Nondestructive Evaluation based on Chemistry and Physics Principles to Unveil the Secrets of

Delhi Iron Pillar

Baldev RajU. Kamachi Mudali*

President, Indian National Academy of EngineeringPresident - Research, PSG Institutions, Coimbatore - 641 004

* Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research,

Kalpakkam - 603 102.

Delhi Iron Pillar – Some FactsM.K.Ghosh (NML Technical journal, 1963)

Built in 320 - 405 AD (Based on style of top decorative work – Gupta Dynasty)

Installation in present place – 12th Century AD

Dimensions (Ms. Cummings)

Total length – 7.1 m

Above ground level – 6.6 m and Below ground level – 0.5 m

Upper diameter (below decorative portion) – 312 mm

Lower diameter – 412 mm

Total weight - ~6 tonnes

Pillar standing on a lead sheet

Base of pillar is flat and 8 small projections shaped as the toes of an elephant

Below the ground – Pitted surface – pits of 100 mm deep and connected to

base at some places, 15 mm thick rust

0.9 m above ground – Pitted (earlier buried portion)

Observations: Since more than 50 years elapsed after the study, it is

important to repeat the efforts to study the underground portion of

the Pillar and pursue preservation with modern methods of corrosion

control to enhance the stability and life of the Pillar

Chemical Analysis of Delhi Iron Pillar(M.K.Ghosh, NML Technical Journal 1963)

TISCO Hadfield NML

C 0.23 0.08 0.28

Si 0.026 0.046 0.056

P 0.18 0.114 0.155

Mn Nil Nil Nil

S Traces Trace Trace

N 0.0065 0.032 ---

Cu --- 0.034 ---

Fe 99.768 99.72 ---

Sp.

gravity

7.672

7.747

(Reg.)

7.81 7.5

Underground

sample

C - 0.03

Si – 0.004

P – 0.436 - 0.480

Mn – Nil

S – 0.008

Elemental analysis

Small pieces from

top rough portion

– one vertical cut

and one

horizontal cut

Observations: The variations in chemical composition including C,

Si, P, Cu etc. need to be studied all over the pillar using insitu and

modern analytical techniques

Sulphur print – No segregation of sulphur

Etching with Stead’s reagent – Cu deposits at cavities & spongy areas

Microexamination

Unetched - Irregular distribution of slag particles and small cavities

Etched

Medium to coarse polyhedral grains; Slip bands in ferrite grains

Small to medium grains with pearlite at grain boundaries

More pearlite in interior regions compared to surface

Surface region is more or less free of pearlite

Oxide films and deformed structure (indication for forging)

Nitride needles in coarse grains

Mechanical properties

YS – 376 Mpa, UTS – 382 Mpa, Elongation – 5%

Hardness – 138 to 177 BHN with increasing pearlite

Ferrite + CW – 148 to 164 BHN; Ferrite + slag – 85 BHN

Observations: The variation in microstrcture and mechanical

property for a single piece cannot be considered representative of

the Pillar due to significant variations observed in the microstructure

of the Pillar at various locations based on our insitu study

Hammer forging of balls of iron – Inhomogeneous (no heat treatment)

– 0.2% / 0.3% / almost no carbon, inhomogeneous P also

Never in molten state – no use of flux

Metal is produced by some kind of puddling process

Hot iron for considerable time for forging – Oxide film would have

been formed and slag would have got incorporated

Three dimensional intermixture of slag, oxides etc. and envelopes

within and around the metal

More dense and less porous iron; Sand is much less

More P and absence of Mn (metal is not deoxidised)

Delhi Iron Pillar - Findings(M.K.Ghosh + Bardgett and Stanners + A.K.Lahiri)

Observations: Solidification microstructure and corresponding

casting methodology need to be evolved with systematic experiments,

modelling and validation.

Rough surface and bulges at the bottom – for better grip

Lead sheet – Cushion bed in case of disturbances and earth quakes

Pb-99%, Mg, Cu and Ag

Iron making in India is well known

Extraction of iron from ore - Treatment process

Beams in the yard of Konark temple and Puri Surya temple

Maximum size – 10.7 m long and 27.5X27.5 mm2 square

Dhar Pillar – 12.9 m in 3 pieces

Observations:

Conservation methodology to be taken up

Iron making and casting methodology need to be studied

Comparison of contemporary and Adivasi iron with Pillar iron

Scientific Investigations on Delhi Iron Pillar

(2000 – 2003)

Insitu Metallography

(microstructure, morphology)

(rust layers, slags/ppts)

Radiometry and Radiography

(defect imaging)

X-ray Fluorescence

(chemical mapping)

Ultrasonics

(velocity, pulse echo)

(flaw detection, defect mapping)

Electrochemical Corrosion

(corrosion resistance mechanism)

In-situ Metallographic Studies on the Delhi Iron Pillar

• 8 locations

– 6 on main pillar

– 2 on upper decorative region

• 2% Nital etchant

• Replica

• At ~100 X

47 m

99 m

142 m

54 m222 m

33 m130 m

Microstructure in Smooth portion of Main Pillar

4.88 m 3.18 m

Fine second phase, slip lines - Intense forging !

4.88 m 3.18 m

• Two different types of second phases– Needle like, Globular

Needle like second phase

Orientation relation with matrix !

Can slag have any orientation relation with matrix ?

Possibility of carbide or nitride need to be understood ?

2.0 m 1.0 m

Specific orientations of second phase observed in all three microstructures –

Iron pillar (optical), carbides in Adivasi iron (optical), cementite in ferrite

grain of a ferritic stainless steel (TEM) – Elongated planar precipitates forming

along <100> and <111> directions on {110} planes

More studies with Adivasi iron material showing similar microstructure and

composition of the Pillar would help to understand the Pillar material.

Adivasi iron

Iron Pillar

Cementite in ferritic steel

Lah

irie

t al

, NM

L Te

chn

ical

Jo

urn

al

Ferritic SS

Microstructural mosaic of ~5 mm length at 2.0 m elevation

Expected Evolution of the Microstructures

Increasing degree of thermo-mechanical treatment

Higher temperature &

slow cooling

Conclusions of the Metallographic Studies

• Surface of the smooth portion of the pillar is essentially free from slag

• Different secondary phases

–different thermo-mechanical treatments

• Smooth portion of the pillar

–Presence of carbide needles (plates) in ferrite matrix

–Also presence of fine globular precipitates - needs to be identified as phosphide ?

• Similar phosphorous based dotted inclusions were reported in underground specimen

• Rough portion – Further study needed to resolve carbides/ carbonitrides / pearlite / slag at the grain boundaries.

• Bell shaped top decoration – Needs further analysis

• Top square - Ferrite+Pearlite - Further study needed to resolve manufacturing technology of the pillar

Two school of thoughts on the fabrication of Delhi Iron Pillar

Horizontal (Radial forging)

Vertical (Axial forging)

Fabrication Methodology of the Pillar

Horizontal (Radial forging)[Prof. Balasubramanian]

Vertical (Axial forging)[Prof. Anantharaman]

Understanding of earlier Proposed Methodologies

• Vertical forging

– Doesn’t explain the near vertical defect

morphology (improper joints between lumps)

• Horizontal forging

– In line with the defect orientations but

– Presence of void/ low density material at centre

is difficult to explain

– May be difficult practically from handling point of

view - such as rotating, lifting etc.

Proposed Mode of Fabrication of the Pillar

Influenced by natural rock structures

305 mm

Next level of defect and

material evaluation by NDT

Interfaces

Elongated Voids

(below capital)

Radiometry and Radiography

Cobalt-60 source with ~ 2 Ci activity

Radiation passing through defects show higher output dose

Teletector is used for dose level mapping at different

locations

Study only up to 360 mm thick (> 1.5 m height) due to dose

constraint

Radiography at regions showing voids and defects (high

dose region)

Cannon Ball strike area

Void below the capital

IMPACT ECHO TESTING

in grids of 250 mm height and

450 sectors, covering the whole

cylindrical portion of the pillar

at 50 mm interval along the vertical direction at two 900 apart locations at each elevation

at the decorated portion on top of the pillar

Major Findings

Impact echo signals confirmvoid type of defects (confirmedby radiography).

Ultrasonic velocity was almostsimilar in radial directions.

Decorated portion exhibited veryhigh velocity as compared tothat in the cylindrical portion.

Ultrasonic Investigations

Experimental setup for Impact echo testing

Impact echo signal from a

defect free location

Impact echo signal showing

defect at 60 mm depth

LOW FREQUENCY ULTRASONICS

Transmission mode using a pair of 250

kHz transducers

Transmitter was kept at four different

locations 90O apart and signals were

acquired at 23 locations 5O apart at each

elevation for all transmitter locations

Keeping the transducer pair at

diametrically opposite locations at

every 50 mm interval in the vertical

direction, covering the whole cylindrical

portion of the pillar.

Major Findings

Peaks corresponding to the diameter at allelevations show the transmittance oflow frequency ultrasonic waves

Ultrasonic velocity was almost similar inthe radial direction at all the elevations

Signals from defects were obtained at various locations.

Delhi Iron Pillar –

Manufacturing Technology

Radiograph showing presence of elongated voids detected in the top region at 0.45m below the capital.

200 m

305 mmNext level of defect and material

evaluation by NDT Interfaces

Metallography – structure

of Main body (4.88 m),

Smaller grains with

pointed slag within

Influenced by the natural rock structures

Proposed mode of fabrication of the Pillar

LRS analysis showed the presence of

- FeOOH (389, 483 & 1002), - Fe2O3 (226,

296, 612 & 1312) and - FeOOH (1311).

Smooth and uniform rust layer homogeneously spread

over the entire pillar, except the base region which was

frequently disturbed by the tourists.

Analysis of Scale and Surface Layer of Delhi Iron Pillar

Electrochemical Investigations on the Delhi Iron Pillar

Corrosion PotentialMeasurement

Insitu Electrochemical Studies

Corrosion Behaviour of Delhi Iron Pillar

Nobler corrosion potential for rust scale

Passivation Current Density, A/cm2

Passive behaviour of rust scale

Less corrosion rates of Iron Pillar

0 5 10 15 20 25 30 35 40 45Time (minutes)

-700

-600

-500

-400

-300P

ote

nti

al

(mV

vs

SC

E)

polished, 0.5 m

polished, 1.29 m

polished, 3.8 m

polished, 4.88 m

polished, crown

Pure iron polished

Observations:Ennoblement of potential for Rust scale ofpillar indicates inertnessEven with applied potential simulatingaggressiveness, rust scale exhibited stablepassive behaviourAfter polishing, the corrosion potential andcurrent behaviour of the pillar at differentregions confirmed its heterogeneityKinetics of Rust scale is very important.

Atmospheric Corrosion Mechanism

Prof. Stratmann’s Theory

• Stage I : Wetting of a dry surface

Fe Fe2+ + 2e

2FeOOH + 2H+ + 2e 2Fe.OH.OH

• Stage II : Reactions at wet surface

FeOOH reduction is completed, and oxygen reduction follows up

• Stage III : Reactions at the drying out surface

Corrosion rate is very high at this stage, thin electrolyte layers

Fe Fe2+ + 2e

0.5O2 + H2O + 2e 2OH-

2Fe.OH.OH + 0.5O2 2FeOOH + H2O

Significant potential changes during wet to dry and dry to wet

transitions decide the role of alloying elements and associated

electrochemical reduction/oxidation reactions

Possible Corrosion Mechanism of DIP

• New conservation techniques and methods should be established on

iron objects of ancient period prior to digging out the pillar for

evolving conservation method for the underground region.

• Soil properties at the pillar site should be studied and methods

should be evolved to alter the soil conditions in order to minimize

corrosion of underground portion of Pillar.

• Exhaust all noninvasive techniques for assessing the underground

portion of the Pillar to decide on the necessity for digging and the

buried portion for direct assessment.

• But since more than 50 years passed after the conservation of the

underground region of the pillar, part of the pillar may be dug out and

the bottom portion reexamined for documentation.

• Action for reexamination and conservation is required.

Preservation of Delhi Iron Pillar

• A nationally coordinated committee is to be formed for identifying

R&D for future work in order to further scientific understanding and

information on history, fabrication, corrosion resistance and

conservation of Delhi Iron Pillar.

• Ancient iron objects like Adivasi iron, Dhar Pillar, Kodachadri Pillar

and Konarak Beams shall be made available for R&D studies.

• Exhaust all methods of NDE prior to scooping out samples from

various parts of the pillar for careful analysis of scale, interface,

pillar composition, microstructure analysis and Accelerator based

mass spectrometry to determine the date of the pillar.

• R&D campaign to fully exploit the possible NDE testing

methodologies with focused experiments such as X-ray tomography

and laser Raman spectroscopy. In-situ metallography for 3D

mapping of a section of the Pillar could be attempted to see finer

details over a larger area of the Pillar.

• Theories about manufacturing (horizontal, vertical, or both) and

evidences of joints in the pillar should be evaluated through

experimental simulation studies.

Way Forward for More Understanding on Delhi Iron Pillar