Correlation of MicrostructEire vvith WIAZ Ernbrittlement

Scandinavian Journai of Metallurgy 2 (1973) 9l-94
Correlation of MicrostructEire vvith WIAZ Ernbrittlement
G. N{. Evans and N.
Christensen
N{etallurgy Division of SINTEF, Trondheim, Norway
611.79 1.05:5i?. 14.01 8.29
Abstract
rvere 3.6 s, 5.1 s and 6.6 s. The electrodes rvere received from
The influence of microstructure on the static fatigue limit (orur) of the
heat-affected zone of C:lln steels has becn evaluated by means of the
implant *eldability test. The results are presented in the form
the manufacturer in the moist condition and were predried
at 300"C for 24 h to produce a total hydrogen level of approximately 7 ppm (fused metal) according to the IIW/IIS pro-
n --
o
"t":
b(I
cedure [4].
- M)!
rihere n and b arc prrilmeters dependent on thc i.)ydrogen content and
-1-/ is r:.c \oiLn:e lrtcuJn oI ntlrricrtsiIe.
In rcirlition, it is confirmed that
6s.'.- A-
B log [H]
*lrcrc..1 and B are prrilnletcrs dcpendcnt on the microstructure and IH]
is the
llW totai hldrogen
In another series, the heat input li.as maintained constant,
varied by the additional use of basic electrodes dried at.150'C
for 24 h (3 ppm) and rutile electrodes ltSO code E i32R22) takcn lrom stock (30 ppm).
The implant test procedure rvas identical to that briefly
content.
Introduction
The inrplant tcsi. in the form proposed by Granjon [1], enables
a quantitative assessment to be made of the factors affecting
cold cracking in the heat-affected zone (HAZ) of metal-arc
rvelds. Thc tcst has previously been applied to evaluate the
effcct of arc cnergy [2] and of steel composition [3] at different
hldrogen levels. The objectivc of the present u'ork \r'as to
cvirluate the elfect of tl-re microstructure in the grain coarsened
region of the HAZ on the extent of embrittlement.
emplo-"-ing ,1 mm dia. electrodes and the h1'drogen level
by
described eiservhere [2, 3], the critical fracture stress. termed
the static fatigue limit (o=""), being chosen as the criterion for
embrittlement. The distance of the notch lrom the top surlace
of the piate *'as standardized at 1.35 mm, 1.85 mm and 1.20 mm
for the progressivcly different diameter basic eiectrcCes, and
1.55 mm for the 4 mm dia. rutile electrodes. Nine implants
uere emplol'ed for
experimental condition and the tests
were terminated if the specimens remained intact after 24 h.
eacl-r
Results
IIetallograplry
Iixperimental
The anailses oi the materials investigated are given in Table I.
the carbon content being the variant. The steels were prepared
as 5 kg laboratory melts and were subsequently forged to 13
mm diameter round bars, homogenized at 1 400'C for 4 h and
then normalizeC at 910'C.
In onc scrics of experiments the heat input u'as varied, by
using 3.25 mm, 4 mn-r and 5 mm dia. basic electrodes (ISO code
E 445820). The gross energy inputs were 0.9 kJ/mm, 1.3 kJ/mm
and 1.7 kJ/mm, respectively, and the corresponding cooling
times (800" - 500'C) in the grain coarsened region of lhe HAZ
The results of point count analyses of the microstructures in
the heat-affected zone of implant specimens have been reported
separately [5]. No evidence could be found of triinned martensite and it appeared that the degree of auto-tempering rvas
identical in all four steels. The reler.ant data for the average
amount of martensite, at distances betrveen 50 and 200 ,rrn-t
Og
l!
0.9
17
t----------a
az
q
l'3
.---l-
J.0
_____-
kJ/mn
0-2r5
0
t70
0.t40
0.7
0.6
Table
L
:
6
Contpositions of steels inuestigatetl
{
ft
Analysis
('1,)
teel
\
0
ta9
U 0.3
l
S
\
0.1
N{n
AI
Nb
0.012
0.021
0.021
0.02 r
0.021
o
0.2
0.t
7
8
L9
L l0
L
L
0.109
6i
0. 1.10
5'7
0.170
0.215
59
59
0.43
0.42
0.41
0.43
0.014 0.005
0.0r6 0.005
0.014 0.005
0.014 0.005
0.005
0.005
0.006
0.005
0.0-14
0.0i0
0.015
0b
12345678
coou NG rN€ (8oo-5a0'c). sec.
Fig.
.1.
Volume fraction of martensite in the grain coarsened region of the
HAZ i5l.
Scand. J.
l[etallurgy
2
92
o - 09 kJ/mm
'.
. - i.r
16 Z
.
tPPa-!.
Mn.
E
\
z\
j
t
i
0
Fig. 2. Effecl of carbon content on static fatigue limit of steels with
lUn at different arc energies.
l
from the fusion boundary, are reproduced in Fig' 1' The
steel
6 "/"
than the other materials and also reacted more strongly to
change in heat inPut.
Variable
lrc
a
Fig. 2. The corresponding hardness values are plotted in Fig'
3
values, and. as can be seen' no evident corrcla-
e\presanlount
decreasing
of
basis
sing the liniting stress (osp1) on the
of rnartensite, ho$,ever. a parabolic relationship $'as exhibitcd
tion exists. each steel shoning an individual trend' on
\
t
\
04 05 uo ut
Fig. 5. l{clationship bctrvecn static fatigue limit and square root of voiume
(Fig. 4). Tlie rcsultant straight line obtained on plotting the
static iatigue limit against the square root of the volume fraction of non-martensitic constituents is shown in Fig. 5. The
rr.*or(r.\rnrrtt')
The eifect of the carbon content of the steel on the static fatigLre
lin.rit (or.r) of the HAZ is shorvn. for the three heat i,.tputr, i,',
against
@
line may be represented by the following equation
etrcrgy
the o.."
02 03
fraction of non-martensitic constituents'
carbon content (0.109 ,'/o) contained less martensite
of lo*est
0l
:
130 + 540 (1
- It)t
(1)
rvhcrc '11 is the voiume fraction of low-carbon auto-tempered
tuartcnsite'
l'aria!'lc hydrogen leuel
Thc inrplant tcst results' for 4 mm dia' electrodes at different
h1'drogen levels' are plotted against the carbon content of the
6,/,Mn
o,urt
E
:\
j
j
-T
-
:
:
3U)
CAREON
500
sfATtc FAfIGUE LIMIT,
Fre. -t. IUaximum hardness plotted against static fatigue
o - os
a-t3
o- t7
kJ/mm
limit'
CoNfENf'wl'% '
Fig. 6. Effcct of carbon content in steels witl.) 1.6
limit at different hydrogen levels"
9i,
Mn on static fatiguo
JPP&-U
€
e
E
*..
a
:
5
30ppm.
I
0
o ot 02 03 04 05 06 07 A6 09
10
I - VOLUME FRAC|ION MARI ENSIT€.
/irg. 4. Rclation bctween static fatigue limit and volume fraction of non'
nlitrtensitic constituents.
Scatttl. J. I
Ic
tallurgJ, 2
o 01 02 03 04 as 06 o7 oa 09
lffi
10
./
Fig. T.Eflect of hydrogen level on the rclationship between static fatigue
limit and microstructure'
Correlation of microstructure with HAZ embrittlement
II.
Trible
Parameters a and b in eq. (2)
93
l
for the three hydrogen
te,-e ls.
Hydrogen
content
(ppm)
Parameters
160
120
20
3
7
30
670
540
310
in Fig. 6. The HAZ was considerably embrittled on using
steels
rutile electrodes, and comparison with Fig. 2 shows that, over
the ranges investigated, the static fatigue limit was more affected by hydrogen than by heat input.
The straight-line relationship exhibited in Fig. 5, for a total
hydrogen content of 7 ppm, is also observed at higher and
iorver contents, as shown in Fig. 7. This relationship may thus
be represented by a general equation of the form
6srL: a+b(l -M)t
(2)
rvhere a and b are parameters dependent on hydrogen level.
The specific values for a and b are given in Table II.
The static fatigue limit was also found, as in previous work
12,31, to be linearly related to the logarithm of the IIW total
hydrogen content. This confirmed relationship is represented
by an equation of the form
o
"",
- A*B
I
log
(3)
[H]
are parameters, at a specific cooling rate, dependent on steel composition. The individual values of A and B
are reported in Table III, together rvith the ratio of BlA, which
is seen to be approximately 0.5.
rvhere
and
-B
8. Circumscribed surface for the correlation between static fatigue
limit, microstructure, and hydrogen level.
.Frg.
proportion of high temperature transformation products having a pronounced influence in the case of a predominantly
martensitic microstructure. The results are represented by eqs.
(2) and (3), and the inter-relationships are such that a and b are
related to the logarithm of the weld metal hydrogen content,
and I and ,B are related to the square root of the volume fraction of non-martensitic constituents. The actual values of the
parameters were evaluated to be, for o"u" expressed in N/mm'?:
a:230-140log [H] and 6:85 -37log [H]
A :230 + 85 (l - M)+ and ,B : 140 + 37 (l - it4lt
On substitution, the three main variables are combined, the
static fatigue limit being defined solely by the hydrogen level
and the microstructure. in the form
o."u
Discussion
Following the work of Hopkin [6], it has become generaily
accepted that the susceptibility to cold cracking in the heat-
:
230
-
140
log [H] + 85
(l -
tv1)+
*
37 log tHl (1
- M)]
(4)
A three-dimensional representation of the findings is shown
in Fig. 8, of note being the fact that the o.u" value for 1009/o
affected zone of steel weldments is dependent on the inter-relation of three main factors, namely (1) hydrogen level, (2) microstructure, and (3) stress.
The present investigation serves to correlate these factors,
the static fatigue limit being a quantitative indication of the
martensite is given by the parameter a whereas the sum d + b
defines the other extreme. Confirmation of a logarithmic dependence on hydrogen level illustrates the deleterious influence
of hydrogen at very low concentrations.
to produce cracking. It has been established,
as expected, that the HAZ is particularly embrittled by the
steels
stress required
found that embritt(Fig'
increase in the
4),
a
slight
lement decreased parabolically
presence of martensite. Furthermore,
it
rvas
Table III. Paron'teters A andB in eq. (3), and ratio BIA (heat
input 1.3 kJlmnt)
Carbon
Parameters
content
Steel
(%)
L7
L8
L9
L l0
0.109
BIA
An interesting feature of the present work is that, rvhen
of different carbon contents (0.1-0.29l.) are compared,
no direct correlation exists between the degree of embrittlement
and the maximum hardness level of the heat-affected zone.
Evidently, the nature of the mixed martensitic microstructure
is the determining factor and not the hardness as influenced
by the carbon content. In consequence, hardness can only
serve as a rough, although readily applicable guide to rveldability.
The present data are limited to the extent that the investigation was confined to the C:Mn system. In addition, the steels
were homogenized, and secondary effects, such as the possible
sJM (71)
0.140
0.170
0.215
800
660
520
430
380
330
270
220
0.48
0.50
0.52
0.51
influence of sulphur [7], were standardized. The results, horvever, serve as a reference basis for future developments and
will enable, for example, the influence oi individual elements
to be evaluated.
,]cand.
l. Lletallurgy 2
94
G. lv[. Euans and N. Christensen
Conclusions
The susceptibility of the HAZ of C-Mn steels to hydrogenassisted embrittlement is parabolically related to the amount
of non-martensitic constituents present. The static fatigue limit
(o""o) may be represented bY
1.
dsrr,:a+b(l-M)\
2. The static fatigue limit can also be expressed in the form
dsrr,: A-Blog[H]
and -B are parameters related to the microstructure
and [H] is the IIW/IIS total hydrogen content.
3. The carbon content of low-carbon martensite is not a
determining factor affecting embrittlement.
4. It follows that the extent of embrittlement is not directly
where
I
related to the maximum hardness of the heat-affected zone.
Acknorvledgments
Financial support for the investigation was provided by the Royal
Norrvegian Councrl for Industrial and Scientific Research, under
project B 2895. The authors are also indebted to Messrs S. O' Prestmo
and R. Gravem for assistance with the experimental work'
References
l.
Granjon, H., The'implants'method for studying the weldability of
Irigh strcngth steels. Metnl Constr. 1 (1969) 509-515.
2. Evans, G. N{. & Christensen, N., Correlation of weld metal hydrogen
content with HAZ embrittlemenl. Metal Constr.3 (197i) 188-189'
3. Evans, G, M. & Christensen, N', Implant testing-€ffect of steel
composition and hydrogen level. Metal Const. 3 (1971) 263-265'
4. Tentative procedure for the determination of hydrogen in mild and
lorv-alloy steel welci metal (Doc. IIS/ IIW-3 l5-68) . Vt/eld. in the .tvorld 7
(1969) 263-26s.
5. R6nningen, J. A., Simonsen, T. & Christensen' N', On the assessment
of ]JAZ microstructures in C-Mn steels. Scanl. J. Metallurgy 2
(1973) 87-90.
6. Hopkin, C. L., A
suggested cause and a general theory for the cracking
of alloy steels on welding. Trans. Instn lyeld' 7 (1944) 76-78.
7. Hewitt, J. & Murray, J. D., Effect of sulphur on the production and
fabrication of carbon-manganese steel forgings. Brit' lVeld. J. l'5
(r968) l5l-158.
Metallurgy Section of SINTEF
N-7034 Trondheim
Norva;'
Scatd. J. lrletallurgl, 2