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AMRA CR 64-04/30

Control of Cast Grain Size of Steel Castings-Effectof S ttre and Nonmetallics on Proerties

Final ReportAugust 1, 1963-January 31, 1968

by

Peter F. WieserNathan ChurchJohn F. Wallace

DISTRIBUTION STATEMENT AApproved for Public'Release

Distribution UnlimitedJanuary, 1968

Department of MetallurgyCase Western Reserve University

University CircleCleveland, Ohio 44106

"Contract No. DA-33-0ii-AMC-273 (Z)

Distribution of this Document is Unlimited

,U.-S.ARMY MATERIALS RESEARCH AGENCYWATERTOWN, MASSACHUSETTS 02172

4 c o JGO~5Oo x

Control of Cast Grain Size of Steel Castings-Effect of Structure and Nonmetallics on Properties

AMRA CR 64-04/3

by

Peter F. WieserNathan ChurchJohn F. Wallace

January, 1968

Department of MetallurgyCase Western Reserve University

University CircleCleveland, Ohio 44106

D/A Prcject No. 1C024401A328

AMCS Code No. 5025.11.294Metals Research for Army Material

Distribution of this Document is Unlimited

U.S. ARMY MATERIALS RESEARCH AGENCY

WATERTOWN, MASSACHUSETTS 02172

i

ABSTRACT

Fhis project was concerned with determining theinfluence of micro-structural refinement on the mechan-ical properties of cast high strength 4335 steel. Thestrength levels investigated were approximately 250,000psi tensile strength and 205,000 psi yield strengthobtained with a liquid quench and temperi-ng treatment.The steel was cast in unidirectionally solidified cylin-ders and conventional sand mold keel blocks.

It was determined that the addition of approximately0.10% Ti to the steel subsequent to deoxidation providedconsideable structural refinement. This structuralrefinement consists of changing the usual columnarstructure to equiaxed and of refining the spacing of thesecondary dendrite arms. Such refinement was accompaniedby improvements in the tensile ductility and impactresistance of the steel. This improvement in propertieswas particularly marked on specimens located with theirlong axis perpendicular to the direction of heat flow.Structural refinement improved both the level of mechan-ical properties and reduced their directionality.

Studies of the micgro-structure of the cast steeldetermined that nonmetallic inclusions were the primaryfactor (in the absence of microshrinkage) in determiningthe level of ductility. The loss in ductility thatoccurred because of the presence of these nonmetallicinclusions was reduced by low sulfur contents and bymeans of combined deoxidation treatments. The bestdeoxidation treatments were found to be calcium-manganese-silicon or zircon-silicon added together with aluminum.Improved ductility was also obtained with the finalsecondary dendrite arm spacing that is obtained by rapidsolidification rates and by grain refinement withtitanium. J

ii

TABLE OF CONTENTS

Page

Abstract

Table of Contents iii

Introduction 1

Procedure and Materials 2

Results and Discussion 6

General Summary 7

Distribution 9

iii

INTRODUCTION

The control of the solidification process is a basic stepin obtaining the potential mechanical properties of a casting.The as-cast structure and presence of heterogeneities such asporosity and nonmetallic inclusions or nonequilibrium segregatesdetermine the degree to which the properties inherent in thecasting are realized. The type and fineness of the as-caststructure is determined by the composition, solidification timeand the state of nucleation during solidification. The presenceof heterogeneities in steel is primarily governed by the thermalgradient in the liquid during solidification, the chemical comp-osition and the deoxidation practice.

Grain refinement is one method to control the solidificationprocess. Nonferrous alloys are frequently grain refined to achievebetter strength, ductility, resistance to cracking and hot short-ness. Similarly, cast irons are frequently grain refined tocontrol the structure and its properties. A detailed review ofthis subject was presented in earlier reports. Grain refinementof steel is commercially applied only in some industries. Theapplication of a moving electromagnetic field during solidificationhas been reported to refine the structure of continuously caststrands and to improve the surface condition, reduce macroporosityand the tendency for hot tearing. Macrorefinement (conversion ofa columnar dendritic structure to an equiaxed structure) has beenobtained in some instances by applying a particular melting,deoxidation or inoculation practice. In spite of these reports,grain refinement has not assumed the important role in the steelfoundry industry which it has achieved in the nonferrous or castiron industry.

Recently, a technique of grain refining high strength, lowalloy steel of the type AISI 4335 was developed. This techniqueconsists of inoculating the melt with titanium. A suitabletitaiuum inoculation reduces the dendrite spacing of the castingand produces an equiaxed dendritic structure in a casting, whichwould normally solidify in a columnar dendritic manner. Suchrefinement improves the tensile ductility as measured by thereduction in area and toughness measured by Charpy V-notchimpact specimens.

The present investigation was undertaken to achieve thefollowing objectives: 1) improvement of the degree of refine-ment without introducing embrittling, nonmetallic inclusions;2) establishment of the relation between wall thickness, orequivalently solidification time and structure of properties;3) influence of melting variables such as deoxidation practiceon the structure, inclusions and properties; 4) determinationof the influence of the mode of solidification (equiaxed versuscolumnar dendritic) on the segregation behavior of solute elementsand on the incidence of microporosity.

1

PROCEDURE AND MATERIALS

The alloy used in this investigation is a high strength, lowalloy steel of AISI 4335 type with the following nominal compo-sition:

C% M__% Si% Cr% Mo% Ni%

0.30 0.60 0.20 0.70 0.20 1.65to to to to to to

0.35 0.80 0.35 0.90 0.30 2.00

The steel was generally melted at a sulfur level of 0.013 percentwith 0.01 percent phosphorus. Exceptions to this analysis willbe referred to in the text whenever necessary. The steel wasmelted from closely controlled charge materials using 50 and100 pound, magnesia lined, high frequency induction furnaces.The primary charge of low sulfur iron was rapidly melted and aCO boil initiated and maintained for at least two minutes byadding pig iron. Pig iron was used to maintain the boil and toadjust the carbon content after blocking the teat with ferro-silicon and ferromanganese. Standard ferroalloys and electrolyticnickel were employed to introduce the necessary alloying elements.A protective atmosphere was maintained during the preparation ofthe heat by directing a stream of argon at the melt through arefractory cover. Final deoxidation was performed by adding0.10 percent aluminum prior to tap. During the phase of theinvestigation of the effect of deoxidation practice all deoxidi-zers were wrapped in light gaqe steel and plunged below thesurface of the melt.

Unidirectionally solidified cylindrical castings andstandard keel blocks were employed as test castings. Thecastings were poured directly from the furnace into moldsthrough a pouring dish. The cylindrical castings were usedin two sizes weighing 45 and 11 pounds. ThLse castings willbe referred to as the large and small cylindrical castingrespectively. The bottom of the mold cavity was formed bya graphite chill block; the rest of the mold cavity was formedby an exothermic sleeve separated from the Chill block by athin dry sand ring. Exothermic hot topping was used for alllarge cylinders, none was employed for the smaller cylinders.The standard keel block cAstings with 1" x 1" x 8" long legswere cast in dry sand molds with a small amount of exothermichot topping.

The castings were usually produced in pairs from a singlemelt, first the base or uninoculated casting was poured, thenthe melt was treated and the inoculated casting poured. Theferrotitanium employed for grain refinement was plunged similarto the deoxidizers mentioned earlier. The time between inoculation

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and pouring was held uniformly at two or three minutes. Thepouring temperature ranged from 28000 to 2950°F for largecylinders as discussed in the text. For small cylinders andkeel blocks, a uniform pouring temperature of 2900OF wasselected. The pouring temperature was closely controlled byimmersion thermocouples.

Macro and microexamination of the structure was conductedin all castings. Humphrey's reagent was used to develop thedendritic structure. The details of the cast structure wereimproved in some cases by isothermal transformation heattreatment. This consisted of austenitizing and quenching tothe isothermal transformation temperature of 1200*F andresulted in ferrite-pearlite dendrites and a martensiticmatrix. The primary dendrite and secondary arm spacingswere measured at various heights of the cylindrical castingsas indicated in the text. The secondary dendrite arm spacingof keel block legs was measured 3/16 inches from the castsurface at 3/4 inches from the bottom of the leg. The amountof nonmetallics was quantitatively determined employing a

* two-dimensional systematic point count method.

A microanalysis was conducted on specially preparedmetallographic specimens employing an electron microprobe,model 400, built by the Materials Analysis Company. Specimensused for microprobe analysis were isothermally transformed toprovide maximum detail in the etched structure. When solutemaxima were determined, the specimens were isothermally trans-formed up to 90 minutes in order to transform all of thestructure, except the areas exhibiting a solute maximum toferrite plus pearlite. The material exhibiting a solutemaximum transformed to martensite during the following quenchin water. The path along which the composition was to bedetermined was marked by micro hardness impressions. Thespecimens were then repolished and analyzed. The microprobetraces were revealed by etching the specimens with 3 percentnital.

All microprobe analyses were made by p6int counting,integrating for ten seconds. The distance between pointsvaried from 2 to 20 microns depending on the accuracy required.In areas exhibiting a solute maximum the distance betweenpoints was two to five microns. The microprobe was operatedwith a potential difference between filament and target of25 KV, the specimen current was 0.03 microamperes, the take-off angle was 350, the beam diameter was held between one andtwo microns. The elements manganese and chromium were analyzedquantitatively using specimens of known composition as standards;qualitative analyses were made for titanium.

The method used to determine solute minima and maximacon!:.isted of conducting point counts along diagonal pathsthrough the dendrite center into the geometrical centerbetween adjacent dendrites of a columnar dendritic specimen.For equiaxed strtctures, the analysis was restricted tospecimens which were slightly over inoculated so that thespecimen contained titanium sulfides instead of the usualmanganese sulfides. The reason for sel-ecting these specimenswas to permit using counts along random paths withoutmistaking concentration peaks from sub-surface inclusionswith true manganese maxima.

Microradiographic techniques were used to determine therelative amount of porosity in various unidirectionallysolidified castings. Specimens 1/2 x 1/2 x 0.0025 inchesthick were radiographed using unfiltered copper and cobaltK-alpha radiation respectively. The two types of radiationwere selected to distinguish between porosity and nonmetallicinclusions. The inclusions in this steel are mostly (FeMn)O°A1203, manganese sulfides and various oxides; they do notshow up in radiographs using unfiltered cobalt radiation.The operating conditions were as follows: specimen and filmto source distance 18 inches; for the copper target, thevoltage was 35 KV, current 23mA, exposure time 17 sec; forthe cobalt target the voltage was 45 KV, current 6mA, exposuretime 25 sec. Contract Process Ortho negative film wasemployed which permitted good resolution of porosity up toa magnification of 50X. The amount of porosity was determinedas the number of pores per unit area of specimen. Conversionto volume percent can be readily performed if the pores areessentially spherical. This was not the case and only a relativemeasure was employed. The resolution obtained with thisprocedure permitted detection of pores of 4 microns indiameter or larger.

Two .212" diameter tensile specimens were utilized: onehad the gage length machined to a 2" radius with a 0.212"specimen diameter; the second had a constant diameter overthe gage length. Ductility measurements were limited toreduction in area for radiused specimens. Both the reductionin area and elongation were determined for other specimens.The tensile properties of cylindrical castings weremeasured both normal and parallel to the chill, referred toas vertical and horizontal specimens respectively. Theproperties of keel blocks were measured with specimens whoseaxes were parallel to the long direction of the keel blockleg.

The heat treatment conducted for the specimens aftersectioning the casting was as follows: homogenizing for2 hours at 2200 0 F, air cool; normalizing for 2 hours at 1750*F,

5

air cool; spheroidizing over night at 11500 F, air cool. Thespecimens were then rough machined and heated to 400 0 F, heldfor 9 hours and air cooled, followed by austenizing for 1 hourat 16501F, cooling to 1500 0 F, holding for 1 hour and subsequentquench in oil, a double temper treatment of 2 hours at 400OFwith a subsequent oil quench concluded the heat treatment.

RESULTS AND DISCUSSION

The detailed results and a discussion of these resultsare presented in the three interim technical reports thathave been submitted on this contract. These reports areall concerned with the area of control of grain size andthe relation between microstructure and mechanical proper-ties in cast, high strength 4335 steel. In addition to theinterim technical reports submitted, three publications inthe technical literature have resulted from this contract.These are the following:

Wallace, J. F., Church, N., and Wieser, P., "Controlof Cast Grain Size of Steel Castings," Modern Cast-ings, April, 1966, p. 129, AFS Trans., Vol. 74, p.113.

Wallace, J. F. and Wieser, P. F., "Grain RefinementCast Steel by Vacuum Melting," Cast Metals ResearchJournal, March, 1966, Vol. 2, No. 1, p. 1.

Wallace, J. F., Church, N, and Wieser, P., "GrainRefinement of Steel Castings," Journal of Metals,June, 1967, p. 44.

This latter paper was granted the outstanding paper awardfor 1966 by the AIME Electric Furnace Conference of theIron and Steel Division. This paper will also appear in the1966 Electric Furnace Proceedings of AIME.

6

GENERAL SUMMARY

This work is summarized by presenting the abstracts from eachof the interim technical reports submitted on this material.

A. Report of September, 1964

Techniques for the quantative evaluation of refinement ofa high strength cast steel have been evolved and used to test theeffectiveness of various inoculants in achieving microstructuraland macrostructural refinement. Of the various materials testedonly titanium additions produced significant refinement of boththe micro and macrostructure.

Comparison of the tensile properties of the basic steelwith that of a refined steel shows that large additions (0.2-0.6 w/o) of titanium are sufficient to decrease the reductionin area sharply. Further data on castings partially refined w'ithsmall (o.10 w/o) titanium additions indicate that an improvementin reduction of area is obtained compared to the results with acolumnar structure solidified under similar thermal conditions.

B. Report of November, 1965

Inoculation techniques were developed to refine the ascast structure of high strength steel. Titanium was found tobe the most effective inoculant. Large titanium additions(0.6%) form sulfides which embrittle the steel. Small titaniumadditions (0.1%) together with controlled superheat and molddesign may produce significant refinement. Under these conditionsthe structure of unidirectionally solidified castings was changedfrom columnar to equiaxed and the reduction in area increased byapproximately 25% for specimens oriented vertically to thedirection of heat .flow and increased about 75% for horizontallyoriented specimens. A small improvement in reduction in areaof keel block castings was obtained by this method. Some cimprove-ment in toughness was also obtained by this grain refinement.The influence of vacuum melting on the cast structure of thesame steel was investigated. A hypothesis explaining themechanism of refinement by vacuum'melting was proposed.

C. Report of May, 1967

The relation of dendrite structure, nonmetallics andmicroporosity to solidification time and refinement by inoculationwas investigated and correlated to mechanical properties of AISI4335 steel castings.

The dendrite spacing is determined by solidification timeand significantly influenced by solute content. Titanium andboron effectively refine the secondary dendrite spacing; selenium,

7

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tellurium and sulfur have the opposite effect. Titaniuminoculation also nucleates equiaxed dendrites, therbyreducing or eliminating the columnar dendritic structure;the elements boron, selenium, tellurium, and sulfur mayincrease the length of columnar dendrites. This effectis hypothesized to result from entrapment and deactivationof nuclei in the segregates of these elements. The amountof nonmetallics or the solidification time are related tothe ductility of unidirectionally solidified castings.Improvements in ductility from grain refinement can beexpressed by the change in dendrite spacing. Refinementof the dendritic structure improves ductility, presumablyby the reduction in particle size and the improved dis-tribution of nonmetallics. Microprobe analysis indicatesno significant differences in the segregation behavior ofthe elements manganese and chromium from inoculations. Ahypothesis concerning the formation of dendrite arms waspostulated based upon measurements of solute distributionand existing theories of dendrite arm formation.

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I1. ORIGINATING ACTIVITY (Corporate author) 2a EOTSECURITY C LASSIFICATION

Case Western Reserve University UncROassifiUP

Cleveland, Ohio 44106. .oup

3. REPORT TITLE

Control of Cast Grain Size of Steel Castings-Effect ofStructure and Nonmetallics on Properties

4. DESCRIPTIVE NOTES (Type of report and inclusive date@)

Final ReportS. AUTHOR(S) (Last name, first name, initial)

Wieser, Peter F.Church, NathanWallace. John F.

6. REPORT DATE 7a. TOTAL NO. OF PAGES 7b. No. or lqir,

_January. 1968 151Sa. CONTRACT OR GRANT NO. Sa. ORIGINATOR'S REPORT NUMMER(S)

DA-33-019-AMC-273 (Z)b. PROJECT NO. AMRA CR 64-04/3

SD/A Project No. 1C024401A328o. 9b. ITNERRMPORT NO(S) (Any othernumbere that may be aessined

AMCS Code No. 5025.11.294d.

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Army Materials & Mechanics ResearchCenter, Watertown, Mass. 02172

13. ABSTRACT

This project was concerned with determining the influence ofmicrostructural refinement on the mechanical properties of cast highstrength 4335 steel. It was determined that the addition of approx-imately 0.10% Ti to the steel subsequent to deoxidation provided con-siderable structural refinement. Such refinement was accompanied byimprovements in the tensile ductility and impact resistance of thesteel. This improvement in properties was particularly marked onspecimens located with their long axis perpendicular to the directiorof heat flow. Studies of the microstructure of the cast steel deter-mined that nonmetallic inclusions were the primary factor (in theabsence of microshrinkage) in determining the level of ductility.Improved ductility was also obtained with the final secondary den-drite arm spacing that is obtained by rapid solidification rates andby grain refinement with titanium.

F D ORM 17DD Casfcto1473

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Steel Castings

Grain Size Effects

Mechanical Properties

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DFORMDD JAN 61473 (BACK)Security Classification

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