Heat is typically required to change the formation and appearance of metals. However, as these heated metals react with air, the said interaction would manifest numerous unwanted changes to the materials such as discolouration, scale, or contamination. The effects of the interaction with air are brought by the oxidation process, which should be avoided to achieve desired metal properties.
One great way to avoid the oxidation process is through vacuum heat treatment. This specific treatment places the metal material in a high-temperature vacuum furnace to effectively remove any reactive element. Additionally, this tightly sealed furnace heats the metal material to the most optimal temperature for it to obtain the intended properties. The soak time of the heated metal material will then depend on the needs of the part and the type of metal that is used. It would typically take around 3 to 24 hours for the whole material to be processed, ensuring its uniformity and repeatability are intact.
Vacuum heat treatment is utilised by a lot of industries due to the following advantages:
Eliminates Contact with Elements
One advantage of vacuum heat treatment is that it eliminates any contact of the metal material to various surrounding elements. The ability of this treatment to isolate the material from oxygen prevents it from having a rough surface, losing metallic lustre, and obtaining soft spots and cracks. The strength of the metal material, as well as its mechanical properties, are also preserved when subjected under vacuum heat treatment since any oxidation, decarburisation, and other processes are avoided.
Improves Mechanical Properties
Another reason why vacuum heat treatment can improve the mechanical properties of a metal is due to its ability to reduce hydrogen to the material. Hydrogen can be dangerous to metals. Fortunately, vacuum heat treatment can reduce the hydrogen content of the material, leading to a significant improvement over its plasticity, toughness, and fatigue strength. The vacuum furnace can readily get rid of the oxides and any free oxygen formed during the whole heating process.
Maintains High Thermal Efficiency
The whole process of vacuum heat treatment has high thermal efficiency, which is very helpful in maintaining the stability and repeatability of the metal from being processed. The high thermal efficiency of the treatment also makes it possible for the material to undergo rapid heating and cooling, preventing it from undergoing oxidation, decarbonisation, and carburisation. Degreasing and degassing are likewise possible with vacuum heat treatment without causing any harm and pollution to the surroundings.
Reduces Workpiece Volatility
Metal materials that are processed with vacuum heat treatment are expected to obtain reduced workpiece volatility. This specific characteristic allows the material to retain its features and properties even if they will be formed and processed again.
All these advantages allow the vacuum heat treatment to be pivotal for industries that strictly rely on metal hardening process. Without vacuum heat treatment, oxidation can still be a huge problem for most industries, making the whole metal processing difficult.
Most heat treatment companies would apply case hardening procedures to a workpiece so that its surface will become harder. Case hardening refers primarily to the process of hardening or coating the surface of a metal workpiece with a thin layer of harder metal while subsequently allowing the base material of the same workpiece to remain soft. The case that surrounds the workpiece can be made from carbon, nitrogen, or both elements.
To date, there are two popular methods of conducting case hardening: nitrocarburizing and carbonitriding. Some vital factors that can determine the most compatible case hardening method for a specific workpiece include its material, part specifications, and intended uses or application.
Nitrocarburizing
Nitrocarburizing is the dissolution of both carbon and nitrogen into a specific workpiece. However, this case hardening method utilises more nitrogen compared to carbonitriding. For this case hardening method, there are two forms that one can apply: austenitic and ferritic.
Austenitic nitrocarburizing is intended to provide a deeper case depth and longer wear life of a workpiece. It is conducted at temperatures ranging between 675 and 775°C so that there will be no phase change occurrences. This form of nitrocarburizing is controlled to produce a surface layer of epsilon carbonitride and another surface of bainite or martensite, which all provide a great support structure for the hard surface layer. Austenitic nitrocarburizing is great for intermediate stress-point contact-resistance applications.
Ferritic nitrocarburizing, on the other hand, can easily provide case hardening without the need to heat metal parts into a phase change. This form of nitrocarburizing is typically conducted between 525 and 625°C to allow the diffusion of nitrogen atoms to diffuse into the steel. With the same temperature ranges, carbon atoms cannot simply diffuse into the workpiece of steel. This process significantly improves scuffing resistance, fatigue properties, and corrosion resistance of the workpiece.
Carbonitriding
In a carbonitriding process, several parts are heated into a sealed chamber before adding nitrogen and carbon. The heating of the parts is done within the austenitic range, which allows a phase change in the crystal structure of the steel and subsequently permits carbon and nitrogen elements to diffuse into them. Ammonia is introduced into the carburizing process so that nitrogen can be diffused into the parts.
Low-carbon, low-alloys steels are hardened well whenever nitrogen is added into them. Nitrogen, which comes in the form of ammonia gas molecules, can help workpieces maintain their needed hardness during high-temperature operations and applications. Compared to nitrocarburizing, carbonitriding can easily provide greater case depths to metal workpieces. Some limitations with this case hardening process, however, are the long process time and tons of resources involved in achieving great case depths.
Harden surfaces of workpieces, which are processed easily and inexpensively, can benefit from carbonitriding. The application of this case hardening method can significantly boost wear resistance and fatigue strength of the involved workpieces.
While both case hardening methods have their own set of differences, they can still enhance the properties of metal workpieces. If you want to know more about these methods, feel free to contact us at Alpha Detroit Heat Treatment.