Corrosion Resistance of ASSAB Stavax ESR Stainless Steel by Heat

Materials Transactions, Vol. 54, No. 5 (2013) pp. 833 to 838
© 2013 The Japan Institute of Metals and Materials
EXPRESS REGULAR ARTICLE
Corrosion Resistance of ASSAB Stavax ESR Stainless Steel
by Heat and Cold Treatment
Lee-Long Han1, Chun-Ming Lin1,+ and Yih-Shiun Shih2
1
Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology,
No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, R.O.C.
2
Department of Mechanical Engineering, National Taipei University of Technology,
No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, R.O.C.
This study compared cryogenic treatment, ultra-cryogenic treatment, and high-low temperature tempering treatment using ASSAB Stavax
ESR, and conducted the following analyses of the prepared specimens: (1) analysis the structure by X-ray diffraction, (3) analysis of the texture
of the processed specimens by optical microscopy, (2) using a hardness tester to analyze the changes in hardness of specimens processed by
cryogenic and heat treatment, (3) analyzing the hydrophilicity and hydrophobicity by the water contact angle test, and (4) analyzing corrosion
resistance by the corrosion resistance test. The experimental results showed that cryogenic temperature affects the amount and shape of the
carbides, which are significantly reduced in cases of high-temperature tempering. The water contact angle test analysis showed that the coating
film’s water contact angle performance is the best, followed by specimens of ultra-cryogenic treatment, specimens of cryogenic treatment, and
specimens of traditional heat treatment. It was found that cryogenic treatment can increase polishability, and empower the specimens with good
mold release performance. The coating-film corrosion resistance test showed that ultra-cryogenic treatment and cryogenic treatment can improve
corrosion resistance; however, the performances of specimens by traditional heat treatment were the worst.
[doi:10.2320/matertrans.M2012377]
(Received November 8, 2012; Accepted February 18, 2013; Published April 5, 2013)
Keywords: corrosive current, cryogenic treatment, corrosion resistance
1.
Introduction
In a working environment of high temperature, high
pressure and high stress, the surface of a mold is vulnerable
to the formation of oxides, and adhesion of plastic materials.
The repeated use of the mold in this situation directly affects
the quality of plastic components, leading to a shortened
mold service life because of wear and tear caused by
adhesion on the mold surface. In particular, the injection
molding temperature is in the range of 200­400°C, and some
plastic materials can easily produce acidic materials at high
temperatures. The material feeding tube and mold of the
injection molding machine is challenged by wear and tear at
high temperatures and corrosion.1) Issues regarding increased
dye life, reduced maintenance costs, and labor waste for
downtime maintenance have become important.
In the case of the Martensite stainless steel mold,2) because
of its excellent corrosion resistance and overall hardening
properties, good ductility and tenacity, outstanding abrasion
resistance and polishing performance, and its exceptional
corrosion resistance,3) the optical mold cavity surface remains
as bright as the original after long-term use.
2.
Experimental
2.1 Preparation of specimens
This study used ASSAB Stavax ESR stainless steel to
prepare research specimens, with its elemental composition
as shown in Table 1. The specimens underwent tempering
heat treatment at 250 and 560°C, and the specimens of
cryogenic and ultra-cryogenic treatment were tempered at
250 and 560°C, after being treated at ¹160 and ¹80°C. All
+
Corresponding author, E-mail: [email protected], Graduate
Student, National Taipei University of Technology
Table 1 ASSAB Stavax ESR stainless steel elemental composition
(mass%).
Element
mass%
Fe
84.32
C
0.38
Si
0.9
V
0.3
Cr
13.6
Mn
0.5
specimens were polished to a mirror-level before conducting
experiments. Prior to conducting experiments, we measured
the surface roughness of the polished specimens using an
¡-step profilometer and determined the surface roughness
value to be Ra0.01 µm.
The numbering of the specimens in this study is shown in
Table 2, with the process shown in Figs. 1(a), 1(b) and 1(c).
2.2 Analysis of the light diffraction instrument
This experiment used the XRD-6000, SHIMADZU X-ray
diffraction instrument at a low grazing angle to analyze the
crystal structure and lattice direction of the coating film.
The X-ray light emitting source was the copper target and
the wavelength was ­ = 0.15406 nm, the current was 30 mA,
the power voltage was 40 kV, and the incident angle was
low at 2°, the diffraction angle range was 20­50°, and the
scanning speed was 1°/min.
2.3 Hardness test
Hardness is representative of material resistance to plastic
deformation. This study conducted the Rockwell hardness
test using the instrument typed Mitutoyo ARK-600 to
measure three points of the specimen. The hardness was
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L.-L. Han, C.-M. Lin and Y.-S. Shih
Table 2 Specimens treatment and No.
Specimens No.
A1
A2
Specimen treatment
vacuum hardening + tempering 250°C
vacuum hardening + tempering 560°C
D1
vacuum hardening + ultra cryogenic ¹160°C + tempering 250°C
D2
vacuum hardening + ultra cryogenic ¹160°C + tempering 560°C
D3
vacuum hardening + cryogenic ¹80°C + tempering 250°C
D4
vacuum hardening + cryogenic ¹80°C + tempering 560°C
(a)
Fig. 2
The surface tension analysis when a liquid is on a solid surface.
(b)
is in contact with air, liquid and solids, as shown in Fig. 2.
When the condensed solid­liquid phase is in contact with the
other phases, it produces physical and chemical interactions,
called surface energy or interface energy, which is the energy
that damages surface bonding. This experiment measured
the contact angle of deionized water. When the contact angle
measurement was greater, the material was more hydrophobic; otherwise, the material was more hydrophilic.3)
(c)
Fig. 1 (a) Cryogenic treatment process (D1, D2, D3, D4), (b) heat
treatment process (A1, A2), (c) cryogenic treatment parameter.
represented by indentation depth. The indenter for hardened
steel and other hard materials was a diamond cone with a
vertex angle at 120° and tip radius at 0.2 mm. The hardness
measurements were represented by HRC marks.
2.4 Water contact angle test
The contact angle is the angle between the tangent lie of
the droplet shape and the solid surface. This crossover point
2.5 Corrosion test
This study used polarization corrosion testing to measure
the surface-corrosion resistance of materials processed by
heat treatment and cryogenic treatment. The experiment
conditions were at room temperature, with silver chloride
as the reference electrode, and platinum (Pt) electrode as the
auxiliary electrode. To simulate the marine environment,
the testing solution was 3.5 mass% NaCl, and the main
operating parameters included a scanning rate at 1 mV/s,
an initial potential at ¹0.8 V, a termination potential at
+0.1 V, and a scanning area at 0.785 cm2. The electrochemical corrosion test determined the corrosion potential (Ecorr)
and corrosion current (Icorr) of heat treatment, and used
cryogenic treatment to determine the corrosion resistance
performance.4)
3.
Results and Discussion
3.1 Microstructural observation
ASSAB Stavax ESR is a high-chromium alloy tool of
stainless steel in the same category as martensitic stainless
steel. The experimental specimens were processed by general
heat treatment, ultra-cryogenic treatment, and cryogenic
treatment. In this experiment, the specimens were ground
and polished to a mirror surface; the surface of the specimen
was corroded by picric acid, hydrochloric acid, and alcohol
solution before using an optical microscope (OM) to observe
the surface microstructure. The experimental results showed
Corrosion Resistance of ASSAB Stavax ESR Stainless Steel by Heat and Cold Treatment
(a)
(b)
(c)
(d)
(e)
(f)
835
Fig. 3 Metallographic structures. (a) 250°C tempering, (b) 560°C tempering, (c) cryogenic temperature ¹80°C tempering 250°C,
(d) cryogenic temperature ¹80°C tempering 560°C, (e) ultra-cryogenic temperature ¹160°C tempering 250°C, (f ) ultra-cryogenic
temperature ¹160°C tempering 560°C.
that ultra-cryogenic treatment and cryogenic treatment can
increase the precipitation of carbides, which can increase
wear resistance performance.5,6)
In tempering experiment A1, martensite bulk iron was
tempered, where a few carbides (Cr,Fe)23C6 had precipitated
along the interface, as shown in Fig. 3(a). In A2 tempering,
the main products were tempered martensite bulk iron and
carbides. The amount of carbides (Cr,Fe)23C6 were significantly less, as shown in Fig. 3(b) because its structure is
more delicate and homogenized after cryogenic treatment.
With decreasing cryogenic temperature, it was shown that the
amount of carbides (Cr,Fe)23C6 increases, and the shape tends
to be spherical. After high-temperature tempering, carbides
(Cr,Fe)23C6 tended to concentrate on the boundary in
nucleation to form small indentations or more complex
tree-like arrays. These types of precipitations are harmful to
mechanical properties.7,8) When the cryogenic temperature
decreased, it condensed into a basic structure, as shown in
Figs. 3(c) and 3(d). D1 and D2 were more significant, as
shown in Figs. 3(e) and 3(f ).
3.2 X-ray diffraction analysis
This experiment conducted XRD analysis on the surface of
specimens processed by heat treatment with incident angles
in the range of 15 to 50°. The residual austenite, after low
temperature tempering, had diffraction peaks at the crystalline surfaces (111), (220) and (311). With decreasing
cryogenic temperature, the diffraction peak (111) became
gradually flat; this indicated that the remaining austenite
became significantly less because of cryogenic treatment.
The diffraction peak of crystalline surface (110) was caused
by the chromium, as shown in Fig. 4. No diffraction peak of
the remaining austenite, after high-temperature tempering,
was observed because the remaining austenite has nearly
completely been transformed into tempered Martensite.9,10)
Fig. 4 XRD diffraction analysis diagram. A1 is tempering at 250°C, D3 is
cryogenic treatment ¹80°C tempering at 250°C, D1 is ultra-cryogenic
treatment at ¹160°C tempering 250°C.
3.3 Hardness test
In this experiment, the Rockwell hardness tester was
employed to measure the hardness of the specimens
processed by heat and cryogenic treatment. Each specimen
was measured by obtaining the average value of the
measurements of five points to avoid the regional effects
of the samples. The experimental results are shown in
Table 3. The experimental results show that the hardness
values of specimens D1, D3, A1, D2, D4 and A2 are
respectively 50, 50, 48, 42, 42 and HRC 41. The hardness
of the material processed by the cryogenic treatment has
improved slightly.
3.4 Water contact angle test
This experiment used deionized water droplets to measure
the water contact angle of the specimens to determine the
836
L.-L. Han, C.-M. Lin and Y.-S. Shih
Table 3
Experimental specimens hardness measurements.
Table 4 Specimens water contact angle.
Specimens No.
Average hardness (HRC)
Average hardness (HV)
Specimens No.
Water contact angle
A1
A2
48
41
484
402
A1
A2
80°
74.5°
D1
50
513
D1
88.3°
D2
42
412
D2
86.6°
D3
50
513
D3
83.7°
D4
42
412
D4
82.3°
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 5 Water contact angle test. (a) 250°C tempering, (b) 560°C tempering, (c) ultra-cryogenic temperature ¹160°C tempering 250°C,
(d) ultra-cryogenic temperature ¹160°C tempering 560°C, (e) cryogenic temperature ¹80°C tempering 250°C, (f ) cryogenic
temperature ¹80°C tempering 560°C.
material hydrophobic or hydrophilic performance. A greater
contact angle represents better hydrophobic performance; and
thus, the material mold release performance is improved. The
experimental results are shown in Table 4. Specimens D1 and
D2 can effectively reduce the remaining austenite to obtain
an even martenite structure with large amounts of carbide
precipitations in spherical shapes. Hence, it can better
improve structural homogenization to easily obtain a smooth
surface, and effectively improve the hydrophobic performance, as shown in Figs. 5(c) and 5(d). The structures of
specimens D3 and D4 are relatively coarser, the carbides are
fewer and have a sharper shape, and the hardness is relatively
lower. Hence, its hydrophobic performance is lower than that
of the specimens processed by the ultra-cryogenic treatment,
as shown in Figs. 5(e) and 5(f ). However, for specimens A1
and A2 because of an uneven structure and few carbide
precipitations, the contact angle is at a minimum, as shown in
Figs. 5(a) and 5(b).
3.5 Corrosion test
This experiment used electrochemical testing to explore
the corrosion resistance of materials processed in different
cryogenic and heat treatment temperatures. The corrosion
resistance of the specimens can be determined by the
corrosion current (Icorr). A smaller value of Icorr indicates
better corrosion resistance. The corrosion potential Ecorr is
determined by the rates of the anodic and cathodic reactions
of corrosion. The higher Ecorr indicates the lower corrosion
rate (higher corrosion resistance) when the cathodic reaction
does not change. This experiment shows that, when the
cryogenic temperature is lower, it can improve lattice size,
which renders the structure more delicate and enables it to
obtain better corrosion-resistance performance. The corrosion
resistance is better in cases of low and medium temperature
tempering than in high-temperature tempering, as the carbide
precipitations at low-temperature tempering are fewer. The
corrosion resistance of specimens processed by ultracryogenic and cryogenic treatment are better than that of
the high-temperature tempering primarily because the
chromium and carbon contents of ASSAB Stavax ESR
stainless steel are relatively higher. After the ultra-cryogenic
and the cryogenic treatment, the carbides Cr23C6 precipitate;
the chromium of these carbides is higher than that of the
martenite stainless steel substrates.11) The precipitations
naturally consume the chromium encompassing the boundary, and result in a chromium-insufficient area. After ultracryogenic and cryogenic treatment, the carbide precipitations
are significantly greater than that in general heat treatment.
Corrosion Resistance of ASSAB Stavax ESR Stainless Steel by Heat and Cold Treatment
(a)
837
(b)
Fig. 6 Corrosion resistance test comparison. (a) 250°C tempering, ultra-cryogenic treatment ¹160°C followed by 250°C tempering,
cryogenic treatment ¹80°C followed by 250°C tempering, (b) 560°C tempering, ultra-cryogenic treatment ¹160°C followed by 560°C
tempering, cryogenic treatment ¹80°C followed by 560°C tempering.
Table 5 Polarization measurements of specimens.
Specimens No.
Ecorr (V)
A1
¹0.43
A2
¹0.44
D1
D2
¹0.36
¹0.37
D3
¹0.43
D4
¹0.45
Thus, when the tempering temperature rises, the diffusion of
chromium is faster than the diffusion of carbon. Because the
concentration of the chromium in the solid solution has not
changed although the carbon concentration significantly
reduces because of the formation of carbides, the chromium
concentration in the crystal boundary and the crystal
chromium is homogenized to reduce the tendency of
corrosion, and improve corrosion resistance.12­14) For hightemperature tempering of the general heat treatment, the
carbide precipitations result in chromium-depleted regions
that reduce corrosion resistance performance. For low
temperature tempering, the homogenization of the chromium
can lead to better corrosion-resistance performance, as shown
in Figs. 6(a) and 6(b). The results of the corrosion potential
(Ecorr) and corrosion current (Icorr) were obtained using
CView2 polarization software, as shown in Table 5.
In a neutral solution, the anodic reaction is dissolution of
metals and the cathodic reaction is oxygen reduction.
Figure 6(a) shows the cathodic current at ¹0.6 V is
compared, D1 indicate the smallest. If the anodic current at
¹0.3 V is compared, D3 > A1 > D1 and A2 > D4 > D2.
The larger current means lower corrosion resistance. Table 5
shows the values calculated using the software. The Ecorr and
Icorr of D1 were ¹0.36 V. The results show that, through
cryogenic treatment and low-temperature tempering, the
internal tissue of the material can be made more homogeneous and dense, preventing corrosive media from directly
penetrating the material in a downward manner during
corrosion.
4.
Conclusion
This study used ultra-cryogenic treatment, cryogenic
treatment and general heat treatment to compare the six
combinations of process parameters for STAVAX ESR
stainless steel to determine the optimal parameters for
STAVAX ESR stainless steel. The comparison items included
structure, water contact angle and corrosion resistance.
According to the research findings, the following conclusions
are provided.
(1) The metallographic test showed that cryogenic treatment can aid in carbide precipitations. With decreasing
cryogenic treatment temperature, the amount of carbides increases and particles become smaller. The
structure is more delicate and homogenized with
decreasing cryogenic temperature. With rising tempering temperature, the amount of carbides increases and
condenses in the substrate structure.
(2) The water contact angle test showed that the specimen
with the maximum contact angle is D1 at 88.3°. Ultracryogenic treatment can effectively reduce residual
austenite to obtain a homogenized martenite structure,
which leads to large amounts of carbide precipitations
in spherical shapes that improve the structural homogenizations and smooth the surface to enhance hydrophobic performance. The greater surface contract angle
can render it easier to release the mold and prevent
adhesion friction; thus ensuring size accuracy and
prolonged mold service life.
(3) The corrosion test showed that the carbon concentration
of various specimens significantly reduces because of
the formation of carbides, rendering the crystal
boundary and crystal chromium homogenized, which
improves corrosion resistance and reduces the tendency
of corrosion. For high-temperature tempering, the
precipitation of carbides can result in chromium
depleted areas, which worsens corrosion resistance.
For low temperature tempering, the chromium concentration homogenization can result in better corrosion
resistance performance. Corrosion resistance is related
to carbide precipitation. Ultra-cryogenic treatment can
838
L.-L. Han, C.-M. Lin and Y.-S. Shih
effectively improve the carbide size and enable the
structure to be more delicate, which improves corrosion
resistance capability.
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