US3073022A - Shot-peening treatments - Google Patents

Description

Jan. 15, 1963 Filed April 3, 1959 U/V Tiff/4 TED J. J. BUSH ETA]. 3,073,022

SHOT-PEENING TREATMENTS 5 Sheets-Sheet 1 LARGE SHOT) SMALLER SHOT} FA T/6UE L /F E lMFROVEMEN T SUBJEC PROCESS FA776UE L/FE- CYCLES UN TREA TED Jan. 15, 1963 J- J. BUSH ET AL 3,

SHOT-PEENING TREATMENTS Filed April 5, 1959 5 Sheets-Sheet 2 PRczwr FAILURE //v POPULA7/0/V 8 & 8 8 8 LIFE CYCLES cfeazye pdlzzir gram/tr field Hills, and

"a high velocity. conventionally,

"a single step process in which the surface of a metal part manner fora predetermined time. peening treatment ordinarily was applied as a single, conthe fatigue 1 generally to at least twice the a conventional shot-.peening operation.

United States 3,073,022 SHOT-PEENING TREATMENTS John J. Bush, Royal Oak, Raymond L. Mattson, Bloom- James GeorgeRoberts, Warren, Mich, assignors. to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Apr. 3, 1959, Ser. No. 803,961

' 16 Claims. (Cl. 29-553) This invention relates to the treatment of metal surfaces and more particularly to a method of increasing the faitigue life of metal parts.

Hammering or peening is well known to cause small, permanent surface deformations which contribute to an increase in strength and hardness ofthe metal of which the part is composed.

In addition to hammering, it has "previously been knownto peen metal surfaces with small, hardened shot particles which impinge on the surface at shot-peening has been is subjected to a high velocity stream of shot in a given Heretofore, the shottinuous treatment rather than 'a series or plurality of shorter treatments of similar intensity. The plurality of treatments of similar intensity provided no material benefit in strength or hardness of the part treated over the benefits obtained from a single, continuous treatment.

However, we have now found that a plurality of shotpeening treatments can be used to materially increase We have unexpectedly found that by using a plurality of treatments in a specific manner we can increase the fatigue life of a metal part normal life obtained from Inaccordance with our invention a metal surface is first shot-peened in a conventional manner and thereafter We have found that when the seca lower intensity and a smaller and features of this invention-will appear more clearly from the following descrip- 'tion'of a preferred embodiment thereof and from the .drawings,. Lin"which: I

, {FIGURE 1 shows a diagrammatic view of the essential steps involved in our invention; 1

. FIGURE 2 shows a bar chart which compares the fatigue! life'ofparts treated by the subject method with the fatigue life of parts which are conventionally peened; Weibull plot of fatigue life of unv eened Yparts, conventionally ,peened parts, and parts FIGURE. 3 is a treated in the subject'manner;

FIGURE 4 is 'a'Weibull plot comparing single and double peened specimens showing fatigue life of the effect of lowering theintensity Iota-second peening treatment;

plot of fatigue life of single and of reducing and FIGURE 5 is a Weibull double peened specimens showing the effect shot size in a second peening treatment.

conventionally, shot-peening is practiced by imparting an impetus ,to the I shot so that the on a given surfacea't a high'velo 'ty, Infonernethod the shot is accelerated to ft he high'velocity by means of a rapidly rotating wheel orimpeller." The .wheel has a central cavity into which the shot is introduced. Several life of a metal part to a much greater degree "than that ever accomplished by means of a conventional, single shot-peening treatment.

shot will impinge atent if desired, by

shot into the wheel passages.

a workpiece is y the methods and apparatus set forth in the SAE Manual with hardened metal shot,

' coverage desired.

' with SAE 70 chilled iron shot using an intensity 3,073,022 Patented Jan. 15, 1953 iCQ "the. wheel. The shot placed in the central cavity of the wheel passes through the passages of the rotating wheel where it is accelerated to a suitable velocity whereupon it isdischarged from the openings on the'circumferential perimeter ofthe wheel. The shot can be emitted from the wheelin the form of a single, unidirectional'stream, employing a control device in the central cavity of the wh'eelto regulatethe point of entry of the The particular size of shot used, intensity of the shot blast, duration of peening, etc. are variable, and the preferred treatment to be used will depend upon the composition, hardness, configuration, etc. of the specific item being treated. However, typically conventionally treated in accordance with on Shot-Peening, SP-84.

Conventional shot-peening is usually accomplished such as cast steelshot, cut steel wire shot or cast'iron shot having a mean diameter of from about 0.005 inch to about 0.13 inch, andin some instances a. shotsize as low as about 0.002 inch can be used. The duration of the shot-peening treatment is variable, depending upon the intensity employed and the For example, a heretofore satisfactory shot-peening treatment for hot-rolled SAE 5160. steel springs which are hardened. and tempered to Rockwell C 48 hardness involves shot-peening to full visual coverage of about a-0.00"20C SAE intensity.

, creases in fatigue life when ver a cess with strength are obtained 'size or the intensity of the secondary treatment, we have This invention comprehends subjecting previously shotwhich In order to obtain the substantial the second shot-peening treatment should be of an SAE intensity which is lower than that initially used or a smaller shot size shouldbe marked improvements in fatigue by singly varying either the shot employed. Although experienced the most significant success in obtaining inboth the shot size and intensity are lowered in the secondary treatment.

m The optimum intensity and shot size for each treatment, of course, isdependent upon thenature of the part being have experienced considerable succonventional However, we

our invention using an optimum,

j shot-peening treatment for the first step and an SAE intensity and shot size which is approximately to that whichis used in the initial treatment. The wordfintensity is used herein to describe the nature of a'shotblast in accordance with the-normal andaccepted practice as described in the Society of Automotive Engineers 1956 Handbook. 'The term intensity thereforerefers to the .v the properties of the blast, e.g., velocity,,size,

usity, kind, of material andzhardness .of the shot. tionally,

posurelto the blast, e.,g., length of time, and shot flow rate.

fused to accomplish the designated as having similar intensities.

eflect produced on a standard-test specimen by a shot peening treatment. The basis of measurement of SAE intensity is the measurement of the degree of curvature of an'initially fiat steel test strip after the test strip is subjected to as'hot-peening treatment. The extent of this curvature on the standard test sampleafter full visual coverage serves as a measurement of. the intensity of the blast, U The degreeof curvature or intensity depends upon shape, den- Add the curvature depends upon the properties ofexangle of impact Thus, varying shot blasts can: ,be samefeffect and are, thefefore,

In practicing 'our invention we; have found that, in general, v,any reductionin the -intensitygerishot;size of the second treatment from, the intensity of the initial,

aotaoa'a conventional peening treatment contributes to an increase in fatigue life of the metal part. The optimum intensity and shot size of the second treatment is primarily dependent upon the intensity of the first peening treatment and the nature of the material. However, it has been established that when the spring steel surface is initially shot-peened in a treatment of approximately 0.016A to approximately 0034A SAE intensity, highly satisfactory increases in fatigue life can be obtained if the secondary treatment is of an SAE intensity of about 0.003A to about 0.011A (0.001C intensity=0.0035A intensity).

The nature of shot used to practice our invention in an economical, commercial manner is as pertinent as in conventional peening treatments. For example, this invention can be practiced using cut steel wire shot, condi: tioned cut steel wire shot, cast iron shot and cast steel shot.

Due to the difference in physical properties in various types of materials which may be treated in accordance with our invention, the individual responses of a number of different materials to a shot-peening treatment is quite variable. Moreover, similar materials may even exhibit varied responses to a shot-peening treatment due to differences in hardness and the like. Accordingly, it is difficult to establish an optimum treatment which will provide the most beneficial increases in fatigue life for all materials using our invention. This is particularly true with respect to the size of shot which is preferably employed in the first and second peening treatments. However, satisfactory results are obtainable when a metal part is initially shot-peened in the known and accepted manner for such treatments to provide optimum fatigue resistance and then shot-peened in a second'treatment using a smaller shot size. We have found that the shot size for the second treatment generally is preferably approximately /3 to /s the size of the shot used for the initial peening treatment. In general, satisfactory results are obtainable when spring steel, for example, is initially shot-peened using a shot having a mean diameter of about 0.023 inch to 0.066 inch and thereafter subjected to a second peening treatment in which the mean diameter of the shot is about 0.007 inch to 0.011 inch in size.

Serving as a specific example of the practice of our invention, leaf-spring specimens were formed of hot-rolled SAE 5160 spring steel. These specimens were finished to a rectangular configuration of 0.192 inch in thickness, 1.5 inch in width and 12 inches in length. The finished specimens were then hardened and tempered to a Rockwell C 48 hardness and then shot-peened in accordance with the invention. After the shot-peening treatment, each of the specimens was fatigue tested by subjecting the shot-peened side to a uniform bending tensile stress over the central six inches of length with a range of zero to 200,000 pounds per square inch at the surface in each cycle.

In the table immediately specimens were subjected following, a number of such to each of the treatments described. FATIGUE TEST RESULTS Primary Treatment Secondary Treatment Mean Group Life 3 SAE SAE SAE SAE Shot Size Intensity 2 Shot Size Intensity 1 Chilled iron shot used. 1 Commonly designated by deflection in thousandths of an inch of a in f tbtnan 1 b r 1 um er 0 g eye as e ore com ets ru ture. No peening. p p

The above table further shows the relative diflferences in mean life produced by varying treatments which include a single peening and a double peening treatment in which larger shot and a larger intensity treatment are used in the second step.

7 We have also found that an increase in fatigue life is also obtained by grit blasting the surface of a previously shot-peened metal part. We have now also found that grit blasting can be used as a secondary treatment for conventionally shot peened parts. Major increases in fatigue life of metal parts can be produced by grit blasting a previously conventionally peened part. Although improvements in fatigue life are obtained in this manner, the overall results obtainable therewith are not as satisfactory as those obtained from the previously described method.

In general, we have found that an air blast carrying metallic particles having an average mean diameter of from about 0.003 inch to 0.017 inch or an SAE grit number of from about G-200 to about 6-40 can be used. These particles can be directed onto the previously shotpeened surface with a conventional grit blasting apparatus, such as that commonly used in the art, employing an air pressure of approximately 70 pounds per square inch to approximately pounds per square inch.

More specifically, a metal leaf-spring specimen generally similar to that previously described, was subjected to shot-peening treatments of an SAE intensity of approximately 0.009C to 0011C using SAE 660 chilled iron shot. Following this shot-peening treatment the specimen was grit blasted with a conventional grit blasting apparatus with an SAE G-80 grit and an air pressure of approximately 70 pounds per square inch under an exposure of about 15 seconds. This treatment of the metal leaf-spring provided more than a 100% increase in the fatigue life of the leaf-spring specimen.

The beneficial results obtainable with our invention are more particularly indicated by the bar chart in FIGURE 2 and the graph shown in FIGURE 3. FIGURE 2, for example, compares fatigue life cycles of untreated metal parts, conventionally peened metal parts, and metal parts treated by the method of our invention. While untreated parts have a median fatigue life of less than 100,000 cycles, parts treated in accordance with our invention display a median fatigue life of over 350,000 cycles.

Although the chart of FIGURE 2 clearly indicates the large improvement in fatigue life resulting from the use of our invention, it is not entirely satisfactory. Of great interest are the probability of failure after testing for x stress cycles and the accuracy with which we can estimate probability.

Information concerning these points is obtained using satistical techniques previously described by Johnson (The Median Ranks of Sample Values in Their Population with an Application to Certain Fatigue Studies, Leonard G. Johnson, Industrial Mathematics, vol. 2, 1951; Fatigue Tests Proved by Three Statistical Checks, Leonard G. Johnson, SAE Journal, March 1958, pp. 72-73; Statistical Estimation of the Minimum Life in Fatigue, Leonard G. Johnson, G.M. Research Laboratories Technical Memorandum 34-948, March 10, 1958; Statistics of Extremes, E. J. Gumbel, Columbia University Press, 1958, p. 302). Briefly information was found on the above points from Weibull plots and confidence bands, respectively. A Weibull distribution function is of the form population occurring at some The minimum fatigue life (a) and the median population curves used in the Weibull plots of FIGURES 3 through 5 are those that best fit the observed fatigue lives. G oodness-of-fit numbers given in FIGURES 3 through 5 are an index to the fit, with a goodness-of-fit number=l describing a condition wherein all (x-a) quantities fall on a straight line. The slope (b) of the best fit straight line is known, as is the characteristic life 0, and the Weibull median population line is completely determined.

FIGURE 3 is a Weibull plot comparing the fatigue lives of the test groups noted in Table I. With percent failure in-population,

1F(:c) as ordinate and life cycles. (x) as abscissa, the Weibull plotsin FIGURES 3, 4 and 5 clearly illustrate the increase in life at different survival levels'by our invention.

Table I I Good- (a) (b) Median Test Group Ilei S-lf- Cycles Slope Lite l Non-Peened 0. 90500 7.76 22,720 Single Peened: SAE 660 at 0.00000 0. 98191 40,000 2.27 80,730 Double Peened: SAE 660 at i 0.00900; SAE

A Weib-ull plotofthe test groups listed in Table II is shown in FIGURE 4. This graph indicates that merely lowering the intensity of the secondary treatment serves to increase median. 1ife..

Table II Good- (a) (b) Median Test Group ness-of- Cycles Slope Ife Fit Single Peened: SAE 660 at 0.0090 0.98191 40, 000 2.27 80,730 Double Peeued: SAE 660 at 0.00000; SAE 660 at-0.00l6C. 0.97183 20,000 5.42 121,000

The Weibull plot in FIGURE is a graph comparing fatigue lives of the test groups listed in Table III. This Weibull plot shows that by solely reducing shot size in the secondary peening treatment, material benefits can be obtained.

Table 111 Good- (41) (0) Median Test Group negs-gf- Cycles Slope Life Single Peened: SAE 660 at 0.0073 0.82292 0 3.32 135,400 Double Peened: SAE 660 at 0.0072A; SAE 70 at 0.0070A- 0. 90017 100, 000 1. 06 149, 300

The 90% confidence bands, also shown on the Weibull plots of FIGURES 3 through 5, estimate the boundaries which enclose the middle 90% of all fatigue lives. Confidence interpolation indicates that the median life at 1% or 50% failure in population level is significantly improved over a single (conventional) shot-peening treatment by:

(1) Secondary peening by using smaller shot and lower intensity than used for the primary shot-peening treatment.

(2) Secondary peening by secondary peening alone.

(3) Secondary peening by secondary peening alone.

using reduced intensity for using smaller shot size for Specification of shot size and intensity for the secondary shot-peening treatment to produce greatest fatigue life is difficult because of the paucity of systematicfatigue data. Nonetheless, some boundaries appear rather clearly. For optimum results on spring steels, for ex; ample, the primary shot size should be between 0.023 inch to 0.066 inch in diameter andthe intensity should be between 0.0l6A to 0.034A, the secondary shot size should be between 0.007 inch to 0.011 inch in diameter, and the intensitybetween 0.003A to 0011A.

It is to be understood that although this invention has been described in connection with certain specific examples thereof, no limitation is intended thereby except as defined in the appended-claims.

We claim:

1. A method of increasing the fatiguelife of a metal part which comprises applying an initial particle blast treatment to a surface of a metal part and thereafter further subjecting said surface to another particle blast of a lesser intensity and smaller particle size than was employed in said initial'particle blast treatment.

2. A method f increasing the fatigue life of a metal part which comprises applying an initial particle blast treatment to a surface of a metal part and thereafter further subjecting said surface to another particle blast of an intensity and particle size which is approximately /3- to /5 that employed in the initial particle blast treatment.

3. A method of increasing the fatigue life of a metal treatment to a surface of a metal part and thereafter further shot-peening said surface with a treatment involving a lower intensity and smaller shot size than was employed in said initial shot-peening treatment.

4. A method of increasing the fatigue life of a metal part which comprises applying a particle blast treatment to a surface of a metal part and thereafter further subjeoting said' surface to a particle blast having, a lesser intensity than the intensity of said former particle blast treatment.

5. A method of increasing the fatigue life of a metal part which comprises applying a particle blast treatment to a surface of a metal part and thereafter further subjecting said surface to a particle blast treatment having an intensity which is approximately /3 to /5 the intensity of said former particle blast treatment.

6. A method of increasing the fatigue life of a metal part which comprises applying a particle blast treatment to a surface of a metal part and thereafter subjecting said surface to another particle blast treatment having an intensity of approximately /3 to /5 that of said former particle blast treatment and in which particles are used which are approximately /3 the size of those used in said former particle blast treatment.

7. The method of increasing the fatigue life of a metal part which comprises conditioning the surface of a metal part by inducing a compressive stress thereon with an initial shot-peening treatment and thereafter further conditioning said surface with a shot-peening treatment having an intensity which is lesser than that of said initial shot-peening treatment 8. The'method of increasing the fatigue life of a metal part which comprises conditioning the surface of a metal part by inducing a compressive stress thereon by an initial shot-peening treatment and thereafter further conditioning said surface with a shot-peening treatment using a shot size which is'smaller than that employed in said initial shot-peening treatment.

9. The method of increasing the fatigue life of a metal part which comprises conditioning a surface of a metal part by inducing a compressive stress thereonwith a shot blast treatment and thereafter further conditioning said surface with a shot blast treatment having an intensity of approximately /3 to /5 that of said former shot blast treatment.

10. The method of increasing the fatigue life of a metal part which comprises conditioning a surface of a metal part by inducing a compressive stress thereon with a shot blast treatment and thereafter further conditioning said surface with a shot blast treatment having an intensity 'of'approximately /3 that of said former shot blast treatment and in which the shot used is approximately /3 to /s the size of that used in said former shot blast treatment.

11. The method of increasing the fatigue life of a metal part which comprises shot peening a metal surface with a blastv of shot from the class consisting of cut wire shot, conditioned cut wire shot, cast iron shot and cast steel shot and thereafter further shot peening said suris of an intensity of approximately /3 to /5 that of said former shot blast treatment. and the shot used therein is approximately 6 to /s the sizeof that used in said former shot blast treatment.

12. The method of increasing" the fatigue life of a metal part whichfcomprises shot-peening a surface .of a

' metal part with a blast of shot from the class consisting mean diameter of shot used is about 0.007 inch to 0.011

inch.

13. The method of increasing the fatigue life of a metal part which comprises shot-peening the surface of a spring steel part with shot having a mean diameter of about 0.023

inch to about 0.066 inch, wherein said shot-peening treatment is of an intensity of about 0016A to 0.034A, and thereafter further shot peening said surface with shot having a mean diameter of about 0.007 inch to 0.011 inch, wherein said shot-peening treatment is of an intensity of about 0.003A to 0.011A.

14. The method of increasing the fatigue life of metal parts which comprises shot-peening the surface of a metal part and thereafter grit blasting said surface of said metal part.

15. The method of increasing the fatigue life of a metal part which comprises shot-peening a surface of a metal part to approximately coverage and subse quently grit blasting said surface with grit having a mean diameter of approximately 0.003 inch to 0.017 inch.

16. The method of increasing the fatigue life of metal parts which comprises shot-peening a surface of a spring steel part with shot from the class consisting of cut wire shot, conditioned cut wire shot, cast iron shot and cast steel shot having a mean diameter of about 0.023 inch to 0.066 inch,wherein said shot-peening treatment is of an intensity of about 0.016A to 0.034A, and thereafter grit blasting said surface of a metal part with grit having a mean diameter of approximately 0.003 inch to 0.017

inch.

References Cited in the file of this patent UNITED STATES PATENTS Pabens Mar. 29, 1932 Palm Jan. 23, 1934 Vorwerk Feb. 6, 1934 Vorwerk Feb. 20, 1934 Minich Apr. 20, 1937 Wallace Feb. 14, 1944 Connor Apr. 17, 1945

Claims (1)

1. A METHOD OF INCREASING THE FATIGUE LIFE OF A METAL PART WHICH COMPRISES APPLYING AN INITIAL PARTICLE BLAST TREATMENT TO A SURFACE OF A METAL PART AND THEREAFTER FURTHER SUBJECTING SAID SURFACE TO ANOTHER PARTICLE BLAST OF AT LESSER INTENSITY AND SMALLER PARTICLE SIZE THEN WAS EMPLOYED IN SAID INITIAL PARTICLE BLAST TREATMENT.

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