Peening weld surfaces with steel shot induces compressive stresses that raise resistance to fatigue and corrosion. By Tom Floyd hot peening improves Testing shot-peened parts, re- Built-in compressive stresses property performance searchers have demonstrated that raise resistance to stress corrosion of welds and weld- peening raises fatigue resistance of by counteracting tensile stresses. mentsS by cancelling residual tensile fillet welds in carbon steel and butt Stress corrosion occurs when tensile stresses that develop in surfaces as welds in constructional alloy steel stresses break up oxide layers that welds cool and replacing them with by 20 to 40 percent, and can double normally protect surfaces of chro- compressive stresses. Compressive fatigue resistance of butt welds in mium-containing alloys. Though stresses raise resistance to fatigue, aluminum plate and 18-percent Ni bare metal re-oxidizes to form fresh stress corrosion, and intergranular maraging steel. Researchers at Rock- oxide coatings, recurring cycles of corrosion. well International (Atomic Di- tensile stresses continually rupture A cold-working process, shot vision) found that shot peening pre- these layers. peening bombards a metal work- vents stress corrosion cracking of As for intergranular corrosion, piece with spherical pellets, com- weldments in austenitic stainless characteristic of stainlesses sensi- monly steel shot. Each impact steel. tized at weld temperatures, peening stretches and densifies surface lay- Fatigue resistance improves be- before welding helps to prevent it ers. Because subsurface material re- cause, when service imposes tensile by breaking up surface grains. This mains unstretched, it exerts an op- stresses in a part, the built-in com- action provides a multitude of nu- posing force, trying to restore the pressive stresses counteract them. cleation sites for chromium carbides surface to its original area. This This action keeps surface stresses in that would otherwise grow prefer- force, compressive stress, preloads compression, allowing the part to entially in grain boundaries, leav- the metal surface against tensile withstand fatigue cycling for long ing chromium-depleted zones sen- loading stresses. periods. sitive to corrosion. PEENING DEVELOPS COMPRESSIVE STRESSES Peened surface 1 Stress Tension 0 Compression ---------- 68 WELD1NG DESIGN & FABRICATION SEPT 85 Controls for peening Operators consider expected ser- LOADS ALTER STRESS DISTRIBUTION vice conditions when determining Peened surface how deep compressive stresses I +-Tension 0 Compressions- , should run beneath surfaces. Shal- low peening, to 0.008 inch, suffices to counteract tensile stresses built up by simple bending and to fore- stall stress corrosion cracking. To combat tensile stresses imposed by complex bends and torsion, and to handle fretting fatigue, peeners rec- ommend compressive depths to 0.015 inch. To adjust depth of compression, operators adjust the energy at which pellets strike work surfaces. They measure kinetic energy on the Al- men Intensity Scale, running tests with Almen strips, of cold-rolled SAE 1070 spring steel hardened and tempered to Rc 44 to 50. Measuring 314 by 3 inches, Almen strips come in three thicknesses: 0.031 inch (N strips, for thin work), 0.051 inch (A strips, for most jobs), and 0.094 inch (C strips, for heavy parts). To run an Almen test, the operator clamps the strip to a jig, called an Almen block, then tightens four screws at corners to hold the strip <I-& u- -..-:I.:--- I.L- ::- LL--..-& llai. I le YUD~LIU~~DLlle jllj VAL-- ule pa1 L, such that the test strip lies along the section of interest, say the surface of PEENING INTENSITY a butt weld. Then the operator starts PREDICTS COMPRESSION DEPTH a peening run, stopping the action periodically to inspect the strip. As peening proceeds, com- pressive stresses build up on the ex- posed side of the Almen strip, bow- ing it to a maximum curvature, called saturation. Curvature is mea- sured with an Almen Gage, a dial gage scaled in mils. The operator records the corresponding arc height-an intensity of 10A means that an A strip is saturated with an arc height of 0.010 inch. Almen intensities relate to com- pression depth, which varies with material and hardness. As the graph shows, an Almen intensity of 6C (0.006 inch on a C strip) corresponds to compression depths of 0.012 inch for steel at Rc 52, 0.015 inch for ti- tanium-6Al-4V, and 0.022 inch for steel at Rc 31. 0.do2 0,004 0.006 0.008 0.010 Almen intensity (C strip), in. Peen all surfaces To raise fatigue resistance effec- SEPT 85 WELDING DESIGN 81 FABRlCATlQN To perform Peenscan inspection, the operator coats the part with fluorescent dye, which glows under black light (left). Peening I for 15 seconds (center) removes some of fhe coating, such that the part continues toglow hazily. Glowing stops after 1 minute of beening (right) to demonstrate 100-~ercentcover&e. With use, shot shatters, breaking Inspection has improved appre- up into sharp-edged fragments that ciably with the development of the nick surfaces. Since peening aims at Peenscan method, which takes the building uniform compressive place of tedious examination by stress into surfaces without abrad- magnifying lens. Peenscan in- ing material or introducing stress spectors use black-light projectors raisers, peening cannot tolerate bro- to determine coverage. They start by ken pieces of shot. We start all peen- coating the part with an alcohol- ing jobs with 100-percent round based fluorescent liquid, which shot, screened to remove broken dries rapidly leaving a thin film of pieces and sized to uniform di- fluorescent material, During runs, ameters. In modern peening ma- p eening gradually removes the chines, classifiers screen out broken film, causing the glow to fade out, as pieces continuously throughout viewed under black light. Complete peening runs. disappearance of the glow indicates 100-percent coverage. Recognized I Operal'ors shot-peen cra;ze-boom welds Peening today by military specifications (MIL-S- to help them resist fatigue in service. Late in the last decade, Peenama- 13165B: Shot Peening of Metal Parts), Booms, of ASTM A618 steel (70,000 tic Division, Metal Improvement Peenscanning eliminates the need lbl in. * tensile), go to draglines. Company, adapted microprocessors for 200-percent coverage in order to to run shot peening production be assured of complete peening. tively, peening must cover weld equipment. Microprocessors pro- Ceramic shot, another new devel- joints, including HAZs, com- gram machines, changing settings opment, gets the call when ferrous pletely-every square inch should to peen parts of different materials, contamination might be a draw- be dimpled overall. Inspectors con- shapes, and sizes. They set, automat- back, as when peening stainless ventionally determine 100-percent ically, all parameters, including steels. Harder than carburized steel, coverage by examining surfaces shot-impingement angle, Almen ceramic shot produces higher Al- thoroughly with a magnifying lens, intensity, air pressure, and motion men intensities and fractures less X 10 power. Some customers call for of the shot streams. The computer than steel shot. Zirconia-silica shot stores settings for repeat runs. It also will probably replace steel shot monitors parameters during peen- within 10 years. ing, and will shut the run down if a Peening shops have devised inno- To raise fatigue resistance effectively, parameter, such as air pressure, vative ways to peen out-of-the-way peening must cover part surfaces, strays out of the preset range. areas and corners of parts. They use determined by X 10 lens or black-light Microprocessor-based systems flexible lances to deliver shot adapt to statistical process control, around corners, and employ inter- inspection of fluorescent-coated parts. which will become simpler when nal shot deflectors to shot-peen in- computer-monitored parameters sides of welded pipe, replace Almen strips, a move 125-percent coverage, even 200-per- planned before the year 2000, Com- Tom Floyd is manager of metallurgy, cent coverage, to make sure that puters will also predict shot de- Cleveland Division, Metal improvement they will get 100-percent coverage. gradation as a function of Almen Company, subsidiary of Curtiss Wright The extra peening adds no benefits, intensity, type of shot, and target Corporation, 18683 South Miles Road, but costs more. hardness. Cleveland, Ohio 44228; (226) 662-8408. 70 WELDING DESIGN & FABRICATION SEPT 85 .