FIELD OF THE INVENTION:
This invention relates to steel casting containing Niobium.
This invention further relates to chromium-Molybdenum-Vanadium steel castings
containing Niobium, for use in steam turbine casing or valve casing applications.
BACKGROUND OF THE INVENTION:
To improve the mechanical properties and weldability of steels, various alloying
elements such as Ti, Nb, Mo, W, B etc. have been added to low and high alloy
steels. Ti and Nb are added as carbide formers and strengthen the alloy steels
by forming fine matrix carbides which interact with dislocations and precipitate at
subgrain boundaries thus reducing the secondary creep rate.
Presently base CrMoV steel castings are being used for turbine casing and valve
casting applications up to around 540°C. Due to increase in power demand and
limitation on C02 emission, there is increasing demand for higher efficiency and
output of the stem turbines. It is possible by increasing the temperature and
pressure of the turbine without much increase in cost.
OBJECTS OF THE INVENTION:
It is therefore an object of this invention to propose a chromium-Molybdenum-
Vanadium cast steel containing Nb, having greater mechanical strength and
ductility at ambient and high temperatures and pressures.
It is a further object of this invention is to propose a chromium-Molybdenum-
vanadium cast steel containing Nb, having greater creep rupture time, rupture
Elongation and reduction of area under high temperature.

Another object of this invention to propose a chromium-Molybdenum-vanadium
cast steel containing Nb, which can be used for manufacturing turbine casings or
other components which are subjected to high temperatures and pressures.
SUMMARY OF THE INVENTION:
According to this invention is provided a chromium-molybdenum-vanadium cast
steel containing niobium. More particularly, the invention provides a heat
resistant chromium-molybdenum vanadium steel consisting essentially of 0.08 to
0.12% by weight of carbon, 0.015% by weight or less of sulphur, 0.02% by
weight or less of phosphorous, 0.30 to 0.60% by weight of silicon, 0.50 to 0.80%
by weight of manganese, 1.20 to 1.50% by weight of chromium, 0.90 to 1.00% by
weight of molybdenum, 0.20 to 0.30 by weight of vanadium, the balance being
iron apart from incidental impurities, with niobium ranging from 0.04 to 0.08% by
weight.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The invention will now be explained in greater details with the help of the
accompanying drawings where:
Figure 1: Schematic diagram showing the double 'V edge used for preparation of
weld joints
Figure 2: Sampling plan for qualification of welding procedures
Figure 3: Larson Miller Parameter vs. stress plots for 1Cr1Mo1/4V steel weld
joints with different Nb contents.
Figure 4: Larson Miller Parameter vs. stress plots for 1Cr1Mo1/4V steel weld
joints with different Nb contents.
Figure 5: Larson Miller Parameter vs. elongation plots for 1Cr1Mo1/4V steel with
different Nb contents.

Figure 6: Larson Miller parameter vs. elongation plots for 1Cr1Mo1/4 steel weld
joints with different Nb.
Figure 7: Effect of Nb content on creep-rupture properties of 1Cr1Mo1/4 V steel
tested at different test conditions.
Figure 8: Effect of Nb content on creep-rupture properties of 1Cr1Mo1/4V steel
tested at 200 Mpa at different test temperatures.
DESCRIPTION OF THE INVENTION:
According to this invention is provided a chromium-molybdenum-vanadium (Cr-
Mo-V) cast steel containing 0.04 to 0.08 % by weight of Niobium.
The cast steel contains 0.08 to 0.12% by weight of carbon, 0.015% by weight or
less of sulphur, 0.02% by weight or less of phosphorus, 0.30 to 0.60% by weight
of silicon, 0.50 to 0.80% by weight of manganese, 1.20 to 1.50% by weight of
chromium, 0.90 to 1.00% by weight of molybdenum, 0.20 to 0.30 by weight of
vanadium, the balance being iron apart from incidental impurities with 0.04 to
0.08% by weigh Niobium being added. The base chromium molybdenum-
vanadium cast steel is subjected to a homogenizing heat treatment of cast steel,
followed by addition of Niobium in percentages of 0.04 to 0.08 by weight. The Nb
addition is carried out in an induction melting furnace.
The casts containing Niobium, thus a prepared have been subjected to various
tests to evaluate their creep/stress rupture, tensile strength.
Three casts were obtained by adding Nb in 0.4 to 0.8% to CrMoV steel. The steel
melts were designated as cast 'B', cast 'C and cast 'D' respectively. The base
melt without Nb is designated as cast 'A'.
A: Base chromium-molybdenum-vanadium cast (CrMov cast)
B: CrMoV cast with 0.04% by weight of Niobium.
C: CrMoV cast with 0.06 % Niobium.
D: CrMoV cast with 0.08% Niobium

The heat treatment for cast 'A' was carried out by solutionising base CrMoV steel
by holding for 3 hours at 940°C followed by forced air cooling. The other casts
'B', 'C and 'D' were heat treated by solutionising the CrMoVNb steel by holding
for 3 hours at 1040°C followed by forced air cooling. Subsequently all the casts
were tempered at 740°C for 5 hours and cooled up to 300°C in the furnace and
then air cooled to room temperature.



CrMoV electrodes were used for welding. Welding, as per the plant practices
detailed out in table 2, was carried out on the castings to get the adequate
number of samples. Weldability studies were conducted. Weldability tests with
180° bend test passed on all the four casts.
Table 2 Details of the welding procedures:
Preheat temperature :300°C(Validity:240°C Min)
Inter-pass Temp. :400°C (Valifity:500°C Max)
State of heat treatment :Quenched & Tempered
Type of Post weld heat Treatment .Quenching and Tempering
Temp. :WQ-930°C(valid from 930°C to 950°C)
:T-720°C (valid from 710°C to 73CTC)
Time : WQ-6 Hours T-8 Hours.
Current range : 180-220 Amp. DC
Voltage range :24-28V
Polarity -.Reverse (valid for DCEP Only)
Joint Preparation : Full penetration with backing
Location of Weld : Root face 2mm, Root gap 5mm,
seam side angle 10°
Weld sequence : Multilayer Welding

All the four casts of CrMoV steel without and with niobium (Nb addition were
subjected to hardness, impact, tensile, hot tensile and creep/stress rupture
testing. Creep/stress rupture testing were carried out at 525, 550, 575 and 600°C
and stresses varying from 100-300 MPa. Microstructural analysis including
scanning electron microscopy was carried out on the as received as well as
creep rupture tested material.
Room and high temperature tensile strength of 1Cr1Mo1/4V steel with Nb
addition was tested and is higher than plain 1Cr1Mo1/4V steel. Among Nb added
steels, tensile strength of 0.06% Nb steel was found to be highest.
1Cr1Mo1/4V castings with 0.06% and 0.08% Nb exhibit higher creep rupture
properties (Fig 3). The creep ductility of these casts is slightly lower than plain
1 Cr 1 Mo 1/4V steel.
On the basis of creep-rupture test results on weld joint samples, the weld joints of
steel with 0.06% Nb are found to be stronger than plain 1Cr1Mo1/4V steel as
well as with 0.04 and 0.08% Nb (Tables 3 & 4 & Figures 3 to 8)

The castings with Nb content exhibit excellent mechanical and creep properties
at high temperature. If a turbine casing is made of Nb containing steels, it is
possible to increase the steam temperature and pressure. The weldability of the
steel also increases, which will help in repair jobs.

WE CLAIM
1) A chromium- molybdenum-vanadium steel containing niobium comprising the
following elements in the weight percentage: -
Carbon-0.08 to 0.12
Sulphur-0.010 to 0.015
Phosphorous - 0.01 to 0.020
Silicon - 0.30 to 0.60
Manganese - 0.50 to 0.80
Chromium - 1.20 to 1.50
Molybdenum - 0.90 to 1.10
Vanadium - 0.20 to 0.30
Niobium - 0.04 to 0.08
Balance - iron with impurities
characterized in that said niobium in Cr- Mo- V Steel develops a carbon nitride
that imparts high temperature strength and fine carbide of molybdenum that
imparts creep strength by precipitation hardening after heat treatment.
2) The steel as claimed in claim 1 wherein the steel is subjected to a heat-
treatment at a temperature 940°-1040° C for 3 hours followed by air cooling.
3) The steel as claimed in claim 1 & 2 wherein the steel is subjected to a tempering
at a temperature of 650°-750° C for 5 hours followed by air cool from 300° C.

The present invention relates to a chromium-molybdenum-vanadium steel containing
niobium comprising the element in weight percentage: Carbon - 0.08 to 0.12, Sulphur -
0.010 to 0.015, Phosphorous - 0.01 to 0.020, Silicon - 0.30 to 0.60, Manganese - 0.50
to 0.80, Chromium - 1.20 to 1.50, Molybdenum - 0.90 to 1.10, Vanadium - 0.20 to
0.30, Niobium - 0.04 to 0.08 and balance - iron with impurities characterized in that
said niobium in Cr-Mo-V Steel develops a carbon nitride that imparts high temperature
strength and fine carbide of molybdenum that imparts creep strength by precipitation
hardening after heat treatment.