The heat treatment of metals has a history dating back thousands of years. The discovery of heat treatment processes undoubtedly helped usher in a significant period of technological advancement. Furthermore, today several machining and metallurgy processes use cutting-edge technologies to produce more precise and higher quality parts thanks to this process.
You will find that various heat treatment methods are used in many industries today. These heat treatment processes help change the physical and sometimes chemical properties of metals for various purposes.
Here you will discover everything you need to know about heat treatment processes, their advantages and the types of steel heat treatment.
What is heat treatment?
To a large extent, heat treatment is one of the most important components of the metal manufacturing process. It includes a group of thermal and metal processing techniques that change the properties of metals.
More specifically, heat treatment by heating and cooling changes the physical, mechanical, and sometimes chemical properties of metals. These properties include ductility, hardness, temperature resistance, formability, machinability, strength, etc.
Heat treatment methods vary depending on the type of metal material and the specific properties required. It is important for manufacturers to know the appropriate heat treatment method that meets their specific needs.
What metals are available for heat treatment?
In particular, it is necessary to study each material to select the correct heat treatment method that will achieve the required properties. This is because not all metal materials are suitable for all types of heat treatment.
You will find that ferrous metals make up the majority of heat-treated materials. Therefore, different types of steel account for about 80% of heat-treated metals. However, some non-ferrous metals are also suitable for heat treatment.
Metal alloys such as copper, aluminum, nickel, brass, magnesium and titanium are heat treated. Below are some of the metals available for heat treatment and the appropriate types of heat treatment:
1. Cast iron, carbon steel, martensitic stainless steel and tool steel
Heat treatment methods – hardening, annealing, normalizing, stress relieving, hardening, nitriding and tempering.
2. Copper and copper alloys
Heat treatment methods – annealing, aging and tempering.
3. Aluminum
Heat treatment methods – annealing, solution heat treatment, natural and artificial aging.
Metal heat treatment steps
In general, there are five basic heat treatment processes for metals. These processes are hardening, hardening, normalizing, annealing and tempering. You will find that each process produces different metal properties.
Separately, it should be noted that the heat treatment of metals consists of three main phases. These phases are heating, soaking and cooling. Check out the individual steps below:
1. The warm-up phase
This is the first step in any metal heat treatment process. Many alloys undergo structural changes when heated to a specific temperature, up to 1200°C, depending on the type of metal. Furthermore, the structure of alloys occurs at room temperature as a mechanical mixture, a solid solution, or a combination of both forms.
Therefore, it would help if you were always extremely careful during this phase. Because incorrect movement during heating can cause cracking or deformation of the metal. Therefore, its main objective is to ensure that the metal heats evenly during the heating phase. Additionally, several factors play a role in determining the heating rate to use. Check it out below:
- The size and cross-section of the metal.
- The condition of the metal.
- The thermal conductivity of the metallic material.
2. The immersion phase
In this phase, the heated metal part is maintained at the temperature at which the desired changes in the metal structure occur. Furthermore, the metal must remain at this temperature until the entire piece is heated evenly.
Soak time, also called retention time, refers to how long you keep the metal at the proper temperature. You can determine retention time by chemical analysis, mass and thickness of the metal – generally speaking, the greater the mass of the metal, the longer the soak time. However, the soaking time varies depending on the metal material, but is generally 2 hours.
3. The cooling phase
After wetting the metal, the next step is to cool it. The main purpose of the cooling phase is to reduce the temperature of the metal back to room temperature. However, the cooling phase determines the resulting properties of the treated metal.
Here too, the composition of the metal can change at this stage, remain the same or return to its original form. Note that this depends on the type of metal, cooling rate, and cooling medium. Therefore, the cooling phase remains crucial in giving your metal certain structures and properties.
Factors affecting the heat treatment process of metals
Several factors influence the metal heat treatment process. Check the following factors:
1. Influence of time and temperature
Perhaps the most important factors in heat treating metals are time and temperature. Most importantly, they play a role in the heat treatment of metals in the heating, soaking and cooling phases.
Furthermore, studies have shown that time and temperature play a crucial role in changing mechanical properties and microstructure during heat treatment. For example, if the rate of steel cooling (quenching) is increased by 1000 °C per minute, a new microstructure, martensite, is created.
2. Extent of deformation
In most cases, different heat treatment processes result in different deformations and changes in the dimensions of the parts. Furthermore, the extent of the deformation depends on:
- The structure of metal material before heat treatment.
- Heat treatment method, placing in the oven.
- The distribution of tensions.
Thus, the degree of deformation is influenced by the heat treatment process of metals.
3. Effect of composition
Alloying elements also affect the heat treatment of metals. Namely, they tend to slow down the diffusion of atoms through the iron lattice and thus delay allotropic transformations.
Additionally, alloying elements such as manganese, molybdenum, nickel and boron increase hardness. Likewise, these elements also allow tempering at higher temperatures to achieve better ductility. Therefore, the metal composition affects the heat treatment process.
Types of heat treatment for steel
There are four basic types of heat treatment that steel undergoes during industrial processing. Check it out below:
1. Hardening
This type of heat treatment improves the hardness and strength of the steel, increasing the strength of the lattice structure. During hardening, the metal is heated to the required temperature of between 800 and 900 °C and then rapidly cooled. The steel type is typically cooled by immersing the hot metal in a quenching medium such as oil, water, or brine.
However, for steel grades with a high content of alloying elements, the required heating temperatures are between 980 and 1110 °C. Most steel materials need to be cooled quickly because the cooling phase affects the degree of hardness.
However, you will find that hardening has no benefits. While the hardening of steel increases during heat treatment, ductility decreases at the same time. This makes the steel material more brittle.
2. Temperament
You will often find that after steel hardens, it becomes stiffer than necessary and too brittle for manufacturing processes. This is due to the strong internal stresses that arise during rapid cooling during heat treatment of steel. Therefore, tempering relieves internal stresses and reduces the brittleness of the steel after hardening.
During this heat treatment, the steel is heated to the required temperature between 125 and 700 °C and then cooled in still air. Therefore, the tempering temperature used in the heat treatment of steel is always lower than the hardening temperature. The cooling rate during tempering also normally has no influence on the metal structure. The higher the tempering temperature used, the softer the steel becomes. However, high-speed steel remains an exception as it tends to become harder rather than softer after tempering.
3. Shine
Due to its low strength and softness, annealing is more suitable for low carbon steels. This method of heat treatment gives the metal properties opposite to those of hardening. Annealing metals helps reduce internal stresses, increase flexibility, soften the metal, and refine its grain structure.
Furthermore, steel annealing includes three main steps:
- Heating the steel material to the required temperature in the range of 260 – 760 °C, depending on the type of steel.
- Holding the steel grade to maintain the temperature for the required time.
- Then let it cool slowly to room temperature.
However, the rate of cooling from the annealing temperature varies depending on the metal. In contrast to hardening, annealing involves slowly cooling the steel to create smoothness.
The metal part is cooled slowly by burying the hot steel part in ash, sand, or other materials that do not conduct heat well. This process is called packaging. Cooling to the annealing temperature is also accomplished by turning off the furnace and allowing the furnace and steel workpiece to cool together. This process is called furnace cooling.
4. Normalize
Normalization involves removing internal stresses from ferrous metals produced by various processes. This includes heat treating, machining, welding, forging, casting or molding. However, normalized steels are harder and more resistant than annealed steels. Therefore, normalizing is usually performed on steel parts subject to impact. Furthermore, this heat treatment method is also used for parts that require maximum toughness and resistance to external stress.
Steel standardization includes the following:
- Heating the steel material to a specific temperature between 810 and 930 °C, depending on the type of steel.
- Immerse the metal for even heating.
- Then let it cool in the open air.
- Keep in mind that normalizing uses higher temperatures to heat treat steel than hardening and annealing processes.
Benefits of Heat Treatment for Metals
Heat treating metal materials can change their properties so that they can be used in various metal manufacturing processes. Below are some of the benefits of heat treating metals:
- Increases hardness on the surface or throughout the part.
- Heat treatment makes metals more flexible and workable.
- Helps improve wear resistance and brittleness of metals.
- Heat treating metals can improve their electrical and magnetic properties. This increases its compatibility with other materials.
University Degree
Heat treatment processes have become an integral part of various manufacturing processes. Before heat treating metals, manufacturers must decide on a suitable metal. Furthermore, different properties must be evaluated based on project requirements. After considering all factors, a quality metal heat treatment service is essential.
Common questions
Which metals are suitable for heat treatment?
In principle, both ferrous and non-ferrous metals are suitable for heat treatment. However, different types of steel make up the majority of heat-treated metals.
What is the difference between Annealing and Normalizing?
Firstly, standardization only applies to ferrous metals such as steel. Additionally, normalizing utilizes a higher temperature and air cooling after the metal is removed from the furnace. Annealing uses a lower temperature and is cooled by packaging or oven cooling.
Can you heat treat stainless steel?
Due to its high nickel content, austenitic stainless steel is difficult to heat treat.
However, martensitic stainless steel is hardened by quenching and austenitization. Austenitizing temperatures are between 980 and 1110 °C.
