Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.1, pp.13-23, 2010
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13
Sensitisation Study of Normalized 316L Stainless Steel
P. Atanda1 , A. Fatudimu1 and O. Oluwole2*
1Materials Science and Engineering Department, Obafemi Awolowo University,Ile-ife,Nigeria.
2Mechanical Engineering Dept, University of Ibadan
*Corresponding author: [e-mail]
ABSTRACT
Austenitic stainless steels with excellent corrosion resistance and good weldability have wide
applications in industry. These iron-based alloys contain a high level of chromium which form
protective oxide film on the surface hence resisting corrosion. The oxide film regenerates when
damaged, making the steel 'stainless'. However, carbide precipitation due to a welding process
or heat treatment can cause the occurrence of chromium-depleted zones at the boundaries,
leading to a phenomenon known as sensitisation, in which the depleted zones become the focus
of the intense corrosion.
The present work was concerned with the study of the sensitization and desensitisation of 316L
steel at the normalizing temperatures of 750- 9500 C and soaking times of 0.5, 1, 2 and 8 hrs.
316L stainless steel was observed to be sensitized when heated to 750- 8500 C and held for short
soaking times of 0.5 – 2hrs before normalizing. Increasing soaking times at these temperatures
to 8 hrs triggered the desensitization process which was fully accomplished at 7500 C but
ongoing at 800 and 8500 C. At 9000 C, sensitization did not occur at 30 mins soaking time but
observed at soaking times of 1 and 2hrs. At a longer soaking time of 8 hrs, there was full
desensitization. At 9500 C, sensitization was already observed at 30 mins. Soaking time and
desensitization was observed to be in progress at 1 and 2 hrs soaking time. By 8 hrs there was
full desensitization. Thus it was observed that at 9500 C, diffusion of Cr was thermally aided
making desensitization fast. The hardness of normalized 316L stainless steel was also observed
to decrease with soaking time and normalization temperature.
Key words: Sensitisation; Normalization treatment; 316L Stainless steel

14 P. Atanda , A. Fatudimu and O. Oluwole Vol.9, No.1
1. INTRODUCTION
The basic 18Cr8Ni (18/8) austenitic stainless steel is so widely used that it accounts for about
50% of all stainless steel production. The stainless character occurs when the concentration of
chromium exceeds about 12wt%, whereas higher chro mium concentrations and the judicious us e
of other solutes such as molybdenum, nickel and nitrogen is then needed to ensure a robust
mater i al1-3 .
The “L” grades are used to provide extra corrosion resistance after welding. The letter L after a
stainless steel type indicates low carbon (as in 316L). The carbon content is kept to 0.03% or less
to avoid grain boundary precipitation of chromium carbide in the critical range (430 to 900oC).
Grain boundary precipitation deprives the steel of the chromium in solution and promotes
corrosion adjacent to grain boundaries. By controlling the amount of carbon, this is minimized4-6.
Chromium is, of course, the primary element for forming the passive film (i.e. high-temperature,
corrosion-resistance chromium oxide). Other elements can influence the effectiveness of
chromium in forming or maintaining the film, but no other element can, by itself, create the
stainless steel.
Nickel in sufficient quantities, is used to stabilize the austenitic phase and to produce austenitic
stainless steels3. A corrosion benefit is obtained as well, especially in reducing environments.
Nickel is particularly useful in promoting increased resistance to mineral acids. When nickel is
increased to about 8 to 10% this is a level required to ensure austenitic structures in a stainless
steel that has about 18% chromium.
Molybdenum in moderate amounts in combination with chromium is very effective in terms of
stabilizing the passive film in the presence of chlorides.
Molybdenum is especially effective in enhancing the resistance to pitting and crevi c e cor rosion4.
Nitrogen is beneficial to austenitic stainless in that it enhances pitting resistance, retards
formation of sigma phase. The change in the microstructure of an alloy can be achieved by heat
treatment.
2. MATERIALS AND METHOD
2.1 Materials
316L stainless steel was obtained from the market for the purpose of this experimentation.
2.2 Methods

Vol.9, No.1 Sensitisation Study of Normalized 316L Stainless Steel 15
2.2.1 Heat treatment
Normalization heat treatments were performed on the samples by varying the
temperature and the soaking period. The following temperatures were used: 750, 800, 850, 900
and 950oC. The different soaking times at these temperatures were 30 mins, 1 hour, 2 hours and
8 hours.
2.2.2 Mounting and grinding
First, each specimen was mounted on thermosetting plastic for ease of grinding and polishing.
The grinding of the specimens was done on silicon carbide paper7-12. The surface of the papers
was flushed by a current of water, which serves not only as a lubricant in grinding, but also
carries away coarse emery particles, which might oth erwise scratch the surface of the specimen.
The silicon carbide papers used in achieving the proper grinding of the specimen were in the
following grades 120, 240, 320 and 400 grits, grinding respectively in that order of grades.
2.2.3 Polishing
The polishing was done on polishing cloth using alumina powder dissolved in water at a
reasonable proportion. The alumina powder was of the grade 1 micron and 0.3 micron
respectively. Light pressure was applied until the surfaces were free of scratches7-12. The samples
were cleaned, dr ied and then examined und er the microscope, using a magnification between 50
and 100 so as to check whether the samples were free of scratches.
2.2.4 Etching
After polishing, the samples were etched with 5gm of FeCl3 + 10ml of HCl + 50ml of H2O7-12.
To etch these specimens, they were washed free of any adhering polishing compound and
plunged into the etching solution, agitated vigorously for 3 minutes. The specimens were then
very quickly transferred to running water, in order to wash away the etchant as rapidly as
possible. It was then exa mined with the naked eye, to see to w hat extent et ching has taken place.
The successfully etched surf ace appeared dull.
After this, the specimens were observed under optical microscope and photomicrographs taken.
3. RESULTS AND DISCUSSIONS
3.1 Results
The photomicrographs of the normalized specimens are shown in Figs. 1 – 5. Figs 1a-d show the
photomicrographs of the normalized specimens from 7500 C each held at the soaking

16 P. Atanda , A. Fatudimu and O. Oluwole Vol.9, No.1
temperature for 30 mins, 1hr, 2hrs and 8hrs respectively. Chromium depleted zones could be
seen here but negligible in the sample soaked fo r 8hrs.
(a) (b)
(c) (d)
Figures 1. (a) Sample heated to 750 oC held for 30mins, (b) Sample heated to 750 oC held for 1hr,
(c) Sample heated to 750 oC held for 2hrs (d) Sample heated to 750 oC held for 8hrs.

Vol.9, No.1 Sensitisation Study of Normalized 316L Stainless Steel 17
Figs 2a-d show the photomicrographs of the normalized specimens from 8000 C each held at the
soaking temperature for 30 mins, 1hr, 2hrs and 8hrs respectively. Chromium depleted zones
could also be seen here and apparently very slightly manifested as well i n the sample soaked for
8hrs.
(a) (b)
(c) (d)
Figures 2. (a) Sample heateed to 800 oC held for 30mins, (b) Sample heated to 800 oC held for
1hr, (c) Sample heated to 800 oC held fo r 2hrs (d) Sample h e ated to 800 oC held for 8hrs.

18 P. Atanda , A. Fatudimu and O. Oluwole Vol.9, No.1
Figs 3a-d show the photomicrographs of the normalized specimens from 8500 C each held at the
soaking temperature for 30 mins, 1hr, 2hrs and 8hrs respectively. Chromium depleted zones
could be seen here as well but also increasingly well formed in the sample soaked for 8hrs.
(a) (b)
(c) (d)
Figures 3. (a) Sample heated to 850 oC held for 30mins, (b) Sample heated to 850 oC held for 1hr,
(c) Sample heated to 850 oC held for 2hrs (d) Sample heated to 850 oC held for 8hrs.

Vol.9, No.1 Sensitisation Study of Normalized 316L Stainless Steel 19
Figs 4a-d show the photomicrographs of the normalized specimens from 9000 C each held at the
soaking temperature for 30 mins, 1hr, 2hrs and 8hrs respectively. Chromium depleted zones
could be seen only in samples 4b and 4c but absent in the sample soaked for 8hrs and 30 mins.
(a) (b)
(c) (d)
Figures 4. (a) Sample heated to 900 oC held for 30mins, (b) Sample heated to 900 oC held for 1hr,
(c) Sample heated to 900 oC held for 2hrs (d) Sample heated to 900 oC held for 8hrs.

20 P. Atanda , A. Fatudimu and O. Oluwole Vol.9, No.1
Figs 5a-d show the photomicrographs of the normalized specimens from 9500 C each held at the
soaking temperature for 30 mins, 1hr, 2hrs and 8hrs respectively. Chromium depleted zones are
negligible in all the specimens here and could be seen to be observed faintly only in 4b and c
which correspond to short soaking times of 1and 2 hrs respectively.
(a) (b)
(c) (d)
Figures 5. (a) Sample heated to 950 oC held for 30mins, (b) Sample heated to 950 oC held for 1hr,
(c) Sample heated to 950 oC held for 2hrs (d) Sample heated to 950 oC held for 8hrs.
Figure 6 shows the micrograph of the untr eated sample.

Vol.9, No.1 Sensitisation Study of Normalized 316L Stainless Steel 21
Figure 6. Untreated Sample.
The photomicrostructures of these heat treated austenitic steel samples have been show a clear
cut difference between the treatment using long time (8 hours) and short times (0.5hr, 1hr and
2hrs). The chromium depleted zones increased considerably up to the 2 hours treatment but
reduced on the samples soaked for 8 hrs. The Rockwell hardness of some of the samples is
presented in Table 1 and Fig. 7.
Table 1. Brinnel Hardness of some Samples.
S/N SAMPLES HARDNESS, HB
1 Untreated Sample 187
2 750 oC held for 30mins 169
3 750 oC held for 1hr 167
4 750 oC held for 2hrs 167
5 750 oC held for 8hrs 166
6 800 oC held for 30mins 164
7 800 oC held for 1hr 163
8 800 oC held for 2hrs 163
9 800 oC held for 8hrs 161
10 850 oC held for 30mins 156
11 850 oC held for 1hr 152
12 850 oC held for 2hrs 149
13 850 oC held for 8hrs 143
14 900 oC held for 30mins 131
15 900 oC held for 1hr 115
16 900 oC held for 2hrs 111
17 900 oC held for 8hrs 110
18 950 oC held for 30mins 109
19 950 oC held for 1hr 107
20 950 oC held for 2hrs 99

22 P. Atanda , A. Fatudimu and O. Oluwole Vol.9, No.1
Fig. 7. Comparative Brinnel Hardness chart of samples in Table1.
3.2 Discussions
From the microstructures observed in Figs. 1a-c, 2a-c, 3a-c, 4a-c and 5a-c, the black patches
form the depleted – zone. It could be observed that the width of the depleted zone increased with
time, leading to a flatter chromium profile near the grain boundaries at the soaking temperatures
of 750-8500 C and for short soaking times of 30 mins to 2 hrs. This heat treatment procedure had
significant effect on the chromium concentration profile. The process of short time heat
treatment brought about sensitization (the breakdown in corrosion resistance which occurs when
austenitic st ainless st eels are r eheated in the te mperature r ange 550 oC to 850 oC). Sensitisation is
associated with the preci pitat ion of the chr o mium- rich carbide su ch as Cr23C6 or Cr7C3 along the
grain boundaries. During the carbide precipitation, interstitial carbon diffuses rapidly to the grain
boundaries. However, chromium diffusion which is much more slower, resulted in the chro mium
– depleted zones at the grain boundaries. In this state the steel is susceptible to intergranular
corrosion.
Figs 4 and 5 show a self – healing processs (desensitization) already taken place, refering to the
return of corrosion resistance of the stainless steels after prolonged heat treatment in the
temperature range which initially caused sensitization. Self – healing began when the chromium
content at the carbide –matrix interface increased due to chromium diffusion from the matrix
further away from the grain boundary . Thus, prolonged soaking at 900 and 9500 C did not have a
deleterious effect on 316L stainless steel. The hardness of the samples was also observed to
decrease with increase in temperature (Fig. 7).

Vol.9, No.1 Sensitisation Study of Normalized 316L Stainless Steel 23
4. CONCLUSION
316L stainless steel was observed to go into sensitization when heated to 750- 8500 C and held
for short soaking times of 0.5 – 2 hrs before normalizing. Incresing the soaking times at this
temperatures triggered desensitization process which was fully accomplished at 7500 C but
ongoing at 800 and 8500 C. At 9000 C, sensitization was observed at soaking times of 1 and 2 hrs
before normalizing. At a longer soaking time of 8 hrs before normalization, there was full
desensitization. At 9500 C, sensitization was already observed at 30 mins soaking time and
desensitization was observed to be in progress at 1 and 2 hrs soaking time. By 8 hrs there was
full desensitization of 316L stainless steel. Thus it was observed that at 9500 C, the diffusion of
Cr was thermally aided making it very fast and initial sensitisation was cancelled out. The
hardness of normalized 316L stainless steel was also observed to decrease with soaking time and
normalization temperature.
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