Scholarly article on topic 'Effect of HFMI treatment procedure on weld toe geometry and fatigue properties of high strength steel welds'

Effect of HFMI treatment procedure on weld toe geometry and fatigue properties of high strength steel welds Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Ebrahim Harati, Lars-Erik Svensson, Leif Karlsson, Kjell Hurtig

Abstract The effects of high frequency mechanical impact (HFMI) treatment procedure on the weld toe geometry and fatigue strength in 1300 MPa yield strength steel welds were investigated. In this regard first the effect of three or six run treatments on the weld toe geometry was evaluated. The fatigue strength and weld toe geometry of as-welded and HFMI treated samples was then compared. Fatigue testing was done under fully reversed, constant amplitude bending load. When increasing the number of treatment runs from three to six, the weld toe radius and width of treatment remained almost constant. However, a slightly smaller depth of treatment in the base metal and a somewhat larger depth of treatment in the weld metal was observed. HFMI treatment increased the fatigue strength by 26%. The treatment did not increase the weld toe radius significantly, but resulted in a more uniform weld toe geometry along the weld. A depth of treatment in the base metal in the range of 0.15-0.19 mm and a width of treatment in the range of 2.5-3 mm, were achieved. It is concluded that the three run treatment would be a more economical option than the six run treatment providing a similar or even more favourable geometry modification.

Academic research paper on topic "Effect of HFMI treatment procedure on weld toe geometry and fatigue properties of high strength steel welds"

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Structural Integrity

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Procedia Structural Integrity 2 (2016) 3483-3490

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ScienceDirect PrOCed ¡0

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21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy

Effect of HFMI treatment procedure on weld toe geometry and fatigue properties of high strength stee1 welds

Ebrahim Harati*, Lars-Erik Svensson, I_,i£iif ]<Larls55on and Kjell Hurtig

Department of Engineering Science, University West, SE]461 86 Trollhättan, Swejen

Abstract

The effects of high frequency mechanical impact (HFMI) treatment procedure on the weld toe geometry and fatigue strength in 1300 Mfa yiekl stoength steel welds weee investigate d. In this regard dirst the effect oo three or six nin treaMents on the weld toe geometry was evaluated. The fatigue strength and weld toe geometry of as-welded and HFMI treated samples was then compared. Fatigue testing was done under fully reversed, constant amplitude bending load. When increasing the number oO treatment runs from three to six, the weld toe radius and widtii of sreatment remained almost constant. However, a sliglitly smaller depth of treatment in the base metal and a somewhat larger depth of treatment in the weld metal wis observed. HFMI uroatment mcremed the fatigue strength gy 26%. Tho treatment did not increase the weld toe radius signiaicantty, bur resulted in a mome uniform weld toe geometry along the weld. A deptii of treatment in the brse metal in die rame of 0.15-0.19 mm aid a width of treatment in the range oh 2.5-3 mm, were achieved. It is concluded ^^^ the three run treatment would be a more economical option than the six run treatmene providing a similar or even more favourable geometry. modification.

Copyright © 201(5 The Authors. Published by Elsevier B.V. This is an opet access article under the CC BY-NC-ND license;

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of dhe Scirntific Committee of ECF21,

Keywords: Fatigue strength; High frequency mocOefical impact treatment; High strength stool; wold too

* Corresponding rnithor. Tel.: +46 520 222 33 41.

E-mail address: ebrahim.harati@hv.se

Copyright © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer review under responsibility of the Scientific Committee of ECF21.

10.1016/j.prostr.2016.06.434

1. Introduction

A weld introduces a change in shape and hence will result in stress concentration. This occurs primarily at the weld toe which is therefore one of the most probable fatigue crack initiation sites. The stress concentration will be low for a smooth transition between the weld toe and base metal, but can be higher if there is an abrupt change in geometry. Increasing the weld toe radius, and thereby decreasing the stress concentration, the fatigue strength will increase (Marquis and Barsoum 2013; Harati et al. 2015; Malaki and Ding 2015).

Fatigue strength of welded components can be improved by using different methods. High frequency mechanical impact (HFMI) is one of the most recent post weld treatment methods which have been used to increase the fatigue strength of welded components (Zhao et al. 2011). The increase in fatigue strength by HFMI is found to be due to the combination of weld toe geometry modification, induced compressive residual stresses and increased surface hardness at the treated region (Mikkola et al. 2016). The depth, width and radius of the HFMI treated region are important geometrical parameters which affect the fatigue strength. Depending on the treatment parameters and materials strength, different values for these parameters have been suggested in fatigue guidelines. A depth of indentation in the range 0.2-1 mm and a width of indentation in the range 2-7 mm have been proposed as optimum values in several studies (Marquis and Barsoum 2013).

The main purpose of this paper is to investigate the influence of HFMI treatment procedure on the weld toe geometry and fatigue strength of 1300 MPa yield strength steel welds. First the effect of treatment on the weld toe geometry is evaluated for two different cases with three or six runs. Then the as-welded and HFMI treated weld toe geometries are compared and discussed.

2. Materials and methods

2.1 Base and filler materials

The base metal was 15 mm thick Weldox 1300 with a yield strength of 1295 MPa and a tensile strength of 1562 MPa. The chemical compositions of the base metal and filler materials are given in Table 1 and mechanical properties of all-weld metal are presented in Table 2.

Table 1. Chemical compositions of base and filler materials (wt.%).

C Si Mn Cr Ni Mo

Weldox 1300 0.25 0.5 1.4 0.8 3.0 0.7

Coreweld 89 a 0.08 0.6 1.3 0.5 2.6 0.7

OK Tubrod 14.11 a 0.03 0.8 1.5 0.04 0.01 -

nominal composition.

Table 2. Typical mechanical properties of all-weld metal.

Welding consumable Rp0.2 (MPa) Rm (MPa) Impact toughness at -40°C (J)

Coreweld 89 910 965 72

OK Tubrod 14.11 420 555 47

2.2 Welding setup

T-shaped assemblies were produced by joining two plates with dimensions of 500 x 200 x 15 mm. Robotic Gas Metal Arc Welding (GMAW), with Ar + 18% CO2 as shielding gas, was used to produce two-sided full penetration fillet welds. OK Tubrod 14.11 was used as filler material in the root bead (bead 1) and a high strength (Coreweld 89) filler was used for fill passes. The energy input was 2.1 kJ/mm. The welding sequence is shown in Fig. 1 (a). The different weld toes were named L1, L2, U1 and U2 (see Fig. 1 (a)).

The welded assemblies were sliced and machined to produce specimens, with dimensions as shown in Fig. 1 (b).

Fig. 1. (a) The welding sequence with five beads. The upper (U1 and U2) and lower weld toes (L1 and L2) in the first and second welded sides

are also shown. (b) Design and dimension of T-shaped fatigue specimens.

2.3 Eigh Frequency Mechanical Imp/ct treatment /nd weld profile investigation

High frequency mechanical impact treatment was performed with the frequency of 20 000 ± 400 Hz and the amplitude of vibration of the sonotrode was 40 ^m. The radii of the indenters were 1.5 mm for the lower weld toes (L1 and L2) and 3 mm for the upper ones (U1 and U2) (see Fig. 1 (a)). The reason for selecting a larger tip radius for the upper weld toe was a larger as-welded upper weld toe radius.

A 3-D measuring system, GOM scanner model ATOS Core 80, was used to measure the weld toe geometries before and after HFMI treatment. The evaluation was done on nine different surface profiles, with a distance of 10 mm along the weld toe lines, for each sample. The main geometrical features: width of treatment (W), depth of treatment in the base metal (Db) and depth of treatment in the weld (Dw) were then measured for the lower weld toes (L1 and L2). The geometry parameters together with part of a treated weld are illustrated in Figs. 2 (a) and (b).

In order to investigate the effect of HFMI treatment procedure on the weld toe geometry, the lower weld toe on one side of a sample was treated with three and the lower weld toe on the other side with six HFMI runs. In each run the entire length of the weld was treated in the direction parallel to the weld toe. During the treatment, the peening head was positioned at the junction of the base material and the weld. The angle between the axis of the peening head and the base metal surface was 45° during the treatment and the indenter tip radius was 1.5 mm.

Fig. 2. (a) Example of a HFMI treated region. A shiny and uniform treated region is seen along the lower weld toe (L1), (b) a surface profile showing the weld toe region after HFMI treatment. Note the definition of toe radius (r), width of treatment (W), depth of indentation in the base

plate (Db) and depth of indentation in the weld (Dw) after the treatment.

2.4 Fatigue testing

The fatigue testing was performed using a 250 kN MTS Servo-Hydraulic machine by applying a constant amplitude bending load condition for as welded and HFMI treated samples. Testing was conducted with a frequency of 29 to 40 Hz at a stress ratio R = - 1. Fatigue assessment was performed using Effective Notch Stress (ENS) approach. The evaluation procedure is explained elsewhere (Harati et al. 2016).

3. Results

3.1 Weld toe geometry

The weld toe radii for as-welded and HFMI treated samples used for fatigue testing are given in Table 3. The radii in as-welded samples represent the average of 63 and in HFMI treated samples of 72 measurements, respectively. The corresponding standard deviations (c) are also given in the Table.

Table 3. Weld toe radii (r) (mm) and corresponding standard deviations (c) for as-welded and HFMI treated samples.

L1 L2 U1 U2

r c r c r c r c

As-welded 1.6 0.62 1.6 0.75 3 1.18 3.4 0.83

HFMI treated 1.7 0.11 1.8 0.13 3.5 0.53 3.3 0.45

From Table 3 it can be seen that the weld toe radius is almost the same for the as-welded and HFMI treated samples. However, a higher standard deviation is found for as-welded samples as compared to HFMI treated samples. This is more pronounced in the lower weld toes (L1 and L2) than the upper weld toes (U1 and U2). Larger radii were measured in the upper weld toes than in the lower weld toes for both as-welded and HFMI treated samples. The depth of treatment in the base metal (Db) was found to be in the range of 0.15-0.19 mm and the width (W) of the HFMI treated region was in the range of 2.5-3 mm. Both represent the average of five measurements. No significant depth of treatment in the weld (Dw) was found. This is seen in Fig. 3 which shows an example of measured depth and width of treatment. The material transfer due to the treatment is also indicated by a dashed arrow.

HFMI treated

As-welded

Fig. 3. HFMI treated and as-welded surface profiles from the same location in a weld toe, showing the depth of indentation in the base metal (Db) and width (W) of the treated region. No significant depth of treatment in the weld (Dw) is seen. Note material transfer from the weld toe base

metal to the weld toe fusion zone as indicated by dashed arrow.

A section of a sample in as-welded condition and two samples, after three and six HFMI runs, obtained using 3D scanning of the weld profiles, are shown in Figs. 4 (a), (b) and (c), respectively. As can be seen from the figure, in the as-welded samples (Fig. 4 (a)) the weld toe region was somewhat irregular, showing some waviness. The HFMI treated samples (Figs. 4 (b) and (c)), on the other hand, had a very uniform profile.

Fig. 4. Comparison of typical weld surface profiles (a) as-welded, (b) after three- and (c) after six HFMI treatment runs. HFMI treatment

produced a more uniform profile along the weld toe line.

The weld toe radius (r) and width of the treated region (W) for the three and six HFMI treatment runs are illustrated in Fig. 5 and the depth of treatment in the base metal (Db) and in the weld (Dw) are shown in Fig. 6.

Fig. 5. Comparison of weld toe radius (r) and width of HFMI treatment (W) between three and six HFMI treatment runs. No significant

change is seen in the measured parameters.

From Figs. 5 and 6, it is seen that the weld toe geometry does not change significantly by increasing the number of treatment runs. The weld toe radius and width of treatment (Fig. 5) remained almost unchanged while the depth of treatment in the base metal was somewhat smaller and the depth of treatment in the weld metal was slightly larger when applying six instead of three runs (Fig. 6). These changes are seen in Fig. 7 which shows an example of surface profiles comparing the three and six treatment runs.

Fig. 6. Comparison of depth of treatment in the base metal (Db) and in the weld (Dw) between the three and six HFMI treatment runs. Note a slight decrease in Db and a slight increase in Dw by increasing the number of treatment runs.

Fig. 7. Surface profiles of three and six HFMI treatment runs. No significant geometry change is seen between the two treatments. Note material transfer from the weld toe fusion zone to the weld toe base metal, as indicated by dashed arrow, for six runs.

3.2 Fatigue properties

Fatigue initiation and propagation with few exceptions occurred from the lower weld toes. The calculated characteristic fatigue strength (FAT), the mean fatigue strength at 2 million cycles and the slope of the S-N curves are presented in Table 4.

Table 4. Characteristic fatigue strength (FAT), mean fatigue strength at 2 million cycles and slope of the S-N curves (m). Sample FAT (MPa) Mean fatigue strength (MPa) m As-welded 306 353 2.94

HFMI treated 315 445 2.79

Considering Table 4, it is seen that fatigue strength of HFMI treated samples is higher than for as-welded samples.

4. Discussion

The main aim of this paper is to investigate the influence of HFMI treatment on the weld toe geometry and fatigue strength of 1300 MPa yield strength steel welds. In this regard first the effect of treatment for two different

cases of three and six treatment runs on the weld toe geometry is evaluated. The weld toe geometry in as-welded condition and after HFMI treatment is then compared and discussed.

Comparing the weld toe geometry after three and six HFMI treatment runs, shows that the weld toe radius and width of treatment did not change significantly by increasing the number of treatment runs. However, the depth of treatment in the base metal was smaller and depth of treatment in the weld metal was slightly larger when applying six instead of three runs. After six treatment runs, some treated material is transferred from the weld toe fusion zone to the weld toe base metal (see Fig. 7). This material transfer results in a slight decrease in Db and an increase in Dw. (see Figs. 6 and 7). The larger depth of treatment in the weld metal than in the base metal can probably be related to the lower strength and hardness of the weld metal. After HFMI treatment the hardness increases in the entire treated region. However, the heat affected zone (HAZ) remains harder than the weld metal after treatment (Harati et al. 2016). Considering the slight change in the weld toe geometry, it seems that the three-run treatment would be a more economical option than the six-run treatment providing a similar or even more favourable geometry modification.

Comparing the weld toe geometry before and after HFMI treatment shows that in this particular case the weld toe radius did not increase significantly due to the treatment. In both the as-welded and the treated condition the weld toe radii are very close to the indenter tip radii which were used for the treatment. Therefore, it seems that the peening tip radius is a determining factor for the treated toe radius. A similar observation was made by Leitner et al. (2015) that measured a weld toe radius of 2 mm after HFMI treatment which was equal to the radius of the indentation tip used.

Previous studies (Aashto 1998; Statnikov 2000; Zhang et al. 2015) reported a depth of treatment in the base metal, Db, in the range of 0.25-1 mm and a width of treatment (W) in the range of 2-7 mm. Results of a research done by Weich (2013) using finite element analysis showed that the geometrical change of the weld toe by HFMI treatment does not provide a major reduction of the stress concentration factor. Although an increase of the weld toe radius generally results in a reduction of stress concentration, increasing the depth of indentation reduces the beneficial effect of an increased weld toe radius. On the other hand, a minimum depth of indentation is required in order to deform the weld toe. Therefore, a depth of treatment in the range of 0-0.25 mm was proposed as an optimum. Thus, it seems that the depth of treatment in the base metal (Db) in this study, 0.15-0.19 mm, is within a reasonable range.

The depth of treatment in the weld, Dw, has only been evaluated or mentioned in few of the relevant studies. Ghahremani et al. (2014) performed HFMI treatment on welded steels with yield strength of about 390 MPa at three different levels termed under- proper- and over-treatment. They observed no difference of the weld toe radius and width of treatment for the different treatment levels. It was found that an average depth of treatment, Dave, which is the average of the depth of treatment in the base metal (Db) and depth of treatment in the weld (Dw), is a key factor relating the HFMI treatment quality to the geometrical features. They suggested that a proper HFMI treatment is achieved when Dave is between 0.25 mm and 0.5 mm. Higher and lower than this range was considered to be over-and under- treatment, respectively. Almost no depth of treatment in the weld was obtained after HFMI treatment in the present study. This is most likely related to the fact that the peening head was directed towards the base metal, rather than the weld during the treatment. Therefore, positioning and orienting the peening tool in such a way that the base metal and the weld are targeted at the same time during the treatment would be a possible solution to achieve a larger depth of treatment in the weld.

When comparing results from literature and the present study it should be kept in mind, though, that all the geometrical features of HFMI treatments reported in literature are for steels with yield strengths lower than 1000 MPa while. However, a much higher yield strength steel was used in this study.

5. Conclusions

The influence of HFMI treatment on the weld toe geometry and fatigue strength in 1300 MPa yield strength steel welds was investigated. In this regard first the effect of three or six treatment runs on the weld toe geometry was evaluated. The fatigue strength and weld toe geometry of as-welded and HFMI treated samples were then compared. By increasing the number of treatment runs from three to six, the weld toe radius and width of treatment remained almost constant. A somewhat smaller depth of treatment in the base metal and a slightly larger depth of treatment in

the weld metal was observed when increasing the number of runs. HFMI treatment increased the fatigue strength by 26%. The treatment did not increase the weld toe radius significantly but resulted in a more uniform weld toe geometry along the weld. A depth of treatment in the base metal in the range of 0.15-0.19 mm and a width of treatment in the range of 2.5-3 mm, were achieved. It seems that the three run treatment would be a more economical option than the six run treatment providing a similar or even more favourable geometry modification.

Acknowledgements

The financial support of the Swedish Energy Agency is gratefully acknowledged. The authors would like to thank ESAB AB, Volvo Trucks and SONATS for their contributions to this paper.

References

Aashto L, 1998. Bridge design specifications. American Association of State Highway and Transportation Officials, Washington, DC Ghahremani K, Safa M, Yeung J, et al, 2014. Quality assurance for high-frequency mechanical impact (HFMI) treatment of welds using handheld

3D laser scanning technology. Weld World 59, 391-400. Harati E, Karlsson L, Svensson L-E, Dalaei K, 2015. The relative effects of residual stresses and weld toe geometry on fatigue life of weldments. Int J Fatigue 77, 160-165.

Harati E, Svensson L-E, Karlsson L, Widmark M, 2016. Effect of High Frequency Mechanical Impact treatment on fatigue strength of 1300 MPa

yield strength steel welds. Submitted to the International Journal of Fatigue. Leitner M, Gerstbrein S, Ottersbock MJ, Stoschka M, 2015. Fatigue Strength of HFMI-treated High-strength Steel Joints under Constant and

Variable Amplitude Block Loading. Procedia Eng 101,251-258. Malaki M, Ding H, 2015. A review of ultrasonic peening treatment. Mater Des 87, 1072-1086.

Marquis G, Barsoum Z, 2013. Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed procedures and

quality assurance guidelines. Weld World 58, 19-28. Mikkola E, Marquis G, Lehto P, et al, 2016. Material characterization of high-frequency mechanical impact (HFMI)-treated high-strength steel. Mater Des 89, 205-214.

Statnikov AS, 2000. Applications of operational ultrasonic impact treatment (UIT) technologies in production of welded joints. Weld WORLD-Lond- 44, 11-21.

Weich D-II, 2013. Henry Granjon Prize Competition 2009 Winner Category C: Design and Structural Integrity; EDGE Layer Condition and

Fatigue Strength of welds improved by mechanical post-weld treatment. Weld World 55, 3-12. Zhang H, Wang D, Xia L, et al, 2015. Effects of ultrasonic impact treatment on pre-fatigue loaded high-strength steel welded joints. Int J Fatigue 80:278-287.

Zhao X, Wang D, Huo L, 2011. Analysis of the S-N curves of welded joints enhanced by ultrasonic peening treatment. Mater Des 32, 88-96.