Plastically deformed steel in a weak magnetic field

LETTERS

remain the

more or less fixed in position.

gas

to

dissociat,ed

escape, and

the

the

earlier is determined

of

must

activation

by the heat of dissociation

of

The linear nature of t’he upper portion of the curve be explained

as follows:

hydrogen

atoms move through

diffusion

rate may

but by t’he probabilit’y form

molecuIes.

Since the iron rapidly,

be determined In

not

the the

by the iron

of the atoms recombining general

one

can

write

to

bhat,

acjax

d = D

D is

the diffusion coefficient and at/ax is the concentration gradient

of

atoms.

hydrogen

of

If

D

is determined

recombination

to

by

form

the

molecular

one can write. D = const/c

or J = (c0nst/c)(ac/ax)

this

in the

drops out and J becomes

expression

independent

for J, $(t) of time.

The

rate of escape would then be a constant. The fact, as shown by Fig. 2, that the time required for complete specimen

de-embrittlement

of a 3116 in. diameter

is about one half of the time for a 3132 in.

specimen

is of

assurance

t,hat the hydrogen

no

significance

two sets of specimens same.

electrolyte prepare

bath

rods

change.

2 is that

therefore

This

the

with

of the same diameter, of

the

bath,

of

When the diameter content

important

of a mechanism

to

uniformity

hydrogen

the lines have

is the

of the current in the

purity

The

is no in the

diameters

reasonable

the

there

thing

would about

the same slopes

show the same heat of activation;

is indicative common

control

in successive rods.

is changed,

undoubtedlg Fig.

and

specimens

concentration of

through

since

concentration

of different

With a set of specimens

it is possible,

and

which

of de-embrittlement,

to all three sizes. work

Wisconsin Departmeld

some

unexpected

these

are

was supported

Alumni Research

by

a grant

from

the

Foundation.

qf Physics and

Department of Mining and

in a weak

investigation

of

to

be

of brittleness

of the experiment,s

were

closely

in this material,

are given

As

noticed.

reIated

to

the

the results

here together

with a

brief discussion. The conditions in diameter Martin

of the experiments

wercl as follows:

with 38 mm gauge length and 10 mm

were machined

from

St. 41 Siemens-

steel rods of 1 in. diamet’er which had been

annealed

for 2 hr at 900°C.

The grain size of the

materia.1 was ASTM 6, the texture

uniform

obvious

the

preferential The N

directions

composition Si

0.003

P 0.010

0.14

of of

the

s 0.031

and no

grains

werr

material

was:

Al 0.014

Mn 0.55

CU

0.19

in weight percentages. The mechanical Losenhausen

tests were carried out on a 35-ton

machine

load \vas determined

of the hydraulic

by a pendulum

and the testing velocity means of a valve.

continuously

was manually

readings that

10-j see-l. specimen

equivalent

the

strains

the volume

remained constant;

were

of t’hc deformed

tests flanges were screwed

The mechanical

The

metal For

to the

on

before it was placed in tests were interrupted

at several points in order to determine

the change in

induction.

magnetic

placing

to it.

calculated

one division on the dial was then

heads of the tested specimen the machine.

by

The change in was measured

to about 413 x 0.01 per cent strain.

the compression

magnetic

regulated

b*v means of a dial gauge clamped

the

assuming

The

In this ma,nner the rate of strain

was maintained at about diameter of t,he tested From

type.

pressure balance,

the

experiments

specimens

in

were x

carried

solenoid.

out

Iry

described

elsewhere,(i) inwhich a magnetic fieldof approsimate1.v 0.15 oersted of 50 c/s.

(maximum)

The output

oscillated

with a frequency

of a sensing coil around

the

H. B. WAHLIN

middle of the specimen was fed to a valve vohmeter.

D. J. MACK

The readings were accurate to approximately

Metallurg~y The University of Wisconsin Madison 6 References 1. Bur. of Stnnrl. Circ. So. 511, September, 1961. “. P. RASTIEN, C.R. ilcrcd. Sci., Pm-is 220, 883-885 (1945). * ltecrivctd May 14, 1959.

phenomena

thought

occurrence

C

substitutes

steel field*

In t’he course of an experimental

0.16

c = S(G#W

deformed magnetic

the effects of plastic deformation, in tension and in compression, on the magnetic induction of steel.

observed.

If one writes,

and

Plastically

Test specimens

where J is the diffusion rate across a boundary,

probabilit,y

689

first be

mentioned

H, molecules in the metal. in Fig. 1 can

EDITOR

In order for

molecules

heat

TO THE

54

per

cent, some inaccuracy being caused l)y fluctuations in the feed current of the solenoid. The plastic deformation always caused a loss of magnetic induction, manifested by a lower reading of the voltmeter. induction

The percentage as given in Figs.

calculated

from

the voltmeter

losses la, lb

of magnetic and lc were

readines. n

The

onlv

0

I

Frc.

la:

between

I 2

I 3

I 5

I

4

Strain,

%

fixed

~~~xi~u~l

stresses

40 30 $

20

2

IO

5

IO

0 h

20 30 40 -2 FIG. lb:

-I

0 Strain,

bet.w;een inoreusing

I %

2 maximum

stresses

correction introduced was one for variations in the field strength; the Lotal ~u~c~~ra~~ of the losses is estimated to be rfrl per cent. Fig. la shows the result of a test in which the specimen was strained alternately t,o a maximum

load of + or -- 2900 kg. The result of t,eri~i~lat~in~ each successive half-cycle on an increased absolute valiue of the load is shown in Fig. lb. Test. results of a,n experiment which commenced with compression are shown in Fig. le. Borne points of interest in the results of the magnetic tests are: 1. The effect of teftsile deformation on the virgm materia,l is much more sc~ve~ethan t,hnt,of c~~~~~~~~~~~~~,v~ deformation. 11. The ef?ect of tensilc deformation may be pa,rtly nullified by subsequent comprcssiw tkformsCon, hut, the opposite is not true. ITT. Cycling a specimen between t’wo limits of load cauws the decrease of pcrmeshilit~v t,o renra,in approximately constant for at. least tht, first few cycles. l\‘. Cycling a, specimen bctwwa limits of load of ever increasing ma~gnitudlccauses an iwreasc of’ the loss of induction. There may be a connection bctawn the featOures obserrtd and the fo~~~at~oi~ of noIl-~~ro~~a~~,i~~ micro-cracks. which phenomenon is known to occur during the early stages of the tensile t.wt in this type of mat,erial (Low(s), Wessel(a), Own et flkt4)). It is known that tho nmgnetizstion at, low field strength takes place by the movement of Rloch r\-a,lls (Bloch(“),

LETTERS

TO

THP:

EDITOR

compression

69 I

was noticed.

A slight ditference between

the initial curves in compression proved

and in tension was

to be due to buckling:

specimens

were particularly

to which

t’he nickel

susceptible.

Acknowledgments The author Rathenau

is much indebted

for his stimulating

to Professor

interest.

tion of the material was determined and his staff of this laboratory. Mr. J. Raadsen,

by Mr. J. Kroonen

The able assistance of

Mr. J. N. Helle and Mr. D. de Graag

during the experiments

was much appreciated. A. TV.

lionilbkltjke

Fig. 2. Xcrocraok formed at O’C 400 x magnifirittion. rwgativr 3 x enlarged.

Becker(G)).

Although

a detailed

t’hat the micro-cracks

tensile plastic

deformation

this movement, induction. formed

in

thus causing a falling-off

If in addition

we assume

tension

be

may

The difference mechanical ductile-brittle

transition

that

without

with this concept.

plastic

it shon-s

the

in tension

well with the known

temperatures

below

temperature

temperature

preceding

compression

at

iron

in

breaks

deformation. usual

in

increases

with

of a Griffith

stable

Grifith

increasing

stress

crack

crack requires

ideas

mentioned

opened

only with up.

A

more stress if it is to

that creation

of vacancies

to check

It was initially might

the

surmised

play the principal

r81e in explaining the phenomena. Keeping plastically deformed specimens for 2 months at 120°C did not, however,

introduce

induction. Jt was realized

any

change

8.

Relation entre la sCgrCgation des impure& et l’autodiffusion intergranulaire dans le fer* Differentes

etudes,

in

the

magnetic

was thus far made of the occurrence

no mention

of microcracks

determiner

laire des impuretes. Bmis

l’hypothese

ou exp&rimentales,

perturbee

des joints de

la segregation

intergranu-

Ainsi ,llcLean et Northcott’r) de

segr6gations

ont

intergranulaires

d’atomes de solute, meme aux temperatures sup6rieures B la temperature

limite de solubilite.

On sait d’autrc

part que la structure des joints depend non seulement de I’orientation aussi

t,hat in the literature

theoriques

ont sugger6 que la structure grains pouvait

were performed

above.

5. 6. 5.

in

be opened up further (Cottrell(*)). A fen- experiments

3. 4.

phenomena

is consistent

being

,“.

tension

(Lo&i’). The finding that the decrease of permeability t,he idea

the

tension.

whereas

yield

References 1.

during

enumerated

behaviour

correlates

behaviour

a crack

closed

points

in magnetic

and in compression

to

of magnetic that

partly

it

during

may act as obstacles

compression, the subsequent above are seen to be compatible

Below

created

SLEIWVYK

Shell-Laboratori/l’n/

Amsterda,m

model is lacking,

is conceivable

G. iV_.

The composi-

de

relative

I’orientation

des cristaux du

reseaux des grains adjacents.

,joint

par

contigus,

mais

rapport

aux

En consequence,

plus

la st,ructure des joints cst pert,urbce (joints s6parant

at temperatures higher than about -90°C. Fig. 2 is a microphotograph of a crack on the surface of an Armco ingot iron specimen that had been successively

des cristaux de forte d&orientation). et plus la tendance a la segregation dcs impure& doit 6tre prononc6e.

electropolished,

nous avons compare

annealed

at’ 960°C and strained

to

4O/‘0 in tension at 0°C. A small number of nickel specimens was tested in the same manner as t’he iron: no restorative effect of

Pour obtenir une preuve directe de cette hypothesc, les phbnomitncs

d’autodiffusion

intergranulairc. a ceux de pr6cipitation d’un solute dissous k unc teneur nettement infirieure & la limite Les techniques autoradiographiqucs dc solubilitc.