PRODEP5

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

The future in metal



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




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

The future in metal



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




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




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




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




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)




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

The future in metal



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.




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.

www.corusgroup.com

Corus Engineering SteelsPO Box 50Aldwarke LaneRotherhamS60 1DWUnited KingdomT +44 (0) 1709 371234F +44 (0) 1709 826233www.corusengineeringsteels.com

Care has been taken to ensure thatthe contents of this publication areaccurate, but Corus UK Limited andits subsidiary companies do notaccept responsibility for errors or forinformation which is found to bemisleading. Suggestions for ordescriptions of the end use orapplication of products or methods ofworking are for information only andCorus UK Limited and its subsidiariesaccept no liability in respect thereof.Before using products supplied ormanufactured by Corus UK Limitedand its subsidiaries the customershould satisfy himself of theirsuitability.

Copyright 2001Corus

PRODEP5

Documents

majority of steels

steel production

adaptability of steel

turbine industry

turbine bladematerial

turbine oftenmeans

steam turbines

steel used ascomponents