18. ‐ 20.5.2010,RoznovpodRadhostem, CzechRepublic, EUTHE TWIP ALLOYS RESISTANCE IN SOME CORROSION REAGENTS Eva MAZANCOVÁ a , Petr KOZELSKÝ a , Ivo SCHINDLER a a VŠB-TU OSTRAVA, FACULTY OF METALLURGY AND MATERIAL ENGINEERING, Tř . 17.listopadu 15, 708 33 Ostrava – Poruba, Czech Republic, [email protected], [email protected], [email protected]Ab st rac t The work deals with corrosion resistance of two high Mn alloys. After cast of the Fe22.3Mn0.5C alloy heat treated at 1200°C/4h/water and forging with total deformation of 25 % were carried out. This represents the TWIP 1 alloy. The TWIP 2 alloy is the TWIP 1 re-heated at 1150°C/3h/air. Final bars corresponde d to 12 mm in diameter. Further the Fe23.1Mn0.73C alloy was laboratory rolled with the final strip thickness of 3 mm representing the TWIP 3 alloy. Exposition of all variants in the 3.5 % water NaCl solution showed corrosion on the boundaries of the twins first of all. The TWIP 3 demonstrated higher corrosion resistance. The worst corrosion attack showed the TWIP 2, where the corrosion traces reached 0.12 mm under surface. After the TWIP 3 alloy exposition in corrosion solution with bubbled hydrogen sulphide (the HIC test), no hydrogen cracks were revealed. The results were confronted with austenite grain sizes, formed twins and mechanical properties. Keywords: TWIP alloys, heat treatment, rolling, corrosion resistance 1. INTRODUCTION Some top austenite steels may show high corrosion resistance such as the Ni-Cr stainless steels. Chromium imparts corrosion resistance and Ni secures high plasticity. Both cardinal elements are expensive due to their strategic importance. The both mentioned elements can be substituted by cheaper Mn and Al in materials where not so extreme corrosion resistance is demanded [1, 2]. It is known both Ni and Mn support austenite area in the iron-carbon equilibrium diagram and are able to stabilize the FCC matrix. The FCC matrix generally shows high plasticity level being connected with more difficult cleavage crack propagation. In comparison with the Fe or Ni, the Mn decreases the matrix density which is attractive e.g. in automotive industry as well as the corrosion resistance, especially in case of the TWIP or TRIPLEX alloys [3, 4]. To the corrosion resistance of the above mentioned alloys special attention has not been practically paid. Since the high Mn alloys are promising materials for automotive industry or for gas transport [3, 5]that is reason why the presented work is focused on corrosion resistance of high manganese alloy of the FeMnC type being treated in three ways and exposed in two corrosion mediums. 2. EXPERIMENTAL MATERIAL AND TECHNIQUE For corrosion investigation one high Mn alloy (Fe22.3Mn0.5C) was used. For laboratory heat production a material of high purity was chosen. Vacuum melting under Ar atmosphere was realized using plasma with followed cast into four bars of 16 mm in diameter. After homogenizing annealin g at the 1200°C/4 h/water the reheated bars (1050°C/10´) were forged into the diameter of 12 mm in 2 steps with the aim to balance the temperature. In each deformation stage the diameter reduction by 2 mm (12.5 %) was carried out. For the forming a multi-stage die was used. One bar was marked as the TWIP 1. The other bar was reheated at 1150°C/3h /air and marked as the TWIP 2.
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8/10/2019 The Twip Alloys Resistance in Some Corrosion Reagents
Some top austenite steels may show high corrosion resistance such as the Ni-Cr stainless steels. Chromium
imparts corrosion resistance and Ni secures high plasticity. Both cardinal elements are expensive due totheir strategic importance. The both mentioned elements can be substituted by cheaper Mn and Al in
materials where not so extreme corrosion resistance is demanded [1, 2]. It is known both Ni and Mn support
austenite area in the iron-carbon equilibrium diagram and are able to stabilize the FCC matrix. The FCC
matrix generally shows high plasticity level being connected with more difficult cleavage crack propagation.
In comparison with the Fe or Ni, the Mn decreases the matrix density which is attractive e.g. in automotive
industry as well as the corrosion resistance, especially in case of the TWIP or TRIPLEX alloys [3, 4]. To the
corrosion resistance of the above mentioned alloys special attention has not been practically paid.
Since the high Mn alloys are promising materials for automotive industry or for gas transport [3, 5] that is
reason why the presented work is focused on corrosion resistance of high manganese alloy of the FeMnCtype being treated in three ways and exposed in two corrosion mediums.
2. EXPERIMENTAL MATERIAL AND TECHNIQUE
For corrosion investigation one high Mn alloy (Fe22.3Mn0.5C) was used. For laboratory heat production a
material of high purity was chosen. Vacuum melting under Ar atmosphere was realized using plasma with
followed cast into four bars of 16 mm in diameter. After homogenizing annealing at the 1200°C/4 h/water the
reheated bars (1050°C/10´) were forged into the diameter of 12 mm in 2 steps with the aim to balance the
temperature. In each deformation stage the diameter reduction by 2 mm (12.5 %) was carried out. For the
forming a multi-stage die was used. One bar was marked as the TWIP 1. The other bar was reheated at1150°C/3h/air and marked as the TWIP 2.
8/10/2019 The Twip Alloys Resistance in Some Corrosion Reagents
18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU
Another Fe23.1Mn0.73C alloy using plasma metallurgy and Ar of high cleanness was prepared. The 1 kg
ingot was cut up and re-melted in radio frequency induction vacuum furnace and cast to cold graphite ingot
mould 32x13 mm in cross section and of 300 mm in length. The annealing at the 1125°C/3h/water followed.
Afterwards the rolling process was realized. The aim of the 1st rolling was a thickness decreasing of the cast
ingot. Firstly, the material was reheated at 1100°C/20´ for an even warming-through and consequently rolled.After the second reduction the thickness corresponded to 8 mm. After the 1st rolling, material was cut in four
sections and subsequently was again reheated at the temperature of 1000°C and rolled by one reduction.
The thickness of the strip was 5.5-6 mm. Then, after reheating at the 1000°C/1.5´ material was again rolled
by two reductions. The final thickness corresponded to 3 mm and the total deformation to 65.1 %. Using
pyrometer the samples temperatures were measured. The rolling process was realized by use of the
laboratory rolling mill of the four-high mill type. The work rolls diameter corresponded to 67 mm reaching 100
revs per minute. The rolled strips were marked as the TWIP 3. Further, at 20°C and -20°C stacking fault
energies were calculated [5-7].
After expositions in two reagents, metallographic investigation and mechanical parameters determinationfollowed. Tensile tests were carried out (at 23°C) using the INOVA tensile test machine (maximal loading of
50 kN and the crosspiece rate in the range of 0.001-500 mm.min-1). In case of the TWIP 3 one
metallographic specimen was also taken. From the bars of the TWIP 1 and 2 two samples were taken in
longitudinal direction with gauge length of 15 mm. One sample of each variant also served for metallographic
analysis by use of the Olympus light microscope X70 as well as the electron microscope SEM JEOL JSM-
6490 LV equipped with the energy dispersion analyser OXFORD INCA Energy 350. The micro-hardness
completed the analysis.
One sample of the TWIP 1, 2 and 3 was used for corrosion test in the 3.5 % water NaCl solution. Four
transverse strips of the TWIP 3 were cut off. The gauge length of one was 33 mm and width of 10 mm and
served for the same corrosion test in the 3.5 % NaCl solution as it was mentioned above. The other three
strips dimensions corresponded to 33x20 mm with the thickness of 3 mm after rolling and were used for the
corrosion test in the H2S. Surfaces of all samples were cleaned in alcohol. Those were exposed in 3.5 %
water NaCl solution for 168 hours. Materials were quite dipped into corrosion solution (23°C) lengthways
lying on thick glass rods of 3 mm on diameter. The test solution of the other corrosion exposition consisted
of 5 % NaCl, 0.5 % CH3COOH in distilled water. The procedure of the experiments was in agreement with
the NACE Standard TM0284-2003, Item No. 21215 for the HIC resistance [6]. The start and finishing
solution pH corresponded to 2.71 and 3.79. The test temperature was kept at the 23°C. The hydrogen
embrittlement was evaluated in agreement with the [6]. Metallographically prepared samples were etched in
glycerine, hydrofluoric-, nitric- and hydrochloric acid.
3. RESULTS AND ANALYSIS
The corrosions tests metallographic and mechanical parameters determination of all samples preceded Tab.
1. Samples of the TWIP 1 showed relatively uniform grain size in all cross-sections. The same could be told
about the TWIP 3 samples since the thickness of the 3 mm displayed none differences between central area
(CA) and the sub-surface area (SS) as the Tab. 1 shows. The 68 % austenite grain size (G γ) difference was
detected between the TWIP 1 and the TWIP 2 samples. This can be ascribed to the TWIP 2 material
reheating at 1150°C. The strength also went down in comparison with the TWIP 1 (Tab. 1). Regarding the
rolled TWIP 3 material, its Gγ was markedly finer than the grain size of the TWIP 2 alloy. However, it is
surprising the micro-hardness of the both last mentioned materials maximally showed 9.5 % difference. In
8/10/2019 The Twip Alloys Resistance in Some Corrosion Reagents
18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU
atoms could be caught in traps during the exposition. The rolled material showed small grain sizes and a lot
of shorter and very narrow deformation twins. The grains and twins boundaries act as numerous hydrogen
traps and the hydrogen resistant is strongly dependent on equally distributed potential hydrogen traps for
hydrogen catching above all. Moreover, the boundaries of deformation twins represent high-angle interface
for crack propagation [11]. It means each boundary performs hindrance leading to a crack deviation andbeing simultaneously attending by a lost of kinetic energy for cleavage crack propagation.
4. CONCLUSIONS
Three TWIP alloys variants marked as TWIP 1, 2 and 3 were investigated. The first material was heat
treated at the 1200°C/4h/water after cast and consequently forged with total deformation of 25 %. The
second alloy represented the TWIP 1 heat treated at 1125°C/3h/air. The TWIP 3 was annealed at
1150°C/water after cast and laboratory rolled at 1000°C. Final thickness was 3 mm and the total deformation
corresponded to 65.1%.
Metallographic evaluation showed the FCC matrix with deformation twins. All materials differ in austenitegrain size (for central area 74, 125 and 13 μm in sequence TWIP 1, 2 and 3), consequently in number,
length and thickness (the narrowest thickness of 0.762, 1.143 and of 0.566 μm again in above mentioned
sequence) of deformation twins in dependence on temperature and faster or slower cooling process.
Strengths were in proportionality with ductility. The lowest strength of 618 MPa connected with 65 % ductility
was found for the TWIP 2, the highest strength of 904 MPa and of 35 % ductility in the TWIP 3 case.
Corrosion influence of the 3.5 % NaCl water solution was the most evident in case of the TWIP 2 (going 0.12
mm under surface) and the least corrosion attack was observed in the TWIP 3 alloy (maximal affected deep
of 0.05 mm). Corrosion was observed in deformation twins interfaces first of all. In the TWIP 3 alloy tested in
the HIC solution none crack were revealed. The fine grains size and the boundaries of deformation twins
acting as numerous hydrogen traps are the reason. The high-angle interfaces of the deformation twins
influence both the formation and the cleavage crack propagation.
ACKNOWLEDGEMENT
Authors acknowledge the Min istry of Education, Youth and Sports of Czech Republic for financial
support of pro ject MSM6198910015 and the RMTVC project CZ.1.05/2.1.00/01.0040
LITERATURE
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