TECHNOLOGY

Shot for Improved Castings

Developed in the 1920s, is well understood in the automotive, machine tool and aircraft industries. Its ability to increase life and reduce porosity can also offer foundrymen the ability to produce lighter weight, longer lasting castings.

A. T DeLitizia Wheelabrator-Frye, InclMaterials Cleaning Div Mishawaka, IN

Every foundryman is familiar with the fatigue strength characteristics has hammering of copper and gold predates blast cleaning required for mold and taken place over many centuries. The the time of Christ while peening or the core sand removal and the cleaning and hammering with a round-edged ham- deburring of castings. Most are also mer was known at the time of the familiar with the equipment needed in Crusades in about 100 AD. A knight's casting cleaning and finishing, whether armor was cold worked to final shape it is air nozzle or centrifugal wheel and hardness. Through this repeated abrasive blast machines. But fewer hammering, improved metal life was foundrymen are fully aware of the use, obtained by increasing its fatigue life. controls and advantages shot peening From the 1920s, there are reports that holds for improving castings. European race cars had the fillets of Shot peening has been used for years their crankshafts hand peened. to eliminate porosity in castings, par- It was also during this period that ticularly in castings for high pressure General Motors Corp engineers were applications. It has also been used on experimenting with ways of increasing austempered ductile irons where the fatigue life of valve springs. The in- mechanical surface preparation of shot creased effectiveness of four wheel peening has increased the fatigue brakes, first introduced in the early '20s, strength of the material. This has re- enabled cars to reach speeds of 55-60 sulted in several companies reporting mph. These higher engine speeds and success in using austempered ductile the stresses of high speed stops resulted iron for gearing applications. The ability in the failure of various components, of shot peening to increase a compo- which had not occurred at the lower nent's fatigue life offers the foundryman speeds. the advantage of producing longer last- Because of the higher speeds the ing castings or reducing casting weight. valve springs were inadequate. GMC

Historical Perspective The development of peening in the prestressing of metals by mechanically working the surfaces to obtain improved

From outward appearances, both blast cleaning and shot peening machines look Shot peening has allowed several compa- The plastic flow and stretching of the identical. The major difference is in the nies to successfully use austempered surface fibers of the metal occurs as each blast media and control devices needed ductile iron for gearing applications like pellet causes a slight indentation when it for shot peening to measure for shot in- the one shown here. strikes the surface of the part. tensity and coverage.

modern castinglOctober 1982 engineers then began experimenting stretching of the surface fibers occurs as surface layer, it is necessary to measure with various types of material and man- each pellet causes a slight indentation for these conditions. This is accom- ufacturing techniques to develop an ac- when it strikes the surface. The depth of plished by the use of an Almen gage, ceptable spring. Springs were typically the layer in which the plastic flow takes almen strips, shot sizing and machine pickled or barrel cleaned after heat treat. place in aluminum, for example, will cycles and conditions. It is important to The tumbled springs showed increased depend upon the impact velocity of the note the x-ray diffraction method of fatigue life over pickled springs. It was shot, physical properties of the alumi- testing can measure the forces induced soon discovered that blast cleaned num and size of the shot. Beyond that into a part by shot peening. The x-ray springs produced the best results and depth, the fibers are not stretched to the curve shown in the accompanying fig- the process was then used on connect- yield point and therefore, retain their ure was generated from shot peened ing rods, connecting bolts and steering elasticity. Since there is a bond between 5160 spring material. Notice the high arms and knuckles. The GM engineers them, the inner fibers pull the outer compressive at the surface and the had developed the first modern day fibers back to a length which is shorter magnitude it obtains 0.009 in. below the approach to shot peening. than that which the stretched fibers surface. The continuation of this curve The use of shot peening to increase would tend to retain. A condition of would eventually bring it to the tension the cyclic life and loading of springs, equilibrium results after peening in side and the resulting equilibrium gen- gears, crankshafts, connecting rods and which the relatively thick core of mate- erated by the low tensile stress at the torsion bars spread throughout the auto- rial under low unit tensile stress re- material core. The use of Almen strips is motive industry. It is now used in the strains and holds the thin layer of sur- then an indirect measurement of the aircraft, turbine, machine tool and oil face fibers in a condition of high unit compressive stresses but its use as a industries. It is used to form aircraft compressive stress. standard, which was developed by J. 0. wings, retard stress corrosion cracking It is this compressed layer that pre- Almen, provides a good control over the and allows components to operate at vents surface cracks from propagating shot peening process. higher stresses over longer periods of into the material core and causing fa- It is important at this time to define time. The current need for light weight tigue failures. Since nearly all fatigue some of the terms used in shot peening: high strength components has renewed and stress corrosion failures originate at Intensity is the resulting impact force interest in the use of shot peening and the surface of a part, the compressed that is absorbed on an Almen strip opened up promising areas for its in- surface layer produced by shot peening which is measured as arc height at full creased use. is an economical way for extending the coverage. It is dependent on shot size, life of a component. shot velocity and angle of impact of the What is Shot Peening? From all outward appearances, the shot on the work. Shot peening is a mechanical method equipment used to shot peen castings is Impact Angle is the angle of impact of imposing a residual compressive identical to the machine used to shot between the work surface and the direc- stress at the surface of a metal part. This blast them clean. The major difference tion of the blast. The peening intensity residual compressive stress is produced is that the shot peening machine uses a varies directly as the size of the angle of by striking the metal surface with a blast more round, uniform media as well as impact. of spherically shaped pellets which re- more control devices which measure for Arc Height is a measure of the curva- peatedly hammer the surface of the part. correct shot peen intensity and cover- ture of a test strip which has been It is not stress relieving as it is some- age. With these controls, a blast cleaning peened on one side only. The arc height times referred to, but the introduction of machine can be used for the shot peen- is measured by the use of Almen test compressive stresses. ing of castings. equipment. The arc-height is expressed These stresses are relatively shallow, as a reading in inches, followed by the generally in the range of 0.005-0.010 in. Measuring Its Effectiveness suffix letter, (N, A or C) to designate the for high strength steel and 0.020-0.025 Since the purpose of shot peening is type of strip used. Thus, 0.013A repre- in. for aluminum. The plastic flow and to produce a uniform thin compressed sents an arc height of 0.013 in, measured

INFLUENCE OF UNDERSIZED SHOT

PEENED 30% COVERAGE 100% + 98% SHOT SHOT & GRIT SHOT

The test graphically depicted here demonstrates that shot size X-ray diffraction can be used to measure the forces induced into should be uniform for the best shot peening results. (Test con- a part by shot peening. This x-ray curve was generated from a ducted by John Straub in the Wheelabrator-Frye laboratories.) shot peened 5160 material. modern castinglOctober 2982 on an "A" strip by using the Almen The group at the right was peened to gauge (the number 2 Almen Gauge is the same arc height as that in group now strictly used). three and to a coverage of 98% with Coverage is a measure of the denting uniformly sized shot. This resulted in of the original surface and should be almost double the average life of the 100% minimum. Many applications re- second group. quire 200% or more coverage. Coverage The above test demonstrates that shot is measured usually by 10 x magnifica- size should be uniform for the best shot tion, the Straub method, the Valentine peening results. This presents a di- method or by the saturation curve lemma for the foundryman who wants method. clean castings as well as longer lasting Shot peening specifications accept castings. But his questions can be the saturation curve method, which es- quickly answered if the foundryman tablishes full coverage by plotting inten- performs a simple series of tests which sity versus time and assuring that the were conducted by a parts supplier to correct intensity (determined by the arc the automotive industry. It consisted of: height of the test strip) falls on the right In tests conducted by a parts supplier to Placing a uniform shot size in the side of the knee of the curve. the automotive industry, Almen blocks cleaning machine. Saturation is that point on the satura- were mounted to a sample casting to Mounting Almen blocks to a sample tion curve where the arc height does not measure for a thin uniform compressed piece part as an example. increase by 10-20% after doubling the surface layer on the part. Shown here is a Plotting a saturation curve. time of exposure and is understood to typical example of a sample casting with Fatigue testing of samples. be at the intensity required for the appli- Almen strips attached. cation. The results showed an improvement In specifying media for shot peening, of six times over unpeened parts and it is necessary to have a minimum of peened under the same conditions as three times over parts that were cleaned 85% full size shot. While in cleaning the second group and was then re- but not control peened. They experi- applications, on the other hand, a good peened with five times as much addi- enced a one and one-half times increase mix might have only 30% full size shot tional undersized shot, simulating the in machine cycle times because of the and the balance distributed over a series size analysis which was discovered in a lower cleaning rates with shot peening of smaller sizes which actually do the production peening machine with a sized media and the time required be- final cleaning. high percentage of undersized shot due fore saturation occurred. The accompanying figure shows the to lack of proper separation. It is inter- Since the compressive forces induced results of fatigue tests run to determine esting to note that the life of these two by shot peening forms a thin layer at the the influence of undersized shot. The groups is almost identical in spite of the surface, any machining that removes test points at the left show an average fact that six times as much shot struck this layer will, of course, remove the life of about 50,000 cycles on non- the third group. Even though the cover- benefits of shot peening. Also, processes peened specimens. The next group was age had been increased from 30% to that heat the part like grinding or heat peened with uniformly sized shot to an over 100% and the arc height had been treating will remove the compressive arc height of 0.011A and a coverage of increased from 0.011A to 0.014A, the stresses. Therefore, those areas where only 30%. This showed a life increase of influence of the additional coverage failure occurs, which are not machined, about 2% to 1. The third group was with undersized shot was negligible. ground or heat treated after shot peening are those that can benefit from shot peening. Castings that are currently shot peened include; camshafts, crank- shafts, gears, railroad wheels and sus- pension brackets. Another example of where shot peen- ing proved beneficial to castings is the use of ductil iron gears for weight sav- ings over steel gears. Ductile iron has a density of approximately 90% that of steel. It has been found that shot peened ductile iron gears are acceptable while unshot peened were not in tests con- ducted for light weight trucks. Shot peened ductile iron gears showed a five times life increase over ductile gears that were not. The foundryman who closely con- trols his cleaning operation and whose manufacturing sequence allows the op- portunity to obtain the benefits of shot peening can expect improved castings. He will experience castings that are uniform in fatigue life, thus longer last- ing and castings that potentially weigh less.

One example of where shot peening has played an important role in casting production is For a free copy of this article circle No. 315 in the use of ductile iron gears which provide weight savings over steel gears. on Reader Action Card.

28 modern castinglOctober 1982