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Short communication

The key role of forging in ancient steel making from whitecast iron

Jang Sik Park⁎

Department of Metallurgical Engineering, Hongik University, Chochiwon, Choongnam, 339-701, South Korea

A R T I C L E D A T A

⁎ Tel.: +82 41 860 2562; fax: +82 41 866 8493.E-mail address: jskpark@shinan.hongik.ac.

1044-5803/$ – see front matter © 2007 Elsevidoi:10.1016/j.matchar.2007.03.013

A B S T R A C T

Article history:Received 19 April 2006Received in revised form28 February 2007Accepted 28 March 2007

Various thermo-mechanical treatments given to specimens of Fe-4.3 wt.% C showed thatforging plays a key role in steel making from white cast iron. Forging causes the brittlecementite phase to fragment continuously in the repeated heat-forge cycles and keeps thedecarburization rate high throughout the entire process. Forging in combinationwith forcedair supply may cut down the processing time considerably and make this steel makingtechnique competitive when no alternative is readily available as in ancient times.

© 2007 Elsevier Inc. All rights reserved.

Keywords:Ancient steel makingWhite cast ironCementiteForging

1. Introduction

In light of its importance in modern iron and steel industry, itis surprising that no cast iron was produced in Europe forindustrial purposes until around the 14th Century AD. This isin sharp contrast to the case in East Asia, where its productionstarted no later than the 4th Century BC in China. If thetechnical diversities available in the presence of cast iron aretaken into account, there is no doubt that the East and theWest had developed iron industries of fundamental differencefor nearly 2 millennia [1–5]. The inherent brittleness of castiron and the consequent restriction in applicabilitymay be themajor factors that made its latent usefulness go unnoticed forsuch a long period in Europe, where the beginning of the use ofiron had startedmuch earlier than in the East. The unique ironindustry based on cast iron, therefore, would not have beenfully established without having the proper means to convertit to more versatile materials such as wrought iron and steel.In fact, the ancient Chinese texts quoted by Needham [1]

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contain passages referring to various processes in which steelwas produced from cast iron either by the direct decarburiza-tion in solid or liquid state or by the combination of cast ironand wrought iron [6].

Despite its significance in ancient as well as inmodern ironindustries, little is known of the technical aspects of early steelmaking from cast iron except a few comments based primarilyon textual evidence. This is especially true with the process ofdirect decarburization of cast iron in the solid state because ithas long been obsolete and is rarely practiced in modern ironindustry. It is suggested that it was the earliest technique, andwas in use as early as in the second century BC. This process asdescribed in the texts [1] begins by the heating of a cast ironlump,which is then forgedwith hammerswhile a cold air blastis directed on it. It is apparent that heating, forging and airsupply constitute the major variables controlling the ancientprocess. Their detailed effects, however, remain obscure.

In an effort to clarify the key factors involved and theirroles in the steel making from white cast iron, a series of

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thermo-mechanical treatments were performed in the pre-sent study. This article presents the continuous microstruc-ture development during the process.

2. Experiments

A majority of cast iron artifacts made in antiquity were castfrom Fe–C alloys of near eutectic composition with no notice-able Si addition. Master alloys for this work were, therefore,made to a target composition of Fe-4.3 wt.% C, using Gliddenelectrolytic iron with the specified purity of 99.9% Fe andspectrographic grade C rods. They were then re-melted andcast in the form of rods 4 mm in diameter to preparespecimens for experiments. A description of the preparationofmaster alloys and specimens has previously been presented[7]. The chemical composition given in this article is based onweight fraction.

The experiments began by heating two specimens for 3 h inan electric resistance furnace set at 1000 °C in air environ-ments. At the completion of the heating one specimen waswithdrawn and quenched in water, and the other was forgedon an anvil while still hot. Two pieces were then cut from theforged specimen, and subjected to the additional treatment,one for an hour at 1000 °C followed by quenching, and theother for 20 min at 1000 °C followed by another forgingtreatment. Both the first and second forging consist of severallight blows with a hand hammer within a few seconds afterthe specimens were withdrawn from the hot furnace. A totalof five specimens were prepared in five different treatments.

All the samples were prepared following a series ofstandardmetallographic procedures for microstructure exam-

Fig. 1 –Optical micrographs showing microstructure of Fe-4.3% Ccast iron structure in the as-cast specimen; (b) white cast iron corein air.

ination under the optical microscope and the scanningelectron microscope (SEM). A solution of 2-vol.% nitric acidin methanol was used for etching.

3. Experimental Results

Alloy compositions were evaluated by two different techni-ques. It was found from spark source mass spectrometry thatthe major impurity elements in two different master alloyswere similarly below 0.005% for Ge, Ni, Co, S, Cu and P. Carboncontent, determined by combustion analysis in one of the as-cast rods was found to be 4.20%, which is slightly lower thanthe target composition of 4.3%.

Fig. 1a presents part of the circular cross section of aspecimen in as-cast state. The microstructure consists pri-marily of white cast iron eutectic with a little proeutecticphase located in tiny areas appearing dark against the lightbackground. The structure indicates that the carbon content isclose to the white cast iron eutectic, as has been determinedby the combustion analysis. The structure change after thetreatment in air at 1000 °C for 3 h followed by quenching isillustrated in Fig. 1b, a cross section of the specimen. It is seenthat a dark surface layer surrounds the bright core. Here thecore corresponds to white cast iron with somemodification instructure caused by the heating and subsequent quenching,while the outer layer was found to be steel of martensitestructure. The thermal treatment is found to have broughtabout three changes; reduction of the specimen diameter by0.5 mm due to oxidation, formation of a steel layer 0.4 mmthick due to decarburization, and coarsening of structure atthe white iron core. It is interesting to note that the white iron

alloys on a circular cross section of a specimen rod. (a) Whitesurrounded by a high C steel layer, after 3 h heating at 1000 °C

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core meets the outer steel layer at a sharp boundary, whichmust have been moving inward during heating.

The effect of forging has also been examined, and theresults are presented in Fig. 2a through d. The specimens havebeen deformed to a plate and their cross section is nowrectangular. Fig. 2a illustrates the effect of forging after the 3 hheating at 1000 °C. This first forging is seen to have produced anumber of cracks in the white iron core. It is important tonotice that the brittle nature of white cast iron, morespecifically cementite, is still maintained at the forgingtemperature, which must be close to 1000 °C because theforging immediately followed the withdrawal of the samplefrom the furnace. Thewhite cast iron interior is now fractured,but is apparently kept from falling apart by the steel layer,which was found to be of fine pearlite structure and served as

Fig. 2 –Optical micrographs showing microstructure at thecross section of specimens after various thermo-mechanicaltreatments in air. (a) Crackedwhite cast iron core surroundedby a layer of high carbon steel, after 3 h at 1000 °C followed byforging while still hot. The cracks were created by forging; (b)white cast iron areas, labeled 1–4, surrounded by layers ofwrought iron or steel with varying carbon content, after thetreatments in order of 3 h heating at 1000 °C-forging — anhour heating at 1000 °C. The areas near crack openings,created by forging, have been decarburized in the secondheating; (c) white cast iron areas, labeled 1–4, surrounded bylayers of wrought iron or steel with varying carbon content,after the treatments in order of 3 h heating at 1000 °C-forging— 20 min heating at 1000 °C - forging. Cracks have beenhealed in the second forging; (d) Magnified view of the whitecast iron core surrounded by layers of wrought iron or steel atarea 3 in (c).

a ductile container. Fig. 2b illustrates the effect of the 1 hheating at 1000 °C at the completion of the first forgingoperation. Fig. 2b should have the same deformation patternas that in Fig. 2a if the initial forging was uniform over thespecimen. This was not the case and a difference is noticed,especially in the outer layer, which is much less intact in Fig.2b than in Fig. 2a. This may not have any significant effect onthe experimental results. It is seen in Fig. 2b that the majorpart of the specimen has been converted to wrought iron orsteel except in the neighborhood of the areas labeled 1, 2, 3 and4 where white cast iron structures remain. Here, the freesurfaces freshly exposed around the cracks play an importantrole in accelerating the decarburization reaction. In particular,the earlier transition to almost pure iron observed in the brightregion between areas 1 and 2 must have resulted from thesubstantial increase in the reaction surfaces provided by thepresence of many cracks there. The effect would be max-imized if the oxygen penetration into the fine crack openingscould somehow be facilitated as by forced air blast.

Another experiment was carried out with the specimenthat had been given the first forging treatment. This specimenmust be similar in both shape and structure to that in Fig. 2a.This time, it was kept at 1000 °C only 20 min before beingforged again. The result of this second forging is presented inFig. 2c, which is thewhole cross section of the specimen that isnow in the form of a long thin plate. Here, four small isolatedwhite iron regions labeled 1 through 4 are surrounded bymultiple layers of varying gray levels. Fig. 2d, a magnified viewin the vicinity of region 3, makes it clear that this difference incontrast originates from the non-uniform carbon contentdecreasing from that of high carbon steel to almost pure ironas one goes from the white iron core to the surface at the topand bottom. Two important facts were observed in Fig. 2c andd; the greater portion of the sample turned to iron and steel,and no cracks were visible under the light optical microscope.It is evident that the application of forging has significantly cutdown the time for decarburization, when compared with thecase of no forging represented by Fig. 2b. In addition, forginghas closed up the cracks generated in the previous forging.

4. Discussion

The experimental results show that forging is a majorrequirement in making steel from white cast iron. The 3 htreatment at 1000 °C in air is seen in Fig. 1b to have produced asteel layer of only 0.4 mm thickness. This kind of a simplethermal treatment is apparently too slow to be practical forsteel making, not to mention the considerable material lossdue to oxidation. It is shown in Fig. 2a, b and c that thisproblem is solved with the introduction of forging whichenhances the reaction rate significantly. Here the extremebrittleness of white cast iron maintained even at the hightemperature close to 1000 °C plays a crucial role by allowingfragmentation of cementite to occur readily upon forging. Thefull advantage of forging, therefore, is guaranteed only if theconversion of cementite crystals to graphite is strictlyprohibited during the process. This does not seem to make aserious problem in ancient cast iron alloys whose Si contentwas naturally very limited, as is also the case in all specimens

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examined in this study. Fig. 2b shows that only 1 h heating at1000 °C of the forged, and thereby cracked, specimen causesthe major portion of it to turn to iron or steel due to theaccelerated carbon loss through the crack openings. It isimportant to notice that the process gets even faster withadditional forging. The 20 min heating with forging is seen inFig. 2c to be much more productive than the 1 h heatingwithout it. The specimen is now mostly iron and steel exceptat several small patches located deep inside. In addition andmore importantly, the cracks generated in the previousforging are found closed up. As a matter of fact, the absenceof visible cracks under the light optical microscope indicatesthat the cracks were healed. This is possible because thesurface layers near the crack openings have turned to iron andsteel and become suitable for welding. It seems that theatmosphere established around the crack openings wasprimarily decarburizing without being too much oxidizingalthough the nature of chemistry involved is obscure. Other-wise, the corrosion product fromoxidation of ironwould act asa barrier to welding and such complete healing would notoccur.

In real situations, as in ancient processes, the size ofsamples would be larger in general and the degree of forgingmuch more extensive than in the present case. As a result thesteel making would require a number of cycles of heating andforging, in which fresh cracks may be generated constantlywhile the old ones are being healed. With the progress of thisprocess, the fraction of ductile iron and steel will increase and,at a certain point, cementite will break inside a ductile matrixwithout creating any free surface. But the rate of decarburiza-tion remains high owing to the continuous fragmentation ofcementite into ever-smaller particles, as in fact currentlyobserved in the traditional Japanese sword making [8]. Theaverage carbon concentration in the final product will then beadjusted by the amount of working that follows. In treatinglarge samples, forced air supply may be required duringforging to promote oxygen penetration through the somewhatconstricted crack openings created deep inside. It is apparentthat hot air is more effective than a cold blast as long as it isreadily available. In ancient times, however, the air suppliedmust have come from ambient atmosphere that is muchcolder than the hot working environment. The term, “coldblast,” referred to in Chinese texts may be understood in thiscontext, not as a strict technical requirement.

5. Summary

In steel making from white cast iron, forging and the brittlenature of cementite constitute the key to success in the rapid

conversion to steel. Forging creates many cracks and pro-motes decarburization from the crack openings. The layersnear the free surface transform to iron and steel, and arereadily closed up in the subsequent forging. In the repeatedheat-forge cycles, fresh cracks are created while old ones arehealed. The decarburization rate is high throughout the entireprocess due to the continuous fragmentation of cementite.The repeated heat-forge operation, often in combination withforced air supply, may cut down the processing timeconsiderably, andmay havemade this steelmaking techniquecompetitive in ancient times.

Acknowledgements

This work was finished while the author had his 1-yearsabbatical leave at the Department of Geology andGeophysics,Yale University, as a visiting scholar from March 2006. Thecomments and suggestions from Dr. Robert Gordon wereinvaluable in revising the manuscript. This work was sup-ported by the 2006 Hongik University faculty improvementplan at foreign countries.

R E F E R E N C E S

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