Carburizing Case Depth and Time at Temperature

Carbon diffuses into steel parabolically -- depth goes with the SQUARE ROOT of time -- so the rate constant times the root of the hours gives the case, and the inversion squares. Doubling a case QUADRUPLES the cycle. On a batch furnace that is the difference between two loads a day and one load every day and a half, and it is the reason a drawing calling for a deep case is a scheduling decision as much as a metallurgical one. The rate constant is entered because it depends strongly on temperature, on the carbon potential of the atmosphere, and on the steel, and a published figure for one combination does not transfer to another. THE TEMPERATURE LEVER IS REAL AND IT IS NOT FREE. Raising the carburising temperature increases the rate constant substantially, so the same case can be reached in far less time -- at the cost of grain coarsening, more distortion to correct in grinding afterwards, and shorter fixture and furnace life. It is a trade a heat treater makes deliberately, and a shop that reaches for it to recover a schedule pays for it in the grinding department. THE SPECIFICATION TRAP IS WHERE SUPPLIER AND CUSTOMER DISAGREE ABOUT A CORRECT PART. 'Case depth 0.030 in' does not say whether it is EFFECTIVE case depth, measured to a stated hardness, or TOTAL case depth, measured to where the case is indistinguishable from the core. They are different numbers on the same part and the total is always larger, so a supplier quoting to one and a customer inspecting to the other will disagree about a part that meets the intent exactly. The drawing has to state which, and for an effective depth it has to state the hardness as well, because that changes the number too. This is the parabolic diffusion relation on an entered rate constant. It does not supply the rate constant for any temperature, atmosphere or steel, model carbon potential, boost-and-diffuse cycles, or the surface carbon concentration, predict the hardness profile or the resulting hardness at any depth (which depends on the quench as much as on the carburising), address distortion, grinding stock, or the retained austenite a high surface carbon produces, or cover nitriding or carbonitriding, which follow different kinetics. The heat treater's own process data, the applicable process standard, and the metallurgist govern.

the parabolic diffusion relation case depth = k x sqrt(time), inverted as time = (depth / k)^2, so doubling a case QUADRUPLES the cycle.

Carburising case depth kinetics. The rate constant k is ENTERED because it depends strongly on carburising temperature, on the atmosphere's carbon potential and on the steel, and a published figure for one combination does not transfer to another.

One diffusion relation on an entered rate constant.

Estimate. AHJ and licensed professional govern.

Field names used by the API: reference_case_in, reference_time_hr, time_hr, target_case_in, hotter_reference_case_in, case_depth_in, time_for_target_hr, time_ratio, depth_ratio, hotter_time_hr

• Rate constant is entered it depends on temperature, atmosphere and steelthe heat treater's process data
• Effective and total case differ and the drawing must say which, and to what hardnessthe applicable process standard
• Hardness profile is not predicted it depends on the quench as much as on the carburisinga metallurgical section