Improved performance of welded high-temperature steels

European Commission Directorate-General For The Information Society And Media Directorate-General For Research And Innovation

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Properties and service performance
Industrial research and development

Informations

• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 8
• Langue: English
• Poids de l'ouvrage: 5 Mo

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* *
Commission of the European Communities
technical steel research
Properties and service performance
Improved performance
of welded high-temperature steels J>
Commission of the European Communities
technical steel researcl
Properties and service performance
Improved performance
of welded high-temperature steels
P. R. Burke
British Steel pic
9 Albert Embankment
London SE1 7SN
United Kingdom
Contract No 7210-KF/803
(1 August 1986 to 31 July 1989)
Final report
PAFL E;;Î?0P. ßiblioth;
Directorate-General
Science, Research and Development N.c.V cc M 35^153",
1991 £¡7TUR133U3EÑ*
ÎYKÙ TÖ to ^ T" Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1991
ISBN 92-826-2365-3 Catalogue number: CD-NA-13303-EN-C
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1991
Printed in Belgium IMPROVED PERFORMANCE OF WELDED HIGH TEMPERATURE STEELS
SUMMARY
Metallurgical failures of high temperature plant are often associated with the heat affected zone (HAZ) of
weldments and usually occur by a creep mechanism. The two common modes of failure in low alloy,
ferritic steel, steam pipelines are Type III and Type IV cracking, which are located in the high temperature
region of the HAZ and in the intercritical, low temperature region of the HAZ, respectively. The features
and mechanisms of Type III cracking are well established and it is known that this failure mode frequently
occurs during post weld stress relief heat treatment due to the exhaustion of creep ductility in weakened
grain boundary regions. Type III cracking is exacerbated by the presence of grain boundary particles
which nucleate creep cavities. Microstructural solutions to the problem therefore involve refinement of
the grain size in this region of the HAZ and/or removal of particles, notably MnS, from the prior austenite
grain boundaries.
Measures to reduce Type III cracking are likely to change the main failure mode in low alloy steel, steam
pipelines to that of Type IV cracking. The Central Electricity Generating Board (CEGB) has developed,
and currently use, low heat input welding procedures which, by restricting grain growth, can eliminate
the problem of Type III cracking. As a consequence, the majority of plant failures occur by Type IV
cracking at times midway through component life (~50000 h).
The present investigation was designed to examine the effects of TiN grain refinement and rare earth
modification of sulphides on the high temperature creep properties of jCriMo^V and 2ţCrlMo steels, in
the normalised and welded condition, using both cross-weld and HAZ simulated samples. Two low
phosphorus casts (0.007% P) were also included in the study to examine the reported effect of this element
on rupture ductility. The rare earth treated 2^CrMo steels also contained high levels of residual elements
(Sb, Sn, As) to investigate the reported segregation of these elements to rare earth inclusions.
Titanium additions of—0.01 and —0.025% with nitrogen levelsaround0.009%producedsignificantresistance
to grain coarsening during high temperature heat treatment and in the HAZ of actual welds in both the
^CrMoV and 2^CrMo steels. This indicates that commercial fabrication of steam pipelines could be
carried out using lower cost, high heat input welding procedures without the risk of developing a coarse
grained HAZ.
In the iCrMoV steels, titanium treatment generally increased the rupture strength in both normalised
and tempered and welded conditions. The majority of cross-weld failures occurred by Type IV cracking and
had rupture lives significantly below the ISO mean data line. The two low phosphorus casts had reduced
rupture strengths but increased rupture ductilities.
Titanium had the opposite effect in the 2±CrMo steels, causing a reduction in the creep rupture strength in
normalised and tempered and welded material. This is probably due to titanium reducing the free
nitrogen available for solid solution strengthening. As with the ^CrMoV steel, the majority of cross-weld
failures occurred by Type IV cracking with rupture lives significantly below the ISO relationship.
Lowering the phosphorus level had little effect on creep rupture strength but improved the ductility at
failure.
Cerium additions, up to 0.1%, to the ^CrMoV steel produced a significant increase in rupture ductility
without any loss of rupture strength, following 950°C normalising and tempering. The rupture ductility in
the base steel after 10000 h service was -5% R of A, whereas in the 0.1% Ce steel the ductility was -45%. The
rupture strength for 10000 h was -20% higher than the ISO mean line.
Ill In the high residual element (Sb, Sn, As), 2}CrMo steels, rare earth treatment resulted in the modification
of sulphide inclusions. At low cerium levels only rare earth sulphides were formed, whereas at higher
levels rare earth sulphides, oxides and oxy-sulphides were produced. Associated with the oxides and oxy-
sulphides were high levels of antimony and arsenic, which together with the removal of grain boundary
MnS particles, resulted in enhanced rupture ductilities without any loss in rupture strength, in the 950°C
normalised and the high temperature HAZ simulated conditions. A combined addition of 0.1% Ti and
0.07% Ce produced a very significant increase in rupture ductility to over 70% R of A for all test conditions,
in the high temperature HAZ simulated material.
Recommendations have been made for the further study of the effects of titanium and/or cerium on the
high temperature properties of welded ^CrMoV and 2}CrMo steels.
- IV CONTENTS PAGE
1.INTRODUCTION1
2.EXPERIMENTAL PROCEDURE 2
2.1 Materials2
2.2 ExperimentalWeldFabrication3
2.3 HAZ Simulation3
2.4 Creep Testing4
2.5 OpticalMetallography4
2.6 Electron y4
2.7 TiN Particle Coarsening5
3. EXPERIMENTAL RESULTS 5
3.1 MetallographyandRoomTemperature Properties 5
3.2 Creep RuptureCharacteristics7
3.3 TiN ParticleCoarsening10
3.4 Grain Coarsening10
3.5 Micro-AnalysisofCeriumSteels11
3.6 AnalysisofBoronLevelsUsingSIMS11
4. DISCUSSION 11
4.1EffectofTitaniumon Microstructures and Room Temperature
Propertiesof*CrMo V Steels11
4.2GrainCoarseningin Titanium-Treated Steels 12
4.3CreepRuptureind *CrMoV Steels14
4.4CreepRuptureinCerium Treated^CrMoVSteels16
4.5CreepeinTitanium d2ţCrMoSteels17
4.6CreepRuptureinCerium TreatedoSteelswith
High Residual Element Contents 19
5. SUMMARY AND CONCLUSIONS21
5.1 Effect of Titanium onMicrostructureandGrain Refinement 21
5.2 Creep Properties of iCrMoVSteels21
5.3 Creep Properties of 2*CrMo Steels22
6. RECOMMENDATIONS 23
7.ACKNOWLEDGEMENTS23
8. REFERENCES23
TABLES27
FIGURES 48 LIST OF TABLES
1. Chemical Composition, Wt. %
2.Mechanical Properties and Grain Size of £CrMoV Steels Normalised from 950°C
3. l Properties and Grain Size of 2|CrlMo Steels Following Normalising from 950°C
and Tempering for 4 h at 700°C
4. Mechanical Properties and Grain Size of |CrMoV Steels Austenitised for 5 min at 1300°C,
Air Cooled and Tempered for 45 min at 680°C
5. Mechanical Properties and Grain Size in 2ţCrMo Steels Following Austenitising at 1300CC
for 5 min, Air Cooling and Tempering
6. Influence of Titanium on Coarse Grain HAZ Grain Size in £CrMoV Single Pass Weld-on-Bar
Samples
7. Analysis of Vanadium Rich Precipitates in -fcCrMoV Steels
8.Influence of Titanium on Coarse Grain HAZ Grain Size in 2-JCrMo Single Pass Weld-on-Bar
Samples
9. Stress Rupture Data - ^Cr^Mo^V Steels with Ti Additions - Normalised from 950°C and
Tempered at 700°C for 4 h
10. Stress Rupture Data - iCr^MoţV Steels with Ti Additions - High Temperature HAZ
Simulated Material, Tempered at 680°C for 45 min
11. Stress Rupture Data - ^Crj-Mo^V Steels with Ti Additions - Intercritical HAZ Simulated
Material, Tempered at 680°C for 45 min
12. Stress Rupture Data - iCr£Mo±V Steels with Ti Additions - Cross Weld Low Heat Input
Samples, Stress Relieved at 680°C for 45 min
13. Stress Rupture Data - Cerium Treated ^Cr^MotV Steels Normalised from 950°C and
Tempered at 700°C Tempered at 680°C for 45 min
14. Stress Rupture Data - 2|CrlMo Steels with Ti Additions Normalised from 950CC and
Tempered at 700°C for 4 h
15. Stress Rupture Data - 2ţCrlMo Steels with Ti Additions High Temperature HAZ Simulated
Material, 1300°C for 5 mins, Tempered at 680°C for 45 min
16. Stress Rupture Data - 2^CrlMo Steels with Ti Additions - Intercritical HAZ Simulation Heat
Treatment, Tempered at 680°C for 45 min
17. Stress Rupture Data - 2^CrlMo Steels with Ti Additions - Cross Weld Low Heat Input
Samples, Stress Relieved at 680°C for 45 min (See Report 1)
18.