Welding of nickel-based alloy C-276

Introduced the welding process of nickel-based corrosion-resistant alloy C-276, according to the weldability of C-276, and the development of viable welding process parameters. The test shows that the GTAW welding priming must be taken certain process measures to prevent oxidation on the back of nickel-based alloys and reduce the generation of welding defects.

0. Preface

Alloy C-276 is a corrosion-resistant nickel-chromium-molybdenum forging alloy; it can prevent the formation of grain boundary precipitation in the heat-affected zone of welding, has outstanding resistance to pitting corrosion and stress corrosion cracking, has good resistance to sulfides and chloride ions after welding can no longer be solid solution heat treatment. Therefore, its welded state can be applied to most chemical engineering treatments and is one of the few materials that can withstand the corrosive effects of chemical media such as wet chlorine vapor and hypochlorite and is widely used in areas with severe corrosion in flue gas desulfurization systems.

1. Characteristics of C-276 alloy

C-276 alloy can be forged, hot forged, and punched. Although the alloy tends to work harden quickly, it can withstand deep punching, drawing, press forming, and punching very well.

C-276 alloy has poor fluidity and can be welded with the welding method of chromium-nickel austenitic stainless steel, such as SMAW, GTAW, GMAW, SAW, and the parts used in corrosive environments are not recommended to be welded with oxyacetylene welding and submerged arc welding methods. Special attention should be paid to certain measures to prevent excessive welding heat input.

C-276 has a high linear expansion coefficient and susceptibility to thermal cracking. Thermal cracking is divided into crystalline, liquid, and high temperature plastic loss cracking.
Crystallization cracks are most likely to occur in the arc pit of the welding channel, forming a fire port crack, mostly along the center line of the weld longitudinally but also perpendicular to the weld wave. Liquefaction cracks appear in the heat-affected zone immediately adjacent to the fusion line, and some also appear in the front layer of multi-layer welds. High-temperature loss of plastic cracking may occur in the heat-affected zone and the weld. Various thermal cracks are sometimes macroscopic, but sometimes there are only microscopic cracks. Thermal cracks occur at high temperatures and do not expand at room temperature.

Liquid weld metal fluidity is poor; the bevel angle of the joint needs to be larger to use the swing process; in addition, its weld metal melting depth is shallow, and the thickness of the blunt edge of the joint should be thinner than other steels. Can not be increased by increasing the welding current to improve its process characteristics; welding current over the recommended range not only makes the melt pool overheating, increasing the sensitivity to thermal cracking but also make the weld metal in the deoxidizer evaporates, the appearance of porosity.

2. Welding process test

(1) Test plate size 400mm × 125mm × 10mm (2 pieces), the base material for Thyssen Krupp VDM company C-276, its chemical composition is shown in Table 1. base material density of 8921kg/m³.

(2) Ws-400 inverter welding machine, manual tungsten arc welding, and electrode arc welding dual-use power source, produced by Era, was selected.

(3) Welding wire for SPECIAL METALS company production of ERNiCrMo-4, manual filler wire, its chemical composition is shown in Table 2, welding rod for SPECIAL METALS company production of ENiCrMo-4, its chemical composition is shown in Table 3.

Table.1 Chemical composition and mechanical properties of C-276

Chemical composition (%)												Mechanical
C	Cr	Ni	Mo	Fe	V	Co	W	Mn	Si	P	S	Tensile strength σb/MPa	Yield strength  σ0.2/MPa
0.005	16.2	allowance	16.2	5.3	0.17	0.4	3.4	0.5	0.092	0.01	0.003	746	347

Table.2 Chemical composition of welding
wire ERNiCrMo-4 (%)

C

Cr

Ni

Mo

Fe

V

Co

W

Mn

Si

P

S

AI

Cu

Nb

Other

0.003

15.91

59.24

15.23

5.77

< 0.01

0.01

3.23

0.39

0.01

0.004

0.001

0.2

<0.01

<0.01

<0.5

Table.3 Chemical composition of welding electrode ENiCrMo-4 (%)

C	Cr	Ni	Mo	Fe	V	Co	W	Mn	Si	P	S	Cu	Other
0.013	16.32	59	15.29	5.4	0.028	0.047	3.35	0.24	0.09	0.01	0.001	0.08	<0.5

The electrode diameter is 3.2mm; the electrode should be dried at 150 ℃ and insulated for 2h before welding; put it into the electrode insulation cylinder when using, and take it as you go; the electrode should not exceed 4h outside the furnace.

(4) Auxiliary tools for the angle grinder, welding rod insulation cylinder, concave groove copper pad, stainless steel wire brush, welding seam measuring tape, etc.

(5) Before welding each test plate, bevel surface and bevel edge within 20mm polished clean with an angle grinder to reveal the metal luster, grinding blunt edge size 0.5-1mm, and then wipe clean with acetone bevel and about 20mm on both sides. The surface oxide can form slag or fine discontinuous oxide if not clean. s, p, etc. can form a low melting point eutectic with Ni, increasing the tendency to thermal cracking.

(6) Generally, do not need to preheat before welding. Still, when the base material temperature is lower than 15 ℃, the joint should be heated to 15-20 ℃ in a 250-300 mm wide area on both sides to avoid moisture
condensation. Generally not recommended after welding heat treatment, but sometimes to ensure that the use of intergranular corrosion or stress corrosion needs heat treatment.

(7) Bevel form and positioning welding due to C-276 large coefficient of linear expansion, welding seam shrinkage after welding seriously. If the bevel angle and gap are too small, the last part of the bottoming channel welded will appear without a gap, resulting in the inability to complete the welding of the seam. Select the 60 ° -70 ° bevel angle, the group gap of 2.5-3mm. See Figure 1; after the assembly of the test plate in the bevel on both sides of the end 20mm from the positioning welding, positioning weld thickness 1-2mm, the test plate reserved for anti-deformation 5 °-6 °.

(8) Nickel-based corrosion-resistant alloy welding process is similar to the welding process to obtain high-quality stainless steel welds.

Priming welding selected manual tungsten arc welding, an electrode for cerium tungsten electrode, model WC-20, specifications ϕ2.5mm, the cone angle of 30 ° -60 °, tip grinding flat, the diameter of about 0.4mm. Argon gas protection, its purity of 99.99%, nozzle diameter ϕ12mm.
Welding process back with a copper pad with a concave groove and through the protective gas to protect the back of the weld to prevent oxidation. To ensure a good fusion with the root, the weld seam should not be too thick when bottom welding. Close the arc into a slow slope, the arc off when filling the arc pit, such as the arc shrinkage or arc pit cracks, the application of angle grinder grinding off re-welding. Specific welding parameters are shown in Table 4.

Figure.1 Bevel form and size

Table.4 Welding parameters table

Level	Welding wire diameter d/mm	Polarity	Welding current I/A	Arc voltage U/V	Welding speed v/(cm.min-1)	Front argon flow rate Q/(L.min-1)	Back argon flow rate Q1/(L.min-1)
1	2.4	DC direct connection	95-110	10-12	10-15	12-14	8-10

The key problem of manual tungsten arc welding is the back protection; if it is in place, it will lead to better back forming. After the test, with the right welding method, the back of the molten metal along both sides of the base material is separated from the trend and cannot be gathered. This phenomenon can be avoided with the left welding method; the back of the very good forming, see Figure 2.

In addition, you can also use a “solar flux” back protection agent, the formation of protective welding coating on the back of the weld to prevent oxidation effectively. At the same time, its wetting behavior can ensure that the molten weld metal is completely through the root of the weld. Its surface adhesion to the molten metal after the melting of the heat has a supporting role, playing a back liner role. When using the back shield, the current must be slightly increased to about 110A using the technique of internal wire filling. The back of the weld will not be unable to be closed when the electrode arc welding is used for priming.

The other layers are selected for electrode arc welding, and the welding process of nickel-based alloys is similar to the welding process for obtaining high-quality stainless steel welds. The interlayer temperature is controlled at 150°C or less. Because the liquid nickel-based alloy melting depth is more shallow and liquid weld metal mobility is poor, the welding parameters must be strictly controlled in the welding changes. The general requirement is to swing the welding rod properly; the swing does not exceed three times the diameter of the welding rod, try to use a flat welding position, and must be short arc welding, with arc break to slightly reduce the arc height and increase the welding speed to reduce the size of the molten pool, which can reduce fire cracking. Welding process parameters are shown in Table 5.

Figure.2 Comparison of molten metal forming using argon gas protection back

Table.5 Welding process parameters

Level	Welding wire diameter d/mm	Polarity	Welding current I/A	Arc voltage U/V	Welding speed v/(cm.min-1)
2-5	3.2	DCRP	80-110	22-24	15-20

(9) Welded joints test results. Appearance inspection without porosity, weld tumor, depressions, edge biting, and other defects; welded joints are well formed; the weld surface is smooth. Using the above process measures and welding parameters, welding seam by radiation flaw detection to achieve JB4730-1994 requirements of Class I film, no fusion, cracks, and other defects. Tensile tests on the test piece σ_b are greater than 800MPa; face and back bending tests align with the performance requirements.

3. Conclusion

C-276 nickel-based alloy, poor mobility; to avoid the generation of welding defects, the need for low heat input, multi-layer multi-pass welding, and short arc welding, manual argon arc welding must take strict back argon protection or the use of back shield and other technological measures to prevent oxidation of the back weld. Tested by practice, these process measures achieve the desired effect.

Author: Yang Xinghua