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ScienceDirect Procedia Engineering 66 (2013) 181 – 191

5th Fatigue Design Conference, Fatigue Design 2013

Residual stresses in welded components following post-weld treatment methods Lasse Suominen1,*, Mansoor Khurshid2, Jari Parantainen1 1

Stresstech Oy, Tikkutehtaantie 1, 40800 Vaajakoski, Finland 2 KTH, Teknikringen 8, 10044 Stockholm, Sweden

Abstract Weld seam is the weakest point of the structure. Weld defects, weld geometry such as toe radius, and residual stresses, which are typically tensile in the critical area, weaken the fatigue strength of the weld seam. These factors become more important with the welding of high strength steels (HSS) because structures containing welded seams loose the benefit from the strength of the steel. The weld seam fatigue strength can be improved locally with several different methods. In this paper residual stresses of the weld area and their modifications for improving the fatigue strength are discussed. The residual stresses of the toe area are changed from tensile up to compressive stresses, which are known to improve fatigue strength, with several methods. High tensile stresses can be relaxed mechanically or thermally as postweld heat treatment. Mechanical treatments are more common especially in large structures. Typical mechanical methods are burr grinding, TIG remelting, hammer peening and needle peeing. A new method is laser re-melting method. All these methods also change the geometry of the weld toe by improving it. This paper concentrates to needle peening (high frequency mechanical impact, HFMI), and presents some results from laser remelting tests. Both surface measurements and depth distributions of stress measured by X-ray diffraction (XRD) method are presented. © Elsevier Ltd. © 2013 2013 The TheAuthors. Authors.Published Publishedbyby Elsevier Ltd. Open access under CC BY-NC-ND license. Selection of of CETIM, Direction de l'Agence de Programme. Selectionand andpeer-review peer-reviewunder underresponsibility responsibility CETIM Keywords: Welding; High Stregnth Steel; HFMI; Residual Stress; Fatigue Strenght Improvement

*

Corresponding author. Tel.: +358-400-252-566; fax: +358-14-333-0099

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of CETIM doi:10.1016/j.proeng.2013.12.073

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Lasse Suominen et al. / Procedia Engineering 66 (2013) 181 – 191

1. Introduction Welding imperfections, defects and geometrical notches have a prominent effect on the fatigue strength of welded joints. Other than these, residual stresses investigated since fifties are also observed to affect the structural durability. Residual stresses often occur in a region of strain gradient which experiences plastic deformations. These conditions occur during fabrication processes, change in microstructure and thermal gradients. In welding these stresses are produced by phase transformations and by thermal gradients In many cases the magnitude of residual stresses can even reach the monotonic yield strength of the base material. It has also been observed that the magnitude of residual stresses is dependent on the geometric parameters of the weld, weld order, welding parameters and the stiffness of the structure [1-6]. It is difficult to evaluate or calculate residual stresses due to welding. In the design rules it is supposed that residual stresses are in the range of the yield strength. Structural failure is usually associated to the combined effect of residual and applied stresses. These stresses can be detrimental or beneficial, depending upon their nature. Tensile residual stresses in welded joints can lead to higher mean combined applied stresses and can negatively affect the fatigue life, while compressive residual stresses decreases the mean combined applied stresses which leads to an improvement in the fatigue life. To modify the tensile residual stress field in welded joints to beneficial compressive stress field, various improvement techniques e.g. shot peening, burr grinding, autofretage, low plasticity burnishing and HFMI are used. All these methods not only induce compressive residual stresses but also change the hardness and decrease the stress concentration at the critical regions. Usually an increase in hardness is observed in the HFMI treated region. The toe is critical area because there are combined both negative factors increased notch effect and high tensile stresses. Highest tensile stresses are typically located in the toe area, Figure 1. Also other welding defects are typical to toe area transition from weld metal to base metal. Due to this the need for local treatment was driving force to develop these peening methods. The HFMI technology was developed at the Northern Scientific and Technological Foundation in Russia in association with Paton Welding Institute in the Ukraine. The equipment used in HFMI is available in different names e.g. Ultrasonic impact treatment (UIT), ultrasonic peening (UP), ultrasonic peening treatment (UPT), high frequency impact treatment (HiFiT), pneumatic impact treatment (PIT) and ultrasonic needle peening (UNP). In HFMI high strength steel cylindrical indenters (available in different diameters) are accelerated against a component with high frequency. These indenters can be single or multiple depending on the manufacturer of the device and end use. The surface layer of the impacted material is highly plastically deformed. Plastic deformations create different kind of lattice defects which increase the volume of the surface. Compressive stresses is created to the surface layer because it can’t expand due to unchanged bulk, Additionally to macroscopic stresses plastic deformation creates microstress around the lattice defect, work hardened the surface layer. In comparison to other peening methods the operation is more user-friendly and the spacing between alternate impacts on the work piece is very small resulting in a finer surface finish. An increase of fatigue life is observed in various investigations due to compressive residual stresses, work hardening and modified local geometry created by improvement methods. One such investigation is done by Yildirim and Marquis where they evaluated published experimental results and have proposed design SN curves for HFMI treated welds. For every increase of 200MPa in yield strength choosing 355MPa as a reference they found 12.5% of increase in strength. The behavior of compressive residual stresses in HSS welds is also an important aspect and it is studied in a number of investigations. These stresses are observed to be stable for a number of cycles in constant amplitude fatigue testing. The stability of these induced stresses during fatigue also explains the enhancement in fatigue life [7-16].


Figure 1. Residual stress distribution in the welded specimens in transverse (left) and longitudinal (right) directions after the welding./[7]

However, limited research has been done on the surface and depth distribution of residual stresses induced by HFMI in HSS welds, which is the main focus of this research work. Also stress measurement results from laser remelting tests will be presented here. Both the surface and depth stress measurements are carried out using XRD. 2. Test specimen The test specimen consists of a base plate with 2 fillet-welded longitudinal stiffeners. Different thicknesses (5, 10, 15 or 20 mm) and grades (S960QL, S960MC, S700MC, S690QL, S420MC) of HSS material have been used to manufacture the specimens. Thickness of the stiffener is equal to the corresponding thickness of the base plate in a specific test specimen. At least the first and last 30 mm of the stiffener (measured from the beginning/end) have been welded with a full penetration weld. These samples experience typically toe crack which initiates in the tip of the weld. These samples have four critical locations A, B, C and D. Basic surface residual stress measurements have been done from all four locations and presented as average values. Detailed geometry and dimensions are shown in Figure 2 below.

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Figure 2. Sample and residual stress measurement location. Four measurement locations; both ends and both sides. Stress measurements were done from samples of 5 and 10 mm.

3. Treatments Following improvement techniques were used: high-frequency mechanical impact peening (HFMI), weld dressing by direct diode laser and welding with low-transformation temperature (LTT) filler metal. HFMI treatment method was mainly two different pneumatic and ultrasonic methods. Four different companies were used for peening. Peening condition as number of pins, their radius, impact time required was not specified but asked the companies to use best parameters based on their knowledge. Goal of the direct-diode laser re-melting is potential improvement technique based on the removal of the harmful sharp discontinuities that are formed at weld toes (stress concentrations) and possibly introducing compressive stresses at the welds. With LTT goal is to avoid the post weld processing. The partial stabilisation of austenite in the weld and subsequent controlled decomposition into martensite can result in compressive residual stresses in the weld toe. 4. Residual stress measurements There are several different methods to measure stresses as sectioning, hole drilling and diffraction methods are few to mention. In this work only X-ray diffraction has been used. Benefit of X-ray diffraction is that with present technology there are no limitations to the size of the sample and also stresses in complicated geometries can be measured. Drawback of X-ray is the low penetration depth which demands etching of the material away to measure depth distributions. In this work residual stress measurement were done from full size samples by a Robot X-ray equipment. Robot performs the goniometer movements. Standard Cr-Ka radiation was used to measure the ferrite interference line (211). Collimator size was 2 mm. Figure 3 shows the measurement arrangement and the directions. Collimator size determines that the first point (zero) is about 2 mm from the toe edge; the center of the measurement spot size of ̋2 mm. First ten points have been measured with 1 mm steps and after that with 2 mm steps.

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Figure 3. Measurement arrangement and directions. Phi -90° corresponds to longitudinal measurement direction to the sample.

5. HFMI treated samples Measured residual stress distribution in as welded and HFMI treated samples are shown in Figure 4 below. These results are average values of the four different locations (shown in Figure 2) in the sample . Scatter is not shown in the graph and it is about ±150MPa. Stress level differences outside of HAZ in base material are due to different surface treatment of the plates before welding. Also in the figure 4 the FWHM surface distributions of the diffraction peaks are presented. These are again average values of four different locations with average scatter of ±0.05 degrees. FWHM describes the microstresses annealing or work hardening of the material.

S420MC 5

600

4

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Figure 4. Surface residual stress and FWHM values of as welded and HFMI treated samples of steels S420 and S700. Plate thickness was 5 mm. Measurement direction was Phi - 90°.

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6. Laser treated Figure 5 shows the results before and after laser treatment from steel S690QL and S960QL. Tensile stress levels of as welded samples are low compared to expected levels (Figures 1 and 4). It is not clear why. Only known difference in the case of Figure 4 is thicker plate. FWHM values of as welded samples are as in the other case of Figure 4.

S690QL

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Figure 5. Surface residual stress level of as welded and laser treated samples of two different steel grades: S690QL and S960QL. Samples are different and plate thickness 10 mm. LTT welded samples. Measurement direction was Phi -90°.

In the goal of using low-transformation temperature (LTT) filler material is to achieve compressive stresses without any post weld treatment. It is affecting only the weld bead itself. Figure 6 results shows that in the toe area there are high tensile stresses. These stresses are lower than in the case of Figure 4 but higher than in the case of Figure 5. Plate thickness has been same as in case of Figure 5 samples. Same annealing effect in the toe area can be seen as in other as welded samples. High compressive stresses in the base material are due to cleaning of the surface by sand blasting before welding.

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400

3.5

300

Stress MPa

100

0 0

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Figure 6. With LTT filler material welded samples. Plate thickness is 5 mm. Measurement direction was Phi -90°.

7. Depth distribution of HFMI treatment Depth distribution has been measured by electropolishing material away step wise. Electropolishing area has been 5*20 mm. Colors from red to blue correspondence changes from tensile to compressive stresses. Dark red line shows the zero stress levels. There are minor differences in the compressive stress areas between different samples. HFMI treatment has created compressive stresses up to about 2 mm depth in the toe area as can been seen in 0 mm depth profile distribution in Figure 7 lower right corner.

Figure 7. Depth distribution of the HFMI treated toe areas. Done in four different companies. Steel grade S700MC and plate thickness 8 mm 025_A etc refers to sample identification numberDepth distribution of fatigued HFMI samples.

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One of main question in the treated weld seam is how steady are the compressive residual stresses in the fatigue. Generally it depends strongly from maximum load in the fatigue cycle. In Figure 8 it is presented four cases of residual stress depth distributions after fatigue testing. In three cases compressive stresses are relaxed totally and one case only partly.

Figure 8. Depth distributions of HFMI treated samples. Steel grade was S700MC and plate thickness 8 mm. 024_A are referring to sample indentification and measurement location.

8. Discussion Nominally same specimens treated with HFMI have been investigated in various studies [8-9, 17] failures have been observed at the four critical locations mentioned in Figure 2 above. In the Figures below failure pictures of the specimens can be found.

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(a)

Failure location HFMI Yildirim and Marquis

(c)

(b)

Failure location HFMI Khurshid et al

Failure LTT specimen

Figure 9. Failure locations.

Knowledge of residual stresses due to improvement methods is very important for fatigue life of a structure. In the literature improvement techniques have been found to enhance the fatigue life of welded joints. In this investigation surface and depth residual stresses have been measured at four critical locations. Residual stress distribution in HFMI , laser treated and LTT joints has been focused on. The joints have been made of different steel grades. In Figure 4 for HFMI treated welded joints mostly compressive residual can be seen in the treated area. There is no clear difference between two different strength of the steel. From FWHM graphs it can be observed that work hardening is little bit stronger in S700 steel as in S400 steel. In Figure 5 in case of laser treated welded joints average level of FWHM values of steel S960QL are higher than steel S690QL as expected based on their strength levels. Laser treatment seems to lower little bit tensile stresses and slightly go to compressive side. However this effect is small compared to HFMI treatment. FWHM values of laser treated area are higher than as welded condition which indicates hardening of the surface layer. In case of LTT it is found that main affects is in the weld bead and slight reduce of the magnitude of residual stresses in HAZ in comparison to as welded specimens. From Figure 4, 5 and 6 it can be observed that significant compressive residual stresses have been created by HFMI. In Figure 7 and 8 depth distribution of residual stresses induced by HFMI can be seen. Compressive residual stresses up to a depth of 2 mm have been induced. The stability of these stresses is quite important and relaxation takes place when locally applied stress is greater than the local yield strength of the material [6-9]. i.e 購鎮墜頂半購槻

(1)

190


購鎮墜頂"銚椎椎鎮沈勅鳥噺 " 計痛 "購津墜陳沈津銚鎮髪 " 購追勅鎚沈鳥通銚鎮

(2)

where"

購槻噺検件結健穴"嫌建堅結券訣建月計建噺嫌建堅結嫌嫌"潔剣券潔結券建堅欠建件剣券"血欠潔建剣堅購追勅鎚沈鳥通銚鎮噺堅結嫌件穴憲欠健"嫌建堅結嫌嫌結嫌

Figure 8 shows that reduction of compressive residual stresses has been observed in the depth direction in two samples 024 and 041. In the case of sample 12 there are no or only minor relaxations in the compressive stresses. These differences are related to the load of the fatigue tests. Similar phenomena have been found in the investigations [8 ,9, 12, 16 and 17]. 9. Conclusions In this work residual stresses in the weld toe area in high strength steels have been studied. Both surface and depth distribution of residual stresses in as welded and post weld treated weld seams have been measured. Also four samples have been measured after fatigue testing. Post weld treatment methods which have been studied are HFMI and laser re-melting. Relative new method low-transformation temperature (LTT) filler material samples have been measured. Clearest strongest effect gives HFMI method. Laser treatment has minor effect and LTT filler material has practically known beneficial effect to the toe area tensile stresses. Results indicate that thin plate is more sensitive to high tensile stresses than thicker plate. This can be related also to different amount of layers. In the fatigue testing compressive residual stress have relaxed different amount.

Acknowledgement The research fund for coal and steel is acknowledged for the financial support through the European project FATWELDHSS.

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