Technical Highlight Vol.27

Vol.27: Flux cored wires for heat resistant steels that conform to AWS specifications

1.Preface

Heat resistant steels, typically Cr-Mo steels, have been widely utilized under such high temperature and high-pressure environments as thermal power plant boilers (hereinafter called boilers).

These steels are classified in Japan by the Japanese Industrial Standard (JIS) and overseas by the standards of both American Society for Testing and Materials (ASTM) and American Society of Mechanical Engineers (ASME).

The classification of welding consumables for heat resistant steels is also regulated by JIS in Japan and by both AWS and ASME standards abroad.

Table 1 shows Kobe Steel’ s line-up of heat resistant steel welding consumables (1.25Cr-0.5Mo and 2.25Cr-1Mo steels) for boilers.

Table 1: Heat resistant steel welding consumables for boilers

Kind of steel	ASTM	SMAW	GMAW	GTAW	SAW Flux/Wire	FCAW
1.25Cr-0.5Mo	A387 Gr.11 Cl.1, 2 A213 Gr.T11,12 A335 Gr.P11,12	[T]CM-A96 [T]CM-B98	[T]MG-S1CM [T]MG-S80B2F	[T]TG-S1CM [T]TG-S80B2	[F]G-80/ [T]US-511 [F]G-80/ [T]US-B2	[T]DW-81B2C[CO2] [T]DW-81B2[Ar-CO2]
2.25Cr-1Mo	A387 Gr.22 Cl. 1, 2 A213 Gr.T22 A335 Gr.P22	[T]CM-A106 [T]CM-B108	[T]MG-S2CM	[T]TG-S2CM [T]TG-S90B3	[F]G-80/ [T]US-521 [F]G-80/ [T]US-B3	[T]DW-91B3C[CO2] [T]DW-91B3[Ar-CO2]

SMAW, SAW, GMAW and GTAW are the welding processes commonly applied to fabricate boilers;however, flux cored wires (FCWs) provide higher efficiency than TIG filler rods (GTAW) and covered electrodes (SMAW) as shown in Figure 1. With their ability to create superb bead appearance even in such severe positions as on fixed pipes, FCWs have become mainstream in most shipyards. Hence, it is expected that FCWs will expand to industries requiring heat resistant steels such as boiler fabrication in the near future.

In this article, FCWs for 1.25Cr-0.5Mo and 2.25Cr-1Mo heat resistant steels that conform to AWS specifications will be discussed.

2.Line-up of FCWs for heat resistant steels

Kobe Steel’s selection of FCWs for heat resistant steels is shown in Table 2. They are classified according to the steels and shielding gases as specified by the AWS while also conforming to ASME’s F-No. and A-No.

Table 2: FCWs for heat resistant steels

	100%CO2	Ar-20%CO2	ASME F-No.	ASME A-No.
1.25Cr-0.5Mo	[T]DW-81B2C [AWS A5.29 E81T1-B2C]	[T]DW-81B2 [AWS A5.29 E81T1-B2M]	6	3
2.25Cr-1Mo	[T]DW-91B3C [AWS A5.29 E91T1-B3C]	[T]DW-91B3 [AWS A5.29 E91T1-B3M]	6	4

AWS also specifies Post Weld Heat Treatment (PWHT) at 690℃ for 1 hour (690℃x1h). Because PWHT is usually carried out in practice, it is necessary to design welding consumables that provide excellent mechanical properties after PWHT by taking the following points into consideration:

(1) minimizing impurities such as P & S in raw materials;

(2) designing for low C and high Mn in order to provide moderate hardenability and to stabilize notch toughness;

Tables 3 and 4 show typical chemical compositions and mechanical properties after PWHT of the deposited metals, respectively. Both chemical compositions and tensile properties satisfy the AWS requirements, and the impact properties at room temperature (+20 ℃) are sufficient.

Table 3: Typical chemical compositions of deposited metals (mass%)

Kind of steel	Trade designation	Shielding gas	C	Si	Mn	P	S	Cr	Mo
1.25Cr-0.5Mo	[T]DW-81B2C	100%CO2	0.05	0.21	0.96	0.009	0.004	1.22	0.50
	[T]DW-81B2	Ar-20%CO2	0.06	0.29	0.97	0.010	0.005	1.30	0.50
	AWS A5.29 B2	Min Max	0.05 0.12	- 0.80	- 1.25	- 0.030	- 0.030	1.00 1.50	0.40 0.65
2.25Cr-1Mo	[T]DW-91B3C	100%CO2	0.06	0.18	0.99	0.007	0.004	2.26	1.00
	[T]DW-91B3	Ar-20%CO2	0.06	0.29	1.12	0.008	0.004	2.38	1.01
	AWS A5.29 B3	Min Max	0.05 0.12	- 0.80	- 1.25	- 0.030	- 0.030	2.00 2.50	0.90 1.20

Table 4: Typical mechanical properties of deposited metals after PWHT (mass%)

Kind of steel	Trade designation	Shielding gas	PWHT condition	0.2%YS (MPa)	TS (MPa)	El (%)	vE+20℃ (J)
1.25Cr-0.5Mo	[T]DW-81B2C	100%CO2	690℃x1h	539	619	23	54
	[T]DW-81B2	Ar-20%CO2		570	654	22	31
	AWS A5.29 B2		677-704℃ x1-1.25h	Min.470	552 - 689	Min.19	-
2.25Cr-1Mo	[T]DW-91B3C	100%CO2	690℃x1h	571	659	22	82
	[T]DW-91B3	Ar-20%CO2		621	696	22	111
	AWS A5.29 B3		677-704℃ x1-1.25h	Min.540	621 -758	Min.17	-

3.Usability of FCWs for heat resistant steels

In Kobe Steel’s FCWs for heat resistant steels, slag forming agents like a rutile (TiO₂) are added in order to improve usability in all position welding. Figure 2 shows applicable ranges of welding current and arc voltage in horizontal fillet and vertical upward welding. Accordingly, welding current can be as high as about 300A in horizontal fillet welding and 240A in vertical upward welding. Also, a wide range of arc voltage canbe used.

Figure 3 shows bead appearances and cross-sectional macrostructures in horizontal fillet and vertical upward welding, respectively. In addition to obtaining sufficient penetration and a sound weld toe, no large particle spatter adhesion appears, the bead shape shows little unevenness, and bead appearance is glossy.

4.Mechanical properties of FCWs for heat resistant steels under various PWHT conditions

Figures 4 and 5 indicate the mechanical properties under various PWHT conditions including the as-welded condition for reference.

With regard to 0.2% offset yield strength (0.2%YS) and tensile strength (TS), the FCWs fully satisfy the lower limits of those of the base metal even under such high temperature and long time PWHT conditions as 690° C x 4h. On the other hand, it is more effective to perform PWHT under higher temperature (650℃→690℃) and longer time (1h→4h) in order to get better impact properties.

5.Diffusible hydrogen contents of weld metals with FCWs for heat resistant steels

Figure 6 compares the diffusible hydrogen content of the weld metals with FCWs for heat resistant steels with that of a solid wire (GMAW) and a covered electrode (SMAW).

The FCWs’ diffusible hydrogen content is from 2 to 4ml/100g which is inferior to that of [T] MG-S1CM (GMAW) but, almost equivalent to that of [T] CM-A96 (SMAW).

Each of the above tests was conducted right after the package of welding consumables was opened. However, because of moisture absorption by or adherence to the welding consumables, diffusible hydrogen content may increase if the welding consumables are left in the packages for long periods after unsealing. It is, therefore, recommended for the consumables to be used promptly once their packages are opened.

6.Stress relief crack resistance of weld metals with FCWs for heat resistant steels

In actual welding of heat resistant steels, PWHT is erformed to improve weld metal impact properties and lso to remove residual stresses. However, stress relief (SR) cracking or reheat cracking may occur during the PWHT process. Two reasons are widely recognized as the cause of SR cracks.

The formula of SR crack susceptibility for precipitation hardening is as follows:

Note: Applicable range of each element: Cr≤1.5%; 0.1%≤C≤0.25%;
Cu≤1.0%; Mo≤2.0%

The above formulae show that Cr, Mo, Ti, V and Nb are the elements that lead to the formation of precipitate and weakening of SR crack resistance. However, it is important to note that these concerns are more relevant to heat resistant steels than carbon steels, because Cr and Mo are principal and unavoidable elements in heat resistant steels.

In this regard, the rutile (TiO₂) is another important aspect in the design of FCWs for heat resistant steels. Although it is commonly utilized in all-position-type FCWs as a slag forming agent, the Ti element decomposed from TiO₂ inevitably mixes into the weld metal, resulting in reduced SR crack resistance. There is also a possibility that V and Nb may mix into the weld metal as they are unavoidable impurities in the raw materials for welding consumables.

FCWs for heat resistant steels must be carefully designed from the SR crack point of view. In Kobe Steel’s FCWs for heat resistant steels, impurities in the raw materials are strictly controlled, resulting in the achievement of excellent SR crack resistance.

Two evaluation methods of SR crack resistance are available: the high temperature/-slow strain rate-tensile test and the C-shaped ring cracking tes t. Figure 7 compares the reduction of area after fracture in the high temperature/-slow strain rate-tensile test.

When the reduction of area after fracture is low, SR crack resistance is poor. The FCW with no control of impurities resulted in a low reduction of area after fracture. By contrast, [T] DW-81B2 FCW, in which impurities are controlled, had nearly the equivalent reduction of area after fracture as [T] CM-A96MBD, a covered electrode that is utilized in the welding of pressure vessels.

The results of the C-shaped-ring cracking test are shown in Figure 8. It can be observed that the crack occurred with the FCW with no control of impurities, which was the same FCW as the one shown in Figure 7. On the other hand, no crack occurred with [T] DW-81B2 FCW, in which impurities are controlled. These results demonstrate that [T] DW-81B2 has excellent SR crack resistance.

7.Notes on usage

In welding, the PWHT condition should be determined by considering the required mechanical properties, even though FCWs for heat resistant steels show good mechanical properties within the range of 650-690℃x1-4h as shown in point 4, above (Mechanical properties of FCWs for heat resistant steels under various PWHT conditions).

For example, under the high temperature and long time PWHT condition like 690℃x4h, FCWs’ 0.2%YS and TS can fully satisfy the lower limit of those of the base metal, and it is advantageous to improve impact properties. On the other hand, if excessive high temperature and long time PWHT is performed, it will cause the formation of a soft structure called the ferrite band and may result in extreme decreases in TS and notch toughness.

It is, therefore, recommended to conduct a confirmation test in advance to determine whether the mechanical properties will satisfy the requirements when excessive high temperature and long time PWHT exceeding 690℃x4h is applied.

Finally, it is not recommended that the FCWs discussed above be applied in the welding of pressure vessels or parts requiring pressure resistance that specify low temperature toughness. The application of these FCWs should be utilized on parts with no strict toughness requirements, or, in other word, with no pressure resistance requirement.

8.Postscript

In this article, the FCWs for 1.25Cr-0.5Mo and 2.25Cr-1Mo heat resistant steels that conform to AWS standard were discussed. They are designed to fulfill requirements not only for chemical compositions and tensile properties but also for impact properties. Furthermore, as they are aimed to decrease the SR crack susceptibility that is peculiar to heat resistant steels, it is hoped that these FCWs will contribute to the improvement of welding efficiency.

Upon reflecting on feedback from customers who apply these welding consumables onsite, Kobe Steel will make utmost efforts to further improve the properties of these FCWs.

[References]
1. The Journal of The Japan Welding Society, 1992 Vol. 61, No. 6,p469-p472
2. API RP 934-A, 2012, Addendum 2, Annex B
3. The Journal of The Japan Welding Society, 1964 Vol. 33, No. 9, p718-p725

Page Top