Mechanical Properties of Heat-Affected Zone of High-Strength Steel Welds.
Hajro, Ismar ; Hodzic, Damir ; Tasic, Petar 等
Mechanical Properties of Heat-Affected Zone of High-Strength Steel Welds.
1. Introduction
Use of high-strength steels (HSS) in various structural application
is beneficial due to the various reasons, while most of them are related
generally to provision of lighter and energy saving structures [1, 2].
While there is no some precise definition of HSS, they can be considered
as one having yield strength equal or higher than 420 MPa. For the
purpose of this paper two HSS are considered, in the range of nominal
yield strength from 690 to 890 MPa, delivered in quenched and tempered
(QT) condition.
Beside challenges which may be faced with application of various
manufacturing technologies to HSS, such as banding or cutting; an
application of welding technologies may become a particular challenge.
This is mainly due to the two reasons: (1) loss of toughness; and (2)
loss of crack resistance. Both problems are well known and particularly
related to cooling time, [t.sub.8/5] [s]; which is principally opposite
to a cooling rate, [v.sub.c] [[degrees]C/s]. Actually, if fusion welding
of HSS is performed with fast cooling rates (e.g. low [t.sub.8/5]),
there is a risk of microstructure hardening and further loss of cold
crack resistance (e.g. hydrogen induced crack). Contrary, if welding is
characterised with high [t.sub.8/5], or slow cooling rates; there is a
risk of microstructural grain coarsening within heat-affected zone (HAZ;
Fig. 1a), following significant loss of toughness. Such generalised
concept is well known, and it is one of primary concerns while welding
HSS. Therefore, it is important to follow recommended [t.sub.8/5]; which
is mostly found within HSS manufacturer's specification. In
addition, according to EN 1011-2, general recommendation is
[t.sub.8/5]=10-25 s [2, 3]; while manufacturers recommendations for
S690QL and S890QL are [t.sub.8/5]=5-15(20) s [7, 8].
Another concern may be related to real welds, which must be
qualified by standard procedures (e.g. according to ISO 15614-1 or ASME
BPVC Sec.IX; etc.); which consist of standardised testing of mechanical
properties, including surface and volumetric non-destructive tests
(NDT). The qualification means that predicted welding technology
parameters are set well to finally provide sound and acceptable welded
joints in production. Thus, a test parts of sufficient size are welded,
following the sampling of test specimens for further mechanical and
technological testing [4, 5].
Somehow, such requirements may be achieved; and overall welding
technology may become qualified. However, such approach may not evaluate
the well-known weakest zone of welded joint--coarse-grained heat
affected zone (CG-HAZ). It is a zone, where, during fusion welding,
maximum (peak) temperature(s) close and over 1300[degrees]C is reached
(Fig. 1b). On such high temperatures heat-affected zone microstructure
tends to overgrow [6, 9], following formation of a quite large grains,
which finally has for consequence significant drop in toughness. Here,
importance of sufficient toughness become particularly important,
because of sufficient resistance to crack initiation and growth [2, 6,
10].
Therefore, even one welding technology (e.g. procedure) may become
qualified in well-known and standardized manner [4, 5], the process of
qualification cannot evaluate the weakest welded joint zone--CG-HAZ.
Regarding its size, in comparison to complete welded joint, weakening
within CG-HAZ should not be neglected easily. However, even everyday
welding engineering practice may not relay on sophisticated approaches
and techniques, such as simulation of welding--thermo cycles; a more
detailed investigation is required, at least to provide general sense of
a weakening effect.
The following experimental procedure, as well as provided analysis,
is one [6] of a possible approaches [10] to gain reasonable sense about
intensity and distribution of a welded joints weakening within HAZ /
CG-HAZ.
2. Performed experiment
Set of welded joints were performed using gas metal arc welding
(GMAW) on two HSS, S690QL and S890QL, both delivered in accordance to EN
10025-6. The general welding parameters are shown on Tab. 1, while the
appearance of welded joints macro-sections are shown on Fig. 2.
Beside real welds on both HSS, a sufficient number of specimens,
10x10x50 mm from both HSS were prepared for welding--thermo cycle
simulation on thermo-mechanical simulator "SmithWeld".
Those specimens were simulated with previously calculated
thermos-cycles, using application "Thermocycle t85" (according
to welding parameters shown on Tab. 1; e.g. preheat temperature and
t8/5), which correspond to CG-HAZ, e.g. with peak temperature 1300
[degrees]C. Appearance of samples before, during and after welding
thermo-cycle simulation are shown on Fig. 3.
The simulated samples were used for testing of hardness and impact
toughness; while they actually represent the weakest zone of welded
joint within heat-affected zone (HAZ).
Sampling of test specimens for tensile and impact toughness are
taken as shown on Fig. 4, including spots for hardness testing on welded
joints macro-sections.
All predicted mechanical testing were performed at room
temperature, e.g. 20[degrees]C; and in accordance to:
* EN 10002-1 and EN 895, for tensile testing of base metal (BM) and
weld metal (WM).
* EN 10045-1 for impact toughness (or general toughness) testing of
base metal (BM), heat-affected zone (HAZ) and weld metal (WM).
* EN 1043-1 for hardness testing HV10 (Vickers method) of welded
joints macro-section (including all three zones: BM, HAZ and WM).
3. Acquisition of mechanical properties
Tab. 2 and 3 shows acquired experimental results, i.e. general
ranges, of tested mechanical properties for both HSS; while only the
shaded cells values were calculated based on relationship provided in
Fig. 5.
Obviously, from Tab. 2 and 3; high hardness has for consequence
high strength and low toughness (as well as ductility); and vice-versa.
Therefore, while considering generalised linear theoretical relationship
[6] between hardness (HV10), strength ([R.sub.m]) and toughness (KV);
the following relationship (regression) analysis were performed (Fig.
5).
4. Results analysis
Based on results, for Rm, HV10 and KV, provided in Tab. 2 and 3,
the analysis (Fig. 6) of mechanical properties distribution is performed
across one side of welded joint (e.g. right-hand of macro-section).
However, it is reasonable to predict symmetry of mechanical properties
distribution perpendicular to the butt welded joint axis.
Distance in [mm] of each zone (CG-HAZ, HAZ) from welded joint
centre-line (e.g. c/l; which correspond to weld metal zone, WM) is
determined from macro-section measurement (Fig. 2a and 2b). Actually, if
considered on mid-thickness; the distances from WM (c/l) of CG-HAZ and
HAZ are approximately 6mm and 8mm for S690QL; 5mm and 7mm for S890QL;
respectively.
The distance between HAZ (rest of HAZ with peak temperatures less
than 1100[degrees]C) and CG-HAZ is based on assumption that temperature
distribution (according to Fig. 1b) is linear within HAZ. Actually, the
width of CG-HAZ is 1,5-2,0mm on both HSS welded joints; based on
theoretical assumption [6] that CG-HAZ is located between
1100-1500[degrees]C (Fig. 1b), with its peak on the 1300[degrees]C.
Finally, according to design requirements [4,5], that reference
value for further comparison analysis are properties of a base metal
(BM); than we can conclude mismatching of mechanical properties as shown
in Tab. 4.
Simply, undermatching (grey shaded cells in Tab. 4) may be
considered as weakening of particular welded joint zone relative to the
base metal; e.g. regarding strength, the weld metal (WM) is weaker in
comparison to the base metal (BM) for 12% in the case of S690QL steel
welded joint; or 4% weaker in the case of S890QL steel.
5. Conclusion
A problem of welded joint mechanical properties distribution,
regarding level of mismatching effect, in real welds is present and it
cannot be evaluated using standardised qualification procedures.
Therefore, more detailed and sophisticated approach is required, at
least by mean of preparation of additional specimens using simulation of
welding--thermo cycle(s) of theoretically known weakest zone--coarse
grained heat affected zone (CG-HAZ).
The presented investigation show one limited approach, mostly
regarding selected high-strength steel(s) (its nominal strength values),
and simulation used only for further evaluation of hardness and
toughness. Also, it is possible to use presented simulation technique
(as a mean of specimen's preparation) for any further mechanical
testing; such as for strength testing which is not done within this
research. For the purpose of this investigation, the strength of
simulated CG-HAZ was recalculated (Tab. 2 and 3) based on the mechanical
properties relationship analysis (Fig 5).
However, the provided investigation show clearly that the higher is
strength of one high-strength steel (HSS) the lower ductility and
toughness are. Beside that; welding may influence significantly a
mechanical properties of welded joint.
Generally speaking, mismatching of properties are always present;
but further important question arise: how intense mismatching effect is
in major welded joint zones?! However, from a point of design
requirements, a particular attention should be drawn to mismatching of
hardness and toughness.
Thus, in the case of investigated high-strength steel's (HSS)
GMAW welded joints, with the nominal strength in the range of 690-890
MPa; there is a significant undermatching of toughness; as well as
overmatching of hardness; within heat-affected zone (HAZ).
The hardness overmatching goes from +(14-52)%, with its maximum
value within coarse-grained heat-affected zone (CG-HAZ). Contrary, the
toughness undermatching goes from -(7-63)%, with its maximum weakening
within coarse-grained heat-affected zone (CG-HAZ).
Such degraded toughness may have for consequence notable loss of
welded joint crack resistance. In addition, while so weakened zone of
welded joint represent up to 40% of complete heat-affected zone (HAZ)
width, it should not be neglected easily.
Finally, it is suggested that further studies and investigation(s)
include wider range of structural steels; for example from 420 MPa, up
to 1300 MPa; and more use of simulation technique. Actually, as long as
simulation of welding--thermo cycles can be used for any peak
temperature within HAZ (not only for CG-HAZ, 1300[degrees]C), it can
provide more precise analysis of mechanical properties distribution.
DOI: 10.2507/28th.daaam.proceedings.086
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Caption: Fig. 1. Welded joint zones, including CG-HAZ regarding
peak temperature distribution
Caption: Fig. 2. Appearance of welded joints macro-sections [6]
Caption: Fig. 3. Appearance of simulated specimens [6]
Caption: Fig. 4. Approximate sampling location for mechanical
testing of real welded joints
Caption: Fig. 5. Mechanical properties relationship(s)
Caption: Fig. 6. Distribution of mechanical properties across
welded joints macro-sections
Table 1. General welding parameters
Welding
HSS process Thickness Preheating [t.sub.8/5]
S690QL GMAW 30 mm 200[degrees]C 6-8 s
S890QL GMAW 20 mm 150[degrees]C 6-7 s
HSS Filler Metal acc. to EN 12534
S690QL G 69 5 M Mn 3 Ni 1 Cr Mo
S890QL G 89 6 M Mn 4 Ni 2 Cr Mo
Table 2. Acquired mechanical properties for HSS, S690QL [6]
Tested welded joint zone
Mechanical
property BM HAZ CG-HAZ WM
Strength / Yield 745-780 -- -- 695-720
stress, [R.sub.p0,2]
[MPa]
Strength / Tensile 857-898 1020 1193 770-779
strength, [R.sub.m]
[MPa]
Ductility, A [-] 0,133-0,159 -- -- 0,144-0,164
Hardness, HV10 262-283 270-421 413 254-274
Impact toughness, KV 184-212 184-187 85-94 114-171
[J] @ 20[degrees]C
Table 3. Acquired mechanical properties for HSS, S890QL [6]
Tested welded joint zone
Mechanical
property BM HAZ CG-HAZ WM
Strength / Yield 935-962 -- -- 878-915
stress, [R.sub.p0,2]
[MPa]
Strength / Tensile 1007-1024 1129 1247 963-984
strength, [R.sub.m]
[MPa]
Ductility, A [-] 0,121-0,131 -- -- 0,138-0,157
Hardness, HV10 327-351 322-455 434 317-351
Impact toughness, KV 147-168 135-145 54-59 99-108
[J] @ 20[degrees]C
Table 4. Mismatch of mechanical properties across welded joint
Mismatch of mechanical properties in comparison
to base metal (BM)
Mechanical
property BM HAZ CG-HAZ WM
Strength (overmatching) (overmatching) (undermatching)
1 1,16 for S690QL 1,36 for S690QL 0,88 for S690QL
1,11 for S890QL 1,23 for S890QL 0,96 for S890QL
Hardness (overmatching) (overmatching) (undermatching)
1 1,27 for S690QL 1,52 for S690QL 0.97 for S690QL
1.14 for S890QL 1.28 for S890QL 0.98 for S890QL
Toughness 1 (undermatching) (undermatching) (undermatching)
0.93 for S690QL 0.45 for S690QL 0.72 for S690QL
0.89 for S890QL 0.37 for S890QL 0.66 for S890QL
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