Salt Bath NitridingEngineering | Diffusion Layer | Hardness | Corrosion Protection | Wear Resistance & Running Properties
Melonite QPQ Process
The Melonite QPQ process is a multi step process that provides a very uniform consistent nitride layer on your components. The first step of the process is a preheat to raise the components surface temperature to about 700 - 800°F in air. The product is then transferred to the MEL 1/TF1 tank containing the liquid Melonite salt to begin the Nitrocarburizing process. The salt melt mainly consists of alkali cyanate and alkali carbonate. It is operated in a pot made from special material, and the pot is fitted with an aeration device. The active constituent in the MEL 1 / TF 1 bath is the alkali cyanate. The Nitrocarburizing process step is conducted in the MEL 1 / TF 1 bath at 896-1166°F, the standard temperature is usually 1076°F. When ferrous alloys are immersed into the bath it creates a reaction with the salt and begins to diffuse nitrogen and a small amount of carbon into the substrate. Because the Melonite process is a liquid nitriding processs, the nitride layer is extremely uniform on inside surfaces as well as outside surfaces. The product is allowed to soak in the MEL 1/TF 1 bath for a predetermined period to achieve the desired cased depth and compound layer thickness. Unlike gas nitriding or gas nitro-carburizing, the substances – MEL 1 / TF 1 and REG 1 - needed for the MELONITE®- and QPQ®-process, do not contain constituents classified as toxic or harmful to the environment. A specially developed cooling bath (AB 1 bath) is used for carrying out the oxidative treatment after salt bath nitrocarburizing. During this treatment, a black iron oxide layer (magnetite) is produced on the surface of the treated parts, which greatly enhances the corrosion resistance. The temperature of the cooling bath is 700 - 800°F. Apart from the oxidative effect, the bath has a positive influence on the dimensional stability of the cooled components.
The parts are then cooled to room temperature and then cleaned (MELONITE®-process).
If the surface of the component after nitrocarburizing is not smooth enough for certain applications, various finishing processes can be used to reduce the roughness. Some proven methods are:
- Lapping with emery cloth grade 360 or finer;
- Polishing or continuous microfinishing with special plastic discs similar to centreless polishing, or on an automated lathe fixed between centre pieces or clamped in;
- Polishing in a vibrating drum. This method is primarily used for small and thin parts;
- Blasting with glass beads;
- Automated blasting with the media;
Mechanical processing can, however, partly reduce the corrosion resistance gained. For this reason, in many cases an oxidative post treatment in an AB 1 bath is carried out after polishing.
As the nitriding process is carried out bath chemistry must be monitored and maintained within strict limits. NCT monitors our bath chemistry daily and by adding specific amounts of the non-toxic regenerator REG 1, the nitriding active constituents levels are maintained in the salt melt and the activity of the MEL 1 / TF 1 bath is kept within very strict tolerances.
This complete process sequence is shown above and is in fact the QPQ®-process. QPQ® comprises MELONITE®-treatment with mechanical processing and oxidative post treatment in a salt melt.
NCT’s Melonite QPQ line is one of the cleanest most advanced lines in the United States. We continue to adhere to our commitment to offering our customers advanced surface treatments with the most up to date technology available. NCT’s production Melonite line is capable of processing large production lots or small individual lots. Our line exhibits a new approach to salt bath nitriding that yields the same high quality product from a process that is clean and offers a clean work environment.
During MELONIZING® a nitrocarburized layer is formed consisting of the outer compound layer (ε-iron nitride) and the diffusion layer thereunder. The formation, microstructure and properties of the compound layer are determined by the base material. The compound layer consists of compounds of iron, nitrogen, carbon and oxygen. Due to its microstructure, the compound layer does not possess metallic properties. It is particularly resistant to wear, seizure and corrosion, as well as being stable almost to the temperature at which it was formed. Compared with plasma or gas nitro-carburizing, compound layers with the highest nitrogen content can be obtained by MELONIZING®. Layers with a high nitrogen content give better protection against wear, and in particular corrosion, than those with a low content.
Depending on the material used, the compound layer will have a Vickers hardness of about 800 to 1500 HV. The graph above shows a comparison of the surface layers produced by various processes and their hardness.
In the metallographic analysis of salt bath nitrocarburized components, that part of the total layer known as the compound layer is defined clearly from the diffusion layer thereunder as a slightly etched zone. During the diffusion of atomic nitrogen the compound layer is formed. The growing level of nitrogen results in the limit of solubility in the surface zone being exceeded, which causes the nitrides to precipitate and form a closed compound layer.
In addition to the treatment parameters (temperature, duration, bath composition), the levels of carbon and alloying elements in the materials to be treated influence the thickness of layer obtainable. Although the growth of the layer is lower the higher the content of alloy, the hardness however increases to an equal extent.
The data shown in the graph above were determined in a MEL1 / TF1 bath at 1076°F. With the usual treating durations of 60-120 minutes, the compound layer obtained was 10-20 μm thick on most qualities of material.
The depth and hardness of the diffusion layer are largely determined by the material. The higher the alloying content in the steel, the lower the nitrogen penetration depth at equal treating duration. On the other hand, the hardness increases the higher the alloying content.
In the case of unalloyed steels, the crystalline structure of the diffusion In the metallographic analysis of salt bath nitrocarburized components, that part of the total layer known as the compound layer is defined clearly from the diffusion layer there under as a slightly etched zone. During the diffusion of atomic nitrogen the compound layer is formed. The growing level of nitrogen results in the limit of solubility in the surface zone being exceeded, which causes the nitrides to precipitate and form a In addition to the treatment parameters (temperature, duration, bath composition), the levels of carbon and alloying elements in the materials to be treated influence the thickness of layer obtainable.
Although the growth of the layer is lower the higher the content of alloy, the hardness however increases to an layer is influenced by the rate of cooling after nitrocarburizing. After rapid cooling in water, the diffused nitrogen remains in solution. If cooling is done slowly, or if a subsequent tempering is carried out, some of the nitrogen could precipitate into iron nitride needles in the outer region of the diffusion layer of un-alloyed steels. This precipitation improves the ductility of nitro-carburized components. Unlike unalloyed steels, part of the diffusion layer of high alloyed materials can be better identified metallographically from the core structure, due to the improved etchability.
But the actual nitrogen penetration is also considerably deeper than the darker etched area visible metallographically. Cooling does not influence the formation of the diffusion layer to any noteworthy extent.
To determine the corrosion resistance of samples and components, a salt spray test (German Standard DIN 50021) and a total immersion test (German Standard DIN 50905/part 4) are often carried out. In the simple salt spray test the parts are subjected to a fine mist of a 5% solution of sodium chloride at 95°F. This test is referred to in the German Standard as SS.
The example above shows the results of a salt spray test conducted in accordance with DIN 50021 SS on hard chrome plated piston rods and MELONITE®-nitrocarburized ones made from unalloyed steel C35. The piston rods were either hard chrome plated to a layer thickness of 15-20 μm or salt bath nitrocarburized for 90 minutes to obtain a compound layer 15-20 μm thick. In the case of the salt bath nitrocarburized piston rods, different variants such as nitrocarburizing plus oxidative cooling, with and without lapping, as well as the QPQ®treatment were tested. After being sprayed for 40 hours, the first corrosion spots occurred on the chrome plated piston rods. After 180 hours the rods showed very heavy corrosive attack over a large area. All nitrocarburized piston rods, however, were still free from corrosion after 40 hours and even after 180 hours the QPQ®-treated piston rods showed no signs of rust.
The graph above shows the corrosion resistance measured in a DIN 50021 SS salt spray test of samples made from material C45 after each stage of treatment.
The graph above shows the respective surface roughness of the samples. In the ground condition, corrosion occurred after only a short time. After 90 minutes salt bath nitrocarburizing followed by oxidation in the cooling bath the corrosion resistance was over 200 hours. Lapping does not change the resistance of the samples. After oxidative post treatment in the cooling bath (25 mins. at 698°F), figures of over 400 hours will be obtained.
The most stringent corrosion test under DIN 50021 is the CASS test in which the test solution additionally contains acetic acid and copper chloride, and the temperature is raised to 122°F. This graph shows the results obtained in a comparison between QPQ®-treated piston rods and hard chrome plated ones with layer thicknesses of 10-12 μm and 30-35 μm.
The test was performed by the Material Testing Institute in Darmstadt, Germany under the following conditions:
Spray solution
5 % NaCl + 0.26 g CuCl2/l;
pH 3.11-3.3;
Temperature 122°F
(1 test cycle = 1 hour).
After QPQ®-treatment, the corrosion resistance is much better than after hard chrome plating. After 16 hours the QPQ® treated samples merely showed corrosive attack on about 10 % of the surface.
For the total immersion test (DIN 50905/part 4) a solution of 3 % common salt and 1 % hydrogen peroxide (H2O2) is used as the corrosive medium. Prior to being dipped into the solution, the samples are degreased.
The table above shows the results obtained on samples made from C45 treated by different surface engineering processes after a total immersion test lasting 2 weeks and carried out in accordance with the Standard.
Wear Resistance & Running Properties
Due to the intermetallic composition of the compound layer, the friction and the tendency to weld with a metallic counter-partner are reduced. Excellent sliding and running properties, as well as greater wear resistance, are the well-known advantages of MELONITE®-treated components. Wear tests and practical application repeatedly confirm the superior wear resistance of salt bath nitrocarburized parts over traditional or induction hardened or hard chrome plated surfaces. In very many cases, the wear resistance of the compound layer is improved still further by an oxidative post treatment. For example, components such as transmission shafts, plug gauges and hydraulic aggregates have a longer service life after MELONIZING® than after hard chrome plating.
The question is often raised as to the wear resistance of the diffusion layer. The data summarized in the graph above shows a comparison of the wear behaviour of rocker arms treated by two different heat treatment processes. It shows the wear on the running surface of the rocker arm which run against a salt bath nitrocarburized camshaft made from chilled cast iron. Although the surface hardness of the case hardened rocker arm was slightly reduced by nitrocarburizing, the much improved wear resistance due to the presence of the compound layer, to approximately 80 hours running time is clearly visible. After 70-80 hours, the wear profile then runs parallel to that of the case hardened only rocker arm, which is attributable to the protection given by the diffusion layer. A spontaneous increase in wear after the loss of the compound layer was not observed. This test again showed very impressively that a high surface hardness does not automatically mean that the protection against wear is also very high. It depends on the respective wear mechanism involved as to how a material or material partnering is to be assessed. Nitrocarburized running partners have proved themselves to be very good under adhesive wear conditions in particular. Their tendency to seize is much lower than that of other surface layers.
The graphs above show the results according to Nieman-Rettig of scuffing load limit tests on gears. These data were established by applying torque to the tooth flank and increasing it until seizure occurred. Nitrocarburizing by the MELONITE®process raised the scuffing load limit of the materials tested by 2-5 times.
Another interesting factor in connection with the wear resistance and running properties is the friction coefficient of the outer surface layer. The interfacial reactions which occur during sliding are not so much determined by the absolute hardness of the running partner but by the material partnering, their microstructural composition, surface geometry and the lubricant used.
To determine the coefficient of friction, tests were carried out in our laboratory on the Amsler machine. The tests were carried out with one disc running at 200 rpm against another disc which was fixed. Both parts were treated equally. To avoid adhesive wear, a load of 5-30 N was applied. Under greater loads the coefficient of friction increased with the load but in the range of 5-30 N it remained constant.
The data summarized in the graph above gives an overview of the friction coefficient of different pairings under dry running conditions, and after being lubricated with oil, type SAE 30. After hard chrome plating, case hardening, and nitrocarburizing followed by water cooling or oxidative cooling in the AB 1 cooling bath, the samples tested had a surface roughness of around 4 μm. Only the surfaces of the QPQ®- treated samples were reduced to a surface roughness of Rm = 1 μm by polishing.
Under dry running conditions, nitrocarburized samples have a much lower coefficient of friction than case hardened or hard chrome plated ones. Due to the oxidation of the compound layer, the coefficient of friction of the nitrocarburized samples increases. In the lubricated condition, the hydrodynamic load supporting film has to be taken into account. With the exception of the QPQ®-treated samples, there is more solid mass because of the surface roughness so that the results presumably lie within the mixed friction range. Under these test conditions, of all variants the QPQ®-nitrocarburized samples had the lowest friction coefficient.The MELONITE®-treatment increases the rotating bending fatigue strength and the rolling fatigue strength of components. These are mainly influenced by: the level of nitrogen in the compound and diffusion layer, the thickness of the diffusion layer and the state of solution of the nitrogen on unalloyed steels.
Furthermore, the state of the microstructure and the strength are to be taken into consideration. Whereas with unalloyed steels the increase in fatigue strength is determined by the rate of cooling, with alloyed materials, however, it has nomentionable effect. The increase in fatigue strength possible after 1-2 hours MELONIZING® is 100 % on parts made from unalloyed and low alloyed steels.In this connection we would like to point out that hard chrome plating reduces the rotating bending fatigue strength of the base material. A similar situation prevails with electro galvanizing. Nitrocarburizing, however, always increases the fatigue strength.
Above is a summary of the results of a fatigue strength test conducted on notched samples made from material C45N. MELONITE®-treatment increased the fatigue strength by more than 50 %. Hard chrome plating, however, reduced the fatigue strength by 20%
