FIELD OF THE INVENTION
The present invention relates to produce DI-pipes based on a study to identify the effect
of change in composition of hot metal (i.e. liquid iron) due to addition of slag fluidiser
containing high boron during sinter making process on the manufacturing process of
ductile iron (DI) pipes, followed by an assessment of the effect of change in boron level
in hot metal on the microstructure and mechanical properties of the produced ductile
iron pipes. More particularly, the invention relates to a process to produce Ductile Iron
(DI) pipes with superior machinability and mechanical properties.
BACKGROUND OF THE INVENTION
In a blast furnace (BF) process of iron making, liquid iron (hot metal) and liquid slag are
produced simultaneously due to the reduction of iron bearing materials (sinter and/or
pellet and/or lump ore) with the help of coke. During this process, the separation of slag
from the hot metal is primarily dependent on slag viscosity which is a function of
percentage of alumina content in slag. Slag drainage rate decreases as the slag
viscosity increases for the slag having high alumina. As a result, blast furnaces, having
high alumina load, lead to high slag volume for dilution of alumina slag. If high alumina
concentration slag is possible to tap, slag rate will drop significantly (at same alumina
load). For this reason, a special type of slag fluidiser was previously developed by Tata
Steel Ltd. By the addition of that particular slag fluidiser in sinter, slag becomes
drainable with the presence of relatively higher percentage of alumina. This special slag
fluidiser contains high amount of boron compound. During a pilot scale experiment, it
has been clearly observed that the use of that slag fluidiser deceases the raw flux
addition which eventually results in drop in coke rate and slag rate. All these factors
lead to high productivity as well as huge savings. However, experiments show that use
of said slag fluidiser also results in increase of boron level in the hot metal up to a
maximum of 200 ppm. If the ductile iron pipes (DI pipes) are manufactured from that
boron-mixed hot metal, the products may become harder as boron addition always
leads to formation of stable carbide precipitate and cementite. This phenomena may
makes the pipes brittle and such pipes may not be used efficiently in potable water
transmission. In fact, it has not been apparent from any of the prior art whether the

presence of boron in the melt of SG iron for Di pipe production has any detrimental
effect or not. Having the background in mind, an investigative project was taken for
assessing the influence of boron in spheroidal graphite iron (SG iron) melt onto the
manufacturing process and final properties of DI pipes.
Ductile Iron pipe (DI pipe) is generally used for potable water transmission and
distribution. The cast iron used to manufacture this kind of DI pipe is characterized by
the spheroidal or nodular graphite within the iron matrix. Usual composition of hot metal
used for ductile iron pipe production is given below:
Table 1: Chemical composition of hot metal (in weight percentage)

The known process route of production of ductile iron pipe is depicted below:
Liquid hot metal is brought from blast furnace (BF) through ladle and put in to an
induction furnace of around 15 ton capacity where temperature is maintained above
1500˚C. For most of the ductile iron pipe (DI pipe)manufacturing units, more than one
induction furnace work simultaneously, in order to maintain a smooth process flow.
Induction furnace is used for mainly two purposes. One is to control the composition of
hot metal and the other is to exert a homogenised fluidity by properly maintaining a
higher temperature.
The composition range for the major elements in the liquid metal from the
aforementioned induction furnace is given in Table 1 in this section.
After that, hot metalis taken for magnesium treatment through the ladle. Mg treatment is
a vigorous reaction process where the total duration of the process is 1.5 –
2minutesand the temperature maintained is around 1450˚C. During this process, Mg
level increases to 0.06 to 0.08 weight % and the graphite flakes are converted to
graphite nodules which eventually provides the ductile property to the cast iron.

After Mg treatment, hot metals are taken in to a ladle and the slag is removed. This
metal is directly sent to the centrifugal caster where casting is done at around 1400˚C.
There are minimum 3– 4 numbers of centrifugal casting machines available at a time for
casting DI pipes of different diameters. The diameter of DI pipes varies from 80– 800
millimetre according to their application. These pipes are then collected in a place and
batch wise sent for annealing. The annealing furnace maintains a temperature range of
850– 900˚C where each pipe stays for 1– 2 hours. During this process, pearlite is
converted to ferrite and carbides are converted to graphite. As a result, nodule size as
well as number of nodules increase.
Effect of B on process route:
There are basically four steps in the whole process and effect of boron needs to be
discussed for each case. The first step is treatment in induction furnace where the main
objective is to control the composition and to exert a homogenised fluidity by properly
maintaining a higher temperature. Thus, specific alloying elements are added in this
step and there cannot be any effect of boron on this step.
Next step is nodularisation. For DI pipe production, specific spheroidising elements such
as magnesium, cesium, cadmium etc. are added to the melt to eliminate sulphur and
oxygen which changes the solidification characteristics and possibly account for
nodularisation [1]. In this step, the graphite flakes are transformed into nodules which
help acquiring properties such as superior ductility, elongation characteristics and
machinability. This type of iron offers unique combination of strength, wear resistance,
fatigue resistance and toughness. However, direct relationship between nodularisation
process and boron has not been apparent in available literatures or patents.
Effect of boron may be seen for next two steps, i.e. centrifugal casting process and
annealing process. It has been established by technical references that boron has a
significant effect on chilling tendency and carbide stabilization in ductile irons [2]. Until
the early 1990’s it was stated that boron levels as low as 10 ppm stabilize pearlite [3].
Now recent investigators have found just the opposite effects. At very low boron level
(especially less than 10 ppm), the volume of pearlite is reduced in pearlitic ductile iron

[4, 5]. However, larger amount of boron has always been found to form very stable
carbides which makes the nodular cast iron brittle [3, 6]. Because of this kind of effect of
boron, the casting process may be affected and during removal of DI pipes from caster,
there may be possibility of formation of crack if boron level is high in hot metal. The
purpose of the annealing process is to convert all the carbides and cementite to
graphite. If boron has a strong relationship with stable carbide formation and cementite
formation, then definitely annealing process would be affected by the addition of boron.
As a result, final properties of the DI pipes will be changed. Previous effort includes an
application (Pub. No.: US 2008/0006349 A1) which was published on Jan. 10, and
discussed on the effect of boron to minimize the influence of nitrogen on the stability of
carbides and pearlite.
In general, the DI pipes have a very good combination of adequate strength (450– 550
MPa), relatively high ductility (must be more than 10%, which is superior to any other
members of the cast iron genre) and reasonable hardness (150-170 BHN). Also, it has
a unique requirement for microstructure having graphite nodule in ferritic matrix with
less than 7% pearlite and less than 7% carbide. Additionally, the microstructure must
contain more than 90% nodularity and the nodule count must be greater than 250
numbers per square millimetre. Therefore, it is imperative to understand whether the
increase of boron in hot metal (as discussed in the previous part) used to make DI pipes
has any adverse effect or not. As such, researchers have never tried to produce boron
added DI pipes with the apprehension that it would make the mechanical properties
worse. In fact, no prior art was found on thedetrimental effect of presence of boron in
the melt of SG iron for DI pipe production as well as effectiveness of boron added DI
pipes over DI pipes without boron.
OBJECTS OF THE INVENTION
It is therefore, an object of the present to propose a process to produce Ductile Iron (DI)
pipes with superior machinability and mechanical properties.
Another object of the invention is to propose a process to produce Ductile Iron (DI)
pipes with superior machinability and mechanical properties, which experimentally

establish the effect of change in composition (for a specified range of boron) of hot
metal on the manufacturing process of ductile iron pipes.
A further object of the present invention is to propose a process to produce Ductile Iron
(DI) pipes with superior machinability and mechanical properties, which allows to
assess a co-relation between the microstructure and mechanical properties for DI pipes
containing different amount of boron.
SUMMARY OF THE INVENTION
The present invention focuses on first assessing the influence of boron in spheroidal
graphite iron (SG iron) melt in a manufacturing process including the final properties of
produced ductile iron pipes (DI pipes). The invention comprises addition of boron at
different levels in SG iron melts to optimize the boron level on the manufacturing
process including assessment of microstructure and mechanical properties of the final
product with the variations in boron level. The study reveals that the presences of boron
up to 200 ppm level in SG iron melt used for DI pipe manufacturing does not have any
deleterious effect on the manufacturing process, microstructure and properties of the
final product. The invention established that the boron added (up to 200 ppm) DI pipes
show better machinability with a superior combination of strength, ductility and hardness
compared to normal (without boron) DI pipes when manufactured according to the
methodology of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Microstructure of DI pipe having no boron (CCM diameter: 350 mm)
Figure 2: Microstructure of DI pipe having 200 ppm boron (CCM diameter: 350 mm)
Table 1: Chemical composition of hot metal (in weight percentage) for making DI pipes
Table 2: Compositional analysis ( in weight percentage) of the plant trials
Table 3:Microstructural feature of the plant trial samples

Table 4: Mechanical properties of the plant trial samples
Table 5: Comparative study of microstructural feature of boron added DI pipes and
without boron DI pipes (after annealing)
Table 6: Comparative study of mechanical properties of boron added DI pipes and
without boron DI pipes (after annealing)
DETAIL DESCRIPTION OF THE INVENTION
During plant level trials in accordance with inventive concept of the invention, hot metal
was brought from the blast furnace to the induction furnace through a ladle. The normal
composition range of that hot metal is given in (Table 1). Generally, hot metal was
processed in the induction furnace. The amount of boron was calculated from the
charge calculation as given below. :-
Charge calculation for boron addition:
• Main basis: boron content in ferro-boron = 16% and boron recovery= 90%
• For a heat size of 1 ton (1000 kg) and for 100 ppm targeted B in the final product,
ferro-boron required = (1000*0.01 )/(100*0.9*0.16) = 0.7kg = 700gm
• Therefore, calculation as elucidated above determines the amount of FeB addition
for achieving a specific amount of B in the final chemistry.
In the plant trial, targeted boron percentages were 100 and 200 ppm as it has been
calculated that the maximum level of boron would be 200 ppm due to the addition of
that special slag fluidiser.
For all the trials at plant level, FeB was added to the induction furnace for complete
mixing (as more than 1500˚C is maintained in induction furnace). The following steps
were followed during heat making and sample collection:
• Small amount of ferro-manganese was added for eliminating the excess sulphur (if
any).

• During 1st trial, 4.9 kg FeB was added and during 2nd trial, 9.8 kg FeB was added to
induction furnace which contains 7 ton of hot metal (amount of FeB added was
found out from the charge calculation as given hereinabove.
• Magnesium treatment was conducted on the hot metal.
• Samples were collected before and after magnesium treatment for comparison
purpose.
• After magnesium treatment, slag was removed and the SG iron was sent to make DI
pipes
• Using that hot metal, pipes were made from three different centrifugal casting
machines of different diameters.
• Samples were collected from as cast metal as well as after annealing for studying
the microstructure and mechanical properties.
• Microstructures were taken by a known microscope without etching. Two different
microstructures are shown in Figure 1& Figure 2 which represent the DI pipe
samples having no boron and 200 ppm boron (two extreme cases). Both the
samples are taken from centrifugal caster of diameter 350 mm to avoid any size
effect.
Observations and results obtained:
• During pulling out of pipes from centrifugal casting machine, no crack was seen.
• Chemical composition analysis reveals that the FeB addition results in increase of
B% in hot metal to 102 ppm during 1sttrial and 198 ppm during 2ndtrial (Table 2).
• During magnesium treatment, no composition change takes place except increase in
Mg in the range of 0.07%
• Boron added DI pipes are having more number of nodules compared to without
boron DI pipes (seen from Figure 1, Figure 2 and Table 5)
• The cast samples as the well as annealed samples of different diameters are
examined under microscope. All the microstructures contain more than 90%
nodularity. The final microstructure (after annealing) consists of maximum 4%
carbide and maximum 4% pearlite which are clearly better level for DI pipe

specification. The nodule counts are also good enough as per specification. The
complete result is shown in Table 3.
• Mechanical properties of the annealed samples are also evaluated and depicted in
Table 4. It is clearly seen that the UTS-ductility-hardness combination is very good
as per specification.
• Table 5 and Table 6 represents consecutively a comparative study of microstructural
feature and mechanical properties of boron added DI pipes and without boron DI
pipes. The table clearly reveals that the boron added DI pipes are always having
higher UTS-hardness combination with similar type of ductility compare to without
boron DI pipes.
• It is seen, during machining the cutting force required is lower for boron added DI
pipes. Also the surface finish is better for boron added samples. It indicates better
machinability for boron added DI pipes over without boron DI pipes. This
phenomena may due to the fact that the average number of nodules is higher for
boron added DI pipes than without boron DI pipes. This factor may be the dominant
factor for improved machinability.

The present invention has the following advantages over the prior art:
Prior arts do not state distinctly the effect of presence of boron on DI pipe manufacturing
and evaluation of its final properties. The present art reveals the following:
• Microstructural observation (seen from Figure 1, Figure 2, Table 3 and Table 5)
indicates that the level of boron up to 200 ppm does not influence the total volume
fraction or shape of graphite nodules. However, number of graphite nodule
increases and size of graphite nodule decreases evidently.
• There is no process anomaly (seen from the plant trials) for boron in DI pipes up to
200 ppm.
• It is found that the mechanical properties (seen from Table 4 and Table 6) achieved
in DI pipes with boron up to 200 ppm show superior combination of mechanical
properties as compare to DI pipes without boron.
• A better machinability is seen for boron added DI pipes over DI pipes without boron
as during machining the cutting force required is lower and surface finish is better for
boron added DI pipes.

Effect of the invention
• Presence of boron maximum up to 200 ppm in hot metal (due to the addition of slag
fluidiser in sinter making stage) does not have any detrimental effect in
manufacturing process of ductile iron pipes.
• The presence of boron up to 200 ppm in SG iron melt used in DI pipe making does
not influence the total volume fraction or shape of graphite nodules. However,
number of graphite nodule is increased and size of graphite nodule is decreased.
• The elongation values are more or less comparable for DI pipes having maximum
200 ppm boron and DI pipes without boron. However, ultimate tensile strength and
hardness values are increased for boron containing DI pipes. As a whole, the UTS-
ductility-hardness combination is found better for DI pipes having maximum 200 ppm
boron than DI pipes without boron.
• DI pipes having maximum 200 ppm boron is having better machinability than DI
pipes without boron.
• During the normal production of DI pipes, up to 200 ppm boron can be added
externally instead of making DI pipes without boron from the point of view of better
machinability and improved mechanical properties.
References:
1. “Quality Control Manual”, Ductile Iron Society Publication, 1991
2. R.A. Grange and H. R. Hribal, “Hardenability Effect of Boron in Carbon Steel” , U.S.
Steel Technical Report 40.061(1), 1972.
3. A. J. Krynitsky and Harry Stem, "Effect of Boron on Structure and Some Physical
Properties of Plain Cast iron," AFS Transactions, 57, p. 475 (1949).
4. R. L. Naro, D. Williams, P. Satre, “Control of Slag and Insoluble Buildup in Ladles,
Melting and Pressure Pour Furnaces”, Ductile Iron News, Issue 1, 2001.
5. R. L. Naro, “Thermodynamic Evaluation of Boron Removal from Ductile Iron Melts”,
Ductile Iron News, Vol. 2, No. 2, 2003.
6. Lyle R. Jenkins, "The Effect of Boron in Ductile Iron", Ductile Iron News, Issue 1,
2001.

WE CLAIM
1. A process to produce Ductile Iron (DI) pipes with superior machinability and
mechanical properties, comprising:-
- producing liquid hot metal in blast furnace using a starting materials containing
iron bearing materials in the form of sinter and/or pellet and/or lump ore;
- transferring the liquid material through a ladle into at least one induction furnace
to control the composition of hot metal including exerting a homogenized fluidity;
- adding boron in the form of FeB into the induction furnace for complete mixing
with the hot metal by maintaining the temperature of the furnace >1500ºC;
- adding spheroidising elements such as magnesium to the melt to eliminate
sulphur and oxygen, and by which eventually graphite flakes are converted to
graphite nodules, the hot metal being transferred through a ladle for this
magnesium treatment;
- transferring the magnesium treated hot metal into a ladle for removal of slag;
- transferring the slag-free hot metal to at least one centrifugal casting machine
for casting of ductile iron (DI) pipe; and
- annealing the casted DI-pipes to convert pearlite to ferrite and the remaining
carbides to graphite.
2. The process as claimed in claim 1, wherein the chemical composition of the hot
metals consists of C,Si,Mn,P,S,Mg, and Fe, respectively in weight-percentage of
3.8 – 4.2, 1.6 – 2.4, 0.3 – 0.6, Max 0.1, Max 0.03, <0.005, and balance.
3. The process as claimed in claim 1, wherein the quantity of boron addition in the
hot metal is determined on the basis of (i) boron content in ferro-boron, and (ii)
targeted boron in the final product ,and(iii) the heat size, and (iv) recovery.
4. The process as claimed in claim 1 or 3, wherein the maximum level of boron can
be added for optimized properties of the DI-pipes is 200 ppm, wherein the main
basis of determination of boron being

a) boron content in ferro-boron and boron recovery being 16% and 90%
respectively,
b) quantity of FeB to be added determined based on a heat size of 1000kg and
targeted boron (B) in the final product 100 ppm is 700 gms.
[i.e. (1000x0.01)^(100x0.9x0.16) = 0.7Kg = 700gms.]
5. The process as claimed in claim 1, wherein the magnesium treatment is done for
a period about 1.5 to 2.2 minutes at a temperature around 1450°C.
6. The process as claimed in claim 1, wherein the casting of DI -pipes in centrigal
caster is done at a temperature about 1400°C, and wherein the annealing
furnace is maintained at a temperature range of 850 - 900°C where each pipe
undergoes annealing step for about 1 to 2 hours.
7. The process as claimed in claim 1, wherein the microstructural features of the
produced DI pipes exhibit percentage nodularity, nodule count, percentage
peralite and percentage carbide respectively in the range of >90, 419-692 per
sqmm, 1-4, and 1-4.
8. The process as claimed in claim 1, wherein the mechanical properties of
produced DI pipes after annealing exhibit UTS, total elongation, and hardness
respectively in the range of 474-554 MPa, 11-15%, and 151-181 BHN.