Research on Welding Process of Inconel601 Nickel Base High Temperature Alloy

In order to study the welding process of Inconel601 nickel based high-temperature alloy, based on the chemical composition, mechanical properties, metallurgical properties, and other characteristics of the material, ERNiCrCoMo-1 welding material was selected as the filler metal. Welding tests were conducted on Inconel601 by controlling heat input and adjusting welding parameters. Mechanical performance testing, corrosion testing, and metallographic analysis were also conducted on the welded joints. The results show that a larger cooling rate is achieved by controlling the heat input, and under the metallurgical action of alloying elements, the deposited metal achieves good crack resistance without the occurrence of crystal cracks in the weld seam, and the weld seam is formed aesthetically; Welded joints have excellent mechanical properties and good corrosion resistance.

0. Preface

Inconel601 is a solid solution strengthened nickel based high-temperature alloy developed by Huntington Alloy in the 1970s. It has strong resistance to strong oxidation medium corrosion and excellent high-temperature strength and is widely used in fields such as petrochemical, aerospace, and environmental power generation. The excellent high-temperature oxidation resistance and high-temperature mechanical properties of Inconel601 have made it highly favored in the petrochemical industry and have been effectively applied in distillers, condensers, heaters, etc., in the field of fatty acid processing. However, due to the poor thermal conductivity and flowability of nickel based alloys and the addition of various solid solution strengthening elements in Inconel601 alloy, welding cracks, structural segregation, and other defects are prone to occur during the welding process. Due to the easy formation of low melting point eutectic compounds between Ni and S, P, N, O, etc., the weld seam is prone to thermal cracking. In addition, Inconel601 alloy contains elements with the effect of qualitative transformation agents such as Ti, Nb, Al, etc., which further increase the difficulty of welding. Welding joints, the weakest link in high-temperature corrosion-resistant alloys, can cause immeasurable losses if corroded and damaged during high-temperature service, leading to equipment failure. Therefore, studying the welding process of Inconel601 alloy to ensure the various performance indicators of the welded joint is of great significance for manufacturing and using heat and corrosion resistant equipment.

1. Welding process and method

1.1 Selection of Welding Materials

The chemical composition of Inconel601 high-temperature alloy is shown in Table 1, and the mechanical properties under the solution annealing state are shown in Table 2. Based on the chemical composition, mechanical properties, and welding characteristics of Inconel601 alloy, welding materials with low carbon content, low impurity elements, slightly low Fe element content, and slightly high solid solution strengthening elements should be selected for welding.
Table.1 Chemical composition of Inconel601 high-temperature alloy material

Table.2 Mechanical properties of Inconel601 high-temperature alloy material

Previously, ERNiCrFe-11 was chosen as the filler wire, which was prone to crystal cracks. The chemical composition comparison between welding wire ERNiCrFe-11 and ERNiCrCoMo-1 is shown in Table 3. Using Figure 1 as a reference guide to evaluating the sensitivity of crystal cracks, as C_req increases, the material becomes less sensitive to crystal cracks. An increase in C_req can effectively prevent the generation of crystal cracks. Due to the continuous aggregation of elements such as Cr and Mo towards the grain boundaries during crystallization, the corrosion resistance of the weld seam will decrease. The addition of more solid solution strengthening elements such as Co, Mo, Al, and Ti not only increases the chromium equivalent (C_req) due to the increase of Mo in ERNiCrCoMo-1 but also inhibits the formation of low melting point eutectic and increases the eutectic temperature, resulting in the formation of γ’- Ni3Al phase and γ’ To improve strength. The low diffusion of Mo element in ERNiCrCoMo-1 has a certain effect on improving the creep strength of welds. At the same time, the addition of Mo element in nickel based alloys can effectively improve their plasticity. Adding 6%-12% Mo element in nickel chromium alloys can give the alloy excellent high-temperature mechanical properties and corrosion resistance. ERNiCrCoMo-1, as a filler metal, has higher crack resistance. According to the welding material list, the mechanical properties of ERNiCrCoMo-1 deposited metal are shown in Table 4. After comprehensive consideration, Inconel601 alloy selects ERNiCrCoMo-1 welding material with higher strength than the base material as the filler metal for welding.
Table.3 Chemical Composition of ERNiCrFe-11 and ERNiCrCoMo-1 Welding Materials

Figure.1 Solidification crack sensitivity based on material composition
Table.4 Mechanical Properties of ERNiCrCoMo-1 Deposited Metal

1.2 Preparation of welding specimens

Due to the low thermal conductivity of nickel based alloys, the flowability and wettability of the deposited metal could be better. Under this condition, the weld penetration is shallow, and the root of the weld is not easily penetrated. Therefore, the penetration of the weld can be achieved by increasing the groove angle and reducing the blunt edge. Add 300 mm × 150 mm × The 8mm Inconel601 alloy specimen is machined into a 70 ° groove and welded in the form shown in Figure 2.

Figure.2 Welding groove size and processing diagram

1.3 Selection of welding process

Inconel601 alloy contains many alloying elements, and inappropriate welding processes and parameters can lead to the formation of crystalline cracks in the weld seam. Although the fluidity of its deposited metal is poor, increasing the penetration depth by increasing the heat input is impossible. If the heat input is blindly increased, it will not only fail to increase the penetration depth but will instead generate microcracks at the weld seam, reducing the service life of the equipment. The formula  shows that the solidification temperature range of the same specimen under the same welding conditions Δ T. Heat transfer efficiency η, the thermal conductivity coefficient K, initial temperature T_O, and liquidus temperature T_L are all constant. At the same time, the length L of the pasting zone is only related to voltage V and current I. Reducing heat input can reduce the length of the gelatinous zone during crystallization, thereby avoiding crystallization cracks. Therefore, welding should be carried out with minimal heat input while ensuring the formation of the weld seam. Select the optimal welding parameters through experiments, as shown in Table 5. The maximum welding heat input Q_max=0.957 kJ/mm can be calculated according to the welding parameters.
Table.5 Welding Process Parameters

Number of welding layers	Current/A	Voltage/V	Welding speed/(mm.min-1)	Shielding gas	Gas flow rate/(L.min-1)
					Positive	Back
1	115	11	100-120	99.99%Ar	13	20
2	130	12	100-120	99.99%Ar	13	20
3	130	12	100-120	99.99%Ar	13	20
4	135	13	110-130	99.99%Ar	13	20
5	100	10	100-120	99.99%Ar	13	20

The welding of Inconel601 alloy is prone to solidification cracks, so it is necessary to clean the groove’s edge strictly. The welding material removes pollutants within at least 50mm of the welding material and groove edge. It prevents harmful elements from entering the weld pool, forming low melting point eutectic substances and generating thermal cracks. At the same time, removing the oxide film on the edge of the groove and the welding material is also necessary. As the melting point of the oxide film is higher than that of the weld metal, the unmelted oxide film enters the molten pool, which is highly prone to internal defects in the weld seam. Welding adopts a swinging method to increase the fluidity and wettability of the molten pool, which can effectively solve the defect of incomplete fusion at the edge of the groove caused by poor fluidity of the deposited metal.

2. Performance testing and discussion

2.1 Non-destructive testing

Using the XXG-3005 inflatable X-ray flaw detector, 100% radiographic testing was conducted on the welding test plate, and the results showed no cracks, pores, or other defects in the weld seam.

2.2 Mechanical performance test

Prepare two sets of horizontally stretched specimens, as shown in Figure 3. Perform tensile test according to GB/T 228.1-2010, and the tensile test results are shown in Table 6. The data shows that the weld area has a higher tensile strength than the base metal, and the sample has good room temperature ductility. The elongation of the specimen after fracture is slightly lower than that of the base metal. From the specimen’s appearance, it can be seen that the necking of the base metal is significantly greater than that of the weld seam, so the plasticity of the weld seam is slightly lower than that of the base metal.

Figure.3 Inconel601 nickel based high-temperature alloy tensile specimen
Table.6 Tensile test results of Inconel601 nickel based high-temperature alloy welded joints

Sample number	Rm/MPa	Rp0.2/MPa	A/%	Z/%	Fracture location
1	656	274	51	46.5	Base material
2	644	268	48	41.7	Base material

Prepare the Inconel601 nickel base superalloy bending sample and verify the plastic index of the weld and Heat-affected zone, the compactness and continuity of the joint by bending the front and back of the weld by GB/T 2653-2008. The test results are shown in Figure 4. Conduct a 180 ° bending test on the sample, and the diameter of the bending center is 4 times the thickness of the sample. After bending, the surface of the sample weld and the Heat-affected zone is smooth, without cracks, openings, and other defects, indicating that although the strength of the weld is slightly higher than that of the base metal, the weld and the Heat-affected zone can still maintain excellent plasticity indicators.

Figure.4 Inconel601 nickel based high-temperature alloy bending specimen
Analysis of the mechanical performance test results showed that the tensile strength slightly increased and the plasticity index slightly decreased due to the addition of elements such as Co and Mo in the welding material, which increased the strength of the weld metal and also improved its thermal strength. It is precisely because the increase of these two elements reduced the plasticity and ductility of non-ferrous metals. The macroscopic manifestation is that the tensile test resulted in a lower shrinkage rate at the weld than the base metal. However, through bending tests, it was found that the reduced plasticity did not have a significant impact on the use of the material and still maintained the integrity of the material under the required bending conditions, thereby ensuring that the weld seam still maintains excellent performance during the bending or forming process.

2.3 Intergranular corrosion test

Intergranular corrosion is an important factor in nickel-based alloys’ failure during service. Intergranular corrosion is based on the difference in corrosion resistance between grain boundaries and grains. According to the theory of poor chromium, the precipitation of Cr₂₃C₆ between grains creates a “poor chromium zone.” Corrosion gradually extends from the “poor chromium zone” to the interior of the metal, damaging the crystal structure, greatly reducing the material’s mechanical properties, and ultimately causing equipment failure. According to the mechanism of intergranular corrosion, the resistance to intergranular corrosion can be determined by measuring the weight loss of the sample under corrosive conditions.
Prepare corrosion samples according to the GB/T 15260-2016A method and use sulfuric acid iron sulfate reagent to test 30mm × 20mm × The sensitivity test of intergranular corrosion was conducted on a 2mm sample with a micro boiling period of 24 hours for a total of 5 cycles. The macroscopic sample after corrosion is shown in Figure 5, and the test results are shown in Table 7.

Figure.5 Macroscopic specimen of Inconel601 nickel based high-temperature alloy after corrosion
Table.7 Intergranular Corrosion Test Results of Inconel601 Nickel Base High Temperature Alloy

Sample number	Corrosion rate/(mm.a-1)
	Cycle 1	Cycle 2	Cycle 3	Cycle 4	Cycle 5	Mean value
1	0.298	0.286	0.258	0.263	0.274	0.2758
2	0.287	0.288	0.264	0.255	0.267	0.2722

According to the experimental results, it can be seen that the welded joint maintains excellent intergranular corrosion resistance, which is due to the use of small heat input during the welding process, resulting in a faster cooling rate and increased undercooling during the crystallization of the weld. During the crystallization of the weld seam, the nucleation rate increases, causing the weld metal to form fine grains. During material sensitization, due to the stabilizing effect of Ti added in the welding material, it can preferentially combine with carbon to form TiC, thereby preventing elements such as Cr and Mo from forming carbides. Therefore, chromium and molybdenum carbides are rarely precipitated in the weld seam. Additionally, the increase in Mo content in the welding material increases the chromium equivalent, compensating for the loss of Cr element due to the formation of M23C6 type carbides and avoiding chromium deficient areas, Improving resistance to intergranular corrosion.

2.4 Metallographic analysis

The microstructure of Inconel601 nickel based high-temperature alloy welded specimens is observed using a metallographic microscope, as shown in Figure 7. The weld is a columnar single-phase austenite structure. Compared with the Heat-affected zone and the base metal, the weld grain is smaller, and there is no continuous precipitation, no obvious carbide, and no intermediate equivalent structure. Observing the weld seam showed no defects such as pores, slag inclusions, and cracks.

Figure.7 Microstructure of Inconel601 nickel based high-temperature alloy welded joint

3. Conclusion

• (1) Using ERNiCrCoMo-1 welding material to weld Inconel601 high temperature nickel base alloy, the tensile strength and plasticity of the weld and the heat-affected zone can be well guaranteed, and it has excellent resistance to crystal cracks, which can effectively avoid the occurrence of crystal cracks.
• (2) Using a smaller heat input to weld Inconel601 alloy and adding solid solution strengthening elements in the welding material improves the intergranular corrosion resistance of the weld seam.

Authors: Cheng Long, Meng Qinghai, Bai Xiaolin, Qiu Yu

Source: China Forgings manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)