10.10232FB-MSAT.0000048834.48163.2e LOW-TEMPERATURE SALT BATH NITRIDING OF STEELS

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Metal Science and Heat Treatment

Vol. 46, Nos. 7 – 8, 2004

UDC 621.785.532

LOW-TEMPERATURE SALT BATH NITRIDING OF STEELS K. Funatani1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 12 – 17, July, 2004. Nitriding technology has gone a long way, from the old gas nitriding to the relatively recently developed plasma nitriding. The latter has replaced the process of “soft nitriding” in the automotive industry based on nitrocarburizing in cyanide salt baths. It seemed that the high toxicity of the initially used compositions for soft nitriding (Tufftride or Tenifer) should have eliminated salt baths from the industry. However, they are still rather widely used. The replacement of old compositions by nontoxic ones has solved fundamental problems of environment protection. Low-temperature nitriding technology also advanced considerably. Salt bath nitriding is a very active process, more intense than that of gas nitriding and nitrocarburizing including the processes of plasma nitriding. The reactivity of the nitriding medium and the final efficiency of the process with allowance for the cost of the equipment have to be taken into account. An additional advantage of salt bath nitriding is the possibility of treatment of stainless maraging steels. The present work is devoted to comparison of the processes of treatment in a nontoxic salt bath and in a gas medium and discussion of the advantages of nitriding in nontoxic salt baths.

process control technology in 1975. Nitriding in salt baths was replaced by gas nitriding for virtually all the parts, and only some items, like exhaust valves, were still treated in salt baths but at subsidiary plants. Though less than half of the earlier used baths for “soft” nitriding based on cyanides are still in operation, there is a tendency for using cyanate baths, which solves virtually all of the environmental problems. The development of a low-temperature process is a recent advancement in slat bath technology. Low-temperature treatment of aluminum extrusion dies and of other forming dies at 480°C is acquiring a wide use and ensures enhanced wear resistance without deterioration of hardness even in repeated nitriding. In addition, low-temperature nitriding reduces the distortion of shafts and crankshafts for automotive applications. One more advantage of salt bath nitriding is the possibility of control of the chemical composition of the bath and of lowering the operating temperature by changing the chemical composition, which promises wider and more effective use of salt bath treatment including the nitriding of stainless and maraging steels.

INTRODUCTION As distinct from the long-used process of gas nitriding, which yields a layer of Fe2N nitrides on the surface [1], the Tenifer-Tufftride process, also known as “soft nitriding” [2], yields a tough and wear-resistant layer. It has been widely used in the automotive industry since BMW started to use salt bath nitriding for treating crankshafts and other parts. The diffusion layer lying under the layer of chemical compounds exhibited high wear and seizure properties and enhanced fatigue resistance. The Toyota Motor Company started the production of automobile parts in 1963 and installed the world’s largest automated salt bath nitriding lines in the engine components plant. However, the requirements on environment protection made the company replace the treatment of engine parts in cyanide baths by soft nitriding in a gas medium. However, salt bath nitriding was still used for treating parts fabricated from special steels, the gas nitriding of which gave unsatisfactory results. The problems of toxicity of cyanide baths were solved by creating nontoxic cyanate-bearing baths in 1970 [3, 4]. Difficulties with the nontoxic baths arose in connection with control of the nitriding capacity, and in many cases the nitrocarburizing was performed by the method of “soft” nitriding in continuous gas carburizing furnaces. In order to improve the process of “soft” gas nitriding, Toyota pursued the goal of creating a compound layer with equivalent chemical composition and final properties and installed a novel 1

COMPOSITION OF SALT BATHS Cyanide-Base Salt Baths Tenifer salt baths contained a high percentage of potassium and sodium cyanides, which are very toxic materials.

IMST Institute, Nagoya, Japan.

277 0026-0673/04/0708-0277 © 2004 Springer Science+Business Media, Inc.

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K. Funatani

TABLE 1. Nitriding Methods, Materials, and Characteristics of Diffusion Layer Method

Tufftride TF1

Thickness of com- Thickness of difpound layer, mm fusion layer, mm

Temperature, K

Steel

853

1015

e, 13

800

1045

e, 13

780

34Cr4

480

The same ²

[2, 3]

² ² 843

X210Cr12

e, 10 –

1015

e, 12

780

843

SCM435

e, 8

171

“Soft” nitriding in gas medium

843

SS2250

g¢, e(?)

353

[7]

“Soft” nitriding in gas medium

793

38CrMoAl

g¢, e : (5)

78 – 97

[8]

40Cr

g¢, e : (4)

63 – 80

Gas nitriding

773

SAE9254

49

[9]

793 (pulsed) 793 (DC)

722M24

(e, g¢ : 5) 12 h

72

[10]

100 110 46 100 200

[11]

² + 0.1Y

(–) (–) (–) (–) (–)

² + 0.1Ce SKD61

(–)

215

e : 1.5

150

Tufftride NS1

Plasma nitriding Plasma nitriding

²

Reference

833 813 793 823 823 823

Low-temperature salt bath nitriding (Palsonite)

753

² En40B En19 Nirtaps 36CrMo

160

² SCM435

e : 4 + (CrN)

106

753

e:4

141

843

²

e

200

843

During the process the cyanides are oxidized due to aeration, and nearly half of them are converted to cyanates. Oxygen-saturated cyanates are active compounds and react with the surface of a steel part forming e- and g¢-nitrides in salt bath nitriding at 570°C. This layer of compounds is not as brittle as Fe2N (x), and its formation is used effectively for raising the wear and seizure resistances. Nitrogen also diffuses into the layers lying below the layer of chemical compounds and contributes into the growth in fatigue resistance. This double effect of nitrogen made the process very popular starting from the 1960s [2]. However, the process in such baths had one more disadvantage, i.e., it was hard to control the chemical composition of the bath. In addition, the subsequent neutralization of the waste salt and the disposal of the rinse water were hazardous for the environment. The ecological problems due to the use of toxic compositions hindered the development of the process, and in many cases it was replaced by gas nitriding.

[6]

[12]

[6] [6]

plified control of the process with the help of the developed regenerator. Nitriding in new salt baths reduced the treatment cost by 30% as compared to the process in the old cyanide-base baths. However, the chemical activity of cyanate-base baths was more difficult to control. For this reason some time passed before the new technology employing cyanate-base baths was installed in heat treatment plants. Low-temperature treatment below 480°C was set up by Nihon Parkerizing in 1973 for nitriding dies for hot deformation and extrusion [5]. The process temperature of 480°C made it possible to prevent a decrease in the hardness during the treatment process. Low-temperature nitriding of extrusion dies was repeated without deterioration of the hardness. Further studies of this group of tools made it possible to lower the temperature to 430 – 440°C and widened the use of nitriding in active salt baths. Reactivity of Salt Baths and Diffusion Rate

Cyanate-Base Salt Baths The necessity for novel chemical compositions providing the same nitriding capacity resulted in a successful solution, i.e., creation of cyanate-base baths bearing no cyanide [3, 4]. The new compositions reduced the problems of salt collection for controlling its reactivity to a minimum and sim-

The reactivity of salt baths combined with their high heat capacity and direct contact with the steel surface create better possibilities for nitrogen penetration than in gas nitriding. This especially concerns cases where the gas process is conducted at reduced pressure and requires additional measures for improving the reactivity.

Low-Temperature Salt Bath Nitriding of Steels

279

Such treatment is recommended for steels TABLE 2. Results of Nitriding of Stainless and Tool Steels with a high content of alloying elements, Thickness Treatment time, Method Temperachromium in particular, which form a protecSteel of diffusion layer thickness, Reference of treatment ture, K and phases layer, mm/h tive oxide film on the surface and delay the beginning of nitrogen diffusion. The process Gas nitriding 773 304 22.37 20 h [13] is accelerated by acid etching, shot blasting, 803 26.85 20 h or cathode ion sputtering. The energy of 843 34.68 20 h plasma, ionization, and some other treatment 873 36.91 20 h techniques, for example nitriding with the use Plasma nitriding 803 SKD61 51.96 [14] 3 h – 100 mm of radicals, can raise the reactivity. 853 NCF601(60Ni) 10.4 (2.9) 3 h – (CrN) [15] The data on the earlier developed Tuff- Gas nitriding SUH35 56.6 0.5 h – 49 mm tride process stimulated its wide use at plants NCF47W 21.2 2 h – 25 mm engaged in mechanical treatment of car parts 843 304 730 1 h – CrN, [16] and in shipbuilding. Today, the parameters ob- Tufftride (TF) g (C + N) tained by the DEGUSSA Company in the development of the Durferrit process have be843 402J2 50 1h come a goal to be reached in other nitriding 843 SKD61 84.9 (5 – 7) processes. However, in order to raise the effi- Palsonite 753 304 169.7 [16] 2 h – CrN, e, ciency of nitriding and reduce its cost, it is g (N) necessary to consider the problems connected 753 402J2 35.4 2h with the chemical efficiency of processes con- Gas nitriding 703 304 (3) 1h [17] ducted in salt baths without hazard to ecology. 833 304 (40) 1h The diffusion rate in nitriding depends on the amount of alloying elements, especially chromium. It can be seen from Table 1 that that nitriding at a temperature below 480°C is very effective the thickness of the diffusion layer depends on the type of the for dies used in hot deformation is an even more important alloying elements. However, the main characteristic of nitrofeature. In addition, the procedure of formation of hardened gen diffusion in carbon steels C15 and C45 is its rate. The layer can be repeated up to ten times without impairing the diffusion rates in a- and g-phases and in e and g¢ compounds hardness. differ, but the nitrogen potential is the highest in salt bath We have already mentioned that nitriding is used for treatment. As follows from Tables 1 and 2, in this case the fitreating some automotive parts (crankshafts, camshafts, and nal result is obtained in the shortest time. various retainers). Such parts as valve springs in high-perforComparing the duration of the treatment as a function of mance engines should possess an especially high heat resisdiffusion rate for various variants of nitriding (see Table 1) tance due to operation at a high temperature [9, 18, 19]. Niwe will see that processes occurring in salt baths are pretrogen is a very effective element for raising the high-temferred. Even at the lowest temperature (about 400°C) a salt perature strength, especially for high-performance parts and bath provides the highest nitriding activity and minimum dutools. ration of treatment. It is important that nitrided stainless steels do not lose It cannot be doubted that safety and preservation of the their corrosion resistance in operation. Parts used in the food ambient are very important, but modern processes in baths industry and biotechnology should also combine high corrowith cyanates do not yield toxic emissions, and a nitriding sion resistance with wear resistance and hardness [20]. plant today is free of waste water. APPLICATIONS As compared to the original Tufftride process, the temperature range of nitrocarburizing in salt baths has widened considerably, which makes it possible to treat parts from automotive valves to tiny pieces for electronics and interdigital transistors. The opinion on the environmental hazard of traditional cyanide-bearing baths has stimulated the use of various gas nitriding processes including reduced-pressure nitriding. Processes conducted at a lower temperature than that used in the traditional Tufftride process and in “soft” gas nitriding have the advantage of minimum distortion. The fact

ADVANCED PROCESSES Gas Nitriding The efficiency of nitriding is controlled by the nitrogen potential Np, which is affected by the conditions arising on the surface of the treated material. The final state of the surface and the chemical composition of the material considerably influence the reactivity of the surface. This primarily concerns chromium-bearing materials that have a coat of chromium oxides on the surface, which hinders the interaction with nitrogen-bearing gases. For this reason many researchers have studied various methods for increasing the

280

surface reactivity, such as shot blasting, oxidizing, or treatment with chlorine- or fluorine-bearing substances. Nitriding at reduced pressure with ionization and without it has some use, but such processes take more time than plasma nitriding. On the contrary, enhanced-pressure nitriding promotes a certain increase in the nitrogen potential and is applied at an industrial scale. In any case, the addition of carbon increases the surface reactivity and results in the formation of e-carbonitride, and a carbon-bearing saturating medium can accelerate the reactions. Specialists in nitriding argue on the interrelation between the formation of the layer of chemical compounds and its properties and the presence or absence of carbon. The general term “nitriding” applied to any process of the kind should be differentiated into nitriding as such, “carbonitriding,” and “nitro-oxinitriding” because each of the processes is characterized by specific duration and by resulting properties. Plasma Nitriding Plasma can accelerate the reactions by increasing the energy of nitrogen ions and additionally activating them as a result of cathode sputtering. The traditional ionization processes have been improved by the development of methods of plasma nitriding with a through case (TC) or active screen (AS) that raise the efficiency and quality of the process. Plasma nitriding is ecologically safe, and various kinds of installations use the process primarily in Europe [10]. Plasma nitriding has the advantage that it is easy to stop the process, in contrast to gas nitriding and especially to salt bath nitriding. The requirements that commercial processes should be performed at reduced temperature, yield durable products, and be low-cost are becoming obligatory. In this connection, processes ensuring a hardened layer at medium and low temperatures, like nitrocarburizing and nitriding, have become especially attractive. Salt Bath Nitriding and Carbonitriding Cyanate salt baths have the same or an even higher nitrogen potential, and the developed method for controlling the composition of baths eliminates virtually any disadvantage of the process. The high reactivity of salt baths based on cyanates is preserved even when the quality of the salt worsens, and modern regeneration techniques are quite economical. Cooling after nitriding in an AS140 salt bath is used for dissociation of – CN, and finishing oxidizing of the surface is performed for raising the corrosion resistance due to formation of an oxide layer. Low-Temperature Nitriding and Carbonitriding Low-temperature processes have become possible due to appropriate choice of the chemical composition and ad-

K. Funatani

vanced methods of chemical control. Diffusion of carbon or nitrogen in plasma nitriding ensures formation of a hardened layer at a low temperature. Nitriding without formation of a layer of chemical compounds is effective for chromium-alloyed stainless steels and can also be recommended for improving the properties of spring and maraging steels. It seems that the use of such process for treatment of such steels will be widened. CONCLUSIONS 1. Gas nitriding and nitrocarburizing, including plasmaassisted processes, are progressive methods, and their further study and advancement should raise the productivity. Successful realization of gas and plasma nitriding depends on appropriate measurement of the composition of the atmosphere and of the temperature. 2. Plasma nitriding presents practical interest but requires solution of the problems of reduction of the cost of the equipment and of increasing the efficiency. 3. Salt bath nitriding is more expedient for many grades of steel than gas nitriding, including the plasma-assisted methods. 4. The use of cyanate salt baths has solved environmental problems due to the elimination of the operations of waste and water disposal; such baths preserve high reactivity in nitriding and carbonitriding. 5. Nitriding in salt baths at a temperature below 480°C yields a thin hardened layer without formation of a layer of chemical compounds. REFERENCES 1. A. Fry, Stahl und Eisen, 43(40), 1274 – 1270 (1923). 2. B. Finnem, Bad und Gasnitrieren, Vol. 18, Betriebsbuecher Carl-Hausner-Verlag, Muenchen (1965). 3. Tufftride Information 15. DEGUSSA Durferrit Abteilung. 4. G. Wahl, Durferrit — Technical Information. Ecology and Economy of Salt Bath Processes. 5. Y. Matsumura and T. Nakaizumi, “N – A hard treatment,” in: Technical Report of Nihon Parkerizing Co. Ltd., No. 1 (1988), pp. 37 – 44. 6. H. Eiraku, K. Shinkawa, Y. Yoneyama, and M. Higashi, “Characteristics of Palsonite (low temperature salt bath nitriding),” JSHT Conf., No. 1, 49 – 50 (1998). 7. E. A. Mattison, K. Frisk, and A. Melander, “Microstructure evolution during the combination hardening process of nitriding and induction hardening,” in: 5ASM-HTSE Europe (2002), pp. 209 – 219. 8. W. Junyi, P. Lin, and Z. Hul, “Effect of rare earth on ionic nitriding process,” in: 1st Conf. Heat Treatment of Materials, May (1998), pp. 57 – 61. 9. S. Kondo, Y. Izawa, O. Nakano, S. Uchida, and M. Onoda, “Influence of white layer produced by gas nitriding on fatigue strength of compressive springs,” J. JSHT, 36(1), 34 – 40 (1996). 10. J. Georges, “TC plasma nitriding,” in: 12th IFHTSE Melbourne, Australia (2000), p. 229; Heat Treatment Met., No. 2, 33 – 37 (2001).

Low-Temperature Salt Bath Nitriding of Steels

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