What Is Solid Solution Heat Treatment?
High-temperature alloys are designed to carry stress at temperatures of 600-1200 °C while resisting oxidation and corrosion. Solid solution heat treatment is the process step that puts the alloying elements into a homogeneous solid solution. The part is heated to a temperature high enough to dissolve precipitates and segregate elements, held to equalize composition, and then cooled at a controlled rate. For precipitation-strengthened alloys such as Incoloy 706, the solution treatment is followed by an aging treatment that precipitates fine strengthening phases from the supersaturated solid solution.
How Solid Solution Strengthening Works
Strengthening by solid solution relies on three mechanisms. First, adding elements with different atomic sizes from the base metal - such as chromium, tungsten and molybdenum - distorts the base lattice and hinders dislocation movement. Second, elements such as cobalt reduce the stacking fault energy of the matrix, making dislocation cross-slip more difficult. Third, elements such as tungsten and molybdenum slow the diffusion rate of the base elements, improving creep resistance at high temperature. Together these effects raise the strength of the matrix itself, before any precipitation is considered.
Precipitation Strengthening Phases
During aging, second phases precipitate from the supersaturated solid solution and strengthen the alloy. The gamma prime phase (γ') has a face-centered cubic structure matching the matrix and precipitates uniformly as fine particles that hinder dislocation movement, producing a significant strengthening effect. It is an A3B-type intermetallic compound; in nickel-based alloys the typical gamma prime is Ni3(Al,Ti). The strengthening effect can be enhanced by increasing the volume fraction of gamma prime, controlling its lattice mismatch with the matrix, adding niobium and tantalum to increase particle size and antiphase boundary energy, and adding cobalt, tungsten and molybdenum to strengthen the phase itself. The gamma double-prime phase (γ'') has a body-centered tetragonal structure with the composition Ni3Nb. Because of its large mismatch with the matrix, it causes strong coherent distortion and very high yield strength, although its strengthening effect decreases markedly above about 700 °C. Cobalt-based superalloys generally contain no gamma prime and are strengthened instead by carbides.
Grain Boundary Strengthening
At high temperature, grain boundaries are the weak link of the alloy. Adding trace amounts of boron, zirconium and rare-earth elements improves grain boundary strength: rare-earth elements purify the boundaries, while boron and zirconium atoms fill boundary vacancies, reduce the boundary diffusion rate during creep, inhibit the aggregation of boundary carbides and promote spheroidization of boundary second phases. Adding an appropriate amount of hafnium to cast alloys also improves grain boundary strength and plasticity. Heat treatment can additionally form chain-like carbides at the boundaries or produce curved boundaries, both of which improve plasticity and strength.
Incoloy 706 Chemical Composition
The composition limits of Incoloy 706 are carbon 0.06% maximum, silicon 0.35% maximum, manganese 0.35% maximum, phosphorus 0.02% maximum, sulfur 0.015% maximum, chromium 14.5-17.5%, nickel 39-44%, molybdenum 1.0-1.6%, copper 0.3% maximum, niobium 2.5-3.3%, boron 0.006% maximum, titanium 1.5-2.0%, aluminum 0.4% maximum, with iron as balance. The niobium and titanium contents support the precipitation of strengthening phases, while the controlled trace elements protect grain boundary quality.
Standard Heat Treatment Cycle for Incoloy 706
Hot-rolled bars, forged bars and forgings of Incoloy 706 are typically treated by heating to 925-980 °C followed by water cooling, then holding at 845 °C for 3 hours, holding at 720 °C for 8 hours, cooling at 55 °C per hour to 620 °C, holding for 8 hours and finally air cooling. Material treated by this cycle typically reaches a Brinell hardness above 285 HB, with a density of about 8.06 g/cm3. The exact cycle should be qualified against the governing specification and the required mechanical properties for the final component.
FAQ
Q: Why is solution heat treatment necessary for Incoloy 706?
A: It dissolves the alloying elements into a homogeneous solid solution and sets up the supersaturated condition required for the subsequent aging precipitation of strengthening phases.
Q: What is the difference between gamma prime and gamma double-prime?
A: Gamma prime is an A3B-type face-centered cubic phase such as Ni3(Al,Ti), while gamma double-prime is a body-centered tetragonal Ni3Nb phase that produces higher yield strength but loses effectiveness above about 700 °C.
Q: What composition does Incoloy 706 have?
A: It contains nickel 39-44%, chromium 14.5-17.5%, niobium 2.5-3.3%, titanium 1.5-2.0%, molybdenum 1.0-1.6%, with iron as balance and tight limits on carbon, silicon, manganese, phosphorus and sulfur.
Q: What is the typical heat treatment cycle for 706 bars?
A: 925-980 °C solution treatment with water cooling, followed by 845 °C for 3 hours and 720 °C for 8 hours, cooling at 55 °C per hour to 620 °C, holding 8 hours and air cooling.
Q: How do boron and zirconium help the alloy?
A: They fill grain boundary vacancies, slow boundary diffusion during creep and inhibit carbide aggregation, which raises grain boundary strength at high temperature.
