Answer: There are four probable causes of this problem.
Probable cause 1:
Consider if you have machined enough stock from the surface of the bar. If stock removal is inadequate, there may be residual surface decarburization from the hot rolling of the bar. It is necessary to machine at least 10% of the surface of the bar to be sure that you are underneath the decarburization caused by the hot rolling. The bar is generally rolled at an approximate temperature of 2200°F (1200°C) without atmosphere and the bar surface will decarburize at that temperature as well as oxidize. The decarburization will cause a nonuniform nitride case to form. An indication that this has occurred is, if the nitrided surface has the appearance of an “orange peel” finish after nitriding, then it is almost a definite indication that decarburization is present.
Probable cause 2:
The second probable cause is for decarburization to have occurred during the pre-heat treatment of the bar stock. If the furnace process atmosphere is not in equilibrium in terms of carbon with the bar stock, surface decarburization can occur. The decarburization can occur particularly with salt bath pre-heat treatment as well as with atmospheres.
Probable cause 3:
The third probable cause is that the control of the gas decomposition has failed thereby exceeding the “limit of solubility of nitrogen in iron.” Simply put, too much nitrogen has diffused into the steel surface. If you consider a glass of water and you add salt to the water, a solution has been made. If you then add too much salt, a saturated solution has been made. The method of dissolving the excess salt in the solution is to heat the water. This will cause the salt to go into solution. On cool down, however, the salt will precipitate out of solution.
As far as nitriding is concerned, the excess nitrogen can be seen as “fingers” at the case/core interface. These appear as white fingers reaching down from the formed case into the core material as seen in the photomicrograph (Figure 1).
Probable cause 4:
The same phenomena will occur with the fourth probable cause as is seen in the above photomicrograph. This can also occur as a result of processing at too high a selected nitriding temperature. The higher the process temperature, the more nitrogen the steel will absorb. Typical nitriding temperatures are in the range of 930°F (500°C) up to 965°F (520°C). The higher the process temperature, the greater is the risk of exceeding the limit of solubility. This ties in with #3.