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Carbide Coating Process and Application in Japanese Cold Forging IndustryPrepared
by: Dr. Tohru Arai, Technical Advisor, Presented by: Horst Glaser,
Product Manager, the TD Center, Columbus, Indiana Introduction In the cold forging industry, product quality and die cost are significantly influenced by die performance. Therefore, a great deal of effort has been expended to improve die performance. Unfortunately, trials with various die surface treatments have failed to obtain fruitful results in many countries. In the Japanese cold forging industry, marked success has been made due to the application of two kinds of carbide coating process. One is the titanium carbide coating produced by the chemical vapor deposition process developed in West Germany and then introduced to Japan. Another is a vanadium carbide coating produced by use of molten salt bath. This process, called the TD Process, was developed by Toyota Central Research and Development Labs of Japan, and introduced to various Japanese industries starting in 1971. In this process, materials to be treated are dipped in a molten salt bath under ambient atmosphere and subsequently cooled in oil or salt, for core hardening. The salt bath for this process is comprised of a mixture of borax as the mass transfer agent, plus compounds containing the carbide-forming elements such as vanadium, niobium, titanium, chromium, manganese, etc. Carbides which can be formed by this process include VC. NbC, TiC, Cr7C3, etc. Complex carbide coating is also possible. Furthermore, the kind of carbides to be coated can be varied by changing bath agents by changing the pot retaining them. However, for most metal forming dies, VC coating is recommended for its coated material properties and ease of operation. The process is applicable to almost any kind of carbon-containing metals, cemented carbides and carbon, as substrate materials. Procedures for carbide coating Dies, finish ground and degreased, are immersed in a salt bath for a definite duration to form the carbide layer. A salt bath furnace is simple in structure. No protective atmosphere is needed and agitation is seldom necessary. Subsequently, the parts are quenched and tempered for substrate hardening. Finally they are dipped in hot water for removal of attached salt. The bath temperature is selected around the hardening temperature of the substrate steel ranging from 1600-1900°F or more. The thickness of the carbide layer is determined by the bath temperature, dipping time, and composition of substrate. Therefore, layer thickness can be precisely controlled by bath temperature and dipping time. To form a .00015 - .0004 (4-10 µm) thick carbide layer which is satisfactory for most dies, 20 minutes to 8 hours are needed in connection with the bath temperature and substrate. Hardening of substrate is one of the most important problems to provide a successful application for cold forging dies. The bath temperature should be at the austenitizing temperature for hardening of steels for example, 1850-1875°F for D2, 1750°F for A2. High speed steel dies (such as M2) should be re-hardened after the coating process, if they were treated at a lower temperature such as 1850° F. The process is quite similar to conventional salt bath hardening and gives no rise to pollution problems. From the results of the experiment made for not only test specimens but also practical parts, it has been confirmed that neither cracks, spall, nor peel of carbide layers is observed, even if they are quenched immediately after withdrawal from a molten bath, as long as optimum treating condition, surface roughness and substrate composition are kept. As the process temperature is similar to the hardening temperature of substrate steels, distortion is the largest concern. However, appropriate countermeasures can minimize distortion. The basic method is to keep preliminary hardening and coating temperature the same. This eliminates size change caused by change of crystal structure, such as from pearlite to martensite and will show less distortion than that of ordinary hardened steels, without a marked decrease in toughness. Thus, the distortion problem can be dissolved to such degree that the punches with the tolerance of .0004 - .0008 (10-20 µm) are produced without meticulous care. Surface roughness of dies is unchanged by the process, except below about 1 µm, thus requiring no further finishing for ordinary applications for piercing punches and cropping knives, etc. However, mirror finishing (5 RMS) assures successful application to the dies for extrusion, ironing, etc. in which severe galling problems are encountered. In comparison with the CVD carbide coating process in which hydrogen gas containing TiCL4 and hydrocarbon is employed, the TD Process needs simpler equipment and has fewer required process controls which results in less coating thickness and adhesion variations.
Properties of coated materials The layers formed are pore free (as shown in fig. 1 which shows the vanadium carbide layer formed on tool steel). Although the distinct boundary line is clearly observed between the layer and the steel substrate, high processing temperature accelerates mutual diffusion of atoms between the layer and the substrate, providing such a large bonding strength that the layer will not exfoliate in severe service, such as cold forging. This process provides a layer, consisting of carbide only, free from a binder phase, and it scarcely decreases the toughness of the substrates. Consequently, carbide coated steels demonstrate the surface characteristic properties inherent with carbides, such as high hardness, excellent wear, seizure, oxidation and corrosion resistance, and the internal strength inherent in the substrate steels.
The hardness of the carbide coating is a function of the carbide forming element. The layer has a very high hardness, (3200 to 3800 Vickers), not only at room temperature but also at high temperature (as shown in fig. 2). 3.2 Wear resistance Fig. 3 shows the results of a simulation test made by H. Kudo[3]. In the test, a .048mm thick and .80 wide low carbon steel strip was fed to the tester as the workpiece model, the surface of which was chipless scratched widthwise by a tool edge model at longitudinal intervals of about .119. Vanadium carbide coated die steel showed practically no wear even after 2,000 scratches, as well as cemented carbide. Vanadium carbide coating reduced the Ft/Fn value, as a measure of the frictional resistance.
Excellent resistance of carbide layers have been recognized in metal forming tests such as shearing of steel sheets and coining of steel sheets and various well-known wear tests.
Backward piercing of 1010 steel cylindrical specimens was carried out with hardened and vanadium carbide coated M2 punches. Resistance to galling was evaluated with appearance of punches withdrawn after indenting, to certain depths. Severe galling was observed on the lane of hardened punches indented, only .200 into unphosphated specimens. On the other hand, vanadium carbide coated punches were indented into unphosphated specimen, 1.5" deep, without any sign of galling. The specimen formed with the vanadium carbide coated punch showed metallic brilliance, because of absence of lubricant. Its surface finish was far superior to that of a phosphate coated specimen. Fig. 4 shows the comparative appearances of guide shoes used in a transverse wedge rolling test, in which an ordinarily hardened shoe and a vanadium carbide coated shoe were used for rolling of carbon steel (1045) heated at 2192°F. No severe pick-up was observed on a vanadium carbide coated shoe. A coating of vanadium carbide considerably improved the resistance to seizure in many other tests, including the ring compression test and the simulation tests for sheet metal forming and forging.
Carbide coating does not reduce the toughness (as shown in Fig. 5). Therefore when steels with high toughness are used as substrates, very tough carbide coated steels can be obtained. According to the results of laboratory tests and industrial applications decrease in toughness was not recognized also in the case, in which pre-hardening treatment had been employed on high speed steel to minimize the size change due to the change of crystal structure. 3.5 Miscellaneous properties Deformation of the substrate could crack the carbide layer. The measured critical strain to induce micro cracks, invisible with naked eyes, was roughly 0.8 -1.4% under tensile stress and 0.6 - 1.0% under compressive stress, depending on the kinds of carbide, the thickness of carbide layer and substrate composition. When exposed to air for a long time at a temperature at approximately 932°F, vanadium carbide layers are oxidized significantly. However, application of such carbide coating to hot forging dies for steels would provide successful results, since the temperature of dies can exceed 932°F only while the dies are in contact with heated blanks. Repetition of cyclic heating to 1292°F by use of high frequency current and cooling in water, provided lesser amounts of heat checking on a carbide coated specimen compare to an ordinarily hardened specimen made of hot working die steel. Industrial application of TD Process to cold forging dies Due to the surprising advancement in wear- and scuffing resistance of tool steels and cemented carbides, the TD Process has been extensively evaluated in the Japanese cold forging industries and elsewhere in the world. TABLE 1: Practical Examples of Life Increases Through Application of TD Process
Table 1 displays a summary of typical life increase reported by users of the process. The table displays the process has a broad range of applications from feeding attachment to the forming of punches and dies, and provides considerable performance improvement. The largest industrial application is punches for piercing and extrusion. Application on various die steels is also evident. In many cases, vanadium carbide has been coated on cold working die steel and high speed steel. However, vanadium carbide on cemented carbide dies is also utilized with notable improvement of die life. The ability to change from cemented carbide to cold working die steel or high speed steel, and from high speed steel to cold working die steel, has been realized in various punches and dies with the application of the TD Process. The process is also successfully utilized in the forging of stainless steel, ball bearing steel and non-ferrous metals. Dies used in warm forging of stainless steel are also treated by this process. Vanadium carbide coating is continuously applied even for hot forging dies. Life increases several times over are not uncommon, due to elimination of galling problems. As a result, a great saving on die consumption has been achieved in Japanese industry. Additional advantages, other than saving on die consumption, were in some cases far greater. These were:
Some of these profits are explained by the following concrete examples
Example 1: Piercing punches Vanadium carbide coating was applied to piercing punches used for making automobile parts made of 1015 steel, as shown in fig. 6. Conventionally hardened M2 punches lasted for less than 1,000 shots. The cemented carbide punches lasted Tor 25,000 ~ 60,000 shots. Life of vanadium carbide coated D2 punches was 34,000 ~ 77,000 shots. Furthermore, vanadium carbide coating considerably improved the surface finish of products.
Example 3: Extrusion punches Vanadium carbide coating has been successfully applied to backward extrusion punches for indenting to .600 depth into 304 stainless steel blanks, .680 in diameter, heated at 932°F. Hardened M2 punches lasted an average 2,500 shots. Vanadium carbide coated punches made of the same steel lasted an average of 3,200 shots. Example 4: Extrusion punches
Vanadium carbide coating improved punch life used for forward-backward extrusion of aluminum, as shown in Fig. 9. Hardened M2 punches only produced about 1,000 parts, because of severe galling. However, vanadium carbide coated punches lasted for about 13,000 parts. Dies used for upsetting of 1015 steel were vanadium carbide coated. Conventional hardened M2 dies lasted for 20,000 ~ 30,000 shots due to severe galling. Vanadium carbide coated dies made of the same steel were still serviceable after 95,000 ~ 100,000 shots since galling was very slight. Example 5: Ironing die The galling problem was surprisingly improved by the use of ironing dies, for making automobile parts from 1010 steel. Only a few hundred parts were produced with an ordinarily hardened M2 die. Replacement of cemented carbide dies (15% Co) increased die lives to 2,000 ~ 4,000 parts. Application of vanadium carbide coating on D2 dies prolonged die lives to average 20,000 parts. Conclusion The TD Process considerably improves wear and galling resistance of steel dies and cemented carbide dies for cold forging, without complex processing and complicated equipment. Since the wear and galling resistance of carbide coated dies is far superior to that of conventional dies, namely, those which are hardened and surface treated by other processes such as plating and nitriding, the TD Process has been accepted as a breakthrough technique which solves galling problems in cold forging. The TD Process has already brought, and will be providing, much benefit to Japanese industry as well as cold forging industries throughout the world, and is now available in the US market. |
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3.1
Layer hardness
3.3
Resistance to scuffing
3.4
Toughness
Example
2: Extrusion punches Vanadium carbide coating has been
applied with satisfactory results to combined forward-backward extrusion,
as shown in fig. 7. Hardened M2 punches lasted for an average of
32,000 shots. Nitriding, either by tufftriding or ion nitriding,
prolonged punch life to 52,000 ~ 72,000 shots. Vanadium carbide
coated M2 punches lasted for 90,000 ~ 110,000 shots and remarkably
improved the surface roughness of products (fig. 8).
