Khare pipe mold steel

A ferritic alloy steel with high ductility and high toughness, and a controlled microstructure for making pipe molds for centrifugally casting pipe consisting essentially of from about 0.12% to about 0.22% carbon, about 0.4% to about 0.80% manganese, about 0.025% maximum phosphorus, about 0.025% maximum sulphur, about 0.15% to about 0.40% silicon, about 0.00% to about 0.55% nickel, about 0.80% to about 1.26% chromium, about 0.15% to about 0.60% molybdenum, about 0.03% to about 0.08% vanadium, and balance essentially iron.

TECHNICAL FIELD 
The present invention relates to ferritic alloy steels used for making pipe 
molds. More specifically, the present invention relates to ferritic alloy 
steels for producing pipe molds with improved service life which are used 
for centrifugally casting pipe. 
BACKGROUND OF THE INVENTION 
Pipe molds that are used for centrifugally casting pipe generally comprise 
an elongated cylindrical member with a "Bell" and "Spigot" end. The "Bell" 
and "Spigot" are separated by a barrel section. 
One of the most commonly used steels for making pipe molds for 
centrifugally casting pipe is the AISI 4130 grade. This steel grade 
according to "AISI 4130," Alloy Digest--Data On World Wide Metals and 
Alloys, November 1954, Revised March 1988, pp. 3, And Kattus, J. R., 
"Ferrous Alloys--4130," Aerospace Structural Metals Handbook, 1986 Pub., 
pp. 1-20 can have the chemistries set forth in Table I: 
TABLE I 
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Alloy Digest 
Aerospace Handbook 
Element Weight % Weight % 
______________________________________ 
Carbon 0.28-0.33 0.28-0.33 
Manganese 0.40-0.60 0.40-0.60 
Silicon 0.20-0.35 0.20-0.35 
Phosphorus 0.04 max. 0.025 max. 
Sulphur 0.04 max. 0.025 max. 
Chromium 0.80-1.10 0.80-1.10 
Molybdenum 0.15-0.25 0.15-0.25 
Nickel -- 0.25 max. 
Copper -- 0.35 max. 
Iron Balance Balance 
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As is seen by reviewing Table I, conventional pipe mold steels such as the 
AISI 4130 grade do not contain vanadium. 
Conventional thinking has been that pipe mold service life is dependent 
primarily on the properties of hardness and strength of the as-heat 
treated pipe mold, therefore, these were the only properties considered 
for making pipe molds with a long service life. 
The element that imparts hardness and strength to pipe mold steels is 
carbon. Hence, pipe molds intended to have a long service life are made 
from steels with high carbon level. Consistent with conventional thinking, 
the AISI 4130 grade had high carbon in the range 0.28-0.33%. 
A departure from conventional thinking was to make the carbon level 
directly related to pipe mold size. Table II is an example this: 
TABLE II 
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Pipe Mold Size Carbon Range 
Aim 
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80 mm (3.2 in.) 
0.24-0.29% 0.26% 
100 mm (4 in.) 0.24-0.30% 0.27% 
150 mm (6 in.) 0.24-0.30% 0.27% 
200 mm (8 in.) 0.26-0.31% 0.28% 
250 mm (10 in.) 
0.27-0.32% 0.29% 
350-1200 mm 0.28-0.33% 0.30% 
(14-40 in.) 
______________________________________ 
The carbon gradient shown in Table II is based on pipe mold size. Small 
size pipe molds with high carbon have a greater likelihood of either 
quench cracking during heat treatment or premature failure during service. 
Larger size pipe molds overcome this by the mass of the pipe molds causing 
them to cool slower during the quenching step. However, regarding the pipe 
molds shown in Table II, conventional thinking is followed in that 
hardness and strength are the primary concerns and high carbon is 
maintained in the pipe mold steel for that purpose. 
There can be problems in fabricating pipe molds from steel that contains 
high carbon if the carbon is not properly accounted for in the heat 
treating process. In heat treating pipe molds, the temperature of the pipe 
mold steel is raised from room temperature to the austenizing temperature, 
then the pipe mold is water quenched. The micro-structure of the pipe mold 
at this stage is such that the pipe mold is very hard and has a great deal 
of internal stresses. This quenching step is followed by a tempering step 
which tempers the hardness, thereby, making the pipe mold softer and 
alleviating many of the internal stresses. The greater the carbon level in 
the pipe mold steel chemistry, the greater the hardness and internal 
stresses. These internal stresses can result in quench cracking during 
pipe mold manufacture or cracking due to thermal fatigue, and distortion 
during pipe production. 
The present invention is a departure from conventional pipe mold steels as 
will be explained in detail in the remainder of the specification. 
SUMMARY OF THE INVENTION 
The present invention is a steel for making pipe molds used for 
centrifugally casting pipe. The steel includes vanadium and reduced 
carbon. The primary properties of the steel that are considered for 
determining the service life of the pipe molds are ductility, toughness, 
and the microstructure, not hardness and strength. Pipe molds made from 
the steel of the present invention have substantially lower internal 
stresses. This makes them very stable, and combined with the other novel 
aspects of the present invention, result in pipe molds with improved 
service life. 
An object of the invention is to provide a steel for producing pipe molds 
with improved service life for centrifugally casting pipe. 
Another object of the present invention is to provide a steel for producing 
pipe molds with improved service life for centrifugally casting pipe, with 
the pipe mold steel having a reduced carbon level and vanadium. 
A further object of the invention is to provide a steel for producing pipe 
molds with improved service life for centrifugally casting pipe in which 
the service life is dependent primarily on the properties of ductility and 
toughness, and the after-heat treatment microstructure of the steel. 
These and other objects of the invention will be described more fully in 
the remainder of the specification.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is a steel for producing pipe molds with improved 
service life that are used for centrifugally casting pipe. Pipe molds made 
from this steel can be used to centrifugally cast both large and small 
diameter pipe. The primary properties that are considered for determining 
the service life of pipe molds made from the steel of the present 
invention are ductility, toughness, and the after-heat treatment 
microstructure rather than hardness and strength. And it has been found 
that the combination of vanadium and reduced carbon in the ranges 
specified for the steel of the present invention promote the desired 
toughness and ductility, and the after-heat treatment microstructure. The 
weight percentages of the steel of the present invention are set forth in 
Table III: 
TABLE III 
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Element Wt. % 
______________________________________ 
Carbon 0.12-0.22% 
Manganese 0.40-0.80% 
Phosphorus 0.025% max. 
Sulphur 0.025% max. 
Silicon 0.15-0.40% 
Nickel 0.00-0.55% 
Chromium 0.80-1.20% 
Molybdenum 0.15-0.60% 
Vanadium 0.03-0.08% 
Iron Balance 
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As seen in Table III, the carbon level of the steel of the present 
invention is lower than the conventional AISI 4130 range of 28-33% and 
even lower than the 24-33% range of Table II. The carbon reduction has 
several beneficial effects in the steel of the present invention. Among 
them, and important to the present invention, are a reduction in hardness 
and strength coupled with an increase in toughness and ductility, and 
increased dimensional stability due to a uniform microstructure. These 
combined benefits greatly improve the service life. 
Regarding microstructure stability, as background, there can be problems in 
heat treating steels. In the heat treatment process, pipe mold steel is 
raised from room temperature to the austenizing temperature. At room 
temperature, the pipe mold steel has the body centered cubic ("BCC") 
microstructure. The BCC microstructure is a cubic structure with three (3) 
equal sides. In this structure eight atoms are present at each of the 
eight corners of the cube with an additional atom present at the center of 
the cube. At the austenizing temperature, the steel has the face centered 
cubic ("FCC") microstructure. The FCC structure is a cubic structure with 
an atom present at each of the eight corners of the cube as well as an 
additional atom present at the center of each of the six faces of the 
cube. 
After austenizing, the pipe mold is water quenched to form some martensite 
which has a body centered tetragonal ("BCT") microstructure. The BCT 
microstructure is a modified B.C.C. structure with two (2) equal sides and 
one (1) elongated side. The greater the carbon level in the steel, the 
longer the elongated side. And the longer the elongated side, the greater 
the internal stresses in the steel that forms the pipe mold. The tempering 
step reduces these stresses somewhat and likewise reduces the elongated 
sides by producing tempered martensite. These internal stresses can result 
in quench cracking during pipe mold manufacture or cracking due to thermal 
fatique, and distortion during pipe production. 
The reduced carbon level of the steel of the present invention provides an 
as-quenched BCT microstructure with shorter elongated sides. The 
as-quenched microstructure, therefore, has less internal stresses than 
conventional pipe mold steels. This reduction in internal stresses in the 
as-quenched structure also means that there is greater stability after 
tempering in pipe molds made from the steel of the present invention. The 
end result being that the pipe molds made from the steel of the present 
invention will be less susceptible to quench cracking during pipe mold 
manufacture or cracking due to thermal fatigue, and distortion during pipe 
production. 
Vanadium is added to the steel of the present invention to give the steel 
fine grain size and prevent softening during heat temper. The fine grain 
size working in conjunction with the low internal stresses resulting from 
the use of reduced carbon further enhances the stability of the steel of 
the present invention. 
Durng heat temper, a certain degree of hardness imparted by the carbon is 
lost. Even though the hardness is not one of the primary properties 
considered for determining the service life of the pipe molds of the 
present invention, the hardness after heat temper in the present invention 
is preferably higher that what it would be in the absence of vanadium. 
When hardness and strength were the primary considerations for determining 
the service life of pipe molds, the heat temper temperature was varied to 
provide a pipe mold of predetermined hardness. Usually, the heat temper 
temperature was between 1050.degree.-1200.degree. F. The specific 
temperature depended on the pipe mold size and the amount of carbon in the 
steel chemistry. Since the main considerations for the present invention 
are ductility, toughness, and microstructure, not hardness and strength, a 
heat temper temperature of approximately 1200.degree. F. can be used for 
all pipe mold sizes. This 1200.degree. F. heat temper also improves the 
uniformity of properties in the finished pipe molds. 
The combination of reduced carbon, vanadium and the other constituent 
elements, along with tempering from 1200.degree. F., bring about a unique 
microstructure. The microstructure thus produced comprises predominately 
lower bainite with some upper bainite and tempered martensite with trace 
amounts, if any, of ferrite. This microstructure has the characteristics 
of high ductility and high toughness. 
The steel of the present invention is embodied in a first pipe mold steel 
designated "Khare I" and a second pipe mold steel "Khare II. The weight 
percentage range and aim chemistries of the constituent elements of the 
Khare I and II steel are set forth in Table IV: 
TABLE IV 
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Khare I Khare II 
Element Range Aim Range Aim 
______________________________________ 
Carbon 0.17-0.22% 0.20% 0.12-0.18% 
0.15% 
Manganese 
0.50-0.80% 0.65% 0.40-0.65% 
0.55% 
Phosphorus 
0.025% max. 
Low As 0.008% max. 
Low as 
Possible Possible 
Sulphur 0.025% max. 
Low As 0.004% max. 
Low as 
Possible Possible 
Silicon 0.20-0.35% 0.25% 0.15-0.40% 
0.23% 
Nickel 0.50% max. Low As 0.45-0.55% 
0.50% 
Possible 
Chromium 0.80-1.10% 0.95% 1.00-1.20% 
1.10% 
Molybdenum 
0.15-0.25% 0.18% 0.40-0.60% 
0.50% 
Vanadium 0.03-0.08% 0.05% 0.06-0.08% 
0.07% 
Iron Balance Balance Balance Balance 
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The Khare I and II steels include vanadium and reduced carbon, and a unique 
microstructure. Khare I steel is preferably for making pipe molds for 
centrifugally casting up to 30 in. diameter pipe; and the Khare II steel 
is preferably for making pipe molds for centrifugally casting pipe with 
diameters larger than 30 in. Even though the Khare I and II steel both 
contain vanadium and reduced carbon, there is a difference in the alloying 
of the two steels. The difference is to account for the mass effect in 
heat treating large mass pipe molds made from the Khare II pipe mold 
steel. 
Pipe molds of the Khare I and II steels have been made. Experiment I sets 
forth the chemistry and properties of the pipe mold made from the Khare I 
steel. Experiment II sets forth the chemistry and properties of the pipe 
mold made from the Khare II steel. 
EXPERIMENT I 
A 10 in. pipe mold for centrifugally casting pipe was made from the Khare I 
pipe mold steel. The ladle chemistry for the steel is set forth in Table 
V: 
TABLE V 
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Element Wt. % 
______________________________________ 
Carbon 0.19% 
Manganese 0.61% 
Phosphorus 0.010% 
Sulphur 0.004% 
Silicon 0.24% 
Nickel 0.19% 
Chromium 0.88% 
Molybdenum 0.18% 
Vanadium 0.05% 
Iron Balance 
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The pipe mold made from the Khare I steel was formed in a conventional 
manner and was then heat treated. The pipe mold was heat treated by water 
quenching from 1600.degree. F. and heat tempering from 1200.degree. F. The 
as-heat treated pipe mold had a wall thickness of 1.5 in. and a weight of 
4100 lbs. 
The pipe mold made from the Khare I steel was tested for properties. Tables 
VI to XI are the results of those tests at the "Bell", "Midlength", and 
"Spigot" of the pipe mold. The "Bell" and "Spigot" tests were conducted on 
a test piece from the barrel section of the pipe mold. The test piece was 
approximately 8 in. long and approximately 12 in. from the start of the 
"Bell" contour or the "Spigot" end. Similarly, the "midlength" tests were 
conducted on a test piece approximately 8 in. long and located at middle 
of the pipe mold. 
The properties at the "Bell" of the pipe mold made from the Khare I steel 
are set forth in Table VI and VII: 
TABLE VI 
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Tensile Tests At The Bell 
Test 
Temp. T.S. 0.2% Y.S. 
.degree.F. ksi ksi % Elong. % RA 
______________________________________ 
Longitudinal Direction 
Room Temp. 96.8 81.2 24.0 73.5 
(+75.degree. F.) 
500 91.0 73.0 22.0 72.0 
600 92.0 73.0 25.0 75.0 
700 86.0 71.5 24.0 79.0 
800 77.5 66.0 21.0 81.0 
900 69.5 62.5 23.0 86.0 
1000 61.5 58.0 24.0 88.0 
1100 51.0 50.0 23.0 91.0 
1200 37.0 35.0 24.0 90.0 
Tangential Direction 
Room temp. 96.8 82.2 21.5 58.5 
(+75.degree. F.) 
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TABLE VII 
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Charpy-V-Notch Impact Test At The Bell 
Test 
Temp. Lat. 
.degree.F. Ft. lbs. % Shear Exp. 
______________________________________ 
Longitudinal Direction 
Room Temp. 164 93 0.089 
(+75.degree. F.) 
+20 161 92 0.088 
Tangential Direction 
Room Temp. 83 79 0.061 
(+75.degree. F.) 
+20 49 49 0.043 
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At the "Bell", the hardness of the pipe mold at the outside diameter is 
Scleroscope No. 30-32 and the grain size is 7-9. The microstructure is 75% 
lower bainite, 10% upper bainite, 10% tempered martensite, and 5% ferrite. 
The properties at the "Midlength" of the pipe mold made from the Khare I 
steel are set forth in Tables VIII and IX: 
TABLE VIII 
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Tensile Tests At The Midlength 
Test 
Temp. T.S. 0.2% Y.S. 
.degree.F. ksi ksi % Elong. % RA 
______________________________________ 
Longitudinal Direction 
Room Temp. 98.2 82.5 24.5 74.5 
(+75.degree. F.) 
500 92.0 75.0 22.0 74.0 
600 92.5 74.5 24.0 74.0 
700 86.5 70.5 23.0 78.0 
800 78.0 66.5 22.0 81.0 
900 68.5 62.0 22.0 86.0 
1000 60.5 57.5 22.0 90.0 
1100 50.5 48.5 24.0 90.0 
1200 38.0 36.0 25.0 91.0 
Tangential Direction 
Room temp. 98.0 82.5 22.0 64.5 
(+75.degree. F.) 
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TABLE IX 
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Charpy-V-Notch Impact Tests At The Midlength 
Test 
Temp. Lat. 
.degree.F. Ft. lbs. % Shear Exp. 
______________________________________ 
Longitudinal Direction 
Room Temp. 172 100 0.093 
(+75.degree. F.) 
+20 163 92 0.090 
Tangential Direction 
Room Temp. 104 100 0.076 
(+75.degree. F.) 
+20 67 58 0.049 
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At the "Midlength", the hardness of the pipe mold at the outside diameter 
is Scleroscope No. 29-30 and the grain size is 7-9. The microstructure 70% 
lower bainite, 10% upper bainite, 15% tempered martensite, and 5% ferrite. 
The properties at the "Spigot" of the pipe mold made from the Khare I steel 
are set forth in Tables X and XI: 
TABLE X 
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Tensile Tests At The Spigot 
Test 
Temp. T.S. 0.2% Y.S. 
.degree.F. ksi ksi % Elong. % RA 
______________________________________ 
Longitudinal Direction 
Room Temp. 99.5 84.2 24.0 74.0 
(+75.degree. F.) 
500 93.5 76.0 22.0 73.0 
600 94.0 75.0 24.0 73.0 
700 88.0 72.5 23.0 78.0 
800 79.0 69.5 22.0 81.0 
900 70.5 64.0 22.0 86.0 
1000 62.5 60.0 22.0 87.0 
1100 52.5 51.0 23.0 90.0 
1200 38.0 37.0 25.0 92.0 
Tangential Direction 
Room temp. 99.5 84.0 22.0 62.5 
(+75.degree. F.) 
______________________________________ 
TABLE XI 
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Charpy-V-Notch Impact Tests AT The Spigot 
Test 
Temp. Lat. 
.degree.F. Ft. lbs. % Shear Exp. 
______________________________________ 
Longitudinal Direction 
Room Temp. 165 100 0.091 
(+75.degree. F.) 
+20 160 92 0.090 
Tangential Direction 
Room Temp. 97 100 0.071 
(+75.degree. F.) 
+20 71 65 0.051 
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At the "Spigot", the hardness of the pipe mold at the outside diameter is 
Scleroscope No. 30-31 and the grain size is 7-9. The microstructure is 70% 
lower bainite, 10% upper bainite, 15% tempered martensite, and 5% ferrite. 
EXPERIMENT II 
A 36 in. pipe mold for centrifugally casting pipe was made from the Khare 
II pipe mold steel. The ladle chemistry for the steel is set forth in 
Table XII: 
TABLE XII 
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Element Wt. % 
______________________________________ 
Carbon 0.13% 
Manganese 0.49% 
Phosphorus 0.008% 
Sulphur 0.004% 
Silicon 0.20% 
Nickel 0.52% 
Chromium 1.06% 
Molybdenum 0.51% 
Vanadium 0.06% 
Iron Balance 
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The pipe mold made from the Khare II steel was formed in a conventional 
manner and was then heat treated. The pipe mold was heat treated by 
normalizing from 1700.degree. F., water quenching from 1600.degree. F. and 
heat tempering from 1200.degree. F. The as-heat treated pipe mold had a 
wall thickness of 3.25 in. and a weight of 33,825 lbs. 
The pipe mold made from the Khare II steel was tested for properties. The 
tensile and impact tests were conducted on an 8 in. long extension from 
the spigot end. These tests were only in the longitudinal direction. 
Tables XIII and XIV are the results of the tests: 
TABLE XIII 
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Tensile Tests 
Test 
Temp. T.S. 0.2% Y.S. 
.degree.F. ksi ksi % Elong. 
% RA 
______________________________________ 
Room Temp. 112.0 99.5 21.0 67.0 
(+75.degree. F.) 
Room Temp. 109.0 96.0 21.0 67.0 
(+75.degree. F.) 
500 102.0 85.5 20.0 61.0 
600 102.0 87.0 20.0 64.0 
700 98.5 85.0 20.0 66.0 
800 90.5 78.0 19.0 69.0 
900 84.5 75.5 19.0 74.0 
1000 77.5 71.0 19.0 76.0 
1100 67.0 64.5 18.0 79.0 
1200 55.0 52.5 21.0 86.0 
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TABLE XIV 
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Charpy-V-Notch Impact Tests 
Test 
Temp. Lat. 
.degree.F. 
Ft. lbs. % Shear Exp. 
______________________________________ 
+75 66 56 0.053 
+75 108 76 0.075 
+75 64 54 0.050 
+20 36 22 0.024 
+20 67 29 0.047 
+20 12 10 0.009 
______________________________________ 
The hardness of the pipe mold at the outside diameter is Scleroscope No. 
31-34 and the grain size is 7-8. The microstructure is 75% bainite, 5% 
upper bainite, and 20% tempered martensite. 
The terms and expressions that are used herein are terms of expression and 
not of limitation. And, there is no intention in the use of such terms and 
expressions of excluding the equivalents of the features shown and 
described, or portions thereof, it being recognized that various 
modifications are possible in the scope of the invention.