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.