Optimisation of the creep behaviour of welded components in modified 9% Cr

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 54
• Langue: English
• Poids de l'ouvrage: 3 Mo

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ISSN 1018-5593
EURO PEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
technical steel research
Properties and ¡η-service performance
Optimisation of the
creep behaviour of
welded components in
modified 9% Cr
Report h
EUR 17895 EN
STEEL RESEARCH EUROPEAN COMMISSION
Edith CRESSON, Member of the Commission
responsible for research, innovation, education, training and youth
DG XII/C.2 — RTD — Actions: Industrial and materials technologies
Materials and steel
Contact: Mr J.-L. Manin
Address: European Commission, rue de la Loi 200 (MO 75 1/10),
B-1049 Bruxelles — Tel. [phone]; fax [phone] European Commission
technical steel research
Properties and in-service performance
Optimisation of the creep behaviour of
welded components in modified 9% Cr
C. Coussement
Belgian Welding Institute
Lakenweverstraat, 21
B-1050 Bruxelles
J.C. van Wortel
ΤΝΟ-ΜΙ
Laan van Westenenk, 501
NL-7300 AM Apeldoorn
Contracts Nos 7210-MA/601 and MA/202
1 June 1992 to 30 September 1994
Final report
Directorate-General
Science, Research and Development
1998 EUR 17895 EN LEGAL NOTICE
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Luxembourg: Office for Official Publications of the European Communities, 1998
ISBN 92-828-3422-0
© European Communities, 1998
Reproduction is authorised provided the source is acknowledged.
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PRINTED ON WHITE CHLORINE-FREE PAPER LIST OF CONTENTS
I. Summary of the R&D Project 5
Zusammenfassung des Forschungs- und Entwicklungsprojektes 9
Résumé du projet R&D 13
IL Objectives of the R&D project7
III. Introduction
IV. Approach-Description of work8
V. Materials used and condition9
VI. Experimental work - Research results 20
VI.l. Modified 9% Cr base materials testing
VI.2. Weldability studies on modified 9% Cr steels2
VI.3. Welding of modified 9% Cr steels6
VI.4. Testing of the modified 9% Cr weldments9
VII. Discussion of the results 38
VIII. Conclusions 43
IX. References - Tables - Figures7
IX. 1. References
IX.2. Tables 51
IX.3. Figures 7OPTIMIZATION OF THE CREEP BEHAVIOUR OF
WELDED COMPONENTS IN MODIFIED 9% Cr"
FINAL REPORT
Period: 1st October 1992 - 30th September 1994
I. SUMMARY OF THE R&D PROJECT
This final report describes the work performed by the end of the project, covering the period
01.10.92-30.09.94, by the Belgian Welding Institute (BWI) and the Metals Research Institute
(TNO-MI), and their subcontractors, on the optimization of the creep behaviour of welded
components in modified 9% Cr.
Three different modified 9% Cr steels from different suppliers were incorporated in this
research project, i.e., BWI used a 20 mm thick GR91 plate material and a thickwalled P91 pipe
material with dimensions φ0 320 χ 36 mm, while TNO-MI used a P91 pipe material with
dimensions φ0 159 χ 20 mm.
At first, the used modified 9% Cr base materials have been fully characterized in order to
verify their chemical composition, their tensile properties at room temperature, their
microstructure and hardness and their mechanical (creep) properties at high temperatures. All
base materials were found to meet fully the ASTM/ASME requirements concerning analyses,
strength and ductility, and moreover they all exhibited an excellent high toughness. Their
microstructure consisted of tempered martensite, although in the plate material some delta-ferrite
was present at mid-thickness due to the manufacturing process (i.e., segregation effects in the
continuous casting). Initial short term uniaxial creep testing indicated that the selected modified
9% Cr steels possessed adequate creep strength, and were thus suitable for use in the test
program. Longer term creep tests on these base materials confirmed this finding.
The preliminary weldability study in the laboratory, using weld simulation and reheat
cracking tests, showed that cracking nor toughness problems were to be expected in the heat
affected zone of the examined modified 9% Cr steels, and this for a wide range of heat inputs,
preheat- and interpass temperatures and postweld heat treatment conditions. Only, a narrow
region in the coarse grained HAZ showed a lower toughness, especially after double cycling with
peak temperatures in the range 750-850 °C. A higher PWHT was found to improve toughness in
this region. The weldability study allowed to select the optimum welding parameters for the
GR91 plate and the P91 pipe, bearing in mind that these represent a compromise between
toughness and creep properties. The effect of the weld thermal cycle on the base metals'
microstructures has been evaluated by detailed microstructural investigations (including
transmission electron microscopic examinations) and hardness measurements.
Welding of the various modified 9% Cr base materials was performed using different
conventional welding processes, such as GTAW, SMAW and SAW and no welding problems
were encountered. Different postweld heat treatment have been applied on these weldments. Most
base materials have been welded in the as-received condition, although some weldments were
also performed on reheat treated (different austenitizing treatments) base material. In addition to the use of the conventional welding processes, the feasibility of the more uncommon process of
electron beam welding (EBW) for welding modified 9% Cr plate and pipe was demonstrated after
several experimental welding trials, which allowed to establish the optimum electron beam
welding parameters for making real butt welds in the GR91 plate and girth welds in the
thickwalled P91 pipe.
The realized modified 9% Cr plate and pipe weldments were subjected to various
mechanical and microstructural investigations, including tensile, bend and impact testing at room
temperature and hardness testing. The SM AW and SAW test welds in the GR91 plates initially
yielded low toughnesses, which mainly proved to be linked to the absence of Ni in that specific
weld filler metal used. It proved nevertheless possible to raise the low toughness to an acceptable
level by applying a new, more appropriate PWHT at higher temperatures, so that rewelding with
another filler metal containing up to 1 % Ni (as is now common practice) was not considered
necessary.
The EB weldments in the GR91 plate showed quite high toughness in the weld pool after
application of the initially selected postweld heat treatment. This was however not the case for
the EB weldments in the P91 pipe, where weld pool toughness was unacceptably low and another
PWHT would have been preferable from the viewpoint of toughness.
In comparison to conventional NDE techniques, optimization and evaluation of advanced
computerized ultrasonic NDE techniques for the detection and sizing of weld defects and the
evaluation of quality of the welded joints was carried out. The calibrations and evaluations were
mainly made on the accidental cold cracks which had occurred in the auxiliary plate/clamp welds
used for assembling the GR91 plate halves to be joined by SAW. The feasibility of the P-scan
technique for the detection and sizing of defects has been demonstrated. Moreover, the tests
performed illustrated that there is no unique sizing method and that the sizing accuracy of a
particular defect is extremely dependant on the use of several methods and on the engineering
judgment and expertise. Specifically, the use of SuperSAFT proved to be the most effective
technique for accurately determining the size of the kind of detected defects (shallow cracks with
a curved geometry). The procedure allows to size cracks over 0,8 mm with a fairly good accuracy,
except for those lying near the border of the plate.
Since the main focus of the project was on optimizing the creep behaviour of modified
9% Cr weldments, the realized test welds have been subjected to an extensive creep testing
programme. Most of this creep testing was performed uniaxially, but also a large multiaxial creep
test on a welded component (cylinder with two welded T-branch connections) has been carried
out.
For the uniaxial creep testing programme, different pre- and post-weld heat treatments have
been applied for verifying how the creep behaviour of the modified 9% Cr welded joints could
be optimized. Although the results indicated that the so-called halftempering procedure of the
base material before welding was giving a somewhat improved creep behaviour of the weldments,
full restoration of the softer fine grained HAZ's poorer creep properties could only be obtained
with a complete new heat treatment after welding, consisting of re-austenitization and tempering.
The influence of the post-weld heat temperature on the creep rupture life was negligible, as was
the use of a high austenitizing temperature of the base metal before welding. As to the influence
of the