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Improving fatigue properties of welded high strength steels
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0003-4978-390X
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In recent years a strong interest has been expressed to produce lighter structures.One possible solution to reduce the weight is to utilize high strength steels and use welding as the joining method. Many components experience fatigue loadingduring all or part of their life time and welded connections are often the prime location of fatigue failure. This becomes more critical in welded high strength steels as fatigue strength of welds does not increase by increasing the steel strength. A possible solution to overcome this issue is to use fatigue improvement methods.The main objectives of this project are, therefore, to increase understanding of the factors that control fatigue life and to investigate how the fatigue strength improvement methods; high frequency mechanical impact (HFMI) treatment and use of Low Transformation Temperature (LTT) consumables will affect fatigue properties of welds in high strength steels. In this regard, Gas Metal Arc Welding(GMAW) was used to produce butt and fillet welds using LTT or conventional fillers in steels with yield strengths ranging from 650-1021 MPa and T-joint weldsin a steel with 1300 MPa yield strength. The effect of HFMI on fatigue strength of the welds in 1300 MPa yield strength steels was also investigated. Butt and fillet welds in 650-1021 MPa steels were fatigue tested under constant amplitude tensile loading with a stress ratio of 0.1 while T-joints were fatigue tested under constant amplitude fully reversed bending load with a stress ratio of -1. The nominal stress approach was used for fatigue strength evaluation of butt and fillet welds whereas the effective notch stress approach was used in case of T-joints. Relative effectsof the main parameters such as residual stress and weld toe geometry influencing fatigue strength of welds were evaluated. Residual stresses were measured using X-ray diffraction for as-welded and HFMI treated welds. Neutron diffraction was additionally used to investigate the near surface residual stress distribution in 1300 MPa LTT welds.Results showed that use of LTT consumables increased fatigue strength of welds in steels with yield strengths ranging from 650-1021 MPa. For butt welds, the vii characteristic fatigue strength (FAT) of LTT welds at 2 million cycles was up to46% higher when compared to corresponding welds made with conventional fillermaterials. In fillet welds, a maximum improvement of 132% was achieved when using LTT wires. The increase in fatigue strength was attributed to the lower tensile residual stresses or even compressive stresses produced close to the weldtoe in LTT welds. Weld metals with martensite transformation start temperatures around 200 °C produced the highest fatigue strength. In 1300 MPa yield strength steel, similar FAT of 287 MPa was observed for LTT welds and 306 MPa for conventional welds, both much higher than the IIW FATvalue of 225 MPa. The relative transformation temperatures of the base and weldmetals, specimen geometry and loading type are possible reasons why the fatigue strength was not improved by use of LTT wires. Neutron diffraction showed that the LTT consumable was capable of inducing near surface compressive residual stresses in all directions at the weld toe. It was additionally found that there arevery steep stress gradients both transverse to the weld toe line and in the depth direction, at the weld toe. Due to difficulties to accurately measure residual stresses locally at the weld toe most often in the literature and recommendations residual stresses a few millimetre away from the weld toe are related to fatigue properties. However, this research shows that caution must be used when relating these to fatigue strength, in particular for LTT welds, as stress in the base materiala few millimetre from the weld toe can be very different from the stress locally at the weld toe.HFMI increased the mean fatigue strength of conventional welds in 1300 MPa steels about 26% and of LTT welds by 13%. It increased the weld toe radius slightly but produced a more uniform geometry along the treated weld toes. Large compressive residual stresses, especially in the longitudinal direction, were introduced adjacent to the weld toe for both LTT and conventional treated welds. It was concluded that the increase in fatigue strength by HFMI treatment is due to the combined effect of weld toe geometry modification, increase in surface hardness and introduction of compressive residual stresses in the treated region.It was concluded that the residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welds. This is based on the observation that a moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm, in fillet welds. Also, a higher fatigue strength was observed for HFMI treated conventional welds compared to as welded samples having similar weld toe radii but with different residual stresses.

Place, publisher, year, edition, pages
Trollhättan: University West , 2017. , 89 p.
Series
PhD Thesis: University West, 11
Keyword [en]
Fatigue strength; Residual stress; Welds; Weld toe geometry; High strength steel; High frequency mechanical impact treatment; Low Transformation Temperature welding consumable
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-11375ISBN: 9789187531606 (print)ISBN: 9789187531590 (electronic)OAI: oai:DiVA.org:hv-11375DiVA: diva2:1135578
Public defence
2017-09-06, 09:15 (English)
Opponent
Supervisors
Available from: 2017-08-23 Created: 2017-08-23 Last updated: 2017-08-28Bibliographically approved
List of papers
1. The relative effects of residual stresses and weld toe geometry on fatigue life of weldments
Open this publication in new window or tab >>The relative effects of residual stresses and weld toe geometry on fatigue life of weldments
2015 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 77, 160-165 p.Article in journal (Refereed) Published
Abstract [en]

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.

Keyword
Weld toe geometry, residual stress, fatigue strength, Low Transformation Temperature filler material
National Category
Engineering and Technology Manufacturing, Surface and Joining Technology Other Materials Engineering
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-7503 (URN)10.1016/j.ijfatigue.2015.03.023 (DOI)000354147300015 ()2-s2.0-84927128911 (Scopus ID)
Note

Available online 2 April 2015

Available from: 2015-04-07 Created: 2015-04-07 Last updated: 2017-08-23Bibliographically approved
2. Effect of high frequency mechanical impact treatment on fatigue strength of welded 1300 MPa yield strength steel
Open this publication in new window or tab >>Effect of high frequency mechanical impact treatment on fatigue strength of welded 1300 MPa yield strength steel
2016 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, 96-106 p.Article in journal (Refereed) Published
Abstract [en]

High frequency mechanical impact (HFMI) is a recent post weld treatment method which can be employed to increase the fatigue strength of welded components. In this paper the fatigue strength of as-welded and HFMI treated fillet welds in a 1300 MPa yield strength steel was compared. Fatigue testing was done under fully reversed, constant amplitude bending load. Finite element analysis was used to calculate the stress distribution in the weld toe region to permit evaluation of the fatigue data with the effective notch stress approach. As-welded samples showed a mean fatigue strength of 353 MPa and a characteristic fatigue strength of 306 MPa. HFMI treatment increased the mean fatigue strength by 26% and the characteristic fatigue strengths by 3%. The weld toe radii in as-welded condition were large. HFMI only increased the weld toe radii slightly but resulted in a more uniform weld toe geometry along the weld. A depth of indentation in the base metal in the range of 0.15–0.19 mm and a width of indentation in the range of 2.5–3 mm, were achieved. Maximum compressive residual stresses of about 800 MPa in the longitudinal and 250 MPa in the transverse direction were introduced by HFMI treatment, adjacent to the weld toe. The surface hardness was increased in the entire HFMI treated region. It is concluded that the increase in fatigue strength is due to the combined effects of the weld toe geometry modification, increase in surface hardness and creation of compressive residual stresses in the treated region.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Fatigue strength, High frequency mechanical impact treatment, high strength steel, weld toe geometry, hardness, residual stress
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9599 (URN)10.1016/j.ijfatigue.2016.06.019 (DOI)000383930200011 ()2-s2.0-84978062474 (Scopus ID)
Available from: 2016-07-24 Created: 2016-07-24 Last updated: 2017-08-23Bibliographically approved
3. Effect of HFMI treatment procedure on weld toe geometry and fatigue properties of high strength steel welds
Open this publication in new window or tab >>Effect of HFMI treatment procedure on weld toe geometry and fatigue properties of high strength steel welds
2016 (English)In: Procedia Structural Integrity, 2016, Vol. 2, 3483-3490 p.Conference paper, Published paper (Refereed)
Abstract [en]

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.

Keyword
Fatigue strength, high frequency mechanical impact treatment, high strength steel, weld toe
National Category
Mechanical Engineering
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9598 (URN)10.1016/j.prostr.2016.06.434 (DOI)
Conference
21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy
Available from: 2016-07-24 Created: 2016-07-24 Last updated: 2017-08-23Bibliographically approved
4. Applicability of Low Transformation Temperature welding consumables to increase fatigue strength of welded high strength steels
Open this publication in new window or tab >>Applicability of Low Transformation Temperature welding consumables to increase fatigue strength of welded high strength steels
2017 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 97, 39-47 p.Article in journal (Refereed) Published
Abstract [en]

Application of Low Transformation Temperature (LTT) consumables in welding is a recent approach to increase the fatigue strength of welds. In this paper high strength steels with yield strengths ranging from 650-1021 MPa were fillet and butt welded using different LTT and conventional consumables. The effects of weld metal chemical composition on phase transformation temperatures, residual stresses and fatigue strength were investigated. Lower transformation start temperatures and hence lower tensile or even compressive residual stresses were obtained close to the weld toe for LTT welds. Fatigue testing showed very good results for all combinations of LTT consumables and high strength steels with varying strength levels. For butt welds, the characteristic fatigue strength (FAT) of LTT welds at 2 million cycles was up to 46% higher when compared to corresponding welds made with conventional filler materials. In fillet welds, a minimum FAT improvement of 34% and a maximum improvement of 132% was achieved when using LTT wires. It is concluded that different LTT consumables can successfully be employed to increase fatigue strength of welds in high strength steels with yield strength up to 1021 MPa. Weld metals with martensite transformation start temperatures close to 200°C result in the highest fatigue strengths.

Keyword
Low Transformation Temperature Welding Consumables, Fatigue Strength, Residual Stress, Martensite Start Temperature
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-10212 (URN)10.1016/j.ijfatigue.2016.12.007 (DOI)000393631300005 ()2-s2.0-85007154275 (Scopus ID)
Available from: 2016-12-05 Created: 2016-12-05 Last updated: 2017-08-23Bibliographically approved
5. Improving fatigue strength of welded 1300 MPa yield strength steel using HFMI treatment or LTT fillers
Open this publication in new window or tab >>Improving fatigue strength of welded 1300 MPa yield strength steel using HFMI treatment or LTT fillers
2017 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 79, no September, 64-74 p.Article in journal (Refereed) Epub ahead of print
Abstract [en]

Fatigue improvement techniques are widely used to increase fatigue strength of welded high strength steels. In this paper high frequency mechanical impact (HFMI) and a Low Transformation Temperature (LTT) filler material were employed to investigate the effect on fatigue strength of welded 1300 MPa yield strength steel. Fatigue testing was done under fully reversed, constant amplitude bending load on T-joint samples. Fatigue strength of LTT welds was the same as for welds produced using a conventional filler material. However, HFMI treatment increased the mean fatigue strength of conventional welds about 26% and of LTT welds about 13%. Similar distributions of residual stresses and almost the same weld toe radii were observed for welds produced using LTT and conventional consumables. HFMI increased the weld toe radius slightly and produced a more uniform geometry along the treated weld toes. Relatively large compressive residual stresses, adjacent to the weld toe were produced and the surface hardness was increased in the treated region for conventional welds after HFMI. For this specific combination of weld geometry, steel strength and loading conditions HFMI treatment gave higher fatigue strength than LTT consumables.

Place, publisher, year, edition, pages
Oxford: Pergamon Press, 2017
Keyword
Fatigue strength; Low transformation temperature welding consumable; High frequency mechanical impact treatment; High strength steel; Residual stress
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-10923 (URN)10.1016/j.engfailanal.2017.04.024 (DOI)000405538800006 ()
Available from: 2017-04-27 Created: 2017-04-27 Last updated: 2017-09-20Bibliographically approved
6. Neutron Diffraction Evaluation of Near Surface Residual Stresses at Welds in 1300 MPa Yield Strength Steel
Open this publication in new window or tab >>Neutron Diffraction Evaluation of Near Surface Residual Stresses at Welds in 1300 MPa Yield Strength Steel
Show others...
2017 (English)In: Materials, E-ISSN 1996-1944, Vol. 10, no 6, 1-14 p., E593Article in journal (Refereed) Published
Abstract [en]

Evaluation of residual stress in the weld toe region is of critical importance. In this paper, the residual stress distribution both near the surface and in depth around the weld toe was investigated using neutron diffraction, complemented with X-ray diffraction. Measurements were done on a 1300 MPa yield strength steel welded using a Low Transformation Temperature (LTT) consumable. Near surface residual stresses, as close as 39 µm below the surface, were measured using neutron diffraction and evaluated by applying a near surface data correction technique. Very steep surface stress gradients within 0.5 mm of the surface were found both at the weld toe and 2 mm into the heat affected zone (HAZ). Neutron results showed that the LTT consumable was capable of inducing near surface compressive residual stresses in all directions at the weld toe. It is concluded that there are very steep stress gradients both transverse to the weld toe line and in the depth direction, at the weld toe in LTT welds. Residual stress in the base material a few millimeters from the weld toe can be very different from the stress at the weld toe. Care must, therefore, be exercised when relating the residual stress to fatigue strength in LTT welds.

Keyword
Residual stress; high strength steel; neutron diffraction; weld toe; low transformation temperature welding consumable
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-11065 (URN)10.3390/ma10060593 (DOI)000404415000027 ()28772953 (PubMedID)
Available from: 2017-06-29 Created: 2017-06-29 Last updated: 2017-09-20Bibliographically approved

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