Hastelloy C-276 alloy is a nickel chromium molybdenum alloy containing tungsten, which is considered a corrosion-resistant alloy due to its extremely low silicon carbon content.
The performance of most corrosive media in both oxidation and reduction atmospheres.
It has resistance to pitting corrosion, crevice corrosion, and stress corrosion. The high content of Mo and Cr makes the alloy resistant to chloride ion corrosion, and the W element further improves the corrosion resistance. Meanwhile, Hastelloy C-276 alloy is one of the few materials that is resistant to corrosion from moist chlorine gas, hypochlorite, and chlorine dioxide solutions, and has corrosion resistance to high concentration chloride solutions such as iron chloride and copper chloride. Suitable for various concentrations of sulfuric acid solutions, it is one of the few materials that can be applied to hot concentrated sulfuric acid solutions.
The physical properties of Hastelloy C-276 alloy are as follows:
Material composition: 57Ni-16Cr-16Mo-5Fe-4W-2.5Co * -1Mn * -0.35V * -0.08Si * -0.01C * * represents a large margin
Executive standards: UNS N10276, ASTM B575, ASME SB575, DIN/EN 2.4819
Density: 8.90g/cm3
The welding performance of Hastelloy C-276 alloy is similar to that of ordinary austenitic stainless steel. Before using a welding method to weld C-276, measures must be taken to reduce the corrosion resistance of welds and Heat-affected zone, such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), submerged arc welding or other welding methods that can reduce the corrosion resistance of welds and Heat-affected zone. However, welding methods such as oxyacetylene welding that may increase the carbon content or silicon content of material welds and Heat-affected zone are not suitable [2].
The selection of welding joint forms can refer to the successful experience of the ASME Boiler and Pressure Vessel Code for Hastelloy C-276 alloy welding joints.
The welding groove is easy to be machined, but machining will bring work hardening, so it is necessary to polish the machined groove before welding.
Suitable heat input speed should be used during welding to prevent the generation of thermal cracks.
In the vast majority of corrosive environments, Hastelloy C-276 alloy can be applied in the form of welded components. However, in extremely harsh environments, C-276 materials and welding components need to undergo solution heat treatment to achieve good corrosion resistance.
The welding of Hastelloy C-276 alloy can choose to use itself as the welding material or filler metal. If certain components are required to be added to the welds of Hastelloy C-276 alloy, such as other nickel based alloys or stainless steel, and these welds will be exposed to corrosive environments, then the welding rod or wire used for welding must have properties equivalent to the base metal.
The solid solution heat treatment of Hastelloy C-276 alloy material includes two processes:
Heating at 1040 ℃~1150 ℃;
Quickly cool to a black state (around 400 ℃) within two minutes, so that the treated material has good corrosion resistance. Therefore, only performing stress relieving heat treatment on Hastelloy C-276 alloy is ineffective. Before heat treatment, all dirt such as oil stains on the alloy surface that may produce carbon elements during the heat treatment process should be cleaned.
The surface of Hastelloy C-276 alloy will produce oxides during welding or heat treatment, which reduces the Cr content in the alloy and affects its corrosion resistance. Therefore, surface cleaning is necessary. You can use a stainless steel wire brush or grinding wheel, then immerse it in a mixture of appropriate proportions of nitric acid and hydrofluoric acid for pickling, and then rinse it clean with clean water.
Test results and analysis
The effect of heat treatment temperature on the grain growth of C-276 alloy pipes. The longitudinal microstructure of seamless pipes of cold-rolled C-276 alloy after being kept at 1040~1200 ℃ for 10 minutes is shown in Figure 1. It can be seen that after heat treatment within the range of 1040~1200 ℃, the recovery and recrystallization of C-276 alloy have been completed. After heat treatment at 1040 ℃, the grain size is smaller and there are a large number of twins in the grain. As the heat treatment temperature increases, the grains gradually grow; When the heat treatment temperature is between 1080~1160 ℃, the grain size is relatively uniform; During heat treatment at 1200 ℃, significant growth of individual grains occurred.
The effect of heat treatment temperature on grain size of C-276 alloy during insulation for 5, 10, 20, and 30 minutes. It can be seen that under the same holding time, the grain size gradually increases with the increase of heat treatment temperature, and the trend of grain growth is the same. At temperatures ranging from 1040 to 1080 ℃, grain growth is faster, while it slows down within the range of 1080 to 1160 ℃, and accelerates again at temperatures ranging from 1160 to 1200 ℃.
The decrease in grain boundary interface energy is the main driving force for grain growth. During the grain growth process, the increase in grain size corresponds to a decrease in the total grain boundary area, resulting in a decrease in the total interface energy of the system. The grain growth rate is related to the grain boundary migration mechanism, and the grain boundary migration rate is closely related to temperature, which is a thermal activation process. The relationship between the large angle grain boundary migration rate M and temperature T satisfies the Arre heni us relationship (2426), i.e., M=Mg exp (- QR/T) equation: M. Is a constant; Q is the apparent activation energy of grain boundary migration, kJ/mol; R is the gas constant, J/(mol · K); T is the thermodynamic temperature, K.
The relationship between the grain boundary movement speed v and the driving pressure P is: v=MP, where M is the grain boundary mobility; And P=y,/D, where y. is the interfacial energy and D is the grain diameter. By integrating dD/dt, it can be obtained that D=y, Mt replaces equation (1) with equation (2), and assuming time t is constant, it can be obtained that D '=A exp (- QR/T) where A is a constant, A=y, M. Taking the logarithm of both sides of equation (3) can obtain: InD=1/2InA-Q/(2RT), where Q is the apparent activation energy of grain boundary migration, kJ/mol; R is the gas constant, J/(mol · K); T is the thermodynamic temperature, K. It can be seen that InD has a linear relationship with 1/T.
Calculate the average grain size of C-276 alloy seamless pipes after insulation at 1040-1200 ℃ for 5-30 minutes, and perform regression analysis according to equation (4) above, as shown in Figure 3. From Figure 3, it can be seen that the linear fitting curves under different holding times are approximately parallel to each other. According to this result, when the holding time is 10 minutes, the relationship between grain size D and heat treatment temperature T is: lnD=0.5lnA-1.887 × According to equation (5), the apparent activation energy of grain boundary migration for C-276 alloy after 10 minutes of insulation at 1040-1200 ℃ is 313.77kJ/mol, which is higher than the self diffusion activation energy of pure nickel in the matrix lattice (about 285.1kJ/mol) (27). This is mainly because C-276 alloy contains more alloying elements, increasing the activation energy of grain growth and inhibiting grain growth.
