Smart Heat Treatment Materials And Processes
SMART HEAT TREATMENT MATERIALS AND PROCESSES
INTRODUCTION
Heat treating (or heat treatment) is a group of industrial, thermal, and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing, and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
Some Basic concepts
· Heat transfer: The energy that is transferred from one body to another as the result of a temperature difference. If two bodies at different temperatures are brought together, energy is transferred — i.e., heat flows from the hotter body to the colder.
· Heat treatment: Materials heated in different ways for getting desired shape or use in various activities such as in agriculture, at home for cooking, different industries, science laboratory, etc.
Why do we need Heat Treatment of Materials? :
There are various reasons for carrying out heat treating. Some procedures make the metal soft, while others increase hardness. They may also affect the electrical and heat conductivity of these materials. Some heat treatment methods relieve stresses induced in earlier cold working processes. Others develop desirable chemical properties to metals. Choosing the perfect method really comes down to the type of metal and the required properties. In some cases, a metal part may go through several heat treatment procedures. For instance, some superalloys used in the aircraft manufacturing industry may undergo up to six different heat treating steps to optimize them for the application.
What Metals Are Suitable for Heat Treating?
Although ferrous metals account for the majority of heat-treated materials, alloys of copper, magnesium, aluminum, nickel, brass, and titanium may also be heat treated. About 80% of heat-treated metals are different grades of steel. Ferrous metals that can be heat treated include cast iron, stainless steel, and various grades of tool steel. Processes like hardening, annealing, normalizing, stress relieving, case hardening, nitriding, and tempering are generally done on ferrous metals.
Copper and copper alloys are subjected to heat treatment methods such as annealing, aging, and quenching.
Aluminum is suitable for heat treatment methods such as annealing, solution heat treating, natural and artificial aging. Heat treatment for aluminum is a precision process. Process scope must be established and it should be controlled carefully at each stage for the desired characteristics.
Evidently, not all materials are suitable for forms of heat treatment. Similarly, a single material will not necessarily benefit from each method. Therefore, every material should be studied separately to achieve the desired result
Examples of heat treatment of materials:
Heat treatment on steel:
- All steel is an alloy of iron and a variety of other elements
- All steel has to be treated to be used in commercial products
- The heat treatment of steel generally always involves annealing, quenching, and tempering.
Heat treatment on copper alloys:
There are four stages for the heat treatment of copper alloys:
· Homogenising
· Annealing
· Stress relieving
· Precipitation hardening
There are 3 stages of heat treatment. They are as follows:
Heating stage: At the time of material heating you must know the materials melting point and boiling point. This heating process takes care of materials have a good condition. During the heating stage, the foremost aim is to make sure that the metal heats uniformly. You get even heating by heating slowly. If you heat the metal unevenly, one section may expand faster than another, resulting in a distorted or cracked section of the metal. You choose the heating rate according to the following factors:
- The heat conductivity of the metal: Metals with high heat conductivity heat faster than those with low conductivity.
- The condition of the metal: Tools and parts that have been hardened, or stressed, previously should be heated slower than tools and parts that haven’t.
- The size and cross-section of the metal: Larger parts or parts with uneven cross sections need to be heated more slowly than small parts to allow the inside temperature to be close to the surface temperature. Otherwise, there’s a risk of cracking or excessive warping.
Soaking stage: The soaking stage is the middle stage. This stage is helpful to maintain temperature and a way to the final stage which is getting the desired shape. The “soaking period” is how long you keep the metal at the appropriate temperature. To determine the correct length of time, you will need the chemical analysis and mass of the metal. For uneven cross-sections, you can determine the soaking period using the largest section.
Generally, you shouldn’t bring the temperature of the metal from room temperature to the soaking temperature in one step. Rather, you’ll need to heat the metal slowly to just below the temperature where the structure will change, and then hold it until the temperature is consistent throughout the metal. After this step of “preheating”, you more quickly heat the temperature to the final temperature that you’ll need. It helps reduce complex shape into simple and also ii avoids damage material due to excessive heat.
Cooling stage: At a cooling stage, According to materials properties or surroundings environment cool material .there are several media to cool material like solid, liquid, ice, room temperature. The rate of cooling depends on the metal itself and the medium for cooling. It follows that the choices you make in cooling are important factors in the desired properties of the metal.
There are also standard processes such as quenching and tempering. In quenching, rapid cooling of metal in the air, oil, water, brine, or another medium takes place. Usually quenching is associated with hardening because most metals that are hardened are cooled rapidly with quenching, but it is not always true that quenching or otherwise rapid cooling results in hardening. Water quenching, for example, annealing copper, as compared to other materials it cools faster.
Heat treatment materials different processes:
· Annealing: Annealing starts with soaking, and then continues by very slowly letting the steel cool in the furnace. The foundry worker turns the furnace off and allows a gentle, controlled drop in temperature. There is thermal consistency throughout the object both while heating and cooling, which means there are few internal stresses: no “zones” of metal with different crystallization properties occur. Metal that has been annealed is generally very malleable, with increased ductility, tensile strength, and elongation. The grain sizes with annealed metals are often very big due to the very slow cooling curve. The annealing process can be done through different annealing process which is isothermal annealing, spherical annealing, recrystallization annealing, complete anneal
· Normalizing: Normalizing a metal means bringing it up to recrystallization temperatures by soaking, and then pulling it from the furnace and allowing it to cool in the atmosphere. Many of the properties of annealed metals are evident in normalized metals, but because there is not quite the same evenness of cooling, the grains tend to be a little less regular. Still, a much smaller temperature differential than is found in freezing metal means a normalized product is less brittle.
· Hardening: The hardening process consists of heating the components above the critical (normalizing) temperature, holding at this temperature for one hour per inch of thickness cooling at a rate fast enough to allow the material to transform into a much harder, stronger structure, and then tempering.
· Quenching and Tempering: When making tools and machine parts, softening the metal may defeat the purpose .For that quenching is used. Tempering: the right mixture of hardness-and-ductility can also be achieved through a process called tempering. Tempering is often done with quenched steel to make it less brittle while preserving some of the hardness. In tempering, a metal is reheated yet again, but now to a lower temperature than in annealing, normalizing, or quenching. Example is martensite.
· Carburizing: also referred to as Case Hardening, is a heat treatment process that produces a surface that is resistant to wear while maintaining toughness and strength of the core. This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components.
Conclusion:Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. To understand the process we need to understand some common concepts like heat transfer. The need for heat transfer is to altering or strengthen materials’ structuring and cooling process.