High temperature alloy strengthening by aging
High temperature alloys are commonly used in the aerospace, energy, and petrochemical industries due to their unique properties. These alloys typically include elements such as nickel, chromium, molybdenum, vanadium, and cobalt, as well as other elements. They are classified into two broad categories: high temperature Creep Resistant Alloys (CrRes) and High Temperature Strength Alloys (HTSA). These alloys are known for their high strength, corrosion resistance, and heat resistance at temperatures up to 1000°C, but they are also susceptible to aging.
Ageing is the phenomenon of time-dependent change of the mechanical and physical properties of an alloy due to the formation of precipitates within the alloy microstructure. Depending on the elements used in the alloy, a wide range of precipitation reactions are possible. As the alloy ages, precipitates form that can reduce its ductility and increase its hardness. This process can significantly decrease the strength of the alloy and its ability to endure heating and cooling cycles without damage.
In order to improve the performance of high temperature alloys, several methods of aging can be used. These include solution treatment, precipitation heat treatment, and aging treatments. Solution treatment involves heating the alloy to a suitable temperature in order to dissolve all the precipitates present in the alloy. After the alloy is cooled, the ammonium sulfate precipitation method is used to precipitate out the precipitates and make them available for inclusion within the alloy structure. Precipitation heat treatment involves an additional heating cycle after the solution treatment, where further precipitation is induced. The aging processes involve an increase in the hardness of the alloy through the precipitation of more densely packed precipitates and an increase in the strength of the alloy.
In certain cases, aging can be beneficial to an alloy as it can increase the strength and wear resistance of the alloy. Aging can also be used as a supplement to solution and precipitation heat treatments, as it can refine and refine the precipitates formed during the heat treatments and make them more stable. Additionally, aging can also be used to reduce sensitivity to tempering treatment and improve corrosion resistance.
When selecting an aging treatment for a high temperature alloy, it is important to understand the differences between them. Aging treatments can vary based on the type of alloy, as well as the amount of time and/or temperature involved. Additionally, it is important to consider the desired result in terms of the desired strength gain, hardening, or reduced tempering sensitivity.
Having an aging treatment at the right time and temperature is important in optimizing the desired result. Commonly used aging treatments are the natural aging treatment and the artificial aging treatment. Natural aging is typically applied after solution treatment, whereas artificial aging is often done after precipitation heat treatment. Factors such as temperature, time, and reaction rate can vary and should be fully considered when selecting an appropriate aging treatment.
The use of aging treatment in high temperature alloy applications can provide significant benefits, such as increased strength and wear resistance. Additionally, aging can also be used to reduce sensitivity to tempering and improve corrosion resistance. As with all material treatments, it is important to consider not only the desired end-use property requirements but also the potential for a premature hardening of the alloy. Having the right aging treatment at the right time, temperature and concentration will ensure that the desired properties are achieved without compromising other properties.
