What Is 1.8550 Steel?
A Complete Guide to 34CrAlNi7-10 Nitriding Steel

The full designation 1.8550 is the EN material number under the European standard DIN EN 10085 — Nitriding Steels: Technical Delivery Conditions. The short name 34CrAlNi7-10 is a direct read-out of the alloy architecture:

The ranges below are the cast analysis limits in EN 10085 for grade 34CrAlNi7-10. Product analysis on finished forgings may carry slightly wider permitted deviations per the standard.

The aluminium content (0.80–1.20 %) is the defining characteristic. Aluminium has the highest affinity for nitrogen of any common alloying element. During gas nitriding it precipitates as coherent AlN platelets, producing surface hardness values — typically 900–1050 HV — that no other standard engineering steel grade reliably achieves.

Mechanical properties are measured in the quenched and tempered (Q+T) condition before nitriding. These core values are fully retained after gas nitriding, as nitriding temperatures (480–530 °C) lie well below the tempering temperature and do not alter the core microstructure.

Gas nitriding is a thermochemical surface hardening process conducted in dissociated ammonia (NH₃) atmosphere at 480–530 °C. Because this lies well below the lower critical temperature (Ac₁), no phase transformation occurs in the core — dimensional stability and core toughness are fully preserved.

Why 1.8550 responds better than conventional steels

Gas nitriding cycle parameters

All 1.8550 forgings must be in quenched and tempered (Q+T) condition before nitriding. The tempering temperature must be set at least 50 °C above the intended nitriding temperature to ensure dimensional stability during the nitriding cycle.

Engineers specifying in non-European standards frequently need the closest equivalent for procurement, import classification, or design validation.

The aluminium content in 34CrAlNi7-10 introduces specific forging process considerations that an experienced manufacturer manages proactively.

Steel melting route — the most critical specification decision

Minimum forging ratio

The forging reduction ratio determines how thoroughly the as-cast dendritic structure is refined. Standard: minimum ≥ 4:1; fatigue-critical: ≥ 5:1; high-severity applications (gyratory crusher shafts, large gear journals): ≥ 6:1. The Jiangsu Liangyi Australian iron ore crusher case study achieved 4+ years of service without fatigue failure at ≥ 6:1 reduction versus 12–16 months for the previous 42CrMo4 design.

Hot working temperature range

Start temperature: 1100 °C. Finish above: 850 °C. The aluminium content means the steel must not be worked below ~850 °C — sub-critical deformation introduces residual stress that persists through heat treatment.

The two most widely specified grades in EN 10085 are 34CrAlNi7-10 (1.8550) and 31CrMoV9 (1.8519). They are not interchangeable — they trade off peak surface hardness against machinability and post-nitriding distortion.

A poorly written purchase specification is one of the most common causes of quality disputes and field failures with nitriding steel forgings. The eight items below form a complete checklist — missing any single item leaves a quality gap.

1. 1
   Material standard: State DIN EN 10085, grade 34CrAlNi7-10, EN material number 1.8550. Include both short name and EN number — together they are unambiguous on any international order.
2. 2
   Steel melting route: State EAF+LF+VD, EAF+PESR, or VIM+PESR explicitly. The most commonly omitted item — and the most consequential for fatigue life. Omitting it allows substitution with the cheapest available route.
3. 3
   Forging standard and minimum reduction ratio: Reference EN 10243-1 or ASTM A668. State minimum forging ratio: ≥ 4:1 general use, ≥ 5:1 fatigue-critical, ≥ 6:1 high-severity applications.
4. 4
   Heat treatment condition: State "quenched and tempered (Q+T)" with target hardness range in HB (e.g., 280–320 HB) and minimum tempering temperature (must be ≥50 °C above intended nitriding temperature).
5. 5
   Nitriding specification: Process type (gas / plasma), temperature range, target effective case depth Nht in mm to HV550 limit, minimum surface hardness in HV, and maximum white layer (compound layer) thickness in µm.
6. 6
   Masking requirements: Identify on the drawing all surfaces, bores, keyways, and threaded features to be masked during nitriding to preserve local ductility at stress concentrations.
7. 7
   NDT requirements: UT class per EN 10228-3 (Class 3 or 4 for fatigue-critical), MT and/or PT for surface. Reference the acceptance standard and reject criteria explicitly.
8. 8
   Material test report (MTR): EN 10204 3.1 (manufacturer-issued test report) or 3.2 (third-party witness). For critical applications, name the acceptable inspection body: Bureau Veritas, SGS, Lloyd's Register, TÜV, RINA, or equivalent.

Grade 1.8550 / 34CrAlNi7-10 occupies a well-defined and irreplaceable position in the engineering steel landscape. When an application demands the highest achievable surface hardness from a nitriding process — combined with a truly tough, fatigue-resistant core capable of surviving shock loads and large cross-sections — this is the steel DIN EN 10085 specifies.

Its aluminium content is what sets it apart. The AlN precipitation mechanism during gas nitriding is simply more powerful than the Cr-only or Cr+V mechanisms of competing grades. The trade-offs — slightly more demanding machinability, careful white-layer management, and the need to specify a competent melting route — are readily managed by an experienced, ISO 9001:2015 certified forging manufacturer.

For procurement and engineering teams sourcing 34CrAlNi7-10 forgings internationally: treat the purchase specification as your primary quality control document. Specify the melting route. Specify the minimum forging ratio. Specify the nitriding parameters and the white layer limit. Require EN 10204 3.1 or 3.2 material test reports. These requirements are the technical barriers between a reliable component and a costly field failure.

Engineers who have applied the specification framework in Section 10 to a specific application are ready to move to procurement. Jiangsu Liangyi Co., Limited manufactures 34CrAlNi7-10 forging parts to customer drawings, with full traceability from steel melt to finished component.