Depending on the nature of carbonaceous atmosphere, carburising is called pack, or solid carburising, liquid carburising, or, gas carburising. But, whatever may be the method used, carburisation always takes place via a gaseous phase.
Method # 1. Pack or Solid Carburising:
In this method, the carburising compound is a solid carburiser, usually charcoal of 3 to 10 mm size (fines are screened off), or coke, semi-coke, or coal. To accelerate the carburising, activator, or energiser normally BaCO3 and, or Na2CO3 in amounts 10-30% of charcoal is added. (Na2CO3 or CaCO3 are avoided as Na2O formed from former may react with carburising box or even pit the steel components and the latter forms the cake). Fig. 8.9 illustrates the effectiveness.
Tar or molasses may be added to bind the energisers. Powdered coke (~ 20%) increases the heat conductivity also of the mixture. Table 8.3 gives composition of some mixtures. Pellets of the mixture could also be used. As a fresh carburising mixture shrinks by almost one fifth of its volume after its first use, fresh mixture is added to the older one. The mixture thus obtained shrinks by a known amount.
When old carburising mixture is used, it is first left exposed to atmospheric air for 1-3 days for it to get reactivated by absorbing CO2 and moisture from air. Normally one part of new mixture to two parts of old is mixed and used.
The components, which are to be pack-carburised, are first cleaned (otherwise carburising shall not occur at least uniformly), and then packed in a box (150-250 mm diameter, or wide) as illustrated in Fig. 8.10. If pack-carburising is done occasionally, then the box may be made of mild steel, or Al-coated carbon steel, or even cast iron.
For regular carburising, box of heat-resisting steel (25% Cr, 20% Ni) is economical in the long run. Carburising mixture has poor thermal conductivity, and thus, one dimension of the box should be smaller, otherwise components packed in the centre of the box are less carburised. The lid should be well-fitting to prevent entry of air. There are provisions to insert test pieces, called witnesses to check carburising results.
First a layer of carburiser, 25 mm deep is spread on the bottom of the box. The components are then put on this layer. This layer acts as cushion too to avoid deformation of components under the load during carburising. There should be a space of 25 mm between the components as well as from the walls of the box (Fig. 8.10). After addition of each carburising layer, the box shall be tapped (to remove the air voids).
The thickness of the top layer should be 50 mm for settling. The box is top-closed with a well-fitting lid, where edges are luted with fire-clay, or a mixture of clay and river sand diluted with water to form a dough. During heating, the clay dries but gets cracked. To prevent entry of air, a better way is to have a rimmed lid 25 mm larger than the box and put on the box (Fig. 8.10 a). The box is then turned up-side down (Fig. 8.10 b). The rim part of lid is then sealed with material such as refractory etc.
The packed boxes are placed in a furnace at about 700°C, and then maintained at 750°C, so that the temperature is uniform throughout the boxes. Thermocouples could be inserted in closed-end tubes welded to box to knowing the heating temperature, or a heating time to the carburising temperature is taken as 7-9 minutes/cm of smaller dimension of the box.
The holding time at the carburising temperature for 150 mm side box is 5.5 to 6.5 hours to get a case depth of 0.7-0.9 mm and is 9-11 hours for 1.2 to 1.5 mm. Though Fig. 8.2 acts as a good guide for obtaining case-depths at the carburising temperature, ‘witnesses’ of the same steel (as being carburised), 5-6 mm diameter and 200-300 mm long are inserted in the container.
One of them is withdrawn after the specific carburising time, and is fractured to measure the case depth. The carburising is continued, if the case depth is less. Higher carburising temperatures of 950-1000°C speed up the carburising process, but pronounced cementite network forms in the case and steeper gradient of carbon is present in steel, and such high temperatures decrease the life of boxes, furnace parts etc. The boxes are cooled in air to 400-500°C, and then opened to remove the carburised parts.
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A uniform temperature throughout the furnace yields a uniform case depth, but even with the best of furnaces, it is difficult to obtain a case depth that has a total variation of less than 0.15 mm from maximum to minimum in any given furnace load. A case depth of less than 0.5 mm could not be specified for pack carburising.
Theory of Pack-Carburising:
The O2 of the entrapped air (in the carburising box) reacts initially as the temperature rises with the carbon of the carburising mixture, say with charcoal, to give
As the temperature rises, the following reaction takes place and the equilibrium shifts towards right, i.e., gas becomes progressively richer in CO-
At the steel surface, the main carburising reaction occurring is-
where Fe(C) is carbon dissolved in austenite.
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This atomic, or nascent carbon is readily absorbed by the steel surface, where it diffuses to interior. CO2 thus formed reacts with the C of the charcoal (reaction 8.12) to produce CO. Thus, the cycle of the reactions continues. Charcoal is the basic source of carbon during pack-carburising.
As the entrapped air inside the box may be less to produce enough CO2 (by reaction in 8.10) particularly in the beginning of the carburising, it is common practice to add energiser, usually BaCO3 which decomposes during the heating-up period as-
The CO2 originally formed then reacts with the carbon in the charcoal to produce the active CO. Thus, BaCO3, makes available CO2 at an early stage to energise the carburising, is called energiser.
It is apparent that in pack-carburising too, the solid carburising agents react, but the process of carburising takes place by gaseous form of Catomic.
Surface Carbon Content:
As per equation 8.12, CO and CO2 attain equilibrium such that at one atmospheric pressure and at the normal carburising temperature of 900°C, it has about 96% CO and 4% CO2. The carbon concentration at the steel surface adjusts in between 0.7% to 1.2%, which is approximately equal to the solubility limit of carbon in austenite at the temperature (i.e. along Acm line).
If CO2 content falls below 2%, some Fe3C is precipitated, but if CO2 content increases (by admitting more air), the steel surface has lesser carbon content. For example, at 900°C, 6% CO2 produces about 0.70% carbon and 14% CO2 produces 0.30% carbon. No accurate control of surface carbon content is available in pack-carburising. Whatever little contra’ is possible, it is by the appropriate carburising temperature.
Case-Depth:
The effect of actual temperature and time on the case-depth during pack-carburising. The best carburising temperature is 900°C. At 950°C, higher case depth is obtained in short time but the cementite network may form at the surface of the steel.
At higher than 900°C, the steel surface absorbs carbon at a faster rate than the rate at which it can diffuse inside, thus producing supersaturated-case which may produce tracks during quenching. In pack-carburising, it is difficult to control exactly the case depth because of many factors affecting it, such as the density of packing, amount of air, etc.
Advantages:
1. It is a cheap, simple method if there are just a few parts to be carburised.
2. Very large and massive parts which are too large for gas or salt carburising can be carburised if a furnace of that size is available. Pack-carburising can be done in large variety of furnaces if these have uniformity of the temperature.
3. Capital investment is very less compared to other two types.
4. No atmosphere controlled furnace is required.
5. No poisonous cyanide or gas is used here.
6. It can be done in any workshop.
Disadvantages:
1. Carburising time is very long as here boxes as well as bad-heat-conducting carburising compounds are to be heated.
2. It is difficult to control the surface carbon and the carbon gradient.
3. It is difficult to control the case depth exactly particularly the light cases cannot be produced within close limits.
4. Handling carburising compounds and packing is a dirty and dusty job.
5. It is difficult to use direct-quenching in pack carburising.
Method # 2. Liquid-Carburising:
When the enrichment of steel surface with carbon takes place by immersing the steel to be carburised in a molten salt maintained at an appropriate temperature (in range 850°C to 950°C), it is called liquid carburising.
The active carburising agent in a salt bath is sodium cyanide (or KCN), though the bath may also have alkaline-earth chlorides such as barium chloride, strontium chloride, etc., sometimes called activators, which help to increase the carbon content in the case, as well as, decrease the melting point and viscosity of the bath.
Though the carburising atmosphere is liquid here, it is again the gaseous form, i.e., atomic-carbon which is absorbed at the steel surface, and then diffuses into interior along-with a negligibly small amount of nitrogen.
The amount of carbon (and nitrogen) picked up by the steel surface depends mainly on the cyanide content of the bath and its temperature. Fig. 8.11 (a) illustrates that the carbon-content of the steel surface increases (while nitrogen decreases) as the cyanide-content increases from 10% to 50% at 950°C, whereas Fig. 8.11 (b) illustrates that carbon-content increases (and nitrogen decreases) as the temperature (of about 50% NaCN bath) increases from 800°C to 950°C.
This thus, requires daily checking of NaCN content of the bath, and replacing equal-volume by fresh salt as required. Plain cyanide salt results in high nitrogen, low carbon case, but presence of chlorides in the bath hinders nitrogen pick-up, and favours absorption of carbon into steel. Fig. 8.12 can be used to assess the case depth in liquid carburising, which is much faster than pack-carburising.
The surface of the bath is kept covered with a layer of graphite powder to reduce radiation losses and gas evolution. Before the parts are immersed in salt baths, they should be preheated to between 100°C to 400°C, partly to remove the traces of the moisture, and partly to use more efficiently the capacity of the salt-bath.
The surface carbon content may become very high due to long carburising time and higher tempera- lures, which when quenched directly from salt bath may show low as-quenched hardness due to larger amounts of retained-austenite. Thus, if parts are to be quenched directly, then the temperature of carburising should be moderate. Bui, when the required case depth is high (> 0.5 mm), then economics require temperature the bath to be high. 900-925°C.
Low as-quenched hardness is quite consequential, particularly in the alloy steels. Re-quenching from a lower temperature is then done. Sub-zero treatment way be done to increase hardness at the cost of decreasing the retained-austenite (nitrogen dissolved in austenite also stabilises austenite).
Thus, depending on the case depth required which controls the cyanide content and the temperature of the bath, liquid carburising baths can be classified in two types:
Low Temperature Baths:
The typical range of composition of such baths is given in Table 8.4. These baths are used in the temperature range of 850-900°C for shallow case-depths in the range of 0.075 to 0.75 mm. Low-temperature bath-carburising is mainly used for small parts, which require small case depths (< 0.5 mm). As the rate of heating is very fast in liquid carburising, greater economy is achieved, if smaller is the case depth.
The parts are normally quenched direct into water after being carburised. Surface carbon content with carburising in a 17-20% NaCN bath could be 1.0 to 1.2% carbon. Such a bath could be prepared by melting together equal parts of sodium cyanide and anhydrous sodium carbonate.
Reactions:
Some important reactions are:
where, Fe(C) indicates carbon dissolved in austenite.
The cyanate content of the bath helps to control the carbon content of the surface of the carburised steel. Actually, the nitrogen absorbed by the steel at a temperature is proportional to the cyanate content of the bath. Now-a-days, proper mixture can be adjusted to give a carbon potential of bath as 0.8-1.0%, so that lesser retained austenite is obtained.
Foil-probe can be used to detect it. It is apparent that for a given carburising temperature, the minimum of the cyanide range results in producing hypo-eutectoid case, whereas the maximum cyanide content results in hyper eutectoid case.
High Temperature Baths:
High temperature baths are used to get deep cases in the range of 0.5 mm to 3.00 mm, and operate in the range of 900° to 950°C. A fresh-cyanide bath is prepared by using pure NaCN, which on melting in the furnace decomposes quite readily to attain a NaCN content of 40-50%, which can be used for case depths up to 0.8 mm.
For case depths up to 1.5 mm, activated baths are used having a NaCN content of about 10%. Normally, these baths also contain barium chloride. The chemical reactions shown for low-temperature baths also occur in the high- temperature baths, but the following reactions are quite effective.
As the temperature rises, it increases the activity of carbon, and decrease the activity of nitrogen:
Some nascent nitrogen (reaction 8.18, 8.20, 8.21) formed is absorbed by the steel surface which then diffuses inside, but the commonly used cyanide content of the baths keeps the activity of nitrogen in bath rather low. Some nitrogen does diffuse inside.
Precautions:
Cyanide salts are very poisonous and all the safety rules directed by the salt supplier or otherwise must be strictly adhered to. Components heated in cyanide baths must be thoroughly neutralised and rinsed immediately after the quenching operation. Neutralisation is necessary to remove poisonous effect of cyanide salt. Neutralisation is done is tanks, in which a 3-5% ferrous sulphate solution is provided.
It takes 5-10 minutes. After neutralising, the articles are thoroughly rinsed in hot water at about 60° to 80°C for a period of 5 minutes. If a part is cyanided in bath having around 10% NaCN and if it is to be martempered, then oxygen bearing salts in martempering bath should not be used. Otherwise normally used sodium nitrite and potassium nitrate bath will become explosively violent.
Advantages of Liquid Carburising:
1. Liquid baths have high heat transfer efficiency to result in large output even in a small furnace.
2. It helps in quick attainment of constant temperature to produce uniform case depth and carbon content.
3. Properly maintained baths eliminate surface decarburisation.
4. Time of carburising is much less as compared to pack carburising and even gas carburising.
5. It is good for mass production particularly small parts with shallow depths.
6. Distortion is minimum.
7. Capital cost is lesser then gas carburising. Labour cost is least.
Disadvantages:
1. All the precautions are necessary as required for using cyanide salts.
2. Moisture sticking to parts may cause explosions.
3. Preheating is essential and so is removal of salts sticking to parts.
4. Disposal of salts is great pollution problem.
Method # 3. Gas Carburising:
Gas carburising has become the most popular method of carburising in the last two decades. When the enrichment of steel surface with carbon takes place by heating the steel in a furnace having gaseous carburising atmosphere, it is called gas carburising.
The main carburising agent in gas carburising is methane (CH4) or propane in the form of natural gas. Liquids are also extensively used as mixture based on methyl, ethyl, or isopropyle alcohols with additions of benzene or other liquid hydrocarbons such as kerosene, acetone, etc., which are dispersed and vapourised.
Methane, etc. are rich in elementary carbon than is necessary and thus, form heavy deposits of soot on components and walls, etc. of the furnace and upset the carburising reactions. Hence, in gas carburising, it is necessary that the hydrocarbon gases are diluted with a carrier gas to avoid soot formation.
Carrier gas can be made by controlled burning of a hydrocarbon gas. The carrier gas commonly used is endothermic base (exothermic gas is decarburising but can be purified and used). Methane can be burnt in air with air to methane ratio of 2.5 to react as-
2CH4 + O2(air) →2CO + 4H2 …(8.24)
and the common endothermic carrier gas has the composition (vol%)
N2 = 39.8%; CO = 20.7%; H2 = 38.7%; CH4 = 0.8%
This gas mixture offers a broad range of carbon control and moderate amount of carbon availability for carburising when operated with a dew point of -7°C and above. During carburising, the hydrocarbon gas to carrier gas ratio could vary between 1:8 to 1:30 depending on the requirements.
Common Ways of Producing Carburising Atmosphere in a Furnace:
1. The liquid hydrocarbon (say ethyl alcohol + white spirit) is allowed to fall in drops on to a plate in the furnace where it gets dispersed and vapourised. In practice, the carbon potential of the atmosphere is controlled by varying the volume of the liquid added.
2. Gas and the air are admitted in the furnace in properly balanced amounts so that the reaction of these two inside the furnace results in a gas mixture which causes carburising.
3. A separate gas producing unit produces the carrier gas by the balanced burning of a hydrocarbon gas such as propane by endothermic combustion to have a carbon potential of 0.35-0.50%. Extra addition of propane is done to this carrier gas before it enters the furnace to have the required carbon potential of around 0.8%.
4. Vacuum carburizing- Hydrocarbons are allowed to enter an evacuated furnace (no air) which can be heated to high temperatures to quicken the carburising process.
Pure nitrogen is inert and can be used as a carrier gas if available at cheap rates. As the hydrocarbons have high carbon content, the ratio of nitrogen to hydrocarbon must be controlled with in ratio of 50:1 to 100:1, but more important is that hydrocarbon gas must be controlled with in limits of about 0.1% to maintain desired carbon potential.
Carburising Reactions:
The important chemical reactions occurring during gas carburising are:
where Fe(C) indicates carbon dissolved in austenite. The gaseous products of reactions 8.26 and 8.27, i.e., CO2 and H2O are both decarburising agents. As the CO2 and H2O contents in the carburising atmosphere increase due to above reactions, the carbon potential of the atmosphere decreases. This necessitates the addition of carburising agent. CH4, and then,
CH4 + CO2 → 2 CO + 2 H2 …(8.28)
and, CH4 + H2O → CO + 3 H2 …(8.29)
The H2 and CO as regenerated by reactions 8.28 and 8.29, react with steel surface according to reactions 8.26 and 8.27 to cause carbon enrichment of the surface, i.e., carburising. It is obvious that the ultimate source of carbon in gas carburising is CH4.
Method # 4. Two Stage Gas Carburising (Boost-Diffuse Carburising):
When the required case is 1 mm, or less (i.e., carburising period is less than 3 hours, or so), then the carburising may be done at a constant carbon potential for the required carburising time at a constant carburising temperature. It is also done when saturated austenite (with carbon) is required at the surface of the steel. Curve (a) in Fig. 8.19 illustrates the carbon gradient obtained in such a case.
In order to have the optimum mechanical properties in the case (say, constant hardness up to certain depth from surface) as well as to reduce the amount of retained austenite at the surface of the steel, it is commonly required to obtain a region of almost uniform (constant) carbon content extending into the carburised zone from the surface. To obtain this, it is common practice to use carburising cycles that consist of two or more combinations of temperature, time and atmosphere composition (carbon potential).
Most widely used cycles consist of two periods:
1. Active Carburising Period:
Carburising is done at a constant temperature starting with a carbon potential in the atmosphere which approaches the value for carbon saturation in austenite (≈ 1.3% C at 927°C, according to Acm line). The excess carbon is also necessary because the demand for carbon by the steel surfaces is greatest when the part first reaches the carburising temperature.
Normally the amount of enriching gas introduced into the furnace in proportion to the carrier gas is about 10-15% of natural gas, of 3-8% of propane or 1-3% of butane at this stage depending on the surface area to be carburised, case-depth required, etc. With increasing carburising time, ease becomes deeper and surface attains the high carbon content. At the end of this period, the enriching gas supply to the furnace is stopped and the charge is held in the carrier gas. The carbon gradient from the surface in the case in reflected by curve (a) in Fig. 8.19.
2. Diffusion Period:
Here, the atmosphere is maintained at a carbon potential equal to the final desired surface carbon content. The desired potential is controlled by decreasing or stopping the enriching gas. It is thus preferred to generate a carrier gas with a composition approximately in equilibrium with the final desired surface carbon content. Actually, in diffusion cycle, the carbon will diffuse back into the atmosphere as well as inwards from the surface.
The modified carbon gradient is illustrated by curve (b) in Fig. 8.19, i.e., the surface layer is partly decarburised because it is in an atmosphere of lower carbon potential. As a ‘thumb rule’, the diffusion period is half the active carburising period. Two-stage gas carburising can be done either in a batch furnace by changing the carbon potential after active carburising period, or in a continuous furnace where different carbon potentials are maintained in different zones.
Control of Gas Carburising:
In gas carburising, the following three variables are to be accurately measured and controlled. In some running plants, the temperature of carburising and the carrier-gas flow rate are fixed.
The time and the flow of enriched gas are varied depending on the case depth and the surface carbon required:
1. Temperature
2. Time
3. Atmospheric Composition
1. Temperature:
Though higher carburising temperature minimises the time as the rate of carbon diffusion, i.e., rate of carbon addition increases, but commonly the carburising temperature is kept at 925°C as it permits reasonably rapid carburising without excessive deterioration of furnace equipment, and the case depth can be controlled more accurately. Sometimes, to reduce the time for deep cases, the carburising may be done at 955°C.
For best results to be obtained in gas carburising, the parts are heated to the carburising temperature in a near neutral atmosphere such as in endo-gas (carrier gas). When the parts have attained uniformly the carburising temperature, then carburising begins with the addition of the enriching gas. The temperature control should be ± 3°C by properly placing the thermocouples.
Fig. 8.20 and table 8.6 gives the relationship between case depth and temperature with time.
2. Time:
The carburising time decreases with the increase of temperature (Fig. 8.20 and table 8.6). Apart from this time at the carburising temperature, quite a few hours may be required to bring the parts to the carburising temperature. Also, if parts are to be quenched directly from the carburising furnace, time is needed for parts to cool from the carburising temperature to about 845°C prior to quenching.
3. Atmospheric Composition:
The control of carbon potential during carburising is obtained by varying the flow rate of the hydrocarbon-enriching gas, while maintaining the steady flow of endothermic-carrier gas.
The flow of the enriching gas can be regulated by monitoring some constituent of the carburising atmosphere as:
(i) CO2 content by infrared gas analysis, or
(ii) Water vapour content by dew point measurement. Increasing the enriching gas decreases the dew point, while increasing the air increases the dew point.
(iii) Oxygen potential using a Zirconia-oxygen probe.
Generally, the generator gas containing 0.1 to 0.3% CO2 and with a dew point of about – 10°C to + 5°C is ideal for use is a carburising atmosphere.
Equation (8.34) gives the carbon potential of the carburising atmosphere. Equation (8.2) can be used as the correction factor for the carbon potential to know the surface carbon content of carburised case.
Method # 5. Selective Carburising:
There are certain parts of which only certain surfaces are to be carburised, a process called as selective carburising, and other surfaces, not to be carburised, must be protected by a coating that is impervious to carburising.
Some of the reasons for selective carburising are:
1. Intended application requires the part to have differential properties.
2. Machining may be required on the remaining surfaces after carburising.
3. Welding is to be done of non-carburised surfaces after carburising.
Methods of Protection:
1. The simplest and cheapest method where leakage may be permitted consists of coating the surfaces not to be carburised with a paste consisting of fire-clay, sand, asbestos with water glass. As such a coating cracks on heating, and carburising gases penetrate through the cracks, it does not altogether prevent the carburising.
2. Ceramic coatings in the form of paint is also a cheap method of protection. The first coat is applied on thoroughly cleaned steel surfaces and allowed to dry before a second coat is applied.
3. The most common and full-proof method is to protect the selected surfaces from carburising gases is to plate copper on these surfaces-minimum 0.013 mm thick. It is relatively easy to plate, is machinable, good conductor of heat and non-contaminating to furnace atmospheres, but steel surfaces must be smooth and fine.
To prevent deposition of copper on surfaces to be carburised but not to be plated, coat is applied of high-temperature microcrystalline wax, or a chemical-resistant lacquer, which are removed prior to carburising. Copper plating may be strengthened by covering it with a layer of water glass. After carburising copper may be removed in a chromic acid bath, or other suitable stripping solution, or by machining it off.
4. Copper and tin pastes can work reasonably well. These pastes are made up of a powder that contains mainly copper suspended in a varnish-type binder. These pastes are good for pack-carburising. Specially compounded pastes are available for gas carburising.
5. The most positive method of ensuring specific areas from carburisation is to have the part with an extra machining allowance greater than total depth of carbon penetration (≈ 3 times DC). These areas are machined after carburising and before hardening, or machined after hardening and high temperature tempering.
Method # 6. Homogeneous Carburising:
Low carbon steels have good formability and weldability. Parts with thickness less than 5 mm such as steel chain links or clutch springs or thin stamping can be easily formed from low carbon steels. These are then carburised throughout to a constant carbon content ≈ 0.65 – 0.70% and heat treated to get desired hardness and strength.
Thus, homogeneous carburising is a process in which a part is thoroughly carburised to a desired level of carbon content. But, this process is economically feasible only for thin sectioned parts. Proper care should be exercised to do carburising for enough long time to carburise the part thoroughly.
Method # 7. High-Temperature Carburising:
Normally, the carburising temperatures used are up to 925°C because the case carbon can be accurately controlled, but chiefly because higher temperatures seriously effect the life of the furnace equipment.
But now with the availability of better furnace materials which can withstand higher temperatures, the high temperature carburising is economical. The advantage of high temperature is that carburising process accelerates markedly at around 1000-1050°C, thus providing the desired case depth in a shorter period of time.
Fig. 8.23 illustrates that time required to achieve a specific case depth decreases sharply with the increases of temperature as the diffusion coefficient of carbon increases exponentially with temperature. The surface carbon content of around 0.90 to 0.95% can be controlled automatically through the cycle by any of the methods.
Long time of carburising at high temperature may cause grain growth which may adversely effect the impact properties of steels. Plain carbon steels are more prone to grain growth than alloy steels. Thus, suitability of a steel for high temperature carburising should be first determined. However, instead of direct quenching from carburising temperatures, the practice of reheating and quenching can partly solve the grain growth problem.
Method # 8. Vacuum Carburising:
Vacuum carburising is basically a high temperature gas carburising process of steel components in vacuum furnaces with a pure hydrocarbon gas for example, methane or a mixture of hydrocarbon gases or nitrogen and a hydrocarbon gas.
Vacuum carburising gives excellent uniformity and reproducibility (due to better process control), improved mechanical properties (as no intergranular oxidation occurs) and reduced carburising time.
Four Main Steps in Vacuum Carburising:
1. Heating to Carburising Temperature:
The components to be carburised are first placed in the heating chamber, kept in trays and the furnace is evacuated (a vacuum of 0.1 to 0.3 torr in graphite furnace or 0.3 to 0.5 torr of hydrogen in ceramic-lined silicon carbide furnace). Components are uniformly heated to carburising temperature, normally 845°C to 1050°C. Thus, furnace provides excellent oxidation and decarburisation resistance.
2. Boost Step:
Carburising gas mixture is back filled in vacuum chamber. Austenite absorbs carbon to its limit of carbon solubility at the carburising temperature.
The hydrocarbon gases crack at the steel surface with the direct absorption of the carbon by austenite as:
CH4 + Fe →Fe(C) + 2 H2 …(8.47)
or C, H8 + Fe → 3 Fe(C) + 4H2 …(8.48)
where Fe(C) means carbon dissolved in austenite. A minimum partial pressure of hydrocarbon gas (10-50 torr in graphite furnace: 100-200 torr in ceramic) in ensured for rapid carburising of steel. Higher than 300 torr causes soot formation.
3. Diffusion Step:
As usual, this step reduces surface carbon content, lowers carbon gradient (case/core). The gases are evacuated to 0.5-1.0 torr vacuum to perform diffusion at the same carburising temperature.
4. Oil-Quench Step:
If carburising temperature is not very high, the carburised parts may be quenched directly in oil under a partial pressure of nitrogen. High temperature carburising normally requires cooling to a lower temperature, stabilising at this temperature prior to oil-quenching.
5. There is a trend to do gas-pressure quenching of the parts.
Advantages of Gas-Carburising:
1. The surface carbon-content as well as the case-depth can be accurately controlled here.
2. It gives more uniform case-depth.
3. It is a much cleaner and efficient method than pack-carburising.
4. Total time of carburising is much less than pack-carburising as the boxes and solid carburiser are not to be heated.
5. Direct quenching reduces the cost of heat treatment and increases productivity.
Disadvantages of Gas-Carburising:
1. Furnaces and gas generators are expensive.
2. Trays and fixture used are expensive.
3. Greater degree of operating skills are required. Fire hazards and toxic cases are to be handled.
Method # 9. Fluidized-Bed Carburising:
With the popular use of Fluidized-bed furnaces for, healing, these furnaces are also used for gas carburising. The mixtures of gases, used for fluidization, are also the source of carbon (and nitrogen in case of carbonitriding). Commonly a mixture of methanol (after vaporisation), N2 and propane or a mixture of propane and air are used. Fine-alumina-powder-bed having components, and the mixture of evaporated methanol. N2 and propane is fed from bottom.
The main reactions are:
The main source of carbon is the hydrocarbon, propane. The reactions are almost under equilibrium conditions, and thus, the carbon potential of the atmosphere is controlled as explained for gas-carburising. It is easily possible to have, here too, two-step carburising process. Fluidised bed process results in faster heat transfer, and carbon potential is attained quickly and can be quickly changed too.
Soot formation does occur in this process but the dynamic-bed passing the components is able to remove it continuously. As the furnace atmosphere can be easily controlled and manipulated, atmospheres of different carbon potentials, and even inert nitrogen atmosphere can be obtained to allow time for lowering the surface carbon content and moderating the carbon content there.