Study of the phenomenon of cracking during stress relief heat treatments in welded joints of quenched and tempered high-strength steels

Directorate-General For Research And Innovation European Commission

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Properties and service performance
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: 11
• Langue: English
• Poids de l'ouvrage: 4 Mo

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ISSN 1018-5593
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European Commission
technical steel research
Properties and service performance
Study of the phenomenon of cracking
during stress relief heat treatments
in welded joints of quenched and
tempered high-strength steels
STEEL RESEARCH European Commission
technical steel research
Properties and service performance
Study of the phenomenon of cracking
during stress relief heat treatments
in welded joints of quenched and
tempered high-strength steels
A. Vinckier, L. Jubin, A. Dhooge, P. Bourges
Lab. Soete voor Weerstand van Materialen en Lastechniek
c/o Belgish Instituut voor Lastechniek
St Pietersnieuwstraat 41
B-9000 Gent
Contract No 7210-KA/201
1 July 1989 to 30 June 1992
Final report
■ ^ü^^ '-h Directorate-General XII
Science, Research and Development
1996 EUR 15576 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 92-827-6166-5
© ECSC-EC-EAEC, Brussels · Luxembourg, 1996
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Luxembourg STUDY OF THE PHENOMENON OF CRACKING
DURING STRESS RELIEF HEAT TREATMENTS IN
WELDED JOINTS OF QUENCHED AND TEMPERED
HIGH STRENGTH STEELS.
Creusot-Loire Industrie, France
Belgian Welding Institute, Belgium
ECSC Agreement Numbers 7210-KA/201, 318 (F5.2/89)
FINAL TECHNICAL REPORT
SUMMARY
An extensive experimental research programme was jointly carried out by Creusot-Loire
Industrie and the Belgian Welding Institute on an industrial heat of a quenched and tempered
Vanadium alloyed high strength steel (E500) as well as on a comparable experimental heats
in order to evaluate the reheat cracking susceptibility and further to study the mechanism of
reheat cracking. Reheat cracking (RC) or stress relief cracking (SRC) is defined as intergranular
cracking along the prior austenite grain boundaries (PAGB) in the heat affected zone (HAZ)
or in the weld metal that occurs during the exposure of welded assemblies to the elevated
temperatures produced by postweld heat treatments (PWHT) or high temperature service. This
type of cracking in welded structures has received considerable attention since the mid-60's.
The widely accepted theories cannot explain the high susceptibility to reheat cracking in these
modern high strength low alloy steels with low Carbon content and of very high purity.
Isothermal tensile tests on weld simulated specimens, Crack Tip Opening Displacement tests
at high temperatures and Tekken tests were performed on this industrial heat as well as on ten
comparative experimental heats in which the content of Carbon, Vanadium and Copper was
slightly varied. A multipass submerged arc weld has further been realized in the 38 mm thick
steel E500 and tested in the as welded and stress relieved condition.
The investigated industrial steel E500 is found to be susceptible to reheat cracking. A clear
correlation was found between the applied test methods : Tekken tests and the isothermal tensile
tests on weld simulated specimens. The susceptibility can be decreased drastically by lowering
or eliminating Vanadium, this however to the expense of the room temperature strength. A
mechanism of reheat cracking is suggested based on grain boundary sliding in combination
with precipitation of extremely small and thin carbide platelets (Vanadium being particularly
deleterious) within the matrix. Measures and recommendations are given in order to avoid
reheat cracking in practice. Multipass welding resulting in a temper effect of the coarse grained
HAZ was found to be particularly beneficial.
Ill CONTENTS
Page
1. INTRODUCTION -j
2. TEST MATERIALS AND EXPERIMENTAL PROCEDURE 3
2.1. Test materials 3
2.2. Experimental procedures 4
3. EXPERIMENTAL RESULTS 6
3.1. Test results on industrial heat E500
3.2.ts on experimental heats 7
3.3. Test results on real submerged arc welded
joints (industrial steerE500)
4. MECHANISM OF REHEAT CRACKING 10
4.1. Identification of the process responsible for
the intercrystalline fracture above 500°C and
associated drop of reduction in area.
4.2. Effect of different microstructural features on
the reheat cracking susceptibility of simulated HAZ 12
4.3. Visualization of grain boundary sliding 15
4.4. Comparison between isothermal tensile test results
and Tekken test results 16
DISCUSSION8
5.1. Influence of the chemical composition
5.2. Mechanism of reheat cracking 20
6. CONCLUSIONS2
REFERENCES 23
TABLES4
FIGURES 36
IV LIST OF TABLES
TABLE 1 : Chemical composition (weight percent) of 38 mm thick testplate in steel
SE 500. Mechanical properties at room temperature (specimen taken out in
longitudinal direction at 1/4 thickness
Impact values (Transverse -1/4 thickness)
TABLE 2 : Mechanical properties of the simulated HAZ at room temperature
(Tp = 1350°C -18/5 = 30 s)
TABLE 3 : Chemical composition of the laboratory melts
TABLE 4 : Mechanical properties of the experimental heats
TABLE 5 : Influence of testing temperature (T) on the RA-value and UTS of an industrial
heat at strainrate: έ = 6. IO"4 s"1 (Tp = 1350°C, t^ = 30 s)
TABLE 6 : Influence of double cycles - First cycle: Tpl = 1350°C, t^ = 30 s,
Test temperature Τ = 600°C, strain rate έ = 6.10"4 s"1
TABLE 7 : Results of CTOD tests,on simulated specimens (Bx2B specimen)
TABLE 8 : Test results (reduction in area) on simulated HAZ (experimental heats)
TABLE 9 : Tekken test results
TABLE 10 : Parameters of SA welding of 38 mm thick plate (wire ESAB OK12.34,
0 4 mm, Flux 10.62)
Preheat/Interpass temperature: 100°C
TABLE 11 : Results of CTOD tests at 600°C of the coarse grained HAZ of K-weld
(BxB specimen)
TABLE 12 : Effect of tempering parameters on HAZ hardness HV10
TABLE 13 : Effect of g at 600°C on RA at 550°C of simulated coarse grained
HAZ with Tp = 1350°C, t^ = 30 s, strain rate έ = 6. IO"4 s"1
TABLE 14 : Effect of exposure to stress and 600°C on RA at 550°C, simulated coarse
grained HAZ, Tp = 1350°C, t^ = 30s, strainrate έ = 6.104 s"1
TABLE 15 : Influence of peak temperature (Tp) on the reduction in area and ultimate
tensile strength at 600°C, strain rate έ = 6.IO"4 s"1
TABLE 16 : Isothermal tensile test results. Test temperature: 600°C,
strain rate έ = 6.10"4 s"1
Influence of grain size
TABLE 17 : Influence of cooling time (t8/5) and holding at temperature - Test temperature:
600°C, strain rate έ = 6.10"4 s1
TABLE 18 : Effect of cooling time tg/5 on RA of simulated coarse grained HAZ at 600°C,
Tp = 1350°C, strain rate έ = 6.10"4 s"1
V LIST OF FIGURES
Figure 1 : Base material microstructure of an industrial heat E500
Figure 2 : TEM-picture of carbon extraction replica of etched sample - Base material
Figure 3 : Optical micrograph of simulated coarse grained HAZ
(Tp = 1350°C; tg/s = 30 s)
Figure 4 : TEM-picture of carbon extraction replica of etched sample - Coarse grained HAZ
Figure 5 : Isothermal tensile test specimen
Figure 6 : Isothermal tensile test: thermal and load cycle
t0 - start of the weld simulation
t, - start of the isothermal tensile test
Figure 7 : Effect oftest temperature on the reduction in area and the ultimate tensile strength
of simulated HAZ (Tp = 1350°C, t^ = 30s), strain rate έ = 6.10^ s"1
Figure 8 : Fracture surface of Specimen H48, strainrate έ = 6. IO"4 s-1,
test temperature 500°C, RA = 16%
Figure 9 : Fracture surface of Specimen H138, strainrate έ = 6.1Q4 s"\
test temperature 600°C, RA = 1%
Figure 10 : Fracture surface of specimen H203: strainrate έ = 6. IO"4 s"1, test temperature:
750°C,RA = 21%
Figure 11 : Microstructure of double cycled specimen, Peak temperature of second cycle
Tp2 =1050°C
Figure 12 : SEM-picture - Fracture surface of CTOD specimen H9 tested at 500°C
Figure 13 : SEM-picture - Fracture surface of CTOD specimen H14 tested at600°C
Figure 14 : Macrograph of SAW weldment in 38 mm thick plate
Figure 15 : Cross section over hot tensile test specimen WT3 tested at 600°C
Figure 16 : Detail of Figure 15 (location A) - Intergranular cracks in coarse grained HAZ
Figure 17 : Detail of Figure 15 (location B) - r cracks in coarsegrained HAZ
Figure 18 : Impact test results - Summary of data
VI Figure 19 : CTOD test at 600°C - specimen CB6. Load-clip gauge displacement curve
Figure 20 : Fractograph of the fracture surface of CTOD specimen CL02
Figure 21 : SEM picture - Fracture surface of CTOD specimen CL02
Figure 22 : Effect of strain rate on reduction in area of simulated HAZ
(Tp = 1350°C, t8/5 = 30 s)
Figure 23 : Influence of test temperature and strain rate on fracture mode
Figure 24 : TEM micrographs of the extraction