Weldability Assessment of Preheated Ductile Iron Microstructures

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 1000-1004

Published Online October 2012 (http://www.SciRP.org/journal/jmmce)

Weldability Assessment of Preheated Ductile Iron

Microstructures

Dakwa Yusufu Kwa1, Joseph Olatunde Borode2*, Itopa Monday Momoh1

1Department of Research and Development, Engineering Materials Development Institute, Akure, Nigeria

2Department of Metallurgical and Materials Engineering, Federal University of Technology-Akure, Akure, Nigeria

Email: [email protected]

Received August 23, 2012; revised September 27, 2012; accepted October 10, 2012

ABSTRACT

When joining ductile iron by submerged arc welding technique, a suitable welding speed and current, among other fac-

tors, need to be skillfully put in place so as to enhance good welding. This work is based on microstructural examina-

tion of pr eheated ductile iron at v arious temperatures ran ging between 100˚C - 400 ˚C. Optical microscope was used for

the microstructure analysis. The result shows varied complex microstructures at the fusion zone, un-mixed zone, par-

tially melted zone and at the heat affected zone.

Keywords: Submerged Arc Welding; Fusion Zone; Un-Mixed Zone; Heat Affected Zone

1. Introduction

The transformation of materials to goods for the satisfac-

tion of human needs is brought into a reality through

manufacturing/fabrication processes. Thus, Manufactur-

ing as an indisp ensable part of production can be consid -

ered as a system in which product design at the initial

stage is processed and delivered as finished products to

the market as the final output fo r u se .

Welding has become a dominant process in manufac-

turing, without which a large number of products would

have to be considerably modified, would be more costly,

or could not perform as efficiently if it were not available.

It is a process by which two materials are permanently

joined together by coalescence, which is induced by a

combination of temperature pressure and some other

metallurgical conditions. An atomic bonding configura-

tion is formed having a natural election configuration be-

tween the two materials by bringing their surfaces to-

gether [1]. The ideal weld bond requires a perfectly

smooth, flat or matching surface, clean surfaces free

from contaminants, materials with no internal impurities,

materials that are both single crystals with identical crys-

tallographic structure and orientation. These conditions

are impossible to achieve in practice, thus, various weld-

ing processes have been developed and preparation de-

signs have been developed to compensate for the imper-

fection.

In solid state welding processes, contaminated layers

are removed by mechanical or chemical action prior to

welding, or by forcing the metal to flow along the inter-

face so that impurities are squeezed out. In fusion weld-

ing, fluxing agents are used to remove contaminants from

the pool of molten metal formed during the welding op-

eration. Welding in a vacuum also prevents contamina-

tion and promotes the removal of contaminants. Coales-

cence, therefore, occurs with considerable ease.

A high quality weld therefore requires a source of sat-

isfactory heat and pressure, a means of cleaning and

keeping the material free from contamination, and pre-

cautions to compensate for, or avoid harmful metallurgi-

cal effects.

The development of commercial industries in the late

19th and 20th century, in addition to lessons learnt by ca-

tastrophic engineering failures that occurred in that pe-

riod, brought abou t a great understanding of the im- por-

tance of welding and weld quality. Fusion welding proc-

esses were rapidly developed and improved upon [2].

High amount of carbon, silicon [3] with combination of

other elements increases the carbon equivalent of cast

iron. High carbon equivalent decreases cast iron’s weld-

ability. The objective of improving weldability may be

achieved by either preheating, or heat treatment, or both.

Ductile iron is one of the numerous ferrous materials

that has its applications both at room temperature and at

elevated temperatures. The majority of the reported dete-

rioration in high temperature operating components are

microstructural degradation [4,5], hydrogen damage, gra-

phitization, thermal shock, erosion, liquid metal embrit-

tlement, and high temperature corrosion of various types

[5].

*Corresponding author.

D. Y. KWA ET AL. 1001

Thus, when joining ductile iron by welding, a suitable

welding speed and current [6-8] must be predetermined

in order to enhance good welding needed to achieve a set

objective(s). This study is, therefore, based on micro-

examination of structure of fusion and heat affected

zones across the weld sections.

2. Experimental Techniques

2.1. Sample Preparation

A ductile iron of chemical composition 93.52% Fe, 3.6%

C, 2.5% Si, 0.007% P, 0.3% Mn, 0.01% S, 0.014% Mo,

0.05% Mg was used in this experiment. After machining

to a rectangular bar shape of cross section 10 mm × 23

mm and a length of 40mm and cut to a V-shape at equi-

distance point from either edge of the samples following

an international standard, the specimen was heat treated

to varying temperature ranging between 100˚C - 400˚C,

held for 30 minute in a muffle furnace of accuracy as

high as ±2˚C in order to attain temperature homogeneity

in the samples. A minimum of two (2) samples each were

treated per treatment for certainty. The samples were

subjected to welding at the respective elevated tempera-

tures so as to avoid weld crack that could ensue had it be

welded at room temperature. A Submerged Arc Welding

(SAW) method was adopted in the joining mechanism

using an electrode coded ECI z308. Earlier, the chemical

composition of the electrode used has been analyzed as

shown in Table 1.

2.2. Microstructural Investigation

Microstructural examination was done across the section

of the specimen from the fusion zone through the par-

tially melted zone, heat affected zone to the base (heat

unaffected zone). Prior to this operation, the welded

sample was grinded using a buehler grinder sequentially

varying the grinding paper from the coarse (60 micron)

to the finest (2400 micron). It was later polished with the

aid of a superfine polishing cloth, a diamond polishing

Table 1. Chemical composition of welding electrode.

Material Composition (%)

Carbon 1.50

Manganese 2.00

Sulphur 0.80

Nickel 0.015

Silicon 94

Iron 2

suspension of 3 micron was also used to enhance the

mirror-like surface of the material before it was etched

with a corresponding etchant—Nital (2% HNO3 + 98%

Ethanol), captured with an optical microscope of model

number 702907 at different magnifications. ×100 magni-

fication was used so as show all necessary sections.

3. Results and Discussion

The base metal and the weld regions were microscopi-

cally viewed and examined in order to see the micro-

structural evaluation along the welded trend of pre-

heated ductile iron. Plate 1 shows the microstructure of

the base metal. It shows the presence of graphite nodules

distributed in a pearlitic matrix and surrounded by fer-

rites forming boundaries. This corresponds to the find-

ings from literature review [9,10]. Plates 2(a)-(g) also

shows the micrographs of the preheated weld joints and its

environs. The micrographs shows that there are two (2)

major weld sections that includes the fusion zones (FZ),

and the heat affected zones (HAZ). Between these two

zones is an interface that separates the zones thereby ex-

plain distinctly the completely melted region and the

unmelted or seemingly melted region. In the FZ, it was

observed that the flux in the electrode only act as a catalyst

to enhance thorough welding while in itself flows to an-

other region that will be explain in due course.

All microstructures of the HAZ, however, show very

different characteristics when compared with the base

(a) (b)

Plate 1. (a) Microstructure of ductile iron showing graphite nodules in a pearlitic matrix. ×200; (b) fusion line view of speci-

men welded without preheating. 2% nital etch, ×100.

D. Y. KWA ET AL.

1002

Plate 2. (a) fusion line view of specimen welded after preheating at 100ᵒC. ×100; (b) fusion line view of specimen welded after

preheating at 150ᵒC. ×100; (c) fusion line view of specimen welded after preheating at 200ᵒC. ×100; (d) fusion line view of spe-

cimen welded after preheating at 250ᵒC. ×100; (e) fusion line view of specimen welded after preheating at 300ᵒC. ×100; (f)

fusion line view of specimen welded after preheating at 350ᵒC. ×100; (g) fusion line view of specimen welded after preheating

at 400ᵒC. 2% nital etch, ×100.

D. Y. KWA ET AL.

1003

metal. Moving away from the fusion line, the peak tem-

peratures and temperature gradients get lower. Cooling

rates are also lower because of the lower temperature

gradients. Varied Structures were, thus, formed.

3.1. Un-Mixed Zone (UMZ)

The distinct regions in the HAZ can be determined as

unmixed zone (UMZ), partially melted zone (PMZ), and

true heat affected zone (HAZ). In the HAZ only phase

transformations take place. No melting takes place as it is

experienced at the UMZ and PMZ. The UMZ region is a

completely melted region, but mixing of filler and base

metal is not achieved as the filler act as a catalyst—en-

hancing welding and diffuse out to PMZ. The increased

cooling rates enhance the transformation o f austenite thus

preventing the precipitation of carbon in the liquid to

graphite but get trappe d in iron carbide which is hard.

3.2. Partially Melted Zone (PMZ)

The PMZ forms next to UMZ as a boundary to HAZ.

Melting is not total in this zone. The temperature has

values between the eutectic and liquids temperature of

cast iron. Structures formed in this zone are mainly by

carbon diffusion out of graphite nodules. At high tem-

perature, the carbon is susceptible to diffusion out of the

nodules increasing to concentration of carbon in the sur-

rounding matrix. This continues until melting takes place.

High carbon concentration in the surrounding supports

eutectic melting, but the high cooling rates cause the

melted matrix to transform. Further away from the gra-

phite nodule, melting does not occur and martensite is

formed by means of phase transformation.

3.3. Heat Affected Zone (HAZ)

No melting takes place at the HAZ. As the distance away

from the PMZ is increased, the temperature gradient also

gets reduced. The amount of martensite, thus, gets re-

duced also. The regions close to the PMZ have a marten-

sitic matrix as shown in Plates 3(a)-(d).

4. Conclusions

Preheating of ductile iron before welding using a SAW

method results in a complex and varied multi-microstru-

ctures depending on the preheating temperature and the

(a) (b)

Plate 3. (a) HAZ of section closest to the base metal in the specimen preheated to 400˚C showing increased pearlite in the ma-

trix. ×400; (b) HAZ of section closest to the base metal in the specimen preheated that would otherwise have been martensite.

×200; (c) HAZ of section closest to the base metal in the specimen preheated to 250˚C. ×200; (d) HAZ closer to the PMZ of the

specimen welded after preheating at 350˚C. ×200.

D. Y. KWA ET AL.

1004

temperature gradient.

- At 400˚C before welding, transformation of austenite

resulting in the formation of fewer hard structures in

the FZ, UMZ, and PMZ, but having a high area frac-

tion of the hard structures (martensite and carbides) at

the HAZ was observed.

- At 300˚C and 350˚C before weld ing produced a h igh-

er area fraction of hard structures in the PMZ and

fewer hard structures in the FZ and HAZ.

- At 100˚C, 150˚C, 200˚C, and 250˚C, the microstruc-

tures at the FZ formed were visually estimated to be

lower and equal relative to other zones.

- Preheating of ductile iron, when carried out using

SAW method using ECI Z308 electrodes enhances

better microstructure that could in turn enhances the

mechanical properties.

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