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TECHNICAL CONFERENCE2016

CZECH REPUBLIC

Contribution to the issue of the influence of welding conditions of creep-resisting steels

used for technological equipment in the energy sector

PROGroup Engineering®

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Contribution to the issue of the influence of welding conditions of creep-resisting steels used for technological equipment in the energy sector

Josef Nejedlý 1)

Abstract

The aim of the contribution is to determine the appropriate conditions of similar

and dissimilar weld joints of creep-resisting steels T/P91 and T/P92. This experiment

examines the properties of the material from the perspective of welding technology,

heat treatment, production and assembly. Weld joints were created using

the following welding methods: gas-tungsten arc welding (GTAW) as well as gas-

metal arc welding (GMAW). Filler metals were used from the manufacturers Oerlikon

and BÖHLER. The Experiment aims to find the optimal result of the structure

and mechanical properties using five variants of heat treatment of weld joints,

which as well were examined with the help of non-destructive and destructive tests.

Furthermore, the results of the tests were compared depending on the method

of welding, the filler metals, the kind of weld joints and the type of heat treatment.

Key Words

Creep-resisting steels, gas-tungsten arc welding, gas-metal arc welding,

similar/dissimilar weld joints, heat treatment, non destructive test, destructive test

1. Introduction The industry plays a very important role in the economics and remains

one of the critical sources of gross domestic product. The growth in our country

is achieved primarily by releasing the European Union market and the influx

of foreign investment capital.

Engineering and technology businesses are investing their resources into new

progressive technologies, which guarantee the quality and functionality

of the product, which must meet European requirements for mechanical resistance

and stability, fire safety, hygiene, health and environmental protection, safety in use,

protection against noise, energy saving and heat protection, sustainability

of resources according to the CPR EU 305/2011.

The key areas of the industry development are innovative technologies as well

as related issues of sustainable production and consumption or, of course, a cleaner

production. The future development of the industry is closely linked to the global

approach to the management of non-renewable and energy sources based

on the principles of sustainable progress. Increase of energy efficiency,

i.e. installation of the equipment with higher efficiency, the introduction of best

available technologies with the maximum limitation of all wastage, is without a doubt

the most important way to reduce energy demand, emissions of substances,

which are harmful for the environment, growth of dependency on imported energy

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as well as increase of the competitiveness of the energy sector and the entire

economics.

Based on the mentioned issues the new creep-resisting materials are being

developed, which allow to work with the new high parameters of steam on the super-

critical and ultra super-critical levels, i.e. working pressure above 260 bar

and temperature up to 600 °C are defined as super-critical steam parameters,

whilst the working pressure above 300 bar and the temperature above 600 °C

are defined as ultra super-critical steam parameters. Therefore these parametrically

designed technological units are called ultra super-critical blocks of power plants

and thus creep-resisting materials play an important role in the designing of these

energy devices.

The aim of this contribution is to establish the appropriate conditions of similar

and dissimilar weld joints of steels T/P91 and T/P92 used for power equipment.

Evaluation of the results of the experimental part is carried out by comparing

the method of welding, the filler metals, the kind of weld joints, the type of heat

treatment and the conditions of the manufacturing plant and the assembly site where

weld joints are made.

2. Background, [1], [2], [3], [4], [5]

Creep-resisting steels are used as structural materials for technological equipment

in buildings, which are operated in high temperature conditions. They are

characterized by high strength, toughness and resistance against creep

and oxidation. The steel must have the ability to transmit long-term external stress

and comply with the value of the ultimate strength in creep for 105 to 2×105 hours.

Research and development of modified (9-12) % Cr steels were initiated

in the 1980s within the European development programmes, such as COST

(Co-operation in the field of Science and Technology). The main objective

is the development of steels for working conditions (620–650)°C. Various kinds

of organizations around the world are involved in the research of modified steels.

However, the main research activities of these materials are centralized in Japan

and the USA and are focused on the materials used for the technological equipment

in the power sector in power plants of a new generation with critical parameters

of steam.

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Research and development of the creep-resisting materials were focused on the fulfilment of the essential requirements, [2] • RmT/10

5 h /600°C ≥ 100 MPa;

• Rp0,2min.= 600 MPa, the application of the creep-resisting steels used for rotors;

• Rp0,2min.= 450 MPa, the application of the creep-resisting steels used for castings;

• Toughness;

• Resistance against brittleness during long-term exposure at working

temperatures;

• Hot workability;

• Foundry properties;

• Weldability;

• Hardenability;

• Resistance against oxidation in steam environment;

• Thermal conductivity;

• Low coefficient of thermal expansion.

Figure 1. A sample of the realization of the technological equipment in the energy sector

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Figure 2. Development of creep-resisting steels, [1]

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3. Experiment of selected weld joints of creep-resisting steels P/T91 - P/T92 3.1 Determination of the experimental section

At present, there are common problems in the operation of power plants

with the critical parameters due to the cracking weld joints connections, which have

been carried out on the modified creep-resisting steels, therefore these weld joints

are critical places, which greatly affect the overall durability, functionality and security

of technological equipment.

With regards to all the mentioned problems related to weld joints of creep-resisting

materials within the construction of new blocks, we decided to carry out this

experiment in cooperation with the following research institutes and business

partners: Faculty of Mechanical Engineering, Department of Welding technology

and Surface Treatment, Foundry Department, PROGROUP ENGINEERING,

PROFLUID, TESYDO.

Figure 3. A sample of the realization of the technological equipment in the energy sector

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N- 140814 3.2 Type of basic material

Properties of steel P/T91 according to EN 10222/2-99

Designation according to chemical composition: X10CrMoVNb9-1

Material number: 1.4903

The chemical composition [hm.%]

C Si Mn P S Cr Ni Mo Nb V N Al

00,8 -

0,12

max. 0,50

0,30 -

0,60

max. 0,025

max. 0,015

8,00 -

9,50

max. 0,40

0,85 -

1,05

0,06 -

0,10

0,18 -

0,25

0,030 -

0,070

max. 0,040

Mechanical properties

Status Normalized annealed and temper

Yield strength ReH or Rp02 min. [MPa] 450

Breaking strength Rm [MPa] 630 -730

Ductility A5 [%] longitudinal 19

tangential 17

The impact of work KV [J]

longitudinal 40

tangential 27

The minimum value of the yield stress Rp02 [MPa] at elevated temperatures

T [°C]

100 150 200 250 300 350 400 450 500 550 600

Rp02

[MPa]

410 395 380 370 360 350 340 320 300 270 215

Physical properties

Density ρ [kg.m-3] 7850

Resistance against degradation processes

Tensile strength in tensile creep RmT [MPa]

T [°C] 500 520 540 560 580 600 620 640 660

RmT/104 289 252 216 182 151 123 99 79 62

RmT/105 258 220 183 150 120 94 73 56 42

RmT/2.105 246 208 171 139 110 86 65 49 35

Technological data

Normalized annealed 1040 – 1090 [°C] cool down in the air

Temper 730 – 780 [°C] cool down in the air or in the oven

The method of manufacture in the electric furnace or in an oxygen converter

Desoxidation fully killed steel

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Properties of steel P/T92 according to EN 10216-2

Designation according to chemical composition: X10CrWMoVNb9-2

Material number: 1.4901

The chemical composition [hm.%]

C Si Mn P S Cr Ni Mo Nb V N Al

00,7 -

0,13

max. 0,50

0,30 -

0,60

max. 0,025

max. 0,010

8,50 -

9,50

max. 0,40

0,30 -

0,60

0,04 -

0,09

0,15 -

0,25

0,030 -

0,070

max. 0,040

Mechanical properties

Status Normalized annealed and temper

Yield strength ReH or Rp02 min. [MPa] 440

Breaking strength Rm [MPa] 620 -850

Ductility A5 [%] longitudinal 19

tangential 17

The impact of work KV [J]

longitudinal 40

tangential 27

The minimum value of the yield stress Rp02 [MPa] at elevated temperatures

T [°C]

100 150 200 250 300 350 400 450 500 550 600

Rp02

[MPa]

420 412 405 400 392 382 372 360 340 300 248

Physical properties

Density ρ [kg.m-3] 7850

Resistance against degradation processes

Tensile strength in tensile creep RmT [MPa]

T [°C] 520 530 540 560 580 600 620 640 650

RmT/104 272 256 240 210 181 153 126 100 88

RmT/105 235 218 202 172 142 113 87 65 56

RmT/2.105 - - - - 129 101 76 56 48

Technological data

Normalized annealed 1040 – 1090 [°C] cool down in the air

Temper 730 – 780 [°C] cool down in the air or in the oven

The method of manufacture in the electric furnace or in an oxygen converter

Desoxidation fully killed steel

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N- 140814 4. Results of the experiment

Experimental weld joints were created with the following parameters in order

to determine appropriate conditions for selected similar and dissimilar weld joints

of creep-resisting steels T/P91 and T/P92:

Method of welding

• Gas-tungsten arc welding (GTAW)

• Gas-metal arc welding (GMAW)

Type of connection

• Similar weld joints

• Dissimilar weld joints

Joint type

• Butt weld (BW)

Filler metals

• Oerlikon

• BÖHLER

Heat treatment

• Variant I

• Variant II

• Variant III

• Variant IV

• Variant V

The Workplace

• The workplace at the manufacturing plant

• The workplace at the assembly site

Non-destructive and destructive tests were performed after the preparation of test

samples and subsequent heat treatment. The test results were compared, depending

on the method of welding, the filler metals, the kind of weld joints, the type of heat

treatment as well as the conditions at the manufacturing plant and the assembly site

where weld joints were made.

Although the experimental project is still in progress and not all the tests have been

performed yet, the achieved test results confirmed significant influence of the heat

treatment on structure and mechanical properties of weld joints.

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The results of the tests have shown that it is necessary to adhere to the following

principles:

• Always perform a Welding Procedure Qualification Record (WPQR)

for the specific geometry of a designed weld joint;

• Observe specific temperature of preheating;

• Adhere to the prescribed temperature, interpass;

• Perform the recommended welding methods;

• Comply with specific thermal input;

• Create welds in a prescribed manner and weld on a prescribed number

of layers;

• Always perform the prescribed heat treatment;

• Always observe the technological discipline at the manufacturing plant and the

assembly site.

A) B) Figure 4. A) Weld of P91 steel, B) Dissimilar weld of P91 – P92 5. Discussion

This contribution intends to point out the importance of all the relevant factors

that have a major impact on durability, functionality and safety of weld joints

of technological equipment in new buildings as well as existing energy blocks under

reconstruction.

6. Conclusion

This contribution examines the properties of the selected similar and dissimilar weld

joints that have been created by using the methods of gas-tungsten arc welding

(GTAW) as well as gas metal arc welding (GMAW).

The aim of the contribution was to determine the appropriate conditions of selected

weld joints of creep-resisting steels T/P91 and T/P92 within welding process based

on evaluation of the resulting changes in the structure and the mechanical properties

of a particular test sample after heat treatment including the different conditions

at the manufacturing plant and assembly site where weld joints were made.

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At present the experimental project is still in progress and not all the tests have been

carried out yet, however the results of the tests showed that heat treatment of weld

joints and compliance with the prescribed technology has a significant impact

on the final structure and mechanical properties of weld joints. The achieved test

results confirmed the significant influence of the heat treatment on structure

and mechanical properties of weld joints.

7. Acknowledgement

We would like to thank the Faculty of Mechanical Engineering, Department

of Welding technology and Surface Treatment, Foundry Department and cooperating

companies PROGROUP ENGINEERING, PROFLUID, TESYDO for cooperation

and taking part in the experiment.

8. Cooperation of research institutes

Brno University of Technology, Faculty of Mechanical Engineering

9. Cooperation of business partners

PROGROUP ENGINEERING, s.r.o.

PROFLUID, s.r.o.

TESYDO, s.r.o.

The Institute of Manufacturing Technology

Department of Welding Technology and Surface Treatment

Foundry Department

http://ust.fme.vutbr.cz

Department of Research and Development

http://www.progroup.cz

Department of Business Development

http://www.profluid.cz

Department of Technical Inspection

http://www.tesydo.cz

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References

[1] FUJITA T.: “Advances in 9-12%Cr heat resistants steels for power plants“. Proc.

of the 3rd EPRI conference on advances in materials technology for fossil power

plants, R. Viswanathan et al. Eds., Swansea 2001, 33

[2]

KUBOŇ Z., VODÁREK V., TOUŠLOVÁ D.: “Creep properties of heterogeneous

P91/P23 welds at 550 and 600°C”. In Materials for Advanced Power Engineering

2006. Liege, Belgium, 2006, Part III/1437, ISBN 3-89336-436-6

[3] VODÁREK V., DANIELSEN H. K., GRUMSEN F. B., HALD J., STRANG A.:

“Electron diffraction studies on (Nb, V) CrN particles in 12CrMoVNbN Steels”.

In Materials for Advanced Power Engineering 2006. Liege, Belgium, 2006, Part

III/1251, ISBN 3-89336-436-6

[4] BLUM R., WANSTONE R. W.: “Materials development for boilers and steam

turbines operating at 700 °C”. In Materials for Advanced Power Engineering 2006.

Liege, Belgium, 2006, Part I/41, ISBN 3-89336-436-6

[5] KLUEH R. L., NELSON A. T.: “Ferritic /martensitic steels for next-generation

reactors”. Journal of Nuclear Materials. September 2007 vol. 371, Issues 1-3, s.

37-52

1) Author

Josef Nejedlý Chartered Engineer AO, CKAIT/CZ International Welding Inspector IWI-C/CZ European Welding Engineer EWE/CZ