Heat Treatment of Metals: An Overview [PDF]

Citation preview

Jeferson Sabio | BSCE 3A Engineering Utilities II | CE 314



Heat treatment is the heating and cooling of metals to change their physical and mechanical properties, without letting it change its shape. In simple terms, it 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 properties of a material. Introduction | Principle | Types | Process | Purpose | Application | Equipment



The process of heat treatment involves the use of heating or cooling, usually to extreme temperatures to achieve the desired result. It is a very important manufacturing processes that can not only help the manufacturing process but can also improve product, its performance, and its characteristics in many ways.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



The properties of heat-treated materials vastly depend on the processes that it has to undergo. Below are those key principles of heat treatment.



1



2



3



Heating



Holding



Cooling



Introduction | Principle | Types | Process | Purpose | Application | Equipment



1



Heating



The first step in the heat treatment process is heating the metal. The temperature depends on the types of metal and the technique used. Uniform temperature is of primary importance in the heating cycle. If one section of a part is heated more rapidly than another, the resulting uneven expansion often causes distortion or cracking of the part.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



2



Holding



The object of heat-treating is to bring about changes in the properties of metal. To accomplish this, the metal must be heated to the temperature at which structural changes take place within the metal. During the holding process, the metal is kept at the achieved temperature for some period of time. The time required depends on the type of metals and also the type of mechanical properties that are expected. The holding time also depends on the part size. If the part is large it is kept in a holding state for more time than the same type of metals having a small part size.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



3



Cooling



After being heated to the proper temperature, the metal must be returned to room temperature to complete the heat-treating process. The metal is cooled by placing it in direct contact with a gas, liquid, or solid, or some combination of these. The cooling must be done in a prescribed manner such as quenching where the metal is rapidly cooled. Different media such as water, oil, or forced air is used to aid in cooling.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



The various heat-treating processes are similar in that they involve the heating and cooling of metals. They differ, however, in the temperatures to which the metals are heated, the rates at which they are cooled, and in the final result. Here are some of the commonly practiced heat treatment techniques in industries:



Annealing



Normalizing



Hardening



Tempering



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Annealing



Annealing is a heat treatment process which is used to soften the metal. This process makes the steel soft so that it can be easily machined.



There are two types of annealing process: Process Annealing Full Annealing



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Annealing



Process Annealing It is done when metal is heated below the critical temperature, keep it for a suitable time, and then cool it slowly. This process is suitable for low carbon steel like sheet metal and wires. No phase transformation occurs during process annealing and it’s considerably cheaper than full annealing



Full Annealing It is done when metal is heated above the critical temperature. This process is suitable for low and high carbon steel. Phase transformation occurs during the full annealing process and it is a costly operation than process annealing.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Annealing



It softens steel and to improve its machinability To refine grain size and remove gases



It removes the internal stresses developed during the previous process To obtain desired ductility, malleability and toughness It modifies the electrical and magnetic properties



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Annealing



It is applied to castings and forgings.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Normalizing



Normalizing is also similar to annealing but the metal is air-cooled instead of other mediums used in the furnace. Normalizing is usually done on carbon steel.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Normalizing



The steel is heated to a temperature of about 40° to 50°C above its upper critical temperature. It is held at this temperature for a short duration. The steel is then allowed cool in still air at room temperature, which is known as air quenching. Promote uniformity of structure



To secure grain refinement To bring about desirable changes in the properties of steel



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Normalizing



It is applied after cold working such as rolling, stamping and hammering.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



The main aim of the hardening process is to make steel hard tough. Generally, material in this condition is too brittle for most practical uses, although this treatment is the first step in the production of high-strength steel. There are three types of hardening process: Case Hardening / Surface hardening Differential hardening Flame Hardening



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



Case Hardening / Surface hardening Case hardening or surface hardening is a hardening heat-treatment process. In the case of hardening, the complete metal piece is heated. But in the case of case hardening, only the outer surface is heat-treated to make it hardened. The inner metal is still soft and ductile. Case hardening is widely used for the tool and die industry where the tool surface needed to hardened but the inner metal piece has to remain ductile. There are three types of case hardening process: Nitriding Cyaniding Carburization Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



Case Hardening / Surface hardening Nitriding



Nitriding is a case hardening process in which nitrogen gas is used to harden the outer surface of the metal. The metal is heated in an ammonia (NH ) atmosphere and then it is cooled. During the whole process, ammonia defuses into nascent hydrogen and nascent nitrogen. This nascent nitrogen diffuses on the outer layer of metal form nitrite which increases the surface hardness.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



Case Hardening / Surface hardening Cyaniding



Cyaniding is a case hardening process in which the metal piece is immersed in a bath of molten sodium or potassium cyanide. After that, the metal piece is cooled into lime water so that cyanide salt is sticking to the outer surface of the metal. This cyanide salt is responsible for hardening the outer surface of the metal.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



Case Hardening / Surface hardening Carburization



In carburization, the hardness of the metal piece is increased by increasing the carbon content. The metal piece is heated below the melting point with the presence of high carbon materials such as charcoal. The heated metal piece then absorbs carbons to make it harder and more brittle.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



Differential Hardening Differential hardening is a kind of hardening process in which different area of the metal piece gets a different heat-treatment process. This is a very popular hardening process for high end cutting tools.



Flame Hardening In flame hardening, only a portion of the metal piece is hardened. This is different from differential hardening where the whole metal piece is getting hardened by the different heat-treatment processes.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening



It increases the hardness of steel.



To resist to wear Allows the steel to cut other metals To increase corrosion resistance



This method is effective for increasing the fatigue limit of medium and small-sized parts such as gears, shafts, wrist pins etc. To give clean, bright and pleasing appearance to the hardened surface



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Hardening It is applied for chisels, sledgehammer, centre punches, taps, dies, and milling cutters, etc.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Tempering



Tempering is done on metals that are already hardened. It is a process generally applied to steel to relieve the strains induced during the hardening process.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Tempering



The steel after being quenched in the hardening process is reheated to a temperature slightly above the temperature range at which it is to be used, but below the lower critical temperature. The temperature here varies from 100°C to 700°C.



The reheating is done in a bath of oil or molten lead or molten salt. The specimen is held in the bath for a period of time till attains the temperature evenly, the time depends on the composition and desired quality of steel. Now the specimen is removed from the bath and allow to cool slowly in still air.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Tempering



To relieve internally stressed caused by hardening To reduce brittleness



Improve ductility, strength and toughness To increase wear resistance To obtain desired mechanical properties



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Tempering



It is applied to cutting tools, tool and gears, which are hardened by the hardening process.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Furnaces in industrial settings heat up materials using fuel and combustion gases. The material may be in direct contact with the fuel and its gases (blast furnaces), indirect contact with the fuel but still in direct contact with the gases (reverberatory), or indirect contact with both fuel and gases (muffle furnaces). Nevertheless, the goal remains the same — attaining a high level of heat.



Below are different furnaces that are used for heating metals in heat treatment process: Box type furnace Bell-type furnace Pit type furnace Vacuum Furnace



Tempering Furnace Salt bath furnace Rotary Furnace



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Box type furnace



Used for heat-treating, calcining, curing, preheating, tempering and other processes, box furnaces feature vertical lift or swing open doors insulating the furnace and ensuring consistent airflow management. Box furnaces, which accept heavy loads, are set up to be loaded by forklift, roller hearth, or other manual processes. There are gas and electric versions available. Able to uniformly treat large material volumes, box furnaces are seen in laboratory and industrial applications.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Bell type furnace



Bell furnaces are electrically heated, gas-fired, or thermal recirculation heating systems featuring a movable dome. This furnace hood can be lifted frequently throughout the vacuum or controlled atmosphere processes. They are used to temper, anneal, normalize, and stress relieve steel plate formed parts. This equipment can be used with multiple bases and still provide reliable seals. These furnaces (multi- or single stack) are used to treat coils, strips, sheets, rods and more.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Pit type furnace



Available in gas-fired or electric-powered configurations, pit furnaces are also known as top load furnaces. Offered in many sizes, pit furnaces heat to different temperatures within a secured working chamber with a controlled atmosphere. These furnaces are often used in automotive and aircraft part manufacturing as well as machine building, wind energy, and mining.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Salt bath furnace



Salt bath furnaces use the high-heat transfer characteristics of convection to achieve very fast heating of metal parts in heat treating applications, as much as 5X the rate of air furnaces. These furnaces are almost invariably electric as electrodes can be immersed directly in the molten salt, although externally heated electric or gas-fired units are sometimes used. The temperature of the bath depends on the varieties of salts used, which are commonly cyanide mixtures and chloride mixtures, with temperatures of 300-2350°F possible.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Tempering furnace



Designed to heat treat ferrous metal products, tempering furnaces increase toughness. Heat treating certain high strength materials can impact the alloy’s formation and energy absorption. To access the best balance of strength and elasticity, tempering furnaces are often used in conjunction with quenching furnaces. It is crucial that these furnaces maintain uniform temperature levels throughout the chamber to achieve the desired material characteristics. Both gas and electric heating types of these furnaces offer indirect fuel contact.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Vacuum furnace



Vacuum furnaces are used in many industries. What distinguishes this type of furnace is the vacuum maintained throughout the heating process to protect heated steel and metal parts. The furnace can be electric or gas heated with pumps preserving the vacuum to prevent oxidation, heat loss, or contamination. These furnaces are used for annealing, brazing, sintering, and heat treatment.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



Rotary furnace Rotary Hearth furnaces have a lined furnace generally welded from steel into a barrel shape. The furnace is mounted on a drive which rotates the barrel throughout the heat treatment. The material sample can be mixed by tilting the furnace during rotation. Highly heat efficient with easy pressurization, these furnaces provide heat uniformity and good material contact. The internal heat source can be gas or electric, with the flow of muffled combustion gases often countercurrent. Applications typically include calcination and oxidation.



Introduction | Principle | Types | Process | Purpose | Application | Equipment



https://www.brighthubengineering.com/manu facturing-technology/30476-what-is-heattreatment/



https://fractory.com/heat-treatmentmethods/#Heat_Treatment_Process_Steps https://www.riansclub.com/heat-treatment/ http://navyaviation.tpub.com/14018/css/Form s-Of-Heat-Treatment-660.htm https://www.theengineerspost.com/heattreatment-of-steel/ https://www.thomasnet.com/articles/processequipment/types-of-furnaces/