MAY 1 6 1968

NATIONAL BUREAU OF STANDARDS REPORT

9829

EXAMINATION OF STOCK POD SECURITY CHAIN

To

Bureau of Research and Engineering

U . S. Post Office Department

Washington, D. C.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 provides measurement and technical information services

essential to the efficiency and effectiveness of the work of the Nation’s scientists and engineers. The

Bureau serves also as a focal point in the Federal Government for assuring maximum application of

the physical and engineering sciences to the advancement of technology in industry and commerce. To

accomplish this mission, the Bureau is organized into three institutes covering broad program areas of

research and services:

THE INSTITUTE FOR BASIC STANDARDS . .

. provides the central basis within the United

States for a complete and consistent system of physical measurements, coordinates that system with the

measurement systems of other nations, and furnishes essential services leading to accurate and uniform

physical measurements throughout the Nation’s scientific community, industry, and commerce. This

Institute comprises a series of divisions, each serving a classical subject matter area: —Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic Physics—Physical

Chemistry—Radiation Physics-- -Laboratory Astrophysics2—Radio Standards Laboratory, 2 which

includes Radio Standards Physics and Radio Standards Engineering—Office of Standard Refer- ence Data. THE INSTITUTE FOR MATERIALS RESEARCH . . . conducts materials research and provides

associated materials services including mainly reference materials and data on the properties of materials. Beyond its direct interest to the Nation’s scientists and engineers, this Institute yields services

which are essential to the advancement of technology in industry and commerce. This Institute is or- ganized primarily by technical fields: —Analytical Chemistry—Metallurgy—Reactor Radiations—Polymers—Inorganic Materials—Cryogenics2—Office of Standard Reference Materials.

THE INSTITUTE FOR APPLIED TECHNOLOGY . .

. provides technical services to promote the use of available technology and to facilitate technological innovation in industry and government. The

principal elements of this Institute are: —Building Research—Electronic Instrumentation—Technical Analysis—Center for Computer Sci- ences and Technology—Textile and Apparel Technology Center—Office of Weights and Measures —Office of Engineering Standards Services—Office of Invention and Innovation—Office of Vehicle

Systems Research—Clearinghouse for Federal Scientific and Technical Information3—Materials

Evaluation Laboratory—NBS/GSA Testing Laboratory.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D. C., 20234.

2 Located at Boulder, Colorado, 80302.

3 Located at 5285 Port Royal Road, Springfield, Virginia 22151.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT MAY 1 6 1968

4213417

NBS REPORT

9829

EXAMINATION OF STOCK POD SECURITY CHAIN

By

W. F. Gerhold

Engineering Metallurgy Section

Metallurgy Division

To

Bureau of Research and Engineering

U . S. Post Office Department

Washington, D. C.

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, ttv whole or in part, is not authoi Bureau of Standards, Washingtc

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

ss accounting documents intended

subjected to additional evaluation

listing of this Report, either in

5 Office of the Director, National

iy the Government agency for which

opies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Examination of Stock POD Security Chain

Reference: (a) Post Office Department, Bureau of Research and Engineering,

letter dated February 19^8, RE-17"68(R).

I n t roduct i on : A 2 ft (approx.) length of zinc-coated stock POD security chain

was submitted under Reference (a). It was requested that an investigation be

initiated to establish the reason for the low impact resistance of this type of

cha i n

.

Specific requirements for this chain are that it be in accordance with

Federal Specification RR-C-271a for a Type I, Grade C, Class 4, welded steel

proof coil chain. Other requirements are that it shall be zinc coated, case

hardened to a depth of no less than 0.005 inch and shall be file hard. The

nominal (trade) size shall be 3/8" and shall conform to the dimensions and

physical requirements shown in Table VII of Federal Specification RR-C-271a.

Mechanical Properties : A one foot length of chain was loaded in tension to a proof load of 3720 pounds. The load was removed and the links were examined

for indications of failure as outlined in paragraph 4. 4.1.4 of Federal

Specification RR-C-271a; there were no failures. The load was reapplied and

increased until failure of the chain had occurred. This load (breaking load)

was 8,240 pounds. The specification requ i rement f or 3/8 Inch nominal (trade)

size of chain is 7^440 pounds minimum. Fracture occurred at two areas on the

fifth link of the ten link segment. All of the links except one contained

secondary cracks at the ends of the links. There were no cracks at the weld

areas on any of the links.

An additional tensile test, not required for specification purposes, was

performed on a single link in order to determine the ductility of the weld.

The link was cut through on the side opposite the weld. A tensile load was

applied to the link and was increased until fracture occurred at a load of

2210 pounds. A sudden decrease in load was noted when the applied load

reached 1700 pounds. This was believed to be the yield load or the load

required to produce initial cracking. There were no failures in the weld area,

but secondary cracks were observed at areas on each side of the weld approximately 1/4 inch from the weld. Fracture occurred at an area approxi mate 1

y

1/2 inch from the weld.

The fracture surfaces on all of the broken links had a granular (cleavage)

appearance and there was no indication of a reduction in area at the fracture.

This indicates that the links failed in a brittle manner.
'

* • -- '

. •

•• i :

•

• ;

•'

:

§

'

"

1.

.

-

‘
2

A compression (or crushing) test was performed on a single link. The

link was placed on one edge between two rams and a compressive load was

applied. This load was increased until fracture occurred. A sudden decrease

of load at 5580 pounds indicated that cracking had initiated or that the

yield load had been reached. Fracture occurred at 8,380 pounds at both ends

of the link; secondary cracks and partial fractures were noted at the weld

area and at the opposite edge. Examination of the areas where complete

fracture was noted, revealed that they were also of a brittle nature.

Chemical Analyses : Sections were cut from two different chain links for

chemical analysis of the carbon content in the core areas of the steel. The

results of these analyses, based on the average of duplicate determinations,

showed that the carbon content of the steel was 0.05$>. Federal Specification

RR-C-271a requires that the chemical composition of the steel be no greater

than 0.35$. There is no minimum requirement.

Meta 1 1 oqraphi c Examination and Hardness Tests : One link was sectioned and

prepared for meta 1 lographi c examination. The depth of the carburized case

was found to be approximately 0.04 inch. Figure 1 shows the gradation of the

carbon penetration from the outer edge, where it is at a maximum, to the core

where it approaches the carbon content of the carbon steel used in the

fabrication of the chain. Examination of the microstructure at a higher

magnification (figure 2) revealed the following mi crost ructura 1 conditions:

a. Tempered martensite plates (dark needlelike structures).

b. Untempered martensite (light gray areas).

c. Nodular pearlite (dark gray areas).

d. Cementite at the grain boundaries.

e. Large prior austenitic grain size as indicated by the size of the

tempered martensite plates and the boundaries outlined by cementite.

This structure, because of conditions b, d, and e above, would tend to

be brittle. We believe it was produced by quenching the chain directly from

the carburizing temperature without a subsequent tempering treatment. The

long length of time or high temperature required to carburize to a 0.04 inch

depth would have produced the large austenite grains. A delay in quenching

or cooling at a rate less than the critical cooling rate could produce

cementite boundaries and pearlite. The darkened martensite plates indicate

that the initial martensite was formed at high temperatures and was tempered

during the cool down, rather than during a subsequent tempering treatment.

The fact that the smaller martensitic areas appear untempered indicates that

no adequate tempering treatment was applied after cool down.
.

• - . i - .. »'•

;•

• T

.

2 \

.

.

. " • i

• .. .

- ..

.. . '

I \<

'

•; .. -v --

1 j
- 3

-

Microhardness determinations (KHN (500 gr.)) were obtained at several

areas across the approximate center of the cross section. These determinations converted to approximately equivalent Rockwell hardness values

revealed: (l) the hardness of the outer case area was R c 63-64; (2) the

hardness of the intermediate zone between the outer case and the core was

R c 50 at the extreme outside to R c 33 adjacent to the core; and ( 3 ) the

hardness of the core was r b 78“79* Specifications require only that the

chain be file hard. In this respect the carburized case was resistant to

ab ras ion by a file.

Reheating, Hardening and Tempering : Since the results obtained from the above

tests and examinations showed that the carburized case was excessively brittle,

an experiment was conducted to determine if additional heat treatment would

improve the ductility of the case without significant decrease in hardness.

Specimens were stripped of the zinc coating by immersion in a 1:1 hydrochloric

acid solution, rinsed and dried. They were then heated to l425°F for 20

minutes, quenched in water, tempered for 1 1/2 hours at 350 °F and air cooled.

Meta 1 lographi c examination of the case in a section cut from one of the

specimens (figures 3 and 4) revealed that the grain had been refined

considerably, with the result that all martensite present had fine plate size.

There was no evidence of cementite at grain boundaries; instead it was

dispersed as fine spheroids. Nor was there any trace of pearlite. All of the

martensite appeared uniformly tempered. A comparison of figures 2 and 4, both

at the same magnification, strikingly reveals the improvement brought about in

the structure by this heat treatment. Microhardness determi nat i ons were

obtained for comparison with those obtained for the stock POD security chain.

The hardness at the surface was R c 63 - 64 . This is sufficiently hard to

resist the abrasive action of a file.

A compression test was performed on a half link which had been given the

additional heat treatment. In contrast to the behavior of the stock POD

security chain, this specimen showed a significant amount of ductility as indicated by the slow decrease in load from the time cracks were initiated

until complete fracture occurred. There was no evidence of a granular

appearance on the fracture surface.

Cone 1 us ? ons : The results obtained from an examination of stock POD security

chain revealed that it met the requirements as set forth in Federal

Specification RR~C-271a for a Type I, Grade C, Class 4, welded steel proof

coil chain and the additional requirements as specified by the Post Office

Department

.
t( ' " *
- k -

However, it was determined that the stock chain was susceptible to failure

due to the extreme brittleness of the carburized case. In order to improve the

ductility without decreasing the hardness, it is recommended that the chain be

given an additional heat treatment after carbur i zat i on . This should consist of

reheating to a temperature of l425°F, holding at this temperature for one-half

hour and then quenching immediately in water. Care should be taken to prevent

surface decarburization. Hardening should be followed by tempering at 350"F

for 1 1/2 hours.

It should also be possible to improve toughness by decreasing the thickness

of the case. Since a minimum case thickness of 0.005 inch is required, it seems

unnecessary to have a case thickness of 0.01+ inch. It is the purpose of the

case to supply sufficient hardness so as to resist cutting by a saw. The core,

on the other hand is designed to support the case and provide toughness. Even

though the suggested heat treatment improves the toughness of the case

considerably, the core is still the principal toughness contributor and therefore reducing its area by increasing the amount of case would normally be

detrimental in a situation where brittleness is a problem. A thinner case

would resist sawing equally as well but for not as long.

USCOMM-NBS-DC
.
Figure 1. Microstructure of a section from a stock POD security chain link.

Carburized case is approximately 0.04 inch thick. Photomi crograph

shows the gradation of the carbon penetration from the outer edge

(darker area) where it is at a maximum to the lighter etching core

area where it is at a minimum approaching the carbon content of

the core. Etched with nital. X 100.

Figure 2. A portion of the area shown in Figure 1 showing the following

conditions: (a) tempered martensite plates (dark needlelike

structures), (b) untempered martensite (light gray areas),

(c) nodular pearlite (dark gray areas), (d) cementite at the grain

boundaries and (e) large prior austenitic grain size as indicated

by the size of the tempered martensite plates and the boundaries

outlined by cementite. Etched with nital. X 500*

Figure 3* Mi cros tructure of stock POD security chain after (l) reheating to

1425°F for 20 minutes and water quench and (2) tempering for 1 1/2

hours at 350°^ and air cool. Matrix of carburized case is tempered

martensite with some small particles (not visible at this

magnification) of FeoC. Core matrix is ferrite with some pearlite.

Etched with nital. X 100.

Figure 4. Portion of carburized case area of stock POD security chain after

additional heat treatments noted in Figure 3« Grain size is very

fine. Spherodized particles of Fe3

C are scattered throughout the

martensite matrix. Etched with nital. X 500.