Nickel is a silver-white metal that was first isolated in 1751 by the Swedish mineralogist Kronstadt. Ni was not made until 1775. From 1825 to 1826, Sweden began the industrial production of Ni, until the Ni refining into alloy steel, Ni industry has a rapid development. Because it has good mechanical strength and ductility, refractory
High temperature resistance, and has high chemical stability, no oxidation in the air and other characteristics, it is a very important non-ferrous metal raw material, which is used to manufacture
Stainless Steel, high
Nickel Alloy steel and alloy structural steel. Widely used in aircraft, radar, missiles, tanks, ships, spacecraft, atomic reactors and other military manufacturing industry. In the civil industry, nickel is often made of structural steel, acid-resistant steel, heat-resistant steel and other large quantities used in various machinery manufacturing. Nickel can also be used as ceramic pigment and anti-corrosion coating, nickel-cobalt alloy is a permanent magnet material, widely used in electronic remote control, atomic energy industry and ultrasonic process and other fields, in the chemical industry, nickel is commonly used as a hydrogenation catalyst.
Due to its excellent properties, nickel has become an indispensable metal for the development of modern aviation industry, national defense industry and the establishment of a modern system of high-level material and cultural life of mankind.
Properties of Ni:
（1）Melting point: 1453℃, boiling point: 3075℃;
（2）Specific gravity: 8.8g/cm3;
（3）Does not oxidize in the air (even if heated to 700~800 ℃);
（4）Strong corrosion resistance, alkali does not work; Except HNO3, other acids have little effect.
Pure nickel purity is 99.0%, cold deformation, weldability and corrosion resistance are good.
nickel alloy
MONEL&Ni-Cu alloy
Nickel alloys can be divided into five categories by use: nickel-based superalloys, nickel-based corrosion resistant alloys, nickel-based wear resistant alloys, nickel-based precision alloys and nickel-based shape memory alloys. Among them, nickel-based corrosion resistant alloys have good comprehensive properties and can resist various acid corrosion and stress corrosion. The earliest application of nickel-based
Corrosion Resistant Alloy is nickel-copper alloy, also known as Monel alloy. Nickel-based corrosion resistant alloys such as Monel have the following welding characteristics:
Welding hot crack problem: Nickel-based corrosion resistant alloy has a high sensitivity to hot crack. The hot crack occurs at high temperature and no longer expands at normal temperature.
Cleaning before welding: The cleanliness of the weldment surface is an important requirement for successful welding of nickel-based corrosion resistant alloys. The main pollutants on the weldment surface are surface oxide and elements causing embrittlement. The melting point of the oxide skin on the surface of nickel-based corrosion resistant alloys is much higher than that of the base metal, and the discontinuous oxide with slag inclusion is often formed.
Limited heat input: Welding nickel-based corrosion resistant alloys with high heat input may have adverse effects, including some degree of annealing and grain growth in the heat-affected zone. High heat input may produce excessive segregation, carbide precipitation or other harmful metallurgical phenomena. This may cause thermal cracking or reduce corrosion resistance. The grain size of the base metal must also be considered in the selection of welding methods and welding processes. When welding problems occur, the welding process should be improved, the heat input should be reduced or the welding method with low heat input should be adopted.
Corrosion resistance: For most nickel-based corrosion resistant alloys, the corrosion resistance does not have much effect after welding. Filling materials with chemical composition close to the base material are usually selected. In most cases, the corrosion resistance of the weld metal is equivalent to that of the base metal.
Process characteristics: The fluidity of liquid weld alloy is poor, if the welding current exceeds the recommended range, it will not only overheat the weld pool, increase the thermal crack sensitivity, but also make the deoxidizer in the weld metal evaporate, and there are pores. Therefore, it is necessary to consider the swing process of welding rod (welding wire); The depth of weld metal can not be increased by increasing the welding current.
INCONEL，Ni-Cr，Ni-Cr-Fe alloy
Inconel is followed by different three digits to indicate alloys with different chemical compositions, such as Inconel600, Inconel625, Inconel718, etc. The first digit is an even number, indicating the solid solution strengthened alloy, and the odd number indicates the precipitation strengthened alloy.
Inconel600 has excellent strength and easy processing characteristics, mainly used in corrosive environment, oxidation resistance in the environment of up to 1177 degrees, heat and corrosion resistant alloy, with good intergranular corrosion resistance, stress corrosion cracking and high temperature halogen corrosion ability; Oxidation resistance, cold and hot processing performance, low temperature mechanical performance, cold and hot fatigue performance. High strength at 650℃, good formability, easy to weld. Mainly used in steam engines, water heaters, nuclear power plant coolers; Caustic soda and VCM production; Pulp and paper industry.
INCOLOY, Fe-Ni-Cr alloy
Incoloy is a nickel-chromium ferroalloy designed to resist oxidation and carbonization at elevated temperatures. There are many types of Incoloy alloys: Common examples are Incoloy800, Incoloy800H, Incoloy800HT, Incoloy825, Incoloy840, Incoloy901, Incoloy925, Incoloy330, Incoloy 25-6Mo, etc.
High-temperature alloy 800 series, is the Inco-corporation developed a heat-resistant nickel-chromium ferroalloy, the early alloy brand is Incoloy 800, for the purpose of corrosion resistance, solution temperature is low, the first used in the military and aviation fields, since the 1950s used in the petrochemical industry, as a conversion furnace tube; Incoloy 800H alloy is developed from Incoloy 800, with high temperature strength as the purpose, high solution temperature, the alloy has good creep resistance and high temperature corrosion resistance, widely used in the petrochemical industry of high temperature equipment, to improve the design level and technical level of petrochemical equipment, played an important role.
Incoloy800 is suitable for F-, Cl-, ammonia acid medium condensation conditions, in H2SO4, HNO3 and their mixed solution also has good corrosion resistance. Resistant to pressure corrosion and polyethylene corrosion. Mainly used in nuclear generators, water heaters, gas coolers, waste steam boilers. Incoloy800H is an antioxidant medium, corrosion resistance, high temperature resistance and high carburizing heat strength, with excellent mechanical properties and corrosion resistance, processing properties and welding properties, is a wide range of comprehensive corrosion resistance of the alloy. Resistance to steam, soft water, steam - air -CO3 mixture, various acid solutions, salts, H2S corrosion. Excellent oxidation resistance, nitriding and argon embrittlement at 600 degrees Celsius. Mainly used for heat exchangers and piping systems in petrochemical plants; High temperature steam pipeline; Heating source with protective layer.
Superalloy 800 series is a kind of corrosion-resistant Ni-Fe-Cr alloy that cannot be precipitated and strengthened. It is a high-quality material with good strength, corrosion resistance, oxidation resistance and carburizing resistance under high temperature conditions. Due to the combined action of high temperature and high pressure, the consequences of any welding defects are extremely serious. In particular, the welding crack not only directly reduces the effective bearing area of the welded joint, but also forms a strong stress concentration at the tip of the crack, so that the local stress at the crack tip is much higher than the average stress of the welded joint. Once it fails, it will bring serious consequences to production. The welding of this material is much more difficult and the quality standard is high. This material has a high density, poor thermal conductivity than steel (about 1/3 of carbon steel, 1/2 of stainless steel), and much higher resistance than carbon steel, so the heat is not easy to dissipate during welding, easy to form overheating and grain growth. Therefore, in the welding process, appropriate measures must be taken, otherwise it will lead to overheating of the welded joint, so that its mechanical properties and heat resistance are sharply reduced. Therefore, in the development of process parameters and welding operations, this feature should be fully considered to increase the cooling speed.
Production practice has proved that the weld metal of nickel-based superalloy has a large tendency of crystallization cracking during welding. In addition, weld porosity, intergranular corrosion of welded joints and other strong problems should also be considered in the development of the process.
1. welding hot crack
Nickel-based superalloys have high sensitivity to welding thermal cracks, and the weld seam may produce macroscopic cracks, micro-cracks or both. From the analysis of chemical composition and structure, the alloying element content is large, the single-phase austenitic structure, the solubility of alloying element is limited. These alloying elements interact with Ni and Fe in the matrix to form eutectic substances with low melting point, which segregate at the grain boundary and produce crystal cracks under the action of welding stress. In addition, the weld metal forms single-phase austenitic columnar crystals with strong directionality when solidifying, and the eutectic substances with low melting point segregate between the columnar crystals and crack easily under the action of stress. The low thermal conductivity of nickel-based superalloy, the long time of the workpiece in the high temperature state and the serious growth of the grain promote the formation of low melting point eutectic.
2. Sensitivity to stomata
There are many factors that cause pores, moisture, oil scale, oxide film on the surface of the workpiece, etc., in the welding heating process if these impurities are not cleaned up, they are adsorbed and decomposed into hydrogen, water gas, welding pool in high temperature liquid, can dissolve more hydrogen, oxygen, nitrogen and other gases. The density of nickel-based superalloy is high, the fluidity of molten pool is poor, which affects the escape of gas, so it is easy to form pores.
3. Corrosion tendency in the weld zone
Because the alloy has two sensitization temperature zones, chromium carbide precipitation occurs in the sensitization state, resulting in grain boundary chromium deficiency phenomenon, resulting in intergranular corrosion and stress corrosion tendency in a certain medium. When welding this kind of steel, it should be cooled quickly to avoid the welding zone staying too long at high temperature to prevent intergranular corrosion.
4. Welded joints and other strength problems
The welded joint can not reach the requirement of equal strength with base material in general condition, no matter it is solid solution strengthened nickel-base alloy or precipitation-strengthened nickel-base alloy. For welded Ni-Fe-Cr solid solution strengthened alloy, the instantaneous strength of welded joint at room temperature and high temperature is not significantly reduced, but the lasting strength at high temperature is greatly reduced, and it can not be improved after heat treatment. The wider the superheated zone, the greater the effect. The production practice shows that this is directly related to the common overheating and serious grain growth in the joint heat affected zone.
C-276 welding and heat treatment
The welding properties of C-276 alloy are similar to those of ordinary austenitic stainless steel. Before using a welding method to weld C-276, measures must be taken to minimize the corrosion resistance of the weld and the heat-affected zone. Such as tungsten gas welding (GTAW), metal gas welding (GMAW), submerged arc welding or some other welding methods that can minimize the corrosion resistance of the weld and the heat affected zone. However, welding methods such as oxy-acetylene welding that may increase the carbon or silicon content of material welds and heat-affected zones are not suitable for use.
For the choice of welded joint form, you can refer to the successful experience of ASME boiler and pressure vessel code for C-276 welded joint.
The best way to weld the groove is to use machining, but machining will bring work hardening, so it is necessary to polish the groove before welding.
Appropriate heat input speed should be adopted during welding to prevent the occurrence of hot cracks.
In most corrosive environments, C-276 can be used in the form of welds. However, in very harsh environments, C-276 materials and welds are subjected to solid solution heat treatment to obtain the best corrosion resistance.
C-276 alloy welding can choose its own welding material or filler metal. If certain components are required to be added to the welds of C-276, such as other nickel-based alloys or stainless steel, and these welds will be exposed to a corrosive environment, then the electrode or wire used for welding is required to have corrosion resistance comparable to that of the base metal.
The solid solution heat treatment of Hastelloy C-276 alloy material includes two processes: (1) heating at 1040℃~1150℃; (2) It is quickly cooled to a black state (about 400 ° C) within two minutes, so that the treated material has good corrosion resistance. Therefore, stress relief heat treatment only for Hastelloy C-276 alloy is ineffective. Before heat treatment, clean the oil on the surface of the alloy and other dirt that may produce carbon in the heat treatment process.
The surface of C-276 alloy will produce oxides during welding or heat treatment, which will reduce the Cr content in the alloy and affect the corrosion resistance, so it is necessary to clean the surface. You can use a stainless steel wire brush or grinding wheel, then dip into the appropriate proportion of nitric acid and hydrofluoric acid mixture pickling, and finally rinse with water.
