NBSIR 77-1275

EXAMINATION OF CAP SCREWS

FROM CURRENCY PRINTING PRESS

FLANGES

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

Failure Analysis Report

June 1977

Prepared for Office of Engineering

Bureau of Engraving and Printing

Department of the Treasury

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Table of Contents

Page

SUMMARY iii

1. INTRODUCTION 1

1.1 Reference 1

1.2 Parts Submitted 1

2. PURPOSE 2

3. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES 2

3.1 Fractographic Examination 2

3.2 Chemical Composition 3

3.3 Dimensional Measurements 3

3.4 Hardness Measurements 4

3.5 Metallographic Examination 4

4. DISCUSSION 5

4.1 Dynamic Loading on Loose Screws 5

4.2 Tensile Strengths Inferred from Hardness 6

4.3 Metallographic Observations 7

5. CONCLUSIONS 8

6. ACKNOWLEDGEMENT 9

TABLES

i

1. Results of Chemical Analyses of Cap Screws 10

2. Results of Dimensional Measurements of Stretched

Type A Cap Screw 11

3. Results of Surface Hardness Measurements on Cap Screws 12

4. Results of Hardness Traverses through Longitudinal

Sections of Cap Screws 13

i

1
FIGURES

1. Fracture surface of type A cap screw.

2. Fracture surface of type A cap screw.

3. SEM fractograph of the fatigue portion of the fracture in type A

cap screw.

4. SEM fractograph of the fatigue portion of the fracture in type A

cap screw.

5. Longitudinal sections through two type A cap screws.

6. Longitudinal sections through two cap screws.

7. Deeply etched longitudinal section through cap of fractured type A

cap screw showing the fracture profile.

8. Part of the longitudinal section shown in figure 7.

9. Unetched longitudinal section through type A cap screw exhibiting an inclusion content representative of the cap screws examined.

10.

Longitudinal section through type C cap screw showing part of one

thread.

11 . Longitudinal section through type C cap screw exhibiting representative

microstructure which consists primarily of tempered martensite.

12 . Longitudinal section through an unstretched type A cap screw exhibiting

microstructure which consists primarily of tempered martensite.

13. Longitudinal section through stretched type A cap screw.
SUMMARY

The Office of Engineering of the Bureau of Engraving and Printing

submitted a number of failed and intact cap screws from printing press

flanges to the NBS Mechanical Properties Section for examination. All

of the cap screw failures examined were due to the initiation and

propagation of fatigue cracks. Based on the results of hardness measurements, some of the cap screws of one manufacturer did not meet the

minimum ultimate tensile strength requirements for DIN 12.9 screws. One cap screw with an ultimate tensile strength significantly below

the specified minimum had yielded apparently while in service.

iii

Examination of Cap Screws from Currency Printing Press Flanges

1. INTRODUCTION

1 . 1 Reference

This examination was requested by Mr. Milton J. Seidel, Chief,

Office of Engineering, Bureau of Engraving and Printing, in a letter

to Dr. Bruce W. Christ, Chief, NBS Mechanical Properties Section.

The request was dated May 26, 1976.

1 . 2 Parts Submitted

Over a period of months, the Office of Engineering of the Bureau

of Engraving and Printing submitted several groups of cap screws from

currency printing press flanges to the NBS Mechanical Properties Section

for examination. Cap screw types A, B, C, and D had a nominal diameter

of 24 mm and a pitch of 3 mm. Type E had a nominal diameter of about

35 mm and a pitch of about 4 mm. The cap screws are used to fasten

flanges to cylinders on currency printing presses. The caps of the screws fit into counterbores in the flanges. The cylinders on the

printing presses rotate at 34 rpm on the Giori presses and 67 rpm on

the American Bank Note presses.

The various groups of submitted cap screws are listed below:

A. May 26, 1976

a. One failed type A cap screw from American Bank Note

printing press no. 304.

b. One new cap screw to be used on an American Bank Note

printing press.

B. July 15, 1976

a. One new type A cap screw.

b. Three new type C cap screws.

c. One threaded shank from a failed type C cap screw.

d. One new type E cap screw (type used on the Giori-80

printing presses).

e. One new type D cap screw.

C. July 19, 1976

a. Six failed type A cap screws — caps only.

1
b. One failed type B cap screw — cap only — from impression

cylinder

.

September 9, 1976

a. Seven failed type D cap screws from Giori-74 printing

press (no. 202).

b. Three intact type A cap screws from Giori-74 printing

press (no. 202).

October 7, 1976

a. One intact type E cap screw.

b. Two used, intact type A cap screws labeled H and Q,

respectively.

c. One apparently unused type A cap screw.

2. PURPOSE

The Office of Engineering of the Bureau of Engraving and Printing

requested that the NBS Mechanical Properties Section examine the submitted

cap screws in order to determine the nature of the failures.

3. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES

3 . 1 Fractographic Examination

The fracture surfaces of all of the failed cap screws were examined

visually and macroscopically . Selected samples were examined with the

scanning electron microscope (SEM) . All of the fractures appeared to have

initiated in fatigue. Actually, in each fracture, a number of fatigue

cracks had initiated adjacent to each other, but on slightly different

transverse planes. After propagating a relatively short distance, the

several fatigue cracks joined to form a common crack front. This common crack front then propagated in fatigue transversely across the cap screw

until the cross section had been reduced sufficiently so that it could no longer support the applied load and fracture was completed in ductile

overload

.

In some of the fractures, fatigue cracks initiated in two regions about

180° apart. When this phenomenon occurred, there were two crack fronts

that propagated toward each other. As in the case of the single crack front,

fracture was completed in ductile overload when the cross section of the

cap screw was reduced sufficiently so that it could no longer support the

applied load.

2
Cracks initiated in either of two locations in the cap screws -- just under the cap or through the root of one of the threads, depending

when in a sequence of failures a particular screw failed. A larger

portion of the fracture of the screws that were among the first to fail

in a sequence of failures propagated in fatigue compared to the fractures

of the screws that failed later in the sequence. Examples of the

fracture surfaces and fracture profiles of cap screws at two different

stages of the fracture sequence are shown in figures 1 and 2.

Representative fracture surfaces were examined with the scanning

electron microscope (SEM). Examples of the features exhibited by the

fatigue portion of the fractures are shown in figures 3 and 4. There

is no evidence of fatigue striations in figure 3, but some apparent

striations can be seen in figure 4. Fatigue striations are often not

resolvable in steel fractures. Dimpled rupture is the fracture mode in the

overload portion of the fracture.

3 . 2 Chemical Composition

A sample from one cap screw from each of three different manufacturers

was submitted to a commercial laboratory for chemical analysis. The

results of these analyses are presented in Table 1. The type A and

type C cap screws were fabricated from AISI 8630 and 4140 steels,

respectively. The type D cap screws were fabricated from a steel

similar in composition to the AISI 5100 series. The chemical composition

of the types B and D cap screws was not determined.

3.3 Dimensional Measurements

One of the type A cap screws that had been used, but was still intact

appeared to have been deformed by being stretched over a distance of

several threads. Longitudinal sections through this cap screw and

through another type A cap screw that did not appear to exhibit any

macroscopic deformation are shown in figure 5. The approximate

region of stretch in the deformed cap screw is indicated in figure 5a

by brackets. The pitch and minor diameter were determined along the

length of a longitudinal section through the stretched screw. The

section was not through the longitudinal axis of the screw; therefore

the minor diameter measurements do not represent the true minor diameter,

but they do show differences caused by the stretching. The results of

these measurements are presented in Table 2. A way from the region of

deformation, the pitch averaged 3.00 mm. Where the deformation was

severe -- at the third thread from the cap — the pitch rose to a maximum of 3.81 mm — about 27% above the average. At this same location, the minor diameter was 18.42 mm compared to 19.91 mm away

from the region of deformation. This cap screw had apparently been

stressed beyond its yield point.

3
3.4 Hardness Measurements

Rockwell hardness measurements were made at various locations on most of the as-received cap screws. The range of hardness and the

number of screws for which the hardness was determined at any given

location are presented in Table 3. Excluding the type B and the type

E cap screws tested, there were some variations in hardness both among

and within the three other groups of cap screws — types A, C, and D. The variation was least among the three groups at the bottom of the screws. The hardness data for the stretched type A cap screw are listed

separately at the bottom of Table 3. This screw is softer than the other

type A screws, the type C screws, and the type D screws at each location

where measurements were taken, and it was significantly softer at the

shank adjacent to the cap at the bottom of the screw.

In addition to the hardness measurements on the as-received screws,

HRC hardness traverses were made on longitudinal sections through the

stretched type A cap screw, the undeformed type A cap screw shown in

figure 5, one type C cap screw, and one type D cap screw. The hardness

measurements were made near the longitudinal axes of the screws. The

results of these traverses are presented in Table 4. The types C and D

cap screws were very similar in hardness, whereas the unstretched

type A cap screw was significantly softer over the entire length. The

stretched type A cap screw was similar in hardness to the unstretched

type A cap screw at the top, but it was significantly softer away from

the top.

3. 3 Metallographic Examination

Longitudinal sections were taken through two type A cap screws, one

type C cap screw, and one type D cap screw. The sections through the two

type A cap screws are shown in figure 5. The other sections are shown

in figure 6. All four sections were heavily etched when photographed

for figures 5 and 6 in order to reveal flow line patterns. Although

there were obvious differences in the flow line patterns among the

different specimens, nothing unexpected was observed.

A longitudinal section through one of the fractured type A cap screws showing the fracture profile appears in figure 7. This section has

been heavily etched to reveal the flow line patterns. In this fracture,

fatigue cracks initiated just under the cap, and they initiated in two

general regions approximately 180° apart. These two regions are indicated by arrows 0 in figure 7. The crack front that initiated at

the left in the figure deviated from the primary fracture where the two

advancing crack fronts meet and followed the path indicated by the

unmarked arrows. For a large part of this path, the crack followed the

flow lines.

4
There were a number of relatively short secondary cracks that were essentially parallel to the fracture and slightly removed from it.

Two such cracks appear in figure 8 which shows part of the longitudinal

section that appears in figure 7, but at higher magnification. The

approximate location of these cracks is indicated by arrow C in figure 7.

These cracks initiated in the fillet between the shank and the cap of the screw. The surface of the cap screw at the fillet appears to be rather

rough, possibly due to machine tool marks. Surface roughness creates

stress concentrations that increase the ease with which a fatigue crack

can initiate.

Sections of cap screws of some of the types were also examined

microscopically. For the cap screws examined, the material in the

type A, type C, and type D cap screws was very similar in regard to

inclusion content. A field showing a representative inclusion content

appears in figure 9. In general, the inclusion content was not considered

abnormally high, although one high inclusion region was found near the

center of the cap of one of the type A cap screws. The low inclusion

content is consistent with the low sulfur and phosphorus indicated by

the chemical analyses.

The type C cap screw section exhibited longitudinal crevices adjacent

to the surface over the entire length of the screw. Apparently the cap screw had been subjected to a surface treatment that had attacked the

"stringer" inclusions as well as the surrounding material to produce the

condition shown in figure 10.

The microstructure of all of the cap screws examined consisted

primarily of tempered martensite. Photomicrographs showing fields

representative of the type C cap screw and the undeformed type A cap screw appear in figures 11 and 12, respectively. The microstructure

of the type D cap screw was similar to that of the type A cap screw

shown in figure 11. The microstructure of the stretched type A cap screw is shown in figure 13. Although this microstructure consists

primarily of tempered martensite, it is different from the others in

that banding is evident.

The microstructure of types B and E cap screws was not examined.

4. DISCUSSION

4 . 1 Dynamic Loading on Loose Screws

The cap screw failures examined in this task were caused by the

initiation and propagation of fatigue cracks due to high cyclic stresses

developed as the cylinder rotates. Cracks initiated either just under

the cap or at the root of a thread. A plausible explanation for the

difference in location of the initiation sites was offered by

Mr. Francis R. Varrese, Manager of Materials Research, Standard Pressed

Steel Company, Jenkintown, Pennsylvania. It was his opinion -- and we

5
concur with that opinion — that when a number of screws in one flange

fail over a period of time and are not replaced, fatigue cracks would

probably initiate under the screw cap in the screws that fail first.

After several screws have failed, those remaining would be more likely

to develop fatigue cracks initiating at the root of a thread. The

change in location of the initiation sites is due to the development

of higher bending stresses in the screws that fail later in the sequence.

The higher bending stresses would be developed partly because there would

be fewer screws remaining to support the applied load and partly because

of a lack of symmetry in the support system caused by the loss of some of

the screws.

The increase in the proportion of fatigue fracture to overload

fracture in the screws that fail last is due to the higher stress

amplitude in the screws that fail last compared to those that fail

early in the sequence. Again, the higher stress amplitude is caused

by fewer screws supporting the applied load.

As indicated in an earlier memorandum"'', inadequate torquing leads

to increased repeated dynamic loading of the screws during rotation

of the cylinder and flange. Such loading can lead to fatigue crack

initiation. Careful attention to proper torquing of the cap screws

should decrease the probability of fatigue failure. In addition, it is

important that there be proper clearance between the fillet at the base

of the screw cap and the bottom of the counterbore in the flange.

Improper clearance may lead to high stress concentrations, stress

relaxation, and subsequent loosening of the cap screws — with an accompanying increase in the dynamic load. A smoother finish on the

fillet at the base of the screw cap would also help to reduce stress

concentrations in this region.

4 . 2 Tensile Strengths Inferred from Hardness

According to the type A cap screw manufacturer’s pamphlet, the

minimum and maximum allowable ultimate tensile strength for a DIN 12.9

screw is 174,000 and 203,000 psi, respectively. It also states that

the type A cap screws are manufactured with a minimum ultimate tensile

strength of between 186,000 and 189,000 psi.

Manufacturer’s information for the types C and D cap screws indicate that the minimum and maximum allowable ultimate tensile

strength for the DIN 12.9 cap screws is 170,000 and 200,000 psi,

respectively. The approximate equivalent HRC hardness values for

the tensile strengths are as follows:

Ultimate tensile strength, psi Hardness, HRC

170.000

174.000

186.000

189.000

200.000

203,000

38

38 1/2

40 1/2

41

43

43 1/2
The results of the hardness traverses on the longitudinal sections

through both the type C and type D cap screws convert to approximate

tensile strengths of 182,000 and 194,000 psi, respectively. Both of

these tensile strength values are well within the allowable tensile

strength range. Hardness measurements on the as-received type D screws

also indicate tensile strengths within the specified range with the

possible exception of one measurement. This one measurement (37 HRC)

,

however, converts to a tensile strength just below the minimum acceptable

in the specification. Since the conversion of hardness data to tensile

strength is only approximate, it can not be said with any degree of

certainty that this measurement is outside of specifications.

Several of the hardness measurements on the as-received type C cap screws converted to tensile strengths below 170,000 psi. These low values

may be at least partially explained by the regions at the surface where

stringers and the material surrounding them had been attacked.

All of the values from the hardness traverse on the unstretched

type A cap screw, and many of the hardness values from the as-received

type A screws were approximately equivalent to tensile strengths below

170.000 psi. Therefore, the tensile strength of many of the examined

type A screws appears to be lower than the specification allows.

Both the as-received hardness values and the hardness traverse

values from the stretched type A cap screw converted to ultimate tensile

strength values significantly below the specified minimum. In fact,

the hardness traverse values were approximately equivalent to ultimate

tensile strength values ranging from 96,000 to 132,000 psi.

The ultimate tensile strength of the type B cap screw as determined

from hardness measurements on the as-received screws ranged from about

126.000 to 129,000 psi. These values are within the DIN specified limits

of 116,000 to 145,000 psi. These limiting values were taken from the

manufacturer's literature.

4 . 3 Metallographic Observations

The microstructures of the unstretched type A, the type C, and the

type D cap screws that were examined consisted of tempered martensite

and appeared to be satisfactory. There were regions at the surface of

the type C screws where stringer inclusions and the material surrounding

the stringers had been attacked and eaten out. These regions could act

as stress concentrators, but since their major dimension is longitudinal,

they would not likely be sources of transverse fatigue cracks. Indeed,

there is no evidence to indicate that these regions contributed to the

fatigue failures.

The microstructure of the stretched type A cap screw also consisted

of tempered martensite, but there was banding in evidence which is

undesirable. Since this screw also exhibited abnormally low hardness.

7
and consequently low strength, it apparently was not properly heat

treated. The unusually low strength is probably what allowed the screw to yield in service.

The inclusion content of all the cap screws examined was similar

and was not considered to be abnormally high.

Nothing abnormal was observed regarding the forging flow line

pattern. Even though one secondary crack was observed to have

followed the forging flow line pattern, in general, the fracture

paths did not appear to be influenced by these flow lines.

There were significant differences among the chemical compositions

of the screws from different manufacturers. Chemical composition,

however, is not believed to have contributed to any of the failures.

5.

CONCLUSIONS

1. All of the failed cap screws appeared to have failed due to the

initiation and propagation of fatigue cracks.

2. Fatigue cracks initiated either just below the cap of the screw

or at the root of a thread, depending on the position of a given

cap screw in a sequence of failures in one flange.

3. Based on hardness measurements, some of the type A cap screws appeared to have ultimate tensile strengths below the specified

minimum. One type A cap screw had an ultimate tensile strength

significantly below the specified minimum.

4. Based on hardness measurements, the ultimate tensile strength of

the types B and D cap screws appears to meet specifications.

5. Hardness measurements on the as-received cap screws indicated

that the ultimate tensile strength of some of the examined type

C cap screws is below the specified minimum. Hardness measurements

on a longitudinal section through a type C cap screw indicated

that the ultimate tensile strength is satisfactory. The low

hardness values on the as-received screws may be due to the

stringers and surrounding material having been attacked.

6. The low hardness-low strength type A cap screw had yielded

apparently while in service.

7. The microstructure of the cap screws examined appeared to be

satisfactory except for the banding in the low strength type A

cap screw.

8
6 . ACKNOWLEDGEMENT

The hardness tests and the metallography were performed by

Leonard C. Smith of the NBS Mechanical Properties Section. The

photographic work was performed by Mr. Smith and Todd Eudy, also

of the NBS Mechanical Properties Section.
Table 1. Results of Chemical Analyses of Cap Screws

Element

Type A

Cap Screw

Type C Type D

Carbon 0.33% 0.41% 0.43%

Manganese 0.91 0.83 0.71

Phosphorus 0.014 0.012 0.024

Sulfur 0.012 0.017 <0.005

Silicon 0.30 0.27 0.31

Nickel 0.52 0.05 0.19

Chromium 0.50 0.89 0.97

Molybdenum 0.21 0.17 0.03

Copper 0.16 0.06 0.20

10
Table 2. Results of Dimensional Measurements of

Stretched Type A cap screw

Thread Minor Diameter, mm Pitch, mm

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

19.40 3.35

18.85 3.33

18.42 3.81

18.97 3.10

19.35 3.07

19.61 3.12

19.81 3.00

19.91 3.05

19.91 3.00

19.94 3.00

19.91 2.97

19.86 3.02

19.99 3.02

19.96 3.00

19.91 3.00
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Figure 1. Fractures of representative cap screws that failed early in a sequence of cap screw failures. Fractures initiated just under

the cap. X 3

a. Fracture surface. Arrows indicate region of fracture

initiation.

b. Fracture profile (horizontal at top).

a

b

Figure 2. Fractures of representative cap screws that failed late

in a sequence of cap screw failures. Fractures

initiated at the root of a thread. X 3

a. Fracture surface. Arrows indicate region of

fracture initiation.

b. Fracture profile (at top).

Figure 3. SEM fractograph of the fatigue portion of the fracture

in a type A cap screw. X 850

Figure 4. SEM fractograph of the fatigue portion of the fracture

in a type A cap screw. Some apparent striations can

be seen. X 1200

a b

Figure 5. Longitudinal sections through two type A cap screws. Etchant: Hot HC1 and H2 O, 1:1 XI 1/3

a. Cap screw has been stretched in the region indicated

by the brackets. In this region, the diameter of

the screw is reduced and the thread pitch is increased.

b. Cap screw exhibits no perceptible deformation.

a b

Figure 6. Longitudinal sections through two cap screws. Etchant: Hot HC1 and H 2 O, 1:1 X 1 1/3

a. Type D cap screw.

b. Type C cap screw.

a b

Figure 6. Longitudinal sections through two cap screws. Etchant: Hot HCl and H 2 O, 1:1 X 1 1/3

a. Type D cap screw.

b. Type C cap screw.

Figure 7. Deeply etched longitudinal section through cap of fractured

type A cap screw showing the fracture profile (horizontal

at the bottom) . The fracture in this screw is just under

the base of the cap. Arrows 0 indicate the locations of the

regions of fatigue crack initiation. Arrow C indicates the

approximate location of the secondary cracks shown in

figure 8. The unlettered arrows indicate the path of the

crack front that initiated at the right where it deviated

from the path of the primary fracture.

Etchant: 5% nital X 4

Figure 8. Part of the longitudinal section shown in figure 7 with a lighter etch. Two secondary cracks are evident. The

approximate location of these cracks is indicated by

arrow C in figure 7.

Etchant: 2% picral X 300

Figure 9. Unetched longitudinal section through type A cap screw exhibiting an inclusion content representative of the

cap screws examined. X 100

Figure 10. Longitudinal section through a type C cap screw showing

part of one thread (white area). The dark, thin

longitudinal regions adjacent to the surface are areas where inclusions and the surrounding material have been

apparently chemically attacked and "eaten out".

Unetched X 40

Figure 11. Longitudinal section through a type C cap screw exhibiting

representative microstructure which consists primarily of

tempered martensite.

Etchant: 2% picral X 100

Figure 12. Longitudinal section through an unstretched type A cap

screw exhibiting representative microstructure which

consists primarily of tempered martensite.

Etchant: 2% picral X 100

Figure 13. Longitudinal section through stretched type A

cap screw. The microstructure consists primarily

of tempered martensite. Banding is evident.

Etchant: 2% picral X 100

U.S. DEPT. OF COMM. I im.'M.K A 1 ION OR l< F.POICI NO. 2. ( »ov 't Access ion BIBLIOGRAPHIC DATA } No.

S H E ET t ,

4. I I I 1.1 AND SI H i l l l.l EXAMINATION OF CAP SCREWS FROM CURRENCY PRINTING PRESS

FLANGES

3. Recipient's Accession No.

5. Publication Date

6. Performing Organization C ock

7. AUTHOR(S)

T. Robert Shives

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task/Work Unit No. 3120541

1 1 . Contract /C>rant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP

)

Office of Engineering

Bureau of Engraving and Printing

Department of Treasury

13. Type of Report & Period Covered

Failure Analysis Repor

14. Sponsoring Agency Cod<

15. SUPPLEMENTARY NOTES

16. ABSTRA( 1 (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or I iterature survey, mention it here.)

The Office of Engineering of the Bureau of Engraving and Printing submitted a number of failed and intact cap screws from printing press flanges to the NBS

Mechanical Properties Section for examination. All of the cap screw failures

examined were due to the initiation and propagation of fatigue cracks. Based on the

results of hardness measurements, some of the cap screws of one manufacturer did

not meet the minimum ultimate tensile strength requirements for DIN 12.9 screws.

One cap screw with an ultimate tensile strength significantly below the specified

minimum had yielded apparently while in service.

17. KEY WORDS (six to twelve entries; alphabetical order, capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Cap screw failures; cyclic loading; fatigue; fatigue failure.

18. AVAILABILITY [ Unlimited

l or Official Distribution. 17 o Not Release to NIIS

Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20102, SD Cat. No. C 1 3

Order F rom National Technical Information Service (N I IS) Springfield, Virginia 22151

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