forge-welded ,.zones showing the presence of slag masses and inclusions and oxide stringers embedded in carbon-free wrought iron and mild steel structures ;.
Some Observations on Corrosion-Resistance of Ancient Delhi Iron Pillar and Present-time Adivasi Iron Made by Primitive Methods A. K. Lahiri T. Banerjee B. R. Nijhawan
This paper outlines the results of metallurgical examination of the ancient Delhi Iron Pillar and the Adivasi iron smelted by primitive methods in tribal Indian villages. Such metallurgical examination has covered chemical, metallographic, physical and X-ray diffraction studies apart from atmospheric corrosion-resistance exposure tests and salt-spray attack, etc. Current investigations on the subject at the National Metallurgical Laboratory, particularly the long-term atmospheric tests, will take some time for arriving at definite conclusions on the complex subject of corrosion- resistance of ancient iron structures, even though valuable light and data on the mechanism by which the latter have withstood ravages of corrosion, have meanwhile been obtained. One of the main conclusions arrived at which appears to provide adequate explanation for high corrosion-resistance of the Delhi iron Pillar is related to the mode of smelting and fabrication of the latter, particularly the trapping of slag and sponge iron during smelting and their intimate kneading with each other during subsequent forging operations. The three dimensional inter-mixture of slog, oxides, etc. and their envelopes, within and around the metal, would afford considerable corrosion-resistance by drastically reducing the breakdown areas where corrosion attacks could be initiated, Corrosion-resistance of the ancient Delhi Iron Pillar appears basically to be related to that of Adivasi iron with its improved corrosion-resistance in corrosion tests conducted so far. When it is borne in mind that even 800 years old iron beams at the Sun-Temple of Konarak, situated on the sea coast, are still in an excellent state of preservation, the hypothesis that the dry climate of Delhi would have materially contributed towards improved corrosionresistance of the Delhi iron Pillar would hardly appear tenable.
HE ancient Delhi Iron Pillar standing practically rustless besides the mighty Kutub-Minar has excited the scientific interest of distinguished metallurgists and archaeologists in exploring the secret of its rust-free preservation through almost 16 centuries. Apart from historical conjectures and hypotheses put forward to explain the corrosion-resistance of Delhi Iron Pillar, considerable scientific work on the subject has been undertaken by many workers and interesting data have thus been collected. Attention of the authors was drawn to almost identical corrosion characteristics of a piece of Delhi Iron Pillar brought to the National Metallurg ical Laboratory for long-term corrosion tests in the Jamshedpur industrial atmosphere and of iron made today in Indian jun gle villa ges by Adivasis by a primitive method which excludes the use of flux durin g smelting of iron ore fines in 3'-4' high mud furnaces located in numbers at the foothills of iron ore deposits. With a view to study comparative corrosion-resistance of the iron samples in the two cases, investigations were initiated at the National Metallurgical Laboratory a few years back and some observations thereon are summarized in this paper.
T
Mr. A. K. Lahiri, Senior Scientific Officer ; Dr. T. Banerjee, Dy. Director ; Dr. B. R. Nijhawan, Director ; National Metallurgical Laboratory, Jamshedpur.
46
Delhi Iron Pillar Before presenting detailed results of atmospheric corrosion studies and metallurgical examination, it may be necessary to refer to some earlier work undertaken at the National Metallurgical Laboratory on the subject during the last few years. A small piece of iron cut from the Pillar at Mehrauli, Delhi, was brought to the National Metallurgical Laboratory for microscopic and chemical analyses. The Delhi Pillar bears the inscription of the fourth century A.D. Chemical composition of the sample piece Carbon ••• 0.26 per cent ••• nil Manganese Silicon ••• 0'056 per cent Sulphur Trace (0'003 per cent) Phosphorus ••• 0'155 per cent. Specific gravity of the iron sample examined was about 7'5. Metallographic examination A small piece from the sample was taken for metallographic examination. The distribution of pearlite was not uniform throughout, as shown in photomicrographs (Figs. 1, 2 and 3). This variation can be tkv -tit •
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X 90
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Fig. 2. Structure, showing higher carbon content than in Fig. I.
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accounted for by possible segregation of phosphorus. It is well established that areas rich in phosphorus diffuse carbon and become poorer in carbon. Portions comparatively rich in carbon (Fig. 3) reveal typical "as-cast" structure showing that enough forging or metal working has not been done on the material. The section of the sample under examination showed extraordinarily high contents of slag. Atmospheric corrosion of sample of Delhi Iron Pillar exposed at Jamshedpur (National Metallurgical Laboratory) Period of exposure
0••
3 years (Aug. '57 to Feb. '61) 4.4652 gm 4'4060 gm 0'0592 gin 4 sq cm
Original weight Final weight Loss in weight ••• Area of the sample Total loss in weight in ... 1'48 gm/dm2 3/ years Further detailed metallographic examination led to the following observations : The sample of the Delhi Iron Pillar has been observed to contain extraordinarily high slag inclusions dispersed in three dimensions in the mass ; the micro-structure of the iron in the adjacent areas revealed wide structural variations starting from Widmanstatten structure to normalised structure as also annealed structure, while in other regions the micro-examination revealed heavily cold-worked and highly distorted structures, a large number of slip bands in the ferritic grains, even sharp needles within the ferritic grains which could be of nitride or carbide and forge-welded ,.zones showing the presence of slag masses and inclusions and oxide stringers embedded in carbon-free wrought iron and mild steel structures ; the welded joints depicted high degree of structural distortion. Furthermore, micro-structures containing up to 0'3% carbon, dropping to 0'2% carbon, as also completely carbon-free ferritic regions were also noted. In effect the Delhi Pillar reveals a wide variation of micro-structures typical of iron and low carbon steels. Slag masses, stringers and inclusions dispersed in three dimensions, cold-working and structural distortion •••
•••
February 1963
Fig. 3. Typical structure of as cast steel showing higher carbon than Fig. 2 (more than 0.2%). X90
effects, etc. were also observed. Blocks of hot metal had apparently been forge-welded and had, in the process, undergone a high degree of structural distortion and cold-working, particularly at the forge-welded joints. Even at the surface of the iron pillar, a high degree of cold work was noted. It appeared most likely that after the metal blocks had been forge-welded, the entire pillar was rounded off through the application of local forge-hammering resulting in surface cold-working and structural distortion ; the slag masses and inclusions on cross-sectioning for micro-studies naturally appeared as stringers and elongated streaks. However, considered on three dimensional basis, these slag inclusions and stringers would represent "envelopes" around the metal grains. Such slag envelopes were dispersed on three dimensional basis round the metal throughout the section. Thus, even when a small piece of 1" or cut away from the surface exposed the metal with internal slag "envelopes" around, the latter were tightly adherent and thereby could afford adequate corrosion-resistance to the exposed metal. The lack of corrosion in the case of Delhi Pillar was obviously principally due to the mode of its manufacture and fabrication resulting in a thorough intermixture and three dimensional dispersion of slag "envelopes" around the metal throughout the section. It would perhaps metallurgically not be logical to disproportionately attribute the superiority in corrosion-resistance of the Delhi Pillar iron to factors hitherto unknown. The metal of the Pillar represented an exceedingly high degree of non-homogeneity in respect of carbon and phosphorus contents, etc. and a total lack of manganese content. The absence of manganese could be attributed to the fact that the steel had not been dc-oxidized and for that matter had never really been fluid or free-flowing during manufacture beyond the pasty sponge stage. In this connection, the work of Herrero and Zubirial could well be referred to. It postulated that the remarkable degree of preservation of the Delhi Pillar can be attributed to a kind of fine closely adherent coating, possibly of slag, which originated in the process of manufacture. It was most probable that the metal had been made through some kind of a "puddling" process to yield spongy unmelted "puddled masses of the 47
metal" which were subsequently forge-welded to one another. The Widmanstatten structure observed could well be the result of prolonged holding of the spongy "puddled" blocks of metal at temperatures above the upper critical range in order to forge-weld them subsequently. During such lengthy soakings, there would presumably have been no precision control over the soaking temperatures and their local variations in the metal prior to forge-welding ; the Widmanstatten structure would thus have developed conforming to the observations of Sauveur2, the noted French metallurgist, reported by Mr. Saville3, to the effect that burning or overheating of steel would assist in the formation of Widmanstatten structure on reheating.
spear and an axe made in the village, were taken for investigation, which covered the following tests : 1. Chemical analyses of the metal. 2. Study of scale on the metal. 3. Metallographic examination and hardness survey. 4. Structural analysis of extracted residues from Adivasi steel. 5. Salt-spray test on test-samples with and without the surface scale removed. 6. Field atmospheric corrosion tests on the testsamples with and without the surface scale removed. 7. X-ray diffraction studies of the rust formed during atmospheric corrosion tests.
Adivasi iron
Chemical analysis of the metal It now appears appropriate to introduce the subject Chemical analysis of the metal forming the spear of Adivasi iron reference to which has been made was carried out by taking drillings from different posiIasi mud earlier in this paper. In the primitive Adi ■ tions and mixing them up. The chemical analyses furnaces the villagers use charcoal but no flux whatsoare given in Table I along with the compositions of ever and the metal obtained is a spongy unmelted mass, a mild steel and a sample taken from the Delhi Iron inter-mixed and enveloped in slag masses —the metal Pillar used for corrosion studies at the National Metalcontains low carbon and corresponds somewhat to lurgical Laboratory. Delhi Iron Pillar compositions. These native, less than 3' high Adivasi furnaces are blown with foot-operated TABLE I air bellows and smelt high grade iron ore fines lying scattered at the foothills of iron ore deposits, to yield Chemical analyses low carbon, semi-steel compositions that are readily Mild forgeable. Charcoal is used as the fuel. The smelting Adivasi Delhi Iron Pillar steel steel temperature is not high enough to ensure a fully molten free-flowing metal—the latter collects in As analysed at As reported the hearth in a pasty sponge form ; it is taken out by Hadfield NML for forging by the village blacksmith for making common household and small agricultural implements. 0'08% 0'12% 1'3% 0'28% Carbon A few pounds of the metal are thus made in batch opera0'056% 0'03% 0'046% 0.42% Silicon tions. The villagers use the metal for making axes, small spears and arrows for their daily use and naturnil 0'42% Manganese nil nil ally so, since they cannot possess the means to get 0'155% 0'114% 0'037% Phosphorus 0.019% their steel quota for their local use through the Iron Sulphur 0.006% 0'003% 0'006% 0'043% and Steel Control perched on the 7th floor of a Calcutta building ! However, it would appear to be possible that the iron pieces made by the villagers Adivasi metal contained high carbon with low phostoday may also possess somewhat superior corrosionphorus and sulphur and naturally no manganese as in resistance not unlike that of the Delhi Iron Pillar. the case of Delhi Iron Pillar. The phosphorus content As in the case of iron currently made by the villagers of Adivasi metal is, however, much lower than that of wherein they do not use any flux, the supposition that for Delhi Iron Pillar. The differences in chemical analymaking the Delhi Iron Pillar also no flux addition was ses as determined at the National Metallurgical Labomade during iron smelting appears most plausible. ratory and that reported by Hadfield of Delhi Iron In either case, the spongy metal was not molten or Pillar confirm our earlier observation that the metal fluid enough to be free-flowing. The slag masses in thereof is highly non-homogeneous and varies widely both the cases were thoroughly inter-mixed with the in chemical composition. metal thereby imparting superior corrosion-resistance thereto. Whilst initial studies on corrosion-resistance Analysis of scale of the Adivasi iron undertaken at the National Metallurgical Laboratory confirm the above trends, comThe surface scale of the Adivasi steel was removed prehensive long term corrosion tests and investigations by dipping the samples in dilute 5% sulphuric acid. are currently in progress. The electron-micro-probe The metal at the metal/scale interface was attacked analysis would provide a valuable tool in these studies and the insoluble scale was thus dislodged from the which do indicate that both in the case of Delhi Iron surface ; the scale was then filtered, washed and Pillar and the metal made today in Indian villages, analysed. It was found to contain in addition to iron the slag inclusions and erelopes puddle-forged toabout 2'4% SiO, and traces of manganese, from which gether and thoroughly inter-mixed in the metal and thus it appeared that the surface scale was not pure iron dispersed therein on three dimensional basis, provide the ancient and the Adivasi iron with their superior oxide but contained entrapped slag forged into the metal and covering its surface ; this was expected since corrosion-resistance. Results of the investigations on the primitive smelting method did not allow the iron Adivasi iron so far conducted at the National Metalto be molten to ensure its adequate separation from lurgical Laboratory will now be presented. the slag. Two representative products of Adivasi furnaces, a 48
NML Technical Journal
Fig. 4. Cementite needles inside the grains. Boiling sodium picrate etch.
Fig. S. X 150
Showing structure of lower carbon area in the steel.
X ISO Nita! etch
Micro-examination and hardness Micro-structure of a specimen from the handle of the spear is given in Figs. 4, 5 and 6 which show that the composition of the metal as expected is not uniform at all. In general, the matrix was of fine pearlite with pro-eutectoid carbide separating as needles with some pockets of low carbon areas. Figure 6 shows a decarburised area around presumably a heavy slag pocket. The heterogeneity of the chemical composition of the metal also explained the variations in hardness values of the metal as given below : Positions
Hardness (V.P.H.N. at 30 kg load)
Low carbon area 110-;--I 12 232--235 High carbon area Such heterogeneity in composition of the metal is attributed to the primitive and crude smelting method with no metallurgical control such as of the smelting temperature. Structural analysis of extracted residues in Adivasi steel X-ray diffraction studies of the carbides and inclusions extracted from Adivasi iron were also carried out. Extraction of carbides Carbides were separated by electrolytic process of separation. A 5% aqueous solution of 1-1CI was used as electrolyte. The specimen was made anode (suspended by ,platinum wire) with platinum foil as the cathode. Electrolysis was continued for about four hours using 0025 amp/cm2 current density. After the electrolysis, the residue was separated from the acid solution by centrifuging. The residue was thoroughly washed with distilled water and alchohol and then dried. X-ray diffraction analysis X-ray diffraction photograph of the residues was taken in a 9 cm powder camera using filtered CoKK February 1963
Fig. 6. Shows decarburisation along the sides of the crack. x150 Nital etch
radiation. Four hours' exposure was given at 30 kV and 30 mA current. Table II gives the X-ray diffraction data obtained. From the table of 'd' values, it is apparent that the extracted residue mostly consists of Fe3 C (cementite). However, some iron oxide lines are also present. The 49
Fig. 7. X-ray powder photograph of the extracted residues from Adivisi steel showing Fe,C line. (Camera dia. 9 cm ; Radiation CoK.:
