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Introduction The migration of our qualified manpower to better avenues in advanced countries is often described as a brain-drain affecting the vitals of our country in its efforts of modernisation. While it could be true to an extent, it is indeed also heartening that many of them hold our country’s flag high abroad by virtue of their individual excellence in their chosen areas - be it in Engineering, Science, Medicine or any other professional field. Their achievements in their countries of adoption in turn also make us realise the vast potential of the Indian brain if utilised for indigenous growth under appropriate environments. It is our bounden duty that each one of us renew our pledge to contribute in this great national task when we enter the New Century such that in the not too distant future the brain drain transforms into a brain source. We would like to introduce one such non-resident Indian in the field of metallurgy and materials science, Dr.Ashok K.Khare. An alumnus of the Indian Institute of Technology, Kanpur, he has discovered two new ferritic alloy steels, called KHARE STEEL I & II, while working for the National Forge Company, Irvine, PA,USA for making pipe molds for centrifugally casting pipes and, as is the common practice in all advanced countries, these steels have been protected through the US Patent no.4,992,239. Thanks to his significant contributions in his field, Dr.Khare has risen to become the current President of ASM International and it was a great event that the Indian Institute of Metals honoured as its chief guest this distinguished alumnus during its Annual technical Meeting held at the campus of IIT, Kanpur itself during November 13 - 16, 1999. Background of the invention The pipe molds used for centrifugally casting pipe generally consists of an elongated cylindrical member with a `bell’ and `spigot’ end which are separated by a barrel section. AISI 4130 low alloy steel, which contains no vanadium, has been one of the most commonly used steels for making pipe molds. It is conventional thinking that the service life of pipe molds is dependent on the properties such as hardness and strength of the heat treated pipe mold. And, carbon is the major element that imparts hardness and strength.Hence, pipe molds meant for long service life are made from steels with high carbon level, and AISI 4130 suited the application. The presence of high carbon in the steel can lead to quench cracking during heat treatment of pipe molds which consists of water quenching them from the austenitizing temperature. Hence, in the present invention, the susceptibility to quench cracking is brought down by reducing the carbon content in the steel. The steel Through the development of this steel, Khare proves wrong the conventional thinking that the service life of pipe molds is dependent primarily on hardness and strength of the as-heat treated pipe molds. Instead the factors which affect the sevice life are considered as toughness, ductlity and the microstructure and keeping these in mind the steel has been designed. The chemical composition of this new steel is as in the Table below. |
| Element | Content,% |
| Carbon Manganese Phosphorous Sulphur Silicon Nickel Chromium Molybdenum Vanadium Iron |
0.12 - 0.22 0.40 - 0.80 0.025 max. 0.025 max. 0.15 - 0.40 0.00 - 0.55 0.80 - 1.20 0.15 - 0.60 0.03 - 0.08 Balance |
| It can be seen that the
carbon level is kept lower than that of AISI 4130 steel which has the
beneficial effects, in combination with vanadium, such as reduction in
hardness and strength coupled with an increase in toughness and ductility,
and increased dimensional stability due to a uniform microstructure. These
contribute greatly to improve the service life of the pipe molds. Also the
reduced carbon level of the steel provides an as-quenched BCT
microstructure with shorter elongated sides and the as-quenched
microstructure has less internal stresses than conventional pipe molds.
This results in greater stability after tempering. The end result is that
the pipe molds made of this steel will be less susceptible to quench
cracking or cracking due to thermal fatigue.
Vanadium contributes a very fine grain structure in the steel and also prevents softening during tempering operation. Since the main considerations for this steel are ductility, toughness and microstructure, and not hardness and strength, a tempering temperature of 1200oF (~650oC) is selected which also improves the uniformity of properties in the finished pipe molds. Khare also suggested to keep carbon level slightly more for pipe molds of diameter upto 30in. than the ones of above 30in. in diameter. Two steel compositions, Khare I and Khare II, have been made as in the following table for the above respective diameters. |
| Element | Khare I | Khare II |
| Carbon Manganese Phosphorous Sulphur Silicon Nickel Chromium Molybdenum Vanadium Iron |
0.17 - 0.22% 0.50 - 0.80% 0.025% max. 0.025% max. 0.20 - 0.35% 0.50% max. 0.80 - 1.10% 0.15 - 0.25% 0.03 - 0.08% Balance |
0.12 - 0.18% 0.40 - 0.60% 0.008% max. 0.004% max. 0.15 - 0.40% 0.45 - 0.55% 1.00 - 1.20% 0.40 - 0.60% 0.06 - 0.08% Balance |
| Even though both Khare I
and Khare II steels contain vanadium and reduced carbon, there is a
difference in the alloying of the two steels to account for the mass
effect in heat treating large mass pipe molds from the Khare II pipe mold
steel.
Properties The pipe mold was heat treated for Khare I steel by water quenching from 1600oF and for Khare II steel by normalizing from 1700oF followed by water quenching from 1600oF. Tempering was done from 1200oF for both the steels. The hardness of the pipe mold of Khare I steel at the outside diameter is Sceleroscope No.29 - 3, the grain size is 7 - 9 and the microstructure consists of 70% lower bainite, 10% upper bainite, 15% tempered martensite and 5% ferrite. For Khare II steel, the hardness of the pipe mold at the outside diameter is Sceleroscope No.31 - 34, the grain size is 7 - 8 and the microstructure is 75% lower bainite, 5% upper bainite and 20% tempered martensite. The typical mechanical properties of the pipe molds made from Khare I and Khare II steel are tabulated in the following table : |
|
|
oF |
ksi |
ksi |
|
|
| Khare I
longitudinal
direction Tangential direction |
Room Temp.
500 600 700 800 900 1000 1100 1200 Room Temp. |
96.80
91.00 92.00 86.00 77.50 69.50 61.50 51.00 37.00 96.80 |
81.20
73.00 73.00 71.50 66.00 62.50 58.00 50.00 35.00 82.20 |
24.00
22.00 25.00 24.00 21.00 23.00 24.00 23.00 24.00 21.50 |
73.50
72.00 75.00 79.00 81.00 86.00 88.00 91.00 90.00 58.50 |
| Khare II
longitudinal
direction
|
Room Temp.
500 600 700 800 900 1000 1100 1200 |
109.00
102.00 102.00 98.50 90.50 84.50 77.50 67.50 55.00 |
96.00
85.50 87.00 85.00 78.00 75.50 71.00 64.50 52.50 |
21.00
20.00 20.00 20.00 19.00 19.00 19.00 18.00 21.00 |
67.00
61.00 64.00 66.00 69.00 74.00 76.00 79.00 86.00 |
| Conclusion A new steel for making pipe molds used for centrifugally casting pipe has been developed.Pipe molds made from this steel have substantially lower internal stresses which makes them very stable with improved service life. Courtesty : METNEWS, Trivandrum |