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Material selection for extrusion and hot-working tools in relation to a new manufacturing technology
Journal o f Mechanical Working Technology, 3 (1979) 47---62 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
47
MATERIAL SELECTION F O R E X T R U S I O N AND HOT.WORKING TOOLS IN R E L A T I O N TO A NEW M A N U F A C T U R I N G T E C H N O L O G Y *
Th. SKAMLETZ and W. SCHEPP
Edelstahlwerke Buderus A G, Wetzlar, (West Germany) (Received May 23, 1978; accepted in revised form July 5, 1978)
Industrial Summary Beginning with the requirements for extrusion and hot-working tools, the essential characteristics in the selection of the steel are described, with consideration being given to economic aspects. Thereafter, suggestions are made for application in the extrusion of light metals, non-ferrous metals and steel. The use of a metallurgical ladle process, and the relating of the processing conditions employed during the forging and heat treatment operations to the required material properties, led to the development of ISO B steels, which are much superior to conventional steels for extrusion-hot-working tools, and which are comparable to remelted steels. Further, these steels have also led to considerable increases in efficiency for hot rolls, dies and pipe moulds.
Introduction At elevated temperatures, extrusion tools are subject to mechanical stresses, changes in temperature and wear. Materials areused, therefore, which meet the requirements for such tools. In addition to t h e best possible resistance against heat cracks and a high resistance to heat wear, a good resistance to tempering -- i.e. heat resistance -- is required from the steels. The correct material selection, therefore, is decisively significant. The determination of the exact initial ultimate tensile strength is also very important for such tools. The material properties of the tools are given b y the composition of the steel and the processing technology employed. Material selection for extrusion tools The most important alloy elements are C, Cr, Mo, W and V as well as -- in some cases -- Co and Ni. Resistance to tempering and heat resistance are determined primarily b y Cr, Mo, W, V and Co; nickel increases the toughness and the hardening-through properties of the material. However, the resistance to heat wear is improved b y means of the elements Cr, Mo, W and V. Later is this *Paper presented at the International Conference on Warm Working, Sunderland, U.K., September 11--12, 1978.
48 paper the particulars of the manufacturing technology employed are discussed. Formerly Cr--W-alloyed steels were used primarily. The higher insensitivity and the advantage in price of Cr--Mo-alloyed steels have continuously promoted the utilization of these materials on a large scale. Even if Cr--Mo--V-alloyed steels are less wear resistant and not as constant in shape as the Cr--Walloyed steels, certain advantages -- besides t h a t of the e c o n o m y in operation -may not be overlooked; W-alloyed steels cannot be used, for example, in cases where the tools have to be cooled. Nevertheless, in certain cases, even in the future it will n o t be possible to dispense with W-alloyed steels. The number of the steel variants will reduce in the future, as, for certain fields of application, special materials -- such as nickel-based alloys, precipitation-hardenable austenites and weldings with hard alloys -- are used. This trend has been considered already in the new edition of the steel application list for steels for tools for bar extrusion presses and tube extrusion presses, which appears in the upper part of Table 1, with indication of the respective qualities. Where diverse steels possess approximately the same properties, Buderus has -with regard to an indispensible reduction of the types -- deleted a number of the steels formerly recommended, in agreement with the Extrusion Press Board of the German Association for Metallurgy. The aim of such actions is to facilitate stockage and treatment of the steels -- for the producer as well as for the consumer -- and to operate in a more economical manner. Amongst others, in this list, the 9% W-steels as per DIN material no. 2581 and 2662 (additionally with 2% Co) as well as the 5% Cr--Mo--W-steel X 37 Cr Mo W 5 1 according to DIN material no. 2606, respectively AISI H 12 -- still used t o d a y in m a n y works -- were no longer considered. Newly added was, however, the steel X 45 Co Cr W V 5 5 5 according to DIN material no. 2678 (Buderus RCW 5), which is superior to the well known W-alloyed steels with regard to resistance to tempering, h o t ultimate tensile strength and heat wear and which represents more than a replacement for the 9% W-steels. If the resistance to wear in respect of stability of shape is not so very important, DIN material no. 2567 from t h e W-alloyed steel range can be recommended. The steels as per DIN material no. 2343, 2344 and 2606 which analytically do not differ very much from each other, have finally led to the deletion of the W-alloyed steel as per DIN material no. 2606. As the resistance to tempering, with respect to the h o t ultimate tensile strength, of DIN material no. 2344 is better than that of DIN material no. 2343, in future quotes for the steel with the lower V-content -- still used t o d a y for extrusion tooling -- should defer in favour of those for DIN material no. 2344. This is already practised in respect of forging tools and die-cast moulds. It has already been pointed out t h a t the ultimate tensile strength of the tooling is very important. It is true that resistance to wear is improved by higher service hardness; however, it must be considered that extrusion tooling is subjected to high temperatures and, therefore, ultimate tensile strength at ambient temperature loses importance with increasing working temperature. If the tooling used for light metals is n o t considered, the working temperatures of extrusion tools regularly surpass the tempering temperatures of the steels employed.
49
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High temperature properties Installation tensile strength* and heat resistance with respect to 0.2% proof stress, as stress characteristic of the toolings, are in a certain relation to each other. As Fig.1 shows, h o t ultimate tensile strength and 0.2% proof stress increase with rising installation tensile strength. However, from this figure, which it will be noted is for DIN material no. 2344, respectively AISI H 13, it is clearly seen that with rising temperature -- presented for the range between 450 ° and 600 ° Centigrade -- h o t ultimate tensile strength decreases. Higher-alloyed steels and steels which are more resistant to tempering are a little more favourable in this respect and consequently are used if higher working temperatures occur. A further consideration is that extrusion tools are thermally stressed for long periods, so t h a t losses in ultin-,ate tensile strength are also possible in these cases, where the tempering temperature of the tooling is above the actual working temperature. This occurs for instance, for sleeves, for which generally -- with DIN material no. 2311 -- a relatively low-alloyed material is used. (Material "Buderus M F R " differs from the real DIN material no. 2311 in so far as
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testing ?ernperoture Fig.1. Hot ultimate tensile strength and 0.2% proof stress as functions o f installation tensile strength and testing temperature, for material no. 2344, where triangular, cross and round points refer to installation tensile strengths of approximately 1600, 1400 and 1200 N / m m 2,
respectively. *Ultimate tensile strength at r o o m -- as opposed to
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--
temperature.
51
a little more Mo is used for improving the h o t tensile strength). If the real characteristics for the loading capacity -- the specific inside pressure -- are n o t considered, then more critical standards, specifically for container sleeves, should be applied in the future. For special cases, for instance for cable covering presses, higher-alloyed steels i.e. according to DIN material no. 2343, have already been used for a considerable time. Steels f o r containers
For conventional design also, i.e. for round containers, DIN material no. 2323, a heat~resistant steel, is used to an increasingly higher degree. Based on experience, the authors still hold the opinion that for specific inside pressures of up to approx. 700 N / m m 2, the 2-part design is to be r e c o m m e n d e d and even if this value should be surpassed, the intermediate liner can be discarded in many cases, if one uses a more load-bearing, higher-alloyed material for the sleeve. Finally, it should be borne in mind that a sleeve represents a unique purchase which is later provided with new liners over a period of many years. Figure 2 gives information on the h o t 0.2% p r o o f stress of the three steels mentioned, for an installation tensile strength of approx. 1100 N / m m 2. The higher-alloyed material is able to benefit from the higher resistance to tempering b y means of the fact that a higher tensile strength is chosen, (Table 2). By this means more favourable heat, resistant properties can be obtained. 800
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tes t i ~ tempe~ture Fig.2.0.2% p ro o f stress o f steels for sleeves as a function o f testing temperature, for aninstallation tensile strength of approximately 1100 N/ram ~.
52
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53 The liner, the real wear part of the container, is of principal importance. For the latter, steels are required which are -- in addition to a high heat resistance and a high resistance to wear with a good thermal conductivity -- as far as possible insusceptible to changes in temperature. Table 2 gives a survey of the materials usually offered at the present time. In this list DIN material no. 2323 was indicated intentionally, since according to previous experience, this steelis at least sufficient for the light easily-extruded metal alloys, although the 5% CT-Mo--V steels are superior as to heat resistance and heat wear. The higher insusceptibility and the lower purchase price are the reason for using DIN material no. 2323 even for the forming of steel. However, also steel as per material no. 2603 -- which is a little more alloyed -- is still used for extruding light metals. For copper and copper alloys, however, as well as for steels, even at the present time, m a n y different material grades are used. There are various reasons for this, dependent upon local circumstances. According to company records, for non-ferrous metals, DIN material no. 2365 and DIN material no. 2603 are used most frequently, although the trend has moved more to DIN material no. 2365 during recent years; the higher insusceptibility to heat cracks is the reason for this trend. In these discussions DIN material no. 2367 should n o t be overlooked. This steel represents a combination of DIN material nos. 2344 and 2365. In the foot-note of the steel application list the remark is made, therefore, t h a t it can be used instead of both the above mentioned steels. Compared with DIN material no. 2365 this steel has a better hardenability and the elevated alloying contribution also increases heat resistance. Actually DIN material no. 2365 still represents the better known steel, and is much more widely used. In the course of the necessary trend towards rationalization it must be established which of the steels is to be preferred. Compared with material no. 2365, remarkable successes were obtained with material no. 2367, for example for liners for the h o t shaping of steel. Wear resistance As martensite-type steel, DIN material no. 2567 can also be offered, because of its superior wear characteristics to those of the above-mentioned steels. On account of a susceptibility to cracks, however, it is n o t suitable for use in connection with liquid cooling. In 1969 when Buderus drew attention to material no. 4980* and explained the characteristic of this steel, the success of the steel, when used for liners, could not have been envisaged. With 26% Ni, 15% Cr, 1.3% Mo and 2.2% Ti, a steel is involved which was developed primarily, and used, for gearing construction until that time. Due to the alloy c o n t e n t and the required shaping, a higher price becomes necessary, when compared with the c o m m o n martensite steels, which, however, generally is more than compensated for by much higher tool life. It must n o t be forgotten that relining costs are saved by higher tool life. It speaks favourably for this material that consumers who have * Material no. 4980 and material no. 2779 may be regarded as identical.
54 tried this steel are t o d a y among the purchasers who repeatedly order the material. The main characteristic of DIN material no. 4980 is the excellent resistance to wear; however, the installation tensile strength is only between 1000 and 1150 N / m m 2, whereas the value is approx. 1400 N/ram ~ for the c o m m o n martensite steels. Decisive, however, is n o t the installation tensile strength -- as mentioned at the beginning -- b u t the properties at the working temperature. The martensite steels are tempered with temperatures b e t w e e n 620 ° and 640 ° Centigrade to obtain the desired service tensile strengths. On the other hand, DIN material no. 4980 is submitted to an age-hardening of 16 hours at 720 ° Centigrade after forging and solution treatment. A yield strength of approx. 700 N/mm 2 occurs, which is inferior to the values for martensite steels at ambient temperature. However -- compared with the normal ho~working steels -for DIN material no. 4980 the 0.2% p r o o f stress decreases much less with rising temperature. Figure 3 shows that the real operational field for this age-hardenable anstenite is at temperatures of from approx. 580/600 ° Centigrade upwards. A further essential property concerning austenitic and martensitic steels is the coefficient of thermal expansion. During shrinkage this fact has to be observed, since austenitic steel expands more during heating than the intermediate liner and the sleeve, which are made from martensitic steel. The shrink allowance must be reduced, therefore, in proportion to the different coefficients of expansion. A further advantage of this steel is that austenitic liners can be manufactured with thinner walls than those made from martensitic steels (i.e. reduction of weight). The heat conductivity of this steel is worse than that of the usual liner materials. An insufficient preheating provokes spoiling of the shrinkfit, where the consequence can be liner-displacement or even cracking of the liner. It is well-known that there is no tooling in regard to steel selection and application, so manifold as the die, the requirements of which depend partly on the design and construction, b u t in first place on the loads that the die is submitted to. On account of the loads occurring, provision of adequate support has also to be observed very carefully. Table 3 cannot lay claim to completeness, as steels are also used which are n o t indicated in this report. The full list leads from Cr--Mo- through CY--W-alloyed steels,austenites and hard alloys to the ceramic materials. For light metal extrusion, D I N material no. 2343 is stillprimarily used, although it would be worthwhile to determine, through the consumer, what advantages would result from application of DIN-material no. 2344 on the basis of the above-mentioned statements. Apart from the somewhat lower price, the reason is obviously to be found in the fact that the requirements can be satisfied with D I N material no. 2343, on the basis of the possibilityof supplementary surface treatment and the small pressing lots often occurring. Extrusion of non-ferrous metals or steels,however, becomes more critical. In this case, not only the kind of pressing alloy, but also the existing shape, must be considered. Martensitic steels,on a basis of Cr--Mo, are frequently in-
55
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testing temperoture Fig. 3. 0.2% p r o o f stress of steels for liners as a f u n c t i o n o f testing temperature.
sufficient and in spite of the higher susceptibility to cracks, one cannot avoid using W-alloyed steels. In special cases, superior heat resistance still leads to the application of austenitic steels. For extruding profiles, strips and tubes of heavy metals and steels at high temperatures, therefore, steels as per DIN material no. 2731 are used, amongst others. DIN material no. 2731 is highly consolidated by means of a directed forging method i.e. the discs are hammer hardforged. The resultant ultimate tensile strength of the material as supplied is relatively low at 1000 to 1200 N / m m 2.
Thermal loading Especially for dies, a careful and radical preheating is indispensable for a good service life. This is also the reason why, particularly for the h o t shaping of non-ferrous metals, hard alloys are still used. The cast stellites, on the basis of Co--Cr--W, n o t only have a high h o t hardness b u t also an excellent shape sta-
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57 bility; t h e y are, however, susceptible to cracks. With Revolta, Buderus has a forgeable stellite in its programme which is n o t as susceptible to cracks as are cast hard-alloys. Revolta is an alloy of C o - - C r - M o which, in addition to a high h o t ultimate tensile strength, is characterized by a better toughness in comparison with cast steUites. Revolta is used for tube and profile dies for nonferrous metals. Limitation of the ultimate tensile strength at the upper side of the die has proven advantageous, as the susceptibility to thermoshocks arising through changes in temperature, is diminished by a lower tensile strength. In addition, it was determined t h a t service-life increases by reduction of die thickness. Also, and specifically to suit Revolta, a careful and radical preheating must be carried out, as has already been mentioned. It must n o t be overlooked that the good h o t hardness and the resulting resistance to heat wear facilitate a higher service-life of the tooling. Revolta can, therefore, be considered as a material which is -- with appropriate manipulation and in spite of the higher purchase price -- more economical in certain cases than the application of the other die steels which could be chosen. Here again there is a permanent group of consumers for this hard alloy, which also includes users of mandrel tips for tube pressing. In addition to the die, the mandrel is likewise submitted to very high thermal loads, as it stays in contact with the hot ingot during the whole of the pressing operation. Above all this is very significant when extruding non-ferrous metals and steels. There are attempts made to maintain the temperature in a tolerable range by means of cooling, as with too-high heating breakdown will begin through reduction in area. At the present time internal cooling of the mandrel is used. It is necessary to pay attention to that fact that a relationship exists between the cooling of the bore and the mandrel outside diameter, because too-abrupt cooling can lead to heat- and resultant tension-cracks. Moreover, the authors hold the opinion that a too-high mandrel tensile strength involves disadvantages; extremely high hardnesses aiming at a high hot-tensile strength and a better resistance to wear, in practice have led to an early breakdown by cracking in several cases. If one uses Cr--Mo--V-alloyed steels almost exclusively for extruding light metal alloys -- and here preferably the 5% Cr--Mo--V-materials -- the same is applicable to pressing mandrels for heavy metal alloys and steels, if water cooling is applied. In special cases, however (for oil cooling), W-alloyed steels as DIN material no. 2567 and DIN material no. 2678 are used. DIN material no. 2678 (of the composition stated in Table 1), in the authors' opinion, is a steel which can be Used universally. In addition to dies for heavy metal shaping (instead of the 9% W-steels as per DIN material no. 2581 and 2662), it is also used to a high degree for d u m m y blocks, as in many cases the CY--Mo--V steels primarily are n o t sufficiently resistant in retaining shape and edges. It is also offered as a housing material for operating hard alloys and nonmetallic special materials. Figure 4 shows the superiority of this material in comparison with the well-known W-alloyed steels.
58
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Fig.4. 0.2% proof stress of tungsten-alloyed hot-working steels as a function of testing temperature, for an installation tensile strength of approximately 1500 N/mm 2.
The 5% Cr-Mo--V-steels are the most usual stamping materials, and from this range, DIN material no. 2343 is most often used. However, especially for higher-loaded extrusion press stems, DIN material no. 2344 is preferred and in comparable cases, in practice, has shown its superiority over DIN material no. 2343. DIN material no. 2714 is still used for press stems, b u t only rarely, as its h o t yield strength is inferior t o that of the 5% Cr--Mo--V-steels and the risk of upsetting is very great, especially for higher-loaded stems. For stronger surface Dressings, for which even the heat resistance of the Cr-Mo--V-alloyed steels is no longer sufficient, martensitic age-hardening steels can be employed. Buderus has such a steel in its production programme under brand designation R H F 105 (DIN material no. 2709), which has already been used frequently for the extrusion section of highly-loaded extrusion press stems, with success. The special advantage of martensitic age-hardening steels is to be found in the excellent elongation and toughness with high ultimate tensile strength and yield point. Ultimate tensile strengths of 1800 to 2000 N/ram 2 are obtained b y means of age-hardening at 480 ° Centigrade. However, with rising temperature a decrease in ultimate tensile strength again takes place, so that the application of this steel as a hot-working steel is limited. Manuf~et~ technology For the usual loading range, conventionally melted steelsare used for extrusion tools. For extreme loading conditions and the highest requirements as to the toughness of the material and its homogeneity, steelsare required which have been remelted according to the V A R - or ESR-procedure.
59
By the application of a metallurgical ladle procedure and a manufacturing technology during forging and heat treating which is related to t h e required material properties, Edelstahlwerke Buderus has succeeded in manufacturing what may be caned isotropic steels, comparable to remelted steels as to the material properties. The Buderus designation of these steels is ISO B; the expression ISO refers to the isotropy of the material properties and the letter B characterizes the metallurgical ladle procedure in which powdery agents bearing Ca are blown in the melt by means of an argon arc. /.0
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The following characteristics especially can be noted: (i) The sulphur content is below 0.005%. (ii) The degree of cleanliness is decisively improved and can be met with operating safety with K4 ffi 0 and K1 ffi 10 as per test sheet STE 1 5 7 0 / 7 1 (Fig.5). (iii) The inclusions detectable in steel are globular and they are n o t stretched during h o t shaping (Fig.6). (iv) Due to the improved degree of cleanliness, the ISO B steels can be cast at lower temperatures and a definite decrease in segregation can be obtained. Figure 7 gives an impression of the improved macrostructure over the cross-sectional area compared with that for a conventional steel with the same dimensions. (v) An increase in toughness characteristics is produced, expressed through elongation, reduction in area and notch impact strength. The toughness characteristics in the transverse direction approach so far to the longitudinal values that it is accurate to describe the material as virtually isotropic (Figs.8--10). Not only for extrusion tools, b u t also for dies and h o t rolls, there is thus the possibility of increasing installation tensile strength in order t o obtain improved properties of wear, with at least an unchanged safety against cracks. The ultimate tensile strengths of h o t rolls were increased 2 0 0 / 3 0 0 N/ram 2 for ingot and billet trains. In addition to the better resistance to wear, the uniform finemeshed surface crack n e t w o r k can be recognized as a remarkable characteristic. ,
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The service life of centrifugal pipe moulds was also considerably increased, as the heat cracks produced during changing temperature loads occurred at a much later time. The same can be reported of die steel, where the improved properties o f the ISO B steels are distinguished by increased safety against cracks and lower susceptibility to hot cracks and led to increases in efficiency, on average, of 30%.
47
MATERIAL SELECTION F O R E X T R U S I O N AND HOT.WORKING TOOLS IN R E L A T I O N TO A NEW M A N U F A C T U R I N G T E C H N O L O G Y *
Th. SKAMLETZ and W. SCHEPP
Edelstahlwerke Buderus A G, Wetzlar, (West Germany) (Received May 23, 1978; accepted in revised form July 5, 1978)
Industrial Summary Beginning with the requirements for extrusion and hot-working tools, the essential characteristics in the selection of the steel are described, with consideration being given to economic aspects. Thereafter, suggestions are made for application in the extrusion of light metals, non-ferrous metals and steel. The use of a metallurgical ladle process, and the relating of the processing conditions employed during the forging and heat treatment operations to the required material properties, led to the development of ISO B steels, which are much superior to conventional steels for extrusion-hot-working tools, and which are comparable to remelted steels. Further, these steels have also led to considerable increases in efficiency for hot rolls, dies and pipe moulds.
Introduction At elevated temperatures, extrusion tools are subject to mechanical stresses, changes in temperature and wear. Materials areused, therefore, which meet the requirements for such tools. In addition to t h e best possible resistance against heat cracks and a high resistance to heat wear, a good resistance to tempering -- i.e. heat resistance -- is required from the steels. The correct material selection, therefore, is decisively significant. The determination of the exact initial ultimate tensile strength is also very important for such tools. The material properties of the tools are given b y the composition of the steel and the processing technology employed. Material selection for extrusion tools The most important alloy elements are C, Cr, Mo, W and V as well as -- in some cases -- Co and Ni. Resistance to tempering and heat resistance are determined primarily b y Cr, Mo, W, V and Co; nickel increases the toughness and the hardening-through properties of the material. However, the resistance to heat wear is improved b y means of the elements Cr, Mo, W and V. Later is this *Paper presented at the International Conference on Warm Working, Sunderland, U.K., September 11--12, 1978.
48 paper the particulars of the manufacturing technology employed are discussed. Formerly Cr--W-alloyed steels were used primarily. The higher insensitivity and the advantage in price of Cr--Mo-alloyed steels have continuously promoted the utilization of these materials on a large scale. Even if Cr--Mo--V-alloyed steels are less wear resistant and not as constant in shape as the Cr--Walloyed steels, certain advantages -- besides t h a t of the e c o n o m y in operation -may not be overlooked; W-alloyed steels cannot be used, for example, in cases where the tools have to be cooled. Nevertheless, in certain cases, even in the future it will n o t be possible to dispense with W-alloyed steels. The number of the steel variants will reduce in the future, as, for certain fields of application, special materials -- such as nickel-based alloys, precipitation-hardenable austenites and weldings with hard alloys -- are used. This trend has been considered already in the new edition of the steel application list for steels for tools for bar extrusion presses and tube extrusion presses, which appears in the upper part of Table 1, with indication of the respective qualities. Where diverse steels possess approximately the same properties, Buderus has -with regard to an indispensible reduction of the types -- deleted a number of the steels formerly recommended, in agreement with the Extrusion Press Board of the German Association for Metallurgy. The aim of such actions is to facilitate stockage and treatment of the steels -- for the producer as well as for the consumer -- and to operate in a more economical manner. Amongst others, in this list, the 9% W-steels as per DIN material no. 2581 and 2662 (additionally with 2% Co) as well as the 5% Cr--Mo--W-steel X 37 Cr Mo W 5 1 according to DIN material no. 2606, respectively AISI H 12 -- still used t o d a y in m a n y works -- were no longer considered. Newly added was, however, the steel X 45 Co Cr W V 5 5 5 according to DIN material no. 2678 (Buderus RCW 5), which is superior to the well known W-alloyed steels with regard to resistance to tempering, h o t ultimate tensile strength and heat wear and which represents more than a replacement for the 9% W-steels. If the resistance to wear in respect of stability of shape is not so very important, DIN material no. 2567 from t h e W-alloyed steel range can be recommended. The steels as per DIN material no. 2343, 2344 and 2606 which analytically do not differ very much from each other, have finally led to the deletion of the W-alloyed steel as per DIN material no. 2606. As the resistance to tempering, with respect to the h o t ultimate tensile strength, of DIN material no. 2344 is better than that of DIN material no. 2343, in future quotes for the steel with the lower V-content -- still used t o d a y for extrusion tooling -- should defer in favour of those for DIN material no. 2344. This is already practised in respect of forging tools and die-cast moulds. It has already been pointed out t h a t the ultimate tensile strength of the tooling is very important. It is true that resistance to wear is improved by higher service hardness; however, it must be considered that extrusion tooling is subjected to high temperatures and, therefore, ultimate tensile strength at ambient temperature loses importance with increasing working temperature. If the tooling used for light metals is n o t considered, the working temperatures of extrusion tools regularly surpass the tempering temperatures of the steels employed.
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High temperature properties Installation tensile strength* and heat resistance with respect to 0.2% proof stress, as stress characteristic of the toolings, are in a certain relation to each other. As Fig.1 shows, h o t ultimate tensile strength and 0.2% proof stress increase with rising installation tensile strength. However, from this figure, which it will be noted is for DIN material no. 2344, respectively AISI H 13, it is clearly seen that with rising temperature -- presented for the range between 450 ° and 600 ° Centigrade -- h o t ultimate tensile strength decreases. Higher-alloyed steels and steels which are more resistant to tempering are a little more favourable in this respect and consequently are used if higher working temperatures occur. A further consideration is that extrusion tools are thermally stressed for long periods, so t h a t losses in ultin-,ate tensile strength are also possible in these cases, where the tempering temperature of the tooling is above the actual working temperature. This occurs for instance, for sleeves, for which generally -- with DIN material no. 2311 -- a relatively low-alloyed material is used. (Material "Buderus M F R " differs from the real DIN material no. 2311 in so far as
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51
a little more Mo is used for improving the h o t tensile strength). If the real characteristics for the loading capacity -- the specific inside pressure -- are n o t considered, then more critical standards, specifically for container sleeves, should be applied in the future. For special cases, for instance for cable covering presses, higher-alloyed steels i.e. according to DIN material no. 2343, have already been used for a considerable time. Steels f o r containers
For conventional design also, i.e. for round containers, DIN material no. 2323, a heat~resistant steel, is used to an increasingly higher degree. Based on experience, the authors still hold the opinion that for specific inside pressures of up to approx. 700 N / m m 2, the 2-part design is to be r e c o m m e n d e d and even if this value should be surpassed, the intermediate liner can be discarded in many cases, if one uses a more load-bearing, higher-alloyed material for the sleeve. Finally, it should be borne in mind that a sleeve represents a unique purchase which is later provided with new liners over a period of many years. Figure 2 gives information on the h o t 0.2% p r o o f stress of the three steels mentioned, for an installation tensile strength of approx. 1100 N / m m 2. The higher-alloyed material is able to benefit from the higher resistance to tempering b y means of the fact that a higher tensile strength is chosen, (Table 2). By this means more favourable heat, resistant properties can be obtained. 800
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53 The liner, the real wear part of the container, is of principal importance. For the latter, steels are required which are -- in addition to a high heat resistance and a high resistance to wear with a good thermal conductivity -- as far as possible insusceptible to changes in temperature. Table 2 gives a survey of the materials usually offered at the present time. In this list DIN material no. 2323 was indicated intentionally, since according to previous experience, this steelis at least sufficient for the light easily-extruded metal alloys, although the 5% CT-Mo--V steels are superior as to heat resistance and heat wear. The higher insusceptibility and the lower purchase price are the reason for using DIN material no. 2323 even for the forming of steel. However, also steel as per material no. 2603 -- which is a little more alloyed -- is still used for extruding light metals. For copper and copper alloys, however, as well as for steels, even at the present time, m a n y different material grades are used. There are various reasons for this, dependent upon local circumstances. According to company records, for non-ferrous metals, DIN material no. 2365 and DIN material no. 2603 are used most frequently, although the trend has moved more to DIN material no. 2365 during recent years; the higher insusceptibility to heat cracks is the reason for this trend. In these discussions DIN material no. 2367 should n o t be overlooked. This steel represents a combination of DIN material nos. 2344 and 2365. In the foot-note of the steel application list the remark is made, therefore, t h a t it can be used instead of both the above mentioned steels. Compared with DIN material no. 2365 this steel has a better hardenability and the elevated alloying contribution also increases heat resistance. Actually DIN material no. 2365 still represents the better known steel, and is much more widely used. In the course of the necessary trend towards rationalization it must be established which of the steels is to be preferred. Compared with material no. 2365, remarkable successes were obtained with material no. 2367, for example for liners for the h o t shaping of steel. Wear resistance As martensite-type steel, DIN material no. 2567 can also be offered, because of its superior wear characteristics to those of the above-mentioned steels. On account of a susceptibility to cracks, however, it is n o t suitable for use in connection with liquid cooling. In 1969 when Buderus drew attention to material no. 4980* and explained the characteristic of this steel, the success of the steel, when used for liners, could not have been envisaged. With 26% Ni, 15% Cr, 1.3% Mo and 2.2% Ti, a steel is involved which was developed primarily, and used, for gearing construction until that time. Due to the alloy c o n t e n t and the required shaping, a higher price becomes necessary, when compared with the c o m m o n martensite steels, which, however, generally is more than compensated for by much higher tool life. It must n o t be forgotten that relining costs are saved by higher tool life. It speaks favourably for this material that consumers who have * Material no. 4980 and material no. 2779 may be regarded as identical.
54 tried this steel are t o d a y among the purchasers who repeatedly order the material. The main characteristic of DIN material no. 4980 is the excellent resistance to wear; however, the installation tensile strength is only between 1000 and 1150 N / m m 2, whereas the value is approx. 1400 N/ram ~ for the c o m m o n martensite steels. Decisive, however, is n o t the installation tensile strength -- as mentioned at the beginning -- b u t the properties at the working temperature. The martensite steels are tempered with temperatures b e t w e e n 620 ° and 640 ° Centigrade to obtain the desired service tensile strengths. On the other hand, DIN material no. 4980 is submitted to an age-hardening of 16 hours at 720 ° Centigrade after forging and solution treatment. A yield strength of approx. 700 N/mm 2 occurs, which is inferior to the values for martensite steels at ambient temperature. However -- compared with the normal ho~working steels -for DIN material no. 4980 the 0.2% p r o o f stress decreases much less with rising temperature. Figure 3 shows that the real operational field for this age-hardenable anstenite is at temperatures of from approx. 580/600 ° Centigrade upwards. A further essential property concerning austenitic and martensitic steels is the coefficient of thermal expansion. During shrinkage this fact has to be observed, since austenitic steel expands more during heating than the intermediate liner and the sleeve, which are made from martensitic steel. The shrink allowance must be reduced, therefore, in proportion to the different coefficients of expansion. A further advantage of this steel is that austenitic liners can be manufactured with thinner walls than those made from martensitic steels (i.e. reduction of weight). The heat conductivity of this steel is worse than that of the usual liner materials. An insufficient preheating provokes spoiling of the shrinkfit, where the consequence can be liner-displacement or even cracking of the liner. It is well-known that there is no tooling in regard to steel selection and application, so manifold as the die, the requirements of which depend partly on the design and construction, b u t in first place on the loads that the die is submitted to. On account of the loads occurring, provision of adequate support has also to be observed very carefully. Table 3 cannot lay claim to completeness, as steels are also used which are n o t indicated in this report. The full list leads from Cr--Mo- through CY--W-alloyed steels,austenites and hard alloys to the ceramic materials. For light metal extrusion, D I N material no. 2343 is stillprimarily used, although it would be worthwhile to determine, through the consumer, what advantages would result from application of DIN-material no. 2344 on the basis of the above-mentioned statements. Apart from the somewhat lower price, the reason is obviously to be found in the fact that the requirements can be satisfied with D I N material no. 2343, on the basis of the possibilityof supplementary surface treatment and the small pressing lots often occurring. Extrusion of non-ferrous metals or steels,however, becomes more critical. In this case, not only the kind of pressing alloy, but also the existing shape, must be considered. Martensitic steels,on a basis of Cr--Mo, are frequently in-
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sufficient and in spite of the higher susceptibility to cracks, one cannot avoid using W-alloyed steels. In special cases, superior heat resistance still leads to the application of austenitic steels. For extruding profiles, strips and tubes of heavy metals and steels at high temperatures, therefore, steels as per DIN material no. 2731 are used, amongst others. DIN material no. 2731 is highly consolidated by means of a directed forging method i.e. the discs are hammer hardforged. The resultant ultimate tensile strength of the material as supplied is relatively low at 1000 to 1200 N / m m 2.
Thermal loading Especially for dies, a careful and radical preheating is indispensable for a good service life. This is also the reason why, particularly for the h o t shaping of non-ferrous metals, hard alloys are still used. The cast stellites, on the basis of Co--Cr--W, n o t only have a high h o t hardness b u t also an excellent shape sta-
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57 bility; t h e y are, however, susceptible to cracks. With Revolta, Buderus has a forgeable stellite in its programme which is n o t as susceptible to cracks as are cast hard-alloys. Revolta is an alloy of C o - - C r - M o which, in addition to a high h o t ultimate tensile strength, is characterized by a better toughness in comparison with cast steUites. Revolta is used for tube and profile dies for nonferrous metals. Limitation of the ultimate tensile strength at the upper side of the die has proven advantageous, as the susceptibility to thermoshocks arising through changes in temperature, is diminished by a lower tensile strength. In addition, it was determined t h a t service-life increases by reduction of die thickness. Also, and specifically to suit Revolta, a careful and radical preheating must be carried out, as has already been mentioned. It must n o t be overlooked that the good h o t hardness and the resulting resistance to heat wear facilitate a higher service-life of the tooling. Revolta can, therefore, be considered as a material which is -- with appropriate manipulation and in spite of the higher purchase price -- more economical in certain cases than the application of the other die steels which could be chosen. Here again there is a permanent group of consumers for this hard alloy, which also includes users of mandrel tips for tube pressing. In addition to the die, the mandrel is likewise submitted to very high thermal loads, as it stays in contact with the hot ingot during the whole of the pressing operation. Above all this is very significant when extruding non-ferrous metals and steels. There are attempts made to maintain the temperature in a tolerable range by means of cooling, as with too-high heating breakdown will begin through reduction in area. At the present time internal cooling of the mandrel is used. It is necessary to pay attention to that fact that a relationship exists between the cooling of the bore and the mandrel outside diameter, because too-abrupt cooling can lead to heat- and resultant tension-cracks. Moreover, the authors hold the opinion that a too-high mandrel tensile strength involves disadvantages; extremely high hardnesses aiming at a high hot-tensile strength and a better resistance to wear, in practice have led to an early breakdown by cracking in several cases. If one uses Cr--Mo--V-alloyed steels almost exclusively for extruding light metal alloys -- and here preferably the 5% Cr--Mo--V-materials -- the same is applicable to pressing mandrels for heavy metal alloys and steels, if water cooling is applied. In special cases, however (for oil cooling), W-alloyed steels as DIN material no. 2567 and DIN material no. 2678 are used. DIN material no. 2678 (of the composition stated in Table 1), in the authors' opinion, is a steel which can be Used universally. In addition to dies for heavy metal shaping (instead of the 9% W-steels as per DIN material no. 2581 and 2662), it is also used to a high degree for d u m m y blocks, as in many cases the CY--Mo--V steels primarily are n o t sufficiently resistant in retaining shape and edges. It is also offered as a housing material for operating hard alloys and nonmetallic special materials. Figure 4 shows the superiority of this material in comparison with the well-known W-alloyed steels.
58
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The 5% Cr-Mo--V-steels are the most usual stamping materials, and from this range, DIN material no. 2343 is most often used. However, especially for higher-loaded extrusion press stems, DIN material no. 2344 is preferred and in comparable cases, in practice, has shown its superiority over DIN material no. 2343. DIN material no. 2714 is still used for press stems, b u t only rarely, as its h o t yield strength is inferior t o that of the 5% Cr--Mo--V-steels and the risk of upsetting is very great, especially for higher-loaded stems. For stronger surface Dressings, for which even the heat resistance of the Cr-Mo--V-alloyed steels is no longer sufficient, martensitic age-hardening steels can be employed. Buderus has such a steel in its production programme under brand designation R H F 105 (DIN material no. 2709), which has already been used frequently for the extrusion section of highly-loaded extrusion press stems, with success. The special advantage of martensitic age-hardening steels is to be found in the excellent elongation and toughness with high ultimate tensile strength and yield point. Ultimate tensile strengths of 1800 to 2000 N/ram 2 are obtained b y means of age-hardening at 480 ° Centigrade. However, with rising temperature a decrease in ultimate tensile strength again takes place, so that the application of this steel as a hot-working steel is limited. Manuf~et~ technology For the usual loading range, conventionally melted steelsare used for extrusion tools. For extreme loading conditions and the highest requirements as to the toughness of the material and its homogeneity, steelsare required which have been remelted according to the V A R - or ESR-procedure.
59
By the application of a metallurgical ladle procedure and a manufacturing technology during forging and heat treating which is related to t h e required material properties, Edelstahlwerke Buderus has succeeded in manufacturing what may be caned isotropic steels, comparable to remelted steels as to the material properties. The Buderus designation of these steels is ISO B; the expression ISO refers to the isotropy of the material properties and the letter B characterizes the metallurgical ladle procedure in which powdery agents bearing Ca are blown in the melt by means of an argon arc. /.0
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The following characteristics especially can be noted: (i) The sulphur content is below 0.005%. (ii) The degree of cleanliness is decisively improved and can be met with operating safety with K4 ffi 0 and K1 ffi 10 as per test sheet STE 1 5 7 0 / 7 1 (Fig.5). (iii) The inclusions detectable in steel are globular and they are n o t stretched during h o t shaping (Fig.6). (iv) Due to the improved degree of cleanliness, the ISO B steels can be cast at lower temperatures and a definite decrease in segregation can be obtained. Figure 7 gives an impression of the improved macrostructure over the cross-sectional area compared with that for a conventional steel with the same dimensions. (v) An increase in toughness characteristics is produced, expressed through elongation, reduction in area and notch impact strength. The toughness characteristics in the transverse direction approach so far to the longitudinal values that it is accurate to describe the material as virtually isotropic (Figs.8--10). Not only for extrusion tools, b u t also for dies and h o t rolls, there is thus the possibility of increasing installation tensile strength in order t o obtain improved properties of wear, with at least an unchanged safety against cracks. The ultimate tensile strengths of h o t rolls were increased 2 0 0 / 3 0 0 N/ram 2 for ingot and billet trains. In addition to the better resistance to wear, the uniform finemeshed surface crack n e t w o r k can be recognized as a remarkable characteristic. ,
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The service life of centrifugal pipe moulds was also considerably increased, as the heat cracks produced during changing temperature loads occurred at a much later time. The same can be reported of die steel, where the improved properties o f the ISO B steels are distinguished by increased safety against cracks and lower susceptibility to hot cracks and led to increases in efficiency, on average, of 30%.