The heat treater does not always know: ·
- What core hardness is required?
- What sort of compound layer is necessary for the workpiece?
- What thickness of compound layer would be required?
- Direct surface indentation with the hardness machine indenter
- Transverse cross traverse on a sliced-out “puck” sample and measure from the surface edge
- What load to use when making the indentation
- Which method of hardness testing to use (Knoop, Vickers or Rockwell Superficial)
A simple analogy can be likened to how much sugar I like in my coffee. The next person may select more or less sugar than I would. The coffee still has sugar in solution, or even to the point of being sickly sweet with sugar that has not all gone into solution, and there is a residue of sugar at the bottom of the cup.
We can put too much nitrogen into the steel surface (saturated solution) and accomplish nitride networking, or we can put too little nitrogen into the surface and have a low surface hardness. Or we can put an incorrect amount of nitrogen in in relation to the analysis of the steel itself (and its alloying elements) and obtain a formed surface metallurgy that is not applicable to the functionality of the nitrided component.
Not everyone understands the meaning “the corner effect.” This means that there is an oversaturation of nitrogen at the corner, which makes it very brittle with susceptibility to chip and fracture. This result is generally caused by excessive nitrogen diffusing in from all around the corner. So flow control of the process gas is absolutely mandatory. The corner effect will only be seen when the corner fractures or when a destructive test is made on the component or test coupon. Remember that the nitrogen is “diffusing” into the steel and not “depositing” onto the steel.
The process method will also play a very significant part in the resulting surface metallurgy of the technology selected for the diffusion process.
- Ammonia will
produce a gas decomposition of two parts nitrogen and six parts hydrogen
↔ 2N + 3H₂). Fixed gas chemistry = Fixed surface metallurgy
- Ammonia diluted with another gas.
- Nitrogen plus hydrogen (such as is used in plasma nitriding) N₂ + H₂. The nitrogen and hydrogen ratios are variable. Variable gas chemistry = variable surface metallurgy.