Salt-bath nitriding can be an economical method of nitriding provided that both the salt-bath chemistry and cleanliness are maintained. It is mandatory that the salt-bath chemistry is checked at the commencement of each shift and the appropriate additions of salt are made to return the bath to its operational strength.
Regular desludging of the bath is necessary to remove precipitated oxide in suspension in the salt and from work baskets and suspension wire. Pre-cleaning is as mandatory as it is with the gas-nitriding procedure, and it is most important after the nitride treatment to remove any trace of the heat-treatment salt in holes or cavities.
Dilution nitriding is a process that has been known since the initial development of gas nitriding by Adolph Machlet. The process has been refined and developed in North America to a very precise science, and it is a process that still uses ammonia as the nitrogen source. It controls the surface metallurgy of the procedure by dilution of the process gas using nitrogen or hydrogen. The next area of process control and process repeatability is to deliver the process gas in a very precise metering system.
The problems that can occur are mainly due to the metallurgical aspect of the procedure such as are described in the gas nitriding section. The problems are not usually hardware or program related, although there is no doubt that problems can ultimately occur.
Ion nitriding is also known as glow-discharge nitriding or plasma nitriding. The process is gaining a great deal of popularity in North America due to legislation on process effluents, European engineering specifications and a growing awareness of the repeatability and metallurgical consistency of the process, which is accomplished by computer control. It is necessary to understand that there are two power system types: continuous DC power and pulsed DC power. From this there are two types of hardware systems: cold wall and hot wall.
Our discussion will continue next time.
1. McQuaid H.W. and W.J. Ketcham, "Some Practical Aspects of the Nitriding Process," American Society of Steel Treaters, 1928
2. Adolph Fry, U.S. Patent 1,487,554, March 18, 1924
3. Pye, David, Practical Nitriding and Ferritic Nitrocarburizing, Chapter 17, "Troubleshooting," ASM International, 2003