Properties and service performance

Directorate-General For Research And Innovation European Commission

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The fracture behaviour of welded joints in higher strength structural steels
Industrial research and development

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• Sciences et techniques

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 7 Mo

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ISSN 1018-5593
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European Commission
technical steel research
Properties and service performance
The fracture behaviour of welded joints in
higher strength structural steels
STEEL RESEARCH European Commission
Properties and service performance
The fracture behaviour of welded joints in
higher strength structural steels
P. Harrison
British Steel, Swinden Technology Centre
Moorgate
Rotherham S60 3AR
United Kingdom
Contract No 7210-KE/820
1 July 1989 to 30 June 1992
Final report
Directorate-General XII
Science, Research and Development
1996 EUR 15842 EN LEGAL NOTICE
Neither the European Commission nor any person acting on
behalf of the Commission 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, 1996
ISBN 92-827-6971-2
© ECSC-EC-EAEC, Brussels • Luxembourg, 1996
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Luxembourg THE FRACTURE BEHAVIOUR OF WELDED JOINTS IN HIGHER STRENGTH STRUCTURAL
STEELS
British Steel pic
ECSC Agreement No. 7210.KE/820
SUMMARY
When welded joints are made in conventional structural steels it is usual for the weld metal strength to
significantly overmatch the parent material strength. In higher strength structural steels it becomes
progressively more difficult to ensure a high degree of strength overmatch in the welded joint and this
leads to the possibility of strain concentration in the weld and HAZ regions at high applied loads. The
HAZ of structural steels is a region in which low levels of fracture toughness can exist and is also a
common location for welding defects. The combination of lowes and defects in a region
which may be subject to strain concentration can lead to poor fracture performance.
In the present work, the possibility of improving the fracture behaviour of welded joints in higher strength
structural steel was investigated in terms of varying both HAZ toughness and weld metal strength levels.
Titanium treated and titanium free steel were used to provide different levels of HAZ toughness and weld
metal strength levels were varied by alloying the weld metal with three levels of Mn, Ni and Mo. Butt
welds were made at 3.0 kJ/mm using a multipass SAW procedure and the HAZ fracture toughness was
assessed using Charpy "V', CTOD and wide plate test specimens.
The results have shown that HAZ toughness can be improved by the use of titanium treatment of the steel
or by PWHT. The fracture behaviour of the HAZ notched wide plate and CTOD tests was shown to be
influenced by weld metal strength mismatch resulting in a tendency for fracture deviation to occur away
from the weld metal for overmatched joints and towards the weld for evenmatched joints. Strain
concentration was observed to occur in the weld metal of the evenmatched welds and this appeared to
result in the development of higher CTOD in the HAZ for a given overall strain level. These high levels of
applied CTOD were not predicted by the CTOD design curve and this suggests that there could be
difficulties in predicting fracture behaviour of such welds using the present PD6493 procedures. The
results therefore suggest a clear benefit of overmatching strength weld metal in situation where high
levels of plastic strain are expected and where the HAZ fracture toughness is high. In situations where
loading was in the elastic region, there appeared to be little influence of weld strength mismatch on
applied CTOD and in this regime the overriding factor influencing fracture performance appears to be the
critical CTOD. Thus, for the best all round fracture performance it appears that the weld strength should
be overmatched and, the HAZ and weld metal CTOD should be high.
In addition to these observations the wide plate and CTOD fracture behaviour appeared to be strongly
influenced by the probabilistic nature of cleavage fracture in the HAZ resulting in variable behaviour and
there was an indication, that the minimum critical CTOD observed in wide plate tests could be lower than
that observed in surface notched CTOD specimens. This led to an inaccurate defect acceptability
prediction using PD6493 Level 2 in one of the wide plate tests.
The Charpy, CTOD and wide plate tests conducted in the present investigation failed to rank the various
weld conditions in the same order and this highlights the practical difficulties in choice of the type and
number of HAZ tests to conduct in order to predict full scale behaviour. Analysis of the data using
probabilistic fracture assessment techniques proved encouraging, but the results suggest that further
work is required in the areas of accurate definitions of HAZ fracture toughness distributions and the
relationship between wide plate and SENB CTOD.
Ill CONTENTS PAGE
1. INTRODUCTION 1
2. OBJECTIVES OF THE RESEARCH 2
3. EXPERIMENTAL PROCEDURE
3.1 Materials
3.2 Welding Procedure
3.3 Specimen Manufacture and Identification
3.4 Tensile Testing 3
3.5 Charpyg
3.6 CTOD Testing
3.7 Wide Plate Testing
3.8 Metallography and Hardness Testing 4
4. RESULTS
4.1 Tensile Tests
4.2 Charpys 5
4.3 Hardness Tests - 5
4.4 CTOD Tests 6
4.5 Wide Plate Tests 8
5. DISCUSSION 10
5.1 Relationship between Strain and CTOD in Wide Plate Tests 1
5.2 Comparison of Results with the CTOD Design Curve 11
5.3 Analyses of Results using PD6493:1991 and First Order 12
Reliability Methods (Form)
5.4 Effect of Weld Metal Strength and HAZ Toughness on Wide6
Plate Fracture Behaviour
5.5 Comparison of Small and Large Scale Test Behaviour 17
6. CONCLUSIONS 18
REFERENCES9
TABLES 21
FIGURES 3
APPENDICES 55
IV LIST OF TABLES
1. Chemical Composition and Identification of Steels
2. Tensile and Charpy Properties of Steels
3. Average Tensile Properties at -10°C
4. Summary of CTOD Results for the Non-Ti Steel (PWHT)
5.y of CTOD Results for the Non-Ti Steel (AW)
6. Summary of CTOD Results for the Ti-Steel (AW)
7.y of Wide Plate Test Results
8. Summary of Wide Plate Test Data Used for PD6493 Analyses
9.y of PD6493 Level 1 and 2 Results
10. Summary of Wide Plate and CTOD Parameters used for Failure Probability Analyses
LIST OF APPENDICES
1. Test Matrix, Weld Identities, Welding Procedure and Cutting and Notching Details
2. Wide Plate Testing Procedure
3. Tensile and Charpy Data
4. Metallographic Sectioning of CTOD Specimens and Weibull Distributions for HAZ CTOD
Data
5.c Sectioning of Wide Plate Specimens
V LIST OF FIGURES
1. Comparison of Lower Bound HAZ Charpy Transition Curves for the Three Welding
Conditions Under Investigation
2. Hardness Traverse for the Non-Ti Steel in the As-Welded Condition (Weld E)
3.s Traverse for the Non-Ti Steel in the PWHT Condition (Weld A)
4. Hardness Traverse for the Ti-Steel in the As-Welded Condition (Weld G)
5. Distribution of CTOD Results for the 3 Conditions Examined
6. HAZ CTOD Results Plotted as Weibull Distributions
7. Gross Section Stress v CTOD and Gross Section Stress v Strain for Different Regions (Non-Ti
Steel, PWHT Condition, Weld Metal Strength Matched Condition)
8.s Section Stress v CTOD and Gross Section Stress v Strain for Different Regions (Non-Ti
Steel, AW Condition, Weld Metal Strength Overmatched Condition)
9. Gross Section Stress v CTOD and Gross Section Stress v Strain for Different Regions (Ti-
Steel, AW Condition, Weld Metal Strength Overmatched Condition)
10. Gross Section Stress v CTOD and Gross Section Stress v Strain for Different Regions (Ti-
Steel, AW Conditions, Weld Metal Strength Overmatched Condition)
11. Average Weld Metal Strain v Average Overall Strain and Average Parent Plate Strain v
Average Overall Strain observed in Wide Plate Tests
12. Relationship between Average Overall Strain and Local Strains Around Tip of Defect (Non-
Ti Steel, PWHT Condition - Strength Matching Series) - Plate 'B'
13.p between Average Overall Strain and Local Strains Around Tip of Defect (Non-
Ti Steel, AW Condition - Strength Overmatching Series) - Plate 'D'
14. Relationship between Average Overall strain and Local Strains Around Tip of Defect (Ti-
Steel, AW Condition - Strength Overmatching Condition) - Plate 'G'
15.p between CTOD and Strain for Various Locations within Wide Plate T3' (Non-Ti
Steel, PWHT Condition - Strength Matching Series)
16. Relationship between CTOD and Strain for Various Locations within Wide Plate 'D' (Non-Ti
Steel, AW Condition - Strength Overmatching Series)
17.p between CTOD and Strain for Various Locations within Wide Plate 'H' (Ti-
Steel, AW Condition - Strength Overmatching Condition)
18. Relationship between CTOD and Strain for Various Locations within Wide Plate 'G' (Ti-
Steel, AW Condition - Strength Overmatching Condition)
19.p between CTOD and Average Strain for Surface Notched Wide Plate Tests with
Different Weld Metal Strengths
20. Comparison of Wide Plate Results with the CTOD Design Curve
VI 21. Assessment of Results According to the PD6493