Overview of Heat Treatment
Heat treatment is a process that uses controlled heating and cooling to modify the crystalline structure of metals and metal alloys. Depending on the material and treatment process, heat treatment can provide numerous benefits, including enhanced hardness, increased temperature resistance, greater ductility, and improved material strength. Heat treatments are a critical aspect of metal fabrication processes, as they allow the materials to gain desirable physical and mechanical properties without altering the shape of the product.
Which Metals Are Available For Heat Treating?
As such, metal alloys such as copper, aluminum, nickel, brass, magnesium, and titanium get heat treated. Below are some of the metals available for heat treatment and the suitable heat treatment types:
1. Cast Iron, Carbon Steel, Martensitic Stainless Steel, and Tool Steel
Heat treatment methods – Hardening, Annealing, Normalizing, Stress Relieving, Case Hardening, Nitriding, and Tempering.
2. Copper and Copper Alloys
Heat treatment methods – Annealing, Ageing, and Quenching.
3. Aluminum
Heat treatment methods – Annealing, Solution heat treatment, Natural, and Artificial Aging.
Phase diagram of an iron-carbon alloying system.
Some common forms of heat treatments include:
Hardening: When a metal is hardened, it’s heated to a point where the elements in the material transform into a solution. Defects in the structure are then transformed by creating a reliable solution and strengthening the metal. This increases the hardness of the metal or alloy, making it less malleable.
Annealing: This process is used on metals like copper, aluminum, silver, steel, and brass. These materials are heated to a certain temperature, are held at that temperature until transformation occurs, and then are slowly air-dried. This process softens the metal, making it more workable and less likely to fracture or crack.
Tempering: Some materials like iron-based alloys are very hard, making them brittle. Tempering can reduce brittleness and strengthen the metal. In the tempering process, the metal is heated to a temperature lower than the critical point to reduce brittleness and maintain hardness.
Case Hardening: The outside of the material is hardened while the inside remains soft. Since hardening can cause materials to become brittle, case hardening is used for materials that require flexibility while maintaining a durable wear layer.
Normalization: Similar to annealing, this process makes the steel more tough and ductile by heating the material to critical temperatures and keeping it at this temperature until transformation occurs.
Heat Treatment Process Steps
In simple terms, heat treatment is the process of heating the metal, holding it at that temperature, and then cooling it back. During the process, the metal part will undergo changes in its mechanical properties. This is because the high temperature alters the microstructure of the metal. And microstructure plays an important role in the mechanical properties of a material.
The final outcome depends on many different factors. These include the time of heating, time of keeping the metal part at a certain temperature, rate of cooling, surrounding conditions, etc. The parameters depend on the heat treatment method, type of metal and part size.
Over the course of this process, the metal’s properties will change. Among those properties are electrical resistance, magnetism, hardness, toughness, ductility, brittleness, and corrosion resistance.
1. Heating stage
This is the first stage of heat treating, where the material is heated to a specific temperature. The goal in this stage is to make sure that the metal heats uniformly. Which means you want to heat the material slowly. Uneven heating results in one section possibly expanding faster than another and you’re left with a distorted or cracked section of the metal.
The heating rate is chosen according to the following factors:
The heat conductivity of the metal. This determines how fast the metal will heat up. (Metals with high heat conductivity heat faster than those with low conductivity.)
The history and condition of the material. Tools and parts that have previously been hardened or stressed, require slower heating than tools and parts that haven’t.
The size and cross-section of the material. Large parts, or even parts with different cross sections should be heated more slowly than small parts. This allows for more even heating, where the inside temperature can reach the same, or close to the same temperature as the surface. Otherwise, there’s a risk of cracking or excessive warping.
2. Soaking stage
The second stage is the soaking stage. This occurs when a specific temperature is reached during the initial heating and the material is held at that temperature for a specific amount of time.
The purpose of maintaining a specific temperature is to ensure that the desired internal structure occurs. The amount of time to keep the material at a specific temperature is called the “soaking period” and depends on the chemical structure of the material, the mass, and the shape (in the case of uneven cross-sections for example).
3. Cooling Stage
The material is then cooled (or quenched), often rapidly, depending on the desired results. The goal of the cooling stage is to bring the metal back down to room temperature. How it’s cooled down affects the end result, including the hardness of the metal.
The metallurgist at this point needs to know the composition of the metal itself and whether a cooling medium, a gas, liquid, solid, or combination needs to be used to quickly cool the part in a way that will achieve the desired effect.
4. Aging Stage
A solution without aging will be ineffective. There will be no mechanical strength or hardness. The aging process MAKES the hardness. Full heat treatment involves heat treating, soaking, cooling, and aging.
FAQs
What metals are suitable for heat treatment?
Generally, both ferrous and non-ferrous metals are suitable for heat treatment. But then, different steel grades account for the majority of heat-treated metals.
What is the difference between annealing and normalizing?
Foremost, normalizing applies to only ferrous metals like steel. Additionally, normalizing uses a higher temperature and air-cooling after removing the metal from the furnace. Annealing uses a lower temperature and cooling either by packing or furnace cooling.
Can you heat-treat stainless steel?
Austenitic stainless steel is difficult to heat-treat due to its high nickel content.
Yet, martensitic stainless steel gets hardened using quenching and austenitizing. Austenitizing temperatures range from 980 to 1110°C.