Heat Treatment Process PDF

TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES -TAGUIG Km. 14 East Service Road, South Superhighway, Brgy. Western Bicutan, Taguig City, Philippines S.Y. 2023-2024 HEAT TREATMENT PROCESS...

TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES -TAGUIG Km. 14 East Service Road, South Superhighway, Brgy. Western Bicutan, Taguig City, Philippines S.Y. 2023-2024 HEAT TREATMENT PROCESS Submitted by: GROUP 3 Abobo, Jr. Daniel R. Bagorio, Patricia Anne H. Bolagao, Peter John P. Loresca, Bryan Anthony C. Macaambac, Xyriel C. Mago, Allysa Pearl F. BETCHT-S-T-3A-T Instructor: Engr. Adonis Atlas Cerbo Table of Contents INTRODUCTION....................................................................................................................... History of Heat Treatment........................................................................................................... Stages of Heat Treatment............................................................................................................. Properties of Metals..................................................................................................................... Types of Heat Treatment.............................................................................................................. Benefits of heat-treating metals................................................................................................... Time-Temperature-Transformation Diagram (TTT).................................................................... Continuous Cooling Diagram (CTT)........................................................................................... INTRODUCTION Whatever your desired traits are, it is very likely that you will never be able to have them all. If you harden a metal, it becomes brittle. If you soften a metal, it loses strength. While improving some features, you degrade others and can make judgments based on the metals end-use. That is why the method known as Heat Treatment. Heat treatment is the process of heating metal without letting it reach its molten, or melting, stage, and then cooling the metal in a controlled way to select desired mechanical properties. Heat treatment is used to either make the metal stronger or more malleable, more resistant to abrasion or more ductile. Stages of Heat Treatment There are three stages of heat treatment: Heat the metal slowly to ensure that the metal maintains a uniform temperature. Soak, or hold, the metal at a specific temperature for an allotted period of time Cool the metal to room temperature The Heating Stage 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. The Soaking Stage The purpose of the soaking stage is to keep the metal at the appropriate temperature until the desired internal structure takes shape. The "soaking period" is how long you keep the metal at the appropriate temperature. The Cooling Stage In the cooling stage, you'll want to cool metal back to room temperature, but there are different ways to do this depending on the type of metal. It may need a cooling medium, a gas, liquid, solid, or a combination thereof. The rate of cooling depends History of Heat Treatment The History 6000 BC Humans learned how to recover metal from their ores, with metals like copper or lead. 1200 BC Beginning of Iron Age brought a huge change to humanity as metalworking advances allowed for new technology, new parts, and new ways of creating machines and architecture. PRESENT Metal working processes are significantly more advanced, but the general processes with metalworking remains the same. Properties of metals that are influenced by heat treatment Mechanical Properties Ductility Corrosion Resistance Hardness Machinability Types of Heat Treatment 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: A. ANNEALING In annealing, the metal is heated beyond the upper critical temperature and then cooled at a slow rate. It is carried out to soften the metal such as aluminum, copper, steel, silver, or brass. It makes the metal more suitable for cold working and forming. It also enhances the metal’s machinability, ductility and toughness. A heat treatment process that changes the physical and sometimes also the chemical properties of a material Process of heating a metal or alloy to an appropriate temperature for a certain period of time and then slowly cooling Increase ductility and reduce the hardness to make it more workable. Metal fabricators use annealing to help create complex parts, keeping the material workable by returning them close to their pre-worked state. TYPES OF ANNEALING: Full Annealing Partial Annealing Subcritical Annealing Figure 1: Annealing Diagram THREE STAGES OF ANNEALING 1. Recovery Stage During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved. It occurs at the lower temperature stage of all annealing processes and before the appearance of new strain-free grains It occurs with softening of the metal through the removal of primarily linear defects called dislocations and internal stresses they cause. 2. Recrystallization Stage During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form. In this stage, the deformed grains are replaced by a new set of undeformed grains New strain-free grains nucleate and grow to replace those deformed by internal stresses. 3. Grain Growth Stage During grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool at a specified rate. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the annealing process. Figure 2: Annealing Process B. NORMALIZING is a heat treatment process used for relieving internal stresses caused by processes such as welding, casting, or quenching. a heat treatment process where a material is heated to a pre-decided elevated temperature, hold at that temperature for a certain period of time (usually 10-20 minutes), and then allowed to cool freely in the air to reach room temperature. Metals Suitable to be Normalized Iron-based alloys like Carbon steel, stainless steel, alloy steel, cast iron, etc. Nickel alloys Copper Aluminum Brass Purpose of Normalizing To remove structural irregularities or impurities and defects from the metal. To improve ductility that has been lost in some metal processing. To reduce the hardness that has been increased by mechanical or thermal hardening processes. To increase the toughness of the metal. To relieve internal stresses. To get an improvement in machinability. THREE STAGES OF NORMALIZING 1. Recovery (Heating) The internal stresses are relieved by heating the material. Steel is uniformly heated to a temperature which a completely austenitic structure. 2. Recrystallization (Holding) The metal is heated to elevated temperatures above the recrystallization temperature of the metal where new grains are formed at this stage, steels are held at a constant temperature for a complete homogeneous structure. 3. Grain Growth (Cooling) The grains develop fully when the material is cooled by air At this stage, the steels are allowed to cool at room temperature usually in the air. The rate of cooling is faster as compared to the annealing process.  Figure 3: Normalizing Before and After Structure Normalizing is used to correct the coarse- grained structure of the steel to improve machinability by cutting and structure before quenching. Figure 4: Normalizing Line in a Diagram Normalizing is accomplished by hitting the steel at least 30 to 50°C above the GSE line after sufficient time has been allowed for Illinois to transform at this temperature it is cooled in steel air DIFFERENCES BETWEEN ANNEALING AND NORMALIZING Annealing Normalizing Material cools at a controlled rate in a furnace Material cools by exposure to air Requires additional furnace time Less expensive Used on steel and cast iron Used on alloy and carbon steels C. HARDENING The use of this will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher, more durable item. Hardened materials are usually tempered or stress relieved to improve their dimensional stability and toughness. Case hardening is a type of hardening process in which the outside of the material is hardened while the inside remains soft. It involves introducing carbon or nitrogen into the outer layer of the material, creating a hardened case while preserving the desired properties of the core. Figure 5: Case Hardening Process METHODS IN CASE HARDENING 1. CARBURIZING – (Carbon addition) Low carbon-steel component is heated up to 850 degrees Celsius inside the furnace. Carbon Dioxide gas is simultaneously added in the furnace while heating. Due to the presence of oxygen, there is a high risk of corrosion during this process. ℎ𝑒𝑎𝑡𝑒𝑑 𝐶𝑂2 → 𝐶 + 𝑂2 2. NITRATING – (Nitrogen addition) Low carbon-steel component is heated up to 950 degrees Celsius inside the furnace. Ammonia gas is simultaneously added in the furnace while heating Depth of hardening is approx. 0.0167 mm/hr. The risk of corrosion is eliminated. Hardness obtained is high but lesser hardening depth. ℎ𝑒𝑎𝑡𝑒𝑑 2𝑁𝐻3 → 2𝑁 + 3𝐻2 3. CYANIDING – (Both carbon and nitrogen addition) Low carbon-steel component is heated up to 900 degrees Celsius in a cyanide bath (mixture of sodium cyanide, sodium chloride and sodium carbonate). 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 𝑎𝑛𝑑 𝑐𝑎𝑟𝑏𝑜𝑛 𝑝𝑒𝑛𝑒𝑡𝑟𝑎𝑡𝑒𝑠 𝑖𝑛𝑡𝑜 𝑖𝑟𝑜𝑛 𝑎𝑛𝑑 𝑓𝑜𝑟𝑚𝑠 𝐹𝑒3 𝑁 𝑎𝑛𝑑 𝐹𝑒3 𝐶 D. TEMPERING Tempering is done to develop the required combination of hardness, strength, and toughness or to relieve the brittleness of fully hardened steels. Tempering is the process of reheating the steel at a relatively low temperature leading to precipitation and spheroidization of the carbides present in the microstructure. Purpose of Tempering The basic purpose of tempering is to make changes in the properties of the steel in such a way that it can be used for a larger number of applications. The properties that are being changed by the tempering are hardness, ductility, toughness, tensile strength, etc. Tempering is a heat-treating method to reduce brittleness and to increase strength of the metal. Some materials like iron-based alloys are very hard and therefore very brittle. When tempered, these metals are heated to a temperature lower than the critical point. This reduces brittleness and maintains hardness. TYPES OF TEMPERING: Differential Tempering − is also called as a graded tempering or selective tempering. The people used this differential tempering in making various tools as well as equipment which are used in battle like swords, knives, etc. Differential tempering results in the sharp as well as hard edges of knives. Austempering − is one of the tempering processes which is particularly used for ferrous metals. The main intention behind using Austempering is to eliminate deformation inside the metal parts. Austempering is a heat-treating process for medium- to-high carbon ferrous metals which produces a metallurgical structure called bainite. It is used to increase strength, toughness, and reduce distortion and cracking. E. QUENCHING Material is heated up to the suitable temperature and then quenched in water or oil to harden to full hardness according to the kind of steels. Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material and final mechanical properties to be achieved. Media for Quenching Air − Air is a popular quenching media used to cool metals for quenching. Oil − Oil is able to quench heated metals much more rapidly than compressed air. Water − Water is able to quench heated metals rapidly as well. It can cool a metal even faster than oil. Brine − Brine is a mixture of water and salt. Brine cools faster than air, water, and oil. The reason for this is that the salt and water mixture discourage the formation of air globules when it is placed in contact with a heated metal. Benefits of Heat-Treating Metals Increased strength and ductility Increased resistance to corrosion Increased malleability and workability during manufacturing (relieving the metal’s internal stresses, if any) Can improve the electrical and magnetic properties of a metal. Time-Temperature-Transformation Diagram (TTT) TTT diagram is a plot of temperature against the log of time for a steel alloy of definite composition. Used to determine when transformation begin and end for an isothermal heat treatment of a previously austenitized alloy. indicates when a specific transformation starts and end. also shows what % of transformation of austenite at a particular temperature is achieved. It depends on the type of heat treatment, time and temperature. the final microstructure of the steel, or any iron-carbon will be changed. the properties also do change. I.e. Strength and ductility. Continuous Cooling Diagram (CTT) A continuous cooling transformation (CCT) phase diagram is often used when heat treating steel. These diagrams are used to represent which types of phase changes will occur in a material as it is cooled at different rates. Figure 6: The cooling rate is from slowest phase to fastest change of rate. Main difference between TTT and CCT diagrams: no space for bainite in the CCT diagram as continuous cooling always results in the formation of pearlite. These diagrams are often more useful than time-temperature-transformation because diagrams. It is more convenient to cool materials at a certain rate (temperature-variable cooling), than to cool quickly and hold at a certain temperature (isothermal cooling).