Applications of advanced low-alloy steels for new high-temperature components

145 Pages·2009·6.3 MB·English

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The new low-chromium steel grades 23 and 24 are candidate materials for components K
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of the new power and petrochemical plants, and for the refurbishment and re-powering N
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of older plants. The mechanical and creep properties of both grades are significantly -
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better than the parent grade 22, but long-term creep performance, microstructural evolu- 5
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tion, welding characteristics and other properties were not fully defined and assessed. It 8
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was also important to improve knowledge of microstructural evolution in order to verify N
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the mechanical behaviour after long-term service. S
The consortium has produced trial components by industrial process routes for both
grades, but the activities have been focused mainly on grade 23, for commercial rea-
sons, and on grade 24 for comparison. New consumables for welding have been devel-
oped and tested. Creep test programmes for base material and welded joints, including
long-term tests, have been carried out, and some tests will continue beyond the end
of the project. The data acquired will also be incorporated in the creep database of the
European Creep Collaborative Committee and will be used in the coming assessments A
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for EN standards. p
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The parallel aim of the project was piping integrity assessment under realistic loading a
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conditions by combined thermal and hydraulic system analysis and stress analysis using io
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the data generated during the project. This work has shown that a P23 pipework system s

will be more durable than an equivalent CMV system providing that good operational o
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European Commission
Research Fund for Coal and Steel
Applications of advanced low-alloy steels for
new high-temperature components
A. Di Gianfrancesco, D. Venditti (1), D. J. Allen, A. Morris (2), S. Caminada (3),
S. Pillot (4), M. M. Rodriguez (5), V. Friedman, P. von Hartrott, D. Siegele (6),
S. Holmström, J. Rantala, J. Salonen, P. Nevasmaa, K. Calonius, P. Junninen (7)
(1) CSM — Via di Castel Romano, 100, I-00128 Rome
(2) E.ON UK — Ratcliffe-on-Soar, Nottingham NG11 0EE, United Kingdom
(3) Dalmine — Piazza Caduti 6 Luglio 1944, 1, I-24044 Dalmine
(4) Industeel — 56, rue Clémenceau, BP 19, F-71202 Le Creusot
(5) ISQ — Av. Prof. Dr Cavaco Silva, 33, Taguspark, PT-2780-994 Porto Salvo
(6) Fraunhofer-Institut für Werkstoffmechanik (IWM) — Wöhlerstraße 11, D-79108 Freiburg
(7) VTT — PO Box 1000, FI-02044 VTT
Contract No RFSR-CT-2003-00037
1 September 2003 to 28 February 2007
Final report
Directorate-General for Research
2009 EUR 23598
EN
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Luxembourg: Office for Official Publications of the European Communities, 2009

ISBN 978-92-79-10006-2

ISSN 1018-5593

© European Communities, 2009
Reproduction is authorised provided the source is acknowledged.

Printed in Luxembourg

PRINTED ON WHITE CHLORINE-FREE PAPER
Table of Contents

(cid:131) Summary 4
(cid:131) Scientific and technical description of the results 14
(cid:131) Objectives of the project 14
(cid:131) Comparison of initially planned activities and work accomplished 15
(cid:131) Description of activities and discussion 17
o WP1: Characterisation of the ‘as-received’ steels 17
o WP2: Development of weldments 29
o WP3: Mechanical data assessment 48
o WP4: High temperature design and assessment 76
o WP5: High temperature design and assessment 90
o Conclusions 110
o Exploitation and impact of the research results 112
(cid:131) List of figures and tables 113
(cid:131) List of References 116
(cid:131) Appendix 1: Welding procedure specification (WPS) 118
(cid:131) Appendix 2: LM and SEM microstructural investigation of aged specimens 131
(cid:131) Technical Annex 134

3
Summary
Introduction
The new advanced low Chromium Grades 23 and 24 steels are and will be strong candidates for
new power and petrochemical plant construction and for the eventual large-scale replacement of
steam pipework on existing power plant.

These new grades are improved version of the grade 22 (2¼Cr1Mo) developed in the years ’60
and applied in worldwide the power and petrochemical plants for large amount of components:
tubes, pipes, cast, forged. In the years ’80-90 several effort have been devoted to increase the
plants performances and therefore materials with enhanced performances were requested.
The steelmaker metallurgists and the material researchers started to develop new chemical
compositions on the base of existing steels with the addition of elements able to give
strengthening by carbides precipitation (V, W, Nb, Ti).
The main differences are described in the following table based on ASTM A213 Standard.

Grade C Mn P S Si Cr Mo W Nb V B Other
min 0.05 0.30 - - - 1.90 0.87 - - - -
22 -
max 0.15 0.60 0.025 0.025 0.50 2.60 1.13 - - - -
min 0.04 0.10 - - - 1.90 0.05 1.45 0.02 0.20 0.0005
23 N: 0.03 max
max 0.10 0.60 0.030 0.010 0.50 2.60 0.30 1.75 0.08 0.30 0.0060
min 0.05 0.30 - - 0.15 2.20 0.70 - - 0.20 0.0015 N: 0.012 max
24
max 0.10 0.70 0.020 0.010 0.45 2.60 1.10 - - 0.30 0.0070 Ti 0,06-0,10

For both the grade V was added in the chemical composition and the other elements were
respectively:
- for Grade 23: W and Nb with lower amount of Mo, with a similar approach used for high
alloyed chromium grade 92 respect to grade 91.
- for Grade 24: more Cr than grade 22, with the addition of B and Ti.
These addition are able to guarantee an increase of creep behaviour with a possible service
temperature up to 560-570°C that means 30-50°C more than grade 22. Otherwise the increase of
carbides former elements increase the problems in the welded joints.

The following figures shown an overview of main components of power and petrochemical
plant:
- small diameter and thin wall thickness tubes (a) for waterwalls of boiler (b) and heat
exchangers,
- larger diameter and thick wall pipes for headers (c) and steam lines (d)
- very large diameter and heavy wall forged components (e) for pressure vessels of
petrochemical power plant reactors (f).
a b c
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d e f

The steam lines are one of the main critical component of a power plant due to:
- high temperature steam and high pressure, from headers to the turbine (blue area);
- low cycle fatigue as consequence of the stress variations during cyclic operating
condition of the power plant;
- the thermal expansion as consequence of the temperature variations during cyclic
operating condition of the power plant generate an additional thermal fatigues stress.

SuperHeater Steam Line
Steam Turbines
Boiler

The additional alloying elements give one increase of the cost of the row materials, but the cost
of the final components are not detrimental for the use of these new materials, because it has to
be take in account the increase of material performances and consequently the reduction of the
wall thickness, as well as, the improvement of the plant efficiency.

The main aims of the project were the following:
- Mechanical and microstructural assessment of base materials,
- Development of weld material and welding procedure to avoid the “bore cracking”
phenomenon,
- Mechanical and microstructural assessment of similar welding,
- High temperature design and assessment of welded components including welded
pressure vessels and pipework,
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- Microstructural modelling to predict changes as a function of time and temperature in
service operation,
- Piping integrity assessment under realistic loading conditions by combined thermal
hydraulic system analysis and stress analysis tools.

Major development needs were:
- to establish knowledge of the base material in term of mechanical and microstructural
evolution during the service life,
- welding consumables with proven high temperature properties needed to be developed
and validated by creep rupture testing, microstructural and mechanical properties
assessment,
- to show that new steels and welds have improved resistance to the in-service bore
cracking phenomenon recently identified in existing CrMoV steel steam pipework.

These required comparison between the material properties after the manufacturing heat
treatment and after additional simulated service aging, the behaviour of the welded joint and the
comparison of low cycle fatigue crack initiation and growth testing, together with creep strain
rate, creep rupture and creep crack growth testing, on new and original pipework steels and
weldments. These tests have been carried out on experimental materials produced in full size
dimensions by industrial process routes.

It has also been necessary to model the effects of plant operational conditions and service
loadings on the integrity of welded pipework geometries and configurations using combined
thermal hydraulic system analysis and structural analysis. This modelling approach has been
successfully defined and verified by the laboratory full scale simulation tests.

In parallel, microstructural modelling and characterisation techniques have been employed to
predict microstructural changes during long term operation of high temperature plant. A large
number of tests have been carried out, and an extensive database of information for steelmakers
(tubes, pipes, plates, welding consumables) and end users has been generated. This approach and
the results obtained will be necessary for component design, manufacture, plant construction,
and maintenance and inspection of power and petrochemical plant.

The Consortium believes that a major increase of the knowledge of the new material has been
acquired, but that not all aspects have been completely clarified, such as the development of a
weld metal with sufficient creep ductility. The partners have gained much knowledge and
experience in the usability and life management aspects of these materials. This knowledge base
can successfully be applied in other projects to come. The acquired understanding of the effect of
the alloying elements in the consumables could be used for further consumable development and
in predicting the long term microstructural evolution. The developed material models will also be
the base for high temperature component simulation.

The results obtained in this project will make a small but relevant contribution to increasing plant
safety and efficiency, and thus towards achieving the Kyoto Protocol target for Europe of an 8%
reduction of CO emission by 2010.
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The main project deliverables for the Consortium partners are:

- A database on the materials properties of grade 23 and 24 steels and welded joints for high
temperature component design and structural integrity assessment. This includes strength,
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toughness, fatigue crack initiation and growth data, and minimum creep rate, creep rupture,
and creep crack growth rate data;

- New welding consumables for P23 steels designed to avoid the risk of premature high
temperature failure due to low creep ductility;

- Weld creep data assessment to improve capability for long term design and operation of
advanced high temperature plant using high and low alloy ferritic steels;

- Comparative high temperature fatigue data to enable optimum steel design and selection for
the prevention of in-service cracking in power plant which is required to operate flexibly and
is hence subject to transient loading;

- Modelling data on the effects of cyclic plant operation on component structural integrity and
the consequential requirements for design, materials selection and integrity assessment;

- Microstructural assessment of materials, weld metals and heat affected zone structures to
study the evolution of service aged and/or tested material and predict the microstructural
changes that will occur in high temperature steels. In conjunction with the materials
properties database, component modelling and assessment, this will provide a sound basis for
plant design, operation and maintenance to prevent the in-service failure of welded
components.

The project was organized in the following work-packages:
WP1: Characterisation of the ‘as-received’ steels (WP Leader: CSM)
WP2: Development of weldments (WP Leader: ISQ)
WP3: Mechanical data assessment (WP Leader: VTT)
WP4: Microstructural assessment (WP Leader: CSM)
WP5: High temperature design and assessment (WP Leader: E.On (ex Powergen))
WP6: Project management (WP Leader: CSM)

Each Work Package (WP) leader has successfully achieved the specific aim of their WP, and a
good network has developed not only between the representatives of each organisation but also
other researchers and technicians involved in the specific tasks. This network will be used by the
partners for further cooperation in the future. The exchanges of specific information and test
results will continue during coming years due to the long term tests still running for a
considerable amount of time.

Two meetings per year have been held, located on a rotating basis at the specific laboratories and
industrial facilities of all the partners, in order to improve the relationships between the different
partners involved in the main activity carried out in each semester.

The summary of the main activities carried out for each work-package and the relevant results
obtained are described below.

3.1 WP1: Characterisation of the “as-received “Steels
The main objectives of the WP1 were:
• The selection of test materials and plate/tube geometries,
• The microstructural characterisation of base material in normalised and tempered condition,
• The mechanical properties of base material including strength and toughness.

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The main conclusions of the work package 1 are following summarised.

The experimental materials for testing have been produced on industrial scale and successfully
qualified by conventional mechanical testing and metallographic characterisation. Grade 23 has
been selected for the main test programme.

3.2 WP2: Development of weldments

The main objectives of WP2 were:
• The development of welding consumables,
• The optimisation of welding procedure (welding technology and parameters),
• The qualification and simulation (using Gleeble testing) of welded joints
• The determination of mechanical properties of welded joints.

The long term performance of similar welded joints in the creep resistant steels is often life-
limiting in the design and operation of high temperature power and process plant. For the new
low alloy steels, long term creep data on welded joints are not currently available, and the
microstructural evolution of welds and base material is not well understood. It was necessary to
investigate these factors to develop design data, welding consumables and welding procedures to
minimise the risks of plant service failures at welds.

Two specific development requirements were addressed in the proposed project. First, problems
of poor weld creep ductility have been reported. Welding consumables with proven high
temperature properties therefore need to be developed and validated by creep rupture testing,
microstructural and mechanical properties assessment. Secondly, the new steels and welds must
be shown to have improved resistance to the in-service “bore cracking” phenomenon now
identified in existing UK CrMoV steam pipework. This requires comparative low cycle fatigue
crack initiation and growth testing, together with creep strain rate, creep rupture and creep crack
growth testing, on new and original pipework steels and weldments.

The main conclusions of the work package 2 are following summarised:
♦ Welding procedures have been developed and optimised for manufacture of thick section
P23 components. An extensive range of weld metal compositions have been investigated
and a comprehensive series of full scale test specimens have been produced.
♦ The overmatching, highly alloyed “state-of-the-art” P23 weld metal B323B, selected for
its high creep strength and used for the manufacture of long term high temperature test
specimens, proved to be a poor choice. This creep-brittle weld metal was found to be
liable to reheat cracking during PWHT of full scale thick section butt welds. The
occasional presence of pre-existing weld reheat defects in the test specimens therefore led
to a pattern of inconsistent and unreliable behaviour.
♦ B323B was deliberately chosen as a test of the viability of the high-strength formulation.
The negative result is valuable, in that it clarifies the pitfalls that can occur and indicates
a compositional range to be avoided.
♦ Parallel work using Gleeble weld thermal simulation together with the BWI tensile reheat
cracking test showed that the P23 weld HAZ is susceptible to reheat cracking, while
confirming that B323B weld metal is extremely susceptible.
♦ Later trials showed that alternative compositions, involving slight deviations from the
P23 specification, could be developed to produce substantially less creep-brittle weld
metals. These successfully survived PWHT without reheat cracking.
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