NATIONAL BUREAU OF STANDARDS REPORT

997 1

THE EFFECT OF CHROMIUM PLATING

ON THE FATIGUE PROPERTIES

OF TRANSFER PLATE SPECIMENS

To

Electrolytic Section

Bureau of Engraving and Printing

Washington, D.C.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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:

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

3120641

NBS REPORT

997 1

THE EFFECT OF CHROMIUM PLATIMG

ON THE FATIGUE PROPERTIES

OF TRANSFER PLATE SPECIMENS

By

T. R. Shives, J. A. Bennett, and L. C. Smith

Engineering Metallurgy Section

Metallurgy Division

To

Electrolytic Section

Bureau of Engraving and Printing

Washington, D.C.

IMPORTANT NOTICE NATIONAL BUREAU OF S

for use within the Government, and review. For this reason, tl whole or in part, is not authc Bureau of Standards, Washing

the Report has been speciticall;

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

ess accounting documents intended

; subjected to additional evaluation

e listing of this Report, either in

re Office of the Director, National

by the Government agency for which

copies for its own use. <3§s>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The Effect of Chromium Plating on the Fatigue

Properties of Transfer Plate Specimens

Tests were conducted on specimens from a stock transfer plate

in order to determine the effect of chromium plating and the effect

of baking subsequent to plating on the fatigue properties of the

material c Tests were conducted in such a manner as to simulate the

conditions under which plates have been failing at the Bureau of

Engraving and Printing,, Specimens in three conditions were tested

in reversed bending:

1. Cyanided and hardened

2. Cyanided, hardened, and chromium plated

3. Cyanided, hardened, chromium plated, and baked

Definition of Terms:

A

N

F S-N Plot

Ksi

Stress amplitude

Mean stress

Cycles to failure

Semilog plot of stress amplitude of

testing vs. cycles to failure.

Thousand pounds per square inch.

Specimens and Specimen Preparation: Specimens (Fig. 1) were machined from

a stock transfer plate furnished by the Bureau of Engraving and Printing.

The thickness of the as-received plate was reduced to 0.180 in. by planing

and grinding material from the back. Cuts were made parallel to the long

edge of the plate. Specimens were machined from the plate with the long

dimension of the specimen parallel to the long dimension of the plate. The

width of the minimum section was 0.390 ± .002 in. at this stage of

preparation

.

Specimens were then sent to the Bureau of Engraving and Printing

where they were divided into two groups and treated as follows:

1. Group 1: Cyanided, hardened and tempered.

2. Group 2a: Cyanided, hardened, tempered and chromium

plated on the front face (plating approximately

0.00025 to 0.0003 in. thick).

- 2-

Group 2b% Cyanided, hardened, tempered and chromium

plated on the front face and subsequently

baked at NBS.

The specimens were then returned to NBS where 0.007 ± <.001 in. was removed from each side of the reduced section by grinding with the

plane of the grinding wheel parallel to that of the specimen. This was done in order to remove any chromium from the specimen sides that may

have been plated there. The final width of the minimum section was then

nominally 0.376 in.

It would be expected that a considerable amount of hydrogen would

diffuse into the basis material during the plating operation. Hence,

approximately one-half of the chromium plated specimens were baked at

400°F for four hours before being tested in fatigue in order to reduce

the hydrogen content. It is probable that the baking procedure changed

the residual stress pattern, particularly near the chromium-basis

material interface.

Fatigue Tests: The specimens were fatigue tested in reversed bending.

About half of the specimens in each condition were tested with zero mean stress. The rest were tested with the mean stress tensile in the back

(mean stress approximately one-half stress amplitude of testing) since

failures start on the back.

Results and Discussion ; The results from the fatigue tests are given in

tables and are shown on S-N plots. An arrow drawn next to a data point

on an S-N plot or next to an value in a table indicates a runout.

This means that the specimen yielding this data did not fail at that

particular combination of stress amplitude and testing cycles. In order

to obtain maximum data from a limited number of specimens, in most cases,

a specimen which ran out at a given stress amplitude was tested at a higher

stress amplitude.

The fatigue test results for cyanide hardened, but unplated , specimens

are given in Table 1 and in Fig. 2. In the finite life portion of the S-N

plot, the median fatigue strength of specimens tested at zero mean stress

was greater than that £or specimens tested at = S^/2 by from about six

percent at Np = 9 x lCr cycles to about 21 percent at W = 2 x 1CT cycles.

The 50 percent runout fatigue limit was approximately 11 percent greater

for tests at S =0. All but one of the cyanided specimens failed due to

a fatigue crack which originated on the back of the specimen. In the one specimen in which failure started on the front (S M

= 0), there was a large

surface pit at the crack origin. Specimens tested with mean stress tensile

in the back would be expected to fail from the back. Specimens tested with

zero mean stress failed from the back because of the poorer surface finish.

-3

-

The fatigue test results for cyanided specimens plated on the front

face with chromium, but not baked after plating, are given in Table 2

and in Fig. 3® In the finite life portion of the S-N curves, there is

essentially no difference between results from tests at - 0 and tests

at S M = S./2. The 50 percent runout fatigue limit results, however,

exhibit about a 28 percent improvement for tests at = S^/2 when compared

to tests at S M

= 0. As in the case of the unplated material, all specimens

but one failed due to a fatigue crack originating at the back,, It is

surprising that tests at = S^/2 yielded the higher fatigue limit.

The fatigue limits for plated, but not baked, specimens are considerably lower than those for unplated specimens which would indicate

that the plating operation has had a deleterious effect on the basis

material, apparently in the form of hydrogen embritt 1 ement . The average

fatigue limit for both stress conditions is approximately 30 percent

higher for unplated specimens than for plated specimens

The fatigue test results for specimens which were cyanided, chromium

plated on the front, and baked for four hours at 400°F are given in Table

3 and in Fig. 4* As with the unbaked specimens, there is little difference

between results from tests at = 0 and = S^/2 in the finite life region

of the S-N ploto The 50 percent runout fatigue limit for tests conducted at

S., = S a /2 is about 6 percent higher than that for tests at S., = 0.

M A M

The baking procedure appears to have been very helpful in eliminating

the effects of hydrogen embrittlement. The average fatigue limit for the

two stressing conditions for baked specimens is approximately 29 percent

higher than the average for unbaked, plated specimens, and only 9 percent

lower than that for unplated specimens. The fatigue limit for baked specimens

tested at = S^/2 is essentially the same as that for unplated specimens

tested at = S‘/2. Since all specimens tested at = S^/2 from both of

these groups failed from the back, the basis ma teria 1 appears to have lost

all evidence of hydrogen embrittlement during baking.

All baked specimens tested at = 0 failed due to fatigue cracks

originating in the plated front surface indicating that the plated surface

is now more subject to fatigue failure than the back. This may be due in

part, at least, to an unfavorable stress pattern induced in the material

during plating and only partially relieved during baking.

Conclusion : The fatigue strength of this transfer plate was severely

impaired during the chromium plating process by hydrogen embrittlement. Baking

of the plated specimens at ^00°F for four hours appears to have completely

eliminated the effects of hydrogen embrittlement, at least in the back of

the specimen, as is shown by the fatigue test results at = S^/2. Even

after baking, the fatigue strength of the plated surface was poorer than that

of the unplated surface. This may be due in part to an unfavorable stress

pattern induced in the material during the plating process and incompletely

relieved during baking.

Table

1

Fatigue

Test

Results

for

Cyanided^

but

Unplated

Specimens

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Table

1

Fatigue

Test

Results

for

Cyanided,

but

Unplated

Specimens

(Con't)

in

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Table

2

Fatigue

Test

Results

for

Cyanided

and

Chromium

Plated^

but

Unbaked

Specimens

to

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Table

3

Fatigue

Test

Results

for

Cyanided

Chromium

Plated,

and

Baked

Specimens

t tl 111

I

0) KA KA KA KA KA KA NA KA KA KA KA KA KA KA KA KA KA KA KA

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Transfer

plate

fatigue

specimen

100 CO ^

t-o

VO o

-d- o

o

X

cu

1/1 0)u

>.

<_>

Fig.

2.

Fatigue

test

results

from

cyanided,

but

unplated,

transfer

plate

speci

mens

.

CO

o

OO < I/) co xC

i/> a>o

>

<_>

LTV O

rLTV VO o

VO

LTV

LTV

Fig.

3-

Fatigue

test

results

from

cyanided

and

chromium

plated,

but

not

baked,

transfer

plate

specimens.

r

Lf\ •> 'H lf\ CO < 
 too vr

LTV VO

LT\ LO

LT\

-cj- X

OJ

Fig,

4.

Fatigue

test

resuHts

from

cyanided^

chromium

piated

and

baked

transfer

plate

specimens.