Inconel 718 disc springs, also called Belleville washers, are specified for continuous service at 500°C because the precipitation-hardened Ni-Cr-Nb-Mo alloy retains about 75% of its room-temperature tensile strength at 700°C, with a reported maximum service ceiling near 650°C for the alloy and 700°C for the disk stock form [S1][S5].
The material is a nickel-chromium-niobium (columbium)-molybdenum age-hardenable alloy (UNS N07718) with a nominal composition of Ni 50.0-55.0%, Cr 17.0-21.0%, Cb(Nb) 4.75-5.5%, Mo 2.8-3.3%, density 0.296 lb/in³ (8.19 g/cm³), and modulus of elasticity 29.0×10³ ksi at 20°C, which makes it dimensionally stable at the temperatures seen in turbine casings, downstream oil-and-gas flanges, and aero engine accessory gearboxes [S1][S4][S5].
Why 500°C Excludes Standard Carbon and 17-7PH Disc Springs
Standard carbon-steel and 17-7PH stainless disc springs lose useful preload above roughly 200°C through stress relaxation and oxidation, while Inconel 718 disc springs are offered by manufacturers for environments from -250°C up to +500°C as standard catalog items [S3]. The alloy also retains about 85% of its ductility down to -240°C, which is why the same part is used across cryogenic LNG/CO₂ cold boxes and hot-turbine sections without a material change [S1].
For sour service the heat-treatment regime is governed by NACE MR0175, with the alloy solution annealed at 1010-1040°C and precipitation hardened at 780°C for up to 8 hours, producing hardness up to 40 HRC and tensile values around 1590 MPa (230 ksi) [S1][S4]. A second high-fatigue cycle (solution anneal 925-980°C, rapid cool, age 720°C/8 h, furnace cool to 620°C/18 h) is used when cyclic loading dominates the duty rather than steady preload [S1].
Sizing a Disc Spring: Series, h0/t, and Deflection Window
Disc-spring design is governed by the ratio h0/t (cone height to thickness) and the spring series, with the practical rule that larger, lighter-duty Series B or C springs generally outlast smaller, heavier-duty parts in dynamic service [S7]. A 500°C application must be sized for elevated-temperature load loss, not for room-temperature force, because the elastic modulus drops as temperature rises, so the same deflection produces less force at operating temperature.
Catalog data illustrates the typical force/deflection envelope: a 63 mm OD x 31 mm ID x 2 mm thick 718 spring with 1.55 mm cone height delivers 2,216 N at 0.7 mm deflection, 2,748 N at 0.93 mm, and 3,645 N at 1.4 mm, while an 80 mm OD x 41 mm ID x 3 mm thick part reaches 9,476 N at 1.71 mm deflection [S4]. The deflection window is normally kept under 75% of h0 to avoid taking the spring past flat, which on 718 parts also means staying below the calculated high-cycle fatigue limit at the operating temperature [S7].
500°C Stress-Relaxation Behavior and Stack-Up Practice
At 500°C the dominant failure mode of any disc spring is stress relaxation, not yield, and published Mubea reference data for Inconel 718 shows a marked increase in relaxation loss above 300°C that must be compensated by pre-load oversizing or by re-tensioning the stack on service intervals [S2]. Mubea reference tables remain the de facto lookup for relaxation and creep of 718 disc-spring stock, and engineers should compare 1000-hour relaxation values, not 1-hour values, when qualifying a part for hot service.
In practice a 500°C duty is usually met with a stacked configuration: springs in series (face-to-face) increase deflection travel while keeping force per spring low, springs in parallel (back-to-back or in same direction) increase force at the same deflection, and alternating stacks with sliding interfaces (often with graphite or MoS₂) prevent galling when the stack is compressed repeatedly [S7]. For more on preload and tolerance budgeting in bolted joints that use these stacks, see the bearing fit selection guide, which covers the same interference/stack-up logic in a different geometry.
Standards, Sour Service, and Sourcing
For oil-and-gas applications the heat-treatment and hardness limits of NACE MR0175 / ISO 15156 are non-negotiable, and one European disc-spring catalog explicitly states that its standard Inconel 718 line is "manufactured from Inconel 718 (NACE MR 0175)" with full lot traceability [S4]. The disc-springs reference disc coupling and industrial valve pages on related stack-up spring geometry, where 718 disc springs are also used as belleville packets inside valve bonnet and coupling assemblies.
For NDT traceability on the raw bar stock used for hot-service 718 disc springs, the thick-section UT inspection guide covers the kV-range and acceptance class logic that typically accompanies a 718 forging or bar-stock certificate. The user-callable spec set for a hot-service 718 disc spring should at minimum include: alloy/UNS code (N07718), heat-treatment cycle (1010-1040°C anneal + 720-780°C age), NACE MR0175 compliance statement, finished dimensions OD/ID/t/h0, force at three deflection points, and a 1000-hour relaxation value at the design temperature [S1][S2][S4].
Material Comparison: 718 versus X-750, 625, and 725 for the Same Duty
Inconel 718 is described as a superior disc-spring choice to Inconel X-750 at higher temperatures when strength matters, while Inconel 625 and 725 are preferred where sour-gas corrosion resistance dominates over hot strength [S1]. A 500°C brief sits inside the 718 design envelope but above the comfort zone of 17-7PH and most stainless disc-spring grades, which is why 718 is the default rather than the exception in this temperature window.
For rotating equipment that bolts down near the hot section, also see the bearing dimension-series decoder for matching the disc-spring stack preload to the bearing's internal clearance budget, and the IEC motor shaft keyway table for the same kind of dimension-driven selection logic when a 718 stack lives inside a motor or generator housing. The default trackable signals are: (a) a 1000-hour relaxation value at 500°C from the disc-spring maker, and (b) a NACE MR0175 / ISO 15156 compliance line on the material certificate.
Spec-level background on the components involved: pressure transmitter.