Technical Highlight Vol.16

TIG welding consumables for 490 MPa high tensile strength steels

1. Preface

Table 1: List of TIG rods for 490 MPa HT steels

Product name	AWS A5.18	ASME F-No.	ASME A-No.
TG-S50	ER70S-G	6	1
TG-S51T	ER70S-6	6	1
NO65G	ER70S-2	6	1
TG-S70S2*	ER70S-2	6	1
TG-S70S3*	ER70S-3	6	1
Note: *= New Product

490 MPa high tensile strength (HT) steels are common in　a wide range of industrial fields. For efficiency and ease of　welding, gas metal arc welding (GMAW) or flux cored arc　welding (FCAW) is generally used, but for root-pass　(one-side) welding of pipe to pipe joints where the　afore-mentioned high deposition methods are problematic,　gas tungsten arc welding (GTAW) is applied in all　position welding. GTAW is also suitable for the welding of such critical members as pipes, flanges of chemical　plants because it yields extremely low oxygen weld　metals that offer high cleanness as well as high quality.

Table 2: AWS A5.18 specifications on TIG rod chemistries (mass%)

Classification	C	Si	Mn	P	S	Ni	Cr	Mo	V	Cu	Ti	Zr	Al
ER70S-2	0.07	0.40-0.70	0.90-1.40	0.025	0.035	0.15	0.15	0.15	0.03	0.50	0.05-0.15	0.02-0.12	0.05-0.15
ER70S-3	0.06-0.15	0.45-0.75									-	-	-
ER70S-6		0.80-1.15	1.40-1.85								-	-	-
ER70S-G	(not specified)
Note: Single values are maximum

Table 3: AWS A5.18 specifications for mechanical properties of weld metals

Classification	Tension test			Impact test
	TS (MPa)	YS (MPa)	EI (%)	Test temp. (°C)	Absorbed energy (J)
ER70S-2	490min	400min	22min	-30	27min
ER70S-3				-20
ER70S-6				-30
ER70S-G				As agreed between supplier and purchaser

Kobe Steel manufactures TIG rods for 490 MPa HT　steels (see Table 1). FAMILIARCTM TG-S70S2 and　FAMILARCTM TG-S70S3 rods have been newly developed to meet market demand. Table 2 lists the　chemistries of TIG (filler) rods and Table 3 details the　mechanical properties of weld metals from the AWS　A5.18 specifications. This article will discuss the　properties of Kobe Steel’s TIG rods in Table 1 in detail and　suggest some tips for better welding results with GTAW.

2. Properties of FAMILIARCTM TG-S50

TG-S50 is classified as ER70S-G in AWS A5.18. It is,　however, a C-Si-Mn type of rod that exhibits excellent　tensile strength and impact properties for GTAW welding　of 490 MPa HT steels. It provides superior usability through fluidity of molten metal and wetting of the bead　surface, and it can be applied to not only the root-pass　welding of pipe joints but also almost all joints as well as　welding positions. It has been Kobe Steel’s most highly reputed GTAW rod and has been adopted in a wide range of applications.

TG-S50 is well-suited to post welding heat treatment　(PWHT), and as a matter of fact, has undergone　PWHT when applied to large-scale structural members　such as chemical reactors. However, users should note　the recommended PWHT condition of around 625°C×5 hours (hr) because the YS decreases during PWHT.

Table 4: Typical chemistry of weld metal (mass%)

C	Si	Mn	P	S	Cu	Ni	Cr	Mo	V
0.17	0.76	1.43	0.011	0.015	0.20	0.01	0.02	<0.01	<0.01

Table 5: Typical mechanical properties of weld metal

	Tensile test at RT*			Absorbed energy (J)
	YS (MPa)	TS (MPa)	EI (%)	-40°C	-30°C	-20°C	0°C
As-welded	485	586	36	163 168 142	192 198 198	216 238 218	244 256 230
Note: *RT = Room temperature

Table 4 shows the typical chemistry of TG-S50 weld　metal, and Table 5, the typical mechanical properties　in as-welded and PWHT (625°C × 5hr) conditions.Figure 1 shows the tensile properties in relation to the　test temperatures. TS increases at a temperature range　of 200 – 350°C in the as-welded condition; this is　known as the blue brittleness phenomenon.

Figure 2 shows back bead appearance after root-pass　welding in the 1G position by TG-S50 (2.4mm dia.).　The groove configuration was of single 60°V, with a　root gap of 3.0-3.5mm. The welding conditions were 110A-12V and without back-shielding.

3. Properties of FAMILIARCTM TG-S51T

Like TG-S50, TG-S51T is a C-Si-Mn type of rod - but equivalent to AWS A5.18 ER70S-6. With higher amounts of C, Si and Mn contents, it yields higher TS and YS levels than TG-S50; however, the impact properties of TG-S51T weld metal are nearly the same as those of TG-S50.

Similar to TG-S50 in usability, bead appearance and ease of use, TG-S51T is also used across a wide range of applications.

Because TG-S51T weld metal retains sufficient tensile properties of 490 MPa HT steels, even over longer periods of PWHT, it can be applied to joints of large-scale structural members that require stress relief annealing.

Table 6: Typical chemistry of weld metal (mass%)

C	Si	Mn	P	S	Cu	Ni	Cr	Mo	V
0.11	0.82	1.55	0.011	0.012	0.20	0.01	0.01	<0.01	<0.01

Table 7: Typical mechanical properties of weld metal

PWHT	L.M.P.*1 (×103)	Tensile test at RT			Absorbed energy (J)
		YS (MPa)	TS (MPa)	EI (%)	-30°C
As-welded	-	523	621	35	168, 174, 160
625°C × 8hr	18.77	422	562	37	194, 176, 176
625°C × 14hr	18.99	414	550	37	188, 172, 191
625°C × 30hr	19.29	396	535	37	160, 178, 172
625°C × 45hr	19.44	382	524	38	184, 156, 158
Note: *1 L.M.P. = Larson-Miller Parameter

Tables 6 and 7, and Figure 3 show the typical chemistry of TG-S51T weld metal, the typical mechanical properties in as-welded and various PWHT conditions and the tensile properties in relation to the Larson-Miller Parameters, respectively.

Figure 4 shows back bead appearance in the 1G position . The groove configuration was of single 60°V, with a root gap of 3.0-3.5mm. The welding conditions were 110A-12V and without back-shielding.

4. Properties of FAMILIARCTM No.65G

NO65G TIG rod, equivalent to AWS A5.18 ER70S-2, contains less C, Si and Mn contents than TG-S50 or TG-S51T; instead, Al, Ti and Zr are specially added. These elements help to decrease the amount of oxygen in the molten pool, allowing NO65G to provide a weld metal of relatively high cleanness. On the other hand, it is a little inferior to TG-S50 or TG-S51T in terms of fluidity of molten metal, because of which its surface tension increases while the oxygen content in the molten metal is decreased.

Because Al, Ti or Zr is an element with high oxygen affinity, slag can be generated on the bead surface after a part of the element combines with oxygen in the molten metal during welding. Users are therefore advised to beware of weld defects, such as poor bead appearance or slag inclusion, caused by the slag.

Each chemical element in NO65G rod is strictly controlled to maintain a narrow range within the AWS A5.18 ER70S-2 specification so that it can meet the tough requirements set out by Japanese clients for mechanical properties in critical structural projects.

Table 8: Typical chemistry of weld metal (mass%)

C	Si	Mn	P	S	Cu	Al	Ti	Zr
0.04	0.50	1.20	0.007	0.012	0.22	0.06	0.05	0.02

Table 9: Typical mechanical properties of weld metal

	Tensile test at RT			Absorbed energy (J)
	YS (MPa)	TS (MPa)	EI (%)	-40°C	-30°C	-20°C
As-welded	584	635	29	Av 165	Av 200	Av 215
625°C × 8hr	545	615	30	Av 138	Av 160	Av 175

Table 8, Table 9 and Figure 5 show the typical chemistry of NO65G weld metal, the typical mechanical properties in as-welded and PWHT (625°C×8 hr) conditions and the absorbed energies in relat ion to the tes t temperatures, respectively.

Figures 6 and 7 show face side bead appearance after root and second passes and back bead appearance after root-pass welding in 1G and 3G (uphill) positions by NO65G (2.4mm dia.), respectively. Figures 8 and 9 exhibit macro-structures of root-pass and second-pass welding in 1G and 3G (uphill) positions, respectively. The groove configuration was of single 60°V, with a root gap of 3.0-3.5mm. The welding conditions were 110A-12V for root-pass without back-shielding and 160A-13V for second pass welding, respectively.

5. Properties of FAMILIARCTM TG-S70S2

Like NO65G, TG-S70S2 is designed to fulfill AWS A5.18 ER70S-2 specifications though it has some key differences. TG-S70S2 is targeted toward a comparatively more versatile set of structures that fall under the AWS A5.18 ER70S-2 requirements, and its chemistries are wider than those of NO65G within the ER70S-2 specifications.

Al, Ti and Zr, increased slag generation and slightly inferior impact properties may result in comparison with NO65G, in which these elements are more tightly controlled. Higher amounts of Al, Ti and Zr cause impact properties to begin to fluctuate.

Table 10: Typical chemistry of weld metal (mass%)

C	Si	Mn	P	S	Cu	Al	Ti	Zr
0.05	0.42	1.05	0.010	0.009	0.11	0.09	0.06	0.04

Table 11: Typical mechanical properties of weld metal

	Tensile test at RT			Absorbed energy (J)
	YS (MPa)	TS (MPa)	EI (%)	-40°C	-30°C	-20°C
As-welded	563	622	28	Av 125	Av 205	Av 247
625°C × 8hr	526	604	30	Av 107	Av 156	Av 180

Tables 10 and 11, and Figure 10 show the typical chemistry of TG-S70S2 weld metal, the typical mechanical properties in as-welded and PWHT (625 °C×8 hr) conditions and the absorbed energies in relation to the tested temperatures, respectively.

Figure 11 shows back bead appearance in 1G and 3G (uphill) positions by TG-S70S2 (2.4mm dia.).

6. Properties of FAMILIARCTM TG-S70S3

TG-S70S3 is a C-Si-Mn type of rod, like TG-S50 or TG-S51T, but it is designed to meet the AWS A5.18 ER70S-3 specifications It is, however, designed to contain lower amounts of C-Si-Mn than TG-S50, resulting in the lower tensile properties of the weld metal. Therefore, PWHT is not recommended but for the as-welded condition only.

Table 12: Typical chemistry of weld metal (mass%)

C	Si	Mn	P	S	Cu	Ni	Cr	Mo	V
0.10	0.53	1.11	0.010	0.015	0.13	0.01	0.02	<0.01	<0.01

Table 13: Typical mechanical properties of weld metal

	Tensile test at RT			Absorbed energy (J)
	YS (MPa)	TS (MPa)	EI (%)	-40°C	-20°C	0°C
As-welded	460	553	36	200 170 190	240 240 245	255 260 286

Tables 12 and 13 show the typical chemistries and mechanical properties of TG-S70S3 weld metal in the as-welded condition, respectively.

7. Comparison of tensile properties

Figure 12 compares the tensile properties of weld metals by five TIG rods that meet AWS A5.18 specifications.

It can be seen that the rod with the highest tensile and yield strength is NO65G, while these properties are lowest in TG-S70S3. These rods are sequenced in terms of tensile properties as follows:

NO65G>TG-S70S2>TG-S51T>TG-S50>TG-S70S3

8. Tips for better welding results in GTAW

An advantage of using TIG welding process for 490 MPa HT steels is the ability to obtain weld metal with high cleanness due to the low oxygen content in the weld metal when utilizing pure 100 % Ar shielding gas. On the other hand, hardness may increase in as-welded and not-reheated zones such as after the final pass in multilayer welding or single pass fillet welding.

When the final pass weld metal of a pipe joint with increased hardness is exposed, for example, to an ambience filled with hydro-sulfide (H2S), the hydrogen, having a small atomic radius, diffuses in the weld metal and may increase the risk of delayed cracks.

When TIG welding is applied to structures that will be exposed to an atmosphere containing H2S, the following tips are recommended to avoid increased hardness and prevent delayed cracking:

(1)Prevent excess hardening by maintaining a low cooling rate of the weld metal while applying high heat input.
 (2)Prevent excess hardening by maintaining a low cooling rate of the weld metal while applying high interpass temperature or preheating.
 (3)Provide an as-welded weld metal with re-heating by PWHT. (This process shall not be utilized on structural members that shall be left in an as-welded condition.)
 (4)It is also effective to apply FAMILIARCTM TG-S35.

Table 14: AWS and JIS classification of TG-S35

AWS A5.18	ASME F-No.	ASME A-No.	JIS Z3316
-	6	1	W35A0U 10

TG-S35 is designed to be used for mild steels with low TS and limited to final pass welding only. It is not classified in AWS A5.18 but in Japanese Industrial Standard (JIS) Z3316 W35A0U 10 (Table 14)

Table 15: Typical chemistry of TG-S35 weld metal and JIS specification

	C	Si	Mn	P	S	Cu	Ni	Cr	Mo	V
TG-S35	0.01	0.02	0.44	0.012	0.008	0.17	0.01	0.01	<0.01	<0.01
JIS Z3316 W35A0U 10	0.02 max	0.20 max	0.70 max	0.025 max	0.025 max	0.50 max	0.15 max	0.15 max	0.10 max	0.05 max

The typical chemistry of TG-S35 weld metal and the JIS specification of the rod chemistry are shown in Table 15.

Table 16: Typical mechanical properties of TG-S35 weld metal

	Tensile test at RT			Absorbed energy (J)
	YS (MPa)	TS (MPa)	EI (%)	-30°C	-20°C	0°C
TG-S35	335	405	37	36 46 32	154 132 136	212 226 228
JIS Z3316 W35A0U 10	250 min	350-450	22 min	0°C
				47J min

The typical mechanical properties of TG-S35 weld metal and JIS specification of mechanical properties both in as-welded condition are shown in Table 16.

For reference, the hardness test result of TG-S35 weld metal in comparison with TG-S50 weld metal is shown in Figure13.The hardness was measured as shown in the drawing below.

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