The Solidification Process and Element Characteristics of Inconel 718, a Nickel-Based High-Temperature Alloy
INTRODUCTION
Nickel-based high-temperature alloys are an important type of alloy materials widely used in aerospace, petrochemical and other fields due to their excellent mechanical properties and good corrosion resistance. Inconel 718 (UNS N07718, alloy 718) is an age-hardenable nickel-based alloy developed in the 1950s. Due to its excellent mechanical properties, high temperature tensile properties, fatigue properties and excellent creep resistance, Inconel 718 is widely used in the aerospace and petrochemical industries.
Inconel 718s solidification characteristics, phase constitution and heat treatment state have an important influence on its mechanical behavior. In order to understand the microstructure of Inconel 718 and explore its relationship with its physical and mechanical properties, theoretical analysis and experimentation of the solidification process of Inconel 718 are necessary.
COMPOSITIONAL ANALYSIS
Inconel 718 is a nickel-base alloy with the main alloying elements of chromium, iron, niobium and molybdenum. The nominal chemical composition (wt%) of Inconel 718 alloy is shown in Table 1. According to ASTM B 637, the chromium content of this alloy ranges from 18 to 21%, the iron content from 4.8 to 6%, the niobium content from 3.1 to 4.37%, the molybdenum content from 2.4 to 3.2%, and the cobalt content from 0.4 to 1.15%. Cobalt and titanium are also added to the alloy to improve its oxidation resistance and corrosion resistance.
Table 1: Nominal Chemical Composition Of Inconel 718
Element Wt%
Ni 50.0
Cr 18.0-21.0
Fe 4.8-6.0
Nb 3.1-4.37
Mo 2.4-3.2
Co 0.4-1.15
Ti 0.2-0.8
Si 0.5
C 0.008
Mn 0.35
Al 0.2
PHASE CONSTITUTION
Inconel 718 alloy is an age-hardenable nickel-based alloy with a complex phase constitution. The as-cast Inconel 718 alloy is composed of gamma (γ) solid solution alloy and a series of intermetallic compounds such as Ni3Nb, Ni3Al, NbMo, CoNi7, Co-Rhenium and CoNi3, which are aged with matrix. Depending on whether the solidifying temperature is changed or not, the longitudinal and transverse sectional structures of the as-cast Inconel 718 alloy and the corresponding microstructures can be divided into four different structures: coarse-grained structure, banded-grained structure, dispersion-strengthened structure and fine-grained structure.
THE SOLIDIFICATION PROCESS
The solidification process of Inconel 718 alloy is a non-equilibrium solidification process, which is affected by many factors such as cooling rate, Si content, Ni content and Ni-Co-Mo-Nb-Ti elements. The solidification process of as-cast Inconel 718 alloy is mainly controlled by nucleation and growth of primary and secondary dendritic α grains. During the process of solidification, the fraction solid transformation, primary γ and γ-Ni3Nb, secondary γ grains and the intermetallic compounds are all involved in the solidification process.
The solidifying temperature of Inconel 718 alloy is related to the cooling rate, chemical composition and grain structure. With the increase of cooling rate, the solidifying temperature of Inconel 718 alloy decreases. In addition, with the increase of the content of Si, Mo and Mn, the solidifying temperature of Inconel 718 alloy will increase accordingly.
HEAT TREATMENT
Heat treatment is an important way to improve the mechanical properties of Inconel 718 alloy. The heat treatment of the Inconel 718 alloy mainly includes the following steps: homogenization annealing and aging treatment. Homogenization annealing is an important preparation process to ensure the uniform mass fraction, uniform organization, homogeneous composition and composition features of the material before being put into use. Age hardening is the process of heat treatment after homogenization annealing in which the material is hardened by means of a precipitation responsible for increased hardness.
CONCLUSION
Inconel 718 is a nickel-based high-temperature alloy, with excellent mechanical properties, high temperature tensile properties, fatigue properties and excellent creep resistance. The solidifying temperature of Inconel 718 alloy is related to the cooling rate, chemical composition and grain structure. The solidification process of Inconel 718 alloy is mainly controlled by nucleation and growth of primary and secondary dendritic α grains. Homogenization annealing and aging treatment are important steps in improving the mechanical properties of Inconel 718 alloy. With an improved understanding of the solidification process and element characteristics of Inconel 718, it is possible to optimize material properties to meet the stringent requirements in a variety of industrial applications.
