The temperature-dependence of the hydrogen yield-point in steel

ACTA

112

T, is the temperature is measured.

METALLURGICA,

at which the surface

The density

tension

heat

p must also be corrected,

using the bulk coeilicient

VOL.

of

increase

of expansion.

structural

5,

1957

vaporization. in surface

The

effect requiring

Using a factor o = 9 and without making any temperature corrections, relatively good predictions

ofo=9.

are made

Stanford Research Institute

for the surface

tension

of many

Table 1 shows some of the comparative

metals.

data.

Metal*

AH,

Al Fe CU Ag

67,600 96,500 81,700 69,400

;: Pt ;;:

68,000 54,400 125,000 46,700 90,700

Bi

47,800

M

P

26.97

2.702 7.86 8.92 10.5 7.28 6.684 21.45 19.3 11.34 9.8

55.85 63.57 107.88 118.7 121.76 195.23 197.2 207.21 209.0

700 1600 1150 970 253 750 2000 1070 350 300

-* Some metals such as Ns, Zn, Cd, and Hg require lower values of w.

4. A comparison of the data on solids In using equation solids, AH,,

(12) for the surface

is the heat of sublimation

tension

of

AH,, Without

=

making

AH, =

AH,

corrections

and using

values of the surface tension

in Table 2.

AH,

ZZ

GXP

uca1c

Correction factors

TO

AH

84,810 72,100 9,373

1852 1235 1610

1800 1235 1440

the

last

two

90% 89.9% 91%

_

1. -

In

110% 109% 110%

1100°K 1040°K 1260’K

columns

are

the

temperature

corrections for AH and p, which are seen to be about 110% for AH and 90% for p. The product of these corrections

is about

unity

at the

meeting

point,

indicating that the effects are equal and opposite and that little is to be gained by using them. It is of interest that the surface tension of solids is about

50%

greater

than the surface

tensions

of

the liquids. Only about 4% of this can be attributed to the AH values, because for most metals the heat of sublimation

is only

about

4%

possible.

The present letter describes a further study

of the temperature-dependence Both commercial

Armco

of the magnitude

greater than the

iron and SAE-1020

were used as base materials. 0.200 in. in diameter,

I p

_

cu Ag AU

iron or steel,

due to hydrogen has recently been a yield-point . shown to exist.(l) A low-test temperature was picked in order that the yield-point would be as large as of

this yield-point.

TABLE 2. Surface tension of solids

=

* Received May 7, 1956.

In a tensile test of hydrogen-charged

were computed. This data is compared with the experimental values of UdinQp 37 4, and co-workers

Metal

1. H. UDIN Metal Interfaces a contribution to a seminar sponsored by the American Society of Metals in Detroit, Michigan, October 13, 1951. 2. H. UDIN, A. J. SHALER,and J. WTJLF Surface Tension of Solid Copper Trans. Am. Inst. Min. and Met. Eng. 185 (1949). 3. E. R. FUNK, H. UDIN, and J. WULF Surface Tension of Solid Silver A.I.M.E. J. Metals T.P. 318. 4. F. H. BUTTNER,H. UDIN, and J. WULF Surface Tension of Solid Gold A.I.M.E. J. Metals T.P. 31873. 5. For an interesting discussion of surface tension, related to the present discussion see JOEL H. HILDEBRANDand ROBERT L. SCOTT The Solubility of Non-Electrolytes, Reinhold Pubhshing Corporation p. 401 3rd Edition 1950. 6. Handbook of Chemistry and Physics. 13th Edition 1721, Chem. Rubber Publishing Co. 1947 (for surface tension of Metals). 7. The AH. and AHap values were taken from J. C. SLATER Introduction to Chemical Physics p. 259, McGraw-Hill Book Company (1939).

The Temperature-dependence of the Hydrogen Yield-point in Steel*

of a given metal,

AH, + AH,

temperature

o = 0.6, the calculated

the use of CIJ= 6 instead

of the solid.

This is equal to the sum of the heat of vaporization, AH,, and heat of melting(‘)

of is a

References

--

840 1360 1034 800 526 368 1819 580-1000 452 388

proportion

solidification

Menlo Park, California

TO 804 1430 1203 804 581 430 1570 1055 450 409

upon

DAN MCLACRLAN JR.

TABLE 1. Surface tension of liquid metals’6V7)

- __-

greater

tension

rod, then polished

steel

Cylindrical

specimens,

were plunge-ground

from J-in.

in a lathe using 1, 0, 3/O emery

papers successively. After degreasing, they were annealed in dry hydrogen for 1 hour at 600°C and cooled in the same atmosphere in a water-cooled extension of the furnace. In order both to eliminate the carbon and nitrogen yield-points

and to allow for the subsequent

absorp-

tion of large quantities of hydrogen, the specimens were prestrained at -78% in a bath of dry ice and alcohol, unloaded, and placed immediately in liquid nitrogen to eliminate possible aging. The Armco iron was strained 6%, the SAE-1020 steel 8%. Both materials were electrolytically charged

with

hydrogen for 4 hours in 4 volume per cent sulfuric acid. As a catalytic surface poison, 0.1 g As,O, was

LETTERS

TO

THE

EDITOR

113

Load-elongation peratures e

3000 I

and 2.

curves

for both

at a series of test tem-

materials

At temperatures

yield-point. As the definite discontinuity

are shown

above

-80°C

in Figs.

1

there is no

temperature is decreased, a appears in the stress-strain

curves for both materials at approximately ---BOY?, and a distinct drop in load appears at somewhat lower temperatures, test temperature

increasing

in magnitude

is decreased.

present what variables are responsible difference in magnitude of the yield-points by the two different materials. was strained

8%,

as the

It is not certain

The SAE-1020

has a grain

at

for the exhibited steel

size equivalent

to

ASTM No. 8, and a 0.12 wt. per cent carbon content. On the other S%,

% Elongation FIG. 1. The temperature dependence of the hydrogen yield-point in prestrained SAE-1020 steel

added

to each liter of acid when charging

1020 steel. hood of remained

the SAE-

The current density was in the neighbor0.2 amp/in.2, and the bath temperature less than 30°C. After charging, the speci-

mens were washed and kept in liquid nitrogen

until

has

hand,

the Armco

a grain-size

range

iron

to

No. 3 to No. 5, and a carbon

content

per cent.

variations

Other compositional

play a significant yield-point.

part in altering

a consistent ability

may also

the size of the curves

of the specimens is by no means

function

probably

ASTM

of 0.012 wt.

It can also be seen from the stress-strain that the elongation

of the temperature.

arises

from

largest surface defect introduced its proximity to the smallest

the

The vari-

severity

of

the

during charging, and cross-section of the

tested.

gage length.

All testing was carried out on an Instron testing machine at a constant rate of cross-arm separation

General Electric Research Laboratory

of 0.05 in. per minute.

was strained

equivalent

H. C ROGERS Schenectady, New York Reference 1. H. C. ROGERS A Yield Point in Steel Due to Hydrogen, Acla Met. 2, 167 (1954). * Received June 6 1956.

Some

Effects of Annealing at High Temperature*

In previous

publications(1~2)

it

has

Ta,O, been

shown

that when Ta20c, films are annealed after formation their corrosion resistance to HP is increased, their ionic

conductivity

is decreased,

and the scattering

of X-rays by the films becomes less diffuse. These effects were attributed to a change in the local ionic configuration in the film. Other phenomena observed

“la Elongation Fro. 2. The temperature dependence of the hydrogen yield-point in prestrained Ammo iron.

at somewhat higher annealing temperatures are the subject of this letter. When Ta,O, films are heated at temperatures above about 3OO”C, there is a marked increase in the capacitance of the film. The increase in capacitance ranges from a few per cent after heating at 300°C to about 15% or so after heating at 600°C if the specimen