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RESEARCH PAPERFatigue strength improvement of steel structures by high-frequency mechanical impact:proposed procedures and quality assurance guidelinesGary Marquis &Zuheir BarsoumReceived:18March 2013/Accepted:29May 2013/Published online:16June 2013#International Institute of Welding 2013Abstract High-frequency mechanical impact (HFMI)has emerged as a reliable,effective,and user-friendly method for post-weld fatigue strength improvement technique for welded structures.During the past decade,46documents on HFMI technology for fatigue improvements have been presented within Commission XIII of the International Institute of Welding (IIW).This paper presents an overview of the lessons learned concerning appropriate HFMI procedures and quality assurance measures.Due to differences in HFMI tools and the wide variety of potential applications,certain details of proper treatment procedures and quantitative quality control measures are presented generally.Specific details should be documented in an HFMI procedure specification for each structure being treated.It is hoped that this guideline will provide a stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing inter-national guidelines relevant to a variety of HFMI technologies and applicable to many industrial sectors.A companion docu-ment presents a fatigue design proposal for HFMI treatment of welded steel structures.The proposal is considered to apply to steel structures of plate thicknesses of 5to 50mm and for yield strengths ranging from 235to 960MPa.Stress assessment may be based on nominal stress,structural hot spot stress,or effec-tive notch stress.Keywords High-frequency mechanical impact (HFMI).Weld toe improvement .Fatigue improvement .Quality control1IntroductionIn 2007,Commission XIII:Fatigue of Welded Components and Structures approved the best practice recommendations concerning post-weld treatment methods for steel and alu-minum structures [1].This recommendation covers four commonly applied post-weld treatment methods:burr grind-ing,tungsten inert gas (TIG)remelting (i.e.,TIG dressing),hammer peening,and needle peening.Burr grinding and TIG remelting are generally classified as geometry improvement techniques for which the primary aim is to eliminate weld toe flaws and to reduce local stress concentration by achieving a smooth transition between the plate and the weld face.Ham-mer peening and needle peening are classified as residual stress modification techniques which eliminate the high ten-sile residual stress in the weld toe region and induce com-pressive residual stresses at the weld toe.These methods also result in a reduced stress concentration at the weld toe.The guidelines also give practical information on how to imple-ment the four improvement technologies,namely good work practices,training,safety,and quality assurance.The improvement techniques described in these recommen-dations are intended to be used both for increasing the fatigue strength of new structures and for the repair or upgrade of existing structures.It has consistently been emphasized that,especially with respect to new structures,weld improvement techniques should never be implemented to compensate for poor design or bad fabrication practices.Instead,improvement measures should be implemented as a means of providing additional strength after other measures have been taken.Doc.IIW-2395,recommended for publication by Commission XIII “Fatigue of Welded Components and Structures.”G.Marquis (*)Department of Applied Mechanics,Aalto University,Espoo,Finlande-mail:gary.marquis@aalto.fiG.Marquis :Z.BarsoumDivision of Lightweight Structures,KTH-Royal Institute of Technology,Stockholm,SwedenWeld World (2014)58:19–28DOI 10.1007/s40194-013-0077-8Simultaneous with the development of the 2007recom-mendations,an increasing number of presentations within Commission XIII focused on high-frequency mechanical impact (HFMI)as a means of improving the fatigue strength of welded structures.From 2002to 2012,46IIW Commis-sion XIII documents reporting HFMI technology or experi-mental studies involving HFMI-based fatigue strength im-provement were presented.HFMI has emerged as a reliable,effective,and user-friendly method for post-weld fatigue strength improvement technique for welded structures.This paper represents an attempt to summarize and synthe-size the knowledge gained both within the IIWand presented in the open international literature concerning quality assurance of HFMI-treated welds.It covers procedure-related and quality assurance-related items such as relevant equipment,proper application procedures,material requirements,safety,training requirements for operators and inspectors,quality controlmeasures,and documentation.All HFMI devices have unique features,and the type of structure being treated (and especially the material grade and welding procedures)will greatly influ-ence the optimal treatment procedures.For this reason,the current document is intended to provide only general guidance especially with respect to operator training,procedures,and inspection.Specific operator training is provided by the tool manufacturers.Specific treatment procedures and requirements can normally be developed in cooperation with the HFMI device manufacturer.It is not the intention of this paper to compare HFMI tools or their efficiency.The goal is only to give an overview of topics which must be considered when preparing an HFMI procedure specification.A companion document [2]presents a fatigue design pro-posal for HFMI treatment of welded steel structures.The design proposal is considered to apply to steel structures of plate thicknesses between 5and 50mm and for yield strengths ranging from 235to 960MPa.Stress assessment may be based on nominal stress,structural hot spot stress,or effective notch stress using stress analysis procedures as defined by the IIW Commission XIII.The design proposal includes a pro-posal for the effect of material strength and a proposal for high R ratio and variable amplitude loading.Several topics for future study with respect to HFMI are given.2High-frequency mechanical impact 2.1BackgroundThe innovation of improving the fatigue strength of welded structures by locally modifying the residual stress state usingas-weldedafter HFMIHFMIAW Fig.1Typical weld toe profile in the as-welded condition and follow-ing HFMI treatment [13,14]Photo courtesy of Applied Ultrasonics.Photo courtesy of Integrity Testing Laboratory (ITL) and Structural Integrity Technologies Inc. (SINTEC)Photo courtesy of Pfeifer Seil-undHebetechnik GmbHPhoto courtesy of PITEC GmbHbc dFig.2Examples of HFMI devices available worldwide.a ultrasonic impact treatment,b ultrasonic peening,c high-frequency impact treatment,and d pneumatic impact treatment (PIT)ultrasonic technology is attributed to scientists and engineers working in the former Soviet Union [3,4].In the past decade,there has been a steady increase in the number of HFMI peening equipment manufacturers and service providers.In 2010,Commission XIII of the IIW introduced the term HFMI as a generic term to describe several related technologies.Alternate power sources are employed,for example,ultrasonic piezoelectric elements,ultrasonic magnetostrictive elements,or compressed air.In all cases,however,the working principal is identical:cylindrical indenters are accelerated against a component or structure with high frequency (>90Hz).The impacted material is highly plastically deformed causing changes in the material microstructure and the local geometry as well as the residual stress state in the region of impact.Various names have been used in literature to describe the devices:ultrasonic impact treatment [5],ultrasonic peeningPhoto courtesy of Integrity Testing Laboratory (ITL) and Structural Integrity Technologies Inc. (SINTEC)a bFig.3a Examples of indenter sizes and configurations and b a double radius indenter developed by the Northern Scientific and Technical Company,Russia for Esonix UIT [18]weld metalHAZweld metalbase metaldefectbase metaldefectdefectshiny defect-free HFMI grooveabcFig.4a Potential introduction of a crack-like defect due to HFMI treatment of a weld with a steep angle or with too large of an indenter and b resulting grooves for a properly treated weld toe (left )and animproperly treated one (right );c micrographs of the induced crack-like defects due toimproper HFMI treatment [18][6],ultrasonic peening treatment[7][8],high-frequency impact treatment[9],pneumatic impact treatment[10],and ultrasonic needle peening[11,12].Figure1shows typical weld profiles in the as-welded condition and following HFMI treatment.In comparison to hammer peening,the operation is considered to be more user-friendly and the spacing between alternate impacts on the work piece is very small resulting in a finer surface finish.2.2EquipmentAs previously mentioned,numerous new HFMI devices have been developed during the past10years,and the number continues to increase.Figure2shows some of the HFMI devices that are in use worldwide.A recent round robin exercise[15]and literature survey[16,17]have iden-tified several HFMI tools which,when properly used,pro-vide the degree of improvement noted in the proposed fa-tigue design guideline for HFMI-treated welded joints[2]. Similar devices can be assigned to this group if they have the same operating principal and are objectively tested and are shown to have the same reliable and beneficial effect on the fatigue strength of welds as in the proposed guideline.Ultrasonic devices consist of a power unit and tool.These normally require compressed air or circulating water to con-trol the temperature of the tool.Other devices known to the authors are pneumatic.The indenters are high-strength steel cylinders,and manufacturers have customized the effective-ness of their own tools by using indenters with different diameters,tip geometries,or multiple indenter configura-tions.Indenters are consumable items which from time to time require replacement or refurbishment.Figure3shows several examples of indenter sizes and configurations which are available.3Procedures3.1Operator trainingWhen delivering new equipment,tool manufacturers normal-ly provide1–2days of operator training.As new applications arise,tool manufactures can provide specialized training or customized procedure specifications.In some cases,HFMI treatment of structures with curvilinear weld toes,e.g.,weld toes in trusses fabricated from circular hollow sections,has proven to be very demanding and will require more expertise than for treating long straight welds or short weld corners.Because HFMI is normally specified as a fatigue strength improvement technology for new structures or during repair and retrofitting operations,it is always essential to consult fatigue experts to ensure that all critical regions in a structure are identified and properly treated.Most fatigue-loaded struc-tures will normally have only a limited number of locations that are critical from a fatigue point of view.Proper identifica-tion of these regions is also important to avoid extra costs and treatment of regions which are not fatigue critical.Additional-ly,the possibility of a failure starting at some other location must always be considered.For instance,if the failure origin is merely shifted from the weld toe to the root,there may be no significant improvement in fatigue life.Some additionalTable1Sample treatment procedure parameters for two HFMI toolsParameter HFMI toolHigh-frequency Impact treatment(HiFIT)[21]Ultrasonic Impact Treatment(UIT)[22,23]Power source Pneumatic Ultrasonic magnetostrictiveNumber of indenters11–4Angle of the axis of the indenters with respect to the plate surface,ϕ(see Fig.5)60°to80°30°to60°[22]40°to80°[23]Angle of the axis of the indenterswith respect to the direction oftravel,ψ(see Fig.5)70°to90°90°(all pins should contact the weld toe)Working speed3to5mm/s5to10mm/s[22]5to25mm/s[23]Other The self-weight of the tool is sufficient[22,23]Minimum of5passes[23]travel speedFig.5Orientation of the HFMI tool with respect to the weld beingtreatedcomments on this topic may be found in the companion fatigue design proposal for HFMI-treated welded joints [2].In the case of multipass welds,it is also needed to treat also the interpass weld toes [19].3.2Weld preparationThe weld cap and adjacent parent material shall be fully de-slagged and wire-brushed or ground to remove all traces of oxide,scale,spatter,and other foreign material.HFMI treat-ment of a convex weld profile or of a weld with a large weld angle can cause the plastically deformed metal to fold over the original weld toe and leave a crack-like lap feature that resembles a cold lap.The weld bead profile should meet the acceptance limits for the weld profile quality level B in ISO 5817[20].This requirement does not imply that the weld must fulfill all quality level B criteria in ISO 5817.However,weld profile-related quality criteria in ISO 5817need to be evaluated.These include undercuts (imperfection 1.7),ex-cessive overfill (imperfection 1.19),excessive concavity (imperfection 1.10)and overlaps (imperfection 1.13).If the weld profile does not comply with these acceptance limits,light grinding before treatment may be desired.It should be noted,however,that HFMI treatment is most effective when the weld toe region itself is treated.Thus,grinding operations which make it difficult for the HFMI operator to distinguish the exact location of the weld toe should be avoided.De-cisions on the need for weld grinding and the proper grinding procedure should be agreed on with an experienced HFMI operator.The need for a proper weld profile before HFMI is illus-trated in Fig.4a which illustrates the formation of a crack-like defect due to improper contact between the indenter and theweld toe.Surface inspection of such a defect reveals a dark crack-like line in the middle of the otherwise smooth and shiny HFMI groove as seen in Fig.4b .Figure 4c shows section micrographs of these defects.The resulting fatigue performance of a welded joint with such defects may actually be less than that of the original as-welded joint.The same type of flaw has been observed in welds with adequate profiles but with improper indenter selection or too severe treatment,i.e.,too many passes over the same region.For specific applica-tions,it may be needed to consult with the HFMI tool manu-facturer in order to select the proper treatment procedures and optimal indenter configuration to avoid crack-like defects.3.3Safety aspectsNoise and vibration from HFMI is significantly less than for more traditional peening equipment.HFMI treatment can be a noisy operation,and it is essential that the operator and others working in the vicinity should use ear protection.Normal protective clothing for working in a fabrication shop is ade-quate but it should include approved eye protection.Vibration from HFMI equipment is usually low enough so that contin-uous operation is permitted without restriction during a nor-mal 8-h work shift.If the vibration of the specific HFMI tool has not been determined,it may be needed to limit the amount of time per day for performing HFMI treatment.Equipment-specific safety issues are provided by the tool manufacturers.3.4Weld toe treatmentSpecific weld toe treatment procedures will vary greatly from application to application and depending on the tool being used.Thus,only general topics can be covered.Table 1weld metalbase metalFig.6The HFMI groove in a shows a thin crack-like defect which reduces or eliminates the effectiveness of the HFMI treatment [21].b shows a defect-free groove but with an individual indenter strike still visible,indicating the need for additional passes [27]Fig.7a Proper profile of an HFMI groove which has no sharp or crack-like features and b an improper HFMI groove profile which shows thepresence of a crack-like feature due to plastic deformation of the materialprovides example procedure parameters for two HFMI tools with alternate power sources and indenter configurations (see also Fig.5).Excessive treatment of a weld toe should be avoided.The American Association of State Highway and Transportation Officials (AASHTO)have developed sample procedures [23]based on research performed at Lehigh University,USA [24,25].3.5Other treatment conditionsHeat treatment and hot-dip galvanizing should not be performed after HFMI.HFMI introduces beneficial compressive residual stresses which may be reduced or eliminated by these opera-tions.The fatigue strength of an HFMI-treated component which is then treated by hot-dip galvanizing may have improved strength with respect to a hot-dip galvanized component without HFMI.In such a case,the fatigue design proposal for HFMI treatment of welded steel structures [2]cannot be used and fatigue strength should be determined by fatigue testing.Static local stresses near a weld toe are the result of both welding residual stresses and dead loads on a structure.If the tensile residual stresses following welding are close to theyield strength of the material (as is normally assumed),the addition of a dead load will cause local yielding but will not result in increased maximum local stresses.HFMI treatment following the application of the dead load will produce compressive residual stresses in the critical weld toe region.On the other hand,if HFMI treatment is performed before the dead loads are applied,the compressive residual stresses following treatment may be partially counteracted by the local tensile stresses due to the dead load.Thus,if significant dead loads are present on the structure during normal usage,it is recommended that the dead loads are applied prior to treatment,i.e.,erect the structure with the welds untreated and then perform the treatment on-site [26].4Quality controlVisual inspection of the HFMI groove following treatment consists of both qualitative and quantitative measures of the treated area.4.1Qualitative measuresVisual inspection following treatment includes an evaluation of the quality of the groove and the groove depth.The resulting groove must be smooth along all defined welds.A smooth and shiny groove without lines is one characteris-tic of a properly treated weld toe (see Fig.4b ).No thin line representing an original fusion line should be visible in the groove.A thin crack-like line such as that shown in Fig.6a is an indication that the weld fusion line has not been treated as previously described in section 3.2.Dye penetrant or simple magnification with a ×3to×10magnifying glass with proper surface illumination (minimum 350lx)will be helpful in assessing the quality of the HFMI groove.Figure 6b shows an HFMI groove which is not smooth and showsindicationsFig.8The HFMI indentation depth following treatment should be 0.2–0.6mm while the resulting width is typically 2–5mmgapFig.9Depth inspection using simple gauges [21].The gap between the base plate and the gauge in the right-hand picture indicates that 0.2mm has not been achievedof individual indenter strikes.Additional passes of the tool would be required to obtain a smooth finish.The HFMI groove must be continuous with no breaks.If the treatment cannot be performed without interruption,e.g.,long welds or around corners,it is recommended that the operation be restarted at least 10mm behind the stop posi-tion.No indications of undercut or porosity in the HFMI area can be visible.Similar qualitative measures have been spec-ified by AASHTO [23,26].HFMI produces significant local cold-forming of the ma-terial near the weld fusion line.If the indenters are directed excessively in one specific location,the resulting plastic displacement of the metal can result in a crack-like featureat the side of the HFMI groove as shown in Fig.7.Failures of this type have been occasionally observed but not studied systematically [28].The crack-like feature should be re-moved by light grinding and the weld toe should be retreated.4.2Quantitative measuresThe depth of the groove is an excellent indicator of the extent of HFMI treatment [29].Depending on the yield strength of the steel and the size of the indenters,typically the optimum HFMI groove will be 0.2–0.6mm in depth and the width will be 3–6mm,see Fig.8[23,26,30,31].However,it should be noted that no single groove dimension is optimal in all situations.AFig.10An example of a HFMI-PS (LETSGlobal —Ultrasonic Peening Procedure Specification)developed for each weld in a structures as a quality assurance measure [19]welded structure with relatively deep undercuts at the weld toe of which requires light grinding of the weld toe before HFMI will necessarily have deeper grooves following HFMI.Also, HFMI grooves in high-strength steel structures will typically be shallower and narrower than grooves in low-strength steel. Groove depth can be checked relatively easily by using simple depth gauges such as is shown in Fig.9.Calipers can be used to measure the width of the groove.The center of the HFMI groove should correspond to the fusion line of the weld.The portion of the HFMI groove in the weld metal must be between 25and75%of the total HFMI groove width[30].In large,complex welded structures,welding heat input will not always be constant along a long weld.For this reason, material hardness at the weld toe may vary and the HFMI treatment may need to be systematically adapted.HFMI groove dimension checks will be needed at regular intervals.4.3DocumentationAn HFMI procedure specification(HFMI-PS)similar to a welding procedure specification should be prepared for the HFMI treatment.The HFMI-PS includes information concerning the component being treated;base and filler material;HFMI equipment type and power settings;number,size and shape of the indenters to be used;special inspection requirements includ-ing HFMI groove dimension,etc.Lopez Martinez and Haagensen have developed an HFMI-PS template which is prepared for each weld in a structure[19],see Fig.10.4.4CalibrationAll of the available HFMI devices have variable power settings which can be adjusted depending on the material being treated and the indenter configuration.As a quality assurance measure,the intensity should be recorded in the HFMI-PS.In some cases,HFMI tool calibration is accom-plished during treatment of a welded joint by ensuring that the resulting HFMI groove dimensions for a specified power setting and treatment time are consistent with predetermined limits.For its own tools,PITEC and other companies have developed a simple test for measuring the intensity of HFMI treatment[32].The concept is similar to that used in the well-known Almen strip test which is common for measuring the intensity of shot peening and blasting operations.The simple equipment used for this test is shown in Fig.11.Residual stress-free flat strips(200mm×20mm×4mm)of S355J2 steel are held in a special fixture.HFMI is applied to the strip via the longitudinal slots.Four to five passes with an HFMI tool with a predefined power setting are applied.Curvature of the strip,which is related to the resulting residual stress,is measured by means of a dial gauge.5DiscussionA great deal of experimental evidence has demonstrated that HFMI can significantly improve the fatigue strength of welded structures.Rarely,but on occasion,test results have been presented which indicate that the HFMI treatment pro-cedure has not been fully understood and/or implemented incorrectly.While HFMI can be considered as environmen-tally friendly,safe,and relatively easy to apply,operators must still exercise safe work practices and understand the equipment and the nature of the post-weld operation which is being imparted to a welded structure.Longitudinal stotsFixtureSteel strip Dial gaugeFig.11Equipment needed to perform the Almen test-type calibration procedure developed by PITEC[32]This paper presents an overview of the lessons learned concerning appropriate HFMI procedures and quality assur-ance measures as discussed primarily with the IIW.Due to differences in the HFMI tools and the wide variety of poten-tial applications,certain details of proper treatment proce-dures and quantitative quality control measures are presented generally.For example,the HFMI groove depth,which is considered to be an important quantitative quality assurance measure,can optimally vary from0.2mm to as much as 0.6mm depending on the steel being treated and the geom-etry of the indenter(s).Travel speed,the number of passes needed to obtain optimal treatment,and the angle of the axis of the indenters with respect to the plate surface(see Fig.5) will also vary significantly depending on the tool being used. Specific details of the treatment process and inspection re-quirements for a structure or component should be docu-mented in an HFMI procedure specification.Qualitative inspection requirements including the shiny appearance of the HFMI groove,the lack of any crack-like lines in the groove,the position of the groove with respect to the original weld fusion line,and the continuity of the HFMI groove are applicable to all tool types and for all welds.Weld preparation prior to HFMI treatment and safety items can also be considered to be universally applicable.It is hoped that this guideline will provide a stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing an international guideline relevant to a variety of HFMI tech-nologies and applicable for many industrial sectors.There are a number of questions which cannot yet be reliably answered nor included into guidelines.These remain as areas for further research studies.For example,what type of repair procedures can be recommended if a crack-like defect(see Fig.6a)still exists after five HFMI passes?When do crack-like defects such as those shown in Fig.7become significant and how should these be removed?Is it possible to develop a catalog of suitable treatment processes for common HFMI devices and typical construction situations? The influence of fabrication processes following HFMI treat-ment also need to be better quantified.For example,if weld repair is required,at what distance from the weld repair does HFMI treatment remain unaffected.What is the quantitative influence of galvanizing on HFMI-treated structures?In refurbishment situations,what is the precise role of the service load history prior to HFMI treatment?6ConclusionsA proposal for procedures and quality assurance for HFMI-treated welded joints in steel has been presented.It was developed based on discussions,presentations,and experi-mental evidence published within Commission XIII of the IIW.The proposal has been reviewed by several HFMI tool manufacturers and has been compared to other available technical documents.The proposal includes brief descrip-tions of HFMI equipment,operator training,weld prepara-tion,safety aspects,treatment procedures,qualitative and quantitative quality control measures,procedure documen-tation,and equipment.Certain details of the precise treat-ment procedures and quantitative quality control measures can vary greatly depending on the specific welded structure being treated.A companion document presents a fatigue design proposal for HFMI treatment of welded steel struc-tures.The proposal is considered to apply to steel structures of plate thicknesses from5to50mm and for yield strengths ranging from235to960MPa.Stress assessment may be based on nominal stress,structural hot spot stress,or effec-tive notch stress.Acknowledgments Support for this work has been partially provided by the LIGHT research program of the Finnish Metals and Engineering Competence Cluster,the Finnish Funding Agency for Technology and Innovation,and the European Union’s Research Fund for Coal and Steel Research Programme under grant agreement no RFSR-CT-2010-00032:“Improving the fatigue life of high strength steel welded struc-tures by post weld treatments and specific filler material.”Cooperation with HFMI companies Pfeifer Seil-und Hebetechnik GmbH,Germany; Structural Integrity Technologies Inc.,Canada;LETS Global AB,Swe-den,Applied Ultrasonics,the Netherlands;and PITEC GmbH,Germa-ny are acknowledged.References1.Haagensen PJ,Maddox SJ(2012)IIW recommendations on postweld fatigue life improvement of steel and aluminium structures.Woodhead Publishing Ltd.,Cambridge2.Marquis GB,Mikkola E,Yildirim HC,Barsoum Z(2013)Fatiguestrength improvement of steel structures by HFMI:proposed fa-tigue assessment guidelines.International Institute of Welding, Paris,IIW Document XIII-2452r1-133.Statnikov ES,Shevtsov UM,Kulikov VF(1977)Ultrasonic impacttool for welds strengthening and reduction of residual stresses.Publ Sci Works:Metall SEVMASH,USSR92:27–28(in Russian)4.Kudryavtsev YF,Trufyakov VI,Mikheev PP,Statnikov EF,Burenko AG,Dobykina EK(1994)Increasing the fatigue strength of welded joints in cyclic compression.International Institute of Welding,Paris,Document XIII-1596-945.Applied Ultrasonics.In:/.6.Integrity Testing Laboratory Inc.In:/.7.Lets Global.In:/.8.Huo L,Wang D,Zhang Y(2005)Investigation of the fatiguebehaviour of the welded joints treated by TIG dressing and ultra-sonic peening under variable-amplitude load.Int J Fatigue27:95–1019.Pfeifer.In:http://www.pfeifer.de/.10.Pitec.In:/.11.Sonats.In:/.12.Bousseau M,Millot T(2006)Fatigue life improvement of weldedstructures by UNP compared to TIG dressing.International Insti-tute of Welding,Paris,Document XIII-2125-06。