COST CONTROL THROUGH SHOT PEENING

Transcription

1 International Conference on Shot Peening andblast Cleaning COST CONTROL THROUGH SHOT PEENING M. C. Sharma Maulana Azad College of Technology Bhopal, India. ABSTRACT Shot peening is the most undervalued process in today's industrial management, which may be due to lack of appreciation of its exceptional merits. The increased fatigue life of a component by means of peening raises other issues like component design, size and material which could all be modified for economy and efficiencies. Various available experimental evidences are presented in this paper which show that by peening, the cost of many components of automobiles and air crafts could be reduced. A method has been suggested by which peening effects are considered in designing a component for economy and efficiency. INTRODUCTION Shot peening is the process of impinging high velocity shots on a metal component resulting in plastic deformation and the setting up of a compressive stress on the peened surface, whereby fatigue life of component is improved. This process is use extensively in the air craft industry, not only to condition the metal surface of the aircraft wings and engine parts, but also for actively forming technique to curve wing skins, which has been increasing so that most medium to large air craft production in the world today utilizes this process. This process forms tapered and sculptured skins more easily, allowing wing design to maximize strength to weight ratios. Additionally the technique does not sacrifice material integrity[l]. There are further more applications of shot peening which could effectively reduce the cost of the various industrial components. 21

2 The weight reduction of automobile is one of the most effective ways in improving fuel consumption. Therefore application of lighter alloys is likely to increase. Wrought magnesium alloy whose density is 66% that of aluminium is expected to substantially increase in this decade. Shot peening to these wrought magnesium alloy will play very important role in enhancing the fatigue strength. CASE ONE Die Castings Part of the air in the die cavity and gas evolved from the liquid metal during filling are apt to be entrapped in castings and forming gas holes in small and dispersed form. These defects are the cause of leakage and the lower pressure tightness of die castings. After shot peening, the tightness of nonferrous die castings can be remarkably increased. Among the automobile industries of large output production, more and more nonferrous die castings are produced and need different shot peening equipment and parameters for peening parts of different shapes and sizes. Table I. Cost comparison of shot peening and vacuum impregnation, Process Shot Peening Vacuum impregnation cost ratio per piece 1 6 manhour ratio per piece 1 10 working area ratio 1 3 man working posts 1 3 recovery ratioof rejected parts % surface quality improve no change The leakage of die castings may also be remedied by vacuum impregnation treatment. As compared with shot peening its equipment is more complex and production cycle is longer. Shot peening also possesses unique advantage of clearing up the fins of castings and improving surface quality[2]. Table 1 gives cost comparison between shot peening and vacuum impregnation techniques

3 CASE TWO Crank Shafts Forged crank shafts have long been used in practice on the basis of their better fatigue behaviour vis-a-vis cast crank shafts. However, recent studies show that cast and peened crank shafts perform better than forged ones effecting cost Shot patring spacificatioa Sam, for botb rna)rrial8 MI 460 Cart rtrtl shots Alman intanrity 1004/r Minimua covtraga writkd by paanrcan proorr. Crank rhaft sin Sema for both mtwlalr (Dotftd area) Dark arao to pew. Journal bratlag dia. 5-4" Ovar all Iangth 36.5' FIG. I. CRANK SHAFT SHOWIN6 MASKED AREA 8 FILLET Fig.1 Crank shaft showing masked area and fillet. Fig.2 Effect of shot peening ground polished forged steel crank shaft.

4 to Ctonk shaft matariol Nodular coat iron 3 2 I 10' d d d Fig.3 Effect of shot peening ground and polished nodular cast iron crank shaft The most highly stressed area of crank shaft is the journal bearing fillet. The high stress point is the bottom side of the fillet when the journal is in the T.D.C during working stroke. Fig. 1 illustrates a crank shaft in which the journal bearing fillets to be peened are shown and the journal bearings have been marked against impingement of the shot. Fig. 2 and 3 show respectively the effect of shot peening on stress Vs Fatigue life of steel crank shafts and nodular cast iron crankshafts for six cylinder diesel engine. The fatigue strength of the forged but unpeened shaft was 320 N/mm where as the fatigue strength of the cast and shot peened shaft was 360 N/mm, an increase of 12.5%. Thus it is possible to substitute a shot peened cast crankshaft for an unpeened forged crankshaft as considerable cost savings due to the fact that castings are cheaper than forgings even with additional peening operation. CASE THREE Stress Corrosion cracking of Hastelloy. Controlled shot peening is a cost effective technique to reduce or to eliminate stress corrosion cracking(scc) failures, which cost the industry millions of rupees[4].

5 A Hastelloy blower wheel used in hot nitrogen atmosphere(at 12O OC) fail in less than six months. Automatic controlled shot peening treatment with S-170 shots to an intensity of 10A -14A (0.25A Amm) followed by glass bead peening for decontamination, with coverage 200% was given to the blower. The above treatment could prevent failure even after three years. The Almen strips were used for intensity control and PEENSCAN for coverage measurement. Hastelloy 82 reboiler heads used in water containing chlorides failed after eight months in the formed dome and the heat effected zones(we1ded) due to SCC. Shot peening treatment with S-230 shots to an intensity of 12A- 14A(0.30A-0.35Amm) followed by glass bead peening for decontamination with coverage 200%, manually peened and coverage examined by PEENSCAN, could prevent failure feven after two years. CASE FOUR Stress Corrosion Cracking of Stainless Steel Components Unpeened stainless steel spools failed in less than one year. The entire inside of spools and exterior welds were peened using automatic control (shot size S-230) with intensity 10A- 14A, coverage 100% followed by peening with glass shot for decontamination. Almen strips were used for intensity measurement and PEENSCAN for coverage. No failure was reported even after four years[4]. A stainless steel slurry stripping column failed due to SCC in top and bottom sections due to chlorides and due to tensile stresses resulting from welding and forming during the manufacture. Tank interior bottom and top ends were shot peening with S-170 shot to intensity 12A-14A followed by glass peening for decontamination with coverage of 200%. This treatment resulted in a satisfactory life of two years and still survived.

6 CASE FIVE Improving of Fatigue Durability of carburized steels with surface structure anomalies by shot peening. Shot peening when applied to improve fatigue durability of carburized steel with surface anomalies (internal oxides precipitated along the grain boundaries and accompanied by non-martensitic micro-structure near the surface which are harmful) was found to be extremely effective[5].thus shot peening is quite beneficial for components undergoing carburizing treatment. CASE SIX The effect of shot peening on contact fatigue beaviour of 40 Cr steel after compound Heat Treatment. The contact fatigue behaviour of 40 Cr steel by shot peening after the compound heat treatment, ion nitriding and induction hardening is far superior to that of induction hardened and carburization hardened steels. The fatigue life has been found to increase 5-8 times under the same contact stress. The compound heat treatment is known for its time and energy saving and less deformation than that of the carburization hardening[6]. Comparison of three types of treatment for stress corrosion cracking prevention[7] is given in Table 2. Table 2 Comparison of three types of treatment for stress cracking prevention Parameter shot Heat sink Wet shot peening weldingihsw) peening 1 I Mitigation of Residual stresses I poor I poor I superior I Recovery of shots I Poor I I superior Mitigation of sensitigation poor superior superior I I 1 I Removal of slag I superior I poor I superior Prevention of transformation poor I better I better

7 CASE SEVEN Influence of shot peening on the fatigue strength of various heat treated steels. Burakowski and Nakonieczny[8], in their exprimental work concluded that 30HGSA steel(0.30%c., 0.1 %Mn, 0.1 %Cr, 1.1 %Si) used in aircraft industry for making different shafts, showed an increase in fatigue strength after shot peening. The improvement depended on preparatory heat treatments and surface heat treatment. The effect of shot peening was about three times higher after toughening (hardening and tempering) than after normalising of the 30HGSA steel. The shot peening, applied after induction hardening, changes the nature of the fatigue cracks, moving the fatigue initiation site from the surface of the sample to a sub-surface. Among the examined technological variants of treatment of f30 HGSA steel the highest fatigue strength was achieved after toughening, induction hardening and shot peening. CASE EIGHT Combined surface treatment by shot peening and power burshing. It was observed by the faculty of engineering Waseda University Japan[9] that large improvement of fatigue strength of machine elements has been obtained by combined processing of both shot peening and power brushing than by either processing alone Material for test pieces was of specification SK5 containing carbon 0.79%. Si 0.25%, Mn 0.41 %, P 0.017%, S 0.008%. METHOD TO CONSIDER FATIGUE STRENGTH OF SHOT PEENED PART IN DESIGN The savings obtainable by the use of shotpeening are so great that the designer must include its effects in calculations if he wants to remain competitive[l01. In automobile suspension coil springs, for instance, the permissible stress

8 range with shot peening is 40% greater than that without peening (SAE 1973). This means that shotpeened springs weigh half as much as unpeened springs would weigh because the weights of springs of equal capacity are (approximately) inversely proportional to the squares of permissible stresses. The saving achieved by shot peening is about 24kg on an average per car and worth perhaps Rs for suspension springs alone, apart from savings on valve springs, gears and shafts and other indirect savings produced by using smaller, lighter components[l0]. When a part is subjected, to variable stress, the designer has to calculate the variable stress component (ov) under any fluctuating cycle. Goodman correlates o (variable stress) and om (mean stress) by a straight line relationship(fig.4- I Endrrauco limit 1 1 I Fig.4 Fluctuating stress cycle Fig.5 Goodman's diagram After shot peening, endurance limit is increased by about 50%. Now under this condition one will have to plot Goodman's diagram and will have to find out the new variables stress component (o v). From Fig.6 values of variables stress component before and after peening are found to be 10kg/mm2 and 1 5kg/mm2 respectively, an increase of 50% in variable stress amplitude. Thus for higher allowable stress amplitude, the component can be designed to required size of lesser dimension and saving in material can b.e achieved. The method proposed by Fuchs[lO] gives more realistic estimates than the often

9 used Goodman diagram. In this method Haigh diagram was used where alternating stresses (Sa) are plotted ovver mean stresses Sm (Haigh 1930). It represents the equation Sa = 2Scat-Sm, or Sa = Scat which ever is greater... (1 ) where Scat = critical alternating tensile stress below which crack will not propagate. The effect of peening on the parts which have propagating cracks can be predicted for long fatigue lives by the relation of fully reversed fatigue limit Sf to the true fracture strength of (Morrow 1968) Fig. 7 and the datain Table 3 show the effect of shot peening on two parts with notches which produce a fatigue strength reduction factor, k = 2, one made of hard steel and the other of mild steel. The diagrams are constructed from the first six lines of the data given in Table Goodman's line after Fig.6 Effect of shot-peening on Goodman's St.iine 29

10 STEEL AT 1930 MP, UTS I020 STEEL AT 440 MPa UT3 t Fig.7 Haigh diagrams for steel parts with notch factors k = 2. peened(p) and unpeened (U), for two different materials. Note different scales. Fig. 7 and the data in Table 3 show that shot peened part of hard steel will survive alternating stress cycles of +780 MPa(Point P). The unpeened part can survive only t 285MPa(Point U). The improvement is 170% or 495Mpa. A smooth specimen of the hardsteel would su'rvive stress cycle of k570mpa (Point Sf). Peening could increase this also to k780mpa. The improvement is only 37% or 21 OMPa. - Line No. Table 3 Estimates for fatigue limit of peened and unpeened parts of two materials (Plotted in Fig.7) Parameter Su (Ultimate tensile strength) o f (True fracture strength) Sy (Tensile yield strength) Sy (Cyclic yield strength) Sf (Fatigue strength at 1 07 cycles) Scat (Critical alternation tensile strength) U (Unpeened notched fatigue strenth) P (Peened notched fatigue strength) Material Material MPa Material Material % of S,,

11 The peened notched part has fatigue strength equal to that of an un-notched specimen. For the same part made of mild steel the improvement obtainable by shot peening is less in relative terms (80%) because the yield strength at point P is only 165% of the fatigue strength Sf while for hardened 4142 steel it was 270% of fatigue strength Sf. The improvement is far less in absolute terms, only 60 MPa for the mild steel part instead of 495 MPa for the part made of Thus greater improvement in parts made of mild steel is expected if they are changed to harder steel and shot peened. CONCLUSION Above case studies clearly indicate that shot peening is one of the most important production tools. This is much more effectively and economically utilized in USA, Europe and Japan etc, where they control various peening parameters (shot size, material, velocity, stand of, peening time and coverage) using computerized numerical control methods, peen scan, suitable peening equipment and processes of peening. Their automobile industries showed increase in fatigue life by shot peening of different components ranging from 200% to 1400%. Air craft industries are forming panels by this method with lesser cost than conventional methods. Further, designing various components, keeping into account the effect of shot peening, will reduce the weight of the component there by direct as well as indirect saving in cost will be achieved. Following data in tabular form provides guide lines to select peening parameters for important materials.

12

13 - C P a F F n u C - rc a 5 tc +- (I] m u a, m a z z 0 =; 03 a," ; ) 0 OC'! - 3 SPRING ALLOYS 33

14 Material SAE 1074 l, " SAE ,I I, SAE 4340 I, Hardness I HRC HRC HRC I, 48HRC 11 I, N I# Res~dual stress Rs Max M Pa Max value Residual stress data for Ni alloys. - - gradient Depth Intensity shot 7 I remark Mpalmm size 1 ' " RS max Rs d=o M Pa Rs. Stress C 4C 4C 230shots 130 shots -330 shots 4C 230 shots 4C 4C 130 shots C 330 shots C I 230 shots 130 shots 330 shots 230 shots 130 shots 130 shots 330 shots 230 shots 130 shots 330 shots 230 shots SP with strain of.050% SP with strair of %

15 REFERENCES 1. Baughman D.L. An overview of peen forming Technology, ICSP-2 Paramus New Jersey p Takemoto M. Prevention of Stress corrosion Cracking of wildment by shot peening ICSP-2 Paramus New Jersey p39. Burnvell N.K. Controlled shot peening to increase the fatigue properties of crankshafts ICSP-2 p361. Paul G Field. Controlled shot peening reduces or eli nates stress corrosion crackmg ICSP-2 p121. Kikuchi M et. al. The improvement of fatigue durabi lity of carburized steels with surface structure anomalies by shot peening ICSP-2 p208. U. Jingpu Z. The effect of shot peening on contact fatigue behaviou~ of 40 Cr. steel after compound heat treatment ICSP-2 p Wang Hu tie Yuam XuDi. The effect of shot peening on pressure tighteners of Pdo~lferrous Uie castings. 8. Bar3ltowski T. and Naltonieczny A. Influence of shot peening on the fatigue strength of various heat treated steel. 9. tiisnrc S et al. Combined surfacc treatment by shot peening and power brushing ICSP-2 Paratnus New jersey p Fochs H.O. The strength of shot peening parts ICSP-1 p323.