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Heat Treatment of Metals : 

Heat Treatment of Metals

Heat Treatment : 

Heat Treatment Metallic materials consist of a microstructure of small crystals called “grains" or crystallites Grain size and composition is one of the most effective factors that can determine the overall mechanical behavior of the metal Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure

Heat Treatment : 

Heat Treatment Involves controlled heating and cooling of the metal or alloy This imparts desirable physical characteristics due to change in microstructure Improvements Toughness Hardness Resistance to shock Fatigue resistance Two broad categories Heat treatment of ferrous metals Heat treatment of non ferrous metals

Heat Treatment (contd) : 

Heat Treatment (contd) Steel and its alloys are hardenable Aluminum, magnesium, copper, beryllium and titanium alloys can also be heat treated Metal is heated to pre determined temperature and then quenched (cooling) For quenching water, oil, brine, liquid nitrogen or air blast is used Heat treatment may consist of more than one cycle

Heat Treatment (contd) : 

Heat Treatment (contd) Carbon steel and its alloys heat treatment is governed by carbon content. Approximately above 0.35% carbon steel is heat treatable Wrought and cast Aluminum alloys which are precipitation-hardenable are termed as heat treatable Non heat-treatable alloys depend primarily on cold work to increase strength Heating to decrease strength and increase ductility (annealing) is used for both heat treatable and non heat treatable alloys

Steel Heat Treatment Processes : 

Steel Heat Treatment Processes Six basic types of processes Stress relieving Hardening Annealing Case hardening Surface hardening Tempering

Stress Relieving : 

Stress Relieving Relieves the stresses developed in parts due to cold work, machining or welding Parts heated to 1100-1200ºF- below critical temperature Held at this temperature for 1 hr per inch of thickness Cooled slowly in still air at room temperature or in furnace

Phase Diagram(Understanding Heat Treatment) : 

Phase Diagram(Understanding Heat Treatment)

Hardening : 

Hardening Part is heated to pre determined temperature- Critical Temperature Temperature at which steel will harden is called its critical temperature Critical temperature depends on the type of alloy and carbon content (1400-2400°F) After heating part is quenched in brine, water, oil or air blast Water or brine is used to quench plain carbon steel Oil used to quench alloy steels Cold air blast is used for high alloy steel Quenching leaves the steel hard and brittle, this brittleness needs to be reduced by Tempering or Drawing

Hardening Temperatures Carbon Steel : 

Hardening Temperatures Carbon Steel

Tempering or Drawing : 

Tempering or Drawing Process reduces some of the brittleness of hardened part Part is heated below critical temperature (300-1300°F) and held until complete penetration is achieved Cooling is done in still air Internal stresses are relieved Toughness and impact resistance increases Hardness and strength reduces

Annealing : 

Annealing Annealing reduces the hardness of the part to make it easy to machine or work Metal is heated to 50-100°F above its critical temperature (normal hardening temperature) Holding time depends upon the shape and thickness of the piece Slow cooling is performed in some insulating material such as ashes or a furnace Primarily used for ferrous metals, but non ferrous metals can also be annealed after they become work hardened

Annealing : 

Annealing

Normalizing : 

Normalizing Process closely related to Annealing Metal; heated slightly above its upper critical temperature Then cooled slowly to room temperature Relieves stresses developed during Machining Welding Forming

Case Hardening : 

Case Hardening Low carbon steel cannot be effectively hardened by conventional heat treatment Part is heated to red heat and small quantity of carbon or nitrogen is introduced in its surface This produces a hard shell on the surface Following three methods Pack method or carburizing Liquid salt method Gas method

Pack Method or Carburizing : 

Pack Method or Carburizing Part is buried in a carbonaceous material in a container Container is placed in a furnace for 15-60 minutes Time controls the depth of the case After removal from the furnace part is quenched

Liquid Salt Method : 

Liquid Salt Method Part is heated in molten cyanide salt bath up to an hour Cyanide is introduced in surface and immersion time determines the thickness of the hard case After holding for desired time part is quenched

Gas Method-Nitriding : 

Gas Method-Nitriding Part is heated in a special airtight chamber Ammonia gas is introduced at high temperature Ammonia gas decomposes into nitrogen and hydrogen Nitrogen enters the steel surface to form nitride Surface becomes extremely hard

Part Being Removed from Cyanide Bath : 

Part Being Removed from Cyanide Bath

Padlock Shackles Being Loaded in Nitriding Furnace : 

Padlock Shackles Being Loaded in Nitriding Furnace

Surface Hardening : 

Surface Hardening Surface hardening is the process that permits the surface of high carbon and alloy steels to be hardened without affecting the internal structure of the metal Three techniques Flame Hardening: Surface is heated rapidly by flame of gas torch and then quenched Induction Hardening: Heating is performed by high frequency electrical current and then quenched Laser Hardening: 3.2 to 15.9 mm Laser beam focused on area to be hardened. Small area gets self quenched within few seconds. Part does not get warped or distorted

Aluminum Alloys Heat Treatment : 

Aluminum Alloys Heat Treatment Preheating or homogenizing, to reduce chemical segregation of cast structures and to improve their workability Annealing, to soften strain-hardened (work-hardened) and heat treated alloy structures, to relieve stresses, and to stabilize properties and dimensions Solution heat treatments, to effect solid solution of alloying constituents and improve mechanical properties Precipitation heat treatments, to provide hardening by precipitation of constituents from solid solution.

Preheating or Homogenization : 

Preheating or Homogenization This thermal operation applied to ingots prior to hot working is referred to as "ingot preheating” Purposes depending upon the alloy, product, and fabricating process involved are Principal objectives is to improve workability The microstructure of most alloys in the as-cast condition is quite heterogeneous. Microstructure is homogenized

Annealing : 

Annealing The distorted, dislocated structure resulting from cold working of aluminum is less stable than the strain-free, annealed state, to which it tends to revert Lower-purity aluminum and commercial aluminum alloys undergo these structural changes only with annealing at elevated temperatures Accompanying the structural reversion are changes in the various properties affected by cold working These changes occur in several stages, according to temperature or time, and have led to the concept of different annealing mechanisms or processes.

Aluminum Heat Treatment to Increase Strength : 

Aluminum Heat Treatment to Increase Strength A three-step process Solution heat treatment. Dissolution of soluble phases Quenching. Development of supersaturation Age hardening. Precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging)

Quenching : 

Quenching The most critical step in the sequence of heat treating operations The objective of quenching is to preserve as nearly intact as possible the solid solution formed at the solution heat treating temperature, by rapidly cooling to some lower temperature, usually near room temperature.

Aluminum Copper Alloy : 

Aluminum Copper Alloy

Aluminum Heat Treatment Designations : 

Aluminum Heat Treatment Designations F As Fabricated - No special control has been performed to the heat treatment or strain hardening after the shaping process such as casting, hot working, or cold working. O Annealed - This is the lowest strength, highest ductility temper H Strain Hardened - (applied to wrought products only) Used for products that have been strengthened by strain hardening, with or without subsequent heat treatment. W Solution Heat Treated - This is seldom encountered because it is an unstable temper that applies only to alloys that spontaneously age at ambient temperature after heat treatment. T Solution Heat Treated - Used for products that have been strengthened by heat treatment, with or without subsequent strain hardening.

Heat Treatment Temper Codes : 

Heat Treatment Temper Codes T1 - Cooled from an elevated temperature shaping process and naturally aged to a substantially stable condition. T2 - Cooled from an elevated temperature shaping process, cold worked, and naturally aged to a substantially stable condition. T3 - Solution heat treated, cold worked, and naturally aged to a substantially stable condition. T4 - Solution heat treated, and naturally aged to a substantially stable condition. T5 - Cooled from an elevated temperature shaping process then artificially aged. T6 - Solution heat treated then artificially aged. T7 - Solution heat treated then and overaged/stabilized. T8 - Solution heat treated, cold worked, then artificially aged. T9 - Solution heat treated, artificially aged, then cold worked. T10 - Cooled from an elevated temperature shaping process, cold worked, then artificially aged.

Strain Hardening Codes : 

Strain Hardening Codes H1 - Strain hardened only H2 - Strain hardened and partially annealed H3 - Strain hardened and stabilized H4 - Strain hardened and lacquered or painted.

Summary : 

Summary Heat treatment basic concepts Steel heat treatment Processes Aluminum Heat Treatment Processes

Questions? : 

Questions?

Toughness and Strength : 

Toughness and Strength Toughness, in materials science and metallurgy, is the resistance to fracture of a material when stressed. It is defined as the amount of energy per volume that a material can absorb before rupturing. Tests can be done by using a pendulum and some basic physics to measure how much energy it will hold when released from a particular height. By having a sample at the bottom of its swing a measure of toughness can be found, as in the Charpy and Izod impact tests. Toughness is measured in units of joules per cubic metre (J/m3) in the SI system and inch-pound-force per cubic inch (in·lbf/in3) in US customary units. Strength and toughness are related. A material may be strong and tough if it ruptures under high forces, exhibiting high strains; on the other hand, brittle materials may be strong but with limited strain values, so that they are not tough. Generally speaking, strength indicates how much force the material can support, while toughness indicates how much energy a material can absorb before rupture.

Slide 40: 

Shock resistance is the property by virtue of which material will withstand impact or thermal shock without failure Fatigue resistance is related to number of stress cycle a material can take before failure

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