Reference format of this paper: Yang Ziwei, Yu Jin, Hong Haoyuan, et al. Study on Microstructure and properties of Inconel 600 thermowell welding [J]. Electric welding machine, 2020, 50 (2): 99-102
Abstract: Inconel 600 alloy is a Ni Cr Fe based solid solution strengthening alloy, which has good high temperature corrosion resistance and oxidation resistance, excellent cold and hot processing and welding performance, and satisfactory thermal sensitivity and high plasticity under 700 ℃ [1-2]. It is mainly used in thermowells in corrosive atmosphere, vinyl chloride monomer production, production and use of corrosive alkali metals, nuclear reactors, catalytic regenerators in petrochemical production and other fields [3]. Parts will produce certain degree of damage and defects in high-temperature service environment, which need to be repaired or replaced in time. However, when the damaged area is small, replacing the whole parts will not only increase the cost, but also increase the time-consuming and reduce the production efficiency [4]. Therefore, the research on the weldability of Inconel 600 thermowell is of great practical value for practical engineering.
0PART Preface
Inconel 600 alloy is a Ni Cr Fe based solid solution strengthening alloy, which has good high temperature corrosion resistance and oxidation resistance, excellent cold and hot processing and welding performance, and satisfactory thermal sensitivity and high plasticity below 700 ℃ [1-2]. It is mainly used in thermowells in corrosive atmosphere, vinyl chloride monomer production, production and use of corrosive alkali metals, nuclear reactors, catalytic regenerators in petrochemical production and other fields [3]. Parts will produce certain degree of damage and defects in high-temperature service environment, which need to be repaired or replaced in time. However, when the damaged area is small, replacing the whole parts will not only increase the cost, but also increase the time-consuming and reduce the production efficiency [4]. Therefore, the research on the weldability of Inconel 600 thermowell is of great practical value for practical engineering.
01Part test materials and methods
The chemical composition and mechanical properties of Inconel 600 alloy are shown in Table 1 and table 2 respectively. The Inconel 600 thermowell shall be cleaned by the combination of mechanical cleaning and chemical cleaning before welding. First, select appropriate fine sandpaper to polish the casing to remove the oxide skin. Then use acetone ultrasonic to clean the sleeve for 30 minutes, wash it with clean water and deionized water, and put it into the oven for drying and welding.
1.2 welding method
Two welding methods, plasma and TIG, are used to weld Inconel 600 thermowell. The shielding gas is high-purity argon (99.99%), gas flow is 1 L / min, and plasma ion gas flow is 1 L / min. the specific process parameters are shown in Table 3.
After welding, the samples were cut by mechanical processing method, and after 400 × 600 × 800 × 1000 × flat grinding and polishing, the metallographic samples were made by 5g CuCl2 + 100 mlhcl + 100ml anhydrous ethanol corrosion. Gx41 inversion and quanta 250F field emission environmental scanning electron microscopy were used to characterize the joint.
02part result discussion
2.1 interface morphology
The thermocouple sleeve is shown in Figure 1. Removing the damaged thermocouple end and connecting the new thermocouple end can not only improve the production efficiency, but also reduce the cost.
The section morphology of plasma and TIG welding joint is shown in Figure 2. The weld grain of manual TIG welding joint is obviously larger than that of plasma welding joint.
2.2 joint microstructure analysis
The analysis of Inconel 600 base metal and XRD is shown in Figure 3. Inconel 600 nickel base alloy has typical all austenite structure and a large number of twins. Its XRD spectrum shows that Inconel 600 is mainly composed of discretionary, reactive (Ni3Al) and Cr23C6 carbide phases. Among them, the reactive (Ni3Al) phase is co precipitated on the reactive matrix, and it is the most important strengthening phase in the nickel base superalloy. At the same time, active metals such as Ti have strong affinity with N, which is easy to form inclusions such as tin, as shown in Figure 4. C is easy to diffuse to the grain boundary, and in order to reduce the free energy of the system and form carbides with Cr, Cr23C6 is mainly used, as shown in Figure 5.
According to the solidification theory, the solidification of liquid metal is a process of nucleation and growth, mainly heterogeneous nucleation. The base metal near the weld pool is in a semi molten state, which is similar to the crystal structure of the new phase formed by solidification. The nucleation work required for nucleation is the smallest, and it can be used as a good base for nucleation. During the nucleation process, the weld pool always preferentially adheres to the grain surface of the base metal in the semi molten state, and forms a common grain with the base metal at the fusion line, that is, the so-called interactive crystallization and epitaxial growth, as shown in Fig. 6a and Fig. 7a. The shape of the weld pool is similar to a hemispherical surface, i.e. the isothermal surface of liquid metal crystallization. According to the crystal crystallization theory, when the optimal growth direction of the crystal is consistent with the fastest direction of heat dissipation (i.e. the direction of maximum temperature gradient), the crystal preferentially grows perpendicular to the isothermal direction toward the weld center, and the columnar crystal at the weld edge preferentially grows in this direction, as shown in Fig. 6B. There are a few equiaxed grains in the plasma welded joint, because in the later stage of solidification, the center temperature is uniform, the surface grains settle, the broken crystal branches rush in during the growth and take them as the core, and they can grow evenly around to form equiaxed crystals. Fig. 7b shows the weld structure of TIG welding joint. It can be seen that the weld is composed of columnar crystal. Because TIG adopts manual arc welding, the weld pool has long heating time and slow cooling speed, so the crystal has sufficient time to grow, forming the weld structure of full columnar crystal. The carbide at the grain boundary is stable, which will grow and coarsen at high temperature. The carbide inside the grain is unstable and will dissolve at high temperature [6]. It can be seen from Fig. 6a and Fig. 7a that the content of precipitate Cr23C6 in the heat affected zone of welding decreased significantly. The study of Ye shows that the dissolution of [7] M23C6 is a diffusion process. The dissolution of M23C6 at high temperature results in the increase of the content of Cr, Mo and other alloy elements and C elements in the grains. The radius of C atom is small, and it moves to the grain boundary through gap diffusion in austenite. Lipnitskii et al. [8] have shown that alloy elements are easier to accumulate and precipitate at the grain boundary. At high temperature, the Cr, Mo and C elements produced by M23C6 dissolution in the grain will concentrate to the grain boundary, resulting in the growth and coarsening of carbides at the grain boundary.
2.3 microhardness of Inconel 600 thermocouple sleeve welding joint
The microhardness of Inconel 600 TIG joint and plasma joint is shown in Figure 8. Because the precipitate Cr23C6 in TIG weld is less and basically dissolved, the precipitation hardening effect on the matrix is lost. At this time, the alloy is mainly solution strengthening and boundary strengthening [9]. According to hall Petch relationship, with the increase of weld grain size, the strength decreases, and the hardness is approximately proportional to the strength, so it can be concluded that the hardness of TIG weld is lower than that of plasma weld.
03part conclusion
(1) The heat affected zone of TIG welding joint is larger than that of plasma welding joint. The heat affected zone of TIG welding joint is 250-300 μ m, and the heat affected zone of plasma welding joint is 200 μ M.
(2) The phase composition of plasma and TIG weld metal of Inconel 600 thermocouple sleeve is similar to that of base metal, which consists of austenite matrix interaction phase, interaction phase (Ni3Al) and carbide phase (Cr23C6). However, compared with TIG weld metal, there are more precipitated phases in plasma.
(3) Due to the effect of welding thermal cycle, the carbide Cr23C6 in the weld HAZ dissolves and the carbide in the grain boundary coarsens.
(4) The microhardness of TIG weld and plasma weld is lower than that of base metal, but the precipitation of TIG weld is less and the weld grain is coarse, so the microhardness of plasma weld is slightly higher than that of TIG weld.
Reference
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[7] Ye X，HUA X M，WU Y Y，et al. Precipitates in coarse grained heat-affected zone of Ni-based 718 super alloy produced by tungsten inert gas welding[J]. Journal of Materials Processing Technology，2015（217）：13-20.
[8] Lipnitskii A G，Nelasov I V，Golosov E V，et al. A molecular-dynamics simulation of grain-boundary diffusion of niobium and experimental investigation of its recrystallization in a niobium-copper system[J]. Russian Physics Journal，2013，56（3）：330-337.
[9] Liu min. study on Microstructure and properties of Inconel 625 alloy during heating [D]. Yunnan: Yunnan University, 2016
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Editor: Tang Huang
Source: welding machine, No.2, 2020
Author: Yang Ziwei, Yu Jin, Hong Haoyuan
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