STEEL FOUNDERS’ SOCIETY OF AMERICA TECHNICAL SERVICE REPORT #104

ANALYSIS OF INCLUSION SOURCES IN BARS CAST FROM ACID MELTED CARBON STEEL

Published by the

STEEL FOUNDERS’ SOCIETY OF AMERICA

Mr. Malcolm Blair Technical and Research Director

August, 1990

IDENTIFICATION OF INCLUSION SOURCES IN BARS CAST FROM ACID MELTED CARBON STEEL

C.E. Bates and John Griffin

ABSTRACT

Defects from an SFSA member company were examined in the current techni-

cal effort to determine the principal source of macro-inclusions. The majori-

ty of the inclusions in the samples examined in this study were composed of

deoxidizers and deaxidation products. Very little iron or manganese oxide

were present to suggest reoxidation during pouring. No mold erosion products

were present since the castings were poured in graphite molds. Typical micro-

graphs of foreign debris are presented.

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IDENTIFICATION OF INCLUSIONS SOURCES IN

BARS CAST FROM ACID MELTED CARBON STEEL

INTRODUCTION

Oxide macro-inclusions have always been troublesome in steel castings. A

general review of the steel casting production process suggests a number of

possible sources of nonmetallic inclusions. Among these are:

1. furnace refractories melted or fluxed during meltdown,

2. metal oxides produced during oxygen blowing,

3. oxidation of the metal and deoxidizers including silicon and

manganese during tapping,

4. formation of aluminum deoxidation products in the ladle,

5. fluxing and erosion of ladle refractories,

6. reoxidation of the metal during reladling,

7. fluxing and erosion of bottom pour nozzles and stoppers,

8. reoxidation of metal during pouring into the mold cavity,

9. reoxidation of metal by reaction with mold binder decomposition

products)

10. fluxing of mold refractories by oxides carried into the mold,

11. loose sand on cores and in mold cavities, and

12. thermal expansion defects resulting in loose sand falling into the

molten metal during pouring.

These inclusions can be classified by their source of oxygen which might

include air, water in refractories, molding sand, and oxidized slag. Mechani-

cal entrapment of oxides such as refractories and sands is also a possibility.

Mechanical entrapment is an obvious source and has often been cited as the

major source of oxide macro-inclusions. However, many sources of inclusions

exist, and a summary is presented in Table I. This table is a compilation of

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information presented in References 1-20. The major inclusion sources include

reoxidation of metal and entrapment of existing oxides in the metal, ladles

and molds,

Reoxidation

There is substantial evidence that reoxidation is a major cause of oxide

Approximately 425 defects from carbon and low alloy steels macro-inclusions.

were removed and examined by optical and scanning electron microscopic tech-

niques to determine the nature and form of oxide nonmetallic materials pres-

ent, Castings came from a wide variety of practices including acid and basic

melting furnaces, large and small pouring ladles, high silica and high alumina

refractory ladle linings and poured in green (clay-water bonded) sand molds

and chemically bonded sand molds.(24)

The results are illustrated in Figure #1. Approximately 80% of the

castings defects consisted principally of reoxidation products and 15% con-

sisted of eroded sand.

tively minor contributors.

Slag, refractory and deoxidation products were rela-

Reoxidation is considered to be the reaction of elements in steel with

oxygen after the steel has been deoxidized. This oxygen may come from the

air, reactions with slag or refractories, or the reduction of water from air,

refractories, or the mold.

The formation of deoxidation and reoxidation products is schematically

illustrated in Figure 2.(16) Deoxidation products are small, typically 10

microns or less in diameter, and are composed almost exclusively of alumina

(Al2O3) in aluminum deoxidized carbon and low alloy steels. Reoxidation

products generally consist of alumina particles in single or multiphase glob-

ules formed by the oxidation of aluminum followed by silicon, manganese and

iron (16,21). The matrix in which the alumina particles lie is typically rich

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in manganese and silicon oxides, but in extreme cases may contain considerable

amounts of iron oxide. The presence of iron oxide may cause gas holes as the

iron oxide is reduced by carbon to produce carbon monoxide gas during solidi-

fication.

Typical oxide macro-inclusions visually appear to be green to white pow-

der. The inclusions consist of a mixture of corundum crystals, silica, and

manganese oxides. The presence of corundum indicates that the inclusion began

as a reaction product between oxygen and dissolved aluminum. Most refracto-

ries are composed of mullite or lower alumina content materials and are not

expected to produce corundum.(1,5,6,8,20,19,20)

Ladle Refractory Attack

Many investigators have concluded that ladle refractories are a major

source of nonmetallic inclusions, Vingas and Zrimsek reported the main

sources of inclusions in castings to be ladle refractories, ladle slags,

oxidation of the deoxidizers, and eroded sand grains.(2,19)

Lyman and Boulger similarly concluded that the major sources of ceroxide

inclusions were reoxidation and refractory materials.(1)

from ladles lined with high alumina refractories contained more alumina inclu-

sions than castings poured from ganister linings suggesting that alumina was

being eroded by the molten metal.

Castings poured

It is certainly possible for liquid metal and the contained oxides to

attack refractories. Liquid steel tapped at 1590 °C (2900 °F) or higher can

produce a molten silica-rich phase in alumina-silica refractories containing

less than 70% alumina. Refractories containing less than 70% alumina have a

simple eutectic that melts at 1590 °C (2900 °F). Progressively higher temper-

atures produce progressively more liquid phase. The liquid lowers the refrac-

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tory strength and can cause sagging or result in refractory erosion during

filling or pouring from a ladle.

Mold Related Inclusions

There are several sources of mold related inclusions as metal enters the

mold cavity. Obvious sources include poor housekeeping and molding practices

that produce sand from loosely compacted molds, brittle sand, loose sand from

rough spots on patterns, debris from overhead conveyors, core or mold crushes

caused by rough handling, improperly sized core prints, and improper core

placement. Correcting these problems lies more in improving the work prac-

tices than in altering the molding or pouring practices. However, Sanders has

shown that inclusion defects cannot be eliminated solely by "good housekeeping

practices". Inclusions were present in steel poured in clean graphite molds.

(22)

Silica sand is a common ingredient of nonmetallic inclusions, although

rarely found by itself. Lyman and Boulger (1) and Caine (23) analyzed inclu-

sions and found many to contain free sand. Excluding the obvious sand grains,

the nonmetallic material was found to contain about 53% silica, 20% alumina,

20% manganese oxide, and 10% iron oxide.(1) Silica sand was a major compo-

nent of the inclusions and could be picked up either by direct erosion or by

having the reoxidation products stick to the sand and then be pulled off by

the flowing metal. Whatever the mechanism, free silica sand was a major

constituent of the inclusion and was found blended with mixed oxide phases.(1)

Air contained in the mold cavity and water in clay bonded sand molds provide

opportunity for oxidation as the metal fills the mold cavity. Since silica is

wet by iron oxide, mold erosion and inclusion formation may be aggravated by

oxidation which produces iron oxide and manganese oxide.(7) These materials

wet and adhere to the sand, and the agglomerate can be eroded off by the

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flowing molten metal.

fluid iron silicate melt which may aggravate erosion.

Iron and manganese oxide dissolve silica to form a

CURRENT STUDY

During the course of several heats of steel, a sample of the steel was

poured into a graphite mold to produce a bar approximately 4" diameter by 9"

long. Each bar was poured at the approximate Middle of the heat. The purpose

of pouring the bar in the graphite mold was to eliminate sand as a source of

inclusions. The bars were then put in a lathe and turned to remove the inclu-

sions. The inclusions were examined with an optical and scanning electron

microscope to determine their nature and probable source.

All of the heats were acid melted with a carbon content of about 0.5%.

Oxygen was blown into the furnace to reduce the carbon to a value of about

0.20-0.24% and then the heat was blocked. After adjusting the composition,

the metal was deoxidized with 6 lbs per ton Hypercal. when higher impact prop-

erties were required, and the remaining heats were deoxidized with a combina-

tion of aluminum, Hypercal, and Calsibar. The weight of the deoxidizing

additions was calculated to add approximately 0.075% aluminum to the metal.

The castings are normally poured into phenolic urethane bonded sand molds

although all of the test bars were poured in graphite molds. The deoxidizers

were placed in the ladle during tapping when the ladle was approximately 1/3

full.

The furnace was lined with 70% alumina brick and the bottom was prepared

with silica mixed with sodium silicate. Ladles were lined with 70% alumina on

both the side wall and bottom using 60% alumina mortar.

The test bars were poured from the ladle when about half way through the

heat. After cooling to room temperature, the bars were turned in a lathe to

remove metal and the dust collected and placed in an envelope.

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Samples of dust from each of the envelopes were placed in a tube and the

tube was filled with epoxy and evacuated to remove all air bubbles. The epoxy

was cured with an application of a small amount of heat under vacuum to firmly

hold the oxides for metallographic polish and SEM specimen preparation. The

debris was examined in a scanning electron microscope, and energy dispersive

x-ray (EDX) analyses of the debris were obtained.

RESULTS AND DISCUSSION

A typical micrograph of dust sample #765 is illustrated in Figure 3 at a

magnification of 60X. Numbers beside the particles indicate the specific

areas analyzed by EDX. The spectrum of area #1 in Figure 2 is illustrated in

Figure 4. Particle #1 consisted almost completely of iron. Only traces of

oxygen, silicon, and magnesia were present. The EDX spectrum from area #2 is

illustrated in Figure 5.

na as seen by the EDX spectrum.

This particle consisted almost exclusively of alumi-

A micrograph of dust sample #810 is illustrated in Figure 6 at a magnifi-

cation of 60X. Again, the numbers and arrows indicate areas analyzed by EDX.

Area #1 in Figure 6 was composed almost exclusively of alumina, although a

small amount of calcium and manganese were present as shown by the spectrum in

Figure 7. Area #2 was similarly composed of alumina and oxygen with traces of

silicon and calcium. (See Figure 8)

The appearance of dust from sample #747 is illustrated in Figure 9. The

debris particles illustrated in Figure 9 were composed almost completely of

calcium oxide, as seen in the EDX spectrum illustrated in Figure 10.

The appearance of a dust sample from #757 is illustrated in Figure 11.

Three areas in Figure 10 were analyzed in the spectrum and are presented in

Figures 12, 13, and 14, respectively. Area #1 appeared to be a complex oxide

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composed of aluminum, silicon, manganese and iron. The presence of the manga-

nese suggests some reoxidation occurring during pouring. Area #2 was composed

almost exclusively of alumina with small amounts of calcium and manganese

oxide. The light area labeled #3 in Figure 10 was iron.

The appearance of the dust sample from specimen #789 is illustrated in

Figure 15. Three areas were chosen for EDX analysis and the results are shown

by the spectra in Figures 16, 17 and 18.

exclusively of iron.

alumina silica and oxygen.

small amounts of calcium and iron.

Area #1 again was composed almost

Area #2 was composed of principally iron with some

Area #3 was composed principally of alumina with

SUMMARY AND CONCLUSIONS

The majority of the inclusions in the samples examined in this study were

composed of deoxidizers and deoxidation products.

nese oxide were present to suggest reoxidation during pouring. No mold ero-

sion products were present since the castings were poured in graphite molds.

Very little iron or manga-

It is recommended that additional studies of this type be conducted where

sections of metal containing the inclusions are removed, mounted and polished

to be sure the entire sample of foreign material is examined, rather than only

the material collected as dust during machining.

SRI-MMER-90-792-7048-000.F

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To: Steel Founders' Society of America Cast Metals Federation Building 455 State Street Des Plaines, IL 60016

Date: August, 1990

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