The nitriding process is perhaps one of the most misunderstood thermochemical surface-treatment processes practiced today.
The nitriding process was just 100 years old in 2003. The patent for nitriding was first granted to Adolph Machlet of Elizabeth, N.J. (see U.S. Patent 1,092,925, dated 24 June 1913) followed by Adolph Fry of Germany in the early 1920s. So the process is not as old as carburizing, for example. However, it is perhaps (as far as chemistry) one of the most simple of all of the thermochemical surface treatments.
In this article, we will describe some of the problems that can occur as a result of and during the nitriding process. It will be necessary to evaluate the process techniques, which are gas, dilution, salt-bath or plasma (ion) nitriding. Table 1 shows the primary nitriding techniques with the process mediums and the operating temperature range.
Based on the information in Table 1, it can be said that the troubleshooting can be categorized into three very distinct categories which are shown in Figure 1.
Our discussion will address issues usually associated with process problems.
One of the major problems with gas nitriding (Fig. 2) is the understanding of surface preparation in terms of surface cleaning. It cannot be over emphasized how important the pre-cleaning of the surface of the steel is. Surface cleanliness is mandatory and a primary requirement to the success of the procedure. Surface contamination can be seen in many forms (Fig. 3). Once the surface contamination has been dealt with prior to the nitriding process, we can then deal with the procedural problems.
If there is no dissociation occurring at the process temperature, check the content of the ammonia storage system and change over to the fullest storage tank. If it is seen that dissociation is occurring but not at the appropriate requirement, something is happening to reduce flow within the system hardware. This could be caused simply by a restriction in the flow line. However, the restriction could be caused by internal oxidation of the pipework if using plain-steel tubing. The other potential cause of internal oxidation could be located within the process chamber itself. By this, it is meant that the process vessel itself could be oxidized or contaminated.
An often overlooked item is the load-support fixturing, such as baskets, trays and load-support furniture. A very simple remedy is to either shot blast or at least glass-bead blast the surfaces. It is recommended not to use any low-alloy or plain-carbon steel as the load-fixturing or support furniture. This material will act in the same manner as a sponge and take the dissociated ammonia away from the work being processed. The degree of gas dissociation will determine the quality of the nitrided surface metallurgy, so it is most important to ensure that the desired dissociation is being accomplished in order to produce the required surface (Fig. 4).
Surface discoloration is usually attributed to ingress of oxygen or air or a surface contaminant being carried into the process on the surface of the workpiece or the load-support furniture. If oxygen is present in the process chamber, it will usually occur on the cooldown portion of the process cycle. Thus, the sealing arrangement of the process vessel will be suspect. If the part is discolored, there will be no adverse effect on the surface metallurgy. Quite the contrary, there will be an improvement in the corrosion resistance of the steel at the point of the contamination.
Some nitriding procedures are now calling for the deliberate oxidation of the nitrided surface as a corrosion-resistant barrier. Some of the trade names for this procedure are oxy-nitride, nitrox, niox and many others.
If it is seen that the nitrided case begins to peel off, this is usually indicative that decarburization is present on the surface of the component. The decarburization is as a direct result of:
- Insufficient surface stock removal at the pre-nitride machining operations
- Decarburization has occurred at the pre-heat-treatment operation
The component should be considered to be scrap, and it is not recommended to be salvaged.
The surface of the steel is seen to be randomly “dimpled” over the nitrided surface. Once again, this problem can be associated with the presence of surface decarburization.
If the nitrided case is seen to be chipping, particularly at corners, it is usually indicative of what is known as nitride networking (Fig. 5). This is an over-enriched area of nitrogen where very hard and brittle precipitates form with the nitride-forming elements in the steel. This problem will usually occur when the nitriding potential of the process gas is too high. The remedy is to check the gas flow and dissociation and reduce the flow accordingly.
This can occur as a direct result of the presence of a surface contaminant on the component. Investigate the pre-cleaning method prior to nitriding and after pre-machining.
This problem is usually due to a low core hardness that is failing to support the nitrided case. Another possibility is that the formed case is too shallow, and this can be remedied by increasing the case depth. The increase of case depth should be cautioned, however. Check what the application of the workpiece is and what sort of load will be applied to the component.
Salt-bath nitriding can be an economical method providing that both the salt-bath chemistry and cleanliness are maintained (Fig. 6). It is mandatory that the salt-bath chemistry is checked at the commencement of each shift and that the appropriate additions of salt are made to return the bath to its operational strength. Regular desludging of the bath is necessaryto remove precipitated oxide in suspension in the salt from work baskets and suspension wire.
Pre-cleaning is as mandatory as it is with the gas-nitride 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 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 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 metallurgical in nature and are described in the gas-nitriding section. The problems are not usually hardware or program related, although there is no doubt these problems can occur.
In part 2, we will discuss the remaining process of ion nitriding and some additional nitriding troubles to address. Additional nitriding (and other metallurgical) discussions can be found in our blog (www.industrialheating.com/pyeblogs), which has been running on Industrial Heating’s website since 2008.
- Some Practical Aspects of the Nitriding Process, McQuaid H.W. and Ketcham W. J., Transactions American Society of Steel Treaters, 1928
- Adolph Fry. US Patent 1,487,554 18 March 1924
- Practical Nitriding and Ferritic Nitrocarburizing, Chapter 17 Troubleshooting. Pye D., ASM International, 2003