3 Steps Of Heat Treatment Process | Basic Of Heat Treatment
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. By heat treating, a metal can be made harder, stronger, and more resistant to impact.Also, heat treating can make a metal softer and more ductile. Some properties are improved at the expense of others; for example, hardening a metal may make it brittle and difficult to machine.
The various types of heat-treating processes are somewhat similar because they all involve the heating and cooling; they differ in the heating temperatures and the cooling rates used and the final results. The usual methods of heat-treating ferrous metals (metals with iron) are annealing, normalizing, hardening, and tempering. Most nonferrous metals can be annealed, but never tempered, normalized, or case-hardened.
Successful heat treatment requires close control over all factors affecting the heating and cooling of a metal.This control is possible only when proper equipment is available. The furnace must be of the proper size and type and controlled, so the temperatures are kept within prescribed limits 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 throughout the heating chamber and surround the metal, as it is being heated. In an electric furnace, the atmosphere is either air or a controlled mixture of gases. In a fuel-fired furnace, atmosphere is a mixture of gases that comes from the combination of the air and the gases released by the fuel during combustion. These gases contain various proportions of carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, water vapor and other various hydrocarbons. Fuel-fired furnaces can provide three distinct atmospheres when you vary the proportions of air and fuel.
They are called oxidizing, reducing, and neutral
STAGES OF HEAT TREATMENT
Heat treating is accomplished in three broad stages:
Stage 1 – Heating the metal slowly to ensure a uniform temperature
Stage 2 – Soaking (holding) metal at a given temperature for a given time and cooling the metal to room temperature
Stage 3 – Cooling the metal to room temperature
The primary objective in heating stage is to maintain uniform temperatures. If uneven heating occurs, one section of a part can expand faster than another and result in distortion or cracking. Uniform temperatures are attained by slow heating. The heating rate of a part depends on several factors. One important factor is the heat conductivity of the metal. A metal with a high-heat conductivity heats at a faster rate than one with a low conductivity. Also, the condition of the metal determines the rate at which it may be heated. The heating rate for hardened tools and parts should be slower than 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 interior temperature to remain close to the surface temperature that prevents warping or cracking. Parts with uneven cross sections experience uneven heating; however, such parts are less apt to be cracked orexcessively warped when the heating rate is kept slow.
After the metal is heated to proper temperature, it is held at that temperature until the desired internal structural changes take place. This process is called SOAKING. The length of time held at the proper temperature is called the SOAKING PERIOD, which depends on chemical analysis of the metal and the mass of the part. When steel parts are uneven in cross section, the soaking 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, metal is slowly heated to a temperature just below the point at which the change takes place and then it is held at that temperature until the heat is equalized throughout the metal. This process is called PREHEATING. Following pre-heat, metal is quickly heated to final target temperature. When apart has an intricate design, it may have to be preheated at more than one temperature to prevent cracking and excessive warping. For example, assume an intricate part needs to be heated to 1500°F (815°C)for hardening. This part could be slowly heated to 600°F (316°C), soaked at this temperature, then heated slowly to 1200°F (649°C), and then soaked at that temperature. Following the final preheat, the part should then be heated quickly to the hardening temperature of 1500°F (815°C).
After the metal has been soaked, it must be returned to room temperature to complete the heat-treating process. To cool the metal, it can be placed in direct contact with a COOLING MEDIUM, either gas or liquid, or solid or any combination of these. The cooling rate depends on metal and the end properties. The rate of cooling depends on cooling medium as well; therefore, the choice of a cooling medium has an important influence on end properties.Quenching is the procedure used for cooling metal rapidly in oil, water, brine or other medium. Because most metals are cooled rapidly during the hardening process, quenching is usually 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 for hardening.Some metals crack or warp easily during quenching while others suffer no effects; therefore, the quenching medium must be chosen to fit metal. Brine or water is used for metals that require a rapid cooling rate, and oil mixtures are more suitable for metals that need a slower rate of cooling. Generally, carbon steels are water-hardened and alloy steels are oil-hardened. Non-ferrous metals are normally quenched in water.
Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.