S. S. Babu, S. A. David and J. M. Vitek

1
Thermo-Chemical-Mechanical Effects on
Microstructure Development in Low-Alloy Steel
Welds.

• S. S. Babu, S. A. David and J. M. Vitek
• Oak Ridge National Laboratory,
• Oak Ridge, TN 37831
• Solid-Solid Phase Transformations 99
• Kyoto, Japan, May 24-28, 1999
• Research sponsored by the Basic Energy Sciences
  Division of the Department of Energy.

Weld quality and property depend on complex
interplay of thermo-chemical reactions and
thermo-mechanical stresses on solid state phase
transformations.
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1. Role of thermo-chemical reactions in liquid
steel
In self-shielded flux cored arc welds (FCAW-S),
large amounts of aluminum are added to the filler
wire. Aluminum reacts with dissolved oxygen to
form Al2O3. Excess aluminum, reacts with
dissolved nitrogen to form AlN compound. After
these reactions, substantial amount of residual
aluminum remains in solid solution depending upon
the amount of aluminum in the wire.
What does residual aluminum in solid solution do?
3
Residual aluminum modifies the microstructure
development.

• High aluminum leads to skeletal d ferrite which
  leads to low levels of weld metal toughness.
• By reducing aluminum in the weld, one can
  eliminate the d ferrite.
• Reduced aluminum leads to inefficient deoxidation
  and increases risk of porosity.
• How can we optimize the aluminum addition to
  improve weld quality, as well as microstructure?

0.53 wt. Al
4
Pseudo-binary Fe-C-Si-Mn-Al phase diagram may be
used to investigate the stability of d ferrite.

• The phase diagram shows the relation between d
  ferrite stability and aluminum concentration and
  supports the experimental weld microstructure.

5
Thermodynamic calculations do not consider the
process effects. Therefore, we need to calculate
the effects of weld cooling rate.
Liquid
Austenite(FCC)
d Ferrite (BCC)

• Diffusion controlled growth calculations were
  carried out for both high and low-aluminum weld
  metal compositions using DicTra software for a
  cooling rate of 10 C/s.

6
Calculations support the presence of residual d
ferrite in high-aluminum welds.
High-Al
Low-Al

• The above analyses are being used to optimize the
  aluminum content in welds.

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2. Role of oxide inclusion composition on the
transition from bainite to acicular ferrite

• In conventional shielded metal arc welds,
  titanium is also added as deoxidizer.
• Weld metal with small additions of titanium (10
  to 30 wt.ppm) can improve the impact properties.
• This is because the titanium rich inclusions
  promote acicular ferrite.
• All the steels have similar hardenability. The
  only difference is the titanium concentration in
  the inclusions.

Weld B - 32 wt.ppm Ti
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Kinetic measurements during cooling showed rapid
transformation below Bs temperature for welds
with Ti-rich inclusions.
Weld A - 7 wt.ppm Ti
50 C/s
Weld B - 32 wt.ppm Ti

• This is related to nucleation potency of
  inclusions and allotriomorphic ferrite at the
  austenite grain boundaries.

9
Indication of competition between bainite and
acicular ferrite can be seen while cooling at 80
C/s.
Weld A - 7 wt.ppm Ti
Weld B - 32 wt.ppm Ti

• Therefore, simultaneous transformation kinetic
  theories need to be coupled with inclusion models
  to describe this competition between grain
  boundary and intragranularly nucleated
  transformations.

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3. Role of thermo-mechanical conditions on
austenite decomposition during welding
? 116 MPa
No Stress

• Elastic stresses affect the acicular ferrite
  microstructure.
• Babu and Bhadeshia, Mat. Sci. Eng.., A156, 1992,
  1-9.
• How does this affect the stress-relaxation
  kinetics?
• How does allotriomorphic ferrite formation react
  to stresses?

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An elastic load was applied after reaching an
isothermal temperature and the stress relaxation
and transformation kinetics were monitored
simultaneously.
Transformation strains
s
s

• The above information will be important for
  estimating the residual stress development in
  steel welds.

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Stress-relaxation is greater during
allotriomorphic ferrite formation than during
acicular ferrite formation.
Fe-0.1C-0.8Si-1.6Mn-0.015Al-0.084O (wt.) steel
weld

• This is related to creep-relaxation of austenite
  before and during transforming to allotriomorphic
  ferrite.
• Further work is necessary to evaluate this in
  detail.

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Microstructural details for stress-relaxation
experiments

• Small alignment of acicular ferrite plates was
  observed.
• Large amount of residual austenite exists after
  allotriomorphic ferrite formation. This
  transforms to martensite on cooling from
  isothermal temperature.

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Summary and Conclusions

• Large concentration of residual aluminum in
  solid-solution leads to residual d ferrite in
  self-shielded flux cored arc welds. The above
  phenomenon was described with thermodynamic and
  diffusion controlled growth calculations.
• Titanium-rich inclusions promoted the transition
  from bainite to acicular ferrite, as well as, the
  transformation rate below bainitic start
  temperature.
• Large stress relaxation occurs during
  allotriomorphic ferrite formation compared to
  that of acicular ferrite formation.
• Results illustrate the complex thermo-chemical-mec
  hanical interactions on solid-state phase
  transformations in steel welds.