Nickel alloys gain their strength from a controlled heat treatment, not from a quench as in steel. This guide covers the two steps that matter most for bars and forgings — solution annealing and age hardening — and the metallurgy behind them.
Why Nickel Alloys Are Different
Carbon and alloy steels harden by forming martensite when quenched. Precipitation-strengthened nickel alloys such as Inconel 625 and 718 harden by a different path: fine particles of gamma prime (γ′) and gamma double prime (γ″) grow inside the grains during a controlled low-temperature hold. The base structure is set first by solution annealing.
Solution Annealing
Solution annealing heats the alloy into the range where precipitated phases dissolve — typically 980–1230 °C — then cools it quickly, by water quench or rapid air cool. The aims are to:
- dissolve carbides and intermetallic phases,
- even out composition across the section,
- restore corrosion resistance after working,
- prepare a clean matrix for age hardening.
For Inconel 625, solution annealing runs 980–1150 °C. For Inconel 718, the usual single solution temperature is about 980 °C.
Age Hardening
Age hardening holds the solution-annealed bar at 650–900 °C so gamma prime and gamma double prime precipitate as a fine, uniform dispersion. Strength and hardness rise sharply while ductility falls in a controlled way. Some grades use a double stage to balance strength and stability:
- Inconel 718 — double stage: 720 °C for 8 hours, then 620 °C for 8 hours. This schedule controls the gamma double prime and delta phases for stable properties.
- Single-stage grades — a single hold in the 650–900 °C band suits many solid-solution and age-hardenable alloys.
Stress Relief and Stabilizing
Not every heat treatment aims at peak strength. Stress relief at 425–870 °C removes machining and welding residual stress (Monel 400 at 540–565 °C). Stabilizing at 600–900 °C, slow cooled, keeps carbides even in niobium-bearing alloys. These steps protect the part in service rather than raising the tensile number.
Practical Notes for Buyers
- Specify the condition, not just the grade: solution-annealed, age-hardened, or solution-annealed plus age-hardened.
- Age-hardened bar is harder to machine; plan the sequence so rough machining precedes aging where the design allows.
- Every heat travels with an EN 10204 3.1 mill certificate that records the actual heat treatment.
Related Reading
- Heat Treatment Services — the conditions Southerly Alloy supplies
- Bar Manufacturing Process — how bars are formed before heat treatment
- Bars and rods by grade — the full nickel alloy bar catalogue
Frequently Asked Questions
Is solution annealing the same as annealing steel?
No. Steel annealing softens by recrystallization; nickel alloy solution annealing dissolves precipitated phases and is followed by a fast cool, then age hardening.
Why does Inconel 718 use two aging steps?
The double stage controls the type and distribution of precipitated phases, which stabilizes strength and reduces the risk of over-aging.
Can a bar be supplied already age hardened?
Yes. Solution-annealed plus age-hardened is a standard final condition for age-hardenable grades.
Are mill certificates provided?
Every heat travels with an EN 10204 3.1 mill certificate.
Request a Quote
Tell us the grade, shape, diameter, condition, and quantity. Southerly Alloy returns a quoted price, lead time, and the certificate package in one reply.