Diffusion annealing of nickel alloy is a kind of heat treatment process. Through diffusion annealing, harmful gases (mainly hydrogen) that are dissolved in nickel alloy at high temperature can be desolventized and precipitated. At this time, it is called dehydrogenation annealing. Diffusion annealing can also heat alloy steel ingots, castings, and billets to a higher temperature close to or lower than the solidus line for long-term insulation, and cool at a certain speed, which can improve or eliminate dendrite segregation in castings and strips in rolled materials. Shape segregation can eliminate or reduce the segregation of chemical components and the unevenness of fiber structure to achieve the purpose of homogenization, so it is also called homogenization annealing. Most non-ferrous metals do not undergo solid-state phase change during the heating process of diffusion annealing. In application, the austenite grain refinement and the oxidation and decarburization of steel must be considered accordingly.
Segregation occurs when the casting is solidified, resulting in inhomogeneity of composition and structure. In the case of ingots, this non-uniformity will be elongated and directional along the rolling direction when rolled into steel, and the most common one is a band-like structure.
The band structure appearing in low carbon steel is characterized by more ferrite in some regions and more pearlite in some regions, and the regions are arranged side by side along the rolling direction. The reason for the band-like structure is the segregation of alloying elements such as manganese in the ingot that affect the stability of austenite. Due to the non-uniformity of the composition and structure, long-distance diffusion annealing is required to eliminate it. The required temperature is high, the time is long, and the cost is high. Therefore, it is only used when necessary. It is mostly used for high-quality alloy steels (such as Inconel 718, Hastelloy C276 , Inconel 625, etc.) and alloys with more serious segregation. For steel parts, diffusion annealing has no effect on the improvement of macrosegregation and inclusion distribution. When the steel ingot is heated for forging and rolling after the billet is opened, the holding time should be appropriately extended to receive the effect of diffusion annealing to eliminate stress. Diffusion annealing is more effective after billeting or forging of nickel alloy ingots, because at this time the as-cast structure has been destroyed and the barriers to element diffusion are greatly reduced.
The heating temperature of diffusion annealing for steel parts is usually 150-300 degrees Celsius, depending on the steel grade and the degree of segregation. Excessive temperature affects the life of the heating furnace and causes excessive burning of steel parts. Carbon steel is generally 1100-1200 degrees Celsius, alloy steel (such as nickel alloy materials) is generally 1200-1300 degrees Celsius, and the heating rate is long controlled at 100- per hour. 200 degrees Celsius.
The holding time of diffusion annealing depends on the thickness of the cross-section of the workpiece. Generally, it is calculated according to the thickness of the interface for 30-60 minutes per 25mm, or 1.5-2.5 minutes per millimeter of thickness.
Since diffusion annealing is carried out at high temperature and the process time is very long, the austenite grains will be very coarse after annealing. In order to refine the grains, a complete annealing should be added after the diffusion annealing. For ingots, post-processing also requires pressure processing, and pressure processing can finely smash the grains. Therefore, in this case, there is no need to supplement a complete annealing after diffusion annealing.
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