Heat Treatment and Cold Forming Processes for Nickel-Based Alloy Incoloy 800 Fasteners

Incoloy 800 bolts and stud bolts keep their strength at high temperature only when three process steps stay inside a narrow window: solution annealing, controlled cold forming and age hardening. Miss one step and the fastener loses creep resistance, cracks during thread rolling or corrodes along grain boundaries. The sequence below gives the numbers that matter for purchase specifications and incoming inspection, while the joint around the bolts still depends on the flange gaskets selected for the medium and temperature.

Solution Annealing at 1100 °C

Solution annealing is one hour at 1100 °C followed by a fast water quench. The temperature sits above the 1050 °C point at which the γ' phase Ni₃(Al,Ti) and carbides such as M₂₃C₆ dissolve completely, so the bar leaves the furnace as a homogeneous single-phase austenitic structure with forming stress removed. The quench freezes that structure and keeps the supersaturated solid solution available for the aging step.

• Heat at ≤220 °C/h; a faster ramp can crack the bar with thermal stress before it reaches temperature.
• Soak 1 h for bar ≤25 mm in diameter and add 0.5 h for every further 25 mm so the core transforms completely (ASTM B408 Annex A1).
• Transfer to water in under 10 s; a longer delay lets carbides precipitate at grain boundaries and raises the intergranular corrosion rate (ASTM G28).
• Hold grain size at ASTM 5-7 (50-90 µm); oversized grains cut fatigue strength.

Metallography under ASTM E112 at 500× should show no undissolved carbides, and XRD should report more than 99.5% austenite with Cu-Kα radiation and FWHM below 0.5°. Furnace uniformity follows AMS 2750D Class 4 requirements, which keeps the soak temperature consistent from load to load.

Cold Forming: Heading and Thread Rolling

Cold forming gives Incoloy 800 fasteners their final shape and adds work hardening before aging. Bolts and studs produced this way are assembled into pipe flanges on high-temperature lines, where head integrity and thread accuracy decide whether the joint stays tight.

• Surface preparation: phosphating plus a soaping coat of 3-5 g/m², holding the friction coefficient at μ ≤ 0.05 (ISO 6892).
• Cold-drawn bar at HRB 75-85 forms without cracking; harder stock splits at the head.
• Cut-off on SKD11 tooling (HRC 60-62) with graphite grease at 40 wt.%; pre-form on YG8 carbide with 20-30% area reduction and an extreme-pressure additive (1% S + P).
• Finish heading at ≤50% total reduction on tungsten carbide CD650 (HV 1500) with a 2 µm nano-MoS₂ coating.

Head quality is checked by macro-etching (ASTM E340) for continuous flow lines and for folds at the thread transition, and by finite element simulation (Deform-3D) showing PEEQ ≤ 1.2 with no localised overstrain.

Thread rolling is chip-less forming: rolls of ASP2060 powdered high-speed steel (HRC 64-66) form a 60° ± 15′ profile per ASME B1.1 at 25-35 rpm (1.2-1.7 m/min). Rolling force follows F = 0.8 × σ₀.₂ × A with σ₀.₂ = 450 MPa. The process leaves -350 to -500 MPa of surface compressive stress measured by XRD, which lifts fatigue life by more than 20%. Root cracks are prevented by keeping pre-rolling hardness ≤ HRB 90 and running 100% magnetic particle inspection (ASTM E1444); three-roller machines hold pitch error below 0.02 mm per 25 mm (ISO 965-1 grade 4h).

Age Hardening at 720 °C

Aging at 720 °C for 16 h precipitates γ' (Ni₃(Al,Ti)) at 10-30 nm with a number density near 1 × 10²² /m³. The precipitates contribute about 300 MPa of strength from Δσ = M × G × b × √(N × d), taking M = 3.06, G = 78 GPa and b = 0.25 nm.

• 700 °C leaves precipitation incomplete; 750 °C over-coarsens the γ' phase and softens the material.
• 8 h falls short of the target hardness; 24 h raises the σ phase risk and drops impact energy below 40 J (ASTM E23).
• Hardness below HRC 20 or above HRC 35 is outside the acceptance band for the aged condition.
• Optional dual-stage cycle: 650 °C × 8 h for a dense 5-10 nm γ' phase, then 750 °C × 2 h to grow it to 20-30 nm and balance strength with toughness; the fatigue limit improves by 15% (R = -1, 10⁷ cycles, ISO 1099).

Inspection Points and Service Performance

Four checkpoints cover the whole route.

Note: values are typical control targets for Incoloy 800 fasteners; the order specification and the material certificate take precedence.

On the mechanical side the difference between routes is large. Solution treatment alone gives HRB 85-90 and a creep rate of 5 × 10⁻⁸ s⁻¹ at 600 °C (ASTM E139). Solution treatment plus cold work plus aging reaches HRC 28, cuts the creep rate to 2 × 10⁻⁹ s⁻¹ and keeps more than 90% of bolt preload after a 1000 h test. Stress-rupture life at 800 °C and 200 MPa rises from 600 h to 1200 h.

HRC 25-30 is the practical strength-toughness balance for these fasteners. Softer material will not hold preload at temperature; harder material loses impact resistance and becomes sensitive to cracking at the thread root.

Frequently Asked Questions

What solution annealing temperature should Incoloy 800 fasteners receive?

1100 °C for one hour, followed by water quench. That is above the 1050 °C point of complete dissolution for the γ' phase and carbides, which is what produces the uniform austenitic structure.

Why must the quench delay stay under 10 seconds?

A longer transfer lets carbides precipitate at grain boundaries. With a delay under 10 s, the intergranular corrosion rate fell by 30% in ASTM G28 testing.

How much cold reduction can the bolt head take before cracking starts?

Total area reduction is held at ≤50% at finish heading, with 20-30% at pre-forming. Bar hardness of HRB 75-85 and a lubricated surface (phosphating plus soaping) keep the head free of folds.

What hardness should be expected after aging?

About HRC 25-30, typically HRC 28 for the 720 °C × 16 h cycle, with no more than 2 HRC between surface and core.

How are rolled threads inspected?

By 100% magnetic particle inspection to ASTM E1444 for root cracks and by pitch measurement; three-roller machines should hold pitch error below 0.02 mm per 25 mm (ISO 965-1 grade 4h).