Heat Treatment of Metals Dr. Siddhalingeshwar I.G.

Chapter 6: Lesson Schedule 01. Introduction to heat treatment process, types of heat treatment: Annealing and its types. 02. Normalizing, hardening and its types, 03. Tempering – Austempering, martempering. 04. Surface treatment- methods of surface hardening. 05. Hardenability, Jominy end quench test, Age hardening of Al & Cu alloys. 06. Time-temperature-transformation (TTT) curves.

We heat treat metals in an attempt to optimize the mechanical and physical properties for a given application. Most people think of heat treatment as a process for hardening metal. This is not necessarily so, as many heat treatments are applied to soften metal in order to allow metal working operations such as deep drawing, cold forging and machining. Where increased strength and wear resistance is required, hardening and tempering treatments are given. Extremely hard steels find applications in cutting tools where highly defined edges must be maintained and heat treatment of these steels is a critical operation. Hard surfaces with ductile base material may be developed by heat treatment.

Learning Outcomes Describe the classification of heat treatment processes, stages, characteristics and applications. Distinguish between the different methods to determine hardenability. Specify a practical precipitation heat treatment in terms of temperature and time that would give intended mechanical characteristics for a given application. Explain the concept of age hardening and how it contributes to the improvement of mechanical properties.

Application and Background Practically nothing can be manufactured without heat treating, a process in which metal is heated and cooled under tight controls to improve its properties, performance and durability. Heat treating can soften metal, to improve formability. It can make parts harder, to improve strength. It can put a hard surface on relatively soft components, to increase abrasion resistance. It can create a corrosion-resistant skin, to protect parts that would otherwise corrode. And, it can toughen brittle products. Heat treated parts are essential to the operation of automobiles, aircraft, spacecraft, computers and heavy equipment of every kind. Saws, axes, cutting tools, bearings, gears, axles, fasteners, camshafts and crankshafts all depend on heat treating.

The Value Of Heat Treating Heat treating adds about $15 billion per year in value to metal products by imparting specific properties that are required if parts are to function successfully. It is very closely linked to the manufacture of steel products: about 80 percent of heat treated parts are made of steel. These include steel mill output such as bar and tube, as well as parts that have been cast, forged, welded, machined, rolled, stamped, drawn or extruded. It is also a vital step in the manufacture of nonferrous products. For example, aluminum alloy automotive castings are heat treated to improve hardness and strength; brass and bronze items are heat treated to increase strength and prevent cracking; titanium alloy structures are heat treated to improve strength at high temperatures.  Miillion = 1000 x 1000 i.e. Thousand Thousands = 1,000,000 = 10 lakhs. Billion = 1000 x 1000 x 1000 i.e. 1000 Million = 1,000,000,000 = 109

Introduction Heat Treatment has been defined in handbook as a combination of heating and Cooling operations, timed and applied to a metal or alloy in solid state in a way that will produce desired properties, that are certain predetermined physical and mechanical properties. As it is expected the desired properties which are needed by heat treatment, are mainly dependent on the microstructure of the alloy, i.e. nature, shape, size and distribution of the phases.

Objectives of heat treatment to increase strength, harness and wear resistance (bulk hardening, surface hardening); to increase ductility and softness (tempering, recrystallization annealing); to increase toughness (tempering, recrystallization annealing); to obtain fine grain size (recrystallization annealing, full annealing, normalising); to remove internal stresses induced by differential deformation by cold working, non-uniform cooling from high temperature during casting and welding (stress relief annealing);

to improve Machinability (full annealing and normalising); to improve cutting properties of tool steels (hardening and tempering); to improve surface properties (surface hardening, corrosion resistance-stabilising treatment and high temperature resistance-precipitation hardening, surface treatment); to improve electrical properties (recrystallization, tempering, age hardening); to improve magnetic properties (hardening, phase transformation)

Heat Treatment Steps Heating Soaking Cooling According to the definition of the heat treatment by handbook the steps of heat treatment can be determined as given in the following steps: Heating Soaking Cooling

Heating must be controlled to the desired temperature using a good insulated proper heating furnace under a suitable atmosphere to prevent the heat dissipation and the oxidation process. Soaking is the essential step to ensure that the temperature of component is equated between its surface and its core. This will depend on the dimensions of the heat-treated component. Cooling will affect the resulted microstructure and then the obtained properties. Accordingly, cooling will be done either in the heating furnace (by switch off the furnace) or by leaving the heat-treated component to be cooled in a certain medium like air, oil or water. Occasionally, salt-bath and low-temperature melted metals are used as a medium for cooling step.

An overview of important heat treatments A broad classification of heat treatments possible are given below. Many more specialized treatments or combinations of these are possible. HEAT TREATMENT BULK SURFACE ANNEALING NORMALIZING HARDENING & TEMPERING THERMAL THERMO- CHEMICAL Full Annealing Carburizing MARTEMPERING Flame Recrystallization Annealing Induction Nitriding AUSTEMPERING Stress Relief Annealing LASER Carbo-nitriding Spheroidization Annealing Electron Beam

Heat Treatment Processes

Heat Treatment Processes Annealing It is a heat treatment process in which a material is taken to an elevated temperature, kept there for some time and then left to cool, usually, in the furnace. Purposes of Annealing Produce specific microstructure: To relieve (relief) internal or residual stresses. To increase softness, ductility, toughness and machinability. Stages of Annealing Heating to required temperature, which is defined according to the objective of the annealing. Soaking at the required temperature by leaving the heat-treated material for soaking of half hour for every 25 mm thickness. Cooling step is done slowly inside the furnace by switch off the furnace after the soaking. This will achieve about 50-100 C/hr cooling rates.

Types of Annealing Processes i. Stress relief annealing It is used to eliminate and/or minimize stresses arising from plastic deformation during machining or forming processes. Stress relief annealing allows these stresses to relax. Annealing temperatures are relatively low so that useful effects of cold working are not eliminated. The given figure shows that heating temperature for the stress relief annealing is between 625-650 C

ii. Recrystallisation and grain growth annealing It is used to eliminate residual and internal stresses and create new grains that will be coarse grains. This process will lead to increase the ductility and the workability. As shown in the given figure, heating temperature for recrystallization is about 650-670 C

iii. Spheroidisation annealing It is long soaking time heating just below the eutectoid temperature of the 727 C (below A1 and A1,3) by about 25-30C. This will produce soft spheroidite structure that could be needed in subsequent forming operations. It is usually performed for plain carbon steel of more than 0.45 wt%. The heating temperature range and the plain carbon steel composition that used by this process are shown in the given figure. It has to be noted that in the previous annealing processes types, the heat-treated steel is heated to temperatures lower than A1 or A1,3 which mean that the steel is never reached to austenite region.

For hypoeutectoid steel T = A3 + (20-40) C iv. Full annealing It is heat treatment process essentially to produce soft steels suitable for all forming and machining processes by heating up the steel to reach g‑austenite phase region, then let the steel to slow cool inside the furnace. The resulting are coarse pearlite and possible proeutectoid phase according to the carbon content of the steel. The heating temperature range of the full annealing is shown in the given figure as the following: For hypoeutectoid steel T = A3 + (20-40) C For hypereutectoid steel T = A1,3 + (20-40) C

Annealing Types of Annealing Process: heat alloy to Tanneal, for extended period of time then cool slowly. Goals: (1) relieve stresses; (2) increase ductility and toughness; (3) produce specific microstructure • Spheroidize (steels): • Stress Relief : Make very soft steels for good machining. To reduce stress caused by: -plastic deformation Heat just below TE -non-uniform cooling -phase transform. & hold for 15-25h. • Full Anneal (steels): Types of Make soft steels for Annealing good forming by heating Process Anneal: to get g , then cool in furnace to get coarse P. To eliminate negate effect of cold working • Normalize (steels): by recovery/recrystallization Deformed steel with large grains heat-treated to make grains small.

Normalizing It is similar to annealing, except that during cooling stage the material is cooled at a faster rate than annealing usually outside the furnaces in air. Purpose of Normalizing Generally to control more precisely a defined grain size to produce finer pearlite mixture and finer proeutectoid phase.

Stages of Normalizing In order to achieve the objectives of normalizing, heat‑treated steel alloys are heated up to austenite region (austenitizing) according to the steel composition as the following For hypoeutectoid steel: T = A3 + (40-80) C For hypereutectoid steel T = Acm + (30-60) C Then, the treatment is completed by cooling into air after predetermined soaking time of 10-20 minutes for every 10 mm material thickness.

Hardening Purposes of Hardening Hardening is the process of heating the steel to temperature sufficient to produce an austenite condition (not essentially austenitizing) depending on the carbon content of steel followed by rapid cooling at rate fast enough to prevent the transformation to any product phase(s) differ than martensite. This cooling type is called quenching process. Purposes of Hardening The main objectives of steel hardening process are inducing high hardness and achieving high wear-resistance for steels. Hardening is done to all heavy-duty carbon steel machines parts and almost all machine parts made of all steel types.

Stages of Hardening In order to achieve the objectives of hardening, plain carbon steels are heated up to temperature depending on the steel composition as the following: For hypoeutectoid steel: T = A3 + (20-40) C For hypereutectoid steel T = A1,3 + (20-40) C

Heat Treatment processes Case hardening Carburizing Nitriding Carbonitriding Chromizing and Boronizing Precipitation hardening Solution treatment (-phase conversion) quenching precipitation treatment (aging)

Furnaces for Heat Treatment Fuel fire furnaces gas oil Electric furnaces batch furnaces box furnaces - door car-bottom furnaces - track for moving large parts bell-type furnaces - cover/bell lifted by gantry crane continuous furnaces

Furnaces for Heat Treatment Vacuum furnaces Salt-bath furnaces Fluidized-bed furnaces Some of the furnaces have special atmosphere requirements, such as carbon- and nitrogen- rich atmosphere.

Quenching Mediums Water Quenching Water is the most common quenching medium.  It is inexpensive, convenient to use, and provides very rapid cooling.  It is used primarily for low carbon steels where the heat must be extracted from the steel rapidly in order to obtain good hardness and strength.  Although water provides a sudden, drastic quench, it can cause internal stresses, distortion, or cracking.  For best results, water should be used at room temperature. Oil Quenching Oil is more gentle than water and is used for more critical parts which have thin sections or sharp edges.  Since oil is more gentle it develops less internal stresses, distortions, or cracking.  However, oil does generally does not produce as hard or strong of a steel as water.  Therefore, the decision must be made by the designer of a part which is more important, hardness and strength or minimizing cracking and distortion.

Quenching Mediums Air Quenching Air quenching is much less drastic than either oil or water.  High speed fans blow room  temperature air over the steel parts.  The slower rate decreases the distortion, internal stress and cracking.  However, it will not be as strong unless special alloys have been added into the metals.  Therefore, air quenching is usually used on high alloy metals such as chromium and molybdenum. Brine Quenching Brine is water with 5 - 10% salt added.  The speed is slightly faster than water and therefore more drastic in regards to cracking and distortion.  It is also effective at removing surface scale from the parts, since the salt causes mini 'explosions' on the part's surface that blows off the scale.  

Surface Hardening Methods Flame hardening Induction heating High-frequency resistance heating Electron beam heating Laser beam heating

Surface Hardening Methods Induction heating High frequency Resistance heating

Induction hardening Induced eddy currents heat the surface of the steel very quickly and is quickly followed by jets of water to quench the component. A hard outer layer is created with a soft core. The slideways on a lathe are induction hardened.

Flame hardening Gas flames raise the temperature of the outer surface above the upper critical temp. The core will heat by conduction. Water jets quench the component.

Case Hardening Case hardening is a process used with mild steel to give a hard skin The metal is heated to cherry red and is dipped in Carbon powder. It is then repeated 2-3 more times before Quenching the metal in water to harden the skin. This allows the surface of mild steel to be able to subject to wear but the soft core is able to subject to Sudden shock e.g. the tool holders

Case hardening Hardening of the surface Improves resistance to surface indentation, fatigue, wear Gear teeth Cams, Shafts, Bearings, Fasteners, Pins Automotive clutch plates Tools and dies

Case Hardening - Carburizing Carburizing involves placing the mild steel in box packed with charcoal granules, heated to 950 º C and allowing the mild steel to soak for several hours. It achieves the same purpose of case hardening.

Case Hardening - Nitriding Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface. These processes are most commonly used on low-carbon, low-alloy steels, however they are also used on medium and high-carbon steels, titanium, aluminum and molybdenum. Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, firearm components, injectors and plastic-mold tools. The processes are named after the medium used to donate. The three main methods used are: gas nitriding, salt bath nitriding, and plasma nitriding.

GAS NITRIDING is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature (below Ac1, for ferritic steels) in contact with a nitrogenous gas, usually ammonia. Quenching is not required for the production of a hard case. The nitriding temperature for all steels is between 495 and 565 °C (925 and 1050 °F). PLASMA, OR ION, NITRIDING, is a method of surface hardening using glow discharge technology to introduce nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material. In a vacuum, high-voltage electrical energy is used to form a plasma, through which nitrogen ions are accelerated to impinge on the workpiece. This ion bombardment heats the workpiece, cleans the surface, and provides active nitrogen. Ion nitriding provides better control of case chemistry and uniformity and has other advantages, such as lower part distortion than conventional (gas) nitriding. A key difference between gas and ion nitriding is the mechanism used to generate nascent nitrogen at the surface of the work.

Case Hardening - Carbonitriding Carbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear. During the process, atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip, increasing the hardness and modulus near the surface. Carbonitriding is often applied to inexpensive, easily machined low carbon steel to impart the surface properties of more expensive and difficult to work grades of steel. Surface hardness of carbonitrided parts ranges from 55 to 62 HRC. Certain pre-industrial case hardening processes include not only carbon-rich materials such as charcoal, but nitrogen-rich materials such as urea, which implies that traditional surface hardening techniques were a form of carbonitriding.

The ideal properties for heat treated steel is hardness, strength, ductility and small grain size. The selection process comes down to: If a steel must be hard and strong, quench rapidly.  However, it will be brittle. If a steel must have great ductility for machining, cool slowly.  However, will not be very strong. If the steel must have both strength and ductility, alloys can be added, but costs will increase.

Hardenability Hardenability is the ability of a steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface, when cooled under a given condition. Hardness is a measure of a material's resistance to localized surface deformation, whereas hardenability is a measure of the depth to which a ferrous alloy may be hardened by the formation of martensite.

Hardenability Hardenability should not be confused with the ability to obtain high hardness. A material with low hardenability may have a higher surface hardness compared to another sample with higher hardenability. A material with a high hardenability can be cooled relatively slowly to produce 50% martensite (& 50% pearlite). Hardenability of plain carbon steel can be increased by alloying with most elements (it is to be noted that this is an added advantage as alloying is usually done to improve other properties).

Typical hardness test survey made along a diameter of a quenched cylinder

Schematic of Jominy End Quench Test Jominy hardenability test Variation of hardness along a Jominy bar (schematic for eutectoid steel)

T T T diagram TTT diagram - The time-temperature-transformation diagram describes the time required at any temperature for a phase transformation to begin and end. Isothermal transformation - When the amount of a transformation at a particular temperature depends on the time permitted for the transformation.

Isothermal transformation diagrams (also known as time-temperature-transformation diagrams) are plots of temperature versus time (usually on a logarithmic scale). They are generated from percentage transformation-vs logarithm of time measurements, and are useful for understanding the transformations of an alloy steel that is cooled isothermally. An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Though usually used to represent transformation kinetics for steels, they also can be used to describe the kinetics of crystallization in ceramic or other materials.

The time-temperature-transformation (TTT) diagram for an eutectoid steel.

The TTT diagram for AISI 1080 steel (0. 79%C, 0 The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at 900°C

The time-temperature-transformation (TTT) diagram for an eutectoid steel. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Application of TTT Diagrams TTT diagram can be used to specify the nature of the final microstructure and approximate percentages of the phases that are existing at room temperature according to certain heat treatment path. Consider three case of heat treatment of Fe-0.77% C eutectoid steel rapid cooled from preheated temperature of 760 C (>727C) as follows: Rapidly cool to 350 C, hold for 104 s and quench to room temperature; Rapidly cool to 250 C, hold for 100 s and quench to room temperature; Rapidly cool to 650 C, hold for 20 s, rapidly cool to 400 C, hold for 103 s and quench to room temperature; In each case, the initial cooling is rapid enough to prevent any no required transformation.

At 350 C g transforms to B; the reaction starts after 10 s and ends at 500 s. By 104 s, 100% of the specimen is bainite and no more transformation is possible. At 250 C, at 100 s the specimen is still 100% g. As the specimen is cooled, M is started at 215 C. Transformation is complete by the time until room temperature is reached at 100% martensite. At 650 C, P begins after 7 s; after 20 s 50% of g is transformed to P. During rapid cool to 400 C little of the remaining g transforms to either P and/or B. At 400 C and after 103 s, the remaining 50% g will have completely transformed to bainite

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