THE VALUE/BENEFIT OF DEPTH OF COMPRESSIVE STRESSES DAN SPINNER: SUPERIOR SHOT PEENING SAM ABSTON: APPLIED ULTRASONICS

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1 THE VALUE/BENEFIT OF DEPTH OF COMPRESSIVE STRESSES DAN SPINNER: SUPERIOR SHOT PEENING SAM ABSTON: APPLIED ULTRASONICS It has long been understood that the addition of compressive residual stresses at the surface and below the surface of metallic components will enhance fatigue life. In 1890, the first patent relative to this phenomenon was issued to B.C. Tilghmann for blasting small particles for surface treatment. Subsequent patents were issued starting in 1927 as listed below: Year Author (s) 1927 E.G. Herbert Achievements / Statements / Proposals Workhardening due to abrasion ( cloudburst process ) O. Föppl German Patent Nr J.H. Frye G.L. Kehl R.Z. v. Manteuffel F.P. Zimmerli H. Wiegand Cold-hammering improves bending fatigue behaviour of structural steel Steel shot blasting of springs Influence of cleaning procedures on fatigue behaviour Improved fatigue strength of sand blasted steel springs Shot blasting and its effect on fatigue fracture life Increased security of surface treated aircraft motor components against fracture As can be seen from the above, shot peening for the purposes of inducing compressive stresses has been around for a very long time. The chart below shows additional work and advancement that took place in the development of technology after WW II: Year Author (s) Achievements / Statements / Proposals J.O. Almen E.W. Milburn H.O. Fuchs R.L. Mattson A.J. Gould U.R. Evans J. C. Straub D. May R.L. Mattson J.G. Roberts Improved fatigue strength of shot peened engine parts; method for measuring peening effects x-ray diffraction applied to shot peened surfaces Residual stress measurement at shot peened springs Improved corrosion fatigue behaviour of shot peened parts Stress peening yields superior enhancements of fatigue strength Analysis of residual stress states induced by strain peening This shows that a large precentage of the knowledge related to shot peening and compressive stress has been around since the late 1950 s, and has been the accepted

2 method of fatigue life extension, as well as corrosion fatigue, and to the modern days as Stress Corrosion Cracking (SCC). While the benefits of shot peening has been an accepted method within industry for providing compressive stress and metal improvement, there is a limited understanding of what depth of compressive stress provides the greatest benefit. AMS and AMS 2430 go into great detail of QC and requirements for peening media, as well as minimum shot sizes to achieve specified intensities, and maximum intensities relative to material, material yield strength, and thickness. No where are there specified intensities for a given part and duty cycle to obtain maximum fatigue life extension and maximum abatement to SCC. These specifications are arrived at by design engineers based on materials knowledge and destructive testing. Many times, however, the old adage bigger is better is applied, and then problems can and do result from over peening. Shot peening, in order to produce compressive stresses, must plasticly deform the surface of the material being peened in order to induce compressive stesses. The illustration below shows what happens as a ball of media impacts the surface. As can be seen in this illustration, the impact creates plastic deformation and induces residual compressive stresses. Each impact creates this phenomenon, and the outward moving residual stresses then push into each other creating residual compression at the surface and below the surface. The magnitude of these compressive stresses and

3 the depth to which they are achieved, depends on the hardness of the material, size of the media,the exposure time of the peening, and the velocity at which the impact occurs. You will also notice that there is an element of work hardening from the cold work that will go to some depth. The depth of this relies on the same variables that control the depth of compression. This is where the difficulty arises. As tiime increases, the amount of work hardening from the cold working increases at the surface and below the surface. As cold working increases, loss of ductility occurs, and once this happens, the effects of the compressive stresses starts to be detrimentaly off-set by the loss of ductility in both fatigue and SCC abatement. The figures below show the stress profiles and the % cold work on a typical 347H stainless steel, shot peened with S-780. The parameters are the same except each successive plot is an increase in coverage in increments of 100% The first shows 100% coverage, the last shows 500% coverage. Whereas no one realistically goes beyond the 300% level, but for purposes of demonstartion, it is included here. The plots were generated by HERCULES shot peening software. Upon review of the plots, you will see that the level of compression varies, but the percent of cold working increases, and at deeper depths. This demonstrates the adverse effects of over-peening in an attempt to achieve ever increasing depths of residual stresses. 100% COVERAGE 200% COVERAGE

4 500% COVERAGE

5 In the ever increasing drive to achieve maximum compressive stress, without the deleterious consequences of excessive cold working, over the last 15 years industry has embraced additional technologies and methods that create higher levels of compressive stresses. One of these methods is Ultrasonics Impact Technology (UIT) from Applied Ultrasonics. Ultrasonic Impact Technology In the 70 s an Ultrasonic Impact Technology (UIT) was developed in Russia to treat the hulls of nuclear submarines. The technology combined many of the attributes that industry has known and used to enhance the properties of base metal and welds, these technologies and methods have providing greater fatigue life for fabricated structures. The technology combined the attributes of ultrasound and mechanical impact to enhance the strength and life of fabricated structures by producing a surface profile change, imparting surface plastic deformation, heat affective zone (HAZ) grain modification, and compressive residual stress. In the mid 90 s, Applied Ultrasonics of Birmingham, Alabama introduced the technology to the US fabrication industry. Through third party validation and research, major university research teams, confirmed the claims made by the Russian scientist, that the technology would impart compressive residual stress into metals to depths greater than (5.0 mm). The studies and research also confirmed the life enhancement capabilities of the technology. This has changed the way industry views materials and post weld enhancement methods and life improvement techniques. The technology makes possible to achieve compressive stress depths greater than (5.0 mm) without the damaging cold work effects of shot peening. These characteristics has earn recognition and utilization of the technology by DOT s across North America, the offshore rig industry, the US Military, the mining industry, and manufactures of automotive and truck components. It currently is utilized by all of these industries for optimum fatigue life. The UIT principle is based on the transmission of ultrasonic frequencies into base metal at frequencies greater than 25 KHz. The technology uses indenter pins that oscillate at 27KHz with a displacement of < 28µ, impacting the base metal surface creating a plastically deformed surface layer while coupling the ultrasonic transducer to the base metal transmitting ultrasound through the thickness of the base metal / weld HAZ. The action of the indenter pins impacting the material surface creates plastic deformation, transmits ultrasound, modifies the grain structure of the HAZ, and induces compressive residual stress depths that can be controlled through parameter selection. The attributes demonstrated by UIT has not been duplicated by any other life extension technology (Shot Peening, TIG Dressing, Toe Grinding, Needle Peening, Hammer Peening, etc.) The pictures below show a typical hand held unit for field application. In addition, there are other tool configurations that permit the application of UIT difficult to reach difficult locations and automated applications such as machine tool or robotic. The tools /

6 systems are sized by frequency (27 KHz, 36 KHz, & 44 KHz) and configured in straight, 45 degree, and 90 degree tool (hand & Machine) geometries. The chart below shows a comparison of what is referred to as legacy technologies or technology currently used for fatigue abatement in components and welded structures.

7 Certifications AASHTO (T4) ABS DNV Veritas Lloyd s 3 rd Party Validation Lehigh University University of Texas Portland State University Sheffield-Hallam University Federal Highway Administration Carderock Stuttgart University Others THE VARIOUS ZONES AND EFFECTS OF UIT TREATMENT Effects of UIT ULTRASONIC RELAXATION (10-:-12mm) IMPULSE RELAXATION (3-:-5mm) PLASTIC DEFORMATION (1-:-1.5mm) AMORPHOUS LAYER (0.02-:-0.1 mm) simp. = (1.2-:-1..5)st simp. = 0.5st Ах=Аоexp(-bx) Ах=Аоsin(wt-kx) [Symbolic presentation in polarization plane, no scale scycles = (0.1-:-0.3)st ZONES Modification of material properties and characteristics Plastic deformation Impulse relaxation Ultrasonic relaxation TECHNICAL EFFECT Amorphous layer: increased wear-resistance, corrosion resistance, yield and ultimate strengths, structural optimization, formation of amorphous structure Cyclic endurance, compensation of deformation, corrosion-fatigue strength Reduction in residual welding stress and strain of up to 70% of the initial state Reduction in residual welding stress and strain of up to 50% of the initial state Physical zones of UIT effect on material properties and condition

8 One example of life extension was the application of UIT on forged steel crank and cam shafts. The parts previously were peened with S-550 cast steel shot. After the conversion to UIT, the life of the crank shaft, determined by running destructive test, was increased by 5X. The chart below shows the magnitude of compression and depth of compression in various zones of a Crankshaft that was treated. It can be seen that a depth of 1.5mm (0.060 ) below the surface, there is still significant compression. This is compared to the normal maximum depth of residual compressive stresses from shot peening of 0.5mm (0.020 ). This underscores the the 5X life extension from UIT compared to shot peening. Depth Cylindrical Section Zone 2 Cylindrical Section Zone 1 Fillet Region (mm) (ksi) (ksi) (ksi) The chart below shows published fatigue life improvementg through shot peening. The far left plot is for forged crankshafts, and shot peening shows an 18% increase in fatigue life from shot peening. The UIT treated forged cranks showed a 5X life increase over the shot peened cranks.

9 60 Improvement of fatigue strength (%) Crankshafts Gears Springs Elements of truck steering system One additional set of data for depth compression provides further evidence that deep compression can be achieved without the deleterious effects of over-peening. This data is from testing done on a forged steel pump component that previously experienced severe SSC failures, and deeper compression, with sub-surface grain modification was considered the best approach. As can be seen, the depth of compression is up to with no ill effects from excessive work hardening.

10 An additional benefit of UIT is sub-surface grain structure modification. The illustration in the photomicrographs below show untreated material on the left and UIT treated material on the right. The untreated material (left photo) has an unorganized grain appearance where the UIT treated (right photo) has a much more organized appearance and more dense structure. This grain modification coupled with stress relief and deep compressive stress enhances fatigue life but also greatly reduces stress corrosion cracking (SCC) in welded components. ` Untrea UNTREATED Untreated grains are larger and with random orientation Treat UIT TREATED Treated grains are more dense and grains become more linear in shape

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12 The above photomicrographs illustrate further the sub-surface grain structure modification, when coupled with the additional Sub-Surface Microstructure (TEM) and The Mechanism of Grain Refinement photomicrographs, show irrefutably that there is a grain modification, and it is largly held in the scientific community that performed the third party validations, that this phenomenon is what leads to the amelioration of Stress Corrosion Cracking (SCC). All of these data were obtained on wleded components. The results of fatigue improvement are Shown Below:

13 R 0.1 R < R 0.5 Cover Plate Transverse Stiffeners Research done by Professor John W. Fisher at Lehigh University Fatigue tests were carried out with 27 full size girders. This study was carried out to demonstrate the effects on fatigue performance of weld details by UIT. FHWA conducted for code modification Dynamic load transverse and longitudinal Transverse stiffeners and cover plates Mild steel and 100kSI high strength steel Testing was performed using requirements set forth by the International Institute of Welding using hydropulse CDM-10 using a 4-point bending method. Stuttgart University Bridge Girder Research Run Out Fatigue Tests were stopped without any failures This means the UIT treatment results in increased fatigue life at a minimum of 2X Stuttgart University IABSE 2005 This study proves that UIT can essentially set back the clock for older bridges

14 The effects of UIT on SCC are illustrated below. This is a weld on stainless steel that is utilized for storage tanks that have stored a mild acid. The weld on the left is untreated and the weld on the right is treated with the UIT process. This graphically illustrates the effects of the relief of tensile stress and the benefits of the grain modification from the deep compression. In review, I feel strongly that we can all agree that the value/benefit of deep residual compressive stress in components extend their life and prevents premature failure from fatigue or SCC. Further, the philosophy that Deeper is better in terms of residual compressive stress is true, but, what must be remembered is that successfully achieving deeper residual stress requires more than simply using a bigger hammer.