825alloy

Alloy 825 is a nickel-iron-chromium solid solution strengthening alloy invented by Inco Corporation with molybdenum, copper and chin. It was developed for use in both reducing and oxidizing media. The alloy has resistance to chloride ion stress corrosion cracking, resistance to pitting, crevice corrosion and many corrosive solutions. This kind of metal in China is a corrosion-resistant alloy with the grade NS142.
825 alloy has many advantages over its Ni-based superalloy. It is a general-purpose engineering alloy with acid and alkali corrosion resistance in both oxidizing and reducing environments. Alloy 825 is resistant to chemicals with a wide range of oxidation resistance, such as nitric acid solutions, nitrates, divalent copper salts, ferric salts and mercury salts, with the exception of chlorides. Alloy 825 is also resistant to attack by many acids and mixed acids of oxidizing and reducing chemicals. It has a high nickel content, plus molybdenum and copper, making it more resistant to hot sulphuric acid, sulfurous acid and phosphoric acid solutions than any common stainless steel. Chromium greatly improves the corrosion resistance of the alloy in oxidizing media such as nitric acid, nitrates, and oxidizing salts. It is resistant to etching of nitric acid solutions of almost all concentrations and temperatures. By suitable heat treatment, titanium can make the alloy more excellent in intergranular corrosion resistance. The alloy is stabilized by the addition of titanium and, due to the lower carbon content, avoids corrosion in the corrosive environment of normal use due to precipitation of carbides in the heat affected zone of the weld. The nickel content of the alloy is sufficient to resist the stress corrosion cracking of the austenite.

1.1 Hot workability of 1.1 825 alloy
The 825 alloy has a narrow thermoforming section and high deformation resistance. After the thermal compression and hot tensile tests of the 825 alloy, it was found that the reduction of the section of the alloy 825 during hot stretching increased first and then decreased with the increase of the stretching temperature. According to the requirements in actual production (in the actual production, the area shrinkage rate is generally greater than 50% as the thermoplastic critical value), the thermal processing critical temperature value of 825 alloy is T=1240 degrees Celsius, and the thermal processing interval of 825 alloy under high strain rate conditions. It is 1050 degrees Celsius <T < 1240 degrees Celsius.

1.2 825 alloy weldability
Alloy 825 is a Ni-Cr-Fe austenitic gold material containing a small amount of AL and Ti. It has corrosion resistance, high strength, high temperature oxidation resistance and good weldability. When 825 alloy is welded, since impurities such as S and P are segregated in the weld metal, the low-melting eutectic formed by S and Ni forms a fused film between the crystals, which easily causes intergranular cracks, so S and P are harmful elements. The alloy 825 has poor thermal conductivity, and the welding heat is not easily diffused, which tends to cause overheating, resulting in coarse grains, increasing the thickness of the intercrystalline interlayer and weakening the intergranular resultant force. It also enables the liquid solidification of the weld metal to be too long, which promotes the formation of hot cracks.
Alloy 825 has high sensitivity to hot cracking (crystal cracking, liquefaction cracking and high temperature deplasticization cracking). When welding, crystal cracks are most likely to occur in the weld crater, forming a crack in the fire. The crystal crack is formed at a slightly higher temperature above the solidus line. Liquefaction cracks mainly occur in the heat affected zone near the fusion line. Liquefaction crack is a microcrack that cracks along the austenite grain boundary. Its small size is mostly found in the recessed area of ​​the weld fusion line and the front layer weld of the multilayer weld. The cause of liquefaction cracks is generally considered to be that the low-melting eutectic on the austenite grain boundary is re-melted at the high temperature in the heat-affected zone or the multi-layer weld-welded layer. The plasticity and strength of the metal drop sharply. It is formed by cracking along the austenite grain boundary under the action of tensile force. High temperature plastic loss cracks generally occur in heat affected zones or welds.

1.3 The weld metal of 825 alloy has poor fluidity.

The weld is not like steel. The weld metal is easy to wet and expand. Even if the welding current is increased, the fluidity of the weld metal cannot be improved, but it is counterproductive. Due to the poor fluidity of the 825 alloy liquid weld metal, it is not easy to flow to both sides of the weld, so it has a good weld appearance. In the welding process, a swinging process is required, but it should be a small amplitude swing, and the swinging distance cannot exceed 2.5 times the diameter of the welding rod and the welding wire. In order to better control the metal filling of the weld joint, the groove angle of the 825 alloy joint should be larger to achieve the swing welding process.

1.4 825 alloy material

The surface of the groove is mainly caused by surface oxide scale and embrittlement. The surface oxide scale of the 825 alloy has a much higher melting point than the base material, and often forms inclusions or fine discontinuous oxides. Because of 5,? The presence of other elements will increase the tendency of hot cracking of Alloy 825. Therefore, the 251^111 area inside and outside the weld must be completely cleaned and polished before welding. After the groove is finished, the 50^01 area inside and outside the groove should be cleaned with acetone solution to thoroughly remove impurities such as grease and perform penetration test.

1.5 825 alloy Corrosion resistance
Alloy 825 is a solid solution strengthened nickel-based corrosion-resistant alloy that may precipitate carbides at the grain boundaries during hot processing or during heat treatment. These carbides adversely affect the corrosion resistance of Alloy 825. The 511101 reactor of a perfume manufacturing company in the country was made of domestically produced 825 alloy. After six months of use, the alloy material was found to be severely corroded. After analysis, it was found that the corrosion crack was intergranular crack, the crack was coarse and the depth was shallow. Metallographic analysis of the material revealed a large amount of carbides in the material. The corrosion product was analyzed and found to be a carbide. Due to the large amount of carbides present in the can material, these carbides are distributed in the material, which reduces the corrosion resistance of the material. The study found that the solution treated tube has better corrosion resistance during use. The recommended solution temperature is 980 degrees Celsius.
825 alloys are austenitic structures from high temperature to normal temperature. Normally, when the intergranular corrosion of austenitic stainless steel is sensitized, the diffusion of carbon to the grain boundaries is faster than that of chromium, so The chromium in the adjacent region is depleted due to precipitation of the M23C6 type carbide between the grains. If the chromium content is reduced below the chromium content limit required for passivation, corrosion along the grain boundaries is accelerated due to the microbattery that constitutes the large cathode-small anode. When the 825 alloy is sensitized at medium temperature, Cr-rich M23C6 precipitates on the grain boundaries, resulting in intergranular corrosion. In addition, the MC phase (TiC) may also cause intergranular corrosion of the 825 alloy. TiC is a high-temperature precipitation phase, which starts from about 800 °C. It forms the fastest at around 900 °C. A large amount of fine TiC is dispersed and precipitated. TiC starts to dissolve with increasing temperature. From 900 °C to 1200 °C, the amount of TiC decreases. High solubility above 1150 degrees Celsius, TiC will dissolve a lot when the temperature exceeds 1150 degrees Celsius. Therefore, in a strongly acidic environment, both M23C6 and TiC have an effect on the corrosion properties of Alloy 825. Therefore, in order to reduce the corrosion tendency, the final hot working temperature of the 825 alloy should be above 1050 degrees Celsius. In order to prevent the tendency of intergranular corrosion of 825 alloy, in addition to the conventional reduction of C content, the proportion of Ti/C and other components should be increased. During the hot working, the deformation of TiC should be repeated in the temperature range where the precipitation of TiC is large, so that the TiC precipitate is formed. In the austenite matrix.