This article discusses the application of Controlled Liquid Ionic Nitrocarburizing (CLIN) processes like TENIFER® and ARCOR® to replace galvanic coatings like chrome, nickel and zinc plating due to excellent corrosion resistance and wear properties. It also highlights economical and environmental advantages of their usage. Due to easy handling, complex plant equipment is not required. The process times are rather short and allow flexible work without building up bigger buffer capacities for the workload.

Introduction

CLIN is a family name of modern and environmentally friendly processes for nitrocarburizing and oxidation of steel and cast iron. Diffusing nitrogen and carbon results in a so-called compound layer, which possesses a nonmetallic character. The outstanding advantage of this edge zone in relation to other coatings is the firm compound diffused on the base material and not applied on the surface. Therefore, they exhibit a very good adhesion, and crack sensitivity is clearly reduced. Depending upon material used, these layers possess hardnesses from 800-1500 Vickers. The compound layer is supported by the underlying diffusion layer. CLIN-treated parts offer eminent protection against wear, seizure, galling, pitting and fatigue.

Process Characteristics

Basically, all kinds of ferrous material – tool steels, mild steels, valve steels, austenitic steels, cast iron or sintered materials – can be nitrocaburized in salt melts without any special preliminary pre-treatment. The process sequence is not complicated. After a short pre-cleaning and preheating in air to 350-400°C (662-752°F), the parts are nitrocarburized in the salt melt, generally for 60-120 minutes. Treating temperature is usually 570-590°C (1058-1094°F). In special cases, lower (480°C) or higher temperatures (630°C) are possible. Water, air, nitrogen, vacuum or an oxidizing cooling bath are used for quenching. Thereafter, the charge is cleaned with hot water in a cascade. For the nitrocarburizing melt, only the following few parameters have to be controlled:

• Chemical composition of the melt
• Treatment temperature
• Treatment time

Salt melts possess an exceptionally high offer of nitrogen in comparison to other treatment media. The nitrocarburizing process starts immediately after immersion into the liquid salt bath. After a few minutes there is already a formation of a compact compound layer. Industrial salts use nontoxic sodium and potassium cyanate as the nitrogen source. Due to reaction on the part surface, alkali cyanate transforms into carbonate whereas the composition of the salt melt only changes slowly. The carbonate decomposition product is recycled into active cyanate directly within the melt by continuously adding the nontoxic, polymeric organic regenerator. Because there is practically no change in volume, no bail-out salt accrues from the desired adjustment of the composition (Fig. 1).

The special characteristic of CLIN-treated parts is the almost mono-phase ε-carbonitride compound layer with very high nitrogen content of 6-11 mass % and carbon content of 0.5-2 mass %. At the usual treatment times of 60-120 minutes, the compound layer reaches 10-20 µm. With increasing alloying proportion the layer growth decreases.

Influence of Post-Oxidation to Corrosion Resistance

CLIN-treated parts are well known for their excellent resistance to wear, pitting and fatigue. Furthermore, the tendency to galling or sticking is remarkably reduced. Corrosion protection is only moderately increased. But if parts are directly quenched into an oxidizing salt melt and followed by an impregnation step (if necessary), corrosion resistance can be dramatically improved. As demonstrated in Figure 2, the average corrosion resistance of an SAE 1035 steel nitrocarburized part shifted from 24 to 810 hours until first sign of corrosion was visible on specimens exposed to a salt-spray test (ASTM B117). In all cases, only single rust spots, never larger areas, were visible when the parts failed.

Figure 3 shows the quality of the compound layer of parts, which passed the complete test time of 1,008 hours. Besides a slight darkening effect on the surface and its pores, the layer itself maintained an excellent condition. This is due to the formation of a thin but compact magnetite layer (Fe₃O₄) on the surface and beneath a predominantly e-carbonitride compound layer. Microsections confirm that the thickness of the magnetite layer is not more than 1 µm. By using liquid oxidizing salts as quenching media, the top of the nitride layer is transformed into magnetite by an exothermic reaction. If the parts are oxidized after cooling down to room temperature, the rise in corrosion resistance will be lower.

Figure 4 shows the salt-spray corrosion resistance of various galvanic processes in comparison with TENIFER (with post-oxidation). Even after a test period of 500 hours, no corrosion attack was visible on the surface of TENIFER-treated piston rods. Depending on the component geometry and roughness, resistance in the salt-spray test reaches up to 500 hours or more. In principle, the corrosion resistance increases with decreasing surface roughness.

Figure 5 shows the corrosion resistance of C45 (SAE 1045) steel samples, which underwent a total immersion test during a period of two weeks (according to DIN 50905, part 4), of various galvanic processes in comparison with TENIFER (with post-oxidation). With an average weight loss of 0.34 g/m² per 24 hours, the TENIFER samples resisted much better than the electrically or chemically plated samples. For the sample coated with 12 µm hard chrome, and even for the 45 µm double-chrome layer, the weight loss was more than 20 times higher in comparison with the TENIFER-treated samples. Only for the triplex layer (37 µm copper, 45 µm nickel, 1.3 µm chrome) is the corrosion resistance comparable with the TENIFER-treated samples.

It is also well known that CLIN processes, like TENIFER and ARCOR when combined with post-oxidation in salt melts, produce far superior corrosion resistance in comparison with other nitrocarburizing processes such as gas or plasma.

Applications

Valves in combustion engines are parts with high thermal-stress, wear and corrosion-resistance demands (Fig. 6). Compared to chrome plating, the manufacturing costs can be reduced by nitrocarburizing because the induction hardening and the final grinding can be omitted. Furthermore, the stem of the exhaust valve need not be made from induction-hardened steel. The valve can be completely manufactured of heat-resistant austenitic steel. More than 250 million valves per year are treated in salt melts. The treatment times for CLIN processes range between 15 and 90 minutes according to specification. Depending upon plant size, the batch size varies from 2,500-4,000 parts. A productivity of less than 1 second per valve is thus accomplished.

The salt-bath nitrocarburizing in combination with oxidizing post treatment is applied more and more for piston rods, hydraulic cylinders or bushings. Materials such as construction steel, unalloyed or low-alloyed steel are used. The required holding time of the salt-spray test is mostly 144 hours without corrosion. In some cases the requirement is 400 hours, which is also obtained. Figure 7 shows a gas spring piston rod, which is employed in several applications, including the automobile and aircraft industry. By substitution of the chrome layer, remarkable cost savings have been achieved. The nitrocarburizing treatment is performed in a fully automated plant. The combination of up to four nitrocarburizing furnaces within one plant enables cycle times of 0.5-0.6 seconds per piston rod.

The driving axle of the windshield wiper was typically zinc plated or galvanized with nickel, but corrosion problems often occurred during operation. Furthermore, on galvanic-coated parts, the helical gearing is relatively soft, so that within the service life it tends to slip. Meanwhile, more than 50 million of these axles are CLIN treated (Fig. 8) per year and are used by almost all leading automotive manufacturers. The thread has a better torsion resistance, which allows the counter nut to be tightened with a higher torque. Depending on the construction and end customer, the corrosion resistance is up to 400 hours in the salt-spray test. The nonmetallic character of the nitrocarburizing layer also leads to a lower friction coefficient at the run of the axle within the aluminum housing. As a result of the high nitrogen available in the salt melt as well as the robustness of the processes, better and more consistent results are achieved under production conditions as compared with other nitrocarburizing processes.

Plant Technology

Meanwhile, it is understood that the heat treatment in liquid salts can be performed in automated, computer-controlled plants. For this purpose, there are open and capsule plants available. The automatic plant shown in Figure 9 is placed in a production facility and treats serial parts for in-house production. A striking feature of this plant is the spotless working environment.

Due to short treatment times, there is no need to create big buffer capacities. The loading of the jigs is performed directly at the machining center. The computerized control system allows the on-line control of the parameter as well as complete batch documentation. Labor costs are reduced to a minimum.

Among loading and unloading and input of the batch data, the user has only to empty the filtration device once or twice a week and to fill up the operating supplies. The plant component is provided with a computerized filling level control, which notifies the user to top up when necessary. The refilling of the salt is performed outside of the capsule in a special apparatus so that the operator has neither to interfere into the heat-treating process nor to work directly at the furnace.

It should also be mentioned that the plant is run waste-water free and is featured with efficient exhaust-air purifying equipment. The prescribed limit values of harmful substances are below specifications. Therefore, there is absolutely no problem of getting authorization for starting new plants.

In addition, an ecological assessment of nitrocarburizing, published by the University of Bremen in 2001, found that from an ecological point of view salt-bath nitrocarburizing (CLIN) is more favorable than gas nitrocarburizing (Fig. 10). If the study is considered objectively, the opinion often expressed that salt-bath technology harms the environment and, therefore, does not conform to present-day environmental philosophy, cannot be confirmed.

Conclusion

CLIN is, in most cases, the ideal alternative for galvanized layers, for distortion-afflicted hardening processes and for gas or plasma nitrocarburizing processes. Applications are also increasing as an alternative to expensive corrosion-resistant steels.

On the basis of the following specific process characteristics, CLIN processes offer excellent reproducibility on a high-quality level.

• No complex pre-cleaning necessary
• Homogeneous and very large offer of nitrogen in the entire melt
• Quick and constant heat transfer
• Only few process parameters are to be considered
• Structure and density of load has only minor effects
• Simple, automatable process engineering

The results achieved under test conditions can usually be easily transferred into series production. IH

The TENIFER® process is known in Europe and German-speaking countries under that name, in English-speaking and Asian countries as TUFFTRIDE®, and in the U.S. as MELONITE®. TENIFER®, TUFFTRIDE® and MELONITE® are registered trademarks of Durferrit GmbH. ARCOR® is a registered trademark from HEF France (CENTRE STEPHANOIS DE RECHERCHES MECANIQUES HYDROMECANIQUE ET FROTTEMENT).

For more information: Dr. Joachim Boßlet, Durferrit GmbH, Mannheim, Germany; e-mail: JBosslet@hef-durferrit.com; web: www.durferrit.com or Danilo Assad Ludewigs, Durferrit do Brasil, Diadema, Brazil; e-mail: danilo@durferrit.com.br; web: www.durferrit.com.br