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Influence of the heat treatment on the corrosion resistance of the
martensitic stainless steel type AISI 420
Article in Journal of Materials Science Letters · August 2003
DOI: 10.1023/A:1025179128333
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J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 2, 2 0 0 3, 1151 – 1153
Influence of the heat treatment on the corrosion resistance
of the martensitic stainless steel type AISI 420
A . F . C A N D E L ÁR I A , C . E . P I N E D O
ˆ
Technological Research Centre, University of Mogi das Cruzes, Av. Candido
Xavier de Almeida Souza 200,
ZIP 08780-0911, Mogi das Cruzes, SP, Brazil
E-mail: pinedo@umc.br
The martensitic stainless steel type AISI 420 is widely
used for applications like cutlery, plastic molds, structural parts and medical devices [1]. This grade of
steel is particularly important, because it is suitable
to hardening after heat treatment such as quenching
and tempering. After hardening it is possible to combine high strength, toughness and corrosion resistance.
Regarding corrosion resistance it is well known that a
minimum of 11% of chromium is necessary to attain
corrosion resistance by the formation of the native protective oxide film [2], and for the martensitic grade the
chromium must be dissolved into the matrix. Therefore, the corrosion resistance of martensitic stainless
steels grade is sensitive to the carbide volume fraction
dissolved on matrix after austenitizing for quenching
and is close related to the carbide precipitation during
tempering [3, 4]. Under such considerations, the heat
treatment is an important processing step to control the
corrosion resistance of this steel.
Taking into account the importance on combining
high strength and corrosion resistance, the present work
present results concerning a detailed study on the influence of the hardening and tempering heat treatment
cycles on the corrosion resistance of the martensitic
stainless steel type AISI 420. The material used as
reference was received as annealed bar with a ferritic
matrix containing M23 C6 chromium carbides with a
homogeneous dispersion, as expected from the phase
equilibrium [5]. The annealed state is considered here as
reference for the highest volume fraction of chromium
carbides. To study the corrosion resistance, the
secondary M23 C6 carbide fraction was varied, by dissolution, using oil quenching from austenitizing temperatures ranging from 900 ◦ C to 1100 ◦ C, for 1 h. Additionally tempering treatments were performed to study the
influence of the carbide precipitation. The heat treatment response was evaluated by Rockwell C hardness.
The corrosion resistance was evaluated by mass loss,
by unit area, using a 0.5 M H2 SO4 solution at room
temperature. These tests were carried out between 10
to 180 min, and after the experiments the corroded
surfaces were examined at a stereomicroscope.
The influence of the austenitizing temperature on
hardness after quenching is shown on Fig. 1. The
hardness increases when the temperature raises up to
1050 ◦ C and lowers for 1100 ◦ C. The hardness increase
is a consequence of the M23 C6 carbide dissolution that
increases the carbon supersaturation and the lattice
C 2003 Kluwer Academic Publishers
0261–8028 distortion of the martensite [6]. The retained austenite fraction at 1100 ◦ C is high enough to decrease the
as quenched hardness [7].
Fig. 2 shows that the corrosion resistance is strongly
influenced by the austenitizing temperature, and, therefore, by the carbide volume fraction. There is a decrease of the corrosion resistance with the increase of
the austenitizing temperature up to 1075 ◦ C for the temperature of 1100 ◦ C the corrosion resistance increase.
Considering that the corrosion resistance should enhance with the increase of the chromium content dissolved into the ferritic matrix, this is an unexpected
result. Therefore, there must be another mechanism
controlling the corrosion resistance that superimposes
the beneficial aspect of the carbide dissolution. This
behavior must be explained as a consequence of the
increase of the internal martensite lattice stresses [8]
Figure 1 Influence of austenitizing temperature on the hardness after
oil quenching.
Figure 2 Influence of the austenitizing temperature on corrosion
resistance.
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Figure 3 Macrography of selected corroded samples.
promoted by the increase of the carbon saturation
when the austenitizing temperature is raised. The decrease of mass loss measured at 1100 ◦ C confirms
the former proposed mechanism. As the volume fraction of retained austenite increases the internal stresses
decrease promoting a beneficial influence on corrosion, sensitive only for the austenitizing temperature
of 1100 ◦ C.
Compared to the reference annealed state, the corrosion resistance is better only for austenitizing temperatures up to 1025 ◦ C. In this range of temperature,
the beneficial effect of the carbide dissolution, and
chromium enrichment of the matrix, is higher than
the deleterious effect of the internal lattice stresses.
For higher temperatures the internal stresses play the
most important role on the corrosion resistance control.
Fig. 3 shows the corrosion surfaces at different austenitizing temperatures, compared to the annealed state.
The material corrodes by localized attack forming small
pits which density varies according to the austenitizing
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temperature. The lower pit density occurs after quenching from 900 ◦ C, and increase with the increase of the
austenitizing temperature.
To confirm the corrosion mechanism proposed, assisted by the internal stresses, the material was submitted to tempering treatments, at 200 ◦ C and 500 ◦ C after
quenching from 1100 ◦ C. Fig. 4 shows that the corrosion resistance is restored after tempering, reaching
values smaller than the standard annealed state. The
smaller corrosion rate is attained after tempering at
200 ◦ C for 2 h; as a consequence of the carbide precipitation that promotes a stress relieve effect on the
martensite lattice. The same effect must occur at 200 ◦ C
for 48 h and 500 ◦ C for 4 h, but in this case the excess of carbide precipitation on tempering impairs the
corrosion resistance.
From the present work it is possible to conclude that
not only the chromium content dissolved into austenite
is important for corrosion resistance. Metallurgical factors such as internal lattice stresses, developed during
dissolution, internal stress level, and further carbide
precipitation on tempering.
References
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4.
Figure 4 Influence of the tempering treatment on the corrosion
resistance of a quenched sample.
5.
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the martensite transformation, play an important role on
the pitting corrosion mechanism. Tempering is useful to
reduce the stresses and to control the corrosion rate by
appropriate temperature and time selection. Heat treatment cycle must combine secondary M23 C6 carbide
8.
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“Heat Treater’s Guide: Standard Practices and Procedures for Steel,”
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J . E . T R U M A N , British Corr. J. 11 (1976) 92.
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Received 22 November 2002
and accepted 16 April 2003
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