1. Outline
Introduction
The Model
Results & Discussion
Conclusion
Hydrogen reduction during steel casting by
thermally induced up-hill diffusion
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net)
Independent Research and Consultancy in Physical Metallurgy
Alcalde Joan Batalla, 4
08340 Vilassar de Mar
Spain
METAL 2010 (17-20 May 2010)
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
2. Outline
Introduction
The Model
Results & Discussion
Conclusion
1 Outline
2 Introduction
3 The Model
4 Results & Discussion
5 Conclusion
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
3. Outline
Introduction
The Model
Results & Discussion
Conclusion
Hydrogen in alloys
Hydrogen is an almost ubiquitous element:
H is a component of air (as moisture).
H is present in polymeric binder components for moulds, welding
cathodes, etc.
H has high reactivity, and affinity with many metals.
H can appear in diverse forms (atomic, molecular or ionic).
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
4. Outline
Introduction
The Model
Results & Discussion
Conclusion
Hydrogen in alloys
Minute amounts of H produce severe embrittlement in some alloys
Reduction of fracture stress and ductility
Porosity
Hydrogen flaking
Metal hydride embrittlement (Ti, V, Zr, ...)
Delayed fracture (in corrosive environtment)
Stress corrosion cracking & corrosion fatigue
... and very diverse mechanisms have been proposed to explain them.
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
5. Outline
Introduction
The Model
Results & Discussion
Conclusion
Hydrogen in alloys
But on the other hand, hydrogen (and other interstitial elements)
have large mobility, and increasingly so at higher temperatures
their solubility in alloys depend on phase and increases with
temperature
are sometimes able to diffuse out of metals at high temperatures
Particularly hydrogen...
H does not form other compounds, except for hydrides in certain
alloys and at relatively low temperatures
H stays either in solid solution or in various traps like dislocations
and grain boundaries or as gas in porosities
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
6. Outline
Introduction
The Model
Results & Discussion
Conclusion
Hydrogen in alloys
Could we use any of that to reduce the incidence of
hydrogen embrittlement in metals?
Let’s consider the redistribution of interstitial
elements during casting
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
7. Outline
Introduction
The Model Interstitial diffusion model
Results & Discussion
Conclusion
Interstitial diffusion model
Characteristics of the model:
Allowing large thermal gradients
Interstitial solubility as function of temperature
Atomic thermal agitation and diffusion
Interstitial site saturation
No exchange with the environtment / (Or a simple estimation of
desorption)
(No description of trapping or other phenomena included yet)
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
8. Outline
Introduction
The Model Interstitial diffusion model
Results & Discussion
Conclusion
Temperature evolution model
Numerical integration of heat equation
δ2 u δu
α 2
=
δx δt
using a finite difference implicit scheme (Crank-Nicholson)
Heat extraction at surface via convection
Cooling conditions ranging from air cooling to severe forced cooling
(h = 6 Wm−2 K−1 to h = 22 · 103 Wm−2 K−1 )
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
9. Outline
Introduction
The Model Interstitial diffusion model
Results & Discussion
Conclusion
Interstitial redistribution model
Starting distribution Ci of interstitial elements determined by initial
conditions. Then, for each cell and time interval...
1 Temperature Ti evaluated
2 Saturation composition C 0 (Ti ) and partial saturation Ci /C 0 (Ti )
determined
3 Displacement of interstitial atoms via random walk, with a mean
√
walk distance ∆x = ∆t · D
4 Atomic flux distribution relative to the partial saturation in adjacent
cells
5 Diffusion described as thermaly activated process via Arrhenius-type
∆H
expression, D = D0 exp − R·T
6 Iterate...
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
10. Outline
Introduction Interstitial redistribution applied to the casting process
The Model Redirection of interstitial diffusive flux
Results & Discussion Redistribution of other interstitial elements
Conclusion
Redistribution of interstitial elements
Steel piece 20cm thick 4 (a) Fast cooling 0s
10s
Starting H 30s
3 60s
content: 2ppm 120s
H content /ppm 240s
2 300s
450s
(a) Fast cooling: 1 600s
Water spray forced 860s
0 0s
cooling 2.1 1h
2 5h
1.9 20h
1.8 60h
(b) Slow cooling: (b) Slow cooling 1.7
Natural convection -10 -5 0 5 10
Thickness /cm
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
11. Outline
Introduction Interstitial redistribution applied to the casting process
The Model Redirection of interstitial diffusive flux
Results & Discussion Redistribution of other interstitial elements
Conclusion
Redistribution of interstitial elements
4
H max
H min
Steel pieces of varying 3.5 Start H
thickness 3
H content /ppm
2.5
Starting H
content: 2ppm 2
1.5
Fast cooling: Water 1
spray forced cooling
0.5
5 10 15 20 25 30 35 40 45 50
Thickness /cm
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
12. Outline
Introduction Interstitial redistribution applied to the casting process
The Model Redirection of interstitial diffusive flux
Results & Discussion Redistribution of other interstitial elements
Conclusion
Redirection of interstitial element flux
Modified casting operation with a severe
Pouring cup
Riser
thermal gradient towards the surface
Feeder being imposed
Casting
Pouring cup
Riser
Feeder
Casting
Interstitial element flux during standard
casting operation Heating Element
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
13. Outline
Introduction Interstitial redistribution applied to the casting process
The Model Redirection of interstitial diffusive flux
Results & Discussion Redistribution of other interstitial elements
Conclusion
Redirection of interstitial element flux
Steel piece 20cm thick 4 0s
10s
Starting H 3.5 30s
content: 2ppm 3
60s
300s
H content /ppm 1500s
2.5 3600s
Surface I: Fast cooling 7200s
2 10800s
Surface II: Kept at
1.5
high temperature
(1500o C) 1
0.5
Temperature gradient 0
-10 -5 0 5 10
maintained
Thickness /cm
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
14. Outline
Introduction Interstitial redistribution applied to the casting process
The Model Redirection of interstitial diffusive flux
Results & Discussion Redistribution of other interstitial elements
Conclusion
Interstitial redistribution: simple desorption estimate
Standard model does not consider Fast cooling standard:
exchange with surroundings
H content reduction: 1.6%
Simple estimation of desorption:
Consider surface condition (S)
No desorption below T = 400o C
Desorption above T = 400o C limited Fast cooling modified:
by diffusion D and surface condition S
H content reduction: 22.7%
But these are only orientative values!!
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
15. Outline
Introduction Interstitial redistribution applied to the casting process
The Model Redirection of interstitial diffusive flux
Results & Discussion Redistribution of other interstitial elements
Conclusion
Comparison with other interstitial elements (C, N, B)
0.08
Mobility defined as mean Carbon
0.07 Nitrogen
random walk distance, ∆x
Mobility respect hydrogen
Boron
0.06
Relative mobility of element 0.05
∆xZ
Z respect H defined as ∆xH 0.04
0.03
0.02
Mobility of other interstitial 0.01
elements very low respect to
0
hydrogen 600 800 1000 1200 1400 1600
Temperature / K
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
16. Outline
Introduction
The Model
Results & Discussion
Conclusion
Further work & Conclusions
A correct understanding of the diffusional fluxes during manufacturing of
metal components is vital.
A model describing the diffusion of interstitial elements including the
effect of large temperature gradients has been presented.
This model allows a better understanding of interstitial element fluxes.
Important aspects like desorption of interstitial elements, trapping of
hydrogen, etc. still need to be incorporated to the model.
Nevertheless, the present work suggests that it could be possible to
reduce the content of some interstitial elements in cast alloys by
imposing large but controlled temperature gradients to the component
during cooling (patent in process).
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
17. Outline
Introduction
The Model
Results & Discussion
Conclusion
Thanks
Thank you for your attention!!
Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Hydrogen reduction during steel casting by thermally induced up-hill diffusio
