Changes in Toughness at Low Oxygen Concentrations in Steel Weld Metals.

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Changes in Toughness at Low Oxygen
Concentrations in Steel Weld Metals.
S. Terashima and H. K. D. H. Bhadeshia Phase
Transformations and Complex Properties Research
Group Department of Materials Science and
Metallurgy University of Cambridge
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Index of Todays Talk.
Today, we are going to.. Section 1 Clarify
an impact-toughness behaviour of steel weld
metal. (780 MPa class in UTS) Find out that
oxygen (or oxides) has important
roles. Section 2 Improve the toughness by
changing size-distribution of oxides.
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Effect of Oxygen on Toughness (UTS 490 MPa
class).
Acicular Ferrite
Coarse Ferrite
In 490 MPa class welds, a peak appeared by
acicular ferrite formation.
Coarse Bainite
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Effect of Oxygen on Toughness (UTS 980 MPa
class).
In 980 MPa class welds, toughness decreased with
increasing oxygen because oxides do not cause
microstructural change in weld metals.
Martensite
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Effect of Oxygen on Toughness (UTS 580-780 MPa).
Some mild contradictions (1) Simply
decreases because oxides initiate cracks or
voids. (2) Makes a peak because coarsened
bainite is formed at lower oxygen level.
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Requirement of 780 MPa Class Weld Metal.
Requirement of 780 MPa class steel (in UTS) is
increasing. Structural thick steel plates
(Ship, Building, etc.) Thin steel plates
(Vehicle, Electronics, etc.) In order to create
weld metals with both high strength and
toughness, systematic understanding is needed
concerning Relationship between toughness and
oxygen content in weld metal.
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Objective.
Relationship between toughness and oxygen
content in weld metal is studied over a wide
range of oxygen content especially from the
viewpoint of microstructures, using weld
metals with 580, 680 and 780 MPa class ultimate
tensile strength.
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How to Change Oxygen Concentration in Welds?
MAG (Metal Active Gas shielded arc welding)
was used to produce weld metals. It is well
known that a composition of a shielding gas
during MAG welding affects an oxygen
concentration of a weld metal. Therefore, a
composition of a shielding gas was changed
Pure Ar 0.99Ar-0.01CO2 0.98Ar-0.02CO2
0.90Ar-0.10CO2 0.80Ar-0.20CO2 Pure CO2
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Relationship between CO2 in Gas and Oxygen in
Welds.
Specimens are prepared with a wide range of
oxygen concentration. High 780
MPa Medium 680 MPa Low 580 MPa
Method Combustion analysis
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Toughness and Microstructure (580 MPa).
Coarsened Structure
Acicular Ferrite
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Toughness and Microstructure (680 MPa).
Acicular Ferrite
Coarsened Structure
Contains Coarsened Structure
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Toughness and Oxygen Concentration (780 MPa).
Complex behaviour. Is there any microstructural
change due to oxygen level ?
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Optical SEM Micrographs (780 MPa).
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Toughness and Microstructure (780 MPa).
Allotriomorphic Ferrite
Acicular Ferrite
Martensite with Bainite
Still not clear between 20 - 140 ppm
O. i.e. Why toughness changed even though
microstructures were the similar ?
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Fractured Surface Tested at 273 K (780 MPa).
(a) 20 ppm O
(b) 110 ppm O
(c) 140 ppm O
10mm
5mm
Inclusions (oxides) at dimples initiated cracks
or voids.
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Toughness and Microstructure (780 MPa).
Martensite with Bainite Cracks or Voids
Martensite with Bainite
Roles of oxygen in welds Shifting
microstructures, and Initiating
cracks or voids.
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Effect of Oxygen on Impact Toughness of Steel
Welds.

• In the 780MPa weld,
• high impact toughness was observed
• not only at the intermediate oxygen level
  (270 and 350ppmO)
• but also at the very low level (20 ppm O).
• (2) In the 580 and 680MPa welds,
• impact toughness was high only at the
  intermediate oxygen level (190ppmO and
  110-210ppmO, respectively).
• (3) Oxygen shows two roles
• one is to change microstructure of the weld
  metals,
• and the other is to degrade impact
  toughness of welds
• by forming oxides which act as crack or
  void initiation site.

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Index of Todays Talk.
Today, we are going to.. Section 1 Clarify
an impact-toughness behaviour of steel weld
metal. (780 MPa class in UTS) Find out that
oxygen (or oxides) has important
roles. Section 2 Improve the toughness by
changing size-distribution of oxides.
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How to Improve Toughness of Steel Welds?
In the literature, Smaller oxides (about 0.5
mm in diam.) initiate acicular ferrite.
Larger oxides (over 1 mm in diameter)
initiate cracks or voids. ---gt If larger oxides
are eliminated from welds, will their
toughness be improved ???
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Objective.
To minimise the oxide size so as to delay
oxide-related damage to the later stages of
stress and strain and to improve impact
toughness of steel weld microstructure.
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How to Change Size Distribution of Oxide?
MAG (Metal Active Gas shielded arc welding)
was used to produce weld metals. It is known
that larger oxides (say, gt 1mm) tend to float to
the surface of a specimen after TIG
(Tungsten Inert Gas shielded arc welding).
Therefore, re-melting by TIG was carried out
after MAG to eliminate larger oxides.
(Re-melting was carried out only for the
as-welded specimen with 350 ppmw O.)
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Relationship between CO2 in Gas and Oxygen in
Welds.
Re-melting decreased oxygen concentration.
Method Combustion analysis
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Size Distribution of Oxides in Steel Welds.
Re-melted 350 ppm O Smaller ones (lt 1 mm)
were decreased by re-melting. Larger ones
(gt 1 mm) were decreased drastically (almost
all) by re-melting. Re-melted 110-140 ppm
O Re-melted had more smaller oxides.
Re-melted had smaller number of larger
oxides, instead.
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Micrographs.
As-welded
Re-melted
Re-melted consisted of a aa Smaller oxides
initiated acicular ferrite even in martensitic
microstructure.
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Acicular ferrite content as a function of oxygen
concentration.
As-welded 20 140 a ab 270, 350
aa 560allotriomorphic ferrite Re-melted a
aa Higher toughness can be expected!
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Impact Toughness.
Re-melted showed good toughness. Re-melted gt
20-140 ppm O Because acicular ferrite appeared
in re-melted. Re-melted gt 270 ppm
O Fractographs???
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Fractured Surface tested at 273 K.
As-welded
As-welded
As-welded
20 ppmw O
110 ppmw O
350 ppmw O
Re-melted
Visible particles were hardly observed at
dimples.
90 ppmw O
Particles at dimples.
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Impact Toughness.
Re-melted showed good toughness. Re-melted gt
20-140 ppm O Because acicular ferrite appeared
in re-melted. Re-melted gt 270 ppm O Because
larger oxides are eliminated from re-melted.
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Effect of Size Distribution of Oxides on
Toughness.
The refinement of oxide particles and a
reduction in the total oxygen concentration
from the usual hundreds of parts per million,
is beneficial to the toughness of strong weld
metals. It is again confirmed that oxides in
steel weld metals can change microstructures
of welds and initiate cracks or voids in
welds.
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Summary and Some Future Works.
Two conventional methods to improve impact
toughness of steel welds are Producing
acicular ferrite (Popular, but not so easy for
stronger welds.) Decreasing oxygen
concentration down to 20 ppm O (Cost would be
high.) According to the present research, the
following method can be proposed Refining
oxide particles (Useful even for stronger
welds, however, difficult to achieve in
practice.)