Many engineering blueprints and drawings do not always state the surface metallurgy that is required for the functionality of an engineered component. Some will simply say nitride, plasma nitride or salt-bath nitride and leave it to the heat treater to be the judge of the surface metallurgy that is necessary for the component.

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?
The drawings will often specify a surface hardness (perhaps 950HV) that is really not obtainable. Drawings do not always specify the load mass for the hardness-test application, nor how to make the reading either by:
  • 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)
This then leaves a great deal of discretion to the heat treater to try and provide their best understanding of the word “nitriding.” The word nitriding simply means the diffusion of nitrogen into the steel surface. But the question begs, how much nitrogen?

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 (2NH₃

    ↔ 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.
This discussion will be continued in the next blog.