FLUIDTHERM

Annealing is a generic term denoting a thermal process of heating to and holding at a suitable temperature followed by cooling at an appropriate rate primarily for the purpose of softening the metallic materials. Annealing is carried out for further cold work, machining or to improve electrical properties. The choice of process, cycle and furnace equipment depends upon material, chemical composition, prior thermal history, property and surface quality requirements after processing. In practice, there are different thermal cycles for steel to achieve various goals of annealing. These cycles fall into different categories that can be classified according to the temperature to which the steel is heated and method cooling is used. Few common annealing practices are ‘Full annealing’, ‘Subcritical annealing’, ‘Spheroidised annealing’, ‘Intercritical annealing’, ‘Isothermal Annealing’ and special processes like Decarburisation annealing etc.

Austempering (STEEL):

Austempering of steel components is a process in which components are heated (preferably in a protective atmosphere) and soaked (till complete austenitisation) and then rapidly quenched, generally in a nitrate/nitrite salt bath maintained at a temperature above the temperature at which martensite transformation would start (for the particular steel components chemistry; typ. > 250° C/482° F) and held therein for the time required for complete transformation (e.g. 15 minutes @ 320° C (608° F) for C80). Further tempering is not required.

Not all steels can be austempered. The selection of the steel is based on the transformation characteristics. Important considerations are

The location of nose that dictates the quench severity required
The temperature at which martensite starts
The time required for the transformation of austenite to bainite.

In addition, section thickness of the steel components is also an important factor in determining whether or not the component can be austempered.

A typical austempering cycle for AISI 1080 (or C80) spring steel pressing is to austenitise at 840°C (1544° F) and rapidly quench into a salt bath maintained at 320°C for 20 minutes. Photomicrograph of the bainite structure that results is shown below.

Etchant : Nital Bainitic Structure Mag : 500X

Carboaustempering:

Carboaustempring is a heat treatment process in which the surface of the part is carburised, then austempered (cooled to a temperature above Ms temperature and held for sufficient time to form bainite). This process when applied to low carbon steel results in a bainite case and low carbon martensitic core. When applied to medium carbon low alloy steels, high carbon bainite is formed throughout the core. The ductility of the bainite is good even at higher hardness levels. Advantages of this process are:

Low distortion
Improved properties of the parts such as higher fatigue strength and better impact properties
No cracking in parts with complex geometry

Boronising:

Boronising is a thermochemical diffusion case hardening process wherein Boron is diffused into the metallic surfaces leading to the formation of MxBy type borides. The boride case is generally characterised by very high hardness, excellent wear resistance, good oxidation resistance and good resistance to aqueous corrosion in chloride containing environment. Boronising of metals can be carried out in solid, liquid or gaseous medium containing boron donor. Pack boronising is a popular process. Pack boronising process involves packing the component in a boronising mixture and heating the pack to a suitable temperature for adequate time to form boride case of useful thickness. The product is reheated, quenched and tempered to have better core properties. Dies in ceramic industries, stamping and bending dies in sheet metal industries, thread guides, rings in textile industries, gun barrels in defence industries are few examples for boronised parts.

Material : AISI 1045
Surface hardness : 1980 HV.2

Case depth : 100 microns

Etchant : Nital Boronised Structure Mag : 250X

Austenitic Nitrocarburising:

Austenitic Nitrocarburising process is a thermochemical diffusion process involving the diffusion of nitrogen and carbon into the surface of ferrous materials. This is conducted at temperatures between 590°C (1094° F) to 720°C (1328° F). In this temperature range the ternary alloy Fe-C-N forms austenite and hence the name austenitic nitrocarburising. The process atmosphere generally consists of N₂, NH₃ and a carbon donor like natural gas or CO₂ or LPG etc. The objective of austenitic nitrocarburising is:

Improved assistance to scuffing, wear and corrosion by the formation of hard compound layer at the skin.
A load bearing hardened case beneath the compound layer achieved by the diffusion of carbon and nitrogen promoting the formation of nitrogen austenite, which is transformed into bainite by re-heating at 350°C (632° F).
Low distortion compared to conventional heat treating techniques.

Etchant : Nital Austenitic Nitrocarburised Structure Mag : 250X

For more details refer Nitrocarburising.

Carbonitriding

In Carbonitriding, ammonia (NH₃) is introduced into the carburising atmosphere. Ammonia decomposes at carburising temperatures and the steel absorbs the atomic nitrogen. In this process both carbon and nitrogen are diffused together. The essential advantage of nitrogen diffusion is that the case has better hardenability. Nitrogen as an austenite stabiliser, reduces the diffusion controlled transformation of austenite to ferrite and pearlite. This higher hardenability allows better control on hardness profiles, use of milder quenchants with lesser distortion. Further this martensite containing Carbon–Nitrogen has better tempering behaviour. Carbonitriding is generally performed at lower temperatures in the range of 815°C (1499° F) – 900°C (1652° F). Carbonitriding at higher temperatures (more than 900°C (1652° F)) can cause increased retained austenite and even porosity at skin due to higher nitrogen activity. This process is generally restricted to produce case depths below 0.7 mm.

Etchant : Nital Carbonitrided case in low alloy steel Mag : 125X

Carburising

Carburising is a thermochemical diffusion process which is done by heating the steel to temperatures high enough to form homogeneous austenite phase in an environment of appropriate carbon source. The low carbon steel usually with a carbon content of 0.15 / 0.25%, when carburised to a surface carbon content of 0.7 to 0.9% and quenched will have a hard case due to transformation of higher carbon martensite. This treatment leads to the formation of hardness gradient and distribution of residual stresses with the compressive stresses in the surface structure due to changes in volume during martensite transformation. The combined effect of these two properties enhances fatigue life, toughness, in addition to wear resistance. The processes are classified according to the carbon sources such as pack carburising (solid compound), salt carburising (liquid carbon source), gas and plasma carburising (gaseous carbon source). Carbon sources for gaseous atmospheres are generally hydrocarbon gases such as methane, propane, alcohol derivatives and other organic compounds. Most commonly used atmospheres are endogases (20% CO + 40% H₂ + 40% N₂) generated by cracking propane or LPG with air at high temperature. Alternatively similar gas chemistry is achieved by cracking methanol and nitrogen in the hot chamber.

Reaction is as follows :

The endogas acts as a carrier gas and by adding propane or LPG, this gas can be enriched and the resulting CO donates carbon to the steel kept at higher temperature. The depth of the case formed depends upon the time, temperature and atmospheric carbon concentration.

In order to reduce the time cycle, a boost/diffuse technique is adopted in Carburising deep cases. During the process, atmosphere is kept at higher carbon potential typically 1.1% C during initial period and then reduced to lower carbon potential typically 0.8% C during balance part of the cycle and during cooling period. Microstructure of a carburised part is given below showing hard case and soft core.

Etchant : Nital Mag : 125X

A typical Carburising cycle is as follows

The atmosphere gas carbon concentration (carbon potential) can be best measured by using an oxygen probe system, dew point analyser or infrared CO / CO₂ analyser or by combination of these instruments.

Brazing

Brazing has three distinct characteristics.

Joining or uniting of an assembly of two or more parts into one structure; is achieved by heating the assembly or region of the part to the joined to a temperature of 450^oC or above.
Assembled parts and brazing filler metal are heated to a temperature high enough to melt the filler metal and not the parts.
The molten filler metal spreads into the joint wet the base metal surfaces by capillary action and anchors the parts.

Commonly used brazing methods are

Torch brazing
Furnace brazing
Induction brazing
Dip brazing
Resistance brazing

Furnace brazing is extensively used when the parts to be brazed can be assembled with brazing filler metal preplaced near the joint. The preplaced filler metal may be in the form of wire, foil, paste or ring. When the brazing is carried out in reducing atmosphere like N2/H2 or dissociated ammonia, separate fluxes are not required in the joint or with filler metal.

Bright brazed steel parts processed in a continuous atmosphere controlled (N2 + H2) mesh belt furnace with copper as the filler metal.

Austinox

Austinox is a proprietary process developed by Fluidtherm Technology. This process is carried out in Fluidised bed furnace.

This is a combination of austenitic nitrocarburising process and an oxidation step for ferrous alloys. The fluidising gas consists of NH₃, N₂ and a carbon donor LPG or Co₂ that acts as a nitrocarburising atmosphere. At the end of the process, steam or humidified nitrogen is sent in the furnace to oxidise the parts. Steam oxidation over the nitro carburised parts results in an adherent oxide with dark blue / black finish that enhances corrosion, wear resistance and appearance.

Etchant : Nital Mag : 500X

For details refer Nitrocarburising

Carbide Network

During carburising, the austenite is super saturated in carbon and then carbide will precipitate at austenitic grain boundaries during cooling forming carbide network. But if the steel is quenched from the carburising temperature, excess carbon will be retained by martensite / austenite structure. As a common practice in carburising, the parts are cooled in the furnace from carburising temperature to hardening temperature, or cooled to room temperature and rehardened from lower temperature. If the carbon is super saturated in austenite (carbon content over the saturation level at hardening temperature), during cooling to the hardening temperature, austenitic will reject the excess of carbon as carbide (Fe₃C) (from solid solution). The grain boundaries provide the necessary substrate and energy situation which favours formation of carbide hence carbide network. Carburised cases containing free carbides as network are relatively weak in response to static bending and impact bending. Fatigue life is affected and further the surface is sensitive for grinding cracks. Formation of network carbides can be eliminated by maintaining the surface carbon potential to the required level which is dictated by the carbon potential of the atmosphere.

Etchant : Nital Carbide Network Mag : 125X

Cyaniding

In cyaniding, the bath usually contains 20 to 40% of NaCN, 40% Na₂CO₃ and chloride salts. The mixture melts around 620°C (1148° F) and remain quite stable liquid at operating temperatures of 815° C (1499° F) / 920°C (1688° F). When the steel treated in this liquid bath, carbon and nitrogen are absorbed by the steel forming a hard carbonitride case on quenching.

Disadvantages are

Cyanide salts are highly poisonous when taken internally (even as fumes) or when in contact with open wounds.
Molten cyanide explodes when contact with water,and hence parts and jigs should be dried properly before it is placed in the liquid bath.
Energy consumption is high and idling time is costly as the bath cannot be shut down and restarted easily.
Disposal of waste is a serious problem.

Decarburisation

The loss of carbon from the surface of a carbon containing alloy due to surface reaction with the oxidising medium that contacts the surface at elevated temperature. When the steel is heated to the austnitising temperature with presence of oxygen in the environment, the following reactions occur.

It can be seen that carbon in the steel is oxidized to CO and transformed to a gas which is lost in the atmosphere. This reaction causes the carbon content at the surface of the steel to be lower than carbon content of the interior. The degree of decarburisation which may be partial or total depends upon:

Chemical composition of the steel
Temperature to which the steel is heated
Time at temperature
Atmosphere surrounding the steel

If the decarbuised steel is hardened, the surface will be soft and the interior will be hard.

Hot forgings, hot rolling, heat treating in open atmosphere are few examples during which surface carbon can get depleted due to reaction of surface carbon with atmospheric oxygen. Decarburisation is not desirable due to:

Reduced wear resistance in hardened steels
Fatigue life is affected in spring steels and steel parts subjected to fatigue loading

Etchant : Nital Decarburised surface of AISI 1080 material Mag : 125X

Normalising

Normailising is a heat treatment process involving heating a ferrous alloy to the austenitising temperature and air cooling to a temperature significantly below the transformation range. Normailising is mainly applied to unalloyed or low alloyed steels. This process refines the grains of the steel that has become coarse in the prior thermal cycling such as hot forging and welding. This grain refinement improves machinability and enhances better response in subsequent heat treatment . In normalising process cooling rate after austenisation is an important factor. Plain unalloyed low carbon steel may be transformed to fine pearlite and ferrite structure after normalising process under certain cooling rate. But low alloyed steels may have bainite and fine pearlite that increases hardness and reduces machinability. Controlled atmosphere heating with controlled cooling can be done in continuous normalising furnace with specially designed variocool section after hot zone, the hardness of the normalised parts designed can be maintained within a close band that improve productivity in machining and consistency of machined part dimensions.

Etchant : Nital Ferrite with fine pearlite in a normalised structure

Etchant : Nital Acicular ferrite & pearlite in as forged material (before normalising)

Retained austenite

When steels more than 0.65 % carbon are austenitised and then quenched, the austenite to martensite transformation does not end at room temperature as Mf temperature is lower than the room temperature. For some steels., Mf temperature is lower than 0° C (32° F). Consequently after these steels are quenched to room temperature, a portion of austenite will remain untransformed. This is referred as retained austenite. Higher carbon content and higher austenitising temperatures increases the tendency for higher austenite retention. Retained austenite is a soft structure compared to martensite. The overall hardness can be low if retained austenite is 10% or higher. If the steel with retained austenite is held for longer time at elevated temperature (say 120° C (248° F)) or at room temperature, the retained austenite is stabilised. Presumably this stabilisation is due to dissolution of martensite nuclei. It appears retained austenite is the primary factor responsible for dimensional instability of quenched and tempered parts. Controlling the heat treatment procedure and sub zero treatments can minimize the retention of austenite.

Etchant : Nital Retained Austenite Mag : 500X

Case Hardening

A generic term covering various processes that are applicable to the steel that changes the chemical composition of the surface by thermo-chemical diffusion processes forming a case. The diffusion process sets up a chemical gradient that reflects as a hardness gradient. The processes commonly used are ‘Carburising’, ‘Nitriding’, ‘Carbonitriding’, ‘Cyaniding’, ‘Nitrocarburising’. The depth of the case depth upon the process completion time, environment concatenation levels like carbon potential, Nitrogen potential etc. and the material chemical composition.

Decarburisation Annealing

A controlled annealing process generally performed on mild steel or silicon steel to remove carbon by preferential oxidation of carbon without oxidising iron. This process is done to improve magnetic properties.

Steel characteristics that interfere with magnetic and electrical properties are

Presence of stresses : Higher residual stresses, poorer magnetic Properties
Carbon content : Higher the carbon content poorer the magnetic Properties
Grains : Finer the grains poorer the magnetic properties

Very low carbon containing steels for use as transformer laminations etc. are very expensive. If these low carbon parts are annealed in N₂/H₂ atmosphere and by controlling the dew point during various stages of heat treatment, carbon content can be reduced to a level lower than 0.005% without oxidising the iron. Further adopting a suitable temperature in the process sequence, the grain size can be increased from ASTM 8 to 9 to ASTM 2 to 3. By controlling the cooling rate the residual stresses can be brought to near zero level. Results after decarburisation annealing are shown below with photomicrographs.

FDC Carburising Process

FDC Carburising is a proprietary process developed by Fluidtherm Technology.

A carburising process done in high carbon potential environment to form fine dispersed carbides (without carbide network or bonecarbide) which will improve the wear resistance of the components significantly. To achieve Fine Dispersed Carbides (FDC) carburising is done in pulsed mode between actual carburising temperature and suitable temperature below lower critical temperature depending on the alloy constitution. When the excess carbon precipitate at grain boundaries, on reheating thin carbide tails are taken in solution leaving heavier carbides. Thus repeat thermal cycling, the precipitated carbides can be made globular without any interconnections. There is marginal hardness increase but the wear resistance of the part increases significantly due to the presence of carbides in the matrix.

Potential applications are:

Bearing races
Shafts
Fuel injection nozzles
Ejector pins
Closing pins
Gear drives, and
Pump shafts & Bushes

Etchant : Nital FDC process – Material : AISI 52100 Mag : 500X

Nitrocarburising

Nitrocarburising is a thermochemical diffusion process involving the simultaneous addition of nitrogen and carbon to the surface of ferrous materials. The process is classified into two groups namely Ferritic Nitro Carburising (FNC) and Austenitic Nitrocarburising depending upon the temperature used. FNC is carried out below 580° C (1076° F) and ANC is carried out between 590° C (1094° F) to 720° C (1328° F). Both the process are carried out by using ammonia (NH₃) and carbon releasing additive to produce a mono phase compound layer of iron nitrides [Fe_2-3(Nc)]. FNC process results in a shallow compound layer and a diffusion layer while ANC process, in addition, a nitrogen austenite layer is developed as substrate between the compound layer and the diffusion layer. The depth of austenitic layer may vary from 5 to 50 microns which on reheating at 350° C (662° F) will get converted to hard bianite. Surface hardness, wear and fatigue resistance are increased by nitro Carburising processes. Any ferrous materials can be nitro carburised, unlike nitriding, which require special nitridable steels. Generally in all steels corrosion resistance is increased by nitrocarburising process. But in stainless steels, there is marginal reduction in corrosion resistance. If the surface hardness alone is the consideration, quenching is not necessary as hardness is imparted by formation of compound layer. But slow cooling allows the nitrogen in the substrate to precipitate as nitride needles especially in plain carbon steels which can reduce the fatigue life. Hence quenching after nitrocarburising retain nitrogen in solution and hence can be adopted to improve fatigue life. A post oxidation after nitrocarburising enhances the corrosion resistance considerably. Since the compound layer formed on the surface constitute for the enhanced wear, corrosion resistance, no further processes like grinding or lapping is allowed for nitrocarburising parts.

Etchant : Nital Mag : 250X

Microstructure showing Ferritic Nitrocarburising surface with compound layer

Refer Ferritic Nitrocarburising/Nitriding for further details

Ferritic Nitrocarburising

This process is a thermochemical treatment that involves diffusional addition of nitrogen and carbon. This process is generally done at temperatures below 580° C (1076° F) and usually for 3 hours. It is also called ‘short nitriding process’. The essential difference between nitriding and nitrocarburising is that in nitro carburising, the surface compound layer contributes for improved properties while the diffused nitrogen layer is important in nitriding process. All ferrous alloys can be ferritic nitro carburised while only nitrodable steels containing Al, Cr, Mo can be used for nitriding process. The process is done in an atmosphere containing NH₃ and a carbon donor such as LP gas, CO₂ or Propane at temperatures below 580° C (1076° F). The ferrous material after nitrocarburising need not be quenched if surface hardness is the only consideration. However if the steel after the process is quenched in oil or water, nitrogen is retained in solution in the diffusion zone which contribute for improved fatigue life.

Hardness after Ferritic Nitrocarburising of few ferrous alloys.

HV.2

Carbon steels 450

Low alloy steels 700

Tool & die steels 1000

Corrosion & heat resistant steels 900

Ductile malleable & grey casting 600

Post oxidation can be done to improve corrosion resistance. The parts after Ferritic Nitrocarburising process should not be ground or lapped before use as these processes can remove the useful compound layer.

Refer Nitrocarburising for further details

Ferrinox

Ferrinox is a nitrocarburising process carried out in fluidised bed furnaces at temperatures 580° C (1076° F) or below for sufficient period in an atmosphere containing NH₃ + carrier gas + a carbon donor. Carbon donors are usually propane, Liquid petroleum gases or CO2. During this processes a hard compound layer mainly of epsilon carbonitrides will be formed on the skin. At the end of the process, super heated steam or humidified nitrogen is sent in the furnace after cutting off the nitrocarburising atmosphere. During this oxidising step, Fe from Fe_2-3NC is oxidized to iron oxide. This oxide is highly adherent to the surface increases the corrosion resistance. Similar oxidation process is carried out in salt bath. The essential difference is that the salt bath oxidation is done around 350° C (662° F). During this step, nitrogen from the nitrogen diffused layer precipitate as fine particles or needles and reduces the fatigue life. In fluidised bed, Ferrinox process after the oxidation step at process temperature, the product can be quenched to retain nitrogen in solution.

Etchant : Nital Ferrinox Mag : 200X

Ferrinox Vs Hard chromium plating (improved corrosion resistance of ferrinox treated parts).

Shock absorber piston rods – hard chromium plating withstood only for 30 hours, the Ferrinox heat treatment crossed 150 hours – salt spray test.

Pulse Nitriding

When a heat treatment process is carried out in fluidised bed furnace, the atmosphere gas has two functions viz. providing necessary environment to carry out the process and to fluidise the aluminium oxide particles. Hence gas consumptions is relatively high in comparison to conventional furnaces. This affects the cost of operations especially in long process cylcles like nitriding. In partially fluidised bed (expanded bed), NH₃ can perculate through the bed, contact the hot job, and can nitride the parts. The good heat transfer from the fluidised bed to the parts is only necessary during heat up phase, but during soak/hold period complete fluidisation is no longer absolutely required. During the pulse down period, parts retain the temperature for a longer period of time and nitrogen diffusion continues as NH₃ molecules are in contact with the part as they perculate through the under fluidised bed. Slowly disuniformity in the bed will start setting and is rectified by a normal fluidisation for a short period

Pulse nitriding process is a process that pulses the flow rate from normal to a predetermined low level periodically so that satisfactory nitriding is done with lower gas consumption.

Recrystallisation Annealing:

Annealing cold worked metal below lower critical temperature to produce a new polygonal grain structure. Recrystallisation annealing does not involve formation of austenite. When the material is cold worked such as cold drawn or cold rolled or cold formed, the ferrite grains are worked and elongated along the direction of working. At this stage material hardness increases and ductility reduces. Cold worked steel, if heated to a temperature below lower level critical temperature, without formation of austenite, the elongated grains recrystallise to form new polygonal ferrite grains. The rapidity and the degree on the level of prior cold work and as the temperature approaches to A, temperature cooling practice after recrystallisation annealing has very little effect on the resultant properties. This process is generally adopted for hypo eutectoid steels.

Marquenching

Same as martempering

Bainite Hardening

See Austempering

Tempering

A heat treatment process of heating a ferrous alloy to a temperature below lower critical temperature after hardening or normalising. The purpose of tempering is

Relieve residual stresses
Improve ductility
Improve toughness

Low carbon alloy steels when normalised, hardness of thin section may be high due to formations of bainite. These steels are tempered at temperature between 500° C (932° F) to 630° C (1166° F) to reduce the hardness and improve the machinablity. Steels after hardening are tempered to reduce the brittleness. Low temperature tempering (150° C (302° F) / 170° C (338° F)) relieve stresses in martensite and result in insignificant drop into hardness, but at higher temperature 170° C to 300° C, hardness drops and ductility increases. This drop in hardness is due to the precipitation of carbide particles from martensite. In high alloy steels like high speed steels or High carbon High chromium steels high temperature tempering results in increase the hardness due to secondary hardening effect. For some steels, very slow cooling rate after tempering at 450° C (842° F) or above may result in brittleness due to a phenomena called temper embrittlement such steels may be quenched in oil or polymer from tempering temperature.

Precipitation Hardening

Some ferrous and non ferrous alloys when heated, enter into a single phase structure. If they are rapidly cooled, the second phase particles are retained in solid solutions. This process is called solution treatment. When they are reheated to a lower temperature, submicroscopic particles precipitate at grain boundaries. This increases hardness and strength when the precipitate particles are fine. For example, aluminium-copper alloys and aluminium-magnesium alloys can be strengthened by solution and precipitation process.

Martensitic Hardening

The Martensite transformation takes place on rapid cooling of the high temperature phase, a process that is referred as quenching. Martensite is the hardest of the transformation products of austenite. It is a supersaturated solution of carbon & iron. This formation is characterised by shear mechanism of austenite lattice. High carbon steel produces plate (needle) martensite while lath (massive) martensite is formed in medium and low carbon steels. Stress levels in the martensite depend upon the work temperature, carbon content and alloys and the quench serenity. Since this is diffusion less transformation, martensite will have the composition of austenite. Hence during austenisation of steel with carbide forming alloys, soak time and temperature of austenisation is important. This determines the alloy partition i.e. Alloy as carbides and alloy in martensite. This alloy partition control the toughness of the tool steels. Martensite has lower density than austenite. Hence transformation is associated with volume growth. Bore shinkage or variation in tooth dimensions in gears are typical examples that martensite growth after the dimensions. To reduce the stress levels in martensite and consequent reductions is dimensional variations several process techniques are adopted such as martempering, hot oil quenching etc.

Martempering

Martempering is a hardening process to transform austenite to martensite by interrupted quenching to minimize distortion.

In martempering, steel is

Austenitised by heating above the upper critical temperature
Quenched into a salt bath maintained at a temperature above Ms temperature and held for sufficient time to equalize the temperature through the entire section without transformation of austenite
Subsequently the entire work piece is cooled in air through the martensite formation range
Then temper the martensite significantly to improve toughness

The result is the formations of martensite with minimum stresses, thus minimising the distortion and avoiding cracks. Heavy sections cannot be martempered because, equialising in martempering bath will be high leading to transformation of bianite near surface or ferrite precipitation at the core.

In modified martempering process the part is quenched in the oil kept at 80° C (176° F) / 120° C (248° F) i.e. Below Ms temperature. The resultant martensite will have lower stresses and the distortion can be low in comparison to cold oil quench. But low alloy carbon steels may have higher retained austenite in this process.

Blueing

Scale free ferrous alloy when subjected to the action of air, steam or other agents at a suitable temperature, a thin blue film of oxide is formed and this improves appearance and marginally enhance the resistance to corrosion. Generally blueing is done between 340° C to 425° C.

Steam Oxidation of PM parts

Many PM parts are steam treated for improved wear resistance, corrosion resistance and sealing capacity. PM parts are heated in steam atmosphere to a temperature between 480° C (896° F) to 560° C (1040° F) to form a layer of dark grey/black iron oxide according to the reaction.

The magnetite oxide (Fe₃O₄) is formed on the surface and interconnecting porosity and hence filling the porosity with a second phase. This process is a simple process if adequate precautions are taken during the cycle. Formation of lower oxides such as ferrous oxide (FeO) and ferric oxides (Fe₂O₅) should be avoided. This process can be done either in batch furnaces or continuous furnaces.

Process steps are as follows

Pre clean the parts to remove lubricants and oil
If batch furnace, load the job at 350° C in air and soak for 15 minutes after temperature recovery. This is to drive out the residual oils and lubricants.
Admit super heated steam and raise the temperature
Soak at process temperature for adequate time – typically at 540° C (1004° F) for 90 minutes
Cooling the furnace in steam ambience to 450° C (842° F)
Stop steam supply, reduce the furnace temperature to 350° C (662° F) and open the furnace.

The important aspects to achieve satisfactory surface quality are :

Residual steam should not condense on parts before it reaches 100° C (212° F).
Air should not enter in the furnace in steam ambience at process temperature which can lead to red dust.

Hardness of the steam heated parts are increased; but the ductility will be reduced. Hence it preferable to steam oxidise the PM parts that has carbon content not more than 0.5 %.

Unetched Steam Oxidation Mag : 250X

Sintering

Powder Metallurgy is a process in which solid components are fabricated by consolidation of metallic materials in powder form. Most common methods include conventional press and sinter. Sintering process involves multiple stage heating and cooling cycle. Sintering may be generally defined as the process wherein powder particles develop metallurgical bonding during heating. There are primarily three steps for sintering ferrous components. During 1^st step which is an intermediate heating step, the lubricant is removed and burned. This step is popularly known as ‘de lube’ or ‘dewax’ step. 2^nd step is high heat step during which the powder particles fuse together, carbon from the premixed graphite dissolved in the ions and the other alloying elements such as copper, nickel and molybdenum diffuse into solid iron matrix with or without actually melting. The 3^rd step of the sintering cycle is the cooling step. Both sintering and cooling are done in controlled atmosphere.

Endothermic or N₂ / H₂ atmospheres are commonly used for sintering of steel parts. Well designed sintering furnaces will have carbon restoration zone to restore carbon in decarburised parts surface. A rapid cooling section can also be introduced between carbon restoration section & cooling section for sinter hardening.

A typical sintering cycle for ferrous powder consists of preheating or dewaxing at 650° C (1202° F), sintering at 1120° C (2048° F) for 25 minutes in reducing atmosphere followed by cooling in the same atmosphere.

Stress Relieving

Stress Relieving treatment is used to remove stresses residual from other manufacturing steps. The treatment involves controlled heating to a temperature below lower critical temperature, holding for sufficient time and cooling slowly to avoid the introduction of thermal stresses. It should be noted that the residual stresses couldn’t be totally eliminated and reduced to zero. Higher residual stresses and rapid heating during subsequent heat-treating operations (such as hardening) can lead to distortion of the parts.

Nitriding

Nitriding is the process of saturating the surface of the steel with nitrogen. Saturation is accomplished in an atmosphere of ammonia gas (NH₃) at temperatures ranging 480° C (896° F) to 580° C (1076° F). Ammonia dissociate to nascent nitrogen and hydrogen when in contact with the hot jobs 2NH₃ ® 2N + 3H₂. Atomic nitrogen formed diffuses into alpha iron and saturate the metal.

Nitrided steel acquire high hardness and retain the hardness even after subsequent heating to 600° C (1112° F). Nitriding increases wear resistance, fatigue and corrosion resistance. Nitriding is usually applied to medium carbon and alloy steels containing carbon and alloy steels containing aluminium, chromine, molybdenum and other elements that are capable of forming nitrides since the process is done at lower temperature, the time cycle will be longer than the caburising cycle. It is preferable to harden and temper the parts prior to nitriding to achieve best properties. The tempering temperature should be minimum of 20° C (68° F) to 30° C (86° F) more thus the nitriding temperature to retain the core properties. The nitrided depth usually between 0.2mm to 0.4mm. No quenching is required as high hardness is obtained directly after this process.

Isothermal Annealing

The practice of annealing so that the transformation occurs at contact temperature is referred to as isothermal annealing as contrast to continuous cooling annealing where transformation to Ferrite and pearlite takes place over wide range of temperatures. This also produces full annealing structure for the purpose of:

Inducing softness

Produce definite micro structure

Alter mechanical properties

Remove stresses

This annealing is very useful for low alloy low carbon steels as alloy limb carbon steels to achieve designed properties within shorter period of time. A typical process cycle is as follows:

Austenitise the steel say 920° C (1688° F)

Transfer the hot steel to another furnace kept at 650° C (1202° F)

Allow austenite to transform to fine pearlite + Ferrite for the time cycle as dictated by the TTT diagram of the specific material. Typically 2 to 3 hours.

Guide line rules for isothermal annealing are

Higher austenitising termperature promote pearlite formation and lower austenitising promote spheroidal carbides

Soft structure is obtained by minimum austenitising temperature and maximum transformation temperature

Furnace time is saved by rapidly cooling the job after completion of transformation time.