Daniel Gaude-Fugarolas @ HSLA2011

Outline
Introduction
The Model
Results & Discussion
A New Method and its Patent
Conclusion

Understanding hydrogen redistribution during steel
casting, and its eﬀective extraction by thermally
induced up-hill diﬀusion

Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net)

Independent Research and Consultancy in Physical Metallurgy

HSLA Steels ’2011
(31 May - 2 June 2011)

Daniel Gaude-Fugarolas,Ph.D, FCPS (dgaude@cantab.net) Understanding hydrogen redistribution and its extraction

Outline
Introduction
The Model
Conclusion

1 Outline

2 Introduction

3 The Model

4 Results & Discussion

5 A New Method and its Patent

6 Conclusion


Outline
Introduction
The Model
Conclusion

Introduction

Introduction


Outline
Introduction
The Model
Conclusion

Introduction: Hydrogen in metals

For a long time, hydrogen has been recognised to be deleterious for
alloys, in particular for some high performance steels
H is ubiquitous, mainly as a component of air moisture (and of
many polymeric mould binders)
H is highly soluble in high T metal, but much less so at low T
When H cannot be kept in solution anymore it is forced to migrate
to various types of lattice defects (traps)
Any excess hydrogen produces:
embrittlement in some alloys (by a reduction of fracture stress and
ductility)
microstructural defects like porosity, ﬂaking, hairline cracks, etc
and enhance corrosion phenomena


Outline
Introduction
The Model
Conclusion

Introduction: Hydrogen in metals

Hydrogen is delt with by...

minimising hydrogen catch up due to moisture and additives
extraction from liquid metal during reﬁning:
Standard: (i.e. ODS): [H] ≈ 1.5 – 2.0ppm
Vacuum degassing: [H] ≈ 0.5 – 1.5ppm
selecting hydrogen tolerant alloys
increasing the tolerance to hydrogen damage of the alloy

Can we still do anything else to reduce the incidence
of hydrogen embrittlement in metals?


Outline
Introduction
Requirements for the interstitial diﬀusion model
The Model
Temperature and phase evolution models
Operation of the interstitial redistribution model
Conclusion

The Model

Let’s start at the beginning:
Let’s try and understand the redistribution of
interstitial elements on the solid metal during
casting and forming

An interstitial diﬀusion model


Outline
Introduction
The Model
Conclusion


Characteristics of the model:

Thermal history including large T gradients (T evolution model)
Incorporate phase transtition (i.e. from FCC to BCC on cooling)
Interstitial solubility as function of T and matrix phase
Atomic thermal agitation and diﬀusion
Interstitial site saturation
At free surfaces: local equilibrium across the surface (Sievert’s law)
(No description of trapping or other phenomena included yet,
but⋆ ...)
⋆ but see Gaude-Fugarolas, D. In proceedings of Metal2011, Czech Republic (2011)


Outline
Introduction
The Model
Conclusion


Numerical integration of heat equation

δ2 u δu
α =
δx 2 δt
using a ﬁnite diﬀerence implicit scheme (Crank-Nicholson)
Heat extraction at surface via radiation and convection⋆
(⋆) Convection conditions may range from gentle air cooling
(i.e. h = 6 Wm−2 K−1 ) to severe forced cooling (i.e. h = 22 · 103 Wm−2 K−1 )


Outline
Introduction
The Model
Conclusion


Phase transformation described using thermodynamic criteria and classic
nucleation and growth kinetic theory

Both the temperature evolution and phase transformation models are based on
previous work and described in:
D. Gaude-Fugarolas. Modelling Induction Hardening. Saarbruchen
(Germany). VDM Verlag Dr. Muller, 2008. ISBN-10: 3639062965.
D. Gaude-Fugarolas. Modelling phase transformations on steel during
induction hardening. In International Conference on “Mathematical
Modelling and Information Technologies in Welding and Related
Processes” - Crimea (Ukraine), 2002.
Jones, S. J. and Bhadeshia, H. K. D. H. Kinetics of the simultaneous
decomposition of austenite into several transformation products. Acta
Materialia. 1997. 28A:2005-2013.

Outline
Introduction
The Model
Conclusion


1 Starting distribution Ci of interstitial elements determined by initial
conditions. Then, for each cell and time interval...
1 Temperature Ti evaluated
2 Phase transitions accounted as function of Ti
3 Saturation composition C 0 (Ti ) and partial saturation Ci /C 0 (Ti )
determined
4 Displacement of interstitial atoms via random walk, with a mean
√
walk distance ∆x = ∆t · D
5 Atomic flux distribution relative to the partial saturation in adjacent
cells
6 Diffusion described as thermaly activated process via Arrhenius-type
∆H
expression, D = D0 exp − R·T
7 Local equilibrium between dissolved and molecular hydrogen,
following Sievert’s law, describes the flux of atoms across the surface
2 Iterate...


Outline
Introduction
The Model
Hydrogen redistribution during a cooling process
Conclusion




Outline
Introduction
The Model
Conclusion

H redistribution: Eﬀect of cooling rate on H concentration

Steel piece 25cm thick 6 Fast cooling Slow cooling

-------------center piece--------
Starting H content: 2ppm 5
0s

H content /ppm
30s
4 150s
(left) Fast cooling: Water 600s
3 1200s
spray forced cooling
1700s
2 2 0h
1h
(right) Slow cooling: 1 1 6h
24h
Radiation and natural 42h
convection 0 0
-12.5 -5 0 5 12.5
Thickness /cm

Fast cooling produces a 3x H concentration!!
And let’s not forget that solubility diminishes with cooling

Outline
Introduction
The Model
Conclusion

H redistribution: Eﬀect of cooling rate on partial saturation

Partial saturation:
Ci /C 0 (Ti ) 45

-----------center piece-----------
Fast cooling Slow cooling
40
35 0s

H partial saturation
Ci /C 0 (Ti ) < 1 Partial 30
600s
900s
Saturation 25 1200s
1500s
Ci /C 0 (Ti ) = 1 Saturation 20 1700s
15 15 0h
Ci /C 0 (Ti ) > 1 1h
Supersaturation 10 10
6h
5 5 24h
0 0 42h
Same conditions -12.5 -5 0 5 12.5
Initial [H] = 2ppm Thickness /cm

Fast cooling produces ≈40+ times supersaturation!!


Outline
Introduction
The Model
Conclusion

H redistribution: Size and transformation temperature

Starting H content: 2ppm
6 Alloy A
Fast cooling 5.5 Alloy B
Start H
5

Max H content /ppm
Eﬀect of thickness (5-50cm) 4.5
(total amount of H to 4
redistribute) 3.5
3
2.5
Eﬀect of FCC to BCC
2
transformation temperature:
1.5
Steel A: 700o C 5 10 20 30 40 50
Steel B: 450o C Thickness /cm


Outline
Introduction
The Model
Redirection of interstitial diﬀusive ﬂux
Conclusion

A New Method

A New Method:


Outline
Introduction
The Model
Conclusion

A new method

A severe temperature gradient forces hydrogen to flow towards the core
region of a component, where it can reach severe supersaturation

Actually, NO!!. The temperature gradient forces hydrogen to flow
towards higher temperature regions!!

Then, why don’t we try instead to redirect the hydrogen flux
towards the surface?
(and eventually get rid of it)


Outline
Introduction
The Model
Conclusion

Redirection of interstitial element flux
Standard casting operation: Interstitial Modified casting operation with a severe
element flux creates enriched regions at thermal gradient towards the surface
the core of the piece being imposed, eliminating interstitials

Pouring cup Pouring cup
Riser Riser

Feeder Feeder

Casting Casting

Heating Element


Outline
Introduction
The Model
Conclusion

Redirection of interstitial element ﬂux: Concentration

5
Steel piece 25cm thick
Starting H 4
content: 2ppm
H content /ppm

Heated Surface
3
Surface I: Fast cooling
Surface II: Kept at high 2
0s
temperature (1500o C) 60s
1 300s
1200s
3600s
Temperature gradient 8400s
0
maintained 2h -12.5 -5 0 5 12.5
Thickness /cm
Final H content 0.99ppm !!

Outline
Introduction
The Model
Conclusion

Redirection of interstitial element ﬂux: Partial Saturation

Steel piece 25cm thick
Starting H content: 2ppm
Surface I: Fast cooling
10
Surface II: Kept 2h at high
H partial saturation

Heated Surface temperature (1500o C)
1 0s
60s Partial Saturation during
300s
1200s treatment below 1.0 !!
0.1 3600s
7200s
7600s
8000s Supersaturation only occurs
8400s
0.01 when reaching room
-12.5 -5 0 5 12.5
temperature, and only to
Thickness /cm
levels similar to those found
during very slow cooling !


Outline
Introduction
The Model
Conclusion

Comparison in H redistribution during cooling
Cooling a 25cm thick plate with 2ppm H:

Slow cool Some H reduction (≈ 25% )
No concentration peaks
Low supersaturation
Long time: 42 hours
Fast cool No H reduction (≈ 1% )
Severe concentration peaks
Severe supersaturation
Short time: 1.5 hours
Directional cool Large H reduction (≈ 50% )⋆
(Patented) Some concentration peaks⋆
Low supersaturation⋆
Short time: 2.5 hours
⋆ Possible to be improved by modifying cooling severity and treatment time

Outline
Introduction
The Model
Conclusion

Conclusion

Conclusion


Outline
Introduction
The Model
Conclusion

Conclusions
A novel physical model had been presented that allows a correct
description of hydrogen redistribution during manufacturing processes

The model predicts that on fast cooling, hydrogen content redistributes
forming concentration peaks that may reach several times the starting
average concentration value and lead to huge supersaturation.

Phase transformation and the temperature at which it occurs greatly
inﬂuence the redistribution of hydrogen

Finally, a method has been presented which enables to reduce hydrogen
content within the metal via the use of imposed temperature gradients.
(PCT patent WO/2010/097755, already awarded in US and Spain.
In process elsewhere).

Outline
Introduction
The Model
Conclusion

Thanks

Thank you for your attention!!


Daniel Gaude-Fugarolas @ HSLA2011

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