AD AMRA CR 64-03/3 IRESEA~ (EFFECT OF MQPZ EL INt _•RFACE REACTIONS ON CASTING SURFACE AND PROPERTIES FI!NAL REPORT July 1, 1963-January 31, 1968 by Charles E. Bates ______ Rodney L. Naro John F. Wallace January, 1968 Department of Metallurgy Case Western Reserve University University Circle Cleveland, Ohio 44106 Contract No. DA-33-019-AMC-270 (Z) Distribution of this Document is Unlimited U.S. ARMY MATERIALS RESEARCH AGENCY WATERTOWN, MASSACHUSETTS 02172 oWO& 1/ 8 001
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AD
AMRA CR 64-03/3
IRESEA~
(EFFECT OF MQPZ EL INt _•RFACE REACTIONSON CASTING SURFACE AND PROPERTIES
FI!NAL REPORTJuly 1, 1963-January 31, 1968
by
Charles E. Bates______ Rodney L. Naro
John F. Wallace
January, 1968
Department of MetallurgyCase Western Reserve University
University CircleCleveland, Ohio 44106
Contract No. DA-33-019-AMC-270 (Z)
Distribution of this Document is Unlimited
U. S. ARMY MATERIALS RESEARCH AGENCYWATERTOWN, MASSACHUSETTS 02172
oWO& 1/ 8 001
Mention of any trade names or manufacturers in this reportshall not be construed as advertising nor as an officialindorsement or approval of such products or companies bythe United States Government.
The findings in this report are not to be construed as anofficial Department of the Army position, unless sodesignated by other authorized documents.
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EFFECT OF MOLD-STEEL INTERFACE REACTIONS
ON CASTING SURFACE AND PROPERTIES
AMRA CR 64-03/3
FINAL REPORTJuly 1, 1963-January 31, 1968
by
Charles E. BatesRodney L. NaroJohn F. Wallace
January, 1968
Department of MetallurgyCase Western Reserve University
F This project investigated the problem of thermal andmechanical stability of mold materials for high strengthsteel castings of the 431'tjype. The ability of variouscombinations of rammed refractory aggreqates and bindersto provide close dimensional control in steel castings wasinvestigated. The aggregates studied included mullite,zircon, silica sand, fused silica, olivine and calcinedalumina. These were bonded with western bentonite andsodium silicate. Close dimensional control was obtainedwith all aggregates bonded with sodium silicate and withthe zircon and mullite aggregate boitded with western ben-tonite. A means of measuring, the tensile strength of moldingmaterials at elelvated temperatures was developed and theresults of this test permit qualitative evaluation of theability. of a mold material to maintain casting dimensions.
The effect of various refractories and different moldcoatings on the surface rdughness op cast steel and on theimpact resistance as measured in reverse bending was alsostudied. The effects of rammed aggregate of silica, chromiteand zircon sands on the surface quiality and fatigue behaviorwere studied. The green silica sand molds were coated witha variety of washes to determine the effect of these coatingsof both surface roughness and fatigue behavior. All fatiguespecimens were heat treated to approximately 185,Q00 pgitensile strength.
Test specimens made in both hexachlorobenzene coatedsilica sand molds and zircon sand produced equivalent resultsin improving the surface finish of experimental test castings.Bending fatigue test results showed that hexachlorobenzenecoated and uncoated zircon provided the greatest overallimprovement in fatigue life and endurance limit. Completelymachined bending fatigue specimens result in only slightimprovements in fatigue compared to sand blasted hexachloro-benzene and zircon specimens with the original cast surfaces.Shot peening these specimens also produced substantial gainsin fatigue strength, compared to machined specimens. Thefatigue failures were initiated at cope surfaces with greaterfrequency than drag surfaces.
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TABLE OF CONTENTS
Page
Abstract ii
Table of Contents iii
Introduction 1
Procedure and Materials 4
Results and Discussion 7
General Summary 8
Distribution 10
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INTRODUCTION
Since molding materials are of primary importance indetermining the surface quality and dimensional accuracy ofsteel castings, these require careful consideration by steelfounders. A molding material should have the followingcharacteristics to produce high quality castings:
1) capability of receiving molten steel without eroding;
2) freedom from chemical reaction with the molten orsolidifying steel;
3) sufficient high temperature strength and a lowcoefficient of thermal expansion in order to controlthe dimensions of the castings to close limits.
Molds are composed of an aggregate or inert refractorymaterial and a binder for this aggregate. Both of thesecomponents serve an important role in the production of highquality castings. Several criteria must be considered for anaggregate including: melting point, availability in the propergrain fineness range and satisfactory thermal expansion character-istics. This latter factor requires both a low coefficient ofthermal expansion and a smooth thermal expansion curve that isfree form crystal inversions. The inversions that occur whenheating ordinary silica molding materials coupled with its largecoefficient of thermal expansion have led to many of the problemsassociated with the use of this aggregate.
The binder employed in the aggregate-binder mixture is alsoa very important consideration. Many casting defects can betraced to deficiencies of the binder rather than the aggregate.The binder must be capable of holding the aggregate in positionduring pattern removal, pouring and solidification of the casting.Maintenance of sufficient high temperature strength,preventionof erosion, mold cracking and spalling are all requirements ofa good binder.
In order to develop and test aggregate-binder mixtures thatwill produce high quality dimensionally stable castings, teststhat predict the performance of the molding materials are needed.The results of standard tests producing quantitative results arebetter for the evaluation of mold materials than those obtainedby examination of miscellaneous castings. Unfortunately, standardtests that predict the thermal stability of mold materials asthe molten metal enters the mold cavity and solidifies are notavailable.
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The reaction of mold refractories with molten steel at themold-metal interface has also been a serious problem. This inter-face reaction is the cause of dimensional inaccuracies, surfaceroughness, sand penetration, and fused sand on steel castings.It has been studied qualitatively by numerous intestigators.Consequences of casting surface defects include a general reductionin mechanical properties, particularly fatigue life. Althoughsteel castings are widely employed in fatigue applications, infor-mation on their fatigue properties has indicated that the endurancelimit in bending fatigue is sharply reduced by the existence ofsurface roughness and discontinuities. Surface defects alsoresult in an impairment of the machaniability of the casting, sinceportions of the casting are usually machined to adjust theirdimensions for service or other applications.
The underlying causes of poor cast surfaces can beseparated into two phenomena: 1) mechanical penetration ofthe molten steel into the molding aggregate, and 2) chemicalreactions occurring between the molten steel and the mold wallinterface.
Mechanical penetration occurs when the pressure of themolten metal is high enough to force this metal into the inter-stices of the mold interface. Many investigators have shown thatmechanical penetration is enhanced by the following conditions:1) high pouring temperatures, 2) high ferrostatic pressure,3) coarse sand grain distributions, and 4) low density molds.Sand grain size and ferrostatic pressure are usually the con-trolling factors governing mechanical penetration.
The influence of sand grain size on penetrating is wellknown. The penetrating pressure is inversely proportional-to the sand grain size. Molten metals do not wel silica sand;accordingly, calculations of the ferrostatic pressure head toforce metal into the sand capillaries show that pouring headsof 30 to 50 inches are required. The majority of all steelcastings are poured without excessive ferrostatic pressure,but considerable burn-on still occurs. Clearly, mechanicalpenetration concepts cannot account for many cases of sandadherence and the subsequent deterioration of surface qualityand casting properties.
The nature of the chemical reactions occurring at themold-metal interface are less clearly understood. The reactionresults in the formation of a fused sand layer adjacent to thecasting. Burn-on, as this layer is often called, and the resultingsurface roughness of the casting has been studied qualitativelyby many investigators. Early investigators conducted superficialtests to determine the characteristics of mold-metal interfacereactions. Although the tests did not simulate actual foundry
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conditions, the reaction produced appeared to be the resultof oxidation of the steel and subsequent reactions betweenthe iron oxide so formed and the silica sand to produce ironsilicate or fayalite. When reducing atmospheres were used,no interface reactions resulted.
Modifications have been made to the original proposedmechanism to account for the fused sand layer and the resultingpoor casting surface finish. When molten steel is poured intoa green silica sand mold, iron oxide is formed at the interfacedue to the oxidizing nature of the mold. This oxide layer canwet the sand and penetrate into it. More recent developmentshave shown that aside from wetting the sand, the oxide furtherdissolves SiO2 and enlarges the voids between the packed sandgrains, allowing molten metal to enter these enlarged voids.
By controlling and eliminating the chemical reactionoccurring at the mold-metal interface, improvements in castsurface finish can be obtained. Methods by which this maybe accomplished are:
1) replacement of SiO2 with another molding aggregate;
2) providing a reducing mold atmosphere with a suitablemold coating;
3) providing a viscous fluxing liquid film between thesolidifying metal and the mold interface.
PROCEDURE AND MATERIALS
Several aspects of the behavior of moldable refractoriesfor steel casting were investigated. These included: thedevelopment and conduct of high temperature sand tests; thecasting of steel into molds and evaluation of resulting castingquality and the measurement of mold def6rmation. Numerousaggregate-binder combinations rammed to different densitieswere studied.
A modified dog-bone type of tensile test was adapted forthe elevated temperature test. Specimens of this type werecompacted in a standard type fixture and tested in a furnaceconstructed around the specimen positioned in a tensile testingmachine. The furnace utilized tungsten heating elementsencased in fused silica with an insulating brick exterior.Specially designed jaws were employed to grip the specimenand thermocouples were employed to record the temperature.Adjustments in the power supplied to the heating elementspermitted obtaining the desired temperature.
The materials selected for testing as molding aggregatesincluded silica sand, fused silica, mullite, zircon, olivineand calcined alumina. Castings were produced in molds rammedfrom each of these materials and examined for dimensionalstability, chemical reaction with the molding material, sandexpansion defects and general surface finish. Each of therefractory materials listed above was bonded green withwestern bentonite, cereal and water. Sodium silicate, setwith both CO2 and silicon metal fines, was also employed asthe binder for some of the aggregate materials. All castingsproduced were in the shape of a scab block developed by theSteel Founders' Society of America with a slight modificationof the gating system. Molds were rammed according to astandard procedure that consisted of both jolting and handramming the drag and hand ramming the cope.
Molds were poured with a low alloy, aluminum deoxidizedsteel directly from a basic-lined high frequency inductionmelting furnace. A standard pouring temperature of 2900'F(15931C), as determined by an immersion pyrometer, wasemployed in all cases. Molds were poured on the same day inwhich they were rammed. Castings were allowed to cool over-night, shaken out, examined, sand blasted and subjected toa final examination for surface condition and dimensions.
Mold deformation caused by the pressure of the moltensteel was measured on a series of green and sodium silicatebonded mold at various distances from the mold-metal interface.A fused silica probe extended from this disc to the outsideof the mold where it was attached to a linear transducer.Movements of the disc with the mold wall were transmitted by
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the probe to the transducer, converted to an electrical signal,and continuously recorded on a calibrated high speed stripchart recorder. The fused silica disc was maintained at theproper distance from the pattern during mold preparation withan accurately machined spacer. This spacer was removed aftersufficient molding material had been packed around the discto support it.
To permit insertion of the silica probe, a 1/4 inchdiameter steel rod was positioned inside the mold during moldpreparation with one of its ends touching the quartz disc andthe other extending to the inside edge of the molding flask.The steel rod was maintained in the proper position duringramming by two supports resting on the pattern board. Afterthe mold had been prepared and the molding flask stripped, thesteel rod was removed. The mold was then jacketed, weighted,and the 6mm-diameter fused silica probe inserted through thejacketed mold into the hole. Using a technique that employeda fused silica disc inserted in the mold and held in placewith a silica rod, mold deformation was continuously measuredat various locations within green sand molds rammed to den-sities of 90 and 100 lb/cu ft ahd in sodium silicate bondedmolds rammed to a density of 100 lb/cu ft.
The materials selected for testing as refractory moldaggregates of the surface roughness included fused silica,silica sand, calcined kyanite, mullite, olivine, chromite andzircon sands. All molds were poured with a nominal AISI 8630,aluminum deoxidized steel directly from a 100 pound basiclined high frequency induction furnace. A pouring temperatureof 30000 F, measured with an immersion pyrometer, was used forall test castings. Castings produced in molds with variousaggregates and coatings were examined for chemical reactions,shakeout behavior and general surface appearance. Individualtest bars were cut from the gates and sand blasted lightly.The surface.. condition of each bar was measured with a profilo-meter after sand blasting.
The metal used for the fatigue studies of the investigationwas AISI 4330 steel. The material was melted and deoxidizedin a similar manner as the bars used for surface conditionevaluation. The steel from each heat was used to cast platebending fatigue specimens and standard test coupons from whichtensile and R. R. Moore fatigeu specimens were machined. Coatingsand refractories which produced the superior cast finishes wereutilized for the production of bending fatigue specimens. Thecast bending fatigue specimens were shaken out and lightlysand blasted to facilitate their removal from the gating system.Fatigue specimens were randomly selected for a visual inspectionof the cast surface and measurements of surface roughness made
using a profilometer. A tensile strength level of approximately185,000 psi for all material was obtained by quenching and
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tempering. Three lots of plate bending fatigue specimens wereshot peened with cut and pre-rounded steel wire shot. A CW-28shot size with a Rockwell C hardness of 46 was blasted ontoeach of the four surfaces of the plates for approximately 2minutes.
For control purposes, bending fatigue specimens fromHeat No. 4 were cast oversize and machined. Specimens to bemachined on cope and drag surfaces only and those to becompletely machined were cast as one-half inch and five-eighths inch plates respectively. All plates were groundon a rotary surface grinder using a selective grindingprocedure to assure equal removal of stock from oppositysurfaces and minimize metal damage. Following machining,the specimens were subjected to tensile, fatigue and hard-ness tests utilizing standard procedures. The fatigue testingwas conducted on two types of fatigue machines. R. R. Moorespecimens were tested in four point reversed bending onstandard R. R. Moore rotating beam machines operating at10,000 cycles per minute. The bending specimens weretested on a constant load amplitude Sonnatag SF-I-U fatiguemachine operating at 1800 cycles per minute. All platebending specimens were tested in reverse bending under fourpoint loading. The dimensions of the unmachined bendingspecimens were measured and machine loading correspondinglyadjusted to account for dimensional inaccuracies of the castplates. All specimens were considered to have failed whencompletely fractured. The plate bending fatigue tests werecarried out at various stress levels to obtain curves ofstress versus cycles to failure. The number of specimensused for a S-N curve ranges from fifteen to twenty. R. R.Moore specimens were also tested at different stress levelsand six specimens were used per curve. The endurance limitfor all specimens was based on ten million cycles.
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RESULTS AND DISCUSSION
The detailed results and a discussion of these results arepresented in the three interim technical reports that have beensubmitted on this contract. These reports are concerned withthe thermal and mechanical behavior of molding materials andsteel castings. They include a study of the dimensionalstability of the molding materials and the effect of themolding materials on the surface roughness and fatigue behaviorof the castings. The reports are dated September, 1964; Novem-ber, 1965; and May, 1967. In addition to the interim technicalreports submitted, two publications in the technical literaturehave resulted from this contract. These are the following:
Wallace, J. F. and Bates, C. E., "An Investigation ofThermal and Mechanical Stability of Mold Materialsfor Steel Castings," Modern Castingsj May, 1966,p. 216, also Trans. AFS Vol. 74, p. 174.
Wallace, J. F. and Naro, R. L., "Effect of Mold Washepon Casting Surface and C-asting Fatigule Propertiesof Steel" Trans. AFS, Vol 75.
The first paper was granted the Award of the Sand Divisionof the American Foundrymen's Society for 1966 as an out-standing contribution to the advancement of the foundryindustry.
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GENERAL SUMMARY
This work is summarized by presenting the abstractcs, frr,- eachof the interim technical reports submitted on this :ýate-rial
A. Report of September, 1964
Test apparatus and tcchniaue were developed to measurethe tensile strength of moldinq material:s at elevated temperatures.The measurements of tensile strength obtained from this e"uipmentpermits a qualitative evaluation of the mechanical stability ofthe mold as molten steel enters and solidifies in the mold. Themold stability obtained with various refractory aggregate-bindercombinations is a significant factor in controlling dimensionaltolerances in the steel casting.
Several rammed mold materials consisting of variouscombinations of refractory aggregates and binders were invest-igated for use in producing high quality steel castings. Aggregatesof mullite, molochite, silica sand, fused silica, calcined kyaniteand calcined alumina were bonded with western bentonite andsodium silicate. Close dimensional control of the castings wasobtained with fused silica, calcined alumina and silica sandbonded with sodium silicate and green, western bentonite bondedmullite. Better casting surfaces were obtained with the fusedsilica aggregate. A modified sodium silicate binder utilizingsilicon metal fines to promote setting was investigated.• Thebond developed was stronger in tension and compression at bothroom and high temperatures than that produced by western ben-tonite or by gassinq identical sodium silicate bonded aggregates.This sodium silicate-silicon fines binder combined with eitherfused silica or silica sand refractories provided a mold forcasting steel. to close dimensions with a good cast surface.
B. Report of November, 1965
A test apparatus was developed to measure the tensilestrength of molding materials at elevated temperatures. Themeasurements of tensile strength obtained from this equipmentpermits a qualitative evaluation of the ability of a mold tomaintain casting dimensions.
Several rammed mold materials consisting of variouscombinations of refractory aggregates and binders were invest-igated for use in producing high quality steel castinqs. Theaggregates selected for study included mullite, zircon, silicasand, fused silica, olivine, and calcined alumina. These werebonded with western bqntonite and sodium silicate. Closedimensional control was obtained with all aggregates bondedwith sodium silicate and with the zircon and mullite aggregatesbonded with western bentonite. The casting surfaces obtained
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with the fused silica and zircon wete generally superior tothose produced in other aggregates.
A technique was developed for measuring and recordingmold wall deformation continuously during pouring and solid-ification of steel castings. Using this tecnnique, molddeformations were measured at various distances from themold-metal interface in molds rammed to different densitieswhen western bentonite and sodium silicate binders wereutilized. Mold deformation was found to be limited to athin layer of the mold adjacent to the casting in all cases.This result indicates the importance of this thin layer indetermining the dimensions of the final casting.
Z. Report of May, 1967
This research investigated the effect of mold coatingsand refractories on the surface quality of cast steel. Theinfluence of surface roughness in reversed bending fatiguewas also examined using as-cast specimens of AISI 4330 steelproduced in green sand molds coated with hexachlorobenzene,aluminum powder, sodium floroaluminate, and a proprietarymold Wash. Plate bending specimens were also produced inunocdted silica, chromite and zircon sand molds and subse-quertly tested. All specimens were heat treated to approx-imately 185,000 psi tensile strength levels as determinedfrom tensile specimens from keel block legs. Control andcomparison tests were made with R. R. Moore, tensile, andmachined plate specimens.
Test specimens made in both hexachlorobenzene coatedsilica sand molds and zircon sand .- duced equivalent resultsin improving the surface fin..h of experimental test castings.Bending fatigue test results showed that hexachlorobenzenecoated and uncoated zircon provided the greatest overallimprovement in fatigue life and endurance limit. Completelymachined bending fatigue specimens result in only slightimprovements in fatigue compared to sand blasted hexachloro-benzene and zircon specimens with the original cast surfaces.Shot peening these specimens also produced substantial gainsin fatigue strength, compared to machined specimens. Thefatigue failures were initiated at cope surfaces with greaterfrequency than drag surfaces.
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(Securty classification of title, body of abstract and indexing anotation must be entered when the overall report is cla•setfed)
I ORIGINATING ACTIVITY (Corporate author) 2. REPORT SECURITY C LASSIFICATION
Case Western Reserve UniversityI Unclassifip-dUniversity Circle 2b. GROUP
Cleveland, Ohio 44106 I3. REPORT TITLE
EFFECT OF MOLD-STEEL INTERFACE REACTIONS ON CASTING SURFACEAND PROPERTIES
4. DESCRIPTIVE NOTES (Type of report and inclusive datee)
Final Report July 1, 1963-January 1. 1968S. AUTHOR(S) (Last none, Brat name, initial)
Bates, Charles E.Naro, Rodney L.Wallace, John F.
6. REPORT DATE 7a. TOTAL NO. OF PAGES 7b. NO. REF
January, 1968 16 ----Ba. CONTRACT OR GRANT NO. 9a. ORIGINATOR'S REPORT NUMSER(S)
DA-33-019-AMC-270 (Z)b. PROJECT NO, AMRA CR 64-03/3D/A IC024401A328
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AMCMS Code No. 5025.11.294d.
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Distribution of this document is unlimited
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13. ABSTRACT
The effect of mold materials and binders and mold washes onthe dimensional behavior, surface finish and fatigue strength of highstrength steel castings was determined. Close dimensional controlwas obtained with all aggregates bonded with sodium silicate and withthe zircon and mullite aggregate bonded with western bentonite. Ameans of measuring the tensile strength of molding materials at ele-vated temperatures was developed and the results of this test permitqualitative evaluation of the ability of a mold material to maintaincasting dimensions. Test specimens made in both hexachlorobenzenecoated silica sand molds and zircon sand produced equivalent resultsin improving the surface finish of experimental test castings. Bend-ing fatigue test results showed that hexachlorobenzene coated and un-coated zircon provided the greatest overall improvement in fatiguelife and endurance limit. Completely machined bending fatigue speci-mens result in only slight improvements in fatigue compared to sandblasted hexachlorobenzene and zircon specimens with the originalcast surfaces.
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Steel Castings
Dimensional Accuracy
Cast Surface Finish
Mechanical Properties
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