Friction, Lubrication, and Wear Technology (1997) Part 11 potx

The results by Thomas (Ref 43) also showed a strong correlation between wear resistance and alloy content (Fig. 17). The low-alloy steel (No. 5 die steel, which is equivalent to steel number 1 in Fig. 15, and 8(a) and Table 8(b)) had relatively poor wear resistance when compared to the 5% Cr hot-work die steel (H12) and the 12% Cr steel. This was true irrespective of the hardness level to which the steels were tempered and even, to a certain degree, of the type of workpiece material. Fig. 17 Influence of initial die hardness on wear of die steels. The wear index is defined as average cross- sectional area of wear depressions in dies. No. 5 die steel: 0.6 C, 0.3 Si, 0.6 Mn, 1.5 Ni, 0.6 Cr, 0.25 Mo; 5% Cr steel: 0.33 C, 0.3 Si, 1.0 Mn, 5.0 Cr, 1.5 Mo, 1.5 W, 0.5 V; 12% Cr steel: 0.1 C, 0.25 Si, 0.7 Mn, 2.4 Ni, 12.0 Cr, 1.8 Mo, 0.35 V. Source: Ref 43 These findings have been verified and further expanded by other researchers. For example, in their carefully controlled forging experiments, Doege, Melching, and Kowallick (Ref 48) and Hecht and Hiller (Ref 50) found low-alloy steels to have far inferior wear resistance as compared to hot-work die steels (Fig. 18) because alloying led to higher hardness and the ability to retain strength at high die temperatures. The work of Netthöfel (Ref 49) is also in agreement with these observations. Fig. 18 Amount of wear of hot-work tool steels as a function of the number of forgings. Equivalent steels are in parentheses. Source: Ref 48 Up to now, most of the discussion of alloying has centered on die steels. Several workers have also investigated the wear characteristics of nonferrous die materials. In Netthöfel's (Ref 49) experiments on forging die wear, it was found that the nickel-base alloy Nimonic 90 had a wear resistance between that of an H12 and H19 steel at a die temperature of 255 °C (490 °F). This is an important finding in view of the fact that the nickel-based alloys are generally many times the cost of the die steel alloys and are also harder to machine. Notthöfel's finding was verified to a certain extent by Ali, Rooks, and Tobias (Ref 42) in their die wear studies in a high-energy-rate forming (HERF) machine (Fig. 19). Although the die were depended on whether the top die or bottom die was examined, it was found that Nimonic 90 was only slightly better than a steel similar to H19 (WEX). Thus, the results reported in Ref 42 and 49 point out the fact that the nickel-base die materials should be reserved for hot-die and isothermal forging applications for which die steels are inappropriate. Fig. 19 Total wear volumes for die materials at a mean hardness of 44 HRC. Source: Ref 42 Die Hardness. Die hardness is another factor whose influence on abrasive wear is easy to quantify. The effect of die hardness is best realized through an understanding of the die wear process itself. Misra and Finnie (Ref 51) have summarized a large amount of work on abrasive wear and concluded that two basic processes are involved. The first is the formation of plastically deformed grooves that do not involve metal removal, and the second consists of removal of metal in the form of microscopic chips. Because chip formation, as in metal cutting, takes place through a shear process, increased metal hardness could be expected to diminish the amount of metal removal via abrasive wear. This trend is exactly what has been observed. The effect of hardness on wear is seen in data on a variety of steels quoted by Kannappan (Ref 40) and by Thomas (Ref 43), which have been discussed previously. From examination of Fig. 17, it is apparent that the dependence of wear rate on hardness is greatest for low-alloy die steels such as 6F2 (No. 5 die steel in Fig. 17). Such a trend has also been reported by Kannappan (Ref 40) in data on several low-alloy and hot-work die steels. Kannappan (Ref 40) has also discussed the correlation between hardness and wear of die steels with microstructures different from the typical die steel structure of tempered martensite. It has been found that the isothermal heat treatment of steels to produce lower bainite results in better wear resistance (Fig. 20). Supposedly, this effect is a result of the fact that isothermal transformation/hardening causes fewer stresses and microscopic cracks (which promote abrasive failure) than does a thermal martensitic transformation. Fig. 20 Relative wear resistance with respect to hardness of selected chromium steels with 0.55% C. Note the difference between the effect of quenching followed by tempering (solid lines) and the effec t of isothermal treatment/quenching to a lower bainitic region (dashed lines). Relative wear resistance is defined as a number directly proportional to the applied interface pressure and the amount of relative sliding and inversely proportional to the total wear volume. Source: Ref 40 Workpiece Temperature. Several researchers have commented on the effect of workpiece temperature on die wear. In his investigation of wear of hammer dies, Thomas (Ref 52) found that in forging of steels, wear increased at first with billet temperature up to 1100 °C (2010 °F) and then decreased with increasing temperature (Fig. 21). The initial increase can probably be attributed to the increase in the amount of scale on the billets, which acts as an abrasive during the die wear process. However, above 1100 °C (2010 °F), the flow stress drops off rapidly enough to minimize the interface pressure during forging and therefore decrease the effect of scale. A similar finding was made by Doege, Melching, and Kowallick (Ref 48), who attributed an increase in die wear as the billet temperature was raised from 800 °C (1470 °F) to 1100 °C (2010 °F) to an increase in the die surface temperature and a simultaneous decrease in wear resistance. Fig. 21 Effect of workpiece temperature on wear. Source: Ref 52 Lubrication/Die Temperatures. The effects of lubrication and die temperature on die wear have been interpreted in a variety of often-conflicting ways in the literature. This is because lubricants and die temperature influence: lubricity, and hence the amount of metal sliding during forging; the interface pressure during deformation; and the heat transfer characteristics between the dies and workpiece during conventional hot forging. The last item is important not only through its influence on heat absorption into the dies, and thus thermal softening and decreased wear resistance of the dies, but also through its effect on the performance of the die and billet lubricants themselves. Investigations into the effect of lubrication on die wear in simple upsetting have shown that wear is greatly increased when the dies are lubricated versus when they are not. This effect is shown in the results of Singh, Rooks, and Tobias (Ref 44) from upset tests in a HERF machine (Fig. 22). The same phenomenon has been demonstrated by Thomas (Ref 43), who upset successive lots of 1000 samples each on a flat die in a mechanical press. In these tests, the amount of wear was greater for the lot involving lubricated compression tests (Fig. 23). From these findings, one might conclude that wear increases with lubrication because of increased sliding and that lubrication is detrimental in forging. Thomas clarified this point, however, by calculating the amount of wear for equivalent amounts of metal flow past a given point; he found that lubrication reduces wear by a factor of 3 when compared to forging without lubrication. Moreover, he emphasized that in closed-die forging, the amount of metal sliding is fixed by die and preform design and not lubrication. Thus, the amount of sliding over the flash land, where wear is usually greatest, depends on the amount of flash that must be thrown and not on the efficiency of the lubricant employed. Because the amount of flash will be roughly the same with or without lubrication, employing lubricants in closed-die forging should reduce abrasive wear of the flash land and other parts of the die cavity. Fig. 22 Effect of lubrication on forging die wear. Source: Ref 44 Fig. 23 Effect of lubrication on forging die wear. Wear index is defined as the average cross- sectional area of wear depressions in the die. Source: Ref 43 The interaction of lubrication and die temperature effects was demonstrated by Rooks (Ref 45) in upset tests on a HERF machine. These tests were run with various bulk die temperatures, dwell times, and cycle times. Dwell time in the HERF operation includes a short forging phase, a somewhat longer "bouncing" phase, and an extended "after-forging" phase during which the dies and billet are in contact under low pressure. Results established that die wear after upsetting of 1000 billets decreased with increasing die temperature. This was correlated with decreased amounts of sliding at higher die temperatures due to an increase in the coefficient of friction. The effects of dwell time and cycle time on die wear were also examined by Rooks (Ref 45). Increasing dwell time increases die chilling. As a result, metal flow is hindered and die wear is reduced. Increased cycle time (time between forgings) tends to have the reverse effect of increasing dwell time (that is, it increases die wear because of lower coefficients of friction and more sliding). However, these effects have been found to be very slight in upset tests, conducted in a HERF machine (Ref 45). A striking die wear feature that Rooks (Ref 45) and Ali, Rooks, and Tobias (Ref 43) noted concerns the generally higher wear experienced by the top die versus the lower die, which is most noticeable in their lubricated upset tests (Fig. 19). This can be attributed to greater chilling on the bottom die because the hot workpiece was placed on it prior to forging. This could, therefore, have been expected to lead to greater friction, less sliding, and thus less abrasive wear than the top die experienced. From a practical standpoint, increased production rate in a forge shop may be expected to lead to lower die life. This is almost certainly a result of increased die temperature. In forging under production conditions, the die surface temperature observed between two consecutive forging blows seems to remain unchanged throughout a production run (Ref 49). During the actual forging operation, the die surface temperatures increase and reach a maximum peak value and decrease again when the dies are separated and the forging is removed. In case the forging "sticks" in one of the dies, the peak surface temperature of that die may increase further and contribute to die wear. Therefore, in conducting die wear studies, it is suggested that an ejector be used to remove the part after forging, so that die temperatures do not increase because a forging sticks in the die. In forging of steel at 1200 °C (2190 °F) with dies at about 250 °C (480 °F), surface temperatures will reach approximately 750 °C (1380 °F) if perfect and ideal contact occurs between the forging and the die. In reality, however, due to scale and oxidation at the die/material interface, the peak surface temperatures during forging reach 500 to 600 °C (930 to 1110 °F) in mechanical presses and 650 to 700 °C (1200 to 1290 °F) in hammers. As an example, die temperatures obtained by Vigor and Hornaday (Ref 53), in forging steel in a mechanical press are given in Fig. 24. It can be seen that the temperature gradient is very large at the vicinity of the die/material interface. Fig. 24 Temperatures at the surface and at various depths in fo rging dies obtained during forging 1040 steel without lubricant. Source: Ref 53 The effects of sliding on die wear are also qualitatively well known in forging practice. These effects are taken into account in designing preforms to ensure that more "squeezing" and less lateral flow and sliding action take place during finish forging. Methods of Improving Resistance to Abrasive Wear From the discussion of the factors that influence abrasive wear, one can deduce methods to improve die performance controlled by this failure mechanism. Perhaps the most direct method is to employ a die steel that is more resistant to wear, that is, one that is harder and that retains its hardness at high die temperatures (Ref 54). This could mean changing from a low-alloy die steel to a chromium hot-work die steel. The decision to make such a change should be based on the suitability of the new die steel itself in the forging operation and the trade-off between expected increases in die life and increases in material (and machining) costs. Coating, hardfacing, and surface treatment of forging dies often can be employed to improve wear resistance as well. Information regarding specific coating and hardfacing alloys (and the methods of their application) and surface treatments such as nitriding and boriding is contained in the following section of this article and will not be reviewed here. However, there are numerous instances of such methods increasing die life. These include the use of chromium and cobalt-base coatings (Ref 48, 55), weld deposits of higher-alloy steels onto low-alloy steels (Ref 56), weld deposits of nickel and cobalt hardfacing alloys on die steels (Ref 57, 58, 59), ceramic coatings (Ref 60, 61), and surface nitriding (Ref 54, 62, 63). Another means of reducing wear in the forging of steel involves reducing the scale on heated billets; scale acts as an abrasive during the sliding that occurs between the dies and workpiece. Thomas (Ref 52) estimates that poor control of scale can reduce die life as much as 200%. Methods of reducing scale are relatively obvious and include the following: • Using a reducing, or inert, furnace atmosphere • Using a billet coating to prevent oxidation • Minimizing time at temperature in the furnace or using induction heating One final means of decreasing the problem of wear is through improved redesign of the blocker shape. This is an important consideration because wear is strongly dependent on the amount of sliding that occurs on a die surface. Thus, it is possible to reduce sliding, thereby reducing wear, by redesigning the blocker shape. Thermal Fatigue Thermal cycling of the die surfaces during conventional hot forging results in the second most common reason for rejecting dies, namely heat checking. Thermal cycling (thermal fatigue) results from the intermittent nature of forging production. The major factors influencing heat checking are: • Die surface temperatures • Surface stresses and strains • Damage accumulation in thermal fatigue • Microstructural effects of fatigue Die Surface Temperatures. Information on die temperatures is best obtained from direct measurements. Surface temperatures for dies used in a mechanical press have been found to reach about 600 °C (1110 °F) in forging of steel cylinders that were preheated to approximately 1175 °C (2150 °F) and upset to 75% reduction in height (Fig. 24). Similar measurements have been made by Kellow, et al. (Ref 64), who upset medium-carbon steel samples in a slow hydraulic press and a HERF machine. Surface thermo-couples were placed at various distances from the axis of the 25 mm (1 in.) diameter billets. Experimental results showed that the temperatures obtained along the initial contact area of the workpiece and the die do not differ significantly between low- and high-speed forging. However, the temperatures obtained outside the initial contact area, where the billet surface extends during deformation, were significantly higher in high-speed forging (900 °C, or 1650 °F) as compared with low-speed hydraulic press forging (550 °C, or 1020 °F). These results are mainly due to differences in heat generation due to friction, which serves as one means of dissipating the energy produced by the forging machine. Other measurements of die temperatures away from the die surfaces themselves demonstrate that large temperature gradients, as well as high temperatures, are induced in forging dies. These measurements include those of Voss (Ref 65), who measured temperatures in low-alloy (6F3) and chromium hot-work steel (H10, H12) radial forging dies preheated to 100 °C (210 °F) before forging (Fig. 25). Measurements away from the surface (at 0.5 mm, or 0.02 in., from the surface) show large temperature gradients. By comparing die temperatures during forging to those between forging blows (Fig. 25), it is apparent that very large temperature changes at the surfaces of forging dies may be expected as well. For this reason, large stresses and large strains due to temperature effects are experienced by the surface layers of forging dies. Fig. 25 Temperatures in dies with air-water cooling of the dies between blows. Initial d ie temperature: 100 °C (210 °F). Initial stock temperatures: (1) 1150 °C (2100 °F), (2) 1050 °C (1920 °F), (3) 950 °C (1740 °F). Upper curves are the temperatures achieved during forging; lower curves are the temperatures reached between forging blows. Source: Ref 65 Materials with high conductivity are less likely to develop large thermal gradients and fail by thermal fatigue than those with poor thermal conductivity. Although conductivity data for the various die materials are scarce, available measurements do show, for instance, that the tungsten hot-work die steels with higher conductivities should be more resistant to heat checking than the chromium hot-work die steels. Surface Stresses and Strains. The stresses and strains that result from the temperature cycles experienced by the forging dies have two main sources: (1) thermal expansion and contraction, and (2) phase changes brought about by temperature cycling. The first of these is probably the easiest to quantify. This is because the thermal stresses and strains are approximately proportional to the maximum temperature difference (T max - T min ) experienced by the dies and the thermal expansion coefficient of the die material. Most die steels have similar thermal expansion coefficients. Therefore, the thermally induced deformation of the dies is controlled primarily by the magnitude of T max - T min . As might be expected, the tendency to heat check can be decreased by reducing T max - T min . This can be done in two ways. First, T min , or the bulk die temperature, can be increased. However, such a change may adversely affect resistance to other forms of die failure. Alternatively, T max can be decreased. The easiest way to do this is by decreasing the workpiece temperature or by using a lubricant with better thermal insulating properties. Figure 26 shows the effects of increasing T min or decreasing T max on the fatigue life (in terms of number of cycles to produce a crack of certain length) of mild steel. It is seen that a 100 °C (180 °F) decrease in T max is much more beneficial in extending the fatigue life than a similar increase in T min . This result is generally true for die steels as well, and can be attributed to the greater reduction of the strain amplitude by decreasing T max . Fig. 26 Effects of maximum (a) and minimum (b) temperatures on the fatigue life of En 25 mild steel. Source: Ref 40 The effects of phase changes on thermal fatigue of forging dies has been examined by Rooks, Singh, and Tobias (Ref 66) and Okell and Wolstencroft (Ref 67). Both sets of investigators have concluded that die surface heating and cooling may lead to reversion of the tempered martensite to austenite and subsequent transformation back to martensite. Because austenite and martensite have different densities, such phase changes lead to strains and stresses that are imposed by subsurface layers of the dies that do not undergo the transformation. As with the thermally induced strains, transformation-induced strains can be reduce either by keeping the maximum die surface temperature below the Ac 1 temperature (the temperature at which austenite forms, which is 800 °C, or 1470 °F), or by keeping the minimum die surface temperature above the martensite start, M s , temperature, which depends greatly on alloy composition, typical values being 280 °C ( 535 °F) for H11 and 380 °C ( 715 °F) for H21. Okell and Wolstencroft (Ref 67) suggested the latter possibility, but specified that it should only be used for the more highly alloyed die steels, which have good hot hardness because they resist overtempering. Microstructural Effects on Thermal Fatigue. Because ductility has a large effect on the number of thermal cycles a forging die can undergo prior to forming cracks, microstructure can have a significant impact on the frequency of heat checking. The most important microstructural variables are cleanliness, grain size, and microstructural uniformity. Die [...]... other hand, dies for thin forgings that must be discarded because of wear (especially at the flash land), and best suited for chromium plating (Ref 112 ) Cobalt Plating As with chromium, various cobalt alloys have been applied to hot-forging die steels, primarily to extend life through reduction of die wear (Ref 113 , 114 , 115 , 116 , 117 ) Typically composed of alloys of cobalt and tungsten or cobalt and. .. 117 1 A Thomas, Wear of Drop Forging Dies, Tribology in Iron and Steel Works, Iron and Steel Institute, London, 1970, p 135 A.K Singh, B.W Rooks, and S.A Tobias, Factors Affecting Die Wear, Wear, Vol 25, 1973, p 271 B.W Rooks, The Effect of Die Temperature on Metal Flow and Die Wear During High Speed Hot Forging, Proceedings of the Fifteenth International MTDR Conference (Birmingham, England), MacMillan,... Hopkins, and K.E Kirkham, The Wear Testing of Hot Work Die Steels, Metall Met Form., Vol 39 (No 2), 1972, p 46 T.M Silva and T.A Dean, Wear in Drop Forging Dies, Proceedings of the Fifteenth International MTDR Conference (Birmingham, England), MacMillan, Sept 1971, p 22 E Doege, R Melching, and G Kowallick, Investigation Into the Behavior of Lubricants and the Wear Resistance of Die Materials in Hot and. .. Forging Dies, Cobalt, Vol 1, 1975, p 17 117 J.K Dennis and D Jones, Brush Plated Cobalt Molybdenum and Cobalt-Tungsten Alloys for Wear Resistant Applications, Tribol Int., Vol 14, 1981, p 17 118 H Moestue, "Hardfacing and Reclamation of Hot Work Dies," unpublished manuscript, Sveiseindustri, Oslo, 1976 119 O Knotek, Wear Prevention, Fundamentals of Tribology, N.P Suh and N Saka, Ed., MIT Press, 1978, p... (No 11) (in Hungarian), Nov 1969, p 480 V.S Burgreev and S.A Dobnar, Electrolytic Boriding of Hammer Forging Dies and Their Heat Treatment, Met Sci Heat Treat., Vol 14 (No 6), June 1972, p 513 H.C Fiedler and R.J Sieraski, Boriding Steels for Wear Resistance, Met Prog., Vol 99 (No 2), Feb 1971, p 101 H.C Child, The Heat Treatment of Tools and Dies A Review of Present Status and Future Trends, Tools and. .. Metal Coated Metal-Working Dies," U.S patent 4,571,983, 25 Feb 1986 111 Heat Treating, Cleaning and Finishing, Vol 2, 8th ed., Metals Handbook, American Society for Metals, 1964 112 S.L Scheier and R.E Christin, Drop Forge Dies Hard Chromium Plating Cuts Cost of Die Sinking, Met Prog., Vol 56 (No 4), Oct 1949, p 492 113 F.A Still and J.K Dennis, The Use of Electrodeposited Cobalt Alloy Coatings to... Sept 1974, p 9 114 K.J Lodge et al., The Application of Brush-Plated Cobalt Alloy Coatings to Hot and Cold-Work Dies, J Mech Work Technol., Vol 3, 1979, p 63 115 F.A Still and J.K Dennis, Electrodeposited Wear- Resistant Coatings for Hot-Forging Dies, Metall Met Form., Vol 44 (No 1), 1977, p 10 116 J.K Dennis and F.A Still, The Use of Electrodeposited Cobalt Alloy Coatings to Enhance the Wear Resistance... flow and shear stresses to a minimum, thereby avoiding shear and spall of the coating Refractory-coated H13 tool steel dies have exhibited significant improvements in wear resistance Electroplating Chromium Plating Chromium is usually applied to metal pieces using electroplating baths composed of chromic acid and some sulfate or fluoride compound (Ref 111 ) Bath temperatures are between 45 and 65 °C (110 ... compositions and crystal structures, and it usually occurs under load with little or no lubrication A common location for adhesive wear is at a surface where wire rope rubs against spools and pulleys, for example Figure 2 illustrates adhesive wear on a steel spool that has been interacting with wire rope Fig 2 Cable wear on large spool, a form of adhesive wear Source: Ref 10 Testing for the types of wear damage... 0.71.2 2.32.8 2.53.6 0.40.7 1 .11. 3 0.51.0 1 .11. 4 0.61.0 2.83.5 0.5-1.0 0.3-1.0 0.5-1.5 0.3-0.8 0.6-1.5 5.5-6.7 0.6-0.9 11. 014.0 0.3-1.0 0.3-1.0 0.51.0 0.40.9 0.81.2 0.30.8 0.61.5 0.40.7 0.30.8 0.41.0 0.20.4 0.30.8 Relative wear rate(c) 88-90 0-1.5 01.2 Hardness range, HB(b) 620-740 500-630 100 -111 (d) 0-1.2 550-650 98-100 3.05.0 0-1.5 520-650 105-109 500-620 110 -120 190-230 114 -120 0-1.0 250-420 126-130 . primarily to extend life through reduction of die wear (Ref 113 , 114 , 115 , 116 , 117 ). Typically composed of alloys of cobalt and tungsten or cobalt and molybdenum, these coatings are applied in. acid and some sulfate or fluoride compound (Ref 111 ). Bath temperatures are between 45 and 65 °C (110 and 145 °F). The kind of plating used for forging dies is called hard chromium plating and. without lubrication, employing lubricants in closed-die forging should reduce abrasive wear of the flash land and other parts of the die cavity. Fig. 22 Effect of lubrication on forging die wear.