iRliS942
Bureau of Mines Report of Investigations/1985
Application of Cast-On Ferrochrome-Based Hard Surfacings to Polystyrene Pattern Castings
By J. S. Hansen. R. R. Jordan. S. J. Gerdemann. and G. F. Soltau
UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 8942
Application of Cast-On errochrome-Based Hard Surfacing to Polystyrene Pattern Castings
By J. S. Hansen, R. R. Jordan, S. J. Gerdemann, and G. F. Soltau
UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary
BUREAU Of MINES Robert C. Horton. Director of Congre::;s Cataloging in Publication Data:
Application of cast-on ferrochrome-based hard surfacings ro polystyrene pancrn cascings,.
(Burcau of 'Iincs report of in \'cstig,Hion s Bibliography: p. Supt, of Docs. nO.: I 28.23:8942. I. Hard.facing. 2. Hard-facing alloys. I. Hansen, J. (J cffrcy 5.). II. Series: Report of investigations (United States. l3ureau of \1ines) ; 8942.
TN23. U43 622s CONTENTS
Ab s t r ac t .•••..•....•••...... ••..•...... •..•...... •...•. 1 In t roduc t ion ...... •...... •...... 2 Background •..••...... •.• " ...... •...... ••...... 2 Process of cast-on surfacing with polystyrene patterns ••••••••••••••••••••••••• 3 Experimental procedure .•..•.....•....•...••..•...... •...... •...... 7 Definition of variables affecting ferrochrome cast-on surfacings ••••••••••••••• 7 Results ...... 10 Binder type .....•..•...••...... •.•...•••..•••.•...... •• 10 Bi nder level ...... •...... 11 Surfacing material particle size ...... •...... •••...... 12 Pour ing temp er a t u re ....•••...... ~ ...... •.....•....•••••....•...... 12 VacullID. level ...... •....•...... •...... •..•...... 14 ~pplication procedure .•••...... ••...... 14 Cas ting thickness ...... ••••...... •••.... 15 Surfacing thickness •...... ••...... •...... ••...... •...•... 15 Compositional variations on ferrochrome surfacings ••••••••••••••••••••••••••••• 21 Steel and white iron substitutes for ferrochrome ••••••••••••••••••••••••••••••• 23 Field trials •...... •...... •..•.•...... •...... •...... 24 Bucket-wheel excavator teeth .••••.•...... •...... •....•...... 26 Plowshares •• ...... 30 Conclusions ••• ...... 32 References .•...•••...... ••••••.••.•...... 33 Appendix.--Ranking guide for defects ••••••••••••••••••••••••••••••••••••••••••• 36
ILLUSTRATIONS
1. Process steps for applying cast-on surfacings to polystyrene pattern cas tings •• It. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 4 2. Hard-surfacing material applied to a polystyrOene pattern corresponding to the casting wear-prone area •••••••••••••••••••••••••••••••••••••••••••••• 5 3. Pattern placed in unbonded sand in double-walled vacuum flask ••••••••••••• 6 4. Stopper rod procedure for preventing slag and air from entering gating s ys tem and cas ting ...... ••.....••••...... •...... 8 5. Pattern incorporating sample ridges for three separate surfacings ••••••••• 10 6. Effect of surfacing binder level •••••••••••••••••••••••••••••••••••••••••• 13 7. Effect of particle size ••••••••••••••••••••••••••••••••••••••••••••••••••• 13 8. Effect of temperature on external casting porosity at two vacuum levels ••• 14 9. Microstructure showing dendrites and interdendritic carbides typical of thin (1/16-in), chromium-rich, white iron surfacings derived from f errochrome ...... 16 10. Effect of increasing surfacing thickness •••••••••••••••••••••••••••••••••• 17 11. Microstructures of surfacings at three thicknesses •••••••••••••••••••••••• 18 12. Ef fec t of surf acing thickness •....•...... 20 13. X-ray radiographs showing inclusions at the surfacing-casting interface of samples surfaced with commercial ferrochrome and number 2 low-impurity CllS tom-mel ted ferrochrome ...... •....•.•.....•...... 21 14. Microstructure of number 3 custo~melted ferrochrome and commercial ferro chrome with 0.1 g added carbon per gram of ferrochrome ••••••••••••••••••• 22 15. Cast iron microstructure of cast-on surfacing with the composition Fe-5C- 21Cr-8. 5Cb-9Mo-O. 3Mrl-O. 6Si...... • ...... 24 16. Cross sections of a 4-in ball valve, 6-in crusher plate, and 3-in cylinder showing ferrochrome cast-on surfacings ••••••••••••••••••••••••••••••••••• 25 ILLUSTRATIONS--Continued
17. One of eight buckets of a bucket-wheel excavator at a Centralia, WA, coal mine...... • . • ...... • . • ...... • . . .• . . • . . • ...... 26 18. Bucket-wheel excavator tooth pattern with surfacing powder applied to outside. .•• .... .•...... • .. . .. •. • ...... •...... 27 19. Radiograph of bucket-wheel excavator tooth showing small voids at casting- surfacing interface...... 28 20. Microstructure of number 2 custom ferrochrome and white iron alloy sur·' faces from bucket-wheel excavator teeth used in field trials...... 28 21. Cross section of bucket-wheel excavator tooth tip showing ferrochrome sur- facing along outside edge...... 29 22. Bucket-wheel excavator teeth after field trials at Centralia, WA, coal mine ...... •..••...... •...... •..•..•...... •...•...... •..•...... 29 23. Plowshare affixed to lower portion of one of five blade assemblies...... 30 24. Weld-rod hard-faced control plowshare and ferrochrome cast-on-surfaced plowshare after field trials...... 31
TABLES
1. Binders investigated for use with ferrochrome powders...... 11 2. Experiments to determine effect of five variables on surfacing integrity with centerpoint checks...... 12 3. Experiments without centerpoint checks to determine effect of five vari- ables on surfacing integrity...... 15 4. Effect of surfacing thickness on surfacing composition and hardness...... 19 5. Dry-sand, rubber-wheel abrasion test results on cast-on surfacings derived from ferrochrome powders...... 20 6. Compositions of white iron and steel cast-on surfacings and selected dry- sand, rubber-wheel abrasion test results...... 23 7. Plowshare field trial parameters and test results...... 31
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT
°c degree Celsius lb pound DPH diamond pyramid hardness lb/ft 3 pound per cubic foot ft foot min minute g gram mm millimeter g/in2 gram per square inch mm3 cubic millimeter h hour pct percent in inch s second in2 square inch vol pct volume percent in Hg inch of mercury (atmospheric pressure) LI ION CAST-ON FERROCHROME-BASED H S ACINGS POLYSTYRENE PATTERN CAST INGS
4 By J. S. Hansen, 1 R. R. Jordan,2 S. J. 3 and G. F. Soltau
ABSTRACT
To advance technology that can conserve critical materials, the Bureau of Mines investigated a cast-on hard-surfacing that wear resistance and confines alloy additions to wear-prone surfaces. Wear-resistant material in form is on a polystyrene , the is embedded in a mold of unbonded s and the wear-resistant material is transferred to a cas sur face at the time of For low-stress abrasion, derived from high-carbon ferro- chrome had higher wear resistance than did commercially available weld- hard The microstructure of the was typi- white iron carbides. Free carbon added to the ferrochrome powder produced more M7C3 carbides than did material contained in surfacings made from ferrochrome alone, and improved the wear resistance. Several white iron powders with greatly reduced chromium contents also white iron contain- M7C3 carbides. One derived from white iron powder had wear resistance to that of weld-deposited hard facings. Among the parameters investigated, surfacing thickness was found to have a or influence on diffusion of elements out of the , microstructure, and wear resistance. The results of field tests on bucket-wheel excavator teeth and plowshares with cast-on hard surfacing are given.
3 Chemical technician. Albany Research Center, Bureau of Mines. , OR. 2
INTRODUCTION
The Bureau of Mines is concerned with part may cause the overlay to crack, and the conservation of critical materials the heat generated during application may and improved performance of equipment result in harmful residual stresses or used in the mining and minerals process loss of heat treatment benefits to the ing industries. One of the major causes part. Excessive dilution of the overlay of failure of this equipment is wear. by iron from the part or absorption of Depending upon the severity of wear, atmospheric gases during application may equipment components may last as long as reduce wear resistance or cause failure several years or as briefly as a few upon impact. hours. The high costs involved, in terms Coatings also are applied by flame and of both labor and raw materials, have plasma spraying of powdered materials. provided the driving force for many ef Such coatings are uniform, and the pro forts to reduce wear. cesses are faster than weld overlaying. Commercially, wear is combated in many On the other hand, the bond strength is ways. The application of wear-resistant weaker than the bond strength of we1d coatings to wear-prone surfaces is one overlaid coatings, and consequently, way that offers significant advantages. sprayed coatings will not stand up under With the exception of manganese steels in impact loading. specific instances, coatings are the only This report describes the results of means available to achieve optimum wear research on a new method of applying resistance and optimum toughness in the wear-resistant coatings. Currently, the same part because these two mutually de method, called cast-on surfacing, is lim sirable properties are inversely related ited to steel castings. Two features are to one another. Through coatings, parts unique: It is performed in conjunction that are made tough by alloying and heat with the process of casting, and it uses treatment can also be made wear resistant polystyrene pattern technology. In the on the surface. In addition, expensive method, a layer of wear-resistant mate and difficult to form carbide and boride rial in powder form is applied to the materials can be routinely and economi surface of an expendable polystyrene pat cally included within surfacings. tern. After low-temperature drying, the Welding an overlay from material con pattern is placed in a flask of unbonded tained in a rod or laid down as a powder sand. Upon pouring, the pattern vapor is the most widely used technique in com izes, and metal fills the cavity. Coin mercial practice for applying coatings. cidentally, the molten metal permeates During welding, both the part surface and and melts the wear-resistant powder in the overlay material melt and fuse to one place. The resulting hard surfacing is another, creating a strong metallurgical metallurgically bonded and can be heat bond. Weld-applied coatings can be laid treated with the casting. Compared with over large areas and in heavy thick conventional coating processes, cast-on nesses. Unfortunately, the process is surfacing eliminates several steps and not without limitations and difficulties. reduces the dangers of residual stress Because weld overlays are usually applied buildup and heat cracking. Although by hand, the weld overlay process is time other hard surfacings are under investi consuming and expensive. Thermal expan gation, this report describes character son differences between the overlay and istics of ferrochrome-based surfacings.
BACKGROUND
The concept of cast-on surfacing of cast-in-place hard facing. The second polystyrene pattern castings is a com technology, polystyrene pattern casting, bination of two existing technologies. is also known as the evaporative pat The first technology, cast-on surfacing, tern casting CEPC) process, full-mold is also known as surface alloying or process, or lost-foam process. Only in a 3
few special applications has commer consumable polystyrene pattern that is cialization been achieved with either embedded in a flask. The pattern vapor- t izes, tern gases exit through the Cast-on methods have been at- sand, and metal fills the cavity t by researchers. vacated by the Researchers have techniques are used to make bimetal partially with carbon steel mill with alloy pickup and gas 18-24). outer and cores of cast iron. Several years after it was Liquid metal has been cast t pre was made more heated solids (2) or sintered compacts variance • ( However,-in general, most efforts which sand to be used to mate to cas have suf- bonded sand. However, more fered from gas from binders, ero- with mold e, sion, and shrinkage of the compact during were created. Vacuum has been used to and after Excessive diffusion eliminate mold and to assist in of material into the cas also gases has been a In summary. researchers have Other inventions and studies describe not been able to achieve satisfactory re- processes in which un sults in cast-on powder is laid loose or pasted methods. Erosion of pour- in a mold (7-13 to be assimilated onto ing, shrink voids in coatings, excessive the casting- surface during pouring. Two dilution of base metal, poor research teams in (1, 14-1 infiltration of base metal into coatings, appear to have been relatively success uneven coating thickness, poor surface ful. Both teams have applied finish, and porosity have been some of molds to eliminate the problems. Moreover, no binder has and both have been found to hold powder on the surface defect-free Co, of molds without rise to gas. chromium carbide, • or fer Steel founders who ted the polys rochromium. Notwithstanding these suc- rene tern cas process have not cesses, the vacuum were never been able to produce castings consistent- because the pro- free of erosion, , carbon cedures were not satis pickup, sand drop, and resid- universal for of cas ual defects. On the positive side, it tyrene pattern casting is the sec- has been demonstrated that cast-on sur ond exis that is incor- cas into the Bureau's cast-on surfac The use of (~). The process entails vacuum has been suggested as a means for metal into an expendable and relieving many defects in both processes.
PROCESS OF CAST-ON SURFACING WITH POLYSTYRENE PATTERNS
For several reasons, the polystyrene be easily applied to shapes and pattern casting process was chosen as a dried before use. If necessary, the pat tial means to overcome some of the terns could be stored for extended dura- difficulties that had been by tions. no earlier researchers of cast-on surfacing. the patterns could be oriented, The process appeared to offer unique ad- and risered so that the because would encounter minimal material in paste pouring and solidification. Reduced turbulence during pouring also rlined numbers in parentheses re was possible. Casting the patterns under fer to items in the list of references vacuum was a condition thought to be es p the sential for good bonding of the 4 surfacing. The application of vacuum to is dried in a forced-air furnace at 45° C a flask containing a pattern proved rela for a minimum of 2 h. A pattern with the tively simple. hard-surfacing material applied to a se The Bureau cast-on surfacing process lected surface is shown in figure 2. consists of several steps that are shown After drying the hard-surfacing materi graphically in figure 1. Prior to hard al, the entire pattern including the gat surfacing, patterns are constructed from ing system and riser is sprayed with a polystyrene. To minimize gas formation, refractory coating. Refractory coatings the polystyrene must have a density can be obtained commercially and contain between 1.00 and 1.25 Ib/ft 3 , which is either zircon or silica powders and small lower than that used in most other appli amounts of binders, alcohol, and water. cations of polystyrene foam. Sprues, The coatings are highly porous to allow runners, gates, and risers can be molded gases to pass. The purpose of the re integrally with the pattern or molded fractory is to improve the surface finish separately and joined to one another with and support the mold cavity after the a nonaromatic rubber cement. Joints and pattern is vaporized. The refractory other fillets are filled with beeswax. also is oven-dried at 45° C for a minimum Hard-surfacing material in powdered of 2 h to drive off the volatiles. form is mixed with a small amount of bin A flask that is double walled on all der and a fluidizer of either alcohol or sides and the bottom is used to form water and applied to appropriate surfaces the mold. The inside walls have screens of the- pattern. Application is done by to permit the passage of gas into the troweling or by alternately spraying the space between the walls, and the out pattern with binder-fluidizer and dusting side walls are vacuum tight. The pattern the sprayed areas with the hard-surfacing is placed in the flask (fig. 3), and powder. After each alternate application unbonded sand, such as one having a of powder, the excess is shaken off be three-screen distribution and an American fore the next layer of binder-fluidizer Foundrymen's Society (AFS) fineness (27) is applied. Subsequently, the surfacing of 75, is compacted by vibration around
--: Cl IIIII/IC . ~ /~~ : - ~~
A polystyrene pat Gates and rise rs are A po ste of powdered hard A refractory coating is sprayed tern is made . attached . mat.erlal is applied to on the entire pattern. wear-prone surfaces. r------
J - ?,:. R 01i 1 J J /-":/~.\\ I 1:1 1 I ~;;- ,... " . I ; )) I J /"' ~ . b . L ·--:::-> I J ~--. - - ~/ : J J J I I L ______J ~------~ I Sand is vibrated around the pattern The flask is sealed with plastic and J in a hollow-walled flask. evacuated. When the mo Iten meta I J is poured, the pat tern vaporizes and I I the hard material is fused to the t casting. I t l... ______
The vacuum is released and the casting drops away. FIGURE 1, - Process steps for applying cast-on surfacings to polystyrene pattern cast ings. 5
FIGl'RE 2. - Hard-surfacing material applied to a pofystyrene pattern corresponding to the casting wear-prone area. the pattern. The AFS 75 sand produces upon pouring as a consequence of the the best balance between a superior pressure differential between the atmos finish obtainable with a finer sand and phere and the evacuated cavity. A stiff improved gas elimination obtainable with plate can be placed under the polyethy a coarser sand. lene to equalize the effect of the atmos Generally, the flask is sealed with a phere and prevent cave-in. sheet of polyethylene plastic over the Castings also can be poured in vacuum entire top. Vacuum is applied to the process flasks that have double walls but flask prior to and during pouring to pro neither a top nor bottom. Polyethylene vide positive gas elimination, harden the sheets are used to seal the open ends, sand, maintain the shape of the molds, and steel reinforcing plates are required and draw the molten metal into the sur just inside both the sheets to prevent facing. Some castings that are located cave-in. near the top of the flask will cave in 6
FIGURE 3. - Pattern placed in unbonded sand in double-walled vacuum flask. 7
EXPERIMENTAL PROCEDURE
Castings for the experiments that casting. In addition, without the stop are described were poured at 1,555 0 to per rod procedure, air was ingested into 1,6200 C under a range of vacuum levels. the casting through a vortex that formed The vacuum was applied to the flask dur in the sprue. The metal head effectively ing the pour and for 10 min thereafter. prevented vortex formation. The pressure during pouring generally in Flat-plate castings were removed from creased by 7 in Hg but decreased to the the flask after an arbitrarily set prepour vacuum level thereafter. time of 2 h. However, other castings Low- and medium-carbon steels were have been removed in as little as 1/2 h poured directly from a 100-lb-capacity with no apparent detrimental effect. All induction furnace into 8- by 12-in pour castings were entirely cleaned by ing basins that were made of sodium sili sandblasting. cate-bonded sand and located over the Deoxidization of the melts was done pattern sprues. During pouring, a graph with ferrosilicon and aluminum. The sil ite stopper rod was placed in the basin icon content of the steel averaged 0."6 hole and removed when the metal head was pct, and the aluminum was under 0.2 pct. 2 to 3 in. The basin and rod proce Melt temperatures were measured with a dure, shown in figure 4, was effective in digital thermocouple just prior to pour floating slag to the top of the basin, ing. Flat-plate castings were inclined thus preventing slag defects in the slightly toward the riser.
DEFINITION OF VARIABLES AFFECTING FERROCHROME CAST-ON SURFACINGS
Commercial high-carbon ferro chrome was Independent one of the first materials to be applied as a cast-on surface. Among its advan Binder type tages, high-carbon ferrochrome is inex pensive, grindable, normally available at Binder level most foundries, compatible with steel, and nonreactive. It melts readily at Surfacing particle size steel casting temperatures while, for the most part, remaining in place. Upon so Surfacing thickness lidification, it precipitates an abun dance of hard carbides that are highly Casting thickness wear resistant. Among its disadvantages are a high strategic mineral content (65- Pouring temperature 70 pct chromium) and a typically high im purity content (up to 3 pct). As will be Vacuum level shown, these disadvantages were at least partially overcome. Powder application technique Several experiences and the literature (5, 13, 15-16) suggested that a system Dependent comb i ning--cast-on surfacing techniques with polystyrene pattern casting would Surfacing porosity contain many independent and dependent variables. Consequently, statistically Casting porosity designed two- and three-level factorial experiments were used to evaluate indi Internal porosity vidual and interactive effects of the independent variables upon the dependent Surfacing roughness variables. A list of the relevant vari ables in cast-on surfacing systems Surfacing contour follows: 8
FIGURE 4. - Stopper rod procedure for preventing slag and air from entering gating system and costing. Proportion of unmelted and seldom in disagreement more than surfa FIGURE 5. - Pattern incorporating sample rid-ges for three separate surfacings. Powdered surfacings have been applied. choke-runner-gate ratio of 1:2:4 was used defects. This gating system was used for to decrease the turbulence as much as all remaining tests. The flat-plate de possible and to prevent erosion of the sign was unchanged except for tests in surfacings. A riser was located opposite which the casting thickness was altered. the gate not only to prevent shrink, but For these, the plate thickness was in to serve as a trap for unvaporized sty creased to 1-3/8 in and 1-3/4 in, but the rene residues. A pattern with surfacing sample protrusion dimensions were kept powders applied to the three samples is the same. shown in figure 5. The rankings of dependent variables Initially, three gating systems with and other results were correlated with choke areas of 0.23 in2 , 0.39 in2 , and independent variables by computer. 0.56 in2 were tried on unsurfaced plates Surprisingly, good surfacings generally to insure adequate filling. Castings were obtained under a wide variety of produced from the gating system with the conditions. 0.56-in2 choke contained the fewest RESULTS BINDER TYPE the proportions listed in table 1 and were troweled on plate samples to an even Seven binders for powders were investi thickness. Either water or alcohol was gated independently of other variables. added to each to activate the binder, in The binders were combined with 50 g of crease fluidity, and improve application. minus 65- plus 150-rnesh ferrochrome in All but the nonaromatic rubber cement 11 TABLE 1. - Binders investigated for use Methyl cellulose and polyvinyl acetate with ferrochrome were j the most suitable among the binders. Both were Pct Minimum sive and formulated and Binder type used drying Both had a sufficient shelf life and time, min could be kept for several days in sealed cellulose ••••••••• 0.5 100 containers. Of the two, cellulose was Nonaromatic rubber cement .9 50 used because it Polyvinyl acetate •••••••• .5 150 did not react with the alcohol-based re Polyvinyl butyral •••••••• .4 25 fractory coating that was applied over Sodium silicate •••••••••• 1.3 25 the pattern and hard Alcohol- Water-based white .8 150 based used Zirconium acetate •••••••• 7.1 100 exclus tests. easy to spread in the listed pro- BINDER LEVEL and all formed smooth surfacings when dry. The minimum drying Upon binder selection, simultaneous time for each was determined by tests were done to evaluate the effects in an oven at 45° C of binder level, hard each at 5-min intervals size, a steady-state change was perature, and vacuum noted. Although 2 h appeared adequate, was employed as the binder, and all for trials were dried 0.3, 0.6, and 1.0 pct of the hard-surfac- in the oven While all the ing were ed. At the binders were with , same time, the other were var- in general those that were alcohol-based ied accord to the format listed in were loosened when alcohol-based refrac- table 2. coatings were applied to the entire Increased weak terns and those that were water-based and statis were loosened when water-based ref in all were gest that if porosity was in Multiple containing each of the the it did not remain there, binders were cast at temperatures rang- but migrated into the casting or to the from 1,560° to 1,5 C and with a casting top. The dense layer vacuum of 25 in may have formed an barrier on observed the a substantial the bottom cast- and the unsurfaced portions of surface the escape of the castings ~rom an average of 1 gases from tern. As to 2.3 and were not stat is shown in visible icant. on of the internal defects ) increased with binder level. However. in retrospect, problem of (The bracketed quantities at each data air entrapment had not been satisfactor point indicate ±1 standard deviation.) ily solved, and vortexes were noted in 6B and show that both tem- the sprues of three cas pour- and vacuum level had a , (The rod technique was in- synergistic effect with binder level on troduced to prevent air entrapment after casting porosity. A full vac- experiments on binder type.) The in uum applied to the flask was more ef ternal ty probably resulted from fective than an 8-in-Hg vacuum in pull sources other than the surfacing binder off gases from the cas ting. Also. because at least one cas containing more gases appeared to have been dis- each binder combination was po solved and subsequently in cast- rosity free. In addition, blank poured at 1,610° C than at 1,560° C. that were used as controls also contained While the data indicate that ty defects. increases with binder level, 12 TABLE 2. - Experiments to determine effect of five variables on surfacing integrity with centerpoint checks (Powder application by trowel, I-in plate thickness, methyl cellulose binder) Surfacing Pouring Vacuum level, Amount of Surfacing powder thickness, in temp, °c in Hg binder, pct size, mesh 3/16 1,580 15 0.6 Minus 65 plus 150. 1/ 16 1,600 30 1.0 Minus 150 plus 270. 5/16 1,600 30 1.0 Minus 35 plus 65. 1/16 1,600 30 .3 Minus 150 plus 270. 5/16 1,560 8 1.0 Minus 150 plus 270. 1/16 1,560 8 1.0 Minus 150 plus 270. 5/16 1,560 8 .3 Minus 150 plus 270. 3/16 1,580 15 .6 Minus 65 plus 150. 1/16 1,560 30 1.0 Minus 35 plus 65. 5/16 1,560 30 .3 Minus 35 plus 65. 1/16 1,560 30 .3 Minus 35 plus 65. 5/16 1,600 8 • 3 Minus 150 plus 270 • 1/ 16 1,600 8 .3 Minus 35 plus 65. 5/16 1,600 8 1.0 Minus 35 plus 65. 3/16 1,580 15 • 6 Minus 65 plus 150 • show that if the binder level is kept Figure 7B shows that the lowest exter below 0.3 pct, little damage occurs. nal casting porosity was obtained at an Cellulose binders may be decreased vir intermediate particle size of minus 65 tually to as little as 0.1 pct, at which plus 150 mesh. External surfacing poros level no effect will be noticed. Po ity increased only at a fine, minus 150- rosity did not increase with surfacing plus 270-mesh particle size (fig. 7C), thickness as might be expected consid probably because surfacing permeability iding that, at the same binder level, was reduced. A similar result would be the total binder in the thickest layer evident in a plot of particle size versus is three times that in the thinnest internal porosity. layer. POURING TEMPERATURE SURFACING MATERIAL PARTICLE SIZE It has been shown (5) that for a given As shown in figure 7A, minus 150- plus surfacing thickness, an unsatisfactory 270-mesh powder resulted in a smoother bond between the surfacing and casting surfacing than did minus 35- plus 65-mesh results when certain minimum pouring tem powder. The effect was more pronounced peratures and casting section thicknesses with 3/16-in-thick surfacings than with are combined. At these minimums, suffi 1/16-in-thick surfacings. The thickness cient heat is not provided to the surfac of the surfacing was a factor because the ing to nullify the chill effect produced ferrochrome did not entirely melt at by the surfacing. At an opposite ex thicknesses greater than 3/16 in. A full treme, certain combinations of maximum 30-in-Hg vacuum also improved surfacing pouring temperatures and casting section finish by 15 to 40 pct at the three lev thicknesses logically can be assumed to els of particle size. promote excess diffusion of the surfacing External casting porosity, internal po into the casting. Because casting sec rosity, and external surfacing porosity tion thickness is usually fixed, the were also affected by particle size. pouring temperature and, to a lesser 13 5 cr> c A ...,.: c 0 '- 4 (f) (f) W Z I <.9 :::> 3 0 A 0:: <.9 Z u 2 3 0' 41 .~ I <..9 B ~ cr> c 0 0.2 0.4 0.6 0.8 1.0 z c 0 ..... 4 r:.y (J)c >-- B thickness, must be vacuum, a considerable bond would be realized, and would be eased. Pouring into an evacu- ated flask increases the inves indicate sprue by an amount to the ranges of pouring temper height of a column of liquid steel in an atures and casting section thicknesses evacuated manometer. When this height, are broad. The range of thick about 52 in, is added to the actual sprue nesses inves were from 1 to 4 • the of the metal pass in and were well above the minimum pre through the choke is at least twice that dicted thickness (i) would result in poor of metal poured similarly into a flask bonding. Likewise, the without vacuum. Some of the vacuum is tures of I, to 1,610° C were well neutralized by the gases produced from within the range investigated in refer the vaporizing polys • and ence 5. The varia- the resistance offered by the tions did not affect bond or is effective in cause a bulence. , the net variety of microstructures resulted (dis- the vacuum is to increase turbulence in cussed later). the metal the mold. Additional- The only notable effect of tem- ly. vacuum creates a vortex in the sprue was produced above 1,580° C and during pouring. and precautions such as in tandem with a vacuum level of 8 in Hg. use of the stopper rod must be taken to The combination caused an increase of insure that air is not entrained in the 1.67 in the external porosity metal. This effect of and In , the results of vacuum level on external casting porosity the vacuum are not clear. The flat-place is graphed in f 8. Similar in- are castable without vacuum. creases were noted for and 1n- Successful of wear-resistant ternal poros without vacuum assist may be in other instances with modifi VACUUM LEVEL cations to variables such as gating. However, surfacings have been applied to in combination with higher the entire exterior of more intricate vacuum had no ,and without vacuum, these sur- s effect on any vari did not fuse to the cas able. A conclusion that might be assumed The implication is that vacuum is neces is that the vacuum could be dispensed sary for at least some applications. with entirely if a increase in could be tolerated. Without APPLICATION PROCEDURE Application technique was investigated, (!) along with thickness, cas Z • and vacu l (/) um level. in a second set of factorial TABLE 3. - without ef of five variables on (Minus 65- 150-mesh ferrochrome surfacings, methyl cellulose binder at <0.3 of surfacing weight, . as noted) 1,560 1 1/8 1 1, 0 1 1 1 1,610 1 1/8 1 1,610 1 1/8 1 1,560 1/8 1 1,610 8 1 1,560 1/8 8 1,560 1/8 1-3/8 1.610 3/16 1-3/8 1,610 1/8 8 1,610 3/16 1-3/8 1,560 3/16 4 1,560 1/8 4 1,560 16 1-3/4 1,610 1/8 4 1,610 16 1-3/4 1.610 1/8 1-3/4 SPRAY APPLICATION 1,560 1 1/8 1 1,6101 1/8 1 1.560 1/8 1 1,560 1/8 1 1.610 1/8 1 1,610 1/8 1 1,560 3/16 1-3/8 18-in-Hg vacuum level. be to patterns by automated meth- thus was caused by of the sur- ods without loss of properties. materials inward. Unexpectedly. the interface, though shifted inward, CASTING THICKNESS remained distinct. The effect of this dilution of the upon wear most As noted previously in the discussion would be negative. of pouring temperature effects, cas thickness effects upon the integrity of SURFACING THICKNESS the surf and bond below a certain minimum thickness were (5). In thickness was impor- this study. the cas thicknesses of 1 tant variable qual- to 1-3/4 in were above minimum of refer i It dictated the microstructure, ence 5 and did not affect integrity. which in turn determined wear resistance. However, that were applied to As thickness was increased, 1- and 1-3/4-in-thick in less and less of the powdered ferrochrome amounts almost twice as thick on melted. the heavier casting than on the lighter At 1/16-in thickness, the entire sur cas the total thickness of was melted. t the the cas remained con- microstructure was highly dendritiC. stant. The apparent thickness increase and few carbides were noticeable. A 16 FIGURf 9. - Microstructure showing dendrites and interdendritic carbides typical of thin (l / 16-in), chromium-rich, white iron surfacingsderived from ferrochrome (upper). The steel of the casting has a Widmanstatten structure (lower) (X 50). 17 microprobe analysis showed that infusion SURFACING WE IGHT, g/ in 2 of iron from the casting into the surfac 5.6 8.3 11 .1 13.9 ing was substantial. The microstructure formed was characteristic of hypoeutec tic. chromium-rich white irons typically having 2 to 2.5 pct C and ~15 pct Cr with Cr:C ratios of 5.5 to 7.5. Such white irons have about 25 vol pct carbides. A representative microstructure of a thin ferrochrome cast-on surface is shown in figure 9. The light. dendritic areas contained 1.5 to 2.0 pct C and 11.2 to 11.9 pct dissolved Cr, and the inter dendritic carbides contained 7.7 pct C. 38.6 pct Fe, and 53.7 pct Cr. The carbon content of the carbides was below the 9.0 pct C content of an ideal M7C3-type carbide. Chromium levels in the casting metal (J) adjacent to the interface ranged from 0.1 W to 0.6 pct. indicating that metal diffu 1-0- 0:::-c.- 0':':: sion from the 1/16-in-thick surfacing C Z 03 into the casting was insignificant. In WL.. o. contrast, carbon levels in the casting (9 metal next to the interface ranged from LLZ 0-u 0.5 to 1.4 pct. Because the diffusion z ", .. : FIGURE 11. - Microstructures of surfacings at three thicknesses. A, 1/16 in thick (X 100); B, 1116 in thick (X 400); C, 1/8 in thick (X 100); D, 1/ 8 in thick (X 400); E, 3/ 16 in thick (X 100); F, 3/ 16 in thick (X 400). 19 TABLE 4. - Effect of surfacing thickness on surfacing composition and hardness Microstructure Constit- Composition, pct Micro thick- material uent Cr C hardness, ness, in DPHl 53.7 7.7 9 {Commercial } Highly dendritic { Carbide. 1/16 ••••• ferrochrome. 2 Matrix •• 11.2-11.9 1.5- 2.0 456 66.2 9.0 1,619 2 Medium dendritic { Carbide. 1/8 •••••• . ..do...... Matrix •• 17.9-19.1 1.4- 2.9 610 77 .6 8.6 1,644 2 Nondendri tic •••• { Carbide. 16 ••••• . .. do...... Matrix •• 20.4 0.0 491 45.7 8.5 55- 567 No. 2 custom } dendritic { Carbide. 1/8...... { ferrochrome. 4 Matrix •• 9.7 1.8 459 No.3 custom }Medium dendrit;c { Carbide. 42.0 9.5 1/8 •••••• { ferrochrome. 5 • Matrix •• B.8-l0.8 1.4- 2.B 375- 551 Carbide. 53.0 10.0 3924 C~:~~~~:~ !~~~} dendritic 5/16..... exothermic { Matrix •• 11.7 2.1 355 { addition. 100-g load, test duration. 266.9 Cr, 6.4 C. too small for indenter; reading includes portion of matrix. 458 Cr, 8.4 C. 554 Cr, 7.8 C. interface into the casting slowed. The In summary, the wider of carbon content of the carbides approached increased surfacing thickness are that the ideal M7C3 composition, and the car chill is more s , solidification bides were and harder. With time is shorter, chromium and carbon mi- er levels of chromium and carbon in the out of the is less, and matrices, the hardness of the matrices carbide and microconstitu- likewise increased. Figure 11 shows the ent hardness are greater. Thin surfac- progression of microstructures with the are dendritic and c while three thicknesses. At the 3/16-in thick thick nondendritic and eu- ness, the carbides acent to the inter tectic The ultimate face were small and sometimes elongated; effect surfacing thickness, through toward the outer the carbides its influence on microstructure and car were and blocky. The nondendritic bide • is of course upon wear outer microstructure was similar to resistance. pereutectic chromium white irons with 30 The low-stress abrasion resistance of to 35 pct carbides. surfacings was measured in a dry-sand, observations of the rubber-wheel to ASTM stan- ferrochrome-surfaced also re dard G-65, The test is vealed a number of inclusions that performed by pressing a sample against a were located near the surfacing-casting revolving rubber wheel as a standard type interface. The inclusions, of sand is between the wheel and silicates and chromite, floated up If all conditions are the ward to the interface while the same from test to test, the volume losses was molten. The inclusions resulted from from sample surfaces. as an indication of the high impurity content that is allow wear resistance, may be compared able in commercial ferrochrome with one another. The results of the that are intended as additions , rubber-wheel tests were aver to steel. the initial stages from two identical tests and are of wear, the inclusions would not be ex shown in table 5. to be detrimental. However, after The first three listed in table wore to the of the 5 were surfaced with commercial ferro r soft inclusions, wear resist- chrome in progressive thicknesses from ance would suffer. 1/16 to 3/16 in. Volume losses to the /0 TABLE 5. - Dry-sand, rubber-wheel abrasion test results on cast-on surfacings derived from ferrochrome powders Surfacing material Surfacing Dry sand test thickness in vol loss mm 3 Commercial ferrochrome 1 ••••• 1/16 21.58 Do. 1 •••••••••••••••••••••• 1/8 14.37 Do. 1 •••••••••••••••••••••• 3/16 9.87 No.2 custom ferrochrome 2 ••• 1/8 8.97 No . 3 custom ferrochrome 3 • •• 1/8 10.97 Commercial ferrochrome 1 with 0.1 g C/g FeCr added ••••••• 1/16 5.69 Commercial ferrochrome 1 with exothermic addition •••••••• 5/16 8.49 NiHard I...... NAp 13.19 1080 (water-quenched)...... NAp 29.10 Weld-applied hard surfacing. ~3/16 11.3 NAp Not applicable. Commercially available part cast entirely of specified material. 166.9 Cr, 6.4 C. 258 Cr, 8.4 C. 354 Cr, 7.8 C. sample surfaces ranged from 21.58 mm. 3 on portions flaked off, leaving a gritty the thinnest surfacing to 9.87 mm. 3 on the finish. thickest surfacing. The table 4 and 5 Surfacings over 1/16 in thick ex.peri data on commercial ferrochromes show a enced gross shrinkage that reached definite correlation between wear resist ance and surfacing thickness, namely, that wear resistance is improved as sur SURFAC ING WEIGH T . 9/,n2 SURFACING WEIG HT . g/,n2 2.8 5.6 8.3 11.1 13.9 2.8 5.6 8.3 11.1 13.9 facing thickness is increased. g' 4 A 41 e' , , I I ' , " , 'I Unfortunately, surfacing thickness can ~ ~ not be increased infinitely to achieve ~g' t;;t 3 ever higher wear resistance. Surfacing u"'0 ~ quality is optimum at thicknesses of ..... ,: "' .... about 1/8 to 3/16 in. At greater thick ~iii ~~2 nesses, steel from the casting is not wx:r sufficiently drawn into the surfacing and melting is not always complete. Porosity is more apparent, and wear resistance de 0' c 0 teriorates. The effect of thickness on " ~ @ '"c surfacing and casting porosity is shown 2 4 vi in figures 1ZA and 12B, respectively. 3 ",' ~ ~ z 0 The high porosity at the 1/16-in thick I .... c.:> z 3 o~ 0 ness could not be linked to any of the a: u c.:> 2 measured variables. ~ "~ u u 2 Roughness and contour were other prop u- u-'" a:'" ~ erties that deteriorated with surfacing ~ '" if) (/) I I , thickness. Roughness, as a measure of 1/ 16 1/ 8 3/ 16 1/ 4 5/1 6 texture, increased slightly between sur SURFACING THICKNESS. ;n facing thicknesses of 1/16 to 1/8 in and FIGURE 12. - Effect of surfacing th ic kness. A, On substantially at surfacing thicknesses surfacing porosit y ; B, on externalca s tingporosity; C, over 3/16 in. Because surfacings over on roughness ; D, on contour. Bracketed quant ities in 3/16 in did not melt or fuse, the outer clude one standard deviation. 21 maximum severity at 3/16-in thickness. This deviation from flatness was termed The shrinkage, in most cases, was charac contour. The effects of thickness on terized by a dip from the plane of the roughness and contour are shown in fig sample running the length of the midline. ures 12C and 12D. COMPOSITIONAL VARIATIONS ON FERROCHROME SURFACINGS Two custo~melted ferrochromes were ob reduced chromium. At thicknesses of 1/8 tained that were more refined in compari in and less, the higher carbon produced son with the commercial ferrochrome. The surfacings with a high proportion of car custom-melted ferrochromes were applied bide. One of the custom ferrochrome sur to flat-plate samples. Figure 13 shows facings is shown in figure 14A. The in X-ray radiographs taken of 1/8-in-thick crease in carbide can be contrasted with sections of surfacings made with one of the carbide contained in commercial fer the custom, low-impurity ferrochromes and rochrome surfacings shown in figures 11A with commercial ferrochrome. The figure and lIB. shows that nearly total elimination of The carbides, although finely divided, inclusions was achieved with the custom contained high levels of carbon. From melted ferrochrome. table 4, it is seen that the number 2 and Perhaps more significant than the elim 3 custom ferrochromes, at 1/8-in thick ination of inclusions, the custom ferro ness, contained carbides with 8.5 and 9.5 chromes contained higher carbon levels pct C, respectively, which compare with than the commercial ferrochrome, although the carbide carbon content of commerical FIGURE 13. - X-ray radiographs showing inclusions at the surfacing-casting interface of samples surfaced with commercial ferrochrome (upper) and number 2 [ow-impurity custom-melted ferrochrome ( lower). 22 ferrochrome at a greater thickness of 3/16 in. The chromium contents of the matrices were between 8.8 and 10.8 pct. As shown in table 5, the increased car bide content and carbide hardness of the number 2 and 3 custom ferrochromes pro duced dry-sand, rubber-wheel abrasion volume losses for 1/8-in-thick surfacings of 8.97 and 10.97 mm 3 , respectively. The losses represent improvements of 24 to 38 pct over commercial ferrochrome surfac ings at the same thickness and are again comparable to commercial ferrochrome sur facings of 3/16-in thickness. Nearly the identical microstructure that was obtained with the custom-melted ferrochromes was also obtained from com mercial ferrochrome powder to which free carbon was added. At a level of 0.1 g of C to 1.0 g of ferrochrome, the interden dritic carbide precipitation in a 1/16- in-thick surfacing was slightly greater than that of either custom-melted ferro chrome. The elevated carbon ferrochrome microstructure is shown in figures 14B and 14C. The dry-sand, rubber-wheel abrasion test volume loss of 5.69 mm 3 generated from this surfacing and noted in table 5 was a 37-pct improvement over that of the number 2 custom ferrochrome. Additional improvements were made to commercial ferrochrome surfacings that were over 5/16 in thick. By applying an outer layer of exothermic material con sisting of 33 pct A1 and 67 pct Fe203' the surfacings were smooth and dendritic upon solidification. Notwithstanding the longer solidification time available, carbon diffusion out of the surfacing did not occur to a large extent, and as shown in table 4, the carbide contained 10.0 pct C. The higher carbon produced a car bide microhardness of 924 DPH in compari son with a hardness of 559 DPH for car bides in a similar dendritic structure from a 1/16-in-thick surfacing. The in creased carbon content of the carbide and heightened microhardness led to a dry FIGURE 14. - Microstructure of number 3 custom sand, rubber-wheel abrasion test volume melted ferrochrome and commerc ial ferrochrome with loss that was lower than that resulting 0.1 g added carbon per gram of ferrochrome. A, 1/ 8 from all other ferrochrome surfaces ex in thick number 3 ferrochrome (X 100); 8, commercial cept the one enhanced with free carbon ferroc hrome with added carbon (X 100); C, com mere ia I additions. This volume loss, noted in ferrochrome with added carbon (X 400). table 5, was 8.49 mm 3 • 23 STEEL AND WHITE IRON SUBSTITUTES FOR FERROCHROME Inasmuch as diffusion of steel into pct Wand 8.0 pct V and had a 3.7 poros ferrochrome surfacings yielded white iron ity ranking. Two compositions that typi structures on solidification, the possi fied the group were selected for dry bility of forming white iron and carbide sand, -rubber-wheel abrasion tests. The containing structures directly from white volume losses for the two, shown in table iron or steel powders was investigated. 6, were 17.63 and 21.39 mm 3 • The volume The compositions and dry-sand, rubber loss for -commercial ferrochrome at an wheel abrasion test results on selected equivalent thickness was 21.58 mm 3 • 1/16-in-thick sur facings are listed in An exceptional white iron with the table 6. composition Fe-5C-21Cr-8.5Cb-9Mo-0.3Mn- The steel compositions formed void 0.6Si produced an exceedingly low (11.70 marked surfacings, with porosity rankings mm 3 ) volume loss in the dry-sand, rubber of 4 and 5. One steel, with the compo wheel test. The microstructure of the sition Fe-0.7C-4Cr-4Ni-14Mn-0.04Si, re white iron, shown in figure 15, consisted sulted in a single-phase structure of low of a matrix of gray iron with 3.1 pct C; hardness (DPH 603). A second steel, with large blocky carbides with 5.2 pct C, the composition Fe-0.7C-7Cr-l.5Ni-l.3Mo- 9.7 pct Mo, and 81.0 pct Cb; and semicon 0.9Mn-0.9Si, resulted in a fine bainitic tinuous interdendritic carbides with 9.2 structure with a blocky, fairly hard sec pct C, 34.3 pct Cr, 43.1 pct Fe, and 13.2 ond phase (DPH 975). Neither steel was pct Mo. Both carbide phases had high deemed suitable for wear testing because hardnesses--the co1umbium(niobium)-rich of the porosity. phase measured 702 to 1,055 DPH and the Several white iron compositions with chromium-rich phase measured 903 to 1,391 carbon contents ranging from 2.3 to 4.6 DPH. pct and chromium contents ranging from Unlike the ferrochrome surfacings, free 2.2 to 26.5 pct produced cast iron micro carbon additions to the Cr-Cb-Mo white structures with few if any carbides. All iron surfacing reduced the proportion surfacings except one were smooth, poros of carbides. In the only abrasion test ity free (ranking of 1), well melted, and on the enhanced-carbon white iron, the dendritic. The exception contained 14.0 surfacing wore through and did poorly. TABLE 6. - Compositions of white iron and steel cast-on surfacings and selected dry-sand, rubber-wheel abrasion test results Dry-sand Compoaitions, pct test vol Remarks loss, mm 3 Fe-0.7C-4Cr-4Ni-14Mn-0.04Si ••••••••••••••••••••• NT Single phase, DPH 603. Fe-0.7C-7Cr-l.5Ni-l.3Mo-0.9Mn-0.9Si ••••••••••••• NT Second phase, DPH 975. Fe-2.3C-10.9Cr-4.8V-1.3Mo-0.3Mn-0.3Si ••••••••••• NT Few carbides. Fe-3.2C-2.2Cr-3.9Ni-0.7Mn-0.6Si ••••••••••••••••• NT No carbides. Fe-3.2C-20Cr-INi-l.5Mo-ICu-0.8Mn-0.5Si •••••••••• NT Few carbides. Fe-3.9C-SCr-14W-8V-ITi-13Mn ••••••••••••••••••••• NT Fine precipitate, over- all DPH = 430. Fe-3.1C-22Cr-lNi-2Mo-0.8Mn-0.6Si •••••••••••••••• 17.63 Few carbides. Fe-4.6C-26.5Cr-3.4Mo-0.4Mn-l.3Si •••••••••••••••• 21.39 Few carbides, overall DPH = 833. Fe-SC-21Cr-8.5Cb-9Mo-0.3Mn-0.6Si •••••••••••••••• 11.70 Cb-Mo carbides plus M7C3 carbides. Fe-7.5C-21Cr-6.1Cb-9.9Mo-0.3Mn-0.6Si •••••••••••• NT Less carbide than above. Fe-lOC-21Cr-6.1Cb-9.9Mo-0.3Mn-0.6Si-0.9Al ••••••• 52.69 Wore through. Cr 7 C3 ••••••••••••••••••••••••••••••••••••••••••• 15.68 Highly dendritic. NT Not tested. 24 However, a better wear factor m.tght be surfacing that dissolved. The hardness expected had the surfacing been thick of the reformed carbides was 634 to 852 enough and not worn through. DPH, and the hardness of the intact Cr7C3 The last surfacing listed in table 6 particles was 1,960 to 2,300 DPH. The was made with minus 50- plus 100-mesh volume loss in the dry-sand, rubber-wheel Cr7C3 powder. The resulting surfacing test was 15.68 mm 3 • No advantage would was dendritic except at the outer sur likely be gained by using carbide powders face, which retained a thin layer of in in place of less expensive ferrochrome or tact Cr7C3 particles. The particles were white iron powders pecause higher rates porous, and the surfacing was rough. of wear could be expected as soon as the Additional M7C3 carbides with 9.5 pct C carbide particles were lost. Control and 55.4 to 57.0 pct Cr were reformed ling the dissolution of the carbides also interdendritically from the portion of would be difficult. FIELD TRIALS6 Many researchers have been at least inherent in other casting processes can partially successful in applying cast-on be avoided because positioning is more surfaces to small, idealized samples that flexible and turbulence is reduced. are oriented and gated preferentially to Several demonstrations were conducted avoid drawbacks such as erosion of the to show the versatility of the Bureau's surfacing during pouring and drift dur cast-on surfacing process and the advan ing solidification. Problems arise with tages of using polystyrene patterns in larger and more complex configurations. the technique. Ferrochrome surfacings Through the use of the polystyrene pat were successfully applied to small jaw tern casting process, the disadvantages crusher plates, 4-in ball valves, and the inside diameter of a 3- by 8-in cylin 6The authors appreciate the assistance der. Cross sections of the three shapes of T. Braaten, assistant maintenance su are shown in figure 16. Additionally, pervisor, Washington Irrigation and De bucket-wheel excavator teeth and plow velopment Co., Centralia, WA, for the shares were surfaced and installed in field testing of bucket-wheel excavator operating "equipment for side-by-side cornr teeth; and L. Wells, Jefferson, OR, for parisons with existing components. the field testing of plowshares. FIGURE 15. - Cast iron microstructure of cast-on surfacing with the compos ition Fe-5C-21 Cr-8.5Cb- 9Mo-0.3Mn-0.6Si. A, 1/ 16-in-thick surfacing (X 100); B, light-colored blocky phase is Cb rich, light colored stringy phase is Cr rich (X 400). 25 FIGURE 16. - Cross sections of a 4-in ball valve (top), 6-in crusher plate (middle), and 3-in cylinder (bottom) showing ferrochrome cast-on surfacings. 26 BUCKET-WHEEL EXCAVATOR TEETH teeth per bucket. Originally, the mine used unfaced teeth that were made with a Bucket-wheel excavators are used to dig medium-carbon steel (Fe-0.3C-1Mn-0.8Si- overburden, coal, and ores at mines and 1.3Cr-0.4Ni-0.3Mo). The maximum life of to transfer stockpiled ores at shipping each tooth was four 8-h shifts. Recent points and smelters. In operation, the ly, the maximum was improved 10 times by wheel, which is oriented vertically, is weld-rod hard facing done at the mine by driven into an ore body or earthen face. mine personnel. As the wheel rotates, buckets attached to To field test the Bureau's surfac the wheel circumference remove large vol ing process, the entire outside surfaces umes of material and transfer them to a of two teeth were surfaced by the cast conveyor belt that is located immediately on technique with the number 2 custom at the back of the wheel. Each bucket is ferrochrome and the Fe-5C-21Cr-8.5Cb- fitted with several teeth that assist in 9Mo-0.3Mn-0.6Si white iron alloy to a cutting the face and protecting the buck thickness of about 1/16 in. The base et edges. Bucket wheels can be over 50 metal of the ferrochrome-surfaced tooth ft in diameter. had the composition Fe-0.2C-0.4Mn-0.3Si- A bucket-wheel excavator is used to re 0.9Cr-0.9Ni-0.1Mo, and the white-iron move a 100-ft thickness of claylike over alloy-surfaced tooth had the composi burden at a coal mine in Centralia, WA. tion Fe-0.2C-0.2Mn-0.5Si-1Cr-1Ni-0.2Mo. The wheel has eight buckets, similar to A pattern with gates, runners, sprue, the one shown in figure 17, with nine and core is shown (upside down for FIGURE 17. - One of eight buckets of a bucket-wheel excavator at a Centralia, WA, coal mine. Nine teeth are fitted on the edge. 27 FIGURE 18. - Bucket-wheel excavator tooth pattern with surfacing powder applied to outside. photographic purposes) in figure 18. The length. After 31 shifts of operation, teeth were heattreated at 875 0 C and oil the custom-ferrochrome-surfaced tooth quenched. lost 2 in of length, and the white-iron The teeth were installed in identical alloy-surfaced tooth lost 1/2 in. In corner positions on successive buckets comparison, the control teeth lost 1-1/4 along with two weld-rod hard-faced con and 1-1/2 in. In terms of length then, trol teeth. The wear of each tooth the white-iron-a11oy-surfaced tooth reg was assessed by noting the loss in its istered a 60-pct improvement over the 28 best control tooth whi.le the custom At most locations, the microstructures; ferrochrome-surfaced tooth did worse than shown in figure 20, were dendritic with the control teeth. The ranking order for few carbides. Thicker surfacings would equivalent thicknesses of the two sur have produced less dendritic structures facings was the same in the dry-sand, and greater carbide proportions. In con rubber-wheel abrasion tests as in the trast, the surfacing on the tip was thin field tests, but the margin of difference ner, less dendritic, and more porous. A was greater in the field trials. cross section of a tooth tip is shown in Upon posttest analysis, several in figure 21. A localized application of sights for future applications were evi exothermic material over the surfacing on dent. Before installation, the teeth the tip corners might have promoted bet were X-rayed, and numerous small voids ter fusion and prevented the loss of fer were found at the casting-surfacing in rochrome powder. terface, as shown in figure 19. The Figure 22 shows the two surfaced teeth voids most likely resulted because the and the better of the two control teeth entire tooth was surfaced, thereby creat after return from the field trials. Much ing an impermeable shell that impeded the of the surfacing of the white-iron-alloy removal of gas from the vaporizing pat surfaced tooth was worn away. In areas tern. (In tests done subsequently, the voids were eliminated when only one side of the pattern was surfaced, thus allow ing better bredthing. _ This practice would be acceptable because currently the teeth at the mine are not turned over, and just one side is subjected to wear.) FIGURE 20. - Microstructure of number 2 custom fer rochrome (A) and white iron alloy (8) surfaces from FIGURE 19. - Radiograph of bucket-wheel excavator bucket-wheel excavator teeth used in field tria Is (X tooth showing small voids at casting-surfacing interface. 100). 29 FIGURE 21. - Cross section of bucket-wheel excavator tooth tip showing ferrochrome surfacing along outside edge. FIGURE 22. - Bucket-wl-Teel excavator teeth after field trials at Centralia, WA, coal mine. Left, weld-rod hard-faced control tooth; middle, Fe-SC-2lCr-8.SCb-9Mo-O.3Mn-O.6Si composition white iron alloy cast-on-surfaced tooth; right, number 2 custom ferrochrome cast-on-surfaced tooth. where a thin layer of surfacing remains, edge between the surfacing and the cast the interfacial voids that were noted on ing near the tip indicates the cracks the X-rays are evident. The voids pro were instrumental in surfacing failure. vided sites for wear initiation and were Some of the cracks were evident after obviously undesirable. It appears that heattreatment, suggesting the heattreat the tooth was removed at the optimum ment requires optimization. The large time. Had it remained in service longer crack that nearly bisects the entire without a surfacing for protection, wear tooth is continuous through to the inside would have been accelerated. cavity and is the result of an ill fit Figure 22 also reveals numerous cracks between the tooth and tooth adaptor on in the surfacing of the number 2 customr the bucket. A greater shinkage allowance ferrochrome-surfaced tooth. The sharp on the cavity core is the remedy. 30 In total, the bucket-wheel field tri many mining environments. An opportunity als have shown re.ason for optimism. The to test and compare cast-on surfacings at cast-on-surfaced teeth remained in ser a local farm arose, and several experi vice for as long as the weld-rod hard mental shares were cast for field trials. faced teeth. In the one instance, a The sandy soil at the farm caused unsur cast-on-surfaced tooth sustained less faced shares to wear out after 25 acres wear than the best weld-rod hard-faced or less was plowed. With commercially control tooth. Relatively simple modifi available weld-rod hard-faced plowshares, cations should improve tooth performance. the farmer was able to plow over 150 Further tests are in progress. acres without having to stop because of plowshare wear. PLOWSHARES Plowshares are about 2 ft long, 4 in wide, and 1/2 in thick. Shares are af Plowshares are made of wrought steel fixed to the bottoms of each of five and most likely will never be cast com blade assemblies like the one shown in mercially. However, the low-stress abra figure 23. The shares make the initial sion that occurs in plowing is similar to contact with the soil and protect the re the low-stress abrasion that occurs in mainder of the blade. The leading edge FIGURE 23. - Plowshare affixed to lower portion of one of five blade assemblies. 31 is tapered to a knifepoint to assist in comparisons on the basis of weight loss cutting the soil. Wear is greatest to are difficult. While table 7 shows that the share assemblies at the front and weight losses for all shares on a compen rear of the plow. sated basis are nearly uniform, the ap Two trials were conducted. The first pearance of the shares was not uniform. involved two plowshares with commercial The most observable difference between ferrochrome cast-on surfaces and one the experimental shares and the control weld-rod hard-faced control; the second shares was that wear along the edges of involved one experimental plowshare with the control shares was even, while wear a white iron alloy cast-on surface and along the edges of the experimental two weld-rod hard-faced controls. Each shares was uneven. The difference is trial involved about 140 acres. The apparent in figure 24. The uneven wear plowshares, positions occupied on the resulted from the uneven application of plow, and weight losses of each share are hard-surfacing powder to the pattern and shown in table 7. from occasional defects at the casting Because the positioning on the plow surfacing interface. The microstructure partially dictates wear conditions, wear of the cast-on hard surfaces appeared TABLE 7. - Plowshare field trial parameters and test results (140 acres plowed) Hard surface Hard surface Plowshare composition Plowshare Wt loss, material • composition position 1 lb Commercial 66.9Cr-6.4C •••••••••••• Fe-0.3C-0.4Mn-0.6Si- 5 1.0 ferrochrome. O.7Cr-1.7Ni-0.2Mo. Do •••••••••••• . . . do ...... · . . do .•...... 4 1.0 Weld-rod hard NA ••••••••••••••••••••• Fe-0.3C-0.3Si-1.2Mn •• 3 .7 surface. 2 Do. 2 ...... NA ••••••••••••••••••••• • .• do ..••..•..••...•. 1 1.0 2 Do...... NA ••••••••••••••••••••• • •• d o ..•.••.•••.•.•.• 3 .8 White iron alloy Fe-5C-21Cr-8.5Cb-9Mo- Fe-O.1C-O.6Si •••••••• 5 1.3 3Mn-0.3Si. NA Not available. lPlowshare positions run in sequence backward from the tractor. 2eontrol tests. FIGURE 24. - Weld-rod hard-faced control plowshare (upper) and ferrochrome cast-on-surfaced plow share (lower) after fie Id tria Is. 32 similar to those observed in lhe bucket hand-cut patterns produced wheel excavator teeth shown in figure 20. many small imperfections, giving rise to , thicker surfacings would have pro- the farmer's observation. He also noted duced a carbides that the attachment holes were and shifted the micro- not which caused the structure more toward the eutectic tal shares to loosen prematurely several composition. times. Besides the irritation involved Al though of wear was with to stop to the loose not the only criteria for j the shares, in the case of the white-iron worth of a plowshare. To be effective, a alloy-surfaced share. produced plowshare must "scour." Scouring is a accelerated wear along an 8- to 10-in process in which the flat surface of the length of the edge. Both the finish share as a result of contact and the hole prob- with the soil. Without a polished sur lem could be cured if the polys face. imperfections or roughness on the patterns were produced on molding ma share causes the soil to across the chines. The positive results were that share ass and a noticeable cast-on surfaces were ap- on the tractor. plied to thin section that The farmer noted that the the cast-on surfaced lasted as tal were slow to scour. The as the control shares. CONCLUSIONS Powdered material can be Vacuum level - Little effect is pro- fused in place to steel casting surfaces on flat-plate sample cast- at the time of pouring by means of the ings. Vacuum may have a greater effect cas process. is lowest from commercial carbon ferrochrome can be formed effect under wide limits of binder type. binder with content, size, than ture, vacuum level, proce dure. and thickness. A condensed effect is description of some of the effects of on or bond quality for these variables is in the the thicknesses (1 to 4 in) investi gated. Diffusion of into the - Cellulose and polyvinyl casting is greatest with thick section are both acceptable binders. castings. Polyvinyl acetate dissolves in alcohol thick- based most Fu- Binder level. - Porosity increases with of the surfacing, dendrite forma increas level. is tion, smoothness, and diffusion of the lowest at 5 to 30 in are t at thicknesses and low pouring (1,5800 less than 1/8 in. Carbide formation, mi Porosity is absent at a binder level of crohardness. and wear resistance are 0.1 pct of This level at thicknesses of 1/8 to 3/16 is readily obtainable. in. Microstructures are similar to Particle size. with a fine chromium white irons containing M7C3-type s carbides. Microstructure from smoothest to eutectic as thickness is an intermediate (minus 65- plus increased. A thickness of 3/16 in is thickness sizes in a - Little effect is 0,560° to are ferrochrome. 33 Carbon additions to ferrochrome increase characteristic of high the carbide content of and chrome white cast iron. Carbides of the also the wear resistance. Some white M7C3 type are interdendriti iron have less chro- cally. The carbon and chromium contents mium than ferrochrome. sur- of both carbides and matrices are low facings with wear resistance. compared with thicker Exothermic compound over surfac- As surfacing thickness is increased that on thin cas sections above 1/8 in, the chill effect becomes fusion. • and solidification time is Optimum surfacings result from minus shortened. Diffusion is decreased, and 65- plus 150-mesh powders with as little carbide precipitation is increased. At a as 0.1 cellulose binder. thickness of 3/16 in, no dendrites are levels are unaffected with pouring tem and the structure peratures from 1,560° to 1,610° C, cast the iron-carbon-chromium eutectic. At thicknesses from 3/4 to 1-3/4 in, and the eutectic, wear resistance, measured thicknesses up to 3/16 in. A by dry-sand, rubber-wheel abrasion tests, exception is at a tem- is subs The wear re perature above 1,600° C and a flask pres sistance of cast-on ferrochrome surfac- sure of less than 15 in Hg vacuum. with eutectic microstruc- thickness influences the tures surpasses the wear resistance of quality of cast-on surfaces more than any many weld-rod-applied hard facings. other variable. quality is op The quality of cast-on surfacings made timum at a thickness of about 3/16 in for from commercial ferrochrome can cas with section sizes between 3/4 be several means. Refined and 1-3/4 in. At surfacing thicknesses ferrochromes with low impurity content than 3/16 in, is exces- reduce inclusions. More carbides are sive and results in an uneven by the addition of carbon to contour. In addition, the outermost pow commercial ferrochrome powders, thereby der does not melt into the , and increasing wear resistance. The quality pores within the powder mixture are not of and with filled. The unmelted also leaves high heat is by add- a finish texture that is equivalent to ing exothermic material to the outside to the mesh size of the increase and so From a wear s an Fe-C-Cr eu- lidification time. with tectic microstructure is desirable. On white iron compositions containing less a microscopic scale, thickness chromium than ferrochrome are of controls microstructure white iron with wear various of chill effect. Thin resistance to that of surfacings (up to 1/8 in thick) melt ferrochrome produced surfacings. readily and completely. Solidification The potential of cast-on to is slow, and diffusion of carbon from the weld-rod hard has been is substantial. The micro- demonstrated in field tests on bucket- structure is highly dendritic and is wheel teeth and plowshares. REFERENCES 1. M. L. Vertical Piston Internal Combustion With Cas Raises Roll Life. Met. Sintered Metal Plug Pat. Mater. (London), Feb. 1975, pp. 47-48. 3,744,547, July 10, 1973. 2. Kura, J. G. Cast Bonding Produces 4. E. Cemented Carbide-Steel Quali Metallic Composites. Mater. Composites for Earthmoving and Mining Ap Eng •• v. 80, No.5, Oct. 1974, pp. 60-61. plications. U.S. Pat. 4,101,318, July 18, 1978. . 3. Panhard. J. L. Methods of Manufac- Crank-Case for Rotary 34 5. Jackson, W. J., D. M. Southall, 14. Steel Cas Research and Trade and D. S. Edwards. Surface of Association (Sheffield, Im Steel Castings. Paper 17 in Advances in provements in or Relating to Casting • Surface The • of South Afr. Pat. 773027, May 20, Institute, London. 1978. pp. 201-218. 1977 • 6. Davis, K. G., and J. G. 15. Davis. K. G., and J. G. The Production of Cas With Abrasion Cast-in-Place Hard Resistant Coatngs. Part I: Cas Centre for Mineral and Energy With Powder Set Into the Walls Ottawa, Canada, Physical of Sand Moulds. Canada Centre for Min search Laboratories eral and Energy Technology, Ottawa, Can (IR), • 1980. 56 pp. ada, PhYSical Metallurgy Research Labora tories MRP/PMRL 78-4 ( ,Oct. 16. Cast-in-Place 1978, 40 pp. Canada for Mineral Technology, Ottawa, Canada, Physical Met 7. Morris, A. D. Method of Making Research Laboratories Report MRP/ Resistant Surfaces. U.S. Pat. 1,072,026, PMRL 80-90 (J), Dec. 1980, 45 pp. Sept. 2, 1913. 17. Shroyer, H. F. Cavityless Casting 8. , V. B. Method of an Mold and Method of Same. U.S. Mold Stool. U.S. Pat. 3,783,933, Pat. 2,830,343, Apr. IS, 1958. Jan. 8, 1974. 18. Duca, A.. M. C. Flemings. and 9. Mikhailov, A. M. Surface Harden H. F. Taylor. Art Cas Trans. Am. of Cas With Al Materials. 's Soc., v. 70, 1962, pp. 801- Russ. Cast. Prod. (Birmingham. .), 810. 1970, pp. 82-84. 19. Stepanov, Yu. A., M. G. Anuchina. 10. Mat the of and V. P. Cas Forma A1 and Substrates in Cast- tion in the Gasifiable Pattern Process. ings. Russ. Cast. Prod. (Birmingham, Russ. Cast. Prod. (Birmingham, .), .), 1974, pp. 255-256. t. 1976, pp. 43 11. Voroshnin, L. G. Increas the 20. Smith, T. R. Method of Cas Wear Resistance of Cast-Iron Cas by U.S. Pat. 3.157,924, Nov. 24, Surface Al Russ.. J. (Mel- ton v. 53, No.5, 1973, 21. Butler, R. D., and R. J. pp. Some Factors Involved in Full Mould Casting With Unbonded Sand Moulds. Br. 12. Solv'ev, V. P.. and A. M. Mik Foundryman, 1964, pp. 178-190. hailov. Structure of and Chromium Dis tribution in Surface-Alloyed Zones. 22. • Yu. A., and A. N• Gav- Russ. Cast. Prod. ( Eng.)~ rishin. Properties of Steel 1974, pp. 202-203. Cas From Moulds With Gasifiable Pat- terns • Russ. Cast. Prod. ( 13. Herfurth, K., and S. Pinkert. .) . 1971, pp. 440-442. Oberflachenlegieren von Stahlguss eine Kombination des Urformens und des Bes 23. Yu. A., A. N. Gavrishin, chichtens (Surface of Steel and V. I. Semenov. Surface Forma - A Combination of Cas and tion on Steel Cas Made With Gasifi Giessereitechnik, v. 19, No. able Patterns. Russ. Cast. Prod. (Bir II, 1973. pp. transl. available mingham, Eng.), pp. 299-301. from Canada Centre for Mineral and Technology, Ottawa, Canada. 35 24. , Yu. A., and D. S. Grish- 27. Kotzin. E. L. .). Metalcast- in. Steel in Moulds With Gasifi- er's Reference and Guide. Am. Foundry able Patterns. Russ. Cast. Prod. (Bir- men's Soc., 1972, 556 pp. mingham, ). 1972, pp. 189-192. 28. American for Tes and 25. • E. Method for Mak Materials. Standard Practice for Con ing Castings. U.S. Pat. 3,581,802. June ducting Dry Sand/Rubber Wheel Abrasion 1. 1971. Tests. G65-81 in 1981 Annual Book of ASTM Standards: Part 10, Metals--Physi 26. W. E. Foundry Process cal, Mechanical, Corrosion Tes Including Heat of Produced Philadelphia, PAt 1981. pp. 1135-1152. in Formation Sand. U.S. Pat. 4.222,429, Sept. 16, 1980. 36 APPENDIX.--RANKING GUIDE FOR DEFECTS A minimum of three observers will make Guidelines for Inclusions, Gas Porosity, rankings from 1 to 5 for casting and sur and Shrink Porosity facing defects. The observers may dis cuss defects but not rankings. The fol Rank according to size and/or number. lowing guidelines will be followed to achieve consistency and to eliminate Ranking Extent of defect doubt. 1 •••••• Not present. GENERAL GUIDELINES 2 •••••• At least one but not more than The general guidelines will be used five small defects (pinhole when specific guidelines for specific de size) • fects are not applicable, for example, on casting defects such as sand drops, mis 3 •••••• 1 to 2 large1 and up to 5 small runs, and residue cavities or on surfac defects, or more than 5 but not ing defects such as erosion, cracking, more than 10 small defects. and nonfusion. 4 •••••• More than 2 but not more than 4 Ranking Extent of defect large defects, and up to 10 small defects, or more than 10 1 •••••• Not present. but not more than 15 small defects. 2 •••••• Noticeable presence of defect but casting or surfacing is ac 5 •••••• Defects of sufficient size and/ ceptable without repair. or number to render casting scrap. 3 •••••• Occurrence of defect is of suf ficient severity to require mi Guidelines for Roughness nor repair. Rank according to the relative grit 4 •••••• Occurrence of defect is of suf tiness of the surfacing. ficient severity to require ex tensive repair--casting is bor Ranking Extent of defect derline between acceptable and scrap. 1. • • • • • Fine. 5 •••••• Occurrence of defect is too se- 2 vere for repair--scrap casting. 3 •••••• Similar to 20-grit sandpaper. SPECIFIC GUIDELINES 4 The following specific guidelines need not be rigidly adhered to if circum- 5 •••••• Similar to 10-grit sandpaper. stances dictate otherwise. However, there should be few such situations. It should not be assumed that a defect is less important because it occurs in an area where it can do no harm and that the 1 Greater than 1/8 in diam, but not ranking can be more lenient because of greater than 1/4 in diam. If large in the location of the defect. Most likely clusions or holes are larger than 1/4 in the defect could have appeared where it diam, downgrade ranking appropriately. would be more harmful. 37 Guidelines for Contour Guidelines for Porosi.ty and Inclusions Observable in Metallography Specimens Rank according to the overall out line of the surfacing irrespective of Rank according to the number of inclu roughness. sions, gas holes, or microshrink holes per X 50 frame. Ranking Extent of defect Ranking Extent of defect 1...... Smooth. 1. . . . • . None. 2 2 ••••.. One per X 50 frame. 3 •••••• Marked by a slight dip or wave. 3 •••••• Two per X 50 frame. 4 4 .•...• Three per X 50 frame. 5 •••••• Marked by a severe dip or wave. 5 •••••• Four or more per X 50 frame. -cr U.S . GPO: 1985-S0S.Q19/20,033 I NT.-6U.OF MINES,P GH . , P A. 279 44