Factors in hydrogen embrittlement of high strength steels

materials

Chemistry

and Pl~ysics,

21 (1989)

437

446

437

SHORT COMMUNICATION

FACTORS

IN HYDROGEN

EMBRITTLEMENT

E. QUADRINI CIRM - Dipartimento 60131 Ancona Received

OF HIGH STRENGTH

Universitci

di Meccanica,

STEELS

di Ancona,

Via

Brecce

Bianche,

(Italy)

September

12, 1988; accepted

October

17, 1988

ABSTRACT The effect of pH and current 39NiCrMo3

steel was studied

The hydrogen in

a

was introduced

sulphuric

acid

density

using

on a hydrogen

the delayed

into the material

solution

at

different

failure

induced

by means of cathodic pH

on UN1

fracture

test at a constant

values.

The

load.

polarization

current

density

impos d on the samples during the embrittlement test ranged from 5 to 20 !? mA/cm . The results obtained showed a noticeable influence of the pH and current density on threshold load values and crack incubation times. This behaviour was attributed to the effect of the pH and current density on rate at which atomic

hydrogen

adsorbs

on the material

surface.

INTRGDUCTXON The susceptibility hydrogen

embrittlement

environment-sensitive Despite

the

have been proposed 'These theories proposed the

mechanical

0254-0584/89/$3.50

the

of

to explain basically

theory,

fall

work

three

proposes

present

into molecular

that

steels, examples

to of

modified

embrittlement after

this

argument

Several

theories

phenomena.

groups.

in the material

on

an enigma.

embrittlement

[5] and most recently

which

documented

performed

still remain

into

strength

of metals.

research

the hydrogen

high

often

most

behaviour

of this phenomenon

in the microvoids

atomic hydrogen

of

amount

by Zapffe and Sims

pressure

pressure

one

is

enormous

11-41, many aspects

in particular

of iron alloys,

The

first

of

by Tetelman results

these, [8], is

from

the transformation

high of

hydrogen.

0 Elsevier Sequoia/Printed

in The Netherlands

438 The second theory, proposed by Troiano and his co-workers 17-101 is the 'decohesion theory', which Proposes that hydrogen diffuses under the influence of a stress gradient to regions where plastic deformation is greater, reducing the interatomic cohesion forces. The third model, proposed by Petch and Stables cl11 is the 'Surface energy theory'.This theory proposes that fracture stress is lowered by a reduction in the surface energy of the material at the internal surfaces of the microcrack. Another model attributed a material's loss of ductility to the temporary or permanent interaction of

hydrogen with

the

dislocations, inclusions and

microcavities, point faults and grain joints [12-131. Since none of these theories can explain on its own the complex phenomena which determine the behaviour of a material toward the embrittling action of hydrogen, it is logical to think of a simultaneous action of the various mechanisms. However, a generally recognized common feature of all theories is that some critical concentration of hydrogen must be reached at potential crack sites for failure to initiate. The critical concentration is influenced by complex environmental and

metallurgical interactions such as

potential crack site, the mechanical resistence of

the

the

nature of

the

i-342 I

the

steel

microstructure [15-161, the state of the stress at the site, etc. The purpose of this work is to determine the effect of pH and current density on delayed failure tests carried out on the sample of UN1 39NiCrMo3 steel. Hydrogen charging was affected by means of cathodic polarization. Indeed, since, is it has

been

observed [17], by means of electrochemical

investigation, that atomic hydrogen formation is influenced by hydrogen ion concentration and current density imposed on the sample during failure tests, it is possible that a variation in these parameters causes more or less marked embrittlement effects.

EXPERIMENTAL Specimens for this investigation were UN1 4ONiCrMo7 steel, the chemical composition of which is reported in Table I. Sample geometry and discussions as regards length and the gage length were similar to those previously used [15].

439

Table I. Chemical c

After

composition

of steel used.

(wt%). Mn = 0.75

= 0.40

cu = 0.17

Ni = 1.74

Si = 0.26

Sn = 0.022

Cr = 0.81

P

= 0.019

Al = 0.028

MO = 0.23

s

= 0.015

mechanical

the

finishing,

specimens

were

austenitized

for

one

hour

at

870°C and oil quenched. The

heat

constant

treatment

a special

quenching

oven

also

specimen

enabled

in

a

tubular

us to achieve

releasing

structure

in Fig. 1. The homogeneity the samples

to Rockwell

other mechanical

failure

out

oven

provided

with

a

device

extremely

allowed

the

drastic

specimen

quenching, to

reach

a

bath very rapidly.

The metallographic

The

carried

supply of argon.

The vertical since

was

influence

of the sample

of the heat treatment

C hardness

characteristics of

after heat treatment

surface

tests:

the values

are reported finishing

tests was found in a recent work

was checked

on

obtained

in Table the

is reported

by subjecting together

with the

II.

results

obtained

in

delayed

[la].

100 pm

1000 MESH

Fig. 1. Metallographic

aspect

of

the steel after oil quenched.

all

Fig. 2. Roughness

profile.

440 Table II. Mechanical

properties

Heat Treatment

of steel used.

Hardness

Yeld Strength

HRC

(MPa)

Ultimate Tensile

Strength

(MPa) oil quenched

55

1650

2050

Tempered

44

1100

1250

at 500°C

In order to reduce made of the sample by

Perther.

In

measurements The

surface

Fig.

polarization

This

the

was

of which

sample

which

was

the

carried

from 10

of

out

profile

obtained

the

pH

to 10

l-l), varying

-1

a check

was

machine

made

from

these

gionl

of

current

the current

quantity

studied

thermostatically

of

cathodic

regulated

glass

circulated.

was

by

Inside

assembled,

imposing

on

the hydrogen

to

the

cylinder the

while

a

in the

cylinder

the

platinum

coil

the

samples

an

ion concentration

electrode

that ranged

. density

was

the

studied

at

a constant

pH

-1 (10

g

ion

from 5 to 15 mA/cm'.

conditions

of oxygen,

means

with the atmosphere.

liquid

cathode

density

In the galvanostatic

by

of 2 cm acted as a counterelectrode.

was

-1

material

in a double-walled

of 1300 mV SSE and varying

The effect

small

the

solution,

thermostatic

constituted

effect

-4

into

acid

round it at distance

potential

type C5D electronic

roughness

of 22O + l°C, and in contact

operation

The

average

introduced

in a sulphuric

cavity

wrapped

2

using a Perthometer

therefore,

is reported.

hydrogen

temperature

this effect as much as possible,

only

indicated, possible

apart

from

process

the

is the

reduction

reduction

of of

a

the

hydrogen. The measurement was applied

of failure

time began at the moment

in which

the whole load

to the sample.

The tests which

did

not

since this is a sufficiently

reach

failure

long period

in 150-200

hours

were

of time for ascertaining

interrupted the treshold

load. Immediately scanning

electron

after

fracture,

microscope.

the

specimens

were

examined

by

means

of

a

441 RESULTS The

AND DISCUSSION effect

of

pH

and

the

current

density

are

results

of

sustained

load

tests and are shown in Figs. 3 and 4. Figure charged

3 refers

varying

tests performed

to results

the pH value, at different

obtained while

current

in the tests

Fig.

4 refers

density

in which

the hydrogen

to results

obtained

in

was the

values. o pH=

d(MPa)

1,2

A p H=2,2 n

pH=

l

pHz4.2

3.2

I

lo-'

10"

10'

10'

10'

10' TIMElmin)

Fig.

3.

Effect

of the applied

stress on time to failure

for different

l

d (MPal

n

pH value.

5 (mAcrf?)

10

020

II

I/

II

11

500

400

300

l

200

0

I(I-’

.

^

10’

10

9

10'

_^1

10' _”

1lJTIMEt

Fig. 4. Effect of the applied density.

stress

on time

to failure

mln

for different

) current

442 It

is

apparent

considerably imposed

effected

current

The

trend

material

from

these

results

by hydrogen

that

hydrogen

ion concentration

induced

fracture

in the electrolyte

is

and by

density. of

the

to hydrogen

curve

shows

a greater

embrittlement

sensitivity

at low pH values

on

and

the

high

part

current

of

the

density

values. The results

may be interpreted

which the atomic hydrogen internal

structure.

the metal during

taking

adsorbed

consideration

on the surface

The elementary

the cathodic

into

reactions

polarization

of metal

the mechanisms penetrates

that take place

can be expressed

by

into its

on the surface

of

as:

(1)

H+ + eads K_

-1

K H

+H ads

H

+H

&H ads

+

K

-2

+e-2

H

ads

K

were H

is

(3)

2

-3

hydrogen

adsorbed

on

the

surface

of

the

metal.

The

hydrogen

ads

adsorbed,

given

structure

of

microcavites These traps

(2)

2

its

a

present

for these

atoms.

atoms

hydrogen

accumulates

reaching

a critical

dimensions divides

(1.04

itself

present During

into the and

zones

damages

their

the

triaxal

mechanical

the

spreading dislocations

lattice,

deformation

of greater

concentration,

the

and in the interstitial

in the crystal

plastic

while

A),

between

on the grain boundaries

always

faults,

the hydrogen

modest

material,

plastic

the

and

the

positions.

constitute

excellent

dislocations

transport

stress.

In these

characteristics

zone

into

fractures

zones

until,

on

and

the

cycle

hydrogen

to

reach

environmental

and

continues. It critical

has

been

concentration

metallurgical hydrogen

the hydrogen

materials

interstitial

that

prior

to

the

time

depends,

characteristics,

in the material

Indeed, the

noted

but

necessary

not

only

also

at the moment

upon

upon the

the the

presence

or

otherwise

of

of stress application.

diffusion

mechanism

testing.

The

jumps, dislocation

for

depends

hydrogen

atmospheres

may

upon move

the initial in

the

or along short circuit

state

lattice paths.

of by

443 In

our

material the

provided

tests,

is simultaneous

surface

interaction

will

the

contact

between

the

major

role

the dislocations

the

hydrogen

the dislocations

with the load application,

have

between

that

in

hydrogen

and the

hydrogen

was

the

present The

transport. atoms

and

on

energy

estimated

at

about 0.3 eV at room temperature. At equal levels equal,

and therefore

upon hydrogen From

the curves

critical

value

of dislocations of

in movement

concentration

depends

is also

exclusively

on the surface.

as the equilibrium

the

concentration

the number

1, it can be seen

reaction

conditions,

the

adsorbed

low pH values,

time

that

the hydrogen

reaction

necessary

value and nucleate

for

shifts

the

adsorbed

towards

hydrogen

the fracture

to

is shorter

is greater

at

the right.

In these

reach

critical

the

(leftward

movement

of

in Fig. 3). from

Furthermore,

[20] that hydrogen upon

of stress,

current

concentration

the electrochemical

permeability

density

values.

is reached

investigation,

does not depend

Also

upon

in this second

more quickly

(leftward

it has

been

the electrolyte

case,

the

movement

observed used,

critical

but

hydrogen

of the curves

in Fig.

4). While

these

propagation,

stress.

to explain

Creep phenomena

of very modest

and in particular

micronotches

on the surface,

deformations

stress relaxation

During

there

where

reaches

the critical In this

is an increase

both

in the

of

the

in points stress

consider

the

when

occur

it

is greater.

phenomena

and

which

is subjected

in the material

corresponding

are induced

nucleation

load values.

moment

proportions

cavitation

tip.

Some

at

phenomena

at the crack

subcritical

in fracture

to already

As a consequence

into the crack

to

during

existing of the

tip with

a

in time.

these deformations

local stress

delays

we must

this behaviour, of a material

rate decreasing

the

the lower threshold

the behaviour

load application,

tension

justify

they do not explain

In order determine

considerations

phenomena

occur

value and consequently way,

at

microvoid

every

in the sites there

moment

population

of and

in which

is a release

load

of

application,

in the

sites

with

stress values. these

cause cavitation.

sites

have

a value

very

close

to

the

stress

necessary

to

444

Some authors suppose (19-201 that during the initial relaxation phenomenon the number of subcritically stressed sites decreases with time, and this reduction is greater in the initial phase of the relaxation phenomenon. In our tests, since for the reasons explained above, hydrogen diffusion is helped at low pH values and at high current density values it will be in these conditions that hydrogen will interact with a greater number of subcritically stressed microvoids. In this way, these microvoids will reach the critical stress necessary for growth as a result of

the reduction in cohesion strength caused by

the

hydrogen. The growth of the microvoids, as well as facilitating hydrogen diffusion in the zones in front of the apex of the crack, will weaken the mechanical resistance of this area and therefore the stress necessary to propagate the fracture will be less (upward movement of the curves in Figs. 3 and 4). From

examination of

the

fractographs of

Fig.

5,

a

predominantly

intergranular type failure can be seen with limited areas of quasi-cleavage. This morphology, which is typical of the fragile fracture induced by hydrogen, does not seem to be modified by test conditions.

Fig. 5. SEM fractography with predominant‘intergranufarfracture.

445 CONCLUSIONS On the basis of the experimental and current

density

substantially incubation

the values

obtained

during

of both

we can affirm

the hydrogen

the threshold

that the pH

embrittlement load

and

of

tests failure

time.

embrittlement

it was

is greater

behaviour

rate of atomic

was

observed

that

the

at low pH values

attributed

hydrogen

Fractographic fracture

imposed on a sample

influence

In particular

This

results

to

adsorption

examination

the

and at high

influence

of

by the surface

highlighted

which was not modified

materials

resistence current

these

to hydrogen

density

parameters

values. upon

the

of the material.

a predominantly

intergranular

type of

by test conditions.

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