Hangbo Alloy Group
Shanghai · Est. 2012
Stress relaxation loses preload at temperature as elastic strain turns to creep. How A-286, 718 and Nimonic bolting hold clamp load in turbines and reactors.
Stress relaxation is the quiet loss of bolt preload that occurs when a fastener is held at a fixed strain while temperature does its work. The elastic strain that created the clamp load gradually converts to plastic (creep) strain, so the force holding a joint together drops over time — even though nothing has moved and no external load was applied. In a turbine casing, a pressure-vessel flange, or a furnace door, that creeping loss of preload is what lets joints loosen, flanges weep, and gaskets fail. Specifying the right nickel alloy for high-temperature bolting is therefore not a metallurgical nicety; it is what keeps the joint sealed. As a nickel-alloy manufacturer, Hangbo produces the precipitation-hardened grades used for bolting and studs in exactly these elevated-temperature services.
Picture a bolt in a rigid joint. The nut is set, the bolt stretches, and that stretch is the clamp load. The joint will not let the bolt get any longer, so the strain is held constant. At room temperature that load would hold for decades. Raise the temperature, and the metal begins to creep: a small fraction of the elastic stretch permanently deforms. Because total strain is fixed, the elastic (load-carrying) part shrinks, and the measured preload falls.
This is distinct from creep in the usual sense. Conventional creep is constant load producing increasing strain — a hanging turbine blade elongating. Relaxation is the inverse: constant total strain producing decreasing load. Both are time- and temperature-dependent, and both accelerate steeply as temperature rises.
A flanged joint does not leak because the bolts are weak; it leaks because the clamp load dropped below what the gasket needs to stay seated. A few percent loss of preload can be enough to break the seal, especially after the first heat cycle. In gas-turbine casings, exhaust manifolds, and reactor heads, a relaxed stud means repeated re-torquing, unplanned shutdowns, and in the worst case, a hot-gas or process-fluid release.
Fastener standards such as ASTM F2281 (nickel alloy bolting) and the relevant sections of ASTM F568 exist precisely because room-temperature tensile strength tells you almost nothing about how a bolt behaves at 600°C. The property that matters is relaxation resistance — how much preload survives after a fixed time at temperature.
The grades that resist relaxation are the precipitation-hardened iron-nickel and nickel superalloys, because their fine gamma-prime or gamma-double-prime precipitates pin dislocations and slow creep:
Hangbo supplies these grades as bar and forgings suitable for studs, bolts, and nuts, with full heat-treatment and mechanical certification.
Good practice starts with temperature. Below about 300°C, relaxation is minor for most alloys; above 400–500°C it becomes the governing design limit, and above 650°C only the strongest superalloys remain viable. Specify a generous design margin, recognize that the first thermal cycle relaxes the most, and plan for at least one controlled re-torque once the joint has been brought to temperature.
Mechanical details help: hardened washers reduce embedment (local crushing of the bearing surface, which itself looks like relaxation), and proper surface finish on nut faces lowers running friction scatter. Most importantly, choose an alloy whose relaxation curve at the service temperature leaves enough residual preload to keep the gasket seated for the intended run length.
It is minor below about 300°C for most alloys but becomes a governing design limit above 400–500°C, and severe above 650°C. The threshold is why high-temperature joints use specialized nickel alloys rather than ordinary stainless, as discussed in our nickel alloy vs stainless steel guide.
Alloy A-286 is the standard choice to about 650°C, with Inconel 718 and Nimonic 80A also widely used, and Waspaloy for service into the 650–760°C range. Hangbo supplies all of these as bolting-grade bar and forgings — see our A-286 fastener guide, Inconel 718 overview, and Nimonic 80A guide.
They are closely related but inverse. Creep is constant load producing increasing strain (a part stretches over time). Relaxation is constant total strain producing decreasing load (a bolt loses preload). Both are time- and temperature-dependent and both accelerate with heat.
Specify an alloy with low relaxation at the service temperature, apply correct initial preload, use hardened washers to limit embedment, and plan a controlled re-torque after the first heat cycle. For the most demanding joints, Waspaloy and similar high-strength superalloys provide the necessary margin.
As a dedicated nickel alloy manufacturer serving aerospace, power generation, chemical processing sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.