J. J. Daly Metal Improvement Company Paramus New Jersey USA

Transcription

1 Shot Peening-Current Trends, Future Prospects J. J. Daly Metal Improvement Company Paramus New Jersey USA The shot peening process is sparingly used compared to other processes such as heat treating, plating, anodizing and various coatings. A suggested reason for this is that it is impossible to evaluate what has been done to a part which has been shot peened without destructively destroying the part. To date there' is no nondestructive method available to us for evaluating residual stress profiles of shot to peened components. Therefore, we must rely on the controls currently available to us as well as the personnel and reputation of the company or department which performs the shot peening process. Additionally, we have seen the process become apparently over complicated 'when in essence it is a simple process. Its low cost makes it cost effective..it is adaptable to most all geometrically piece p'art shapes and it is relatively easy to apply. Over complication of the process leads to unfamiliarity and that leads to resistance to accept the process. In fact, the process was always simple to apply, where its influence on fatigue performance was complicated to understand and predict. Today the apparent over complication of the process stems from a much better grasp of its mechanics.

2 Although the approach remains simple, a more sophisticated approach to parameter selection and control now enables us to obtain optimum improvement and repeatability. Evaluation of the shot peening process is best performed under controlled operating conditions in the laboratory or the field on actual shot peened hardware compared to unpeened hardware. In many cases, this is not possible due to the size of the part, the cost of the test, or the time frame required for evaluation. Alternatively, and in most cases, fatigue test specimens are used in lieu of parts and the type of testing or the specimen used in many cases I have seen does not represent the actual loading on the part or the surface finish of the peened part. In this case, fatigue test results may be erratic, inconclusive and in many cases give erroneous data. As an example, we know that shot peening does not show up well on high stress low cycle fatigue. When accelerated tests are performed by increasing the stress level above the design operating stress level, the S and N curves in many cases show that both the shot peened and non-shot peened curves meet near the yield point and in many cases will cross over yielding data which is inaccurate for comparison of shot peened versus non-shot peened components. Shot peening has proven to be of great value in extending fatigue life of components which loaded in bending or torsion are to a lesser extent in axial loading. Fatigue tests are conducted in axial loading when in actual practice bending or torsional loading causes fatigue failure. Additionally, fatigue specimens in many cases are erroneously polished and do not represent the actual surface finish of the component. Most finished machine components have a final surface finish of 125RMS compared

3 to polished specimens having a finish of between 8 and 32RMS. Fatigue cracks primarily initiate at notches, and in many cases notches from machining. In reviewing fatigue test data where axial tests are performed on smooth versus shot peened specimens, one can draw the erroneous conclusion that super finishing yields better fatigue results than shot peening for all fatigue applications. In pure axial loading, the entire cross section of the part is loaded in tension. The compressive stresses induced on the surface of the part by shot peening are balanced by subsurface tensile stresses which under loading may become undesirable causing premature subsurface failure. In axial loading conditions low shot peening intensities are recommended which yield depths of compressive stress that do not exceed 5% of the cross section of the part. Higher.intensity shot peening, yielding up to 20% of the part in compression have proven beneficial for bending and torsion applications as well as minimizing fatigue failures from foreign object damage and corrosion pitting. Subsequent shot peening to a lower intensity with a small shot size or polishing further improves fatigue life by bringing the maximum value of compressive stress closer to the surface as well as providing an improved surface finish. However, this improvement is offset with additional costs and economics then becomes a factor. I believe that we who recognize the value of shot peening have to do more to address the resistance of those who question the reliability of the shot peening process because of their concerns over the lack of

4 tangible data concerning controls. The advent over the past five years of computer monitored shot peening machines and the introduction of fluorescent Dyescan media for measuring coverage through the Peenscan process certainly is a step in the right direction to give industry this assurance. This brings us to the subject of coverage on a given component versus coverage on the Almen strip. It is incorrect to correlate a direct relationship between coverage on a component and coverage on an Almen strip. The only direct, relationship would be if the component is of the same alloy, the same hardness as the Almen strip. Otherwise there is no correlation. Coverage can only be measured by visual inspection or the use of fluorescent media, and the Almen strip is solely an instrument to be utilized for the purpose of measuring the kinetic energy, more commonly called intensity, and to control this kinetic energy from component to component. There are too many shot peening callouts of improper shot size and intensity on components which were established either through standardization within the particular company, or it is specified due to the fact it worked well in the past on other components. When erroneous callogts are specified, maximum fatigue life benefits are sacrificed and in many cases fatigue life is reduced. It's for this reason that it is recommended to employ the following steps before specifying the shot size and intensity. Step 1: Determine the Proper Depth of Compression. This depends on the type of loads to which the component is exposed, such as axial torsion, bending, stress corrosion,

5 fretting fatigue, corrosion fatigue, and also will the component be subject to mechanical damage in service. Remember the normal rule of thumb is that no more than 5% of the total cross-section of the area of the component to be shot peened should be in compression to allow for equalization of tensile stresses in the core to compensate for the compressive stresses at or near the surface. This 5% rule of thumb applies particularly to thin sections and axial loading. There are exceptions to the 5% rule, depending upon the thickness of the section and whether or not the part is subjected to mechanical damage, fretting fatigue, or corrosion fatigue, in which case a deeper depth of compression should be considered to give optimum fatigue results. I recommend, however, that no more than a maximum of 20% of the cross-section of the coniponent to be shot peened should be in compression. In order to determine the relationship between intensity and depth of compression for different materials at different hardness levels, many residual stress curves have been' developed to be used for such references. Work is progressing by Metal Improvement Company to establish a computer program which will alleviate and simplify this research process. Step 2 : Media, consisting of size, material, hardness and uniformity. The shot size has to be small enough to get into the radii or changes in sections. Size also plays an important role in surface finish requirements. As an example, the larger the ball for a fixed intensity, the better will be the surface finish. The trade-off is cost since a larger ball will take longer to obtain full coverage than a smaller ball.

6 tion concerns, there are many nonferrous media materials available today -- ceramic, glass, stainless steel. Next is hardness of the media. It is a proven fact, and enough tests have been run to indicate that the hardness of the media should be as hard or harder than the component to be shot peened. One area where this has proven most success- fu 1 is shot peening o f case hardened carburized gears where hard shot, 55 Rc and above, produces a much higher magnitude of compres- sive stress than the use of softer shot. Next is the uniformity of the media, which is important to obtain good surface finish and avoid surface damage. This is accomplished with automatic mechanical shot separators, an integral part of the machines, readily avail- able on the market today. Step 3 : Kinetic Energy, more commonly called Intensity, which is controlled by utilization of the Almen strips located in fatigue critical areas, the locations of which should be specified by design or structures engineers. The Almen strip, developed by John Almen back in the early 19301s, is the most effective, inexpensive, practical method to measure intensity. As intensity is directly related to depth of compression, its control provides assurance that the requested depth is achieved. Though also directly related to surface finishing, the selection of optimum intensity for some materials may call for a compromise between maximum depth of compression and surface roughness. Step 4: Surface Finish. To obtain optimum surface finish at a fixed intensity, the larger the shot, the bet-

7 ter the finish. Frequently "dual shot peening" is used to obtain better finish as well as improvement in fatigue life. Dual shot peening is the use of a larger shot, following with smaller shot size at a lower intensity. This not only improves surface finish but brings the peak compressive stress closer to the surface of the material being shot peened. Step 5: Coverage. This can be accomplished by visual inspection utilizing a 10-power magnifying glass. However, on many components such as case hardened carburized gears and parts of.complex configura- tions and internal surfaces, visual inspection and the use of a boroscope are expensive, time consuming and ineffective. For this reason, the Peenscan method of inspection was developed utilizing fluorescent tracer liquid which exhibits a distinct ' change in color under a U.V. light depending upon different coverage rates obtained. Step 6: Equipment. Great progress has been made in the last five years in the utilization of computer monitored shot peening machines with full automation and shot flow sensors. These computer monitored. machines provide a distinct computer readout on a printer indicating that all of the parameters in the machine are controlled and within required specifications. I predict that within the next ten years most of the mechanical shot peening machines used in industry today will become obsolete and industry will solely depend on computer monitored machines. In summary, we must not only continue to emphasize and educate the metalworking industry worldwide on the

8 benefits, versatility and low-cost advantages of the shot peening process, but must also contribute time and effort to the education within the lecture halls on shot peening and its benefits.