Comprehensive Guide to Nickel-Based Alloys: Monel Alloy
What is Monel Alloy?
Nickel-copper alloy, also known as Monel Alloy, is the earliest developed and most widely used type of nickel-based corrosion-resistant alloy. The alloy was first developed by the American International Nickel Company in 1906, and China began to produce and use it in the 1950s. Monel alloy can be divided into solid solution strengthening type and age hardening type according to the strengthening method. Solid solution strengthening Monel alloys include Monel 400, Monel 404 and Monel 405; age hardening Monel alloys include Monel K-500, M-30C, Monel 505, etc. According to the forming method, Monel alloy can be divided into deformation alloy and casting alloy, and the grades and compositions are shown in Table 1 and Table 2. There are three main ways to express the Monel alloy grade in the American standard system: one is the SAE (Society of Automotive Engineers) system grade, which is represented by the letters K, R, H, S plus Monel alloy. "K" monel alloy is age-hardening type, "R" monel alloy is free-cutting type, "H" monel alloy is medium-strength type, and "S" monel alloy is high-strength type. The second is the digital brand of International Nickel Company, which is represented by a two-digit or three-digit monel alloy suffix. The third is the "ASTM-SAE Unified Number System" brand, abbreviated as "UNS", represented by the letter N + five digits. In China, it is represented by "NCuxx xx xx".
1. Comparison of the organization, properties and applications of three types of monel alloys
Monel alloy has developed more than 20 grades, among which Monel 400 (general monel alloy), Monel K-500 (age-hardening monel alloy), and Monel 505 (high-silicon monel alloy) are widely used, corresponding to China's NCu30, NCu30-3-0.5 and NCu30-4-2-1 grades respectively.
The structure of Monel 400 alloy is a typical single-phase austenite structure with excellent corrosion resistance. The alloy generally does not produce stress corrosion cracks and has good cutting performance. It has excellent corrosion resistance in fluorine gas, hydrochloric acid, sulfuric acid, hydrofluoric acid and its derivatives. At the same time, it is also corrosion-resistant in neutral solutions, hot concentrated alkaline solutions, water, seawater, atmosphere, organic compounds, etc. It can be used for seamless water and steam pipes in power plants, seawater exchangers and evaporators, pump shafts of equipment using seawater and hydrochloric acid, uranium extraction and isotope separation equipment, reactors, etc. Like most nickel-based, iron-based, cobalt-based, and copper-based alloys, Monel 400 alloy also exhibits medium-temperature brittle mechanical properties, and its plasticity is lowest in the temperature range of 600-850℃. The copper content in Monel 400 alloy is very high. If Cr and Al elements are not added, a stable oxide film cannot be formed, so it is very easy to oxidize at high temperatures. Generally, the maximum continuous working temperature in air does not exceed 600℃, and the maximum temperature under anhydrous ammonia and ammoniation conditions does not exceed 585℃. The alloy can be used in both solid solution and cold working state. The solution treatment temperature is generally between 870 and 980℃, but the solid solution strengthening effect is not obvious, and the strength is much lower than that of Monel K-500 alloy, so it is not suitable for high-strength components.
In order to improve the wear resistance of the alloy, researchers added Si element to Monel 400 alloy in the 1980s to improve the wear resistance of the material. They developed NCu30-4-2 alloy and Monel 1-505 alloy, which belong to the SMonel series. The microstructure after aging is face-centered cubic austenite matrix and Ni3 silicon phase. Ni3Si phase is the main strengthening phase of silicon-containing Monel alloy, which will precipitate during solidification, but will precipitate significantly during aging. The alloy has the characteristics of high hardness, high strength, excellent wear resistance and anti-adhesion. It is mainly used to manufacture precision friction parts that require stable operation, such as aviation fuel devices and rudders, rudder shafts of air-to-air missiles, and fuel accessories of aviation engines. The recommended heat treatment process is 950℃×2h solution treatment + 600℃×8h aging treatment. Since silicon tends to react with hydrofluoric acid to generate fluorosilicic acid, silicon-containing materials will be corroded by hydrofluoric acid, so the alloy cannot be used in hydrofluoric acid medium. The addition of Si increases the difficulty of melting and forming, and its plasticity is much lower than that of Monel K-500 alloy. It cannot be welded and formed, and its current application range is relatively small, mainly used in the aviation field.
At the same time, in order to improve the strength of nickel-copper alloys, researchers added a certain amount of Al and Ti elements to Monel 400 alloy to develop Monel K-500 alloy. The physical properties of Monel K-500 alloy are basically similar, but compared with Monel 400 alloy, its thermal conductivity and magnetic transition point are lower (see Table 3). Therefore, the application fields of the two are basically the same, but due to the addition of titanium and aluminum, its comprehensive performance is better than Monel 400 alloy. It has great application potential in the fields of petroleum, chemical industry, navigation, atomic energy, metallurgy, textile, printing and dyeing, papermaking, food machinery, medical equipment, etc. The main products are pump shafts, oil well tools, scrapers, etc., valve internals, fasteners (bolts), springs, etc.
The microstructure of Monel K-500 alloy after aging is mainly composed of face-centered cubic austenite matrix and Ni. 3 (Al, Ti) dispersed precipitates, which is completely different from the strengthening phase composition of Monel 505 alloy. This alloy is the best corrosion-resistant material among the halogen elements in high-strength marine materials except titanium alloy. Therefore, it is widely used in foreign ship equipment manufacturing. It is reported that the fasteners, bolts and nuts of the North Sea oil pipeline and the Thames Bridge in the UK are mainly made of Monel K-500 alloy. This material is widely used in the propellers, key shafts and some parts of pumps of ships. Many equipment and systems of the French "Charles de Gaulle" aircraft carrier also use this material. The US Navy Virginia submarine uses Monel K-500 to manufacture propeller shafts, fasteners and certain external equipment, with excellent application performance. The military has purchased in bulk. In addition to being widely used in the marine field, Monel K-500 alloy is also widely used in my country's petrochemical industry, mainly as valve springs and pump shafts for fuel injection pumps. Our research found that compared with stainless steel springs, Monel K-500 alloy can better guarantee the service life and reliability of the pump in alkaline polymer mother liquid medium when used as a petroleum polymer injection pump. However, when used as a marine fastener, it is necessary to avoid using it in combination with high-potential materials. In summary, Monel K-500 alloy and Monel 505 alloy are new grades developed by adding different strengthening elements to Monel 400 alloy. However, the comprehensive performance of Monel K-500 alloy far exceeds that of Monel 505 alloy, and it has a broader market for use.
2. The influence of elements on alloy properties
The main elements in Monel K-500 alloy are nickel, copper, aluminum, titanium, etc. Copper can improve the corrosion resistance of the alloy in reducing media. The main role of aluminum and titanium is to improve the strength and mechanical properties of the alloy. Iron can significantly improve the resistance of Monel K-500 alloy to turbulent impact corrosion in seawater and improve the processing performance of the alloy; Manganese can also improve the corrosion resistance of the alloy in flowing seawater and enhance the beneficial effects of iron, such as refining the structure; Sulfur can improve the cutting performance of Monel K-500 alloy, but excessive sulfur content will reduce its welding and forging performance, so it needs to be controlled within 0.03wt%; The effect of carbon on Monel K-500 alloy is quite complicated. On the one hand, it can improve the strength of the alloy, and on the other hand, it can reduce the plasticity of the alloy. It has a very small solubility in the alloy and can easily form primary and secondary carbides with titanium. The amount of carbide precipitation is proportional to the carbon content, which has a significant effect on the forging performance of the alloy. The content of parts with large deformation should be controlled below 0.15wt%.
3. Strengthening mechanism of Monel K-500 alloy
Generally speaking, the main strengthening methods of nickel-based alloys are grain boundary strengthening, solid solution strengthening and precipitation strengthening. The strengthening mechanism of Monel K-500 includes solid solution strengthening and precipitation strengthening, among which aging precipitation strengthening is the main strengthening method. Table 4 shows the solid solution strengthening coefficients of elements such as C, Ti, and Mn on Monel K-500. It can be seen that the strengthening effect of C is obvious. Therefore, Monel K-500 usually has a higher carbon content (C ≥ 0.10%), which can effectively improve its interstitial strengthening. Compared with Monel400 alloy, the tensile strength and yield strength of Monel K-500 alloy after aging treatment are more than 2 times higher under the premise of the same solid solution strengthening. The strengthening of the alloy is mainly due to the dispersion precipitation of γ' phase at medium and low temperatures.
