The Twip Alloys Resistance in Some Corrosion Reagents

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18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU

THE 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,

T ř . 17.listopadu 15, 708 33 Ostrava – Poruba, Czech Republic,[email protected], [email protected] , [email protected]

Abstract

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 corresponded 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 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.




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




relation to the reached strengths (46 % difference as the Tab. 1 gives) a higher strength value would be

awaited in the TWIP 3 material. Regarding the proportionalities among strength and reached ductility of all

investigated materials those are in agreement with [7].

Table 1 Mean values of detected parameters (SS=sub-surface area, CA=central area)

alloy HV0.2 Gγ Re Rm A[-] [μm]

SS CA CA SS [MPa] [%] TWIP 1 317 301 74 74 429 749 58TWIP 2 269 243 117 125 406 618 65TWIP 3 266 266 13 13 468 904 53

a) b) c)

Fig. 1 a, b, c Micrographs of the a) TWIP 1, b) TWIP 2, c) TWIP 3

Microstructures of all studied alloys are presented in Fig. 1a, b and c (transverse sections). After deformation

the microstructure of all samples showed the FCC matrix with more or less developed deformation twins as

the X-ray measurement confirmed [8]. In all materials the thickness of the deformation twins were measured

using the SEM at magnification of 3000x. The TWIP 1 matrix showed the shorter and thinner deformation

twins of 0.762 μm in thickness (seldom) and/or of 0.950 μm (more frequently), while the maximal strip

thickness corresponded 8.095 μm. In the TWIP 2 matrix the thicknesses of twins were lying in extend of

1.143 - 5.3 μm, resp. 20 μm sporadically. The TWIP 3 matrix showed above mentioned range of 0.566 –

0.941 μm. Those results are quite in agreement with the way of heat treatment and deformation conditions.

The higher deformation and faster cooling is used, the finer grain size is obtained and the finer and shorter

deformation twins are formed [9, 10]. The obtained results confirm this proportionality. Twin boundaries

correspond to special high-angle interfaces representing strong barriers for the propagation of other non-

coplanar deformation twins or for gliding dislocations [11] being also reason of reached different strength

levels of the TWIP 1, 2 and 3 alloys (Tab. 1). A difference in energy per unit fault area between faulted and

perfect structure the SFE expresses. The presented TWIP 1 and 2 alloys show the same calculated SFE

lying on the level of 22.2 mJ.m-2 at 20°C similarly determined like in [12, 13] and 19.2 mJ.m-2 at -20°C

according the concept of the works [13, 14]. In case of the TWIP 3 the SFE was 24.1 mJ.m -2 (at 20°C),

10μm30μm 30μm




respectively 21.1 mJ.m-2 (at -20°C). The differences between the TWIP 1, 2 and the TWIP 3 the chemical

compositions give. The higher Mn-addition of the TWIP 3 alloy makes the SFE higher, while the C-addition

shows much lesser impact [12, 15, 16]. At the temperature of -20°C the SFE exceeds 18 mJ.m-2. By this, it is

expected the austenite stability of investigated materials under given conditions [13, 14]. Unfortunately, the

calculation does not consider deformation conditions. Deformation and consequently the grain size play animportant part.

a) b) c)

Fig. 2a, b, c Micrograph of the corrosion attack of the a) TWIP 1, b) TWIP 2, c) TWIP 3 matrix

After samples immersion in the 3.5 % water NaCl solution those showed even unnoticable corrosion as the

macrographs of the Fig. 2 present. In the immediate vicinity of few observed sulphides and/or oxisulphides

localized point corrosion was detected unlike the 25Mn6Al high manganese alloy presented by Hamada [17]

who observed pit corrosion in minority BCC phase being more prone to pitting in chloride ion environments

than the FCC matrix. Our materials were fully austenitic. The corrosion attack was observed in the grinded

transverse sections (grinded before exposition) and hardly visible corrosion over the total circumferences. It

can be supposed, an oxides immunizing film protected the material. Using the SEM, corrosion positions were

revealed at the boundaries of deformation twins as the Figs. 2a, b, c show. The longitudinal metallographic

samples observation showed the deepest corrosion attack in case of the TWIP 2 maximally reaching 0.12

mm under surface, then of the TWIP 1 maximally detectable 0.09 mm under surface and the shallowest

corrosion trace was found in the TWIP 3 sample maximally reaching 0.05 mm under surface. Reason of

those differences may lies in number of twins. During the same exposition time the chloride ions were evenly

redistributed among the twins of each matrix. The TWIP 2 showed the thickest, longest and the lowest

number of twins unlike the TWIP 3 where numerous of very narrow and short twins were formed during

intensive rolling process.

The hydrogen susceptibility evaluation [6] was without cracks detection. It is truth, the samples dimensions

for 96 hours hydrogen sulphide exposition should be bigger according the NACE Standard [6], however

hereby the corrosion conditions were much stricter, because in smaller material volume more hydrogen




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

[1] SCHUMANN, H.: Wesensgleichheit der γ/ε/α-Umwandlung in kohlenstoffarmen austenitischen Mangan und Chrom-Nickel-Stählen. Neue H ű tte, 17, 1972, Heft 4, pp. 203-210.

[2] FLOREEN, S.: An exhibition of chromium substitution in stainless steels. Metal. Trans. 13A, 1982, pp. 2003-2013.

[3] FROMMEYER, G., BRÜX, U.: Microstructures and mechanical properties of high-strength Fe-M-Al-C light-weight TRIPLEX

steels. Steel Research Int ., 77, 2006, No. 9-10, pp. 624-633.

[4] MAZANCOVÁ E.: New material types for automotive industry–physical engineering properties of high strength material alloyed

with manganese and of metal hydrides alloys for hydrogen storage (in Czech). Monography , 1st. ed. Ostrava, VŠB-TU Ostrava,

2007, 74 p.

[5] KUNZ, M.: Verformungsmechanismen hoch manganlegierter austenitischer TWIP-Stähle. Der Ander Verlag, the 1st ed., 2008,

pp. 97-100.

[6] Evaluation of pipeline and pressure vessel steels for resistance to hydrogen-induced cracking. NACE Standard TM0254-2003,

Item No. 21215 , NACE International, Houston, 2003, 13 p.




[7] UEJI, R. et al.: Tensile properties and twinning behaviour of high manganese austenitic steel with fine-grained structure. Scr.

Mater ., 59, 2008, pp. 963-966.

[8] MAZANCOVÁ, E. et al.: Microstructural analysis of high manganese alloy. Metallurgical Journal , 2009, LXII, No. 6, pp. 61-65.

[9] INAGATI, M.: Role of annealing twins for grain refinement in controlled rolling of low carbon microalloyed steel. Trans. ISI Jap.,

23, 1983, pp. 1059-1067.

[10] UMEMOTO, M., OHTSUKA, H., TAMURA, I.: Transformation of pearlite from work hardened austenite. Trans. ISI Jap., 23,

1983, pp. 775-784.

[11] SEVILLANO, J.G.: An alternative model for the strain hardening of the FCC alloys that twin validated for twinning induced

plasticity steel. Scr. Mater ., 60, 2009, pp. 336-339.

[12] MAZANCOVÁ, E., KURSA, M., MAZANEC, K.: Chosen physical metallurgical parameters of high manganese alloys and their

influence evaluation. Metallurgical Journal , 2008, LXI, No. 7, pp. 71-74.

[13] DUMAY, A. et al.: Influence of addition elements on the stacking fault energy and mechanical properties of an austenitic Fe-Mn-

C steel. Mat. Sci. Eng. A, 483-484, 2008, 483-484, pp. 184-187.

[14] PHIU-ON, K. et al.: Effect of solution treatment and test temperature on tensile properties of high Mn austenitic steels. SteelResearch Int., 80, 2009, No. 1, pp. 29-38.

[15] ALLAIN, S. et al.: Correlations between the calculated stacking fault energy and the plasticity mechanisms on the FE-Mn-C

alloys. Mater. Sci. Eng. A, 2004, 387-389, pp. 307-317.

[16] MAZANCOVÁ, E., MAZANEC, K.: The stacking fault energy evaluation of the TWIP and the TRIPLEX alloys. Metallic Mater., 47,

2009, 6, pp. 1-6 .

[17] HAMADA, A.S.: Acta Univ. of Oulu, 2007, C281, htt://herkules.oulu.fi/issm03553213/.

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The Twip Alloys Resistance in Some Corrosion Reagents