UNS S29225 Cr-Mn-N Steel Round Bar

Product Description

UNS S29225 (often recognized under the commercial brand name BÖHLER P558 or European Werkstoffnummer 1.3808) is an ultra-clean, high-nitrogen, low-nickel/essentially nickel-free austenitic stainless steel. Standardized under ASTM F2581, it is specifically engineered for critical biomedical applications and surgical implants.

S29225 utilizes a unique metallurgical approach: rather than relying entirely on a high manganese matrix to replace nickel (like UNS S29108), it balances a tight combination of high nitrogen, carbon, and a moderate manganese profile to stabilize a completely ferrite-free, non-magnetic austenitic microstructure.

Chemical Composition (Mass Weight %)

Unlike conventional austenitic stainless steels where high carbon is strictly avoided due to sensitization risks, S29225 intentionally balances elevated carbon alongside interstitial nitrogen. This specific pairing broadens the thermal window for austenite stability down to lower solution-annealing temperatures.

Core Engineering Advantages

• High Pitting and Crevice Resistance: Due to its combined levels of Chromium ($16.5–18.0\%$) and Molybdenum ($2.7–3.7\%$) bound together with high Nitrogen, S29225 features an extraordinarily high Pitting Resistance Equivalent Number (PREN). Its defense against localized pitting and crevice corrosion in physiological saline fluids vastly outperforms conventional standard reference alloys like 316L (ASTM F138).

• Nickel-Allergy Prevention: Because its nickel threshold is tightly capped at a maximum of $0.05\%$, this alloy eliminates the risk of adverse tissue reactions, localized inflammation, or systemic nickel hypersensitivity in patients.

• Microstructural Cleanliness: Typically melted via premium routes (such as Electro-Slag Remelting / ESR), the material guarantees a fine-grained matrix entirely free of delta ferrite and detrimental intermetallic precipitates, yielding exceptional fatigue limits.

• Completely Amagnetic: It displays ultra-low magnetic permeability and remains stably non-magnetic even after undergoing severe mechanical profiling or cold work-hardening.

Mechanical Properties (At Room Temperature)

Like other specialized high-nitrogen austenites, UNS S29225 cannot be hardened via heat treatment; its strength profile is elevated solely through cold working (strain hardening).

Thanks to interstitial solid-solution strengthening via nitrogen and carbon, its baseline mechanical yield is remarkably robust even in the soft-annealed state:

Note: In the cold-drawn / cold-worked condition, the tensile strength cleanly scales past 1100–1300 MPa depending on the reduction percentage, enabling high-fatigue cross-sections for slim-profile applications.

Processing & Fabrication

• Machining: High interstitial nitrogen and carbon increase mechanical resistance and the rate of work-hardening. Sharp carbide tools, rigid machine setups, positive feed rates, and continuous chip removal are necessary to prevent rapid tool degradation.

• Surface Condition: Round bars are typically delivered centerless ground, polished, or peeled to eliminate surface defects, complying with strict surgical implant criteria.

Target Applications

UNS S29225 round bars are manufactured primarily for use in fields demanding high tissue safety, strong fatigue limits, and absolute non-magnetic clarity:

• Surgical Implants: Intramedullary pins, bone reconstruction plates, spinal fixation hardware, and orthopedic screws.

• Medical Instrumentation: Surgical drills, guide wires, forceps, and complex diagnostic components designated for use within active MRI fields.

• Horology & Luxury Goods: High-end watch cases, bracelets, and internal mechanical movements requiring non-magnetic shielding and absolute hypoallergenic properties for direct skin contact.