Disc springs manufactured to DIN EN 16983 (formerly DIN 2093) are produced from Ck75 (DIN 1.1248, also designated C75S or SAE 1075) for thin sections and from 51CrV4 (DIN 1.8159, AISI 6150) for thicker sections where through-hardening and fatigue performance dominate [S10].
The two grades represent different metallurgical routes to the same product class: a high-carbon unalloyed steel versus a chromium-vanadium alloyed spring steel, with the alloy content directly driving hardenability, toughness, relaxation resistance, and ultimately cycles to fatigue failure [S2][S4].
Composition and Metallurgical Background
C75S is a high-carbon unalloyed spring steel with roughly 0.70-0.80% carbon, the grade commonly referred to simply as C75 or spring steel in commercial catalogues [S2]. It is the default choice for shallow, thin disc springs because its carbon content gives high surface hardness after quench-and-temper, while remaining machinable and economical in coil or strip form.
51CrV4 is a chromium-vanadium alloyed steel with carbon typically in the 0.47-0.55% range, chromium at 0.90-1.20%, vanadium at 0.10-0.25%, and silicon capped at 0.40% [S5]. The chromium improves hardenability and through-section consistency, while vanadium refines grain size and resists tempering softening, the combination that gives 51CrV4 better fatigue resistance than plain carbon grades at thickness above the unalloyed steel's hardenability limit [S2][S5][S7]. A closely related variant, 50CrV4, is described by disc-spring manufacturers as having "high strength, yield ratio, good toughness, high fatigue strength" and as the correct pick for "high stress amplitude and strict fatigue performance" discs [S6].
Disc-Spring Thickness Split and Why It Exists
The 51CrV4 material is described by Mubea, the dominant disc-spring OEM, as "the most regularly used disc spring material" with a "lower level of relaxation than non-alloy steels" and is applied in discs up to about 40 mm thick [S4]. For standard DIN 2093 / DIN EN 16983 disc springs, C75S and 51CrV4 are explicitly named as the two base materials, with grade selection driven by thickness [S10].
The thickness split exists because C75S relies on a high carbon content to reach surface hardness, and as section thickness increases the unalloyed steel cannot fully through-harden, leaving a softer core that becomes the fatigue crack initiation site under cyclic loading. 51CrV4's chromium content lifts hardenability enough to achieve uniform martensite across the disc thickness, so thicker discs (typically those above roughly 2-3 mm) are specified in 51CrV4 to preserve fatigue margin. The standard material designation summary on [S8] lists 51CrV4 / 1.8159 as the chromium-vanadium alloyed grade used in the rolled condition for disc springs, and the same source groups C75S / 1.1248 alongside it as the carbon-steel counterpart. For the wider spring-steel family, the alloyed and carbon grades share the basic property of a "high elasticity limit, fatigue resistance" and the chromium-vanadium chemistry is the explicit fatigue-resistance lever [S7].
Fatigue Behaviour and Hardening Response
Direct Paris-law and threshold data for 51CrV4 in railway leaf-spring service are published in the 2024 open-access study by Gomes and co-authors, which characterises mode-I crack growth in 51CrV4 at variable stress ratios and derives Paris and Walker coefficients for design use [S1]. The study confirms that fatigue crack propagation in 51CrV4 is transgranular, that crack initiation in this grade is dominated by surface defects and internal inclusions (oxides, carbides), and that shot-peening plus higher tempering temperature both raise cycles to failure, exactly the levers a disc-spring designer uses when specifying a peened and stress-relieved 51CrV4 disc [S1].
For the disc geometry specifically, Mubea's published material statement is the cleanest engineering rule: 51CrV4 has lower relaxation than non-alloy steels and is the most commonly used disc-spring material, which is why the thicker end of the DIN 2093 / DIN EN 16983 catalogue is 51CrV4 and the thinner end is Ck75 / C75S [S4][S10]. Suppliers marketing standard materials side by side describe 51CrV4 as offering "better hardenability, toughness, and fatigue resistance" versus C75 directly, attributing the gain to chromium and vanadium additions [S2]. A wider comparison by Nifty Alloys (2025-08) ranks 51CrV4 (AISI 6150) above 55Si7 for elastic limit, fatigue strength, stress-relaxation resistance above roughly 150-200°C, and through-thickness hardenability, while flagging 55Si7 as more economical and adequate for service below about 250°C [S5]. SPIROL's disc-spring material white paper frames the same trade-off as a balance between raw-material durability under heat treatment and the dimensional/cost envelope of the application [S3].
Standards, Supply Condition, and Surface State
Disc-spring steels for European supply are commonly delivered to DIN EN 10132-4 for cold-rolled strip, a standard that SCHNORR explicitly cites for its safety washers and load washers and that governs the chemical and mechanical tolerances both 51CrV4 and C75S are ordered against [S9]. The disc itself is then made and tested to DIN EN 16983 (the successor designation of DIN 2093), which is the standard that lists Ck75 (1.1248) and 51CrV4 (1.8159) as the two prescribed base materials, with grade switching driven by disc thickness [S10].
Supply condition matters as much as grade. Both 51CrV4 and 55Si7 are typically stocked soft-annealed for blanking and machining, then oil-quenched from approximately 840-870°C and tempered to working hardness; a common buyer error is comparing a soft-annealed data sheet of one grade against a quenched-and-tempered data sheet of the other, which produces an artificial fatigue or strength advantage that does not exist in service [S5]. The same principle applies to C75S in disc springs: hardness, relaxation, and fatigue numbers are only meaningful when the heat-treatment condition matches the in-service condition. Surface state is the other lever, and shot-peening is widely applied to disc springs specifically because it raises the propagation threshold by introducing compressive residual stress at the surface, which is also the initiation site in 51CrV4 components [S1].
Decision Matrix: When to Specify 51CrV4 vs C75S
Specifying C75S / Ck75 (1.1248) makes sense when the disc is thin (DIN 2093 / DIN EN 16983 light series at the lower thickness range), the service temperature stays well below about 150°C, the duty cycle is moderate, and cost-per-piece is the dominant commercial driver [S2][S10]. Specifying 51CrV4 (1.8159) is the right call when the disc is thick enough that through-hardening with a plain carbon steel cannot be guaranteed, when the application involves sustained cyclic loading at elevated temperature, when relaxation loss directly affects preload (clamped bolted joints, safety washers, valve spring stacks), or when published fatigue data exists for 51CrV4 in similar mode-I conditions and that data is needed for fitness-for-service [S1][S4][S5]. Mubea's published guidance is the cleanest single line: 51CrV4 is the default disc-spring material for thicker sections, with the lower-relaxation property as the technical reason [S4]. For the broader spring category, including disc springs operating as Belleville washers in industrial assemblies, the chrome-vanadium grade is also the preferred pick when the comparison is line-by-line on hardenability, toughness, and fatigue resistance versus C75 [S2][S7].
Trackable signals to watch for the next design review cycle: the publication of additional DIN EN 16983 fatigue S-N curves specifically tagged to 51CrV4 versus C75S at matched thickness, and any expansion of the Mubea or SCHNORR standard-material tables to call out the thickness threshold at which they switch from Ck75 to 51CrV4 in current production runs [S4][S9][S10].
The underlying component specifications are covered under disc coupling, spring washer, and advanced material.
See also our earlier report, How to size an AGV battery for full-shift operation.