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The significance of softened HAZs in high strength structural steels


H G Pisarski and R E Dolby 

Paper printed in Welding in the World, Vol 47, No 5/6, 2003 pp.32-40 (May/June 2003).


The development of lean alloyed high strength steels of 500-800 MPa yield strength has shown that conventional welding processes and procedures can result in significant HAZ softening. Previous work on the significance of softened HAZ regions is reviewed and a recent investigation at TWI will be described in which the importance of HAZ softening in a 550 MPa QT steel has been assessed using fracture toughness and surface notched mini-wide plate tests. Submergedarc welds at 2.4kJ/mm were made using consumables of different strength level which both overmatched and undermatched the parent plate yield strength. Softening of approximately 45 HV 10 was observed in the HAZ and the CTOD fracture toughness tests showed that the transition temperature of the HAZ for the overmatched weld was 60°C higher than the equivalent HAZ in the undermatched weld.

The work shows that overmatching weld metals can concentrate strain into softened HAZs and if flaws are close to the HAZ, low cleavage resistance can be measured. This was confirmed by the results of the mini-wide plate tests. The implications of these findings are discussed in the context of welded steels of 500 - 800 MPa yield strength.

1. Introduction

Softened zones in the HAZs of TMCP or QT steels have been the subject of concern for more than 20 years. The issue arose particularly in the late 80s when improved processing methods were developed to produce tonnage steels in the420-550MPa yield strength range, using lean low carbon microalloyed compositions. Accelerated cooling and direct quenching were later developments which resulted in higher strength grades with very lean compositions, and softened HAZs have been found to be a feature of most weldments in these grades.

The features and characteristics of HAZ softening in TMCP/QT steels are well described by Denys. [1] He noted that softening normally occurred in the 650°C-1100°C peak temperature regions of the HAZ corresponding to the subcritical, intercritical (Ac 1 -Ac 3 ) and fine grained austenite zones. Hardness drops of more than 25HV5 were found compared to parent material levels. Denys observed that micro-tensile tests, or tests on simulated specimens, can be used todetermine the changes in yield and work hardening properties across softened HAZs and discussed the engineering significance of such zones for a weld metal which either undermatched or overmatched the parent material yield strength. Figure 1 shows his schematic view of the likely HAZ fracture propagation paths depending on steel type and whether undermatching or overmatching is present. He observed that the extent of the softened zone increased with increasing heat input. His main conclusion was that the engineering significance of softened HAZs can be determined realistically only by conducting wide plate tests which assessed the complex interaction between the various zones(i.e. weld metal, HAZ and parent plate).



Fig.1. Predicted crack propagation paths (Denys - Ref [1] )


Lundin et al [2] studied various high strength TMCP steels to examine the extent of softening and the width of these zones, and Fig.2 shows that hardness drops ( ΔHV) can exceed 40HV5, and usually approach a saturation value with increasing Δt 800-500 . Softened zone widths increased lineally with Δt 800-500 for a given steel ( Fig.3). The hardness drop in the HAZ is a function of steel chemistry as well as steel processing route and, in the as welded condition, TMCP steels containing Cu or Nb show an enhanced or reduced ΔHV, respectively. [2,3]


Fig.2. Hardness decrease as a function of cooling time (Lundin, Ref. [2] )


Fig.3. Softened zone width as a function of cooling time (Lundin, Ref. [2] )


The effects of softening on joint tensile strength, fatigue strength and buckling have been studied in Japan. [4,5] The only deleterious effect found was at high heat input (14 kJ/mm) where softening caused a 10% reduction in joint tensile strength using small test pieces. However, using wide plates, no reduction in joint tensile strength was found for the same high heat inputs due to the expected constraint effect on the soft zone of surrounding high yield strength material. This Japanese work was extended to examine the effect of weld overmatching using finite element analysis. It was shown that for an overmatch (weld metal yield to parent metal yield ratio, M), of 1.2, the applied CTOD at the tip of the surface flaw for a given overall strain of 0.4% on a butt weld was 0.15mm, whereas, for an undermatch situation (M=0.9), the applied CTOD was 0.30mm. The conclusion was that a degree of overmatch was desirable in butt welds to give protection against fracture from HAZ flaws close to softened zones.

Recently, softened HAZs have been studied in some detail in a Japan National Project which is targeted at developing 800 MPa steels of 1µm grain size. These developments are using very lean chemistries and employ processing schedules which create much heavier deformation in the steel bars or plate than would normally occur in controlled rolling. These routes have been successful in producing very fine-grained steels in the laboratory. Subsequent welding tests have shown that these steels exhibit softened HAZs to varying degrees and microstructures and hardness changes as affected by changes in welding process and procedure have been investigated. [6,7]

In summary, the features of HAZ softened zones are well understood. Such zones must be expected in most TMCP steels, but ΔHV will depend on detailed chemistry and steel processing route and the softened zone width will depend on welding process and procedure. The engineering significance of softened zones depends on a complex interaction of the mechanical behaviour of the weld metal, HAZ and parent metal, and must be assessed by tests which assess this interaction.

Despite this understanding, there have been few published investigations which have studied mechanical properties such as the fracture toughness of HAZ softened zones. It is clear that a key factor which must be considered in such an investigation is the degree of mismatch in strength between the weld deposit and the parent steel. The schematic diagrams of Denys, shown in Fig.1, indicate that the degree of overmatch or undermatch will have a major effect on the detailed variation of yield strength across the weld and on the width and actual hardness drop in the softened zone. In very high strength steels, the degree of weld deposit overmatch (or undermatch) depends critically on the choice of welding process or consumables, and so, in any realistic assessment of the properties of softened zones in high strength grades, mismatch must be an experimental variable.

Measurement of softened zone HAZ toughness is an important first step in the procedure for determining the significance of flaws in real structures. The most widely used flaw assessment procedure in Europe and elsewhere is BS7910:1999. In this version of the standard, detailed changes in yield strength across the weld and HAZ are not taken into account in the analyses, and it is assumed that the flaw tip is present in homogeneous material of uniform yield strength. For flaws with their tips in the HAZ, the guidelines recommend that the yield strength of the lowest strength region be used in the calculations, since this gives more conservative answers for critical flaw size calculations. However, new assessment procedures have been recently developed to enable strength mismatch between weld metal and parent metal to be considered, [8] and these procedures are now coming into more general use.

HAZ softening is an additional complexity and before assessment procedures can be developed which take into account the complete strength mismatch between weld metal, HAZ and parent metal, there is a requirement for much more workon numerical analysis methods and the generation of appropriate toughness data. A start has been made in this area and TWI and EWI have recently managed a study on strength mismatch in a 550 MPa QT steel grade which showed HAZ softening following submerged arc welding. The aim was to determine how fracture toughness and flaw assessment procedures for HAZ cracks were affected by strength mismatch. The approach adopted was to vary the deposit strength level for the same parent steel. The steel selected was a QT grade of nominally 550 MPa yield and HAZ softening was anticipated. The HAZ toughness was measured using both small-scale bend specimens and mini-wide plate tests, and the work is described in Sections 2 and 3. Section 4 reviews the results of a parallel and related project in a 700 MPa QT steel in the UK, and Section 5 discusses the findings and the implications of both studies in the context of tests for assessing the toughness of soft HAZ regions and the engineering significance of flaws sited in soft HAZs, with varying strength mismatch between weld metal and parent plate.

2. Fracture toughness of softened heat affected zones

2.1 Materials and welding procedures

The parent steel selected was a commercial, 25mm thick roller quenched and tempered grade, RQT 501, produced by CORUS, with a nominal yield strength of 550 MPa ( Table 1). The plate was welded by the submerged arc process at a heat input of 2.4kJ/mm, and it was decided to post weld heat treat the welds before testing in order to simplify the numerical analyses by reducing the residual stresses substantially.

Table 1 - Chemical analysis of 25mm thick parent plate (RQT 501)

Element, wt%
0.11 0.28 1.29 0.012 0.003 0.02 0.18 0.01 0.022 0.01 0.05

The target mismatch levels between weld metal and parent plate were 0.75 and 1.25, where mismatch is defined as the ratio of room temperature weld metal yield strength to parent steel yield strength. Butt welds were made using Oerlikon S2 wire and OP121TT flux to achieve undermatching and a cored wire (Oerlikon Fluxochord 45) with OP121TT flux to produce the overmatched deposit. The joint preparation was designed to facilitate testing of the HAZ. One sidehad a machined square edge, and the other a 20° bevel. A backing bar was used to support the weld root bead, and a typical macrosection is shown in Fig.4. After welding the panels were given a PWHT of 580°C for one hour.



Fig.4. Typical macrosection


2.2 Weld property assessment

Tensile tests conducted after PWHT showed that the average parent steel yield strength at room temperature was 566MPa and that the actual mismatch ratio, M, achieved, was 0.65 and 1.51 for the undermatched and overmatched situations, respectively.

Hardness tests carried out on macrosections are shown in Fig.5. HAZ softening is apparent from the traverses with a maximum hardness drop of ~45HV10 for the overmatched and undermatched welds.


Fig.5. Hardness traverses, 11mm below weld cap


The room temperature tensile properties of the HAZ were established using Gleeble simulation. The specimens were cycled to simulate grain coarsened HAZ, intercritical HAZ and grain refined HAZ microstructures, and the results after PWHT at 580°C for one hour are shown in Table 2. The results confirm the hardness traverses, with the HAZ undermatching the plate yield strength.

Table 2 Tensile results (at RT) from HAZ Gleeble simulation tests after PWHT at 580°C for 1 hour

HAZ region simulatedPeak temperature* °CLower yield strength, N/mm 2Tensile strength, N/mm 2
Grain coarsened 1339 451** 632
Intercritical 768 483 597
Grain refined 952 436 542
* Cooling time, 800-500°C, 32s
** Best estimate of 0.2% proof stress

Square section single edge notch bend specimens (SENB BxB where B=25mm) to BS 7448 : Part 2 : 1997 were notched from the original plate surface into the HAZ adjacent to the vertical side of the weld. Notching and fatigue cracking were conducted to try and place the final crack tip into the grain coarsened HAZ. To achieve this, the crack depth varied in each specimen and a/W ratios ranged from 0.27 to 0.50.

Post test metallography was carried out to check on the fatigue crack tip position and fracture initiation microstructure. In undermatched welds, some fatigue cracks were found to grow away from the HAZ and into softer weld metal. Additional tests were carried out in order to increase the dataset for fatigue cracks in the HAZ.

The fracture toughness specimens were instrumented with a double clip gauge arrangement mounted on knife edges. This enabled crack mouth opening displacement (CMOD) to be estimated. CMOD was then used to calculate J in accordance with the draft annex to ASTM E1290. CTOD at the initiation of fracture was estimated from J, taking strength mismatch into account.

CTOD (mismatch) was calculated from J using equations developed by Wang and Pisarski on this project. The general equation is:-


m is dependent on a/W and strain hardening, whilst σ nom is a function of tensile properties of the mismatched weld.

m = -0.111 + 0.817 (a o /W) + 1.36R

R = σ UTS / σ Y , where σUTS is the average of the weld metal and parent plate tensile strengths, and σ Y , is the average of the weld metal and parent plate yield strengths.


The partitioning parameters λ are given by:-


2.3 Fracture toughness test results

The test data for the surface notched specimens are shown in Fig.6 in terms CTOD (mismatch). All results are for specimens in which post weld metallography showed that cleavage fracture initiation was from the HAZ. Results from tests initiating in the weld metal are excluded.



Fig.6. HAZ mismatch CTOD data and mini-wide plate results


The results show that the specimens taken from the overmatched weld generally gave lower fracture toughness and higher transition temperatures than those from undermatched welds. There is substantial scatter but the shift intransition temperature is of the order of 50°C.

The data were analysed further to establish toughness distributions, and to find the median (50 th percentile), allowing a better comparison of shift in transition temperature. This was done by converting J CMOD to K J values using:


Applying the Master Curve maximum likelihood (mml) procedure [8] which assumes that fracture toughness data follow a Weibull distribution (shape factor 4 and shift parameter 20MPam 0.5 ), the data were replotted in Fig.7. This shows that the 200 MPam 0.5 HAZ transition temperature is approximately 40°C higher in the overmatched weld compared to the undermatched weld.



Fig.7. HAZ K J data with predicted median (P f = 0.5) curves


It is interesting to compare the CTOD values obtained using the procedure developed in this work which allows for mismatch, i.e. from J & CMOD, with those obtained according to the procedures currently recommended in BS 7448 :Parts 1 and 2 which assume homogeneous material. In the latter case, the highest yield strength material present was used to calculate the elastic component of CTOD. Figure 8 compares the two methods, where all the surface notched and through thickness notched data have been included with a / W ratios varying from 0.27 - 0.5, even though BS 7448 requires a / W ≥ 0.45.



Fig.8. Comparison of CTOD (mismatch) with CTOD (BS 7448)


In general, there is little difference between the two methods of determining CTOD, below values of 0.3mm. At higher CTOD values, the BS:7448 procedures tend to underestimate CTOD by up to 20%.

3. Assessment using mini-wide plate tests

Welded specimens of 100mm width and 485mm length were prepared for testing to check the behaviour observed in the fracture toughness tests. Specimens were produced for both the undermatched and overmatched types and a semi-elliptical notch was introduced, positioned so that the final fatigue crack tip could be located in the HAZ close to the fusion boundary. The initial notches were made by electro-discharge machining and these were extended byfatigue in four point bending to grow the cracks to final size. Crack depths were monitored using ultrasonic time of flight diffraction equipment. The average crack depth to thickness ratio achieved was 0.24 with an average crack depthto length ratio of 0.18.

CTOD values were estimated using a pair of clip gauges mounted on knife edges and adopting the following equation:

CTOD = V1 - [(V2 - V1)x(Z1 + a o )(Z2 - Z1)]

V1 and V2 are lower and upper clip gauge openings
Z1 and Z2 are lower and upper knife-edge heights
a o is original crack depth

The mini-wide plate specimens were tested at -70°C where the mode of fracture from the HAZ was expected to be cleavage. The temperature also took account of the lower constraint of the tension specimen compared to the fracture toughness bend specimens.

The results are included in Fig.6 and support those of the CTOD tests described earlier, with the overmatched weld giving the lowest HAZ toughness.

4. Related work

In a closely related UK project at CORUS, the effect of mismatch on the HAZ toughness of a different QT steel was studied by Harrison. [9] He used a nominal 700 MPa QT grade of 25mm thickness which exhibited HAZ softening but, in contrast to the work described in Sections 2 and 3, he produced overmatched and undermatched welds by using a constant strength weld deposit and a parent steel in two heat treatment conditions, QT and normalised.

In experiments using the flux cored arc welding process at 1.25 kJ/mm, he achieved an undermatched weld (M=0.95) when the steel plate was in the QT condition and an overmatched weld (M=2.15) after the steel plate had beennormalised, both panels being in the PWHT condition. Based on a previously established hardness-strength correlation, the average yield strengths for the different regions are shown in Fig.9 for the undermatched and overmatched panels, after PWHT.


Fig.9. Yield strength of weld regions for undermatched and overmatched panels (Harrison, Ref. [9] )

Through thickness notched HAZ CTOD tests to BS 7448: Part 2 were carried out and the results are shown in Fig.10. This indicates that, in this case, the overmatched weld had a consistently higher HAZ toughness than the undermatched weld. Surface notched HAZ CTOD tests gave a similar result, but with more scatter.



Fig.10. CTOD mismatch data from Corus (Harrison, Ref. [9] )


5. Discussion

5.1 Toughness of softened HAZs

The TWI/EWI experimental work reported here shows that measured HAZ toughness values in a 550 MPa QT steel exhibiting HAZ softening, are affected significantly by the mismatch in strength between the weld deposit and the parent plate. Taking the closely related CORUS work on a 700 MPa steel into account, the effect of the strength mismatch on HAZ toughness is seen to depend on the method of achieving the mismatch.

In the TWI/EWI project, weld metal strength was varied and the plate strength was constant, whereas in the CORUS work, the weld strength was constant and the plate strength varied. In the former case, an overmatch of the weld metal strength to the parent steel strength led to the lowest HAZ toughness whilst, in contrast, an undermatch situation in the CORUS experiments led to lowest HAZ toughness.

How can these differences be reconciled? The simplest explanation lies in the qualitative explanation given by Denys. [1] The results in the present work support the argument that where the flaw is sited close to the fusion boundary or HAZ of a steel showing HAZ softening, strain will be concentrated in local regions of lower yield strength.

There are three main cases to be considered:-

  1. The weld deposit undermatches in strength both the HAZ and parent steel.
  2. The plate undermatches both the HAZ and weld deposit.
  3. The HAZ undermatches both the weld deposit and the parent steel.

The experimental work described covers all three of these scenarios and both the TWI/EWI and CORUS investigations showed that the lowest HAZ toughness was associated with case (c) above. Thus, where distinct HAZ softening is apparent, relative to both plate and weld deposit, the lowest HAZ toughnesses have been recorded. The issue of whether the weld deposit overmatches or undermatches the parent steel strength is not the determining factor.

The explanation for case (c) being the worst case appears to fit Denys' explanation for likely crack paths in mismatched welds, i.e. locally soft regions will concentrate strain during deformation and increase the risk of cleavage fracture. In cases (a) and (b), a lower strength weld deposit or parent plate will absorb strain preferentially, reducing the risk of cleavage fracture in the HAZ, leading to higher HAZ toughness values.

5.2 Defect tolerance of softened HAZs

The implications of the toughness results on defect tolerance are now considered. A number of defect assessment procedures, such as BS 7910:1999, are based on a failure analysis diagram (FAD) which considers failure with respect to both fracture and plastic collapse. These recognise that failure may not be completely brittle or completely ductile, but a mixture of both failure modes. Strength mismatch between the weld metal and parent material will affect theestimation of plastic collapse, and usually a conservative approach is taken with plastic collapse being assumed to be governed by the lower strength material present. Consequently, in normal situations where the weld metal has ahigher strength than the parent material, the plastic collapse assessment is based on the strength of the parent material irrespective of the crack location.

When the HAZ strength is less than that of the parent plate, the situation is less clear. One could base the assessment on the tensile properties of the softened HAZ and follow the principle of employing the lower strength materials, but estimating tensile properties of softened HAZs is difficult and impractical in most cases. Furthermore, in the opinion of the authors this is unnecessary because if a flaw is present in a narrow softened zone, constraint provided by stronger material either side would elevate its yield strength and inhibit further yielding. Consequently, plastic collapse is likely to be governed by the weld metal or parent material, whichever has the lower yield strength.

It is possible to estimate plastic collapse for a strength mismatch condition, but such procedures are not yet in common use and can be complicated to apply. Nevertheless, in TWI-EWI project mentioned earlier, finite element analyses were undertaken to estimate limit loads for mismatched welds. On the basis of these studies, a simplified mismatch correction factor was developed which can be applied to the plastic collapse axis of the FAD (i.e. L r ) axis, (see Fig.11).



Fig.11. Strength mismatch corrected FAD for a 4mm deep flaw


The equation developed for through-thickness cracks located at the fusion boundary of a middle crack tension specimen is:-



P h is the homogeneous material (based on parent material properties) reference stress solution (such as can be derived from equations in BS 7910:1999). M is the strength mismatch ratio (ratio weld metal to parent material yield strengths) h is the half weld width W is half the specimen width a is half the through-thickness crack length

No general solution could be found for semi-elliptical surface cracks. However, a simple modification to the above equations was found to work reasonably well for long semi-elliptical cracks in the materials and specimen geometries considered in the project. This involved replacing the 'W' term by material thickness, and the 'a' term by surface crack depth.

The above discussion centres on considerations of failure by plastic collapse in mismatched welds with softened HAZs. The assessment of failure by brittle fracture in, say, BS 7910:1999, is based on the fracture toughness of the material in which the flaw is present. In the case of softened HAZs, the worst case toughness will be found when the weld deposit overmatches the plate in strength as discussed in Section 5.1. The fracture toughness tests used for the assessment must of course match the practical situation. Thus, if the weld metal undermatches the plate, the test specimen for measuring HAZ toughness must match the anticipated strength mismatch in the final weld.

5.3 Influence of softened HAZs on structural integrity

The overall effect of strength mismatch on the behaviour of cracks in softened HAZs can be best illustrated by the following example. This considers the significance of an axial surface crack in the HAZ of the longitudinal seam weld of a pipe with a diameter of 900mm and wall thickness of 25mm. The weld width is assumed to be 25mm. The pipe material is assumed to have a yield strength of 566MPa and overmatching and undermatching weld metal are considered with yield strengths of 850MPa and 373MPa, respectively. These give mismatch ratios of 1.51 and 0.65, respectively, and are assumed to be the same materials as those considered in Sections 2.1 and 2.2 above. For the purpose of the example, the HAZ contains a reference flaw 4mm deep and 50mm long. In the overmatched weld (M = 1.51), the HAZ is softened with respect to the parent pipe, as in Fig.5. A lower bound estimate of fracture toughness from the Master Curve mml procedure (see Fig.7), based on a lower 5th percentile (P f = 0.05) provides K mat values at -40°C of 193MPam 0.5 and 253MPam 05 for the HAZ in material where M = 1.51 and M = 0.65, respectively.

Using the mismatch corrected FADs in Fig.11, together with the Level 2A flaw assessment procedure in BS 7910:1999 (facilitated using TWI software Crackwise 3), estimates were made of the maximum allowable hoop stress in the pipe containing the reference flaw.

Five conditions were considered and the results are presented in Table 3. Since the seam weld is as-welded, yield magnitude residual stresses were assumed to be initially present. These were relaxed to lower stresses, depending on the level of applied stress in accordance with BS 7910:1999.The first two conditions in Table 3 ignore strength mismatch and assume parent material strength. (This is the same as a standard defect assessment where the weld overmatches the strength of the parent material). As expected, the HAZ with the higher fracture toughness (K mat = 253MPam 0.5 ) can tolerate a higher hoop stress without failure (539MPa). However, as this higher fracture toughness was measured in an undermatched weld, an overestimate (non-conservative) of the failure stress isobtained. Repeating the analysis and assuming that plastic collapse is controlled by the undermatched weld metal, the hoop stress at failure is predicted to be lower at 389MPa. As pointed out earlier, this is a conservative estimate. Abetter estimate is obtained if allowance is made for strength mismatch by using a mismatch corrected FAD. In this case, the maximum hoop stress is predicted to be 419MPa. This is lower than the maximum hoop stress predicted using the mismatch corrected FAD for the overmatched weld (613MPa), despite fracture toughness being lower.

Table 3 Maximum allowable hoop stress for an axial flaw 4mm de ep and 50mm long

CaseConditionK mat
MPam 0.5
P m Max,
1 Homogeneous, based on parent material tensile properties 193 496
2 Homogeneous, based on parent material tensile properties 253 539
3 Homogeneous, based on lowest weld metal material tensile properties 253 389
4 Mismatch correction, applied to overmatched weld, M=1.51 193 613
5 Mismatch correction, applied to undermatched weld, M=0.65 253 419

Thus, for the particular example chosen, it is shown that local HAZ softening in overmatched welds can result in a significant reduction in fracture toughness. However, defect tolerance is not necessarily compromised because of the beneficial effects of overmatching on the assessment of plastic collapse.

Nevertheless, the sensitivity of the assessment to the choice of fracture toughness cannot be over emphasised. Figure 12 shows how the maximum allowable hoop stress changes with fracture toughness for both the overmatched and undermatched conditions. The maximum hoop stress for the overmatched condition may be higher or lower than that for the undermatched condition depending on the relative measured HAZ toughness for the two situations. Inspection of the fracture toughness transition curves in Fig.7 shows that fracture toughness values below 115MPam 0.5 become increasingly likely at temperatures below -40°C in the overmatched weld (M=1.51). Indeed, the results from two mini-wide plate tests at -70°C (see Fig.6) confirm that fracture is the dominant mechanism in the overmatched weld (M=1.51) since fracture occurred at a lower fracture toughness (in this case CTOD) and stress than in the undermatched weld (M=0.65).


Fig.12. Maximum allowable hoop stress versus softened zone HAZ toughness


6. Conclusions

The HAZ fracture toughness of a nominal 550 MPa QT steel showing HAZ softening ( ΔHV~45) has been assessed using bend specimens and mini-wide plate tests. Two sets of welds were made giving weld metal yield strength overmatch (M=1.51) andundermatch (M=0.65) compared to the parent plate. The results were compared with related UK work on a 700 MPa QT steel.

  • The fracture toughness of softened HAZ regions depended on the mismatch in strength between the weld deposit and parent plate.
  • The worst-case fracture toughness of the softened HAZs occurred when the HAZ undermatched in strength both the weld deposit and the parent plate. Higher toughnesses were measured when either the weld metal or the parent steel undermatched the HAZ in strength.
  • The lower HAZ fracture toughness in overmatched welds in TMCP/QT steels which have HAZ softening does not necessarily compromise the flaw tolerance in structures such as pipelines, where plastic collapse is the dominant failure mode.
  • However, a sensitivity analysis has shown that the tolerance to flaws in softened HAZs depends critically on the fracture toughness of the HAZ region and tolerance reduces rapidly in situations where cleavage is the dominant failure mechanism.
  • In assessing the toughness of softened HAZs, the test specimen must match the practical situation in terms of yield strength mismatch between weld deposit and parent steel.

7. References

  1. Denys, R 'The effect of HAZ softening on the fracture characteristics of modern steel weldments and the practical integrity of marine structures made by TMCP steels' EVALMAT 89, Kobe, Japan, 20-23 November 1989, ISIJ 1989 vol 2 pp 1013.
  2. Lundin, C D, Gill, T P S and Qiao, C Y, 'Heat affected zones in low carbon microalloyed steels'. Recent trends in Welding Science and Technology Proceedings, 2nd International Conference, Gatlinburg, May 1989. Eds S A David, J M Vitek, A S M International 1990.
  3. Shiga, C 'Effects of steelmaking, alloying and rolling variables on the HAZ structure and properties in microalloyed plate and line pipe'. The Metallurgy, Welding and Qualification of Microalloyed (HSLA) Steel Weldments Proceedings International Conference, Houston, November 1990. Ed J T Hickey AWS 1990.
  4. Komizo, Y 'Performance of welded joints in TMCP steel plates'. Welding International 1991 5 (8) p 598.
  5. Yurioka, N, 'TMCP steels and their welding'. Welding in the World 1995, 35 (6) p.375
  6. Otani, T, Tsukamoto, S, Arakane, G, Ohmori, A 'HAZ characteristics of Ultra-fine grained high strength steel welded by high power CO 2 Laser'. 7th International Welding Symposium, November 2001, Kobe, JWS p.773.
  7. Ito, R, Hiraoka, K, Kosugai, T, Shiga C 'Microstructural characteristics of welding HAZ in Ultra-fine ferrite grained steel', Ibid.
  8. SINTAP - Structural Integrity Assessment Procedures for European Industry' Final Procedure, November 1999, Brite Euram Project BD95-1426. Contract BRPR-CT95-0024
  9. Harrison, P L 'Weld Strength Mismatch Effects - multipass weld evaluation' SINTAP report BS/24, March 1999.

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