Welding Conditions.

Welding method: MAG(Metal Active Gas Shielded Arc Welding)

Number of Pass: 1 (Single pass)Shielding gas: Ar-xCO2

Preheat: NonePost Weld Heat Treatment: None

SteelPlate

SteelPlate

Bead

X (%) 0 1 2 10 20 100

Heat Input

(kJ mm-1)

580 MPa 3.5 3.4 3.1 3.1

680 MPa 3.5 3.4 3.1 3.1 3.1

780 MPa 3.5 3.4 3.1 3.1 3.1 3.2

Traveling Speed (mm s-1) 3.3 3.3 3.3 3.3 3.3 3.3

Sample Preparation for Charpy Impact Test.

2m

m

SteelPlate

Bead

Half size specimens(10 mm x 5 mm in cross section)

Hammer

5 mm

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

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.

Combinations of Plates and Wires

Chemical compositions of Welds.

Nitrogen Concentration in 780 MPa welds.

Unit : mass ppmMethod: Inert gas fusion thermal conductivity detection method.

20

110

140

270

350

560

O N

36

35

33

36

36

35

Vickers Hardness of Weld Metals.

Hardness change suggests a microstructural change.

Especially for 780 MPa class, martensite formation is suggested.

0 100 200 300 400 500 600200

250

300

350

400Test temperature: room temp.Load:98N

Oxygen concentration, ppmw

Vic

kers

har

dnes

s, H

V

Strength of weld: High

Low Medium

Acicular Ferrite Content in Weld Metals.

0 100 200 300 400 500 6000

20

40

60

80

100

Oxygen concentration (ppmw)

Aci

cula

r fe

rrite

co

nte

nt (

%)

Strength of weld: HighLow

Medium

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.

Misorientation Distribution Diagram.

0 10 20 30 40 50 600

0.1

0.2

0.3

0.4

0.5

Misorientation angle (degree)

Fra

ctio

n (a

rb.

unit)

110ppmw O

Oxygen concentration: 140ppmw O

High-strength weld metal

10ppmw O

Vickers Hardness for Austenitised Samples.

0 100 200 300 400 500 600200

250

300

350

400

Load:98NTest temperature: room temp.High-strength weld metal

Oxygen concentration, ppmw

Vic

kers

har

dnes

s, H

V

Austenitised and water quenched

As-prepared

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.

Vickers Hardness for Austenitised Samples.

ObjectiveTo interpret further by establishing that of 100% martensite.

ExperimentalWeld 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.

Bayesian neural network analysis.

Aci

cula

rfe

rrite

cont

ent

Mea

nox

ide

size

0

1

2

3

4

5

w

Input valuables

Input: AF content & Mean oxide sizeOutput: Toughness

Then, assess the independent roles of these variables. A greater w indicates a greater influenceon toughness.

Welding Conditions.

SteelPlate

SteelPlate

Bead

X(%) 0 1 2 10 20 100

Heat Input (kJ mm-1) 3.5 3.4 3.1 3.1 3.1 3.2

Traveling Speed (mm s-1) 3.3 3.3 3.3 3.3 3.3 3.3

Welding method: MAG(Metal Active Gas Shielded Arc Welding)

Number of Pass: 1 (Single pass)Shielding gas: Ar-xCO2

Preheat: NonePost Weld Heat Treatment: None

778 MPa (in UTS)

Conditions for Re-melting.

After MAG, re-melting was carried outbecause re-melting can float some oxides on a surface of welds.

Re-melting method: Tungsten Inert Gas Shielded Arc WeldingNumber of path: 1 (Single pass)Shielding gas: ArHeat input: 2.9 kJ mm-1

Travelling speed: 1.0 mm s-1

Pre-heat conditions: For 3.6 ks at 473 KPost 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.

Sample Preparation for Charpy Impact Test.

Specimens with special size(10 mm x 2 mm in cross section)

Hammer

2 mm

2m

m

SteelPlate

Bead

Vickers Hardness.

Hardness data show agood relationship with microstructures.

i.e.

As-welded:20 – 140: ’ + b

270, 350: a

560 ： allotriomorphic ferrite

Re-melted: ’ + a

(Higher toughness can be expected !)

0 100 200 300 400 500 600200

250

300

350

400Load: 98 NTest temperature: Room temp.

Oxygen concentration (ppmw)

Vic

kers

ha

rdn

ess

(H

V)

Re-meltedAs-welded