A method of specimen corrosion protection for high temperature creep testing

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Ames Laboratory Technical Reports

Ames Laboratory

10-1959

A method of specimen corrosion protection for

high temperature creep testing

J. R. Bohn

Iowa State University

Robert E. Uhrig

Glenn Murphy

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Recommended Citation

Bohn, J. R.; Uhrig, Robert E.; and Murphy, Glenn, "A method of specimen corrosion protection for high temperature creep testing" (1959).Ames Laboratory Technical Reports. 11.

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A method of specimen corrosion protection for high temperature creep

testing

Abstract

The determination of mechanical properties of materials at elevated temperature presents difficulties, particularly when the material to be tested is subject to oxidation. Various methods have been employed to permit the evaluation of high temperature creep properties. The method described in this paper was developed on the basis of modifications of a technique developed for the protection of high temperature fatigue specimens. The method involves encasing the creep specimen in a flexible capsule which is capable of withstanding exposure to the atmosphere for extended periods at temperatures up to 1000C.

Extensive testing of materials such as uranium and tantalum has provided the basis for claims relative to the effectiveness of this technique.

Disciplines

Chemical Engineering | Chemistry

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A METHOD OF SPECIMEN CORROSION

PROTECTION FOR HIGH TEMPERATURE

CREEP TESTING

by

J. R. Bohn, R. E . Uhrig and

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UNCLASSIFIED

IS-48

V

Chemistry-General ('(JC-4~ TID 4500, August 1, 1959 UNITED STATES ATOMIC ENERGY COMMISSION

Research and Development Report

A METHOD OF SPECIMEN CORROSION PROTECTION FOR HIGH TEMPERATURE

CREEP TESTING by

J. R. Bohn, R. E. Uhrig and Glenn Murphy

October 1959

Ames Laboratory

at

Iowa State University of Science and Technology F. H. Spedding, Director

Contract W -7405 eng-82

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2 IS-48

This report is distributed according to the category Chemistry-General (UC-4) as listed in TID-4500, August 1, 1959.

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IS-48 3

TABLE OF CONTENTS

Page

ABSTRACT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

INTRODUCTION. . . . . . . . . . . • . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . 5

ENCAPSULATION... . . . .. . . 7

PROTECTION. . . • . . . . . . . . . . . . . • . . . . . . . . . . . . . • . . . . . . . . . . . .. . 15

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f~ 15..;48

A METHOD OF SPECIMEN CORROSION PROTECTION FOR

HIGH TEMPERATURE CREEP TESTING

by

J. R. Bohn, R. E. Uhrig, and Glenn Murphy

5

Abstract--The determination of mechanical properties of materials

at elevated temperatu~;presents difficulties, particularly when the

material to be tested is subject to oxidation. Various methods have

been employed to permit the evaluation of high temperature creep

properties. The method described in this paper was developed on the

basis of modifications of a technique developed for the protection of

high temperature fatigue specimens. The method involves encasing

the creep specimen in a fjlexible capsule which is capable of withstanding

exposure to the atmosphere for extended periods at temperatures up to

100C1'

c.

Extensive testing of materials such as uranium and tantalum has

provided the basis for claims relative to the effectiveness of this

technique.

INTRODUCTION

Interest in high temperature properties of materials which in

themselves do not possess the ability to withstand corrosion of the

atmosphere has been stimulated by current nuclear applications. The

inadequate means of conventional testing equipment to provide the

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6

retarded advancements in this field. In creep and fatigue tests, where

the periods of testing are usually long, the problem of oxidation is most

severe. A highly satisfactory means of oxidation protection for fatigue

specimens was developed by Bohn and Murph}P and it is from this work

\

that this method of protection for creep specimens has evolved .

. Oxidation is obviously prevented if oxygen is not allowed to come

in contact with material under test. This implies that an oxygen tight

barrier be placed between the specimen and the oxygen laden atmosphere.

The barrier may be placed (a) surrounding the appropriate components

of the testing machine; (b) on the surface of the specimen; or (c) surrounding,

but not in intimate contact with the specimen.

The most common approach taken in the design of oxidation protection

equipment for high temperature creep studies has been based on (a). The

unusual cost and complexity of operation and maintenance of this type of

equipment poses obvious disadvantages.

Limited success was obtained using method (b) for creep applications

prior to this investigation. The method involved electroplating copper,

nickel and silver, alone and in multiple .layer combinations, onto the

specimen. Diffusion of oxygen through the plating at higher temperatures

caused scaling and reduced the effectiveness of the technique.

A technique for oxidation protection based on (c) was developed and

proven satisfactory for temperatures up to 1000

oc.

The technique

( 1) Bohn,

J.

R. and Glenn Murphy, High-temperature fatigue testing with application to uranium. American Society for Testing Materials

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I

7

provided a dependable low maintenance system which utilized

commerCially available basic high temperature creep testing equipment.

The method of protection involved encasing the creep specimen in a

flexible capsule capable of withstanding prolonged exposure to the

atmosphere .at high temperatures. Provisions were made to attach

extensometer linkages through the capsule walls to the specimen gage

points. A compound system of gage points was used to render the

desired coupling.

ENCAPSULATION

The capsule consisted of several sections of bellows welded to

machined parts which comprised the gage point mounting rings and

specimen end-adapters. The build-up of the various components is

shown in Fig. 1. An assembly drawing of the capsule is given in

Fig. 2, which illustrates in more detail the functions of the various

components and provides also an index to their description. The

details of the components are given in Figs. 3, 4 and 5. Single-ply

seamless stainless steel bellows were used. In several minor forming

operations the end convolutions on each bellows were formed to match

the corresponding weld flanges on the appropriate machined parts.

The creep specimen used in this investigation is shown in Fig. 6.

OveirtJdlO_ capsule dimensions were based on this specimen.

Fabrication of the capsule was accomplished readilyusing

shielded-arc welding techniques with the aid of specially designed

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8

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9

A. SPECIMEN

END

ADAPTERS

B. END BELLOWS

C. GAGE POINT FLANGE

D. GAGE POINT

MOUNTING RING

E. CENTER BELLOWS

F. CREEP SPECIMEN

G. EXTENSOMETER

.

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SPECIMEN

(F)

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14

were discussed in detail by Bohn and Murphy( 2 ) thus the only concern here will be the necessary description.

Several pre-welded sub-assemblies were fabricated prior to the final encapsulating operation. The specimen end-adaptors(A) were welded to the end bellows(B) and the center bellows(E) was welded to the gage point mounting rings(D). Bellows of sufficient length for the center assembly were not commercially available, and necessitated welding two short sections of bellows together. It was found that a thin washer(O. 020 in) placed between the two sections of bellows aided in producing a uniform welded joint.

The center sub-assembly was placed on the specimen and the inner gage point set-screws were tightened securely into the specimen gage point holes, then the two end sub-assemblies were screwed onto the specimen and weldedto the center assembly. The lengths of the bellows were adjusted so that they were held in a slight state of compression in the final assembly. This was done to insure a good mechanical coupling between the weld flanges of the various components, which was found to be a prime requisite in obtaining

s~ welded joints. All of the welding operations were performed in a welding chamber which was initially evacuated using a mechanical fore pump and then purged with argon to a pressure of one atmosphere.

The argon acted to support the arc in welding and was ultimately

sealed in the capsule. The apparatus provided a semi-remote welding

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operation, in that the work was positioned and rotated by a constant

speed drive mechanism, whereas, the d~ctrode was held in a fixed

position. Positioning the electrode was accomplished from the exterior

of the chamber. Fig. 7 is a view of the welding apparatus.

PROTECTION

Several disadvantages are inherent in this method of oxidation

protection. They include: (1) the uncertainty of leak-tightness of

capsule welds, and (2) the exact determination of the specimen

temperature.

The first has not presented any real difficulty in this investigation,

since visual inspection of the welds usually indicated whether or not

a tight seal had been achieved. Additional leak checks were occasionally

made when prompted by doubt in visual inspection. One method involved

replacing the welded assembly in the weld chamber and

over-pressuring the chamber with argon to about 10 psig. After approximately

one hour the assembly was removed and immersed as rapidly a.s

possible in alcohol. Pin-hole leaks, if present, were detected by the

appearance of bubbles. Another method employed was to evacuate the

chamber, with the capsule included, immediately after welding. If

the pumping time to obtain the minimum fore pump pressure was_

abnormally long, it was assumed that a capsule leak was preseRt

Experiments were performed using a modified capsule to

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16

[image:19.601.19.524.76.721.2]

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external capsule temperature. Provisions for thermocouple

temperature measurements were made through seals in the capsule walls. As might be expected, the magnitude of the temperature difference varied as

a

function of the test temperature. (3 ) The observed difference between the specimen and external capsule

temperatures at 800°C did not exceed .:i:_

soc;

however, it is suggested that the appropriate temperature calibrations be made for the

particular conditions of testing desired. CONCLVSIONS

From the investigation reported it. is concluded that encapsulation is a practical and effective means of protecting creep specimens to be tested .at high temperatures. The method of protection is limited temperature-wise only by the temperature restrictions -o~ the material used in construction of the capsule and the testing machines.