Technical Highlight Vol.21

Vol.21: TRUSTARC™ DW-A62LSR（A5.29 E91T1-GM)　improves notch toughness of HSLA weld metal after PWHT

In the construction of structures such as spherical tanks and pressure vessels, weldments are subjected to postweld heat treatment (PWHT) in order to reduce the residual stresses induced by welding and for improving the fracture toughness and fatigue properties of the welds. As these structures have grown larger in size and are being operated at even-higher pressures, in tandem with recent growth in energy demand, the steel materials used have been increasingly strengthened. To company with such a trend, TRUSTARC™ DW-A62LSR, a rutile type flux cored wire (FCW) for HT610 or higher class steel materials, has been developed and confirmed to satisfy the following requirements:

As welded：TS≧621MPa (90ksi), vE≧27J at-60℃
PWHT： TS≧586MPa (85ksi), vE≧27J at-40℃

Table 1　shows the typical chemical compositions of deposited metal with TRUSTARC™ DW-A62LSR.

Table 1: chemical compositions of deposited metal (mass%)

C	Si	Mn	P	S	Ni	Others
0.05	1.14	1.29	0.007	0.008	2.59	Mo, Ti, B

Figures 1 and 2 show the relationship between PWHT conditions and mechanical properties of the deposited metal.

The effect of heat input (cooling rate at 540℃　［℃/sec］, calculated by Rosenthal's equation) on the tensile strength and absorbed energy of deposited metal in as welded and PWHT conditions was studied and the results are shown in Figures 3 and 4, respectively.

A butt joint weld test was conducted under the conditions shown in Table2.

Table 2: Welding conditions

Welding wire	DW-A62LSR (1.2mmΦ)
Base metal	TS610MPa class steel (60mm thick)
Dimension of groove	After welding the face side, the groove of reverse side was machined to a shape of 50° angle and 35 mm depth.
Welding position & parameters(heat input)	(1) Flat (1G): 270A-28V (1.2 kJ/mm) (2) Horizontal (2G): 260A-28V (0.8 kJ/mm) (3) Vertical-up (3G): 220A-24V (2.4 kJ/mm)
PWHT	As welded & 620℃x 8 hours (LMTP18.7x10³)
Preheating & interpass temperature	90-110℃ and 140-160℃
Shielding gas	80%Ar-20%CO2; 25 liter/min

Figure 5 shows the macrostructures of the welded joints in 1G, 2G and 3G positions. The test results of mechanical properties in as welded and PWHT conditions are shown in Tables 3.

Table 3: Mechanical properties of welded joint (Location: center)

Position	PWHT condition	Tensile properties			Notch toughness
		0.2%PS [MPa]	TS [MPa]	El [%]	Absorbed energy [J]
					-60℃	-40℃
1G	AW *1	713	748	22	67	81
	PWHT *2	627	692	22	41	61
2G	AW *1	722	752	22	81	91
	PWHT *2	678	721	27	47	62
3G	AW *1	640	706	24	61	90
	PWHT *2	619	686	28	31	64

*1 AW: as welded　*2 PWHT: 620℃ x 8 hours

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