US2958925A

US2958925A - Shot peen inspection technique

Info

Publication number: US2958925A
Application number: US811119A
Authority: US
Inventors: James G Roberts
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1959-05-05
Filing date: 1959-05-05
Publication date: 1960-11-08
Anticipated expiration: 1977-11-08

Description

J. G. ROBERTS 2,958,925

SHOT PEEN INSPECTION TECHNIQUE Filed May 5, 1959 ARC HE IGHT (we/45s) I l l I l I i I I I l I I g I l I I l l I EXPOSURE TIME (Mm/um) 3 INVENTOR JAMES G ROBERTS ATTORNEY United States Patent Office 2,958,925 Patented Nov. 8, 1960 SHOT PEEN INSPECTION TECHNIQUE James G. Roberts, Warren, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed May 5, 1959, Ser. No. 811,119

Claims. (CI. 29-90) This invention relates generally to shot peening of metal parts, and more particularly to the method of determining when a part has received full coverage or has been fully treated by the peening process.

Cold working of the outer skin of metal parts by peenj ing has been known for several centuries. Peening with tions are uniformly dispersed and the skin has had full coverage, the fatigue life of the article is greatly increased as to twisting and bending resistance. If the article is over peened, however, minute cracks occur in the surface due to the fact that the surface is stretched so greatly that the ductility is exhausted. These cracks are very likely to start a spreading fatigue fracture under even very low stresses.

As explained in chapter 2 of the publication entitled Shot Peening by the American Wheelabrator and Equipment Corporation, Second edition, 1946, the relationship between the fatigue life of an article and the stress applied is a geometrical one and when the stress reaches a certain critical value for the material concerned, a small increase in stress results in a very large decrease in fatigue life. Shot peening raises the amount of initial stress required to reach this critical value. Full coverage of the article by shot peening is very much desired since the final degrees of the process leading to full coverage may increase the fatigue life as much as several hundred percent.

Since full coverage is highly desired but over peening is very undesirable, the problem of determining this point of full coverage is very crucial. This problem has caused considerable difiiculty when dealing with high hardness materials.

Accurate determination of this point of full coverage not only helps prevent over peening, but also aids research workers who have had difficulty in the study of high hardness items such as bearings. This is true since one variable property may not be studied exclusively when the properties are spuriously altered depending on the degree of full coverage obtained in the peening process.

Furthermore, it has been noted that there is a time interval between the moment at which full coverage is first reached and that moment when over peening begins. It is therefore desirable, for low cost production economy, to be able to determine the minimum time at which full coverage is reached when using high hardness materials.

It is therefore an object of this invention to provide a method to enable one to determine when full coverage of a metal article by shot peening has been reached. A further object is to provide a method to enable one to readily determine this time on metals having surfaces of a hardness above Rockwell C(RC)55. More specifically, it is an object of this invention to set forth a procedure to determine when full coverage, by shot peening has been obtained for high hardness steels. These and further objects will become more apparent from a reading of the descriptive matter to follow when read in connection with the accompanying drawings wherein:

Figure 1 is a plan view of a metal test strip shot peened in accordance with the present invention;

Figure 2 is an elevational view of Figure 1 illustrating the arc height of the test strip; and Figure 3 is a graphical representation of the relationship between strip arc height and exposure time to shot peening of two strips in accordance with the present, invention.

Broadly, the method in accordance with the present invention involves the determination of full shot peening coverage of a high hardness article by the use of test strips which are dimensionally equivalent. For convenience, standard Almen test strips may be used along with a relationship which has been discovered as existing between the peening characteristics of soft and hardsurface metals.

The time required for full shot peening coverage of a production item having a hardness of RG55 or greater is found by utilizing two dimensionally equivalent test strips: one of the material and having a hardness equivalent to that of the production item above, and the other having a specific hardness in the range of RC44-50. The full coverage times are obtained for these strips in the manner well known in the art and explained more fully hereinafter, and from these two times a ratio having a value greater than 1 is set-up. This ratio is then multiplied by the full coverage time required for shot peening a production item equivalent to the one above, a

but having a specific hardness equal to that of the test strip in the range of RC4450. This latter time may be conveniently found by visual determination or by calculations based on the data which have been collected in the past on materials of this hardness and which are well known. to those having ordinary skill in the art, The numerical product obtained from these factors is equal to the time required for shot peening the high hardness production items to full coverage.

The process involved may be understood more readily in the light of the following background information.

The method of treating metals by shot peening has been studied as a science and one result of this study has been the development of Almen test strips to be used for shot peening experimentation. The industry has adopted these test strips as standards. The two strips developed by Almen are labeled A and C and the specifications of these strips are given in the chart below.

It is seen that the only difference between the two is thickness, thus providing a test strip for both high intensity and low intensity peening. For convenience,

the Almen test strips were utilized in the conductance: of experiments associated with the present invention, but

it is obvious that test strips having dimensions differing from those of the Almen strips will Work very well in the proposed method as long as the dimensional characteristics are alike, i.e., that the strips are dimensionally equivalent for shot peening purposes.

Almen Test Strip When these test strips such as strip in Fig. 1 are shot peened, the compressive forces setup on the peened surface 12 cause the strip to bend in an opposite direction creating an arc height as shown by the arrow 14' in Figure 2.- This bending continues until a crucial point is reached at'which time the arc of the strip no longer changes. This complete cessation in change holds true if theshot is uniformin size and other conditions, such as velocity, are ideal. Under conditions as normally encountered, however, the cessation'in bending is not quite so 'clearly pronounced, but the full coverage point may still be readily determined as the point of drastic decrease in rate of arc height change. This point in the peening process at which the arc height increases negligibly with further peening is known as the full coverage'point, and the arc height is substantially at its maximum value at this time. This point may be determined as shown by the -x" marks on the respective curves in Figure 3. The corresponding test strips, a and y; for each curve as shown in'Figure 3 are the relatively soft (RC44-50) and the hard (RC 55) strips, respective'ly.

Inproduction operat ons, the only conditionwhich is allowed to be a variable-is time, since velocity of shot is easilycontrolled using the air-pressure or'cent rifugalwheel-thrower methods of propulsion; and the same shot iS' recircuIated continuously through the short throwing machine thus involving no variable in overall shot size.

The time of peening is related to are height of the test strips to determine at what time full coverage is accomplished.

This point of full coverage may be readily determined for metal articles other than test strips, if the articles have a hardness between about Rockwell C44 and Rockwell C50. The time maybe determined by either visual means or by estimation calculations based on the data which have been collected in the past on materials of this specific hardness. The present invention utilizes this 7 factor in the peening determination of hard surfaces as described hereinafter.

Many production items today possess hard surfaces, such as are encountered with high hardness steels, of Rockwell C55 and above. Although it is equally desirable to shot peen surfaces of this type, until now there has been no easy, inexpensive, reliable method of determinging the time of exposure corresponding to full coverageof the article. Visual means are not really ade quate to determine the stopping point for surfaces having a hardness of about Rockwell C55 or harder. Even under moderate magnification (20x), visual observations are inadequate and unreliable to determine if the surface has been completely covered by peening or if the surface has been over-peened. Furthermore, even if it were possible using high power magnification apparatus, this type of apparatus, if used in production, would be, cumbersome, time consuming, and costly.

The method which has been invented to overcome this difiiculty utilizes related properties found existing between the high hardness materials and the materials for which the time of peening may readily be determined. Two physically equivalent Almen test strips, both of either the A or C type, are first heat treated. One strip is hardened to a hardness within the range of Rockwell C44-50 and the other strip is brought to the hardness of the production item which may be Rockwell C55 or above. These two strips are shot peened under like conditions and the separate times necessary to bring about full coverage are related to obtain a ratio. Full coverage is obtained when the arc height of the alrnen strips, as measured by a standard No. 2 Almen specimen gauge, ceases to change with further peening. The time required for full coverage .of the hard metal surface such as a high hardne s steel is consider y longer than. h

4 required for the softer material of RC44-50. The ratio of the high hardness time to the RC44-50 time (high hardness time) (RC44-5O time) may be called the Almen stripfaetgr (ASF).

(Ttsyl (1) (Ttsa) where Tt s'y is the tirrre g required to shot peen to full coverage the test strip having a high hardness (RG55 or above), and Ttsa is the time required to shot, peen to full coverage the test strip having a hardness in the range of RC44-50.

The next step is to determine the time required for full coverage of the production item if it had a hardness in the range of RC44-50 and'equal to that of the test strip a. (For convenience, call the peening time required for the productionitem of this hardness Ta.)

This can be done by estimation on the basis of existing data which hasbeen collected due to much past experimentation with metals of this hardness, or' by actual- 1y imparting a hardness of RC445O to the standard item or; an item having an equivalent peening area, and then visually determining the time necessary for full coverage" when shot peening the item'under equivalent production peening conditions.

It has been found that the time required for obtaining full'coverage of the high hardness production item (Ty) is a multiple of the time required for the physically identical Rockwell C44-SO article (Ta) and may be accurately found by multiplying the following factors:

, y) (2) (Ttsa) Obviously this mathematical set-up of the factors is arbitrary and may be altered without departing from the scope of the invention'as long as the true relationship is t (A (T retained. A sample'deter'mination as actually found by this method is given below. I

Example Two C Almen test strips were hardened to Rockwell C62 and Rockwell C50, respectively, and were given a shot peening treatment with SAE 230' chilled iron shot at 26 psi. air pressure, i.c., corresponding to the nominal Almen. intensity of 0.010A. The ratio of the time re-. quired for full coverage as determined by means of.

ASF

mission gear with a hardness of RC62. Since the time 7 required for this same gear having a hardness of RCSO (Ta) can be readily found by calculations or by peening and visual determination, this time (Ta) is now simply multiplied by theASF. previously'determined. Thetime of 16 minutes is used for (Ta).

Therefore:

Ty=TGEAR=(ASF) (Ta) (2.7) (16) :43.2 minutestime required for shot peening to full coverage a steel gear with a hardness of RG62. This is just one example of how this new and useful method may be quickly and easily used to obtain a reliable and economical determination of the time required for full shot peening coverage of a hard-surfaced article.

Various changes and modifications of the embodiments of my invention as disclosed above may be made by those skilled. in the art without departing from the principles and spirit of my invention as set'forthin-the appendedclaims.

me hod mis s stniin is. fu l 1 1 .ps inaw emsei for a high hardness steel article comprising the steps of shot peening test strips to experimentally determine the ratio of the shot peening time required for a test strip having a hardness of Rockwell C44-50 and the time required for a test strip having a hardness equal to that of said high hardness steel article, and shot peening the high hardness steel article for a time proportional to said ratio.

2. A method for shot peening a high hardness article with a hardness of RCSS or above to full coverage comprising the steps of peening a test strip having a hardness equal to that of said high hardness article to full coverage, peening a physically similar test strip having a hardness of Rockwell C44-50 to full coverage, and finally peening the high hardness article for a time calculated from the ratio of the test strip times and the time normally required to peen under equivalent conditions a similar article having a hardness of Rockwell C44-50.

3. A method for determining the minimum full coverage time for shot peening a high hardness steel article comprising the steps of shot peening to full coverage a test strip of the same high hardness steel and a test strip having a hardness of Rockwell C44-50, and then shot peening said high hardness steel article for a time equal to the product of the time required for fully shot peening under equivalent conditions a steel article with a hardness of Rockwell C44-50 and a multiple determined from the ratio of the times required to shot peen said test strips.

4. A method for obtaining full shot peening coverage of a high hardness steel item in a minimum amount of time comprising the steps of shot peening a test strip having a hardness equal to that of said high hardness steel item to full coverage and then shot peening said high hardness steel item for a time equal to a predetermined multiple of that time required 'for full coverage of the test strip.

5. A method for shot peening to full coverage in the optimum time a steel with a hardness of about RC55 or above by subjecting said steel to the driven shot for a time expressed by the equation:

Where Ty is the shot peening time sought for the high hardness steel,

Ttsy is the shortest time at which further shot peening of a test strip of the high hardness steel results in negligible increase in arc height,

Ttsa is the shortest time at which further shot peening of a test strip under equivalent peening conditions to those of said high hardness steel test strip, which is dimensionally equivalent to the high hardness test strip but which has a hardness of Rockwell C44-50, results in negligible increase in arc height,

Ta is the minimum shot peening full coverage time under equivalent peening conditions to those associated with time Ty, for a steel having a hardness'of Rockwell 044-50 and having an area equivalent to that of the high hardness steel-to be shot peened, and

( a) Ttsa) 6. A method for determining full shot peening coverage on a high hardness steel article comprising the steps of shot peening a test strip of the high hardness steel for the shortest time required to produce substantially the maximum arc height of said strip (Ttsy), shot peening under equivalent conditions a dimensionally equivalent test strip having a hardness of Rockwell C4450 for the shortest time required to produce the maximum arc height of said equivalent test strip (Ttsa), shot peening to full coverage to obtain a determinable time a first article physically equivalent to the high hardness steel article and having a hardness of Rockwell 044-50 (Ta), and shot ASF=the ratio of peening under conditions equivalent to those of said first article the high hardness steel article for a time (Ty) expressed by the equation:

( y) Ttsa) 7. A method for shot peening a high hardness metal to full coverage in a minimum time comprising the steps of shot peening to full coverage a metal having a specified area and a hardness of Rockwell C44-50 in the minimum time and recording the time required, shot peening two dimensionally equivalent test strfps having a hardness of Rockwell 044-50 and a hardness equal to that of the high hardness metal to be shot peened, respectively, to determine a greater-thanone ratio from the time required for the two strips, and shot peening the high hardness metal having an area equivalent to that of said specified area, and under equivalent peening conditions, for a time which is equal to the product of the time required for shot peening said metal and the ratio determined from the test strips.

8. A method for preventing over peening when shot peening a high hardness steel comprising the steps of shot peening the high hardness steel for a time expressed by the following equation:

Ty- (Ta) Ttsa) (Ta)(ASF) Ty is the shot peening time sought for the high hardness steel,

Tzsy is the time at which further shot peening of a test strip of the high hardness steel results in negligible increase in arc height,

Ttsa is the time at which further shot peening of a test strip, under equivalent conditions to those of said high hardness steel test strip which is dimensionally equivalent to the high hardness test strip but which has a hardness of Rockwell C44-50, results in negligible increase in arc height,

Ta is the shot peening full coverage time under equivalent peening conditions as those associated with time Ty, for a steel having a hardness of Rockwell C44-50 and having an area equivalent to that of the high hardness steel to be shot peened, and

where 9. A method of shot peening a high hardness steel to full coverage in the minimum time comprising the steps of determining the respective minimum times required to shot peen test strips of a specific hz'gh hardness steel and of a steel having a specific hardness between Rockwell C44-50 to substantially the maximum arc height of each, and then shot peening the high hardness steel for a time which is mathematically derived from the minimum times required for the test strips.

10. A method for determining the optimum shot peening time required to obtain maximum fatigue life for a high hardness steel, comprising the steps of experimentally determining the peening time (Ta) necessary for full coverage of steel (Sa) having a hardness in the range of Rockwell C44-50, and then shot peening under equivalent conditions a high hardness steel dimensionally equivalent to Sa for a time which is a multiple of the full coverage time for Se and is equal to:

(Ttsy) Ttsa) Where Ttsy=the minimum shot peening time required to obtain substantially the maximum arc height for a high hardness steel test strip, and Ttsa=the minimum shot peening time required to obtain substantially the maximum arc height for a test strip having a hardness equivalent to Sa when Ttsa and Ttsy are shot peened under like peening conditions.

No references cited.

ASF=the ratio of