HI \ W W | ‘ l l H“ N L 115 385 THS fin SURVEY G? METALLURGECAL imifififi MS EH! ‘2 E‘QRGNG EN£B"S"R‘2" $30 1.2133818 {14* "ha fiery» {2f 3‘91. S. :6“; NS?" Inf" ia- 225%; :‘f' “ f5? 11'12915 This is to certify that the thesis entitled SURVEY OF I‘ETAIJJURGICAL PROBLEMS IN THE FORGING INDUSTRY presented by John F. Lederer has .been accepted towards fulfillment of the requirements for degree in METALLURGICAL ENGINEERING 0-169 A SURVEY OF METALLURGIChL PROBLEMS IN THE FORGING INDUSTRY By John F. Lederer m ABSTRACT Submitted to the College of Engineering Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MnSTER OF SCIENCE Department of Metallurgical Engineering _/ l957 I- //'/l’ /I‘:‘:. 1’ / : I '0 . / .f"/ //j’ 7. / 7’ ,1,"- TELESIS nBS'I‘RnC T Lack of sufficient information pertaining to the forging industry has stimulated the writing of this thesis. It is a review of metallurgical problems pertinent to the forging industry. One of the main purposes of this paper is to stimulate interest in research projects on the part of industry and institutions of higher learning. Areas where research is necessary are specified in each section. The problems are divided into three areas, namely, heating for forging, aspects of die blocks, and other phases of forging. The heating section includes such problems as overheating, burning, underheating, scaling, decarburization, and rate of heating. Research in this phase of forging embraces improved methods of heating and increased rates of heating, scale free heating (with and without controlled atmosphere), magnitude and effects of overheating, underheating, and burning. In die blocks, the major considerations are limited compositions available, lack of knowledge concerning wear- characteristics, lubrications, and surface conditions, and effects of scale and speed of deformation on dies. Nec- essary research projects include development of new compositions, study of wear characteristics, improved 2 understanding of lubrication and surface conditions, and new and improved methods of preparing dies, especially casting the impression in the block and spark erosion. Study of dies through the use of radioactivity is also promising. Other fields of study are hydrOgen embrittlement and flaking, improved understanding of forgeability, in— cluding the effects of speed and mode of deformation and conditions of material. New developments entail the use of lead in forgings for improved machinability, forging from the cast state, and investigations of properties and forging characteristics of the vacuum melted and vacuum cast materials. Fundamentally, the study of these subjects requires increased willingness of industry to allot funds and to cooperate with the institutions of higher learning in arriving at solutions. Progress will advance when all involved exhibit renewed and increased interest in research in forging. A SURVEY OF‘METALLURGICAL PROBLE'I-ZS IN THE FORGING INDUSTRY By John F. Lederer A THESIS Submitted to the College of Engineering Michigan State University of agriculture and applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Metallurgical Engineering ACKNOJLEEGEMENTS The author wishes to express his appreciation to Dr. A. J. Smith for his inspiration and guidance through- out this thesis. Also, the author offers his utmost gratitude to the Drop Forging Manufacturers (Federal Drop Forge, Lansing Drop Forge, Lindell Drop Forge, and Nelling Drop Forge) who sponsored the fellowship under which this study was pursued. To the Drop Forging As- sociation, Ladish Drop Forge, Atlas Drop Forge, Oldsmobile Forge, and the four members already mentioned, the author is indebted for the plant tours, discussions, and whole- hearted cooperation in acquainting him with the Drop Forging Industry and its problems. TABLE OF CONTENTS PILGE IIIJIFQUJCTION...OOOOOIOIOOOIOOOOOOOOOOOOOOOOOOOOO. l EYLEJ-‘II‘IDIG ;:OR FORGIIIGOOOOOOOOOIOO00.0.00...00...... 5 LXSPECTS 0;? DIE BLOCKSOOOOOOOIOOOOOO000.000.0000.. 24 OTHE: PHIoSES OF 50RGI1§G990900000000000000..000000 58 SUI‘min-IRIQQOoooo0000000000oooooc00000000000000.0000 58 BIBLIOGR‘F‘PHYOOOOO...0.0.0OOOOOOOOOOOOOOOOOOOOOOOO 63 INTfiODUCTION It is necessary to be acquainted with the entire scope of a process or industry in order to fit its prob— lems into their respective importance. It would be poor economy to exert time solving a problem which occurred only a few times in the production of many thousand pieces in contrast to one which happens with greater frequency. As the first recipient of the Drop Forging Manu- facturers of Lansing's Fellowship, the author was con- fronted by just such a dilemma. fihat are the most important and pressing metallurgical problems requiring study in the Drop Forging Industry? Since the answer to this question was not uncovered by discussions or pre— liminary reading, it was decided that a review of the metallurgical problems was in order. It will be of value to future holders of the Drop Forging Fellowship, as well as providing a comprehensive picture to the people of industry. It is desired that this paper encourage awareness of the scientific metallurgical work required in the Drop Forging Industry. Kyle defines forging as ”the working of metal, either hot or cold, to some definite predetermined shape 2 by hammering or pressing, or by a combination of these processes" (l).* It is one of the oldest mechanical methods in the art of working metals according to the Manual of Open Die Forging (2). Its use dates back many centuries,having been developed with the earliest discov— ery of metals. Biblical reference indicates that the ancient inhabitants of western Asia were among the ear— liest users of iron and other forgeable materials. The Greeks and inhabitants of India and China were well versed in the art of hammering metals (3). One of the best illustrations of forging practice was the worlds famous swords of Damascus. The thirteenth century saw the development of the first mechanical hammers using open dies (4). Nasmyth perfected the steam hammer in 1842 (British patent $9382). Many of the original features of Nasmyth's steam hammer are still incorporated in today's hammers, although camouflaged by modernization. With the steam hammer came the application of simple im- pressions in the die face and the forerunner of the modern closed die process. Many other milestones have dotted the history of forging, including the board drop hammer, trip hammer, upsetter, and forging press to mention only a few. il- Numbers in parentheses refer to the bibliography. 3 Today, there are forging hammers rated at 50,000 pounds and larger ones in construction. (The Ladish Com- pany has a counterblow hammer rated by some at 100,000 pounds, but this is controversial.) Two forging presses in operation in this country are rated at 50,000 tons each, with plans almost complete for a 75,000 ton press. A 200,000 ton press is a not too distant reality. Even with these developments, it has been stated by Bishop (6) “The Drop Forging Industry is not one which readily lends itself to great improvement by research". Others have commented that it is still more of an art- than a science. It is with this thought as a spur that the scientific person should proceed into this field which holds so much potential development. Rather than include a long list of definitions in an appendix, the author has inserted the necessary defini- tions in the body of the text to insure continuous reading. Complete glossaries of forging terms are available in references (1), (2), (3), and (4) of the bibliography. These are standard reference texts on forging and can be found in any library. This thesis is in the form of a review of problems existing in the forging industry. all problems are not 4 covered; only those which it appears will benefit industry most by their solution. In the arrangement of this thesis, the author has attempted to group problems which are closely related into the same section. All problems concerned with heat- ing are in one chapter; die block problems in another. A third chapter deals with a group of problems not neces- sarily closely related, but still arranged in a logical manner as to importance. Most of the discussions will pertain to the forging of steel. Where other parts of the industry are specifi- cally concerned; this will be indicated. Many of the remarks will apply to both ferrous and nonferrous forgings even though not specified. Much has been written of the light alloys and titanium due to the government's spon- sorship of work in these fields. The inclusion of problems relating to these fields would be a complete thesis in itself. Therefore, only passing mention will be made in this direction. The problems introduced in this paper are those which should be given first consideration. Their solutions through research and development would enhance the progress of the forging industry. HfinTING FDR FORGING One of the most important phases in the manufacture of a high quality forging is the heating to forging tem- perature. Eetals are susceptible to many evils while being heated. Among these factors are overheating, burning, underheating, decarburization, scaling, and adverse rate of heating (7). If these factors were kept under complete control, the manufacture of a quality forging would be greatly simplified. OVEnHLaTING Overheating is "A term applied when, after exposure to an excessively high temperature, a metal develops an undesirably coarse grain structure but is not permanently damaged. Unlike a burnt structure, the structure produced by overheating can be corrected by suitable heat treatment, by mechanical work, or by a combination of the two' (8). Since overheating is not a permanent defect, it can be corrected, but special care and handling are required. The difficulty involved is in the detection of an overheated piece. Unless close control is observed, occasional pieces in an overheated condition will be produced. This coarse grained product, with its undesirable attributes and poor physical pr0perties will not be suitable for service, yet will not be rejected in an inspection or test. 6 Susceptibility to overheating is a function of the chem— istry of the metal. It can also be developed by improper heating. Some materials resist overheating and burning in a reducing atmosphere as compared to an oxidizing atmosphere. Overheating is a time dependent process, and this must be considered in each Case, especially those involving complex changes. Generally, high and medium carbon steels are more likely to be overheated than low carbon steels. although special applications may call for coarse grained steels, their low resistance to shock and greater tendency to distortion are dissuading factors (9). "The fracture test has become useful to study the effects of forging temperature and overheating on steels, particularly alloy steels" (10). It is quite evident that for the more complex alloys, overheating must be carefully controlled if a serviceable forging is to be produced. There is no test or procedure whereby a manufacturer can measure the results of overheating. This is an area where quantitative and qualitative information on the degree and effect of time, temperature, and composition should be made aVailable through research. Then, the true effect of thesephenomena on the finished forging will be better appreciated. "Burning is defined as heating the metal to a temperature so close to its melting point as to cause permanent injury to the metal by intercrystalline penetration of oxidizing gases, or by incipient melting” (9). The injury which is caused by burning cannot be cor- rected by any known heat treatment. It may be considered as the stage following overheating. For most materials, burning is avoided by keeping the temperature below a certain maximum. Charts have been compiled for many materials, such as shown in the Metals Handbook, 1948, page 349, which gives the maximum forging temperature to avoid burning. As in overheating, the detection of burned steel is not readily revealed by examination. Sparking is some- times visible during burning, but this is not a necessary or sufficient condition for burning to occur (8). Certain metals when used as alloying agents increase the suscepti- bility to burning (Ni, Co, and Mo), while others (Cu, 1, Cr, and N) have the opposite effect on steels (9). The (I) mechanism and compositions necessary to control burning are known only qualitatively. Resistance to burning is excellent in a strong reducing atmosphere if the carbon content is below 0.84 per cent (4). The same relation as to carbon content exists in burning as in overheating with the higher carbon being more easily burned. nesearch is needed to determine the effect of time, temperature, composition, and their relative importance on a burnt structure. UNDERHEATING The term underheating is self—explanatory. It is used to denote a condition where the temperature is so low that plastic deformation will not proceed without rupture. This defect is not as common as overheating and burning since the hammer operator can often detect it by the manner in which the stock flows under the blow of a hammer. Good furnace control is a valuable aid in keeping underheating in check. Portevin states "Most hot metals can be deformed to a great extent without rupture or tearing — at least within certain limits of temperature where the material is sufficiently" forgeable or susceptible to considerable deformation without cracking. It is, in short, a tendency toward cracking which limits the scope of these shaping processes, crackability being in a way the reverse of forgeability" (ll). Difficulties arise due to a tempera- ture gradient existing in th material. While the surface is in the safe thermal zone of working, the center is below it when underheated. 9 The theoretical study of plasticity is a compara- thmly young branch of science and still developing. The fines of plasticity have not been utilized to any great mdent in the study of forging. Efforts to apply these nfles have achieved only moderate success. The research hastmen largely concerned with the forces developed from much predictions of the power requirements are made. ldttle concern has been accorded to the flow of material using the rules which have been developed. This is partly due to their intrinsic complexity. If greater knowledge of metal flow were available, better establishment of both upper and lower limits of forgeability would be possible. Research on the loss of temperature during forging has developed the following relationship (12). Temperature £3 T : $229 inOC change 5 S is the smallest of the three dimen- sions in mm. of a piece with a rectangu- lar cross section. This formula gives the temperature change in a ten sex:orui interval when the starting temperature is 115000. {Drily' radiation in still air is considered in this case, .f23r* if‘tbe piece were resting on a steel plate at 300°C, ‘tkie ‘temperature drop increases by a factor of 1.7. 10 Research on the plastic flow of metals and the rate and mechanism by which a forging loses heat (en route to hammer, during working, and between blows), similar to that stated above, would provide data to minimize under- heating problems. OXIDATION AND SCALING The oxidation layer formed on the surface of a metal heated to a high temperature in air or in an oxi— dizing atmosphere is denoted as scale. Many metals form scale according to the parabolic relation x2; kt x is the weight of scale formed, t is time, and k is the rate constant (8). This equation has been shown valid when a dense and ad- herent scale is formed. In this case, the specific volume of the oxide is equal to or greater than that of the base metal. The only known exceptions are tungsten and molybdenum. The rate constant k is temperature dependent and may be expressed by the Arrhenius equation k = 5 exp (- g_) RT A and Q are constants, R is the gas constant, T is the absolute tempera- ture (8). Those metals with oxides having lower specific volume than that of the metal lose weight by a relation which increases ll ililiesarly with time (8). The noble metals have oxides vvitii higher dissociation pressures than the partial pressure of oxygen in air at the oxidation temperature, tknas :fonning no superficial oxide. Oxidation is a diffusion phenomenon. It occurs ix; an outward movement of metal ions from the base metal, EHK1 an inward movement of the oxygen ions from the at- Imosphere. In both cases, the ions must pass through the already formed oxide layer. The phase diagram of the iron-oxygen system shows three phases - wustite, mains- tite, and hematite. The wustite phase is unstable de— composing below 560°C. The accompanying figure shows diagram- matically the manner in which scale appears on steel. Wustite Magnetite Hematite, FeO Fe304 Fe203 Iron. Oxygen Fe v——————Fe-————+<: 02 02 80-90% 10-20% .5-2% HIGH TEMPERATURE SCALE Magnetite Hematite Iron F9304 F8203 Oxygen Fe 02 LOw TEMPERATUOT SGnLE (Per Lewis) (13). l2 hialloy steels the alloying element is often fmnm.cnmentrated in the outer layer of the base metal amitheinnermost oxide layer. This has been demonstrated xflmn aflicon, nickel, chromium, and copper are the alloy— hwgagents. Lead is suspected of behaving in a similar nmnnervflwn it is disseminated in steel for free machining There is considerable difference in the power of numerous elements to diffuse through the scale. This leads to the conclusion that distribution of the alloying element in the scale is due to the relative rates of diffusion of iron and the element considered. It has been shown by Wagner that as the electrical conductivity of the oxide decreases, so does the rate of scaljlug. Since oxides of highest melting point have lowest conductivity, those metals which form these oxides give the best resistance to oxidation when used as alloying agents (8). ’The nmial composition exerts its influence on the seedling;3ohenomenon,greatly affecting scale composition. It may also cause great difficulty in scale removal (13). .Faxztors directly responsible for scaling are oxygen, water vapo r, caroon dioxide, and sulphur arranged in the 13 ordercfl‘their relative importance. The reaction of othergwmes may also assist in the promotion of scale. Smallemmunts of sulphur gases in an oxidizing atmosphere also, it is quite causeeimarked increase in oxidation; harmflfl.in the form of H28 or organic compounds (14). Sulpmncis usually present when gas or oil are used for fuel. The amount of scale formed on a forging during heating is given as approximately ten per cent of the and between two to seven per weight for large forgings, cent of the weight for small forgings. Nork done by Lee- Bird has shown a figure between two to three per cent to be reasonable for small pieces (13,14). Showalte: states "Scale is more than lost metal; it is lost dollars" (15). The effect of scale is felt'in 1) lost metal, increased cleaning, many ways such as: especially in the ex— pensive high alloy materials, 2) pickling, blasting, tumbling, and grinding, 3) higher re- longer heating times jection, 4) exceeded tolerances, 5) due in: Insulating value of scale especially on large pieces,6) unacceptable surfaces,7) higher machine cost, mozwe firrishing passes, more restrikes, and more finish machining,€) fluxing of furnace bottom.9) lastly, it 14 shorflnm die life. This alone is important enough to ewnmethe remark that, "the avoidance of scale,which is :mssflfle with induction heating, offers definite advanta- ges if only in toe interests of prolonging die life"(16). This hithe face of knowledge that "Scale has a serious abrasive action on dies and punches, but with other factors it is difficult to assess the exact affecting die wear, This first reduction in die life due to scale" (14). quotation is remarkable due to the incompatibility of the above two statements. English drop forgers consider the removal of scale important enough to warrant its removal before forging. This is done at forging temperatures when no attempt is made to control it during heating. Scraping and brushing are the two mechanical means of removing scale before or during the forging. The applied methods are blowing and sprayimmg. water at 1500 - 1800 psi pressures has been used.snnccessfully and is described in reference 13. Another method used is that of passing the part through rapid thermal shock which (a G an induction coil giving it removes the scale (17). The nature and. effects of scale are well established. The research necessary is that of establishing the conditions where scaling can be reduced to no n impo rtance . 15 LEG mam: gm ION Decarburization is "a loss of carbon from the mnfitce of a ferrous alloy as a result of heating in a medium that reacts with the carbon" (8). As in scaling, decarburization is an effect of heating in an adverse atmosphere. The reactions and types of atmospheres which cause decarburization are well known. As applied to forg- ing, decarburization goes hand in hand with scaling since they are both greatly affected by furnace conditions. Much of the present day heating is accomplished in equip- ment where little control of decarburization is possible. Elimination of decarburization is not important if scal- ing is uncontrolled since the scale contains the affected It becomes increas- areas, and it is usually removed. ingly important to control decarburization when scale is being regulated, especially if a forging relatively scale free of close tolerances is required (ie. induc- tdcui heating, radiant gas heating, etc.). The opposite is relatively unimportant, though effect, car-burization, exaHgXHss of forging failures by this mode have been descrfilxai (18). Carbon restoration after foraing is de— manding; more attention as higher strength forging-gs are 'beirmgrnanuféctured. Research in decarburization must be alcnng tins same lines as in scaling, for these two effects have a high degree of interrelationship. l6 RATE OF HEATING The rate and evenness of heating becomes increas- nmdy hmmrtant as attempts are made to increase the speaicfi‘the process of heating. Past experience has dichflmd preheating and slow heating to temperature to insure a product free of clinks and fine cracks. Also, this was deemed necessary to insure free metal flow during forging. Work done before World War II in England Additional indicated that more rapid heating was feasible. work of more recent origin (19,20,21) has further sub— stantiated these claims by rapid heating with gas using Some of the advantages listed for radiation techniques. 1) uni- this type heating besides speed are as follows: form metal temperature 2) little scale formed 3) grain growth is retarded 4) decarburization is reduced and 5) more favorable flow of metal in dies. The claim of improved forgeability with this rapid has not been proved. There is some evidence that heating soak is needed as indicated by some experience a short with induction heated billets (22). {The main danger of too rapid heating is that of causing internal clinks. These are due to thermal ex- pansion of the outside layer causing stresses sufficient to fracture the interior. Any residual stresses set up 111 thus uniterial oy processing to the billet stage prior 17 to mflmating add to this effect. Claims made originally oyeiPolish worker E. Terlecki on rapid heating have been dmmkedtw'English investigators and found substantially true. The following table shows some of Terlecki's (22) times for heating to 120003 without clinking. The pieces werecflwxged cold into the furnace at 135000. Heating times for various steel pieces from cold to 120000 ready for forging (according to Terlecki) (22). Heating Size Height Type of Steel time iingot) (approx.l Hr. Min. 36 in. dis. 8 tons 0.2% carbon 4 15 30 in. thick slab 12 " 0.2 " 3 20 21 in. dia. 23 9 18/8 austenitic 2 30 15% in. slab 2g " 18/8 " 1 36 7.2 in. dia. 310 lo. N-Cr. V-Go high- 5 40 speed It is stated that the radiative capacity of the furnace 'walls depends on the difference between the fourth powers (If the absolute temperatures of the walls and work pieces. This 1J3 shown by the following formula: Radiation Capacity of the RC . Ewa)4 — (TprE-la furnace walls where T is the absolute temperature of the furnace walls and T”- is the absolute temperature of the " work piece, a, a proportionality constant. l8 Thus,gnmhin; up the wall temperature greatly increases For example, theemwunt of heat transference to the work. felony the furnace wall temperature from 110000 to lAOOOG would nearly double the rate of heat absorption tw'thexmuk.piece (22). Jork by the Selas Corporation of America in heating die blocks has shown that remarkable gains in heating times can be obtained (23). Blocks which took over twenty-four hours to come to temperature in the conventional furnace have been heated in less than four hours. ”7‘1"“. .7 JiLiuLi’..:ii.J a discussion of possible solutions Before-entering to the above proolems, it is wise to review the type of equipment used in the industr". The common heating fur- naces in use are as follows: 1) batch furnace - usually slot type, 2) pusher and automatic - conveyer type, 3) rotary hearth.furnace, 4) induction furnaces. In small plants the slot furnace is almost in ex— cliuaive use. Larger plants have the pusher type with the indiurtion and rotary hearth (controlled atmosphere) used only':flor high production continuous runs. The larger plenots vvere careful of the furnace temperature and used temperature controls to regulate it. In none of the small forges visited, did the author see any temperature l9 contnfls being used. Some operators of the smaller forges gmkecfl'trying controls but with little success, and fhufljy reverted to the age old method of the experienced operatu”s eye and knowledge. The use of temperature recorders my small forge shops is known to be successful (24, 25). The main obstacle to overcome is that of edu— cating the operators of the furnaces in the use of instruments. The importance of good temperature control cannot be over-emphasized. although the forging range of the low carbon low alloy steels is wide, many of the newer alloys have narrow ranges of forgeability. It is impor- tant to forge rapidly at the highest possible temperature. Siebel's deformation formula is P = (F) (Kf) where P is the pressure required for deformation, F is partial cross section of compression, and Kf is the mean tensile strength. To take into account losses caused by internal and extedunal friction, the mean tensile strength is replaced by the deformation resistance Kw K - Kf w — LE where N is the forming efficiency. 20 It hswell known that the deformation resistance is con- tnflledtw'temperature. It decreases as the temperature huneases becoming constant in the range 17000 - 23000? it is clear that the deforma- Ibr many steels. Therefore, tunifbrmula given above is controlled almost exclusively tn the temperature. Consequently, if materials are to be deformed easily with minimum wear and tear on equip- ment, especially on dies, the temperature should be held Therefore, temperature and time at as high as possible. temperature must be closely controlled. Automatic tempera- ture controls can assure these results consistently, the human operator adsinisters sporadic treatment whereas, due to his approximate method. a recent survey by the Drop Forging Association indicates that the furnaces in Operation.today are running far below their possible Only a few furnaces are equipped with efficiencies (26). This sug- preheaters to aid in raising this efficiency. gestms‘that a thorough study of operation using temperature contdfixls, with and without preheaters, would yield valuable Lnfornurtion as to optimum operating conditions. Estab- lisflnnent;<1f the economy of using heat savers could be ascertained. iStuéies of the newer types of heating devices should Induction heating with its many be made at this time. 21 advanhwes can certainly do a bigger Job than it is pres- afifly doing. any process, which offers the following: ID faster heating 2) cleaner heating,3) uniform heating, 10 little scale (increased die life, closer tolerance, less lost material,5) ized (requires unskilled labor),7) improved surface con- less decarburization,6) mechan- dithnLED cool working conditions,9) compact and adaptable saving space,lO) lower noise level,and 11) should be more little energy wasted, as does induction heating, thoroughly inVestigated. The main disadvantages are the high initial cost, higher operating cost, and its lack of adaptability for heating various sized pieces. The first two mentioned are offset by the many advantages while the latter can be overcome by development of an adjustable inductor, especially for the small sizes between 1" to 4". These features are illustrated in references 279 28’ 29: 309 and 310 The newer fast heating radiant gas furnaces (19,20, 21” j52, 33) have recently started to receive their Just attenrtion. They have the advantage of low initial cost octuiled.1vith speed of heating which reduces the scale and decaJQNArization making them quite attractive for use in forging. 22 Other promising methods which need more research finders they can be adapted to forging are as follows: 1) glass baths such as used in Italy for heating extrusion billets (54L 2) increased use of salt baths especially on larger forgings where the drag out of salt is minimized (35L 3) resistance heating which is considered more economical than most others if successful. The main limitation of this last method is that current has to be fed by contact pieces at the ends. a method has been developed in sweden using a plastic like sponge which is a good conductor of electricity and may be the answer to this difficulty (22). a comparison of different methods of heating for forging would be an important step in the solution of heating problems. Factors necessary for consideration would.be as follows: Economic status of each method, relative'speed, effect on material heated, (especially vfluare speed.is high), starting condition of material (ie. yvith regard to residual stress and shape), and neces- sary'cuontrol. With this knowledge, choice of the correct metluxd of heating would be simplified and the means to the prmnhoctiorlof‘better quality forging would be close at harui. 23 A means to evaluate the relative importance of eachibcet of the forging process is required. Labora- tonynwans for separating the various effects are being ckweloped. C. L. Koloe of the General blectric Company has developed a technique for forging in an inert at- .MU mosphere (56). This allows the factors of heating to be singled out from those of forging by controlling the atmosphere while working the metal. Research similar to this must be made available; then, and only then, will the true importance of heating be put in its proper perspective. AQPLCTS OF DIE BLOCKS DEVELO KENT an important factor to success in the manufacture ofeifbrging is the quality of the die which forms the piece. Iflodern machining facilities producing superior dies are the secret of success in many small forges. The forge shop is responsible for the care and maintenance (including replacement) of the die following the initial die purchase. This places the burden of research and development squarely on the forging industry if its present competitive position is to be maintained. The requirements for the die blocks have risen steadily as the magnitude of equipment and tonnage produced has increased. Until 1915, the is blocks in use were annealed carbon steel. after machining they were heat treated (quenched and tempered by-color) to the desired hardness. They were usually distorted and had decarbur- ized surfaces. This led to the present day use of alloy steels (23) . Early experience with nickel steels in the pre- klandenedcondition led to their modification with chromium. ‘ft tine same time, molybdenum steels became available and tlisey'xvere developed for use in die blocks. The nickel- Cklrwamiunknmlybdenum.and chromium-molybdenum-vanadium are 25 mn>of the most widely used alloy steels in service today. Theaflloying agents mentioned increase the strength at elevated temperature and improve hardenability. In addition, molybdenum aids in reducing temper brittleness. The majority of die blocks are supplied in the '— prehardened condition. attempts to produce a free- machining steel in the hardened condition by alloy ad— ditions have been unsuccessful. Development of tool steels and carbide cutters has simplified the machining, but the demand for high hardness has exceeded this pro- gress. Hardness values from 55-60 Rockwell "C" have necessitated the use of die inserts, which are hardened after machining, in many of the large hydraulic presses. his is especially true when forging the light metals - aluminum, magnesium, and titanium. The major characteristics of a good die block are: l) tough enough to resist work stresses.2) heat treated and, of such composition to resist softening in service and.51'tendency toward fire checks,3) adequate wear re- sistdxng qualities,and 4) no internal defects developing after (lies are sunk (37). These general features are diffixnalt to achieve in any one composition or alloy. Ctheri'the emphasis is on one factor and the others are :neglrx3ted. The features desired depend on the design of :- anew-QT..— ~ o... '- 26 umgnam, type of metal and the equipment used for mryng(ie. hammer, press, upsetter, etc.). sharp corners, quick change of section size, and vmy Hun sections all require special attention and 'MEnzmssiole should be avoided. Choice of material has "as ' rule, it is the a. agxeatbearing on die life. constituents which determine the relative forg— alloying ingense in producing the part. By increasing the alloy- hwrconstituents, we increase the forging difficulties wear on the impression of the dies" (37). Other variables are section size, close tolerances, and heat- ing procedures. With so many changing components, it is difficult to type dies in reference to their wear characteristics. Dies for aluminum, magnesium and titanium require hardeq~ : n 8 0.2 h :3 Forgeable / 1' 0.1 // I P 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Per cent Manganese EFFECT OF MENGM'IESE AND SULPHUR ON WRGEA- BILITY (Anderson, et al) (75). Sulphur is the principal impurity which affects hot workability. The role of manganese is that of a desulphuriZer and deoxidizer and alloying agent. The amount of manganese needed is given by the following formula which holds for above .03% S and .06% En %Mn required : 1.25 (%s)-+ .03 It was found that varying the carbon and aluminum 51 (used for deoxidizing) had no effect on hot forgeabil- ity. Ihrig (66) has made a list showing the qualita— tive effects of various elements on the hot working characteristics of steels. these effects. The following table shows Little or Beneficial Detrimental no Effect Effect Effect Oxygen Manganese Sulphur Carbon Nickel Selenium Phosphorus ' Chromium above Silicon 9,2: Cobalt Nitrogen Vanadium Molybdenum Titanium Uolumbium Lead Tin Chromium be- low 9% EFFECT OF VfiRIOUS ELEMENTS ON "0T WORKING CHaRnCTERISTICS (per Ihrig) (66). It is interesting to note that in all these in- vestigations of hot workability, the hot twist test was used as a measure of hot working. These studies show the necessity for research on the many minor constituents to determine the quantitative 52 effect of these elements on the forgeability of steels. Clark (65) recqnized the fact that "deformation characteristics and path of fracture in steels is a function of temperature and rate of application of stress". Using the hot twist test, in which the rates of defor— - mation were of the same magnitude as in forging, he found that the material which failed above the maximum given by the twist test did so by intercrystalline failure. Also, the rate of defornation was no longer critical above a certain rate. Sachs (76) states that "a very pronounced, but comparatively simple, speed effect exists in the tempera- ture range of true hot working. The higher the speed, the larger is the flow stress, while the ductility is generally unlimited and no hardening is retained after forming." He states further that in reference to the magnitude, if the forming velocity is doubled, the re- sulting flow stress and forming forces increase about 10 to 20 per cent. This simple relation allows a test to be conducted at slow speed. after adding a certain amount to account for the difference in speed between the test and the actual process in estimating the force and power required for a particular operation, the ex— perimenter should have a conserVative estimate. This is 53 due to the temperature increase of the part when formed at high speeds. The flow stress or resistance to deformation de- creases as the temperature increases. nt any given tem- perature, it will tend to level off in value. In general, it has a higher value under dynamic than static loading. It also increases as the speed of deformation increases. Reference 64 presents graphs showing this effect. Ellis (71) demonstrated that grain size has a minor effect on deformation resistance. The fine grained steels have greater resistance at forging temperatures. Impact effects also enter into consideration in forgeability. Under static loads, the stresses follow known rules. In rapid force application, the relation is more complex. Sachs (76) distinguishes two periods, an initial and stationary period, under such conditions. The stationary period, in which there is no acceleration or deceleration follows the laws of mechanics for stress prediction. It is during the initial period, when the velocity is changing that no rules for stress distribu- tion have been advanced for plastically deformed materials. When the fast moving tool collides with the work, energy is released by the tool. It is dissipated in two ways; 54 some goes into deformation of the equipment, and the rest into plastic deformation of the metal. The en- ergy deforming the material accelerates the particles near the surface where the tool and piece contact, but this effect diminishes with depth of pentration into the piece. Very high stresses occur at the surface in the initial period during the forming Operation. Consequently, the higher the velocity, the more the deformation will be concentrated at the end of the form- ing period. In forging, if the blows are light and fast, the deformation will be concentrated near the contact area, while with a slowly applied load, the de- formation is uniformly distributed. Cook (77) has studied the effect on the mode of deformation. He considered these aspects: a) the ef- fect of tool geometry and forging schedule on the mode of deformation and strain distribution within forged stock, and b) the extent to which mechanical properties are dependent on forging strains. His results show clearly that tool geometry is important, and that in- homogeneous strains can lead to variations in mechanical properties. This article concludes that plasticine models can give quantitative results on the measurements 55 of strains in the interior of forged models. These re- sults can be correlated with that of steel, for proof is given that plasticine is a satisfactory model for steel at forging temperature. It should be clear that the use of plasticine models in research can aid in giving the necessary in- formation needed to clarify the effects of speed and mode of deformation. This is the direction for research if progress is to be accomplished in understanding these effects. QTHE£.POS;IBILITIES Recent advancements in the technique of adding lead to steel have resulted in a better distribution of lead in ingots (78). Lead gives improved nachinability by al- lowing faster removal of metal by deeper cuts and faster operating speeds. The possibility of using lead in forg- ixngs seems an obvious procedure. Reevaluation of this technique is a research project worth time and talent. The dependability of a forging is based to a large exterm;on the planned directionality of flow lines in thee;for5ed part. Grain flow is inherent in the forging and.:is retained regardless of subsequent heat treatment. 56 Flow lines impart improved ductility, and impact strength in a direction parallel to their course in the forcing, while these properties perpendicular to flow lines are reduced. They are macroscopic and can be developed by simple polishing and etching. This fiber developed by etching has an unknown origin. Eany people believe that fiber is chiefly the ex- tension of constituents in the metal, both metallic and nonmetallic, in the direction of working. Others believe flow lines are connected with dislocations. No definite proof is availeole for either view. Full understanding of flow lines is necessary if an appreciation of their effect is to be attained. Research on the origin and effects of fiber would secure the answer to this problem. New and better materials are being developed with each passing year. The forging industry must keep a- breast of these developments. hesearch on the new vacuum cast and vacuum melted metals and alloys is neces— sary to determine their place in the forging industry. This research on forging characteristics should include their advantages and disadvantages over conventional materials as well as economic justification for their use. 57 another new technique worthy of mention is the practice of casting small ingot molds and fo ging directly from the the cast state as done in Germany. This would allow the forger to by—pass the work done by the steel mills and avoid the delays.in mill scheduling required for reduction to billet form. also, the forging manu- facturer, without too large an investment, could remelt the scrap resulting from flash (this amount varies be— tween 50-50 per cent on each piece) which otherwise is wasted. This utilization of material would have consid— erable economical value in lowering the cost of a forg- ing. This method requires research and development to prove that sufficient forging effect and properties can be achieved from the cast state. The researcher should keep uppermost in his mind the fact that a great number of rejections are caused by defective steel. For the very best forging methods cannot make a good forging from defective steel, while jpoor forging practice can asks poor forgings from steel of'excellent quality. SUMMARY There are many areas for research in the forging industry. It is a field old in age, but young in develop— ment. For too many years, the industry has relied on the merit of its acknowledged superior product. The forging industry has allowed products, with lesser pro- perties, to narrow the gap of superiority through re— search. If the industry wishes to maintain its present position, the only alternative is renewed and increased progress through research. Generally, the basic problems have been known for many years. However, due to the empirical nature of drop forging development, little has been accomplished which would lead to fundamental understanding of cause and effect relationships. The attitude of indifference exhibited by forging manufacturers, which accepts such phenomenon as scale formation as a necessary evil is not conducive to continuing progress. Heating, which has been a part of the industry since its inception, requires much development before it can taKe its place as a science. Amidst some of the newer equipment, heating furnaces seem to be a product of the middle ages. 59 Specifically, the areas recuiring research in heating are: i) 2) 3) 4) 7) Development of a test that will determine the effect of overheating and burning. Development of quantitative and qualitative information on the effects of time, tempera- ture, and composition on overheating and burning. Research on scaling and decarourization to establish conditions where these phenomena q can be re uced to nonimportance. \ ( Investigation on the newer types of raaid heating equipment (induction heating, radiant $l gas heating, resistance heating) with special attention to the rates of heating and their effect on the material being heated. Study of furnace efficiencies, with and with- out preheaters, using temperature controls to establish their adaptability for forging furnaces. Study of temperature losses during th forgin cycle to minimize the effects of underheating, Development of a method or methods to evaluate the relative importance of each facet of heat— ing. 6O Concerning die blocks, the following require re- search: 1) 3) 4) U1 V Study of die wear using radioactive tracers or fully activated dies. Establishment of the effect of scale on die life. Effect of die finish on die life. Development of a means for measuring relative extension of die life over the normal die life. Review of the merits of chromium plating and other coatings with special attention to type of die plated and manner and method of plating. Investigation of other methods of surface 'treatment (ie. nitriding, peening, high heat treatment, etc.). Study of various aspects of lubrication of dies. Methods of completin: the finished impressions especially spark erosion and cast die blocks. In regard to other possibilities for research, the following are important: 1) Research on hydrogen diffusivity and solubility under various conditions and the effect of structure and stress on these properties. This is necessary to designate the exact role of hy- drogen in flake formation and embrittlement. s: 2) Evaluation of a test for forgeability in closed dies. 3) Effect of alloying agents on hot workability. 4) Use of plasticine models for quantitative strain measurements to clarify the effects of speed and mode of deformation. 5) Ee-evaluation of lead in forging to promote improved machinability. 6) Study of the origin and effects of flow lines. 7) Investigation of forging from the cast state. 8) Betermination of the forging characteristics of vacuum cast and melted alloys plus any of newer metals and alloys. It is fortunate that many of these remarks do not apply to the entire forging industry. although a number of larger firms are doing research, there is no free exchange of information in this highly competitive industry. Much of this research could be accomplished in institutions of higher learning. Other progress will re— quire close cooperation between industry and institutions of higher learning. Problems should be approached from a practical as well as theoretical viewpoint in order that results will have immediate significance. The founding 62 of a drop forging research laboratory, similar to those in Hanover, Germany and Sheffield, England, under the auspices of the Drop Forging Association, would pro- vide a center where common problems of both the small and large forger would be evaluated and solved. n supply of current and past information would allow many of the manufacturers to solve problems without taxing their own organization's time and money. Hesitstion on the part of the forging industry to allot funds for research can lead only to decreased utilization of forged products by future purchasers. an awakening now can avert this dismal future which awaits the drOp forging industry. \J) v 6) 7) 8) 9) lO) BIB LIOGRHPHY Kyle, P. E., The Closed Die Forging Process. 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