Sikalidis C. (ed.) Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

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Reinforcement of Austenitic Manganese Steel with

(TiMo) Carbide Particles Previously Synthesized by SHS

379

it, we expect the stiffness and the fatigue resistance, two characteristic-lacks of the Hadfield

steel, will be increased.

Tensile

strength

(MPa)

Yield

strength(0.2%)

(MPa)

Elongation

(%)

Reduction of

area (%)

Impact

energy

(J)

Conventional

steel

636 371 35 41 162

Reinforced

steel

575 482 16 22 128

Table 5. Mechanical properties of the reinforced steel

3.2.3 Wear resistance

It has been evaluated by means of the pin (of martensitic steel) on disk test on the

conventional and reinforced steel samples, during 30 hours under a normal stress of 90 MPa

(N/mm2) and 150 rpm. The results, in wear rate of the disks, are in the table 6. From them,

we can deduce the wear resistance (under stress) of the reinforced austenitic steel is twice

the conventional.

Hadfield steel Reinforced steel

Wear rate (mm

/N·m) 6.8·10

-8

3.5·10

-8

Table 6. Wear behaviour

4. Conclusions

From the figures 15, 16 and table 2, we can deduce the grey particles of the masteralloy

synthesized by SHS are complex carbides of (TiMo)C type. The reaction synthesis is the sum

of the next sequential (almost infinitesimal in time) partial reactions:

Formation of a 68%Ti-32%Fe intermetallic eutectic and it melting at 1086 º C aprox. due to

heat generated by an electrical resistance; Diffusion of solid C in the Ti-Fe liquid and

formation of TiC (very exothermic reaction with spontaneous generation of heat, rising the

temperature, while Fe remains as binder phase);Replacement, in the TiC cell, of some Ti

atoms by Mo ones, because the atomic radii values are similar (1,40-1,47 Ǻ of the Ti, 1,39-

1,45 Ǻ of the Mo) and at high temperature the combination of C and Mo is very favourable.

Then, TiC+Mo+Fe → Fe(TiMo)C.

From the microstructure (fig. 23 and 24), the spectra (fig. 25, table 4) and the microhardness

values (point 3.2.2.1), we can deduce that the particles inserted within austenite grains of the

reinforced steel are (TiMo)C carbides.

We can thus assure that these carbides are just the same grey particles present in the carbide

of the masteralloy Fe(TiMo)C added to the steel. The small difference in composition is not

representative because the EDS semicuantitative analysis is not exact. Likewise, the small

change in shape (less rounded in the steel) is due to erosive action of the molten steel.

The mechanism of formation of the composite steel+carbides is the next: When the

masteralloy Fe(TiMo)C contacts the liquid steel (iron base), the binder Fe of the former melts

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

380

and adds up to the metallic alloy, while the (TiMo)C particles, separated already from the

binder and wetted by the liquid, remain embedded within it. Due to its high refractoriness

and thermal stability, the carbide particles are not changed much even after the heat

treatment of the solid material.

The polygonal titanium-molybdenum carbides, inserted metallurgically within the very

ductile and tough austenite grains, decrease some mechanical characteristics (elongation,

reduction of area, etc.). However, the impact energy values of the new material are

sufficiently high for the expected service conditions of the typical components made in

Hadfield steel (railway crossings, crushing jaws, hammers), so that, there is no risk of

component fracture if this reinforced steel is used.

Besides, the beneficial effect of the carbides on the properties of the reinforced austenite

such as deformability (expected to be lower, due to lower elongation), stiffness (expected to

be higher, due the same reason as before), fatigue resistance (expected to be higher, due to

higher yield strength), and wear resistance (clearly higher) are very interesting and

promising in order to solve the characteristic shortcomings of the austenitic Hadfield steel.

In summary, the main conclusions of the present experimental work are:

a. The powder metallurgy process SHS allows the manufacture of a masteralloy

Fe(TiMo)C, wettable by molten steel and composed by particles of complex carbide in

an iron binder.

b. The addition of this masteralloy to a molten Hadfield steel bath and the subsequent

solidification and heat treatment of the mix, allows the insertion of the complex carbide

particles (TiMo)C of the masteralloy within the austenite grains of the steel.

c. This insertion produces a reinforcement of the austenitic matrix, with an appreciable

increase in yield strength and wear resistance, but keeps the values of the other

properties, such as elongation and impact energy, within adequate levels.

In this manner, we can contribute to improve the behaviour of a material (figure 1),

optimizing its properties by means of a modification of it microstructure through the

combination of two metallurgical techniques of fabrication, the powder metallurgy (SHS)

and the liquid metallurgy, for casting production.

5. Acknowledgment

This work is the result of more of 10 years of research and experimentation of a team

established with the arriving of the doctor Sergei Vadchenko, an authority in the SHS field,

to Inasmet (now Tecnalia). Any years later, arrived the doctor Ara Sargysan (also an

specialist in powder metallurgy) substituting to the former and the development of the

works increased, achieving very promising results as demonstrate the present chapter. So

that, the author gratefully acknowledge these researchers, the rest people of the team and,

also, the companies have financially supported several projects in this field (Fundiciones del

Estanda, Talleres ZB, JEZ Sistemas Ferroviarios).

6. References

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Roncal, J. (2005). Desarrollo de Aceros Reforzados con Carburos Primarios Vía

Reinforcement of Austenitic Manganese Steel with

(TiMo) Carbide Particles Previously Synthesized by SHS

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Surface Equilibrium Angle for Anisotropic

Grain Growth and Densiﬁcation Model

in Ceramic Materials

Sergio Cava

, Sergio M. Tebcherani

, Sidnei A. Pianaro

Elson Longo

and José A. Varela

Universidade Federal de Pelotas

Universidade Estadual de Ponta Grossa

Universidade Estadual Paulista

Brazil

1. Introduction

Sintering is the main operation in powder technology. This explains the attention of scientists

and practical engineers towards sintering.

Currently there are two main cardinally different approaches to describing sintering: classical

physical sintering theory and phenomenological. The ﬁrst rests on physical constants

at the level of individual particles, and the second is based on continuum equations

for solid (viscous medium) mechanics, and it requires for its implementation presence

of empirical coefﬁcients and it is used for describing macroscopic problems of powder

technologyGalakhov (2009).

Experimentally, the most common way to investigate the relative energy of the grain

boundary consist in establishing a relationship of the surface free energy of the material

with its grains geometryJin et al. (2000); Kinderlehrer et al. (2002); Xin & Wong (2003). This

relationship was ﬁrstly quantiﬁed by HerringHerring (1951) who found out that the surface

free energy is stationary when the balance process is reached. In this way, it is possible

to determine the whole interface energy by observing the geometry of a certain number of

interfaces among crystals of well-known orientation.

Thus, the ﬂow of atoms j

of a system is related to the intrinsic diffusion coefﬁcient D and the

gradient of chemical potential difference between the atom and the vacancy, in accordance

with the Herring equation Herring (1950).

= −

∇

(

− μ

)

(1)

where

represent the relative term of material mobility through the grain boundary or

diffusion by the lattice, k

is the Boltzman constant, T is an absolute temperature, Ω

is the

atomic volume, μ

and μ

are the chemical potentials of atom and vacancies, ∇(μ

− μ

) is

the gradient between the chemical potentials difference that leads to the mass transport.

2 Ceramic Materials

In a general way, Hansen et al. (1992) has obtained a relationship denominated as a ﬂow

general equation:



αγC



(2)

where: α is a proportionality constant that relates the gradient with λ and is just dependent

of the three-dimensional geometric relationship between sources and material drains along

the limit of the grain boundary, being λ the linear average distance of deﬁned diffusion for

the intersection between a pore and the center of the neck among two grains, γ is the surface

energy, G is the average grain diameter, C

and C

are geometric proportionality constants

where K deﬁnes the medium curvature of pore.

In surface techniques, the excess of the grain boundary free energy per surface free energy

(γ

/γ

)Dhalenne et al. (1983); Kingery (1994); Moment & Gordon (1964); Readey & Jech

(1968); Shackelford & Scott (1968); Wolf (1983) is a function of the dihedral angle of

experimental surface measured by the intersection Ψ

giving the equation:

= 2cos

(3)

The determination of materials structures in polycrystals ceramics with high surface area are

being facilited microscopy techniquesMunoz et al. (2004); Saylor et al. (2000); Saylor & Rohrer

(1999). The liquid phase formed during sintering process is found by means of determination

of dihedral angleBelousov (2003; 2004).

Under appropriate experimental conditions, surface dihedral angles, relative grain boundary

energies, and surface diffusivities determined from Atomic Force Microscopic (AFM)

measurements are consistent with data acquired by more laborious techniquesSaylor & Rohrer

(1999).

The densiﬁcation equations for pressure less solid state sintering can be easily extended to

describe the densiﬁcation behavior during hot-pressing or hot-isostatic-pressingBeeman &

Kohlstedt (1993); Shi (1999).

In this paper the use of the AFM technique to determine the dihedral surface angle for

compacts sintered in solid-phase with anisotropic grain growth of 0.5 Mol% MnO

-doped

tin dioxide obtained by the chemical method of the polymeric precursor is described.

2. Experimental procedure

The polymeric precursor solution was prepared by the Pechini method, which has been

used to synthesize polycationic powders Cassia-Santos et al. (2005); Cava, Sequinel, Berger

& Tebcherani (2009); Cava, Sequinel, Tebcherani, Michel, Lazaro & Pianaro (2009); Cava

et al. (2006; 2007); de Lucena et al. (2005); Gonzalez et al. (2004); Pontes et al. (2004); Simoes

et al. (2004); Simoes, Ramirez, Perruci, Riccardi, Longo & Varela (2005); Simoes, Ramirez,

Riccardi, Ries, Longo & Varela (2005); Simoes, Ries, Moura, Riccardi, Longo & Varela

(2005). The process is based on the metallic citrate polymerization using ethylene glycol. A

hydrocarboxylic acid, such as citric acid, is used to chelate cations in an aqueous solution.

The addition of a glycol such as ethylene glycol leads to the formation of an organic ester.

Polymerization, promoted by heating the mixture, results in a homogeneous resin in which

metal ions are uniformly distributed throughout the organic matrix.

384

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

Surface Equilibrium Angle for Anisotropic Grain Growth and Densiﬁcation Model in Ceramic Materials 3

In the experiments, tin chloride dehydrated SnCl

.2H

O (Merck) was ﬁrst dissolved in

distilled water. Subsequently ammonium hydroxide - NH

OH (Synth) was added in order

to form tin hydroxide - Sn(OH)

Besso (n.d.), according to equation 4.

SnCl

+ NH

OH → Sn(OH)

+ NH

Cl (4)

The presence of chloride in the solution was analyzed after decantation by means of the

addition of silver nitrate in the ﬁltrate liquid. Due to the silver chloride formed as a white

solid insoluble in water Baccan et al. (1990) it is possible to predict the absence of Cl

−

within

tolerance limit for the following equation.

(

aq)

+ Cl

−

→ AgCl

(s)

(5)

With the controlled addition of citric acid anhydrous - C

, dissolved at 50

◦

Cfor1h,the

formation of tin citrate took place. The citric acid/metal molar ratio was ﬁxed at 3:1.

Manganese acetate (Carlo Erba) was added at 0.5 mol% to obtain the doped compositions 0.5

Mol% MnO

-doped tin dioxide. The polymerization occurred upon the addition of ethylene

glycol - C

. The mass ratio of the citric acid/ethylene glycol was set at 60:40.

This mixture was then stirred at 80

◦

C for 1h until the solution became completely transparent.

This solution was further heated at 130

◦

C to promote polymerization and remove excess

solvents.

The powder obtained was heat-treated in an oxygen atmosphere at 400

◦

C for 4 hours, in order

to oxidate the remaining organic matter. The powder obtained in this way is referred to as the

“precursor”. In the furnace, the precursor was heat-treated at the 600

◦

Cduring15hours,in

an Al

boat, and then cooled to room temperature.

This obtained powder was compacted by uniaxial pressing (SCHWING SIWA-15T) using

pressure of 15 MPa followed by isostatic pressing (CARLZEISS-JENA) using pressure of 210

MPa forming cylindrical disks of approximately 6.0 mm of diameter and 6.0 mm of height,

where the green density reached 60% of the theoretical density. The sintering process was

performed in a dilatometer (NETZSCH 402E) using a constant heating rate of 2.5

◦

C/min

under atmosphere of synthetic air, reaching the ﬁnal temperature of 1350

◦

C and soak time

ranging from 30-120 minutes.

The d isks of sintered 0.5 Mol% MnO

-doped tin dioxide were observed by Transmission

Electronic Microscopy (TEM) employing a Philips CM200 equipment.

A standard procedure for TEM sample preparation starting from bulk samples which

included cutting, grinding and dimpling was used.

Atomic force microscopy (AFM) was used to obtain an accurate analysis of the sample

surface and the quantiﬁcation of very important parameters such as roughness and average

grain size. A Digital Instruments Multimode Nanoscope IIIa (Santa Barbara, CA) was used.

AFM imaging was carried out in the contact mode, using a triangular-shaped 200-μmlong

cantilever with a spring constant of 0.06 N/m.

3. Results

The curves of linear shrinkage and shrinkage rate are depicted in the Fig. 1. By means of

this analysis it was possible to verify that the 0,5 Mol% manganese doping in tin dioxide is a

sufﬁcient amount to obtain sintered samples with densiﬁcation closer to theoretical densitiy

(25%). The derivative of the curve represents a sintering in solid phase without chemical

substance transformation.

385

Surface Equilibrium Angle for Anisotropic Grain Growth

and Densification Model in Ceramic Materials

4 Ceramic Materials

Fig. 1. Linear shrinkage and shrinkage rate of the sintered 0.5 Mol% MnO

-doped tin dioxide

samples.

The sintered tin dioxide tend to form an equilibrium angle of 120

◦

in the bulk interior due

to the vectorial action of the resultants in agreement with the micrograph of transmission

electronic microscopy of the Fig. 2

Fig. 2. Micrograph of transmission electronic microscopy of sintered sample of 0.5 Mol%

MnO

-doped tin dioxide.

By means of atomic force microscopy (AFM) an accurate analysis of the interaction among the

grains, surface roughness and porosity was obtained, according to Fig 3.

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Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

Surface Equilibrium Angle for Anisotropic Grain Growth and Densiﬁcation Model in Ceramic Materials 5

(a) (b)

Fig. 3. AFM micrographs of the sintered 0.5 Mol% MnO

-doped tin dioxide samples. Soak

time of (a) 30 minutes, (b) 50 minutes, indicating the pore detail.

Data of dihedral angles was obtained by means of AFM using procedures of section analysis,

according to F ig. 4. An imaginary line is inserted under the micrograph by means of a cursor

(indicated by an arrow in the Fig. 4a). The porosity and dihedral angles are determined when

the cursor moves along the line. The dihedral angle was d etermined by extracting and adding,

two adjacent angles were subtracted from 180

◦

(a) (b)

Fig. 4. Determination of dihedral angle and porosity by AFM. a) Micrograph with imaginary

line and lecture cursor represented by an arrow. b) Representation of the cross-section

attained by the cursor.

In this way, the obtained dihedral angles are of statistic signiﬁcance. An average of 300

angles per sample was considered in order to obtain a more precision value (

±2.1

◦

). Thus,

the dihedral angles are changed when the soak time is raised, in accordance with Fig. 5.

387

Surface Equilibrium Angle for Anisotropic Grain Growth

and Densification Model in Ceramic Materials

6 Ceramic Materials

Fig. 5. Angular evolution according to the time required to reach the equilibrium angle.

SnO

.0.5Mol%MnO

. AFM Dihedral angle Thermal treatment. Er=± 1.38. FFT linear =

129.22

◦

With the soak time ranging from 30 up to 120 minutes, the angle is decreased from 145

◦

129

◦

. However, the angle remain practically constant around 129.22

◦

(±1.38

◦

)forthermal

treatment with soak times higher than 60 minutes. In this way, the dihedral equilibrium angle

was determined.

This process is due to the migration of the manganese to the surface of tin dioxide caused

by the higher temperature of thermal treatment. The exudation causes disequilibrium in the

vectorial composition among the interfaces energy. Consequently, the chemical potential of

SnO

grain boundaries is reduced as a attempt to return at the equilibrium.

The equilibrium of the grain growth in the surface is attained through thermal treatment with

soak time higher than 60 minutes. This process is named as equilibrium dihedral angle.

Tab. 1 shows the number of times (fraction) that the dihedrals angles was extracted from

AFM micrographs in function of the values of those angles and, for each thermal treatment.

The angles were counted at each 10

◦

starting from 90

◦

up to 160

◦

. In this way, the frequency

in each angular interval was determined.

Fig. 6 was plotted from results of Tab. 1 by means of Gaussian ﬁtting. In this plot, the angular

fraction appearances (in percentage) is a function of the angular intervals obtained by AFM

micrographs.

A displacement of the plots was made in order to verify the effect of manganese migration into

the SnO

surface. Thus, the maximum point of each plot was moved to a s ame point related

to the angles axis to evaluate the width at half maximum of each plot. Such width is reduced

when the time of thermal attack is increased, in accordance with the Table 2). Consequently,

the manganese exudation is more effective and the SnO

dihedral surface angles became more

uniforms along the whole analyzed surface for samples heat treated with soak times higher

than 60 minutes.

Thus, by means of Fig. 6 it is possible to observe that the manganese is moved to

surface in a homogeneous way related to the time of thermal attack. Furthermore, the

388

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications