1. Traveling Speed (mm s-1)
Welding Conditions.
Welding method: MAG
(Metal Active Gas Shielded Arc Welding)
Number of Pass: 1 (Single pass)
Shielding gas: Ar-xCO2
Preheat: None
Post Weld Heat Treatment: None
Steel
Plate
Bead
X (%) 1 2 10 20
100
Heat Input
(kJ mm-1)
580 MPa
3.5
3.4
3.1
680 MPa
780 MPa
3.2
Traveling Speed (mm s-1)
3.3

2. Sample Preparation for Charpy Impact Test.
2mm
Steel
Plate
Bead
Hammer
5 mm
Half size specimens
(10 mm x 5 mm in cross section)

3. EDS Analysis for Particles at the Dimples (after Charpy).

4. A Method to Estimate Particle Size-Distributions.
Find particles on a SEM image.
Identify a particle as an oxide if an oxygen peak is observed by EDS.
Measure a size of an oxide from a magnified SEM micrograph.
Count the number of oxides up to a total area of 0.2 mm2.
An example of an oxide.
An example of a non-oxide.

5. Combinations of Plates and Wires

6. Chemical compositions of Welds.

7. Nitrogen Concentration in 780 MPa welds.20 36
110 35
140 33
270 36
350 36
560 35
Unit : mass ppm
Method: Inert gas fusion thermal conductivity detection method.

8. Vickers Hardness of Weld Metals.
Hardness change suggests a microstructural change.
Especially for 780 MPa class, martensite formation is suggested.

9. Acicular Ferrite Content in Weld Metals.

10. Method to Estimate Acicular Ferrite Content.
(1) Carry out Vickers micro-hardness test
(with a load of 0.49 N)
(2) Obtain standard microhardnesses of each of the phases
(3) Measure the volume fraction of acicular ferrite using point counting on micrographs.
(4) When there was doubt about the interpretation of the microstructure, the microhardness was measured and compared with the standard values to assist identification.

11. Misorientation Distribution Diagram.

12. Vickers Hardness for Austenitised Samples.
Slight decrease
(20 to 140 ppmw)
is due to an increase in bainite at the expense of martensite.
Which is consistent with the dilatometer data.

13. Vickers Hardness for Austenitised Samples.
Objective
To interpret further by establishing that of 100% martensite.
Experimental
Weld metals: High-strength alloys (20, 110 & 140 ppmw O)
Heat treatment: Austenitised at 1173 K for 300 s
and rapidly quenched into water.
Interpretation
If bainite is formed in the initial welds, carbides in bainite cannot dissolve into austenite at austenitisation. Hence, hardness after water-quench is considered to reflect an amount of martensite in the initial welds.

14. Bayesian neural network analysis.
Input: AF content
&
Mean oxide size
Output: Toughness
Then, assess the independent roles of these variables.
A greater sw indicates a greater influence
on toughness.
Acicular
ferrite
content
Mean
oxide
size

15. Traveling Speed (mm s-1)
Welding Conditions.
Welding method: MAG
(Metal Active Gas Shielded Arc Welding)
Number of Pass: 1 (Single pass)
Shielding gas: Ar-xCO2
Preheat: None
Post Weld Heat Treatment: None
Steel
Plate
Bead
X(%) 1 2 10 20
100
Heat Input (kJ mm-1)
3.5
3.4
3.1
3.2
Traveling Speed (mm s-1)
3.3
778 MPa (in UTS)

16. Conditions for Re-melting.
After MAG, re-melting was carried out
because re-melting can float some oxides on a surface of welds.
Re-melting method: Tungsten Inert Gas Shielded Arc Welding
Number of path: (Single pass)
Shielding gas: Ar
Heat input: kJ mm-1
Travelling speed: mm s-1
Pre-heat conditions: For 3.6 ks at 473 K
Post weld heat treatment: None
To ensure the same cooling rate during both MAG and TIG welding,
pre-heat was carried out only before re-melting by TIG.

17. Sample Preparation for Charpy Impact Test.
2mm
Steel
Plate
Bead
Hammer
2 mm
Specimens with special size
(10 mm x 2 mm in cross section)

18. Vickers Hardness. Hardness data show a
good relationship with microstructures.
i.e.
As-welded:
20 – 140: a’ + ab
270, 350: aa
560：allotriomorphic ferrite
Re-melted: a’ + aa
(Higher toughness can
be expected !)