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Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels for Turbine Blading and Bolting Applications
Technical Paper Prod/EP5

Developments in the Processing of Alloy and Stainless Steels for Turbine blading and 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 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. The majority of installed steam turbine equipment powered by fossil-fuel boilers operates with a maximum steam temperature in the range 530-565°C, nuclear boiler steam temperatures are in the range 350-550°C. Presented at: The Third International Charles Parsons Turbine Conference - ‘Materials Engineering in Turbines and Compressors’ 25th - 27th April 1995.
Page 1 of 12

material in new designs.

Thus steel must be adaptable and operate over a wide range of temperatures from 565°C down to near ambient temperatures.

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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.

Figure 1 R Values of Durehete 1055 Casts

R Value = P + 2.43 As + 3.57 Sn + 8.16 Sb + 0.13 Cu R Value 0.2 0.15 0.1 0.05

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 turbine’s 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. 0 1979 1989 1990 1991 1992 1993 1994 1995 Year of Manufacture 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)

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.

Page 2 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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)

Figure 2 Electric Arc Furnace with Submerged Tap Hole

Water Cooled Roof

Electrodes

Slag Door

Water Cooled Panels Tap Hole Slag

Launder Liquid Steel Hearth

Figure 3 Electric Arc Furnace with Eccentric Bottom Tapping

Electrodes Water Cooled Roof

Slag Door Slag

Water Cooled Panels

Liquid Steel

Hearth

Sliding Gate Mechanism

Page 3 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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)

Figure 4 Ladle Furnace

Alloy Additions

Electrodes

Auto Sampling & Temperature Dip Fume Off-take

Lid Movement

Water Cooled Lid

Ladle

Argon Bubbling Car Movement

Ladle furnaces may also have a vacuum facility and this type of unit is generally known as a vacuum arc degassing (VAD) unit.
Page 4 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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.
Figure 5 Tank Degasser

Figure 6 V.A.D. Unit

13 Alloy Hoppers

Electrodes

Vacuum Exhaust

Conveyor Heat Shield Insert Gas Stirring Ladle

Water Cooled Heat Shield Tank Seal

Vacuum Leak

Car

Steam Ejectors

Inert Gas Stirring

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.

Page 5 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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 7 Stainless Steelmaking - A.O.D.

Fume Extraction A.O.D Vessel

Oxygen/ Argon/ Nitrogen Submerged Tuyere

Tilt Mechanism

Figure 8 Stainless Steelmaking - V.O.D.

Oxygen

Water Cooled Oxygen Lance

Oxygen Lance Drive

Water Cooled Heat Shield

Tank Seal Steam Ejectors

Refractory Splash Shield

Inert Gas (Argon/Nitrogen)

Page 6 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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. Ladle Refractory 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, Inert Gas Shrouding subject to a minimum reduction ratio of 6:1, as covered in BS 5S100.

Figure 9 Ingot Teeming

High Alumina Runner-Wave Refractory

Page 7 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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.

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 inline 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)

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
Page 8 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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.

Table 1 Examples of Turbine Manufacturers’ Ultrasonic Testing Specifications Compared With BS.5S100

Turbine Manufacturer 1

Application Forging Stock Blading Bar Intermediate Blading Bar Blading Bar Blading Bar Plate

Process Air Melt Air Melt Air Melt Air Melt ESR Remelt (ESR/VAR) Air Melt/ Remelt Air Melt Remelt Air Melt Remelt 2.0mm FBHE 2.0mm FBHE 3.0mm FBHE 3.0mm FBHE 2.0mm FBHE 2.0mm FBHE 3.2mm FBHE Standard Grade 3.0mm FBHE 2.0mm FBHE 2.0mm FBHE 1.2mm FBHE

Acceptance Standard

2 3 4

BS.5S100

Bar, billet or slab, alloy and ferritic/martensitic stainless <200mm section

Higher Grade 2.0mm FBHE 1.2mm FBHE 1.2mm FBHE 0.8mm FBHE

Single Indications Multiple & Linear Indications

Page 9 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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.
Figure 10 M.P.I. Indications on Turbine Blade. X1

Figure 11 Ferrite Stringers with Associated Carbides. X350.

Figure 12 Ferrite Stringers with Associated Carbides. X900.

Page 10 of 12

The future in metal

Corus Engineering Steels

Developments in the Processing of Alloy and Stainless Steels 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.

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.

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, 910 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
Page 11 of 12

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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www.corusgroup.com

Care has been taken to ensure that the contents of this publication are accurate, but Corus UK Limited and its subsidiary companies do not accept responsibility for errors or for information which is found to be misleading. Suggestions for or descriptions of the end use or application of products or methods of working are for information only and Corus UK Limited and its subsidiaries accept no liability in respect thereof. Before using products supplied or manufactured by Corus UK Limited and its subsidiaries the customer should satisfy himself of their suitability. Copyright 2001 Corus

Corus Engineering Steels PO Box 50 Aldwarke Lane Rotherham S60 1DW United Kingdom T +44 (0) 1709 371234 F +44 (0) 1709 826233 www.corusengineeringsteels.com

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