1. 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

2. Index of Today’s 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.

3. 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

4. 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

5. 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.

6. 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”.

7. 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.

8. 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

9. Relationship between CO2 in Gas and Oxygen in Welds.
Specimens are prepared with a wide range of oxygen concentration.
High: MPa
Medium: 680 MPa
Low: MPa
Method: Combustion analysis

10. Toughness and Microstructure (580 MPa).
Coarsened
Structure
Acicular
Ferrite
10mm
20ppmw O ac
Test temperature: 293K
233K
253K
273K
10mm
110ppmw O aa

11. Toughness and Microstructure (680 MPa).
Acicular
Ferrite
Coarsened
Structure
10mm
90ppmw O ac
Test temperature: 273K
213K
233K
253K
10mm
210ppmw O aa
10mm a
280ppmw O
Contains
Coarsened
Structure

12. Toughness and Oxygen Concentration (780 MPa).
Test temperature: 273K
213K
233K
253K
Complex behaviour.
Is there any microstructural change
due to oxygen level ?

13. Optical & SEM Micrographs (780 MPa).
110 ppmw O
alb
aub
140 ppmw O
10mm
270 ppmw O aa
350 ppmw O
20 ppmw O
560 ppmw O a

14. Toughness and Microstructure (780 MPa).
Martensite
with Bainite
Acicular
Ferrite
Allotriomorphic
Ferrite
Test temperature: 273K
213K
233K
253K
Still not clear between
ppm O.
i.e.
Why toughness changed
even though microstructures were the similar ?

15. 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.

16. Toughness and Microstructure (780 MPa).
Martensite
with Bainite
Cracks or Voids
Martensite
with Bainite
Test temperature: 273K
213K
233K
253K
Roles of oxygen in welds:
Shifting microstructures,
and
Initiating cracks or voids.

17. 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 ppmO, 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.

18. Index of Today’s 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.

19. 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.
---> If larger oxides are
eliminated from welds,
will their toughness be
improved ???
Test temperature: 273K
213K
233K
253K

20. 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.

21. 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, > 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.)

22. Relationship between CO2 in Gas and Oxygen in Welds.
Re-melting decreased oxygen concentration.
Method: Combustion analysis

23. Size Distribution of Oxides in Steel Welds.
[Re-melted & 350 ppm O]
# Smaller ones (< 1 mm) were
decreased by re-melting.
# Larger ones (> 1 mm) were
decreased drastically
(almost all) by re-melting.
[Re-melted & ppm O]
# Re-melted had more
smaller oxides.
# Re-melted had smaller
number of larger oxides,
instead.
Re-melted
All oxides
Oxide diam.:
0.5 – 1 mm
1 - 3 mm
> 3 mm
< 0.5 mm
As-welded

24. Micrographs. As-welded Re-melted 110ppmw O alb aub 270ppmw O 350ppmw O
5mm
110ppmw O
alb
aub
10mm
270ppmw O aa
350ppmw O
5mm
140ppmw O
aub
alb
10mm
90 ppmw O aa
Re-melted consisted of
a’+ aa
Smaller oxides initiated
acicular ferrite even in
martensitic microstructure.

25. Acicular ferrite content as a function of oxygen concentration.
As-welded:
20 – 140: a’ + ab
270, 350: aa
560：allotriomorphic ferrite
Re-melted: a’ + aa
# Higher toughness can
be expected!

26. Impact Toughness. Re-melted showed good toughness.
[Re-melted > ppm O]
Because acicular ferrite appeared in re-melted.
[Re-melted > 270 ppm O]
Fractographs???

27. Fractured Surface tested at 273 K.
As-welded
As-welded
As-welded
20 ppmw O
5mm
5mm
110 ppmw O
5mm
350 ppmw O
Re-melted
Visible particles were hardly observed at dimples.
90 ppmw O
5mm
Particles at dimples.

28. Impact Toughness. Re-melted showed good toughness.
[Re-melted > ppm O]
Because acicular ferrite appeared in re-melted.
[Re-melted > 270 ppm O]
Because larger oxides are eliminated from re-melted.

29. 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.

30. 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.)