3 stages of the heat treatment process | Basic heat treatment
Roberto Magalhães
To share
Heat treatment is the method by which metal is heated and cooled in a series of specific operations without melting it. The purpose of heat treatment is to make a metal more useful by changing or restoring its mechanical properties. Through heat treatment, a metal can become harder, stronger, and more resistant to impact. Additionally, heat treatment can make a metal softer and more ductile. Some properties are improved at the expense of others; for example, hardening a metal can make it brittle and difficult to machine.
HEAT TREATMENT THEORY
The various types of heat treatment processes are somewhat similar because they all involve heating and cooling; they differ in the heating temperatures and cooling rates used and the final results. The usual methods of heat treatment of ferrous metals (metals with iron) are annealing, normalizing, hardening and tempering. Most non-ferrous metals can be annealed, but never tempered, normalized or hardened.
Successful heat treatment requires tight control over all factors that affect the heating and cooling of a metal. This control is only possible when the appropriate equipment is available. The oven must be of adequate size and type and controlled so that temperatures are maintained within the limits prescribed for each operation. Even the furnace atmosphere affects the condition of the metal being heat treated. The furnace atmosphere consists of the gases that circulate through the heating chamber and surround the metal as it is heated. In an electric furnace, the atmosphere is air or a controlled mixture of gases. In a fuel furnace, the atmosphere is a mixture of gases resulting from the combination of air and gases released by the fuel during combustion. These gases contain various proportions of carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, water vapor, and other miscellaneous hydrocarbons. Fuel-fired furnaces can provide three distinct atmospheres when you vary the proportions of air and fuel.
They are called oxidants, reducers and neutrals.
HEAT TREATMENT STAGES
Heat treatment is carried out in three major stages:
Step 1 – Heat the metal slowly to ensure an even temperature
Stage 2 – Immersion (holding) of the metal at a certain temperature for a certain time and cooling the metal to room temperature
Step 3 – Cool the metal to room temperature heat treatment steps
WARMING STAGE
The main objective in the heating stage is to maintain uniform temperatures. If uneven heating occurs, one section of a part may expand more quickly than another and result in distortion or cracking. Uniform temperatures are achieved by slow heating. The heating rate of a part depends on several factors. An important factor is the thermal conductivity of the metal. A metal with high thermal conductivity heats up more quickly than one with low conductivity. Furthermore, the condition of the metal determines the rate at which it can be heated. The heating rate for hardened tools and parts should be slower than for unstressed or untreated metals. Finally, size and cross-section figure into the heating rate. Parts with a large cross-section require slower heating rates to allow the internal temperature to remain close to the surface temperature, preventing warping or cracking. Parts with irregular cross-sections experience uneven heating; however, these parts are less likely to crack or warp excessively when the heating rate is kept slow.
IMMERSION STAGE
Once the metal is heated to the proper temperature, it is held at that temperature until the desired internal structural changes occur. This process is called IMMERSION. The period of time maintained at the appropriate temperature is called IMPULSION PERIOD, which depends on the chemical analysis of the metal and the mass of the part. When steel parts have an irregular cross-section, the immersion period is determined by the largest section. During the soaking stage, the temperature of the metal is rarely brought from room temperature to the final temperature in one operation; instead, the metal is slowly heated to a temperature just below the point at which the change occurs and is then held at that temperature until the heat is equalized throughout the metal. This process is called PREHEATING. After preheating, the metal is quickly heated to the desired final temperature. When separates have a complex design, it may be necessary to preheat at more than one temperature to prevent cracking and excessive warping. For example, suppose a complex part needs to be heated to 815°C (1500°F) to harden. This part can be slowly heated to 600°F (316°C), soaked at this temperature, then slowly heated to 1200°F (649°C), and then soaked at that temperature. After final preheating, the part must be rapidly heated to the hardening temperature of 815°C (1500°F).
COOLING STAGE
After the metal has been soaked, it must return to room temperature to complete the heat treatment process. To cool the metal, it can be placed in direct contact with a COOLING MEDIUM, whether gas or liquid, or solid or any combination of these. The cooling rate depends on the metal and final properties. The cooling rate also depends on the cooling medium; therefore, the choice of a cooling medium has an important influence on the final properties. Quenching is the procedure used to quickly cool metal in oil, water, brine, or another medium. Because most metals are cooled rapidly during the hardening process, quenching is often associated with hardening; however, quenching does not always result in increased hardness; for example, to anneal copper, it is usually quenched in water. Other metals, such as air-hardened steels, are cooled at a relatively slow rate to harden. Some metals break or deform easily during quenching, while others suffer no effect; therefore, the quenching medium must be chosen according to the metal. Brine or water are used for metals that require a fast cooling rate, and oil mixtures are better suited for metals that need a slower cooling rate. Generally, carbon steels are hardened in water and alloy steels are hardened in oil. Non-ferrous metals are typically quenched in water.
heat treatment steps
