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Page 1 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Developments in the Processing of Alloyand Stainless Steels for Turbine bladingand Bolting Applications
By H. Everson, British Steel Engineering Steels,
J. Orr, British Steel Technical - Swinden Technology
Centre
Abstract
The majority of special steels for turbine applications are
made by the electric arc furnace route. Careful selection
of raw material is necessary to ensure undesirable
residual elements are kept to a minimum. Secondary
refining techniques such as vacuum arc degassing and
ladle furnace technology, coupled with improved casting
methods, have given significant quality improvements so
that it is now possible to use air melted steels to replace
remelted steels in many applications with resultant cost
savings. Close compositional control and uniform heat
treatment have improved the consistency of the finished
product.
Introduction
Steel played a significant role in the early stages of
turbine and jet engine development and retains its
dominant role as the first choice material for blading and
bolting in steam turbines and the compressor section of
land based gas turbines.
Steel still has its part to play in current jet engine
construction in the form of shafts, gears, bearings and
rings but has now been replaced as a turbine blade
material in new designs.
The majority of installed steam turbine equipment
powered by fossil-fuel boilers operates with a maximum
steam temperature in the range 530-565C, nuclear
boiler steam temperatures are in the range 350-550C.
Thus steel must be adaptable and operate over a wide
range of temperatures from 565C down to near
ambient temperatures.
This Paper was presented prior to the formation of
Corus plc following the merger of British Steel and
Koninklijke Hoogovens. Corus Engineering Steels is
the new name of British Steel Engineering Steels
referred to throughout the text of this paper.
Presented at: The Third International Charles Parsons
Turbine Conference - Materials Engineering in
Turbines and Compressors 25th - 27th April 1995.
Technical Paper Prod/EP5
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The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
The increasing size of turbines has placed greater
demands on the integrity of the steel used as
components become larger, rotational speeds increase
and containment forces rise.
Developments in steel production and inspection
techniques over the past 20 years have given significant
improvements in consistency and integrity to satisfy the
increasing expectations of the turbine industry.
The majority of steels used in turbines have been
specifically developed to cater for the variety of service
temperatures, stresses, pressures and corrosive
conditions that exist in the turbines environment and
demonstrate the adaptability of steel as a material.
However, the wide range of steels in a turbine often
means that each type is wanted in a variety of sections
and product forms, consequently order quantities per
grade and size are small and production is predominantly
via the flexible electric arc steelmaking and ingot cast
route followed by rolling to the desired profile.
Raw Materials
Scrap is the principal raw material charged to the
electric arc furnace. The steelmaker exercises careful
control over the selection of scrap to restrict the level of
residual elements to required levels and also to optimise
alloy content from the scrap in order to control costs.
The introduction of portable, accurate instrumented
analysis equipment has aided scrap segregation and
quality assurance procedures have been introduced into
the scrap supply chain.
A recent development at British Steel Engineering Steels
has involved the careful selection of very low residual
scrap and fig. 1 illustrates the significant improvement in
residual control achieved in the case of Durehete 1055
bolting grade over recent years.(1)
Page 2 of 12
R Value
0.2
0.15
0.1
0.05
01979 1989 1990 1991 1992 1993 1994 1995
R Value = P + 2.43 As + 3.57 Sn + 8.16 Sb + 0.13 Cu
Figure 1 R Values of Durehete 1055 Casts
Year of Manufacture
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Page 3 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Electric Arc Melting
The size and power rating of electric arc furnaces has
continued to increase such that many units are now in the
90 to 180 tonne range with power ratings up to 120 MVa.
Higher power ratings have enabled a reduction in melt
down time and the arc furnace is now used primarily as
a rapid melting unit but operates in conjunction with a
secondary steelmaking unit.
Submerged/eccentric bottom tap holes, see figs. 2 & 3,
or sliding gate valves are now a common feature on the
arc furnace and prevent slag carry-over into the refining
ladle with resultant improvements in steel cleanness and
analysis control. (2-4)
Water CooledRoof
Electrodes
Tap Hole
Launder
Liquid Steel Hearth
Water CooledPanels
SlagDoor Slag
Figure 2 Electric Arc Furnace with Submerged Tap Hole
HearthSliding GateMechanism
Water CooledPanels
Water CooledRoof
Figure 3 Electric Arc Furnace with Eccentric Bottom Tapping
Liquid Steel
Slag
SlagDoor
Electrodes
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The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Secondary Steelmaking
Ladle Furnaces
During slag free tapping from the arc furnace into the
ladle a clean synthetic slag is added to maintain low
oxygen levels which is necessary if inclusion levels are to
be minimised and a consistent yield from ferro alloys
additions is to be achieved.
The composition of the slag is controlled to achieve the
desired sulphur level. Heating is provided by three
electrodes, see fig. 4, and enables precise temperature
control. Inert gas bubbling is used to stir the molten
steel and promote better mixing of the steel and alloying
additions and to homogenise temperature.
Turbine steels often have a complex, carefully balanced
composition to optimise properties. Close analytical
control is therefore important to ensure that every cast
meets tight composition ranges and to ensure that cast
to cast variability is minimised hence giving consistency
in downstream processing characteristics and service
performance. The majority of ladle furnaces are
equipped with computer controlled, conveyor fed,
metered alloying systems which give precise alloy
additions. Coupled with the fact that the use of a
synthetic slag gives a predictable alloy yield in the
molten steel this gives much more accurate
compositional control. At the end of secondary
steelmaking very gentle stirring promotes the flotation of
inclusions leading to a clean product. (5)
Ladle furnaces may also have a vacuum facility and this
type of unit is generally known as a vacuum arc
degassing (VAD) unit.
Figure 4 Ladle Furnace
Car Movement
Alloy Additions Electrodes Auto Sampling &Temperature Dip
FumeOff-take
Water CooledLid
ArgonBubbling
Ladle
Lid Movement
Page 4 of 12
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Page 5 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Vacuum Degassing
The simplest and lowest cost degassing process is tank
degassing in which a ladle of the molten steel is placed
inside a vacuum chamber. There are facilities to stir the
molten metal by either inert gas bubbling or
electromagnetically, see fig 5.
Degassing is used to promote hydrogen removal and to
give a clean steel. In the majority of alloy steels
hydrogen control measures are needed to prevent
hairline cracking during subsequent processing stages.
The VAD unit has an advantage over other methods in
that it has a reheating facility and thus extended
treatment times can be utilised, see fig. 6.
13 Alloy Hoppers Electrodes
Conveyor
Vacuum Leak
Car
Ladle
Insert GasStirring
Heat Shield
VacuumExhaust
Figure 6 V.A.D. Unit
Figure 5 Tank Degasser
Water Cooled HeatShield
Tank Seal
SteamEjectors
Inert GasStirring
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Page 6 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Stainless Steels
The majority of stainless steels used in the manufacture
of turbines are martensitic stainless grades with carbon
contents more than 0.05%, typically between 0.10% and
0.2% carbon. This level of carbon is sufficiently high for
electric arc furnace/VAD refining to be practicable and
this is the standard route for martensitic stainless steels.
Many stainless steels for turbine applications require low
levels of phosphorous and other residual elements.
Phosphorous removal cannot easily be effected in high
chromium melts therefore low phosphorous scraps or
base mix practices are used to make very low
phosphorous stainless grades.
Low carbon austenitic stainless steels are produced
using an argon oxygen decarburisation (AOD) unit or a
vacuum oxygen decarburisation (VOD) unit to achieve
the low carbon levels required, see figs. 7 & 8. (6-8) Figure 8 Stainless Steelmaking - V.O.D.
Oxygen
Oxygen LanceDrive
SteamEjectors
Tank Seal
Water CooledOxygen Lance
Water CooledHeat Shield
RefractorySplashShield
Inert Gas(Argon/Nitrogen)
Figure 7 Stainless Steelmaking - A.O.D.
FumeExtraction
A.O.D Vessel
TiltMechanism
SubmergedTuyere
Oxygen/Argon/Nitrogen
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Page 7 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Ingot Casting
Specialised turbine steels are invariably uphill teemed to
obtain good surface quality. The molten metal is teemed
from the ladle through a high alumina erosion-resistant
refractory runner system that feeds into typically
between four and eight moulds at a time. To prevent
re-oxidation the liquid stream is protected by an inert
gas and the development of these clean steel practices
has contributed to the significant improvements in the
cleanness of specialised turbine steels, see fig. 9. (9)
Continuous Casting
In Western Europe over 90% of steel production is
continuously cast. Significant developments have taken
place in continuous casting technology, particularly in
areas such as submerged pouring systems, mould
design and optimum cooling conditions, such that many
special steel users outside the turbine industry now
specify continuously cast material.
The majority of specialist steels used in turbine blading
and bolting are used in relatively small quantities,
however, should there be more rationalisation in the
industry with respect to the number of grades used,
there is no reason why continuously cast turbine steels
should not be made on a regular basis.
Similar martensitic stainless grades used in the oil and
gas industry are already made as direct cast rounds for
subsequent seamless tube rolling. Continuously cast
steels are allowed for high integrity aerospace use,
subject to a minimum reduction ratio of 6:1, as covered
in BS 5S100.
Figure 9 Ingot Teeming
Ladle Refractory
Inert Gas Shrouding
High AluminaRunner-WaveRefractory
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Page 8 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Reheating, Rolling and Cooling
Turbine blading and bolting steels tend to have high
hardenability and are therefore prone to cracking if
heated or cooled incorrectly. Ideally it is more economic
to direct hot charge ingots to the rolling mill, but as 20
or 30 ingots from a large cast may be destined for as
many different sizes, slow cooling, ingot annealing and
careful reheating of ingots is necessary, in order to meet
roll mount sequences.
Ingot heating regimes are important factors in the
prevention of deleterious microstructural features that
can give rise to problems in finished components. For
example, the 12%Cr steels can be prone to primary
carbide stringers and delta ferrite formation both of
which can produce undesirable indications on magnetic
particle inspection of the finished component. All
reheating operations are optimised to ensure that
phases that would interfere with the integrity of
inspection at the final stages are controlled.
The majority of grades are direct rolled to a final primary
size but for small sized products these may require
rolling to an intermediate size for subsequent re-rolling.
Cooling from primary or secondary rolling has to be
carefuIly controlled to prevent cracking of the hard
martensitic microstructure.
Heat Treatment
The majority of turbine blades and bolts are machined
from rolled or forged sections that have been fully heat
treated. Final heat treatment is a major factor in
determining the properties and performance of the
finished component. Modern heat treatment furnaces
utilising improved thermal insulation, anticipatory
temperature controls in conjunction with computerised
burner controls, give very uniform temperature
distribution and freedom from overshoot condition.
These factors result in much greater consistency of
properties and performance.
Bar Inspection
Major strides have been made in the development of in-
line inspection techniques over the past decade.
Thermal imaging, eddy current and magnetic methods
are used for surface inspection, in-line high speed, high
sensitivity ultrasonic equipment has been developed for
internal inspection, all of which result in greater product
assurance.
Continuous development of in-line surface and internal
inspection techniques has resulted in increased levels of
product assurance. British Steel Engineering Steels uses
an infra-red technique called Thermomatic to surface
inspect primary rolled products to tight defect
thresholds. An in-line ultrasonic facility is also available
for internal inspection and can be complemented by
additional hand-held ultrasonic inspection as required. (10)
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Page 9 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Table 1 shows the ultrasonic acceptance standards of a
number of turbine manufacturers for rolled or forged
products. In general the acceptance standard for air
melt products is 3.0mm to 3.2mm flat bottomed hole
equivalent (FBHE) compared with 2.0mm FBHE for
remelted steels.
Modern ladle refined air melt steels produced by British
Steel Engineering Steels to ultra clean steel practices are
capable of meeting the 2.0mm FBHE remelt steel
standard and many turbine manufacturers now order air
melted steel in the place of remelted steel thereby
achieving significant savings.
The improvement in cleanness of air melt steels for high
integrity applications is reflected in the improved
standard requirements for aerospace steels as published
in British Standard 5S100.
Turbine AcceptanceManufacturer Application Process Standard
1 Forging Stock Air Melt 2.0mm FBHEBlading Bar Air Melt 2.0mm FBHE
Intermediate Blading Bar Air Melt 3.0mm FBHE
2 Blading Bar Air Melt 3.0mm FBHEESR 2.0mm FBHE
3 Blading Bar Remelt 2.0mm FBHE(ESR/VAR)
4 Plate Air Melt/ 3.2mm FBHERemelt
Standard Grade Higher GradeBS.5S100 Bar, billet or slab, alloy Air Melt 3.0mm FBHE 2.0mm FBHE Single
and ferritic/martensitic Remelt 2.0mm FBHE 1.2mm FBHE Indicationsstainless
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The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Final Component Inspection
Turbine blading and bolting are usually subject to a final
magnetic particle inspection when finish machined. The
introduction of clean steel practices has essentially
eliminated rejections due to significant MPI indications
caused by non metallic inclusions on finished
components made from air melt steel. However, many
high chromium grades used in turbine steels have a
composition balance prone to delta ferrite and carbide
formation which can give rise to significant indications
on machined surfaces. Figure 10 shows the effect of a
ferrite stringer with associated carbide on magnetic
particle inspection of a turbine blade. Figures 11 and 12
show the microstructures responsible. These features
can be minimised by the optimisation of casting
practices, ingot design, re-heating, homogenisation and
thermo-mechanical working technique.
Certain grades with unfavourable composition balances
which are prone to significant amounts of delta ferrite or
retained austenite in their microstructure may require
testing by dye penetrant techniques to distinguish
between indications caused by rejectable cracks and
those caused by microstructural features inherent in the
material composition.
Page 10 of 12
Figure 11 Ferrite Stringers with Associated Carbides. X350.
Figure 10 M.P.I. Indications on Turbine Blade. X1
Figure 12 Ferrite Stringers with Associated Carbides. X900.
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Page 11 of 12 The future in metal
Corus Engineering Steels
Developments in the Processing of Alloy and StainlessSteels for Turbine Blading and Bolting Applications
Summary and Conclusions
Steelmaking developments using lower residual scrap
and secondary refining techniques have led to much
tighter compositional control. Together with modern heat
treatment facilities this has enabled greater consistency
of mechanical properties and hence subsequent material
performance to be achieved.
Significant improvements in steel cleanness, closely
controlled re-heating and rolling practices and
developments of in-line automated inspection
techniques have eliminated significant MPI indications on
final inspection of turbine blades.
Turbine manufacturers who have used a remelted
product in the past now find that modern air melted
steel gives satisfactory material properties for many of
their end-use applications.
References
1 Everson H, Orr J, Low Residuals in Bolting Steels,
Institute of Materials - EPRI Workshop, Clean Steels
- Super Clean Steels, London, 6-7 March 1995.
2 Broome K A, Eccentric Bottom Tapping, SMEA
Conference: Quality Steel - Advances in Secondary
Steelmaking and Casting, Paper No 1, Sheffield, 9-
10 April 1992.
3 Shelbourne A, Submerged Taphole on the Electric
Furnace, ibid, Paper No 4.
4 Marsh F, Use of the Slidegate Taphole Valve on the
Electric Arc Furnace, ibid, Paper No 2
5 Davies I G, Broome K A, Thomas K, Major
Improvements in Steel Cleanness Process Route
Modifications and the Introduction of Ladle Furnace
Operation, Clean Steel III Conference, Balonfused,
Hungary, June 1986.
6 Choulet R J, Mehlman S K, Status of Stainless
Refining, Metal Bulletin International Stainless Steel
Conference, 12th November 1984.
7 Broome K A, Beardwood J, Berry M, The
Production of Carbon, Low Alloy and Stainless Steels
using VAD, VOD and LF Secondary Steelmaking
Facilities at Stocksbridge Engineering Steels,
International Conference - Secondary Metallurgy,
Aachen, September 1987.
8 Everson H, Clarke M A, Influence of Steelmaking
and Primary Processing Factors on Availability and
Properties of Stainless Steels, Stainless 87: Institute
of Metals, York, September 1987.
9 Morgan P C, Control of Oxygen During Steelmaking
and the Production of Ultra-Clean Steel, Clean
Steels for Aerospace Applications Seminar: Institute
of Metals, London, April 1988.
10 Cope A D, Davies I G, Fretwell I, Hardman A, The
Ultrasonic Inspection of Clean Steels, ATS
Steelmaking Days, Paris, December 1988.
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