metal from coming into contact with oxygen and other gases that can cause unwanted reactions and defects.
Gas media with added active components (for example, ammonia in nitriding) make it possible to control the diffusion process and prevent cracks and surface irregularities.
4. Proper cooling and quenching
Cooling after thermochemical treatment plays an important role in preventing surface defects. Rapid cooling (quenching) is often used to achieve high hardness, but if done incorrectly, it can cause thermal stresses that lead to cracks and distortion.
To prevent such defects, step quenching or quenching media with a controlled cooling rate are recommended. For example, cooling in oil or water at a specific temperature can help avoid thermal shock and ensure uniform temperature distribution in the metal.
5. Controlling the composition of the chemical medium
The chemical composition of the medium used in thermochemical treatment plays a key role in shaping the properties of the metal surface. In carburizing, for example, it is important to maintain the optimal carbon level in the medium to avoid an overly hard and brittle structure.
Regular analysis and adjustment of the medium’s composition helps avoid defects such as excessive porosity or microcracks and ensures high surface quality.
6. Using high-quality equipment
Modern thermochemical treatment equipment also helps minimize surface defects. Vacuum furnaces, controlled-atmosphere chambers and automatic temperature and time control systems ensure precise process execution, reducing the risk of defects.
7. In-process quality control
To reduce the likelihood of defects, in-process quality control is recommended at every stage of thermochemical treatment. Non-destructive testing methods such as ultrasonic testing or radiography make it possible to detect potential defects early and adjust the process.
Surface defects during thermochemical treatment can be avoided by paying attention to material preparation, strict control of temperature and time, protective atmospheres, proper cooling and modern equipment. A comprehensive approach and compliance with all process requirements make it possible to produce a high-quality part with the required performance properties while minimizing the risk of defects.