in a batch-type furnace, the workload is charged and discharged as a single unit or batch. in a continuous furnace, the work enters and leaves the furnace in a continuous stream. continuous furnaces are favored for the high-volume production of similar parts with total case depth requirements of less than 2 mm (0.08 in.).
temperature. the maximum rate at which carbon can be added to steel is limited by the rate of diffusion of carbon in austenite. this diffusion rate increases greatly with increasing temperature; the rate of carbon addition at 925°c (1700°f) is about 40% greater than at 870°c (1600°f).
the temperature most commonly used for carburizing is 925°c (1700°f). this temperature permits a reasonably rapid carburizing rate without excessively rapid deterioration of furnace equipment, particularly the alloy trays and fixtures. the carburizing temperature is sometimes raised to 955°c (1750°f) or 980°c (1800°f) to shorten the time of carburizing for parts requiring deep cases. conversely, shallow case carburizing is frequently done at lower temperatures because case depth can be controlled more accurately with the slower rate of carburizing obtained at lower temperatures.
therefore, for best results, the workload should be heated to the carburizing temperature in a near-neutral furnace atmosphere. in batch furnaces, parts can be heated in endogas until they reach the furnace temperature; then carburizing can commence with the addition of the enriching gas. many new continuous furnaces are being built with separate preheat chambers to ensure that the load is at a uniform temperature before entering the carburizing zone. in continuous furnaces that lack positive separation between heating and carburizing stages, the best that can be done is to:
carbon potential. the carbon potential a furnace atmosphere at a specified temperature is defined as the carbon content pure iron that is in thermodynamic equilibrium with the atmosphere. the carbon potential of the furnace atmosphere must greater than the carbon potential of the surface of the work pieces in order for carburizing to occur. it is the difference in carbon potential that provides the driving force for carbon transfer to the parts.
carbon diffusion. the combined effects of time, temperature, and carbon concentration on the diffusion of carbon in austenite can be expressed by fick’s laws of diffusion.
fick’s first law states that the flux of the diffusing substance perpendicular to plane of unit cross-sectional area is proportional to the local carbon gradient perpendicular to the plane. the constant of proportionality is the diffusion coefficient d, which has the units (distance)2/time. fick’s second law is a material balance within elemental volume of the system; the flux carbon into an elemental volume of iron minus the flux of carbon out of the elemental volume equals the rate of accumulation of carbon within the volume. combining the two laws leads to a partial differential equation that describes the diffusion process.
alloy effects. the various alloying elements found in carburizing steels have an influence on the activity of carbon dissolved in austenite. a definition of carbon activity (ac) is:
ac = (wt% c)γ
where γ, the activity coefficient, is chosen so that ac=1 for an amount of carbon in solution that is in equilibrium with graphite.
chromium tends to decrease the activity coefficient, and nickel tends to raise it. as a consequence, foils of a chromium-bearing steel equilibrated with a specific furnace atmosphere will take on more carbon than pure iron, and nickel-bearing steels will take on less carbon. it is also true that carbides are produced at lower carbon potentials in chromium-bearing steels than in carbon steels.
the primary effect of alloying elements on the diffusion of carbon is due to their effect on the driving force for the surface reaction. to obtain the true driving force, the surface carbon content in an alloy must be converted into the equivalent carbon content in pure iron. methods of correcting the activity coefficient of carbon for alloy content are available. however, the quantity of experimental data upon which such correlations are based is rather limited. therefore, predictions should be verified by experiments, particularly when an alloy contains substantial amounts of more than one alloying element.
for a particular alloy, the amount of retained austenite in the case increases as the case carbon content increases. an appreciable decrease in case hardness is usually found when the amount of retained austenite exceeds about 15%, but for applications involving contact loading, such as rolling element bearings, the best service life is found when the retained austenite content is quite high, for example, 30 to 40%. in other applications, especially when dimensional stability is critical, the retained austenite content should be low
pit gas carburizing furnace specifications:
|name||unit||pit carburizing furnace(type)|
|loading basket size||diameter||mm||300||300||450||450||600||300|
|furnace and air power loss||mw||≤7||≤8||≤11||≤14||≤16||≤18|
|the biggest load||mg||50||100||150||220||400||500|
|furnace heating and air time
|furnace out dimension||l||mm||1620||1620||1750||1750||1700||1700|