Technical Highlight Vol.6

Kobelco flux cored wires for stainless steels : meeting diverse market needs with a wide range of products

Kobelcoux cored wires for stainless steels : meeting diverse market needs with a wide range of products

1. Global demand for stainless steels and associated welding consumables

Figure 2: Transition of the scale of penstock in Japan.

Figure 1 : Global stainless steel production
Note : China is included in Asia prior to 2007.

In the years since the financial crisis of 2008, global stainless steel production recovered smoothly due to large increases in consumption in China, reaching a level of 30 million tons in 2011 (see Figure 1). It is forecast to expand to 40 million tons in 2014 and 45 million tons in 2020 because a constant increase in worldwide demand is expected.

When worldwide demand is examined by country, China has accounted for the largest amount since 2008 and about 40 % in 2011, far outstripping demand from Japan (slightly more than 10 %) and the USA (just under 10 %). Forecasts of future demand suggest increasing consumption from emerging markets such as India and Turkey in addition to China.

As for what types of stainless steel will be in demand in 2020, austenitic steels are expected to slightly decrease from the current 60 % of total demand to about 50 % while ferritic as well as duplex steels will increase.

By contrast, while data on global demand for welding consumables for stainless steels is not available, it can be calculated from the data on stainless steels. Information on welding consumables for stainless steels in Japan is available from the Japan Welding Material Association, which reports that domestic consumption of welding consumables for stainless steels reached around 7,500 tons in 2011. Accordingly the ratio of welding consumables to stainless steels (unit ratio) is calculated as about 0.2 %. Hence, using the same unit ratio of 0.2 %, worldwide demand of welding consumables for stainless steels is forecast to be a little over 60,000 tons in 2011, increasing to 80,000 tons in 2014 and 90,000 tons in 2020.

2. Features of Kobelco flux cored wires for stainless steels

Figure 2 : Schematic cross section of Kobe Steel FCW
for stainless steel

Figure 2 : Schematic cross
section of Kobe Steel FCW
for stainless steel

Flux cored wires (FCWs) provide high deposition rate as well as excellent weldability in general. The high deposition rate helps to decrease total welding time, to improve weldability, and to minimize the time spent on treatment-after-welding such as removing spatter and fume sticking to steel plates. Especially when used with austenitic stainless steels, FCWs provide welded portions with beautiful appearance and high corrosion resistance: important factors on austenitic stainless steel structures.

Figure 2 shows a schematic cross section of Kobe Steel FCW for austenitic stainless steels. The outer sheath is drawn to become as thin as possible so as to allow for high current density, which in turn, increases efficiency by raising the amount of welded metal per unit of time even if the same welding current is applied.


Table 1 : Wide range of FCWs for stainless steels
Steel type or Application Feature and key note for application Product name AWS
Classification
Main
Chemistry
Applied
position *1
304 General DW-308 E308T0-1/-4 20Cr-10Ni F, HF
DW-308P E308T1-1/-4 20Cr-10Ni F, HF, VU, OH
304H Bismuth free ; High temperature operation DW-308H E308HT1-1/-4 19Cr-10Ni-0.06C F, HF, VU
304, 304L Low carbon (0.04% max.) ; General DW-308L E308LT0-1/-4 20Cr-10Ni F, HF
DW-308LP E308LT1-1/-4 20Cr-10Ni F, HF, VU, OH
304, 304L Gauge plate DW-T308L E308LT0-1/-4 20Cr-10Ni F, HF
Low Cr(VI) in fume DW-308L-XR E308LT0-1/-4 20Cr-10Ni F, HF
Low Cr(VI) in fume DW-308LP-XR E308LT1-1/-4 20Cr-10Ni F, HF, VU, OH
Cryogenic temperature (≥27J/-196℃) DW-308LTP E308LT1-1/-4 20Cr-10Ni F, HF, VU, OH
DW-308LT E308LT0-1/-4 20Cr-10Ni F, HF
Bismuth free ; Solution treatment DW-308LH E308LT1-1/-4 19Cr-10Ni F, HF, VU
TIG rod for root pass welding without back purging gas TG-X308L R308LT1-5 20Cr-10Ni F, HF, VU, OH
316, 316L General DW-316L E316LT0-1/-4 19Cr-12Ni-2.3Mo F, HF
DW-316LP E316LT1-1/-4 18Cr-12Ni-2.8Mo F, HF, VU, OH
Gauge plate DW-T316L E316LT0-1/-4 19Cr-12Ni-2.3Mo F, HF
Low Cr(VI) in fume DW-316L-XR E316LT0-1/-4 18Cr-12Ni-2.3Mo F, HF
Low Cr(VI) in fume DW-316LP-XR E316LT1-1/-4 18Cr-12Ni-2.3Mo F, HF, VU, OH
Bismuth free ; Solution treatment DW-316LH E316LT1-1/-4 19Cr-12Ni-2.3Mo F, HF, VU
Bismuth free ; High temperature operation DW-316H E316T1-1/-4 19Cr-12Ni-2.3Mo-0.06C F, HF, VU
Cryogenic temperature (≥27J/-196℃) (316L) DW-316LT E316LT1-1/-4 18Cr-13Ni-2.3Mo F, HF, VU, OH
TIG rod for root pass welding without back purging gas TG-X316L R316LT1-5 19Cr-12Ni-2.3Mo F, HF, VU, OH
Dissimilar
metal and
overlay welding
General DW-309L E309LT0-1/-4 24Cr-13Ni F, HF
DW-309LP E309LT1-1/-4 24Cr-13Ni F, HF, VU, OH
Gauge plate DW-T309L E309LT0-1/-4 24Cr-13Ni F, HF
Low Cr(VI) in fume DW-309L-XR E309LT0-1/-4 24Cr-13Ni F, HF
Bismuth free ; Overlay welding on low alloy steel DW-309LH E309LT1-1/-4 24Cr-13Ni F, HF, VU
TIG rod for root pass welding without back purging gas TG-X309L R309LT1-5 24Cr-13Ni F, HF, VU, OH
General DW-309MoL E309LMoT0-1/-4 23Cr-13Ni-2.3Mo F, HF
DW-309MoLP E309LMoT1-1/-4 23Cr-13Ni-2.3Mo F, HF, VU, OH
General (310S) DW-310 E310T0-1/-4 26Cr-21Ni-0.18C F, HF
High ferrite content DW-312 E312T0-1/-4 29Cr-10Ni-0.12C F, HF
321, 347 General DW-347 E347T0-1/-4 19Cr-11Ni-0.6Nb F, HF
Bismuth free ; High temperature operation DW-347H E347T1-1/-4 19Cr-10Ni-0.6Nb-0.06C F, HF, VU
Bismuth free ; Low carbon DW-347LH E347T1-1/-4 19Cr-10Ni-0.6Nb F, HF, VU
TIG rod for root pass welding without back purging gas TG-X347 R347T1-5 19Cr-10Ni-0.6Nb F, HF, VU, OH
317L General DW-317L E317LT0-1/-4 19Cr-13Ni-3.3Mo F, HF, VU
DW-317LP E317LT1-1/-4 19Cr-13Ni-3.3Mo F, HF, VU, OH
Bismuth free ; Solution treatment DW-317LH E317LT1-1/-4 19Cr-14Ni-3.4Mo F, HF, VU
Duplex stainless steel Lean duplex (ASTM S32101, S32304) DW-2307 E2307T1-1/-4 25Cr-8Ni-0.13N F, HF, VU
Standard duplex
(ASTM S31803, S32205)
DW-2209 E2209T1-1/-4 23Cr-9Ni-3.3Mo-0.14N F, HF, VU
TIG rod for root pass welding without back purging gas TG-X2209 --- 23Cr-9Ni-3.3Mo-0.14N F, HF, VU, OH
Super duplex (ASTM S32750, S32760) DW-2594 E2594T1-1/-4 26Cr-10Ni-3.8Mo-0.24N F, HF, VU
Martensitic
stainless steel
for hydro turbine
All position type DW-410NiMo E410NiMoT1-4 12Cr-4Ni-0.6Mo F, HF, VU, OH
Metal type FCW MX-A410NiMo EC410NiMo 12Cr-4Ni-0.6Mo F, HF
405, 409 Ferritic 13Cr-Nb DW-410Cb E409NbT0-1 13Cr-0.6Nb-0.06C F, HF
Buffer layer for 13Cr overlay welding DW-430CbS E430NbT0-1 17Cr-0.9Nb F, HF
For car exhaust system 17Cr-Nb ferritic metal type FCW MX-A430M --- 17Cr-0.7Nb F, HF
Ni alloy Alloy 625 and 825; Overlay welding ; Dissimilar joint DW-N625 ENiCrMo3T1-4 Ni-21Cr-8.5Mo-3.5Nb F, HF, VU
Cladding and girth welding of clad pipe (5G, 6G) DW-N625P ENiCrMo3T1-4 Ni-21Cr-8.5Mo-3.3Nb Pipe 5G, 6G
Alloy 600 and 800 ; Dissimilar joint DW-N82 ENiCr3T0-4 Ni-21Cr-3.0Mn-2.5Nb F, HF
Alloy C276 DW-NC276 ENiCrMo4T1-4 Ni-16Mo-15Cr-3.3W F, HF, VU
Note: *1: Applied position : F : flat ; HF : horizontal fillet ; VU : vertical upward ; OH: overhead ;
Figure 3 : Comparison of spatter generation

Figure 3 : Comparison of spatter generation

Kobelco FCWs for stainless steels are also highly reputed for a stable arc with both 100 % CO2 and Ar-CO2 mixed shielding gasses, leading to very little spatter generation as shown in Figure 3. This feature is obtained not only by the appropriate flux design (and excellent quality control in Kobelco’s manufacturing plants in Japan and the Netherlands) but also the special wire surface treatment that enables stable wire feedability. Another feature of FCWs for stainless steels such as PREMIARCTM DW-308L, PREMIARCTM DW-316L (for flat position and fillet welding), is how they are designed to handle slag formation : slag can be easily peeled off at the optimum time after welding, which prevents the formation of temper color on the bead surface (see Figure 4).

Figure 4 : Slag removability and bead appearance right after welding (DW-308L)

Figure 4 : Slag removability and bead appearance right after welding (DW-308L)

When temper color forms on welds that require aesthetic appearance and cleanliness, pickling treatment, an acid treatment to remove color from a weld metal surface, is called for as a countermeasure. By avoiding the formation of temper color, the time spent on acid treatment is reduced, boosting productivity.


4. Up-to-date Kobelco FCWs for stainless steels

Kobelco’s technologically advanced FCWs for stainless steels are singular products developed exclusively by Kobe Steel; they are trusted and preferred by users around the world. Table 1 (on page 4) lists the wide range of FCWs available for stainless steels.

3-1. Low Cr(VI) FCWs for stainless steels : “XR series”

Figure 5 : Relationship between flux components and Cr(VI) in welding fume

Figure 5 : Relationship between flux components and Cr(VI) in welding fume

FCWs can generate a higher amount of fumes than other conventional welding processes do, increasing safety risks. The welding fume is an oxide that forms when metal vapor generated by the arc cools and solidifies in the air. In the case of stainless steel welding, the fume contains 5 to 20 % Cr oxide, a portion of which exists as harmful Cr6+, notated as Cr(VI).

The toxicity of Cr(VI) has recently been re-evaluated in accordance with moves toward regulating it more strictly in the workplace. For example in 2010 the American Occupational Safety and Health Administration (OSHA) cut the amount of airborne Cr(VI) allowed in workplace by 90 %. It goes without saying that the most effective method to reducing Cr(VI) associated with stainless steel welding is to install more powerful ventilation systems to remove fumes. On the other hand, if welding fumes contained less Cr(VI) to begin with, less effort would be required removing it via better ventilation like a local ventilation.

Figure 6 : Recommended range of welding parameters of DW-308LP-XR

Figure 6 : Recommended range of welding parameters of DW-308LP-XR

To reduce Cr(VI) in welding fume itself is also effective. Kobe Steel has developed a new FCW series, “XR series” for flat position/horizontal fillet welding as well as for all position welding that drastically reduce the Cr(VI) content in the welding fume. The highly versatile XR series FCWs target three types of stainless steels, namely 308L, 316L and 309L.

As shown in Figure 5, controlling the content of Na and K, added to flux as arc stabilizers, can reduce Cr(VI) content in the welding fume. In order to maintain stable weldability, however, the content of other additives, such as fluorides as well as Na and K, may have to be adjusted.

Figure 7 : Cr(VI) emission rate of DW-308LP-XR

Figure 7 : Cr(VI) emission rate of DW-308LP-XR

One of the new XR series FCWs is the all-position PREMIARCTM DW-308LP-XR. It is designed to apply both 100% CO2 and Ar-CO2 mixed shielding gas. Figure 6 shows the recommended range of welding parameters and Figure 7, the Cr(VI) emission rate (as measured by ISO 15011-1 and ISO 16740), respectively. It shows that the XR series emits Cr(VI) at a rate of just 1/6th that of the conventional DW-308LP. For more information on PREMIARCTM DW-308L-XR and PREMIARCTM DW-316L-XR, please refer to the Product Spotlight column of KOBELCO WELDING TODAY, Vol. 14, No. 3 issued in 2011.

Figure 8 : Bead appearance and macrostructure of butt joint, 3G position welding with DW-308LP-XR

Figure 8 : Bead appearance and macrostructure of butt joint, 3G position welding with DW-308LP-XR

A butt joint was welded in the vertical upward (3G) position with DW-308LP-XR under the conditions listed in Table 2; Figure 8 shows the bead appearance and macrostructure, respectively.

Table 2 : Welding conditions of butt joint in 3G position
Groove shape
and
pass sequence
Location Welding
current
(A)
Arc
voltage
(V)
Interpass
temperature
(℃)
Plate thickness : 15mm
Groove shape : Single V
Groove angle : 60℃
Back side : 3 passes
Final side : 1 pass
Back 160 28 <300
Final 160 28 <300

3-2. FCWs for duplex stainless steels

Figure 9 : Corrugated partition members
in a chemical tanker Figure 10 : Stonecutters Bridge in Hong Kong

Figure 9 : Corrugated partition members in a chemical tanker
Figure 10 : Stonecutters Bridge in Hong Kong

Duplex stainless steels have a two-phase microstructure that is 50 % ferritic and 50 % austenitic. Advantages include high strength, superb resistance against pitting corrosion, crevice corrosion as well as stress corrosion cracking (SCC).

Three different duplex stainless steels are available in the market: (1) standard duplex stainless steels, typically ASTM S31803, S32205 and JIS SUS329J3L; (2) lean duplex stainless steels, which, while inferior to other duplex stainless steels, are nearly equivalent to 304L and 316L in corrosion resistance and lower in cost due to reduced Ni and Mo contents, and (3) super duplex stainless steels, which contain higher amounts of Cr, Mo and N in order to withstand more highly corrosive environments. Because of their excellent pitting corrosion resistance, they are widely applied in desalination plants, oil and natural gas drilling and refining, flue gas desulfurization systems, and corrugated partitions in chemical tankers (Figure 9). Duplex stainless steels are even spreading to more general structures such as the Stonecutters Bridge in Hong Kong (Figure 10) and the roof of New Doha International Airport in Qatar.

FCWs for duplex stainless steels include the newly-developed PREMIARCTM DW-2209 for standard duplex stainless steel, PREMIARCTM DW-2307 for lean duplex stainless steel and PREMIARCTM DW-2594 for super duplex stainless steel. Table 3 shows the chemistries of all weld metals and Table 4, the mechanical properties of DW-2307 and DW-2594, respectively.

Table 3 : Chemistries of all weld metals of FCWs for duplex stainless steels (mass%; 80%Ar-20%CO2)
  C Si Mn P S Cu Ni Cr Mo N PRE
DW-2307 0.03 0.5 1.3 0.02 0.003 0.06 7.9 24.6 0.03 0.15 27.1
AWS A5.22
E2307TX-Y
≤0.04 ≤1.0 0.5-2.5 ≤0.04 ≤0.03 ≤0.75 6.5-10.0 22.5-25.5 ≤0.8 0.10-0.20 -
DW-2594 0.03 0.5 1.2 0.02 0.004 0.03 9.6 25.8 3.8 0.24 42.2
AWS A5.22
E2594TX-Y
≤0.04 ≤1.0 0.5-2.5 ≤0.04 ≤0.03 ≤0.75 8.0-11.0 23.0-27.0 2.5-4.0 0.08-0.30 -
Note : PRE : Pitting Resistance Equivalent=Cr+3.3Mo+16N
Figure 11 : Bead appearance and macrostructure by DW-2594 in 3G position. (80%Ar-20%CO<small>2</small> shielding, 160A-26V)

Figure 11 : Bead appearance and macrostructure by DW-2594 in 3G position.
(80%Ar-20%CO2 shielding, 160A-26V)

Table 4 : Mechanical properties of all weld metals of FCWs for duplex stainless steels
  Tensile properties Notch toughness at
20℃(J)
0.2%PS
(MPa)
TS
(MPa)
El
(%)
DW-2307 571 750 29 58
AWS A5.22
E2307TX-Y
- ≥690 ≥20 -
DW-2594 712 900 25 60
AWS A5.22
E2594TX-Y
- ≥690 ≥20 -

One feature of both duplex stainless steels and their associated welding consumables is the high N content, which may cause blow holes in weld metals, or pits and worm holes on weld metal surfaces when the high N dissolved in a molten metal does not remain within the solidifying weld metal in solid solution state. In Kobelco FCWs for duplex stainless steels, the flux components are optimized so as to resist gas cavities in spite of the high N content. Figure 11 shows the bead appearance and macrostructure of a butt joint by DW-2594 in the 3G position. No defect such as a worm hole or pit is visible.

3-3. Flux cored TIG rod “TGX series”

Figure 12 : Back bead of pipe back-bead welding by TGX wire Figure 13 : Back bead (left) and surface bead (right)

Figure 12 : Back bead of pipe back-bead welding by TGX wire
Figure 13 : Back bead (left) and surface bead (right)

In root pass welding of stainless steel pipes by TIG rod, a back shield of pure Ar gas is usually required to prevent oxidation in the back bead, which could render it unsound. There are two common back shielding methods: whole pipe shielding and local weld zone shielding. However, with either method, the amount of time and Ar gas required for shielding is enormous and expensive.

In another example of Kobe Steel’s leading FCW design technology, the TGX series of FCW filler rods for TIG root pass welding eliminate the need for expensive back shielding. Another highlight is how they allow operators to work safely inside pipes without the danger of oxygen deficiency.

Figure 14 : How to maintain proper key hole

Figure 14 : How to maintain proper key hole

The flux inside the TGX filler rod produces an appropriate amount of slag that completely covers both the back and surface sides of the bead, protecting them from exposure to air and preventing oxidation even without a back shield. The slag covering both sides of the bead is easily removed with a light tap and leaves a beautiful bead as shown in Figures 12 and 13.

Because the TGX series FCWs are seamless, they are handled in almost the same way as solid TIG rods.

In order to secure a sound back bead with TGX filler rod, it is essential to form a key hole during welding, so that a sufficient amount of molten slag will flow to the back side of the groove and cover the back side of the bead (Figure 14).

Table 5 shows the recommended groove shapes, based on the wall thickness and root gap.

Table 5 : Recommended groove shape for root pass welding by TGX filler rod
Groove shape Single V (70℃)
1.0mm shoulder
Wall thickness (mm) 4 6 10
Root gap (mm) 2.0 2.5 3.0

The feeding speed of TGX filler rod differs slightly from that of conventional TIG filler rod. It has to be fed at a high pace and little by little, with attention paid to not feeding too much at one time.

Since the TGX series were launched in the mid 1980s, they have been reputed as one of Kobelco’s benchmark products due to their reliability and for what they’ve achieved. To meet new market needs, the series has been expanded with PREMIARCTM TG-X2209 for duplex stainless steel as well as PREMIARCTM TG-X308L, TG-X316L, TG-X309L and TG-X347.

3-4. DW-T series, suitable for thin stainless steel sheet

Figure 15 : Relationship between welding speed and leg
length by DW-T series

Figure 15 : Relationship between welding speed and leg length by DW-T series

Because the thickness ratio of thin sheets applied to stainless steel structures is much higher than to carbon steel structures, low current welding is more important in stainless steels than in carbon steels. In the past, 0.9 mm dia. FCWs or solid wires were mainly used. However 1.2 mm dia. FCWs have long been desired due to their reasonable cost and better availability. The DW-T series has been developed under these circumstances and is highly evaluated in the markets now.


Figure 16 : Optimum welding parameter range of DW-T series

Figure 16 : Optimum welding parameter range of DW-T series

The DW-T series, 1.2 mm dia. offers the following :
(1)Suitability for small leg length as shown in Figure 15 as well as low current welding as shown in Figure 16. Even 100A welding is possible.
(2)Thin sheet welding from 1.0 or 2.0 mm in thickness is possible whereas it was difficult with conventional 1.2mm dia. wires.
(3)Excellent arc re-start, eliminating the need for a wire edge cut at arc re-start during tack welding.


4. Postscript

As one of the most efficient welding processes, FCWs are forecast to spread further into ever more applicable fields; accordingly, new types of FCWs will have to be developed to meet future needs.

Kobe Steel’s FCWs for stainless steel are some of the most reliable welding consumables in the world and has been highly evaluated and supported from the markets as well. The designing and manufacturing technologies cultivated so far have been utilized for developing not only FCWs for stainless steels but also for nickel alloys as shown in Table 1.

The quite recently developed welding process of FCWs for stainless steels exploits pure Ar shielding gas. The extremely low spatter and low carbon content featured in this newly-developed process will be introduced in the next issue of KOBELCO WELDING TODAY.


References:
【1】 International Stainless Steel Forum (ISSF), Home Page


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