Heat treatment Processes

Heat treatment of sttel

• Hardnening • Normalizing• Annealing• Tempering• three basic processs in heat treatment are

hardening annealing and normalizing tempering is done to reduce the increase in hardness and to improve the toughness induced due to hardening

Hardening

• Steel hardening can be be done by heating above lower critical temperature and quenching in a suitable liquid

• In this process hardness is increased but toughness is greatly reduced

• Fully hardened steel is not tough.but differnet hardness levles are required for different uses as an example ball bearing requires hardness and little and toughness spring required to be more tough

Tempering

• Tempering is the process that will increased the toughness while the hardness that is tested by charpy impact reducing toughness testing machine

• To improve the toughness hardened steel is temperedFor tempering steel components are heated below the lower critical temperature which they are begin to harden and allowed to cool in air or other suitable medium

• amount of tempering is controlled by controlling the temperature at which heating is done and cooling in a suitable medium Air water oil

• Fully hardened steel has typical martnesitic structure of plain carbon steel and toughness is very low By Tempering of the 343 C heating to the temperature indicates considerable increase in toughness rec carbo Bide precipitate482 C reduces the precipitation of Carbide from martensite definite coalescence of the carbide particles has taken place

• for steels susceptible to temper brittleness.• Temper colours formed on a cleaned surface are

still used occasionally as a guide to temperature. They exist due to the interference effects of thin films of oxide formed during tempering, and they act similarly to oil films on water. Alloys such as stainless steel form thinner films than do carbon steels for a given temperature and hence produce a colour lower in the series. For example, pale straw corresponds to 300°C, instead of 230°C (Table 1).

Temper Colour Temperature °C Objects

Pale straw 230 Planing and slotting tools

Dark straw 240 Milling cutters, drills

Brown 250 Taps, shear blades for metals

Brownish-purple 260 Punches, cups, snaps, twist drills, reamers

Purple 270 Press tools, axes

Dark purple 280 Cold chisels, setts for steel

Blue 300 Saws for wood, springs

Blue 450-650 Toughening for constructional steels

Annealing

• is a stress reliving process

• Two principal functions induced by annealing is it softnens steel and improve ductility.

• also requires heating the steel in a furnace and cooling.

• Annealing is carried out by heating the steel 75 degress above upper critical temperature and keep at the temperature for one hour and heat is shut off and allowed to cool in the furnace slowly when it is removed from the furance all stress are finally relived. under the influence of high annealing temperature stress are relived and during slow cooling process stresses are not raised again. Thus when finaly removed they retain more softeness and ductility

Full annealing is the process of slowly raising the temperature about 50 ºC (90 ºF) above the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC (90 ºF) into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon).It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate of about 20 ºC/hr in a furnace to about 50 ºC) into the Ferrite-Cementite range. At this point, it can be cooled in room temperature air with natural convection.The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.

Normalizing

• improve the mechanical properties

• removes undesirable coarse grain structure that may have occurred during mechanical working process of forging or rolling or mechanical working

• similar coarse grain structure occur in castings while solidifying

• normalizing produces more close grain structure better adopted to subsequent heat treatment .

• some times normalizing along sufficient to obtain the required mechanical properties to steel and no further treatment required

• Normalizing is done by heating the items to be normalized 65 C above upper critical temperature and controlled cooling in Air after heating and slow cooling in the air reduces the size of the grain size and removes the grain variation in the structure

difference between full annealing and normalizing

• is that fully annealed parts are uniform in softness (and machinablilty) throughout the entire part; since the entire part is exposed to the controlled furnace cooling

• . In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part. This may not be desirable if further machining is desired, since it makes the machining job somewhat unpredictable

Process Annealing•  used to treat work-hardened parts made out of low-

Carbon steels (< 0.25% Carbon). allows the parts to be soft enough for further cold working without fracturing. Process annealing is done by raising the temperature to just below line A1on the diagram. This temperature is about 727 ºC so heating to 700 ºC should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.

Stress Relief Annealing

•  is used to reduce residual stresses in large castings, welded parts and cold-formed parts. Such parts tend to have stresses due to thermal cycling or work hardening. Parts are heated to temperatures of up to 600 - 650 ºC and held for an extended time (about 1 hour or more) and then slowly cooled in still air

Spheroidization•  is an annealing process used for high

carbon steels (Carbon > 0.6%) that will be machined or cold formed subsequently. This is done by one of the following ways:

•   1.Heat the part to a temperature just below the Ferrite-Austenite line, line A1 or below the Austenite-Cementite line, essentially below the 727 ºC line. Hold the temperature for a prolonged time and follow by fairly slow cooling

. 2.Cycle multiple times between temperatures slightly above and slightly below the 727 ºC line, say for example between 700 and 750 ºC and slow cool. 3.For tool and alloy steels heat to 750 to 800 ºC and hold for several hours followed by slow cooling.

Spheroidization contd

Case hardening or surface hardening

• Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy. Case hardening is usually done after the part in question has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process.

Carburizing

• Carburizing is a process used to case harden steel with a carbon content between 0.1 and 0.3 wt% C. In this process steel is introduced to a carbon rich environment and elevated temperatures for a certain amount of time, and then quenched so that the carbon is locked in the structure; one of the simpler procedures is repeatedly to heat a part with an acetylene torch set with a fuel-rich flame and quench it in a carbon-rich fluid such as oil.

• Carburization is a diffusion-controlled process, so the longer the steel is held in the carbon-rich environment the greater the carbon penetration will be and the higher the carbon content. The carburized section will have a carbon content high enough that it can be hardened again through flame or induction hardening.

• It's possible to carburize only a portion of a part,• The carbon can come from a solid, liquid or gaseous source; pack

carburizing. Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers.

• gas carburizing involves placing the parts in a furnace maintained with a methane-rich interior.

Nitriding

• Nitriding heats the steel part to 482–621 °C in an atmosphere of ammonia gas and dissociated ammonia. The time the part spends in this environment dictates the depth of the case. The hardness is achieved by the formation of nitrides. Nitride forming elements must be present for this method to work; these elements include chromium, molybdenum, andaluminium. The advantage of this process is it causes little distortion, so the part can be case hardened after being quenched, tempered and machined.

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Cyaniding

• Cyaniding is a case hardening process that is fast and efficient; it is mainly used on low carbon steels. The part is heated to 875-900 °C in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide.

• This process produces a thin, hard shell (between 0.010 and 0.030 inches) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted. It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous.

Carbonitriding

• Carbonitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be quenched then the part is heated to 775–885 °C; if not then the part is heated to 650–790 °C.