Aaron Den Boer - 701 Seminar - November 20 2012 - Course Notes

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Aaron Den Boer - 701 Seminar - November 20 2012 - Course Notes

Characterization and

Dissolution of Ferroniobium

Aaron Den Boer

Supervisor: Dmitri Malakhov

McMaster University

November 20, 2012


Outline

‣ Introduction

‣ Problem Definition

‣ Completed Work – Characterization of FeNb

‣ Current Work – Modelling Solid Nb Dissolution in Liquid Fe

‣ Future Work – Validation of Binary Model

2


Introduction

• Microalloying produces steels (called High Strength Low Alloy

steels) with superior toughness, corrosion resistance, and increased

strength, as compared to some other steels

• Microalloying with elements such as Nb, Ti, and V improve these

properties through grain refinement, and solid solution and

precipitation hardening

3


Introduction

• Evolution of microalloyed HSLA steels has led to the development

of ferroalloy additions in steelmaking

• Microalloying is performed by adding ferroalloys to the liquid steel

during ladle metallurgy FeNb, FeTi, FeV

• One aim of ladle metallurgy is alloying to adjust for target chemical

analysis; the ladle temperature is around 1600°C

• Once these ferroalloys have assimilated into the melt, casting will

result in the formation of fine precipitates at late stages of

solidification

http://www.danieli.com

Porter, Repas, 1982

4


Introduction

• How do these fine precipitates form during solidification

i. “Homogenous” liquid melt

ii.

Phases with high melting

temperatures (i.e. Nb-C,

Ti-N, Ti-C-S) precipitate out

Fe – 0.05 Nb – 0.1 C

(Nb,Ti) (C,N)

Jun, Kang, Park, 2003

5


Ferroniobium Manufacturing

• A mixture containing alumina, sodium aluminate, iron and niobium

is heated up to 2200°C and held for several minutes

• The melt is completely liquid and cast into a mold or poured on a

flat surface where it solidifies.

• Since freezing of several tonnes of FeNb is characterized by a low

cooling rate, solidification is assumed to proceed according to the

Gulliver-Scheil formalism

• Thermo-Calc can be used to simulate the solidification behaviour

and predict the phase portrait of the Fe-Nb system

Sousa, 2002

6


Fe–Nb Phase Diagrams

TCFE2

TCFE6

Eutectic

Distectic

C14

µ

C14

µ

http://cst-www.nrl.navy.mil/lattice/index.html

Laves C14 → (Fe,Nb) 2 (Fe,Nb) 1

µ → (Fe,Nb) 7 (Nb) 2 (Fe,Nb) 4

7


Fe–Nb Phase Diagram TCFE2

2

T STEEL

1

BCC

2

µ

1

Voss, Palm, Stein, Raabe, 2011

8


Problem Definition

• Coarse Nb(C,N) and (Nb,Ti)(C,N) particles were observed on the

fracture surface of bend test specimens

• These coarse particles are detrimental to the mechanical properties

of HSLA steels

• Particle size: >10µm

Abraham, Klein, Bodnar, Dremailova, 2006

9


Problem Definition

• Abraham et al proposed that the coarse particles were inherited

from multiphase ferroalloys

i. Phases with high melting temperatures are released into melt

via disintegration of ferroalloy

ii. Finite time → non-homogeneous melt

µ Phase

BCC Phase

Abraham, Klein, Bodnar, Dremailova, 2006

10


Characterization of FeNb + 0.12 wt% C

ICP Analysis

&

Carbon/Sulphur Analysis

Element

wt%

Nb 68.14

Fe 29.38

C 0.12

(Ti, Cu, Al, Mn, P, Si) 2.37

Total 100.00

11


Nb–C Phase Diagram

FCC

HCP

M 2 C carbides, HCP structure

(Nb, Fe,…) 2 (C,N,Va) 1

MC carbides, FCC structure

(Nb, Fe,…) 1 (C, N, Va) 1

12


Characterization of FeNb + 0.12 wt% C

5

1

2

4

3

Location

Elements (at%)

Predicted

Fe Nb Ti Al Si Mn Phase

1 7.60 91.92 0.48 0.00 0.00 0.00 BCC

2 40.93 52.91 0.40 0.89 4.45 0.42 µ

3 35.34 62.16 0.89 0.48 0.83 0.31 η

4 1.92 96.94 1.14 0.00 0.00 0.00 HCP

5 11.09 13.94 0.94 72.94 0.92 0.18 Al203

FeNb + 0.12 wt% C

Phase Image Analysis TCFE2 EBSD

HCP (Red) 2.10 1.83 3.21

BCC (Yellow) 14.70 24.10 9.19

µ 80.80 73.85 87.6

Total 97.60 99.78 100

13


1600°C Isothermals for Fe–Nb–C

TCFE2

TCFE6

Nb C + L

2

Nb C + L

2

NbC + L

NbC + L

14


Characterization of FeNb + 0.7 wt% C

4

3

2

1

6

5

FeNb + 0.7 wt% C

Phase Image Analysis TCFE2

HCP 12.10 11.86

BCC 1.60 13.83

µ 78.50 73.13

LAVES 6.73 0.00

Total 98.93 98.82

Location

Elements (at%)

Predicted

Fe Nb Ti Al Si Mn Phase

1 0.89 98.47 0.64 0.00 0.00 0.00 HCP

2 43.00 51.71 0.41 0.88 3.59 0.41 µ

3 52.18 39.63 0.30 0.99 6.43 0.47 Laves

4 7.29 92.20 0.51 0.00 0.00 0.00 BCC

5 8.30 86.74 4.40 0.55 0.00 0.00 BCC

6 0.89 98.47 0.64 0.00 0.00 0.00 HCP

15


Modelling a Binary System

• Isothermal mass transfer phenomena governs kinetics of

dissolution – Mass Balance

• Determination of the time to complete dissolution (TCD) of pure

niobium spheres in pure liquid iron

16


Introduction to Model

• To solve total flux equation, one must know the precise

relationships between the total flux, N, and the fluid field quantities

(v, ρ)

N

A J

A vC

A

total flux

diffusive flux

bulk flux

• This mass transfer coefficient, k, is used to incorporate the bulk

flow and diffusive flow into one parameter

• Both stagnant and convective fluid conditions are bundled up into

dimensionless numbers, specifically the Sherwood number (Sh)

• In a stagnant fluid, v = 0 and Sh = 2 Sh = 2 + f(v,ρ)

Sh

kd


D

k


2R

D


k

DSh

2R


*

C C

*

NA

D kC C

R




17


Results: Stagnant Fluid


R R DC

C

2 L *

0



S

i




t

kA




i * *

s

C

C C C

exp

t

VL

18


It’s more complicated…

• If velocity is non-zero, how can one handle the total flux

formulation

N

A J

A vC

A

total flux

diffusive flux

bulk flux

• Fluid conditions are handled through the Sherwood number for

both natural and forced convection

19


Convective Fluid

• There are two types of convection that must be considered:

1. Natural Convection

o

the dissolving solid will develop concentration gradients near the

interface, forming a density gradient in the liquid

1/4

Sh20.59 Grm

Sc

R

final

dt

N 3/4

C

C R

Natural

R

0

RdR

1 2 0

t

final

2. Forced Convection

o A velocity field is applied and fixed externally on a system

Sh20.6Re

Sc

1/2 1/3

R

final

dt

F 1/2

C

C R

Forced

R

0

RdR

final

1 2 0

t

20


Results: Convective Fluid

21


Future Work

• Modelling ternary dissolution of Nb-C phases in pure liquid Fe

– Range of stoichiometry

– Solubility product and flux balance at interface

Experiments:

• Dissolution of Fe-Nb and Fe-Nb-C to validate the binary and ternary

models

22


Conclusions

• Thermodynamic databases should be used with caution in relation

to ferroalloys

• The poisonous effect of carbon in ferroniobium should not be

marginalized with respect to size and fraction of niobium carbides

• The solution of the dissolution equation for the Fe-Nb system with

different fluid conditions resulted in a noticeable effect on the TCD

and followed a consistent, logical pattern

• Consideration of the Fe-Nb-C system for modelling of dissolution

and experiments

23


The End…

• With gratitude to the following people and groups:

– Dr. Dmitri Malakhov

– Dr. Gord Irons

– Jim Garrett

– John Thomson

– Chris Butcher

– Doug Culley

– Ed McCaffery

– Xiaogang Li

– Fellow graduate students

– SRC

– CCEM

24


Meta-stable phase, η

2

1

Location

Elements (at%)

Predicted

Fe Nb Ti Al Si Mn Phase

1 34.48 62.46 0.77 0.56 1.73 0.00 ETA

2 35.19 61.02 0.67 0.59 2.52 0.00 ETA

2002 - THE IRON - NIOBIUM PHASE DIAGRAM AND THE VISCOSITY OF LIQUID - Korchemkina, Shunyaev 25


EBSD of FeNb + 0.7 wt% C

26


FeNb solidified at CBMM

27

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