Liquid Nitriding

Liquid nitriding or nitriding in a molten salt bath employs the same temperature range as gas nitriding, which is 510 to 580°C. The case hardening medium is a molten, nitrogen-bearing, fused-salt bath containing either cyanides or cyanates.

Unlike liquid carburizing and cyaniding, which employ baths of similar compositions, liquid nitriding is a sub-critical casehardening process; thus, processing of finished parts is possible because dimensional stability can be maintained. Also, liquid nitriding adds more nitrogen and less carbon to ferrous materials than that obtained through higher-temperature diffusion treatments.

The liquid nitriding process has several proprietary modifications and is applied to a wide variety of carbon, low-alloy steels, tool steels, stainless steels, and cast irons.

Liquid Nitriding Applications

Liquid nitriding processes are used primarily to improve wear resistance of surfaces and to increase the endurance limit in fatigue. For many steels, resistance to corrosion is improved. These processes are not suitable for many applications requiring deep cases and hardened cores, but they have successfully replaced other types of heat treatment on a performance or economic basis.

In general, the uses of liquid nitriding and gas nitriding are similar, and sometimes identical. Gas nitriding may be preferred in applications where heavier case depths and are required. Both processes, however, provide the same advantages: improved wear resistance and antigalling properties, increased fatigue resistance, and less distortion than other case-hardening processes employing through heating at higher temperatures.

The degree to which steel properties are affected by liquid nitriding may vary with the process used and the chemical control maintained.

Liquid Nitriding Systems

The term liquid nitriding has become a generic term for a number of different salt processes, all of which are performed at subcritical temperature. Operating at these temperatures, the treatments are based on chemical diffusion and influence metallurgical structures primarily through absorption and reaction of nitrogen rather than through the minor amount of carbon that is assimilated.

Although the different processes are represented by a number of commercial trade names, a typical commercial bath for liquid nitriding is composed of a mixture of sodium and potassium salts.

• The sodium salts, which comprise 60 to 70% (by weight) of the total mixture, consist of 96.5% NaCN, 2.5% Na₂CO₃, and 0.5% NaCNO.
• The potassium salts 30 to 40% of the mixture, consist of 96% KCN, 0.6% K₂CO₃, 0.75% KCNO, and 0.5% KCl. The operating temperature of this salt bath is 565°C. This bath is widely used for nitriding tool steels, including high-speed steels, and a variety of low-alloy steels, including the aluminum-containing nitriding steels.
• Another bath for nitriding tool steels has a composition as follows: 60 to 61% NaCN, 15.0 to 15.5% K₂CO₃, and 23 to 24% KCl.

Several special liquid nitriding processes employ proprietary additions, either gaseous or solid, that are intended to serve several purposes, such as accelerating the chemical activity of the bath, increasing the number of steels that can be processed, and improving the properties obtained as a result of nitriding.

Cyanide-free liquid nitriding salt compositions have also been introduced. However, in the active bath, a small amount of cyanide, generally up to 5.0%, is produced as part of the reaction. This is a relatively low concentration, and these compositions have gained widespread acceptance within the heat-treating industry because they do contribute substantially to the alleviation of a potential source of pollution.

Three processes, liquid pressure nitriding, aerated bath nitriding, and aerated low-cyanide nitriding, are described in the sections that follow.

Liquid Pressure Nitriding

Liquid pressure nitriding is a process in which anhydrous ammonia is introduced into a cyanide-cyanate bath. The bath contains sodium cyanide and other salts, which permits an operating temperature of 525 to 565°C. Because the molten salts are diffused with anhydrous ammonia, a new bath does not require aging and may be put into immediate operation employing the recommended cyanide-cyanate ratio, namely 30 to 35% cyanide and 15 to 20% cyanate. Except for dragout losses, maintenance of the bath within the preferred ratio range is greatly simplified by the anhydrous ammonia addition, which serves continuously to counteract bath depletion.

Aerated Bath Nitriding. Aerated bath nitriding is a process in which measured amounts of air are pumped through the molten bath. The introduction of air provides agitation and stimulates chemical activity. The cyanide content of this bath, calculated as sodium cyanide, is maintained at preferably about 50 to 60% of the total bath content, and the cyanate is maintained at 32 to 38%. The potassium content of the fused bath, calculated as elemental potassium, is between 10 and 30%, preferably about 18%. The potassium may be present as the cyanate or the cyanide, or both. The remainder of the bath is sodium carbonate. The outer compound layer provides wear resistance, while the diffusion zone improves fatigue strength.

It should be noted that only chromium-, titanium-, and aluminum-alloyed steel respond well to conventional bath nitriding. Plain carbon (nonalloyed) steels respond well to aerated bath nitriding but not to conventional nitriding. Thus, the aerated process should be specified for nitriding all plain carbon steels because test data show that plain carbon steel will not develop adequate hardness in a nonaerated nitriding bath. However, the full effect of nitriding will not be realized unless alloy steel is selected.

Aerated Cyanide-Cyanate Nitriding. Another aerated process for liquid nitriding is a high-cyanide, high-cyanate system that is proprietary. The cyanide content of the fused salt is maintained in the range of 45 to 50% calculated as potassium cyanide, and the cyanate content is maintained in the range of 42 to 50% calculated as potassium cyanate. Makeup salt consists of a precise mixture of sodium and potassium cyanides that are oxidized by aeration to the mixed cyanate. The ratio of sodium ions to potassium ions is important in duplicating the integrity of the compound zone and the diffusion zone.

Aerated Low-Cyanide Nitriding. Environmental concerns have led to the development of cyanide-free processes for liquid nitriding. In these processes, the base salt is supplied as a cyanide-free mixture of potassium cyanate and a combination of sodium carbonate and potassium carbonate, or sodium chloride and potassium chloride. Minor percentages of cyanide develop during use in these compositions. The problem is overcame in one process, by quenching in an oxidizing quench salt that destroys the cyanide and cyanate compounds (which have pollution capabilities) and produces less distortion than that resulting from water quenching.

Effects of Steel Composition. Although the properties of alloy steels are improved by the compound and diffusion layers, relatively greater improvement is achieved with plain carbon steels of low and medium carbon content. For example, the improvement in fatigue strength of unnotched test bars of 1015 steel nitrided by this process for 90 min at 565°C and water quenched (to further enhance fatigue properties) is roughly 100%. Improvement obtained with similarly treated test bars made of 1060 steel is about 45 to 50%.

Operating Procedures

Among the important operating procedures in liquid nitriding are the initial preparation and heating of the salt bath, aging of the molten salts (when required), and analysis and maintenance of salt bath composition. Virtually all steels must be quenched and tempered for core properties before being nitrided or stress relieved for distortion control. So, prior heat treatment may be considered an essential part of the operating procedure.

Prior Heat Treatment. Alloy steels usually are given a prior heat treatment similar to that preferred for gas nitriding. Maintenance of dimensional and geometric stability during liquid nitriding is enhanced by hardening of parts prior to nitride treatment. Tempering temperatures should be no lower than the nitriding temperature and preferably slightly above. Depending on steel composition, the effect of core hardness is similar to that encountered in gas nitriding.

Starting the Bath. Case-producing salt compositions may vary with respect to manufacturers, but they are basically sodium and potassium cyanides, or sodium and potassium cyanates. Cyanide, the active ingredient, is oxidized to cyanate by aging as described below. The commercial salt mixture (60 to 70% sodium salts, 30 to 40% potassium salts) is melted at 540 to 595°C. During the melting period, a cover should be placed over the retort to guard against spattering or explosion of the salt, unless the equipment is completely hooded and vented.

Externally versus Internally Heated Salt Baths. Salt baths may be heated externally or internally. For externally heated salt baths, startup power should be limited to 37% of total capacity until signs of melting are apparent on all sides of the salt bath. For internally heated salt baths, natural gas flame torches having a moderate flame are effective in melting a pool of molten salt for a conductive path between electrodes.

Aging the Bath. Liquid nitriding compositions that do not contain a substantial amount of cyanate in the original melt must be aged before use in production. Aging is defined as the oxidation of the cyanide to cyanate. Aging is not merely a function of temperature alone, but also depends on the surface-to-volume ratio of the molten bath. It is the surface air (oxygen)-to salt contact that oxidizes cyanide to cyanate.

Molten salts in conventional baths should be aged by being held at 565 to 595°C (1050 to 1100°F) for at least 12 h, and no work should be placed in the bath during the aging treatment. Aging decreases the cyanide content of the bath and increases the cyanate and carbonate contents. Before nitriding is begun, a careful check of the cyanate content should be made. Nitriding should not be attempted until the cyanate content has reached at least the minimum operating level recommended for the bath.

The ratio of cyanide content to cyanate content varies with the salt bath process and the composition of the bath. The commercial NaCN-KCN bath, after aging for one week, achieves a ratio of 21 to 26% cyanide to 14 to 18% cyanate. The bath used in liquid pressure nitriding operates with a cyanide content of 30 to 35% and a cyanate content of 15 to 20%. The aerated bach is controlled to a ratio of 50 to 60% cyanide to 32 to 38% cyanate. The aerated noncyanide nitriding process is controlled to a ratio of 36 to 38% cyanate to 17 to 19% carbonate removed from all baths. In normal operation, overheating of any bath (above 595°C) should be avoided.