Metals typically undergo various heat treatment processes to alter their overall properties and characteristics. Generally, heat treatment is a group of industrial, thermal, or metalworking processes that are utilised to change the physical or chemical properties of a material. Aside from metals, other materials like glass can also maximise the benefits of different heat treatment processes.

There are a lot of processes that are under heat treatment. Some of these processes include annealing, case hardening, carburising, precipitation, strengthening, tempering, and quenching. A wide variety of properties, such as ductility and toughness, that is found on a specific material can be easily modified by one or more of the mentioned heat treatment processes.

Heat Treatment Processes for Metals

Metals such as steels and other alloys are hugely utilised in the manufacturing industry. Their versatility, durability, and longevity have encouraged a lot of manufacturers to maximise them and fabricate them into different equipment, appliances, devices, and other products.

The fabrication of metals has been carried out through different heat treatment processes. One of the most common heat treatment processes for metals is annealing. This heat treatment process heats and slowly cools metal to eradicate stress and makes it softer. Annealing likewise modifies the structure and ductility of the metal. Precipitation hardening, on the other hand, subjects the metal at elevated temperatures to increase its yield or even high-temperature strength.

Carburising is another heat treatment process that adds carbon to the surface of the metal by heating it below the melting point and exposing it with carbon-rich elements. This process helps the metal gain a harder surface and withstand abrasion. Quenching, alternatively, is a heat treatment process that cools the metal by immersing it into water or oil. This is done to fix the whole metal structure in a solidified state.

Protective Atmosphere Normalising

Another heat treatment process or technique that can be utilised by manufacturers would be a protective atmosphere normalising. The purpose of undergoing this heat treatment process is to refine the grain size of the metal and subsequently improve the uniformity of its microstructure. This process can also enhance the overall machinability of the metal.

Protective atmosphere normalising is carried out by heating the metal under suitable temperature and subsequently still-air cooling it to eliminate the heat or thermal influence from hot rolling or forging. The metals under this process are typically heated about 37°C above the upper critical temperature, which is around 900°C. They are then held at the given temperature so that all their parts are evenly heated before cooling them in air.

Cold worked metals are expected to soften when normalised, while metals that have undergone annealing will become reinforced when subjected under this heat treatment process. The microstructure that is produced by this heat treatment process can be pearlite, pearlite in a ferrite matrix, or pearlite in a commentate matrix.

Materials that have been welded, segregated in castings, and forged can all utilise the refinements of protective atmosphere normalising, especially when it comes to their microstructure and grain size.

Different manufacturing industries maximise all the features that can be found on metals. For one, almost all metals can be easily fabricated into any shape, size, or design. Some of them can also withstand intense heat, pressure, force, corrosion, and other damaging elements on their surroundings. Their durability and longevity make them suitable for a wide variety of applications.

But before these industries can take advantage of the properties of metals, these materials must undergo certain steps first. Heat treatment is a process that places metals under several controlled heating and cooling operations, helping the materials achieve notable changes to their physical and structural properties. This process is primarily composed of three basic stages, which can convert an ordinary piece of metal into a properly forged and fabricated metal component.

Heating

Different metals possess a wide variety of distinct properties and characteristics. Some of them can react erratically to heat, while others do not. Some metals may warp and distort when placed under high temperatures at a fast rate, while others do not easily get affected by extreme temperatures. These notable differences are necessary to make the very first stage of the heat treatment process a success.

The first stage of the heat treatment process involves the heating of the metal optimally and uniformly. Considering the properties of metals is crucial so that this stage will effectively process them before proceeding to the next stage. These properties typically include conductivity, overall condition, size, and the cross-section of the metal. To achieve a high-quality final product, the metal must be heated slowly so that all sections of the material can achieve property changes without getting distorted or cracked.

Soaking

Once the heating stage is done, the metal material must then undergo the soaking stage. The main purpose of this stage is to hold and keep the metal at a specific temperature and time until its internal structure changes. Holding the metal under a specific temperature will make its physical appearance red as heat is distributed evenly to the material.

The time required for the soaking stage would normally depend on the mass of the metal. Chemical analysis can likewise help obtain the needed soaking period. But in general, the temperature of the metal must not be placed instantly under soaking temperature. Instead, the entire metal must be first heated slowly below the temperature where structural changes might occur. Once the metal is evenly heated, the temperature may now be raised to the final temperature needed so that internal changes can now occur.

Cooling

The final stage in the heat treatment process is the cooling stage. For the metal to be used in various applications, it must be cooled and returned to room temperature. After all, it would be impossible for industries to process and use metal sheets if they still possess very high temperatures. The type of cooling medium that is available for metals include solid, liquid, and gas.

Common elements that are considered in choosing the correct cooling medium for the metal are its size and type. To get the desired properties of the metal, the medium for cooling as well as the rate of the process must be calculated correctly. Rapid cooling can be done through brine or water. A slower cooling process, alternatively, can be done through oil mixtures. Carbon steels can be cooled by water, while alloy steels can be hardened and cooled by oil. Nonferrous metals can likewise be quenched with water.

Heat treatment is a type of metalworking process that is utilised to change and alter the physical and even the chemical properties of a specific metal material. This process typically deals with heating or chilling material to extreme temperatures so that the same material can be either hardened or softened. Numerous factors such as strength, hardness, toughness, machinability, formability, ductility, and elasticity can all be affected by the heat treatment process.

To date, there are various techniques that industries maximise to get their desired products. These techniques include annealing, hardening, case hardening, normalising, tempering, precipitation strengthening, and quenching. While these techniques often promote the transformation of material, there are instances where the heat treatment process can cause distortion.

Types of Distortion

Heat treatment can cause two types of distortion. One type of distortion is dimensional distortion. This type of distortion occurs whenever the metal material changes volume whenever its crystal structure changes. Whenever the heated parts are subjected under one heat treatment technique, their internal crystal structure changes. Subsequently, the processed parts will be tempered, resulting in a change of volume that is insufficient to offset all the changes before heating and transformations.

Another type of distortion is shape distortion or warpage. This type of distortion is primarily caused by either processing or design issues. Some of the reasons why a metal material warp during heat treatment include rapid heating, overheating, non-uniform heating, cooling, or agitation, oil contamination, huge mass and section change, and asymmetric characteristics.

Causes of Distortion

Distortion generally can be caused by various factors. A material that is placed under a heat treatment process may have surface tearing or burnishing, which then creates stress risers on the part. Excessive stress from machining, particularly from drilling and milling, can likewise distort the material. If there are holes, slots, or varying sections on a part, then these specific areas will most likely quench faster and cause differential part cooling.

Overheating of a material due to heat treatment can lower its mechanical properties, which promotes the sagging or creeping of parts depending on the furnace orientation. Additionally, failure to normalise the material and improper annealing can both lead to its distortion. Excessive case depth of case hardening can also cause material distortion since deeper case depths increase the chance of causing the warpage.

There are three major elements that are considered to be the defining factor for distortion and all the causes mentioned above. These elements are part design or geometry, metal production method, and the overall quality of the material.

Effects of Distortion

Distortion of materials can cause numerous effects. The outside or inner diameter of the material grows, which requires it to be subjected under finishing operation. The length of a material may also shrink or grow, depending on its part geometry. Additionally, materials with long parts that are distorted will bend unevenly. And as the distortion occurs, materials with cross holes and slots may shrink and cause stress riser. The concentricity of the material likewise gets lost as its thin tubing adopts and oval shape.

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.