NAK80 vs P20 – Composition, Heat Treatment, Properties, and Applications

Introduction

NAK80 and P20 are two of the most commonly specified mold steels in injection-molding and tooling industries. Engineers, procurement managers, and manufacturing planners frequently weigh the tradeoffs between them when choosing a steel for core/cavity inserts, hot-runner components, and prototype tooling. Typical decision contexts include balancing surface finish and polishability against cost and machinability, or choosing between improved corrosion resistance and standard mechanical performance.

The central practical distinction is that NAK80 is a low‑carbon, nickel‑bearing pre‑hardened mold steel optimized for high polishability and improved resistance to environmental corrosion, while P20 is a chromium‑molybdenum alloyed mold steel developed for good machinability, uniform hardenability, and broad availability. This difference drives choices in finishing, processing, and long‑term part performance.

1. Standards and Designations

• P20:
• Common designations: AISI/UNS (often referenced as AISI P20), DIN/EN equivalents exist in tool steel listings, and various national specifications (e.g., JIS/GB listings referencing mold steels).
• Classification: Alloy tool steel / pre‑hardened mold steel.
• NAK80:
• Proprietary/commercial grade name widely used in the plastics mold industry (sold by multiple steel makers under the NAK80 designation).
• Classification: Pre‑hardened mold/tool steel with low carbon and elevated nickel content, marketed as a polishable, corrosion‑resistant mold steel.

Note: Neither grade is a stainless grade in the strict sense (i.e., both are not in the austenitic stainless family), but NAK80’s nickel content provides improved corrosion resistance relative to conventional P20.

2. Chemical Composition and Alloying Strategy

The following table lists typical composition ranges reported in commercial datasheets. Compositions vary by supplier; always verify with the mill certificate for a given lot.

How alloying affects behavior: - Carbon: Greater carbon in P20 yields higher as‑hardened strength potential but reduces polishability and increases the risk of hard phases that complicate finishing and increase the chance of weld cracking without preheat/controlled procedures. - Nickel: NAK80’s elevated nickel improves toughness, ductility, and resistance to environmental corrosion (and contributes to a fine, stable microstructure favorable for high‑gloss polishing). - Chromium and molybdenum: Both grades include Cr and Mo to provide hardenability, temper resistance, and wear resistance. P20’s balance favors machinability and through‑hardening; NAK80’s Cr/Mo is tuned to complement Ni for surface finish and corrosion behavior. - Microalloying elements (V, Nb, Ti): Present at low levels to control grain size and precipitation behavior; impact grindability, toughness, and temper response.

3. Microstructure and Heat Treatment Response

Typical microstructures: - P20: In the pre‑hardened (as‑supplied) condition P20 commonly contains tempered martensite and tempered bainite with a relatively higher proportion of carbides (due to higher carbon) that are distributed throughout the matrix. After conventional quench and temper cycles, P20 attains a martensitic microstructure with alloy carbides. - NAK80: With lower carbon and higher nickel, NAK80 in the pre‑hardened condition shows a tempered martensitic or bainitic matrix with fewer and finer carbides. The nickel stabilizes a tougher, more ductile matrix and reduces the formation of coarse chromium carbides problematic for mirror finishing.

Heat treatment response: - Normalizing: Both steels respond to normalizing to refine grain size, but NAK80 is typically supplied vacuum‑treated and pre‑hardened to minimize post‑machining distortion; normalizing is less common for NAK80 parts intended for high polish. - Quenching & tempering: P20 is designed to be heat treated to higher hardness levels (and often is supplied in a pre‑hardened condition for production molds). Hardening P20 requires care with austenitizing temperature to manage retained austenite and carbide dissolution. NAK80’s lower carbon content limits maximum hardened hardness but provides a more forgiving temper response and better dimensional control for polishing. - Thermo‑mechanical processing: Not typically applied in mold‑steel contexts compared to structural steels; both grades rely on controlled heat treatment rather than hot‑rolling processing paths for final properties.

4. Mechanical Properties

Values vary significantly with heat treatment and supplier. The table shows representative delivered‑condition ranges often seen in commercial data.

Interpretation: - Strength: In delivered photos both can provide similar tensile/yield ranges; P20’s higher carbon gives a marginally higher hardening potential if fully quenched and tempered, but NAK80’s alloying yields comparable tensile values in commercial pre‑hardened conditions. - Toughness and ductility: NAK80 generally exhibits better toughness and a more ductile fracture tendency at comparable hardness due to nickel and lower carbon, which reduces brittle carbides and improves shock resistance. - Hardness: Both are offered in similar pre‑hardened ranges optimized for mold production; maximum attainable hardness after full hardening is typically higher for P20 because of its higher carbon, but this comes with tradeoffs in finishability.

5. Weldability

Weldability considerations hinge on carbon equivalent and microalloying.

Two common measures: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Dearden–Bennett / Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - P20 typically has a higher carbon content, which increases $CE_{IIW}$ and $P_{cm}$, raising the risk of hydrogen‑induced cold cracking and requiring preheat, interpass temperature control, and post‑weld heat treatment for critical tooling repairs or thick sections. - NAK80’s low carbon and higher nickel reduce the propensity for hard, brittle weld‑heat affected zones; nickel improves toughness and tolerance to thermal gradients, making NAK80 more forgiving for welding and local repairs. Nevertheless, due to mold accuracy requirements and potential sensitization/contamination, welding should be planned and validated (including filler selection and PWHT where needed). - Both grades benefit from low hydrogen consumables, preheating where recommended, and trial welds to establish parameters. For high‑precision molds, welding is often avoided in finished surfaces or followed by re‑machining/polishing.

6. Corrosion and Surface Protection

• Neither NAK80 nor P20 is a stainless steel series designed for long‑term immersion corrosion resistance. However, NAK80’s elevated nickel content yields noticeably better resistance to mild corrosive environments (humid, mildly aggressive molding atmospheres, and repeated cleaning) compared to P20.
• When corrosion resistance is required, both steels are typically protected with surface measures:
• Electroplating (nickel, hard chrome) for wear and corrosion protection.
• Chemical or physical passivation is not applicable as with stainless steels.
• Regular maintenance and drying/dessicant control in tooling storage mitigate rusting.
• PREN (pitting resistance equivalent number) is relevant only to stainless grades; for informational purposes: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to NAK80 or P20 as marketed mold steels (they are not chromium‑rich stainless grades with significant nitrogen content).

7. Fabrication, Machinability, and Formability

• Machinability:
• P20: Historically favored for machinability. Higher carbon content, predictable carbide distribution, and established tooling parameters make P20 straightforward to mill, drill, and EDM prior to final heat treatment. Carbide‑forming elements and microstructure are compatible with standard high‑speed tooling.
• NAK80: Slightly more challenging to machine than P20 in some cases because of nickel’s work‑hardening tendencies and tougher matrix; however, modern tooling and cutting parameters mitigate differences. NAK80 machines well when using high‑quality carbide tooling and adjusted feeds.
• EDM and surface finishing:
• Both steels are EDM‑friendly. NAK80 often yields superior mirror polish and edge integrity owing to fewer coarse carbides and a fine matrix—important for optical surfaces and glossy plastic parts.
• Formability/bending:
• As mold steels used in block form, forming is not a primary concern; however, for thick sections and welding clipping, NAK80 tolerates deformation and repair welding better than high‑carbon alternatives.
• Surface finishing:
• NAK80 is engineered for high polish grades (RA and mirror finish) with lower tendency for micro‑pitting and “orange peel” after repeated cycles. P20 can be polished to high gloss but may require more corrective grinding and passivation.

8. Typical Applications

Selection rationale: - Choose NAK80 when surface finish, corrosion tolerance, and minimal polishing effort over the mold lifecycle are primary requirements. - Choose P20 when cost, standardized processing, and maximum hardenability or higher final hardness are more important.

9. Cost and Availability

• Cost: P20 is generally less expensive on a per‑kg basis due to simpler alloying and wide production volume. NAK80 typically carries a premium for alloy content (notably nickel), vacuum melting, and marketing as a polishable mold steel.
• Availability: P20 is widely stocked in many mill sizes and forms (blocks, plates, pre‑hardened plates). NAK80 is broadly available but may have longer lead times or limited availability in very large sections depending on the supplier and regional inventory.
• Product forms: Both are available as pre‑hardened plates, blocks, and sometimes through‑hardened bars; NAK80 is more commonly specified in pre‑hardened condition to preserve surface quality.

10. Summary and Recommendation

Choose NAK80 if: - Mirror finish/polishability and cosmetic part quality are critical. - The mold will be exposed to humidity, regular cleaning, or mildly corrosive environments and you require improved corrosion tolerance without plating. - Frequent on‑mold re‑polishing and long cosmetic life are expected and you want to minimize finishing cycles.

Choose P20 if: - Budget and material availability are primary constraints. - You need a well‑understood, easy‑to‑machine mold steel suitable for a wide range of molding applications. - Higher hardenability or the option to quench/temper to higher hardness is required and surface cosmetic requirements are moderate or will be addressed by coatings/plating.

Final note: For production decisions, obtain and compare the mill certificates, confirm the supplier’s recommended machining and heat‑treatment practices, and run trial machining/polishing and, where applicable, weld trials. Actual part performance depends on specific alloy heat, section size, and the applied thermal and mechanical processing steps.