NATIONAL BUREAU OF STANDARDS REPORT

4585

Progress Report

on

A PROPOSED SPECIFICATION FOR

DENTAL CHROMIUM-COBALT CASTING ALLOYS

by

Duane F. Taylor

W. T. Sweeney

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 March 28, 1956 4585

Progress Report

A PROPOSED SPECIFICATION FOR

DENTAL CHROMIUM-COBALT CASTING ALLOYS

by

Duane F . Taylor*

W. T. Sweeney**

* Technologist, Dental Research Section, National Bureau

of Standards.

** Chief, Dental Research Section, National Bureau of

Standards

.

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association

the Army Dental Corps, the Air Force Dental Service, the Navy

Dental Corps and the Veterans Administration.

(Task 7756-41 BV No. 55-14)

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

the publication, n Approved for public release by the

unless permission i director of the National Institute of

25, d.c. Such pe Standards and Technology (NIST)

cally prepared if 1 On October 9, 2015

• in part, is prohibited

tandards, Washington

sport has been specifieport for its own use,
..

, . ...
A PROPOSED SPECIFICATION FOR

DENTAL CHROMIUM-COBALT CASTING ALLOYS

Abstract

The widespread use of chromium-cobalt casting

alloys in the construction of partial dentures has

presented a need for a specification for alloys of

this type. The major requirements of the specification proposed herein are a composition of not less

than 85$ chromium* cobalt and nickel* yield strength

of 60*000 psi* tensile strength of 90*000 psi* elongation of 1.5 and hardness of 50 (Rockwell 30N) . The

alloys are divided into two types* one with a minimum

fusion temperature of 2400 °F and one with a maximum

fusion temperature of 2400 °F.

Chromium-cobalt base alloys were introduced for dental use

in 1929 [ 1 ]* and since that time have shown gradual and steady

gains in popularity. While the desirable physical properties of

these materials and their relatively low cost have promoted their

use* the necessity of using specialized equipment and techniques

has restricted their application almost entirely to commercial

dental laboratories. In recent years* as dentists have come to

rely more upon the services of commercial laboratories and of auxiliary personnel in general* the chrome-cobalt alloys have nearly

preempted the partial denture field.
' ' » I
2

The increasing popularity of these materials has prompted

the appearance on the market of many new products of this type

which exhibit wide variations in their suitability for dental use.

As the number of available products has multiplied, a growing need

has been felt by the dental profession and the laboratories for

a standard or specification as an aid in the selection of a material. The interest of the Federal dental services has been particularly pronounced since such a specification would facilitate their

purchase of these alloys

.

Much of the research on the properties of these materials has

been directed toward properties which are of interest for their

industrial use, such as creep resistance, but which have little

pertinence for their dental application. Several groups of investigators have worked with the dental alloys in the last twenty

years, but few results have found their way into the literature,

and as yet there is no accepted standard for these materials.

The accompanying specification is submitted as representing

an initial approach to this problem, and has been approved by the

Specifications Committee of the Dental Materials Group of the

International Association for Dental Research. The values proposed in this specification are based upon the work done by Paffenbarger. Caul, and Dickson [1], and more recently by Taylor,

Leibfritz, and Adler [2] at the National Bureau of Standards, and

upon that done at the University of Michigan by Bush, Ingersoll,

Mahler and Peyton [3]. The tensile test specimen design and

spruing arrangement recommended are essentially those developed

- 3

- by the Michigan group.

As far as possible the following principles were observed

in drafting this specification:

1. The values specified should be selected to eliminate materials

having properties inferior to those readily obtainable in

currently available products* without at the same time excluding products of known clinical acceptability.

2. The specification should limit the variation in composition

and properties to types within which acceptable products may

be used interchangeably without major alterations in equipment

or technique.

3. The specification should parallel as far as possible the existing specifications for other materials used for the same

purpose [4],

4. The test methods specified should be selected to simplify

testing and reduce the required interpretation of the observations as far as possible without sacrificing accuracy or

thoroughness

.

Certain of the individual requirements of the specification

made on this basis would seem to merit further discussion.

This specification divides the chromium-cobalt base dental

casting alloys into two types on the basis of their liquidus

temperatures. The dividing temperature has been taken as 2400 °F

on the basis of advice from the manufacturers of investments for

these materials that alloys melting above 2400 °F cannot be cast

with routine success into gypsum -bonded investments. Since the

techniques required for the water-mixed* gypsum-bonded investments
,

.

.
- 4 - differ greatly from those employed for materials utilizing ethyl

silicate or some other high temperature binder, this seemed to

constitute a logical basis for a division into separate types.

The composition requirement that the alloy contain no less

than 85$ by weight of chromium, cobalt, and nickel serves two

purposes. It defines the general group of alloys considered by

the specification somewhat more closely than does the title, and

at the same time it is an attempt to compensate for the lack of

a satisfactory test for corrosion resistance under oral conditions.

In the absence of such a test it is felt that this composition

requirement gives seme assurance of corrosion resistance without

unduly imposing penalties in the form of reduced mechanical

properties. All widely used, clinically successful alloys meet

this requirement without difficulty.

The specimen design used for the determination of tensile

properties was selected as approximating dental dimensions, and,

on the basis of the results of Bush et al, as showing greater

uniformity of properties and freedom from casting failure than

other designs. The chrome-cobalt alloys are particularly hard

to cast in the rod form used for testing dental golds.

The numerical values required for the tensile properties

correspond approximately to those of Federal Specification

QQ-G-540 for dental casting golds, type IV. The requirements for

yield strength and elongation are set slightly lower than those

for golds since some base metal alloys of known clinical suitability are unable to meet those requirements. The hardness test

- 5 - requirement is in terms of a Rockwell superficial hardness scale

(30N), rather than the Brinell test employed for golds, because

the diamond indenter used in the Rockwell test is not subject to

the deformation that occurs in the ball indenter of the Brinell

tester when used on these hard materials.

- 6 -

BIBLIOGRAPHY

1. Paffenbarger, George C., Caul, H. J. and

Dickson, George. Base metal alloys for

oral restorations. J.A.D.A. 30, 852-862

June 1943.

2. Taylor, Duane F., Leibfrltz, Walter A., and

Adler, Alfred G. Physical properties of

chromium cobalt dental alloys. (in preparation) .

3. Bush, S. H., Ingersoll, C. E, and Peyton,

F. A. Navy contract N6-0nr-232 Progress

Reports, School of Dentistry, University

of Michigan.

4. Federal Specification QQ-G-540 for Gold;

Casting, Inlay, Dental.
; \

l
- 7

- PROPOSED SPECIFICATION FOR

DENTAL CHROMIUM-COBALT CASTING ALLOYS

1. SCOPE AND CLASSIFICATION

1.1 Scope. This specification covers chromium-cobalt

casting alloys of the type used in removable partial

denture prostheses.

1.2 Types. The chromium-cobalt alloys covered by this

specification shall be of the following types:

Typo I. High fusing.

Type II. Low fusing.

2. APPLICABLE SPECIFICATIONS

2.1 Specification. There are no other specifications

applicable to this specification.

3 . REQUIREMENTS

3.1 Color. The color of the alloy shall be the color

specified by the purchaser.

3.2 Composition. The alloy shall contain a total of not

less than 85$ by weight of chromium, cobalt and nickel.

3.3 Physical properties. The requirements for physical

properties for each type are listed in Table 1.

Table 1

REQUIREMENTS FOR PHYSICAL PROPERTIES

Type

Fusion

temperature

Yield

strength

Tensile

strength

Elongation

(one inch

gage length)

Hardness

Rockwell

30N

°F psi psi %

min. min. min. min.

I 2400 60, 000 90,000 1.5 50

(min.

)

II 2400 60, 000 90,000 1.5 50

(max.

)

- 8 - 3.4 Manufacturer's directions. Instructions for use shall

accompany each package. The instructions shall include

casting temperature of the alloy, burnout temperature for

the investment, a description of necessary auxiliary materials and equipment, and any special precautions for satisfactory use.

4. SAMPLING, INSPECTION, AND TEST PROCEDURES

4.1 Sampling. The manufacturer shall provide a set of six

specimens of the type described in 4.4.1, cast from each

of the lots or batches of alloy included in the delivery.

The specimens shall be submitted in the "as cast" condition

and with sprues and buttons attached. An additional six

ounces of material shall be chosen at random from each lot

or batch of material included in the delivery.

4.2 Inspection. Compliance with the requirements outlined in

3.1, 3.4 and 5* shall be determined by inspection.

4.3 Test procedures.

4.3.1 Composition. At the option of the purchaser, certification by the manufacturer that the alloy contains not

less than 85% of chromium, cobalt, and nickel may be considered to satisfy the composition requirement. Otherwise

the composition shall be determined by wet analysis. The

determined values for metallic constituents shall be recorded as the nearest 0.5 percent. When a determined value

falls midway between a half and a whole number, the whole

number shall be recorded.

* Analyses for chromium, nickel and cobalt shall be made by

procedures given in J. Research NBS 53*353 (1954), ASTM Methods

of Chemical Analysis of Metals (1950) Chemical Analysis of Iron

and Steel, by Lundell, Hoffman and Bright, or by methods demonstrated to be of equal accuracy.
i .
- 9

- 4.3*2 Fusion temperature. The fusion temperature shall be the

liquidus temperature as determined by cooling curves . The

manufacturer shall determine and certify the fusion temperature. The purchaser may, at his option, accept such certification as satisfying the fusion temperature requirement.

4.3.3 Physical properties.

4. 3. 3.1 Preparation of specimens. Physical properties other

than fusion temperature shall be determined on cast specimens

tested in the "as cast" condition without subsequent heat

treatment. The specimens shall be of the type and dimensions

indicated in Figure 1. They shall be cast in accordance

with the manufacturer's directions for partial dentures

except that where those directions conflict with the provisions of this specification, the specification shall be followed. It is suggested that two, three or four such patterns

be symetrically arranged around a central sprue, and be provided with gates, risers, and vents in a manner similar to

that indicated in Figure 2. If so desired, a continuous

ring of adequate cross-section may be substituted for individual risers

.

4. 3. 3 .

2

Compliance. An alloy shall be considered to meet the

requirements for tensile properties and hardness when all

of the recorded values for three or more of a set of six

specimens meet the requirements of Table 1

.
-

.

-

'

.

'

'

.

1
10

4. 3. 3. 3 Yield Strength. The yield strength shall be determined

using an averaging extensometer and a one-inch gage length.

An initial load calculated to produce a stress of 5000

pounds per square inch shall be applied to the specimen and

the strain gage read. The load shall then be increased to

produce a stress of 60,000 pounds per square inch and a

second reading taken. The load shall be applied at a rate

of not less than 50 nor more than 100 pounds per minute.

The maximum strain permitted between the limits specified

shall be 0.0025 inch per inch. The determined value of

strain shall be recorded to the nearest 0.0001 inch per

inch. When the determined value falls midway between two

numbers, the even number shall be recorded.

4. 3. 3.

4

Tensile strength. After the loading for the yield

strength (4. 3. 3. 3) das been completed and the strain gages

have been removed, additional load shall be applied to the

specimen until rupture occurs. The determined value for

tensile strength shall be recorded to the nearest 1000

pounds per square inch. When the determined value falls

midway between two numbers, the even number shall be recorded .

4. 3. 3.

5

Elongation. The elongation shall be determined on the

specimens used in the tests for yield strength and tensile

strength. Elongation shall be measured on a one-inch gage

length. Castings that break outside of the gage marks shall

not be repulled, but the elongation between the marks shall
.
11

be recorded if it equals or exceeds the minimum specified

value. The determined value shall be recorded to the nearest 0.1 percent. When the determined value falls midway between two numbers the even number shall be recorded.

4.4.6 Hardness. The Rockwell 30 N number shall be determined

on each tensile specimen. After fracture the tensile

specimens shall have a flat surface wet ground on their

rod portions. Five indentations shall be made on the flat

surface of each specimen no nearer than one -eighth inch

from the fractured end or from the tapered end. The high

and the low readings shall be discarded and the remaining

three shall be averaged. The average hardness number of

each specimen shall be recorded to the nearest whole number.

When the determined value falls midway between two numbers*

the even number shall be recorded.

5. PREPARATION FOR DELIVERY

5.1 Packaging. The alloy shall be packaged in accordance with

acceptable commercial practice.

5.2 Marking. Each package shall be marked with the name of the

manufacturer or contractor* name of the material* type and

minimum net weight of contents* together with a serial number or combination of letters and numbers which shall refer

to the manufacturer’s records for the particular lot or batch

of alloy. The date of manufacture Bhall be indicated either

as a part of the serial number or as a separate item.

Instructions for use . Instructions for use shall accompany

each package (3.4).

5.3

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FIGURE

THE NATIONAL BUREAU OF STANDARDS

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