Power Suppliers are Increasing Demands on Steel Grade P91 or X10CrMoVNb9-1

Economic pressure is forcing power plant operators to increase their effectiveness by increasing temperature and steam pressure to provide energy at the lowest prices possible.

For current power stations, the characteristic steam temperature at the turbine intake is between 540°C - 565°C, although in the last few years, there has been an increase of the working medium to go above 565°C. In some cases, the water vapour is used with a temperature of up to 600°C. In the case of using the steam in the state of supercritical parameters (610°C/30MPa), compared to the current water vapour parameters (540°C/18MPa) it is possible to achieve an increase of effectiveness of energy devices by up to 8% at the current decrease of CO2 emissions by up to 20%.

Železiarne Podbrezová is adapting to the requirements of the segment. Today it is offering its customers seamless tubes destined for the critical parameters of above 600°C usage. This is provided by the steel grade P91 and T91 or X10CrMoVNb9-1 and corresponding equivalents.

P91 grade - key factors

P91 grade is ferrite-martensitic steel (9% chromium, 1% molybdenum) microalloyed with vanadium, niobium and with controlled content of nitrogen. Tubes made out of P91 grade are destined for boiler superheaters, power plant tube heaters, headers and duct pipes for high-temperature applications, and eventually for furnace ductwork in petrochemical assembly.

P91 grade - advantages
•Significantly improved strength and material creeping behaviour at higher temperatures
•Improved material corrosion resistance compared to other boiler type grades
•Decrease of tube weight and subsequently tube components and boilers alone
•Increased material resistance against thermal fatigue
•Good heat transfer and low expansion coefficient compared to austenitic steel grades

P91 standards

In 1983, the ASTM/ASME approved the T/P91 tubes and since then they have been approved by many national standards.
•A213/T91: seamless ferritic and austenitic alloy-steel boiler, superheater and heat exchanger tubes
•A335/P91: seamless ferritic alloy-steel pipe for high-temperature service
•A234/WP91: piping fitting of wrought carbon steel and alloy steel for moderate and high-temperature service
•A200/T91: seamless ferritic, alloyed tubes to be used in the refinery field
•NF A 49213/TUZ 10CDVNb 09-01: steel tubes; seamless unalloyed and Mo and Cr-Mo alloyed steel tubes for use at high temperatures
•DIN 17175/X10CrMoVNb9-1: seamless steel tubes for elevated temperatures
•BS 3604-2/P91: steel pipes and tubes for pressure purposes: ferritic alloy steel with specified elevated temperature properties
•BS 3059-2/P91: specification for carbon, alloy and austenitic steel tubes with specified elevated temperature properties
•EN 10216–2/X10CrMoVNb9-1: seamless steel tubes for pressure purposes. Part 2 - Non-alloy and alloy steel tubes with specified elevated temperature properties

Tubes for power suppliers from Železiarne Podbrezová

Železiarne Podbrezová is the leading manufacturer of seamless tubes and pipes to be used in boiler production, especially in Central Europe but also worldwide. Two production plants (in Slovakia and Spain) are served by the steel from its own production.

Dimension range:
•OD (outside diameter) from 12.7mm to 139.7mm
•WT (wall thickness) from 0.89mm to 12.50mm
•Tubes made up to 21.3mm diameter are produced by the cold drawing process

Basic facts and numbers:
•Revenues: €216m
•Number of employees: 3,200
•Production capacity: 110,000 ton seamless steel tubes
   
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More Efficient Heat Exchange Thanks to Steel Tubes

Requirements on increasing the level of efficiency of high-performance steam generators have caused an occurrence of accidents on tube walls due to overheating and cracks have even been caused by burning through a pipe wall. In order to solve the issue, Železiarne Podbrezová has developed tubes with protrusive projections on the inner side.

Today, there is still the prevailing use of tubes with smooth surface for the purposes of construction as well as reconstruction of energy type facilities, despite the fact that tubes with surface structure (tubes with interior ribbing) positively influence the performance of energy type facilities.

Tubes with interior ribbing are intended for high-performance steam generators and high-pressure boilers and are also used for burners in the equipment.

Interior ribs cause separation of steam and liquid as a result of centrifugal power, whereby the fluid phase (water) is forced to the tube wall, where it is stored longer as a water film. Using this method prevents occurrence of boiling on the interior tube wall which prevents exchange of heat and may also be the cause of occurrence of critical zones of commencing distortion of steel tube. Admissible share of water steam while retaining the heat flow and using ribbed tubes thus increases in the interval of 20% - 40%, when using smooth surface tubes, to 70% - 90%.

That phenomenon results in increasing the contact surface area between the liquid part and tube wall, which brings about the following benefits:
•Improves the level of heat transfer even with a higher share of steam
•A very good level of heat transfer is retained also at lower medium flow
•Temperature of tube walls is reduced and so is the risk of distortion of tubes
•It also prepares the room for improvement of heat transfer by a suitable optimisation of rib geometry

Last but not least, ribs have an interesting attenuating effect, which prevents sedimentation of solid particles present in the medium, contributing to limitation of the occurrence of corrosion processes, and it has a favourable impact on the increase of tube lifespan. That is a clear representation of the general economic and ecological contribution of such tubes.

Currently, there is an increasing demand for tubes with interior ribbing for construction of new super critical boilers. On the other hand, for the purpose of reconstruction of older energy type facilities, it is possible to apply ribbed tubes to increase the output of the facility while retaining the facility's dimensions.

The heat transmission degree can be optimised by a suitable choice of rib geometry, depending on existing operating conditions of the equipment (boiler or heat exchanger). Železiarne Podbrezová, manufacturer of seamless steel tubes, has many years of experience in production of tubes with interior ribbing. The entire production process, i.e. the melting of steel, production of rolled tubes, tubes drawing, as well as the final heat processing, is carried out by Železiarne Podbrezová. Each tube is tested by ultrasound or eddy current in order to ensure the required quality features.

Tubes are supplied in quality grades in accordance with EN 10216, JIS G3461 - 3462, ASTM, ASME A/SA 192, 209 210 A 213 and other corresponding standards.
   
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Heat Treatment of Seamless Steel Tubes Pipes P24, P91

Economic pressure to deliver energy at the lowest possible prices forces operators of electric power plants to increase their efficiency by means of increasing the temperature and steam pressure. For current power engineering it is characteristic that the temperature of the steam entering the turbine is 540°C to 565°C.

However, in recent years, there is a tendency to increase the temperature of working medium temperature above 565°C. In some cases, steam temperatures up to 600°C are being used. In the case of using steam in a state of supercritical steam parameters (610°C/30MPa), compared with the current parameters of water vapor (540°C/180MPa), it is possible to achieve an increase of the energy efficiency of equipment up to 8%, while reducing CO2 emissions by up to 20%.

Implementation of these conditions requires the replacement of an obsolete system with a new one, which will be resistant to the conditions of operation, mainly in terms of having higher creep resistance.

One of the structural elements of thermal power plants which is made from steel tubes is membrane walls. For their production steel T12 is normally used, which does not comply with the supercritical parameters of operation. Therefore, in this group of steels research began in the 80s, resulting in the original Japanese steel T/P23 and its European variant under the name T/P24 7CrMoVTi10-10.

These new steels meet the criteria for supercritical operation. Another one is the quality of the P91 (X10CrMoVNb9-1). These are ferritic-martensitic steel (9% chromium, 1% molybdenum) microalloyed with vanadium, niobium and controlled nitrogen content. Tubes made from grade P91 are designed for boiler superheaters, tube heaters for power plants, and tube collectors for high temperature use respectively for furnace tubes in petrochemical aggregates.

Requirements for creep resisting steel complicates the necessity to resist aggressive environments. When operating at high temperatures and pressures in the steels used, material creep phenomenon is applied.

Boiler quality tubes that are used for higher operating temperatures and higher operating pressures need to ensure the creep resistance of heat treatment which ensures their appropriate starting structure. It is a precipitation hardening process, which is caused by the presence of fine precipitates based on carbides and carbonitrides of alloying elements (V4C3, Mo2C respectively. Cr7C3), which forms a barrier for the movement of dislocation by grid.

One of the annealing furnaces in the tube mill of Zeleziarne Podbrezova meets all the requirements for heat treatment of tubes made of creep resistant grades. In the hot part of the furnace in the second to fourth zone are placed ceramic radiant burners, allowing the increase of annealing temperature from normal temperature max. 980°C up to 1,080°C.

Back in the hot part of the annealing furnace is additionally installed electrical heating by electric coils, which ensures the retention of the tube at the required temperature before intensive cooling. At the beginning of the cold part of the furnace is installed a powerful rapid cooler, allowing the tubes to cool down in a protective atmosphere at the speed of 350°C/min., depending on the tube diameter.

A modernised furnace provides the perfect conditions for heat treatment of boiler tubes in such a way so that their properties meet the requirements of the legislation under which the tubes are supplied and meet customer requirements.
   
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ŽP Research and Development Centre Ltd

Company Zeleziarne HAS established a subsidiary company ZP VVC Ltd. Its goal is to professionalize the research and development activities with a focus on manufacturing programme of ZP but also other companies in the ZP GROUP. Finally, it is expected that the research and development activities will be for external customers too.

The business activities

We implement the following activities:
•Research and development in the area of natural sciences and engineering, which fits into the disciplines of physical metallurgy, materials, limiting states of materials, Mechatronics and focuses derived from it
•Research and development in manufacturing of electrical steel: liquid phase, alloying, refining, continuous casting
•Research and development in the theory of hot and cold forming: the optimization of relevant technological processes with a goal to achieve the desired microstructure, development of new materials with desired complex strength, plastic and fracture properties with special regard to their application in demanding thermal-stress and deformation conditions in energy, engineering applications, corrosive environments
•Analysis of the microstructure and mechanical properties of steel, including violations and limiting states of materials
•Research and development of tool materials and metallurgical ceramics
•Modelling and simulation of technological processes of continuous casting and hot rolling
•Research in the area of ecology
•Microstructure and substructural analysis, microanalysis and chemical analysis and evaluation of mechanical and selected physical properties of materials
•These activities are carried out for external clients, including professional and technical and consulting activities, as well as continuing educational activities

The department is also involved in:
•Conceptual and strategic proposals in the area of research and development, carrying the latest global knowledge into the production cycle; developing international cooperation and initiate participation in the of projects under EU programs
•Collaboration with universities and research institutes
•Guarantees of legal protection of achieved solutions and their presentation in journals and at conferences at home and abroad

The focus of departments - Department of Metal Forming
This department deals with the evolution of the microstructure of materials in the process of thermomechanical processing with application focus on technological conditions of the tube mill of the ZP. Optimizes the basic technological parameters from the rotary heath furnace all the way up to the stretch-reducing mill, depending on the chemical composition of steel in order to achieve performance properties of rolled tubes. It studies and addresses the issues of static and dynamic recrystallization in thermomechanical processing in order to achieve the desired state of the structure and substructure material.

It also uses the dislocation theory of plasticity to optimize cold forming, while respecting the technological possibilities of the cold drawn tube mill of the ZP. By the deformation hardening, it optimizes the conditions of deformation and strain rate in order to achieve rational technological processes - intensive deformation, reducing the number of draws to get the final size, the intermediate annealing.

Department of Physical Metallurgy

The department deals with the physico-chemical nature of steel production in close cooperation with the Steelmaking plant of ZP for critical technology nodes: scrap management, EOP, secondary metallurgy - ladle furnace and ZPO. Main attention is paid to the optimization of individual technological parameters of production to ensure high quality and reproducibility of production of continuously cast blooms.

Department of Materials Engineering

The department deals with chemical and structural concept of steel produced at the ZP a.s., including its substructural and microstructural analysis. Analyzes the strength, plastic and fracture properties and their relation to chemical composition and microstructure of steels. We study and address the theoretical issues of limiting states and violations of materials, fractographic signs, life expectancy and causes of possible breakage of real structures.

The department also deals with the heat treatment of steel, by optimizing the chemical composition and microstructure to achieve the required mechanical properties. Specifically, it deals with the evaluation and analysis of mechanical properties at higher temperatures, with particular emphasis on the creep characteristics of boiler quality.

It also deals with the increase of performance properties of tool materials in the ZP and specifically addresses surface engineering, including the Zn-Fe system and corrosion resistance.

The department, in conjunction with the Gork operation, provides development of methodologies and development of instrumentation laboratories for metallography, ELMI and REM, microanalytical methods and evaluation of mechanical properties of materials.

Department of modelling and simulation processes

The department deals with the use of mathematical methods for modelling the key technological processes in the ZP. Key areas include thermodynamics of thermally active processes in the continuous casting of steel at ZPO, thermal deformation processes respectively deformation processes in hot forming respectively or during the precision tubes drawing.

The department provides software and programmes that are needed for research activities of ZP VVC Ltd. Collaborates to solve challenges in the area of applied mathematics and statistical processing of experimental results. It also provides web site content of ZP VVC Ltd.
   
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Structure and Tube Properties from Steel Grade X10CrMoVNb9-1

Železiarne Podbrezová has a long tradition in the production of boiler tubes. The production capacity is constantly increasing and the company focuses on tubes made of grades (especially class 15) 16Mo3, 13CrMo4-5, 14MoV6-3, 10CrMo9-10, 7CrMoVTiB10-10, which by their properties fully comply with standard EN 10216-2, including creep resistance at temperatures ranging from 530°C to 590°C, respectively 600°C. In recent years, we have proceeded to the production of tube steel grade X10CrMoVNb9-1, which is designed for use in ultrasupercritical conditions in energy plants and up to temperatures of 620°C at an operating load to 30 MPa. In this white paper we explore the strength, toughness and plastic properties of this grade.

Download this free white paper to find out more.
http://www.power-technology.com/downloads/whitepapers/pipes/filestructure-and-tube-properties/    
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Železiarne Podbrezová - Steel Tubing for Power Engineering

Modern thermal systems (including steam generators in nuclear power plants) work with a high level of effectiveness and require suitable construction materials in order for them to be safe and work without any problems for a long period of time. Steel tubes are important construction materials used in these types of facilities.

Steel tubes for power generating facilities

Železiarne Podbrezová produces a wide range of steel tubes. Tubes for power generating facilities (frequently referred to as boiler tubes) are from the set standards point of view classified into a group of tubes for pressure usage at higher temperatures.

Production and testing of these tubes goes through a very tough screening process. In terms of European Standards this is a three-level structure. The appropriate directive for pressure equipment is at the level of being a law. This is closely connected with the whole set of standards for construction of appropriate power generating facilities and they in turn determine the standards for individual construction materials (tubes, sheets, etc.). Producers and suppliers of tubes must have the proper certification for the system and production.

Pressure seamless steel tubes

Železiarne Podbrezová produces pressure seamless steel tubes according to EN 10216 standards (except steel tubes made out of rust-resistant steels) and their corresponding standards and directives.

Tubes for pressure services have a wide application in many industries. Use for a particular kind of tube (depending on pressure value, temperature and the kind of transported medium) is specified in the appropriate regulations for pressure services.

Boiler tubes

Boiler tubes are used for power plant construction, such as boiler components and steam superheaters in power plants, heating plants and boiler rooms.

Heat exchanger tubes

Heat exchanger tubes are used mainly for heat exchangers in which energy is transferred from one medium to another. This heat transfer can be combined with or without medium phase change. Heat exchanger tubes are used in the power and chemical industries and in heating or air conditioning devices.

Tubes for heat exchangers are supplied as cold drawn and can be delivered with internal riffling.

Fine grain steel tubes

Železiarne Podbrezová produces fine grain steel tubes. These pressure services tubes are made from steel with feritic grain size 6 and less. Fine grain steel tubes are used when the required characteristic of impact strength should be guaranteed.

Steel tubes for low temperature

We also manufacture steel tubes that are suitable for use in apparatus and appliances using low temperatures.

Steel tube grades

Here is a brief overview of the technical parameters of steel tubes we supply:
•Carbon steel: St 35.8, St 45.8, P 235GH, P 265GH (DIN, EN), Grade A, Grade B, Grade C, according to US standards
•Mo steel: 15Mo3, 16Mo3, T1, T1a, T1b (DIN / EN / US)
•Cr-Mo steel: 13CrMo4 4, 13CrMo4-5, T12, P12, 10CrMo9 10, 10CrMo9-10, X11CrMo9-1, T22, P22, 7CrMoVTiB10-10, T24, (X11CrMo5, T5)
•Cr-Mo-V steel: 14MoV6 3, 14MoV6-3
•Cr-Mo-V-Nb steel: X10CrMoVNb9-1, T91, P91

Other steel is available as per agreement.

Seamless steel tube dimensions

The means of production and dimension interval of our seamless steel tubes are as follows.

Hot rolled:
•OD: 21.3mm - 139.7mm
•Wall: 2.3mm - 12.5mm (depending on the D)
•Length: commonly 5m - 6m, as per agreement 22m or 24m

Cold drawn:
•OD: 12.7mm - 76.2mm
•Wall: 0.89mm - 5.6mm
•Length: commonly 5m - 6m, as per agreement up to 18.3m

Comments regarding the dimensional assortment:
•After agreement we can make steel tubes according to US standards
•After agreement we can also make steel tubes that are cold drawn with other dimensions
•Besides boiler tubes we also deliver heat exchanger steel tubes as drawn tubes for low and high temperatures

Production and testing of steel tubes

Tubes for all segments of power engineering and machinery are delivered according to the appropriate CSN (STN) DIN, EN, ASTM A, ASME SA, GOST, JIS standards and others. In EU countries we should not be using the original national standards but the EN standards.

Železiarne Podbrezová owns all necessary system and production certificates and has the necessary technological equipment for production and testing of steel tubes used in power generating plants.
   
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