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

Introduction

P20 and NAK80 are two widely used mold and tool steels for plastic-injection and die-casting tooling. Engineers, procurement teams, and manufacturing planners commonly weigh trade-offs between total part cost, tool life, surface finish, and maintenance when selecting between them. Typical selection contexts include: choosing a cost-effective material for large-volume tooling (where toughness and machinability dominate) versus choosing a stainless-capable mold steel for high-gloss or corrosion-sensitive parts (where surface finish and passivation matter).

The principal practical difference that drives many design decisions is surface behavior: NAK80 is engineered to deliver superior surface finish retention, corrosion resistance, and polished appearance for high-gloss plastic parts, while P20 is a more conventional pre-hardened mold steel optimized for affordability, machinability, and general-purpose toughness. Because they occupy overlapping application spaces (mold bases, cavities, core inserts), designers routinely compare them for final part aesthetics, mold maintenance, and production environment constraints.

1. Standards and Designations

• P20
• Common designations: AISI/SAE P20, DIN 1.2312 (close equivalents), various supplier trade names (pre-hardened mold steel).
• Classification: Alloy tool steel, typically supplied in pre-hardened condition.
• NAK80
• Common designations: Trade name (NAK80) used by several Japanese and global suppliers; sometimes referenced as a martensitic stainless mold steel grade.
• Classification: Martensitic stainless tool/mold steel (stainless mold steel).

Applicable standards to check on mill certificates and purchase orders include ASTM/ASME specifications for mold steels and the relevant national standards (EN, JIS, GB) for tooling and stainless tool steels. Always confirm supplier-specific designations and certificates because trade names map differently between producers.

2. Chemical Composition and Alloying Strategy

The following table gives representative nominal composition ranges widely cited in supplier datasheets. These values are intended as typical ranges—always verify specific mill certificates for procurement.

Element	P20 (typical nominal range, wt%)	NAK80 (typical nominal range, wt%)
C	0.25–0.35	0.03–0.12
Mn	0.35–0.60	0.10–0.60
Si	0.20–0.35	0.10–0.80
P	≤0.03	≤0.025
S	≤0.03	≤0.020
Cr	1.30–1.60	11.0–13.5
Ni	0.30–0.60	1.0–4.0
Mo	0.30–0.50	0.30–0.60
V	trace–small	trace
Nb (Cb)	—	trace
Ti	—	trace
B	—	trace
N	—	trace (in stainless variants)

How alloying affects properties: - Carbon: primary hardenability and strength contributor. P20’s higher carbon (relative to stainless low-carbon variants) supports higher hardness and wear resistance after quench/tempering, but raises HAZ cracking risk during welding. NAK80 is often kept lower in carbon to balance corrosion resistance and avoid excessive brittleness. - Chromium and nickel: In NAK80, elevated Cr and Ni provide stainless/passive behavior and improve corrosion resistance and polishability. In P20, modest Cr and Ni improve hardenability and strength but do not confer stainless properties. - Molybdenum and vanadium: Improve hardenability, secondary hardening, creep resistance, and carbide stability — beneficial for both grades to enhance wear resistance. - Other microalloying (Nb, Ti, B): Refine grain size and control precipitation; in stainless tool steels these are often present in trace amounts to control properties.

3. Microstructure and Heat Treatment Response

Microstructure (typical): - P20: Tempered martensite with fine alloy carbides (Cr/Mo carbides). Often supplied prehardened (e.g., ~28–32 HRC) in a normalized and tempered state; microstructure optimized for machinability and toughness. - NAK80: Martensitic stainless matrix with chromium-rich carbides and a passive chromium oxide surface layer when polished; often produced to a prehardened condition suitable for polishing and corrosion resistance.

Heat treatment response: - P20: - Can be supplied prehardened; if hardened and tempered, quench-and-temper cycles produce tempered martensite. Normalizing refines grain size. - Nitriding is commonly applied for surface hardness and wear resistance; nitriding behavior depends on alloyed nitride-forming elements (e.g., V, Cr). - NAK80: - Heat treatment aims to balance hardness with stainless characteristics. Typical routes include solution anneal and quench followed by tempering; careful control required to avoid sensitization and to retain corrosion resistance. - Stainless behavior complicates high-temperature treatments; heat-to-temper windows differ from carbon-alloy steels and may require vacuum or controlled atmosphere to avoid decarburization and oxidation.

Thermo-mechanical processing can influence grain size and distribution of carbides; for both steels, tighter process control yields better polishability and more uniform hardness.

4. Mechanical Properties

The table below lists typical property ranges for commonly supplied prehardened conditions and heat-treated states. Verify with supplier certificates for exact figures.

Property	P20 (typical)	NAK80 (typical)
Tensile Strength (MPa)	800–1100	700–1000
Yield Strength (0.2% offset, MPa)	600–900	500–850
Elongation (%)	10–18	8–18
Impact Toughness (Charpy, J)	moderate (depends on temper)	moderate to good (depends on condition)
Hardness (HRC)	28–32 (prehardened), can be heat-treated higher	30–36 (prehardened stainless variants)

Interpretation: - Strength: P20 in comparable prehardened conditions typically provides slightly higher nominal strength attributable to higher carbon and its alloying mix; however, heat treatment can narrow differences. - Toughness/ductility: Both grades trade hardness for toughness; P20 formulations tend to prioritize general-purpose toughness and machinability, whereas NAK80 is balanced for polishability and corrosion resistance with adequate toughness. - Hardness ranges overlap; the grade chosen should align with required wear resistance and final surface finish targets.

5. Weldability

Weldability depends on carbon equivalent and alloy content. Use of carbon-equivalent formulas helps assess HAZ cracking susceptibility and preheat/postheat requirements.

Common formulas: - IIW carbon equivalent (qualitative guide): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International welding eletrotechnical carbon equivalent (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: The higher carbon and presence of Mo/Cr raise carbon-equivalent numbers relative to low-carbon steels, increasing susceptibility to HAZ hardening and cold cracking unless appropriate preheat and post-weld tempering are used. P20 often requires controlled welding procedures or use of compatible low-hydrogen filler metals. - NAK80: As a martensitic stainless steel, NAK80’s Cr and Ni content change weldability behavior; stainless martensitics can be prone to HAZ and weld-metal cracking if not preheated and if interpass temperatures and cooling rates are not controlled. Nickel improves weldability to an extent, but the stainless matrix requires careful filler selection and often post-weld heat treatment to recover toughness and corrosion resistance. - In practice: Both grades can be welded, but welding plans (preheat, interpass temp, post-weld temper) and qualified filler metals are essential. For cosmetically critical surfaces, as-welded areas should be avoided or ground and re-polished.

6. Corrosion and Surface Protection

• P20: Not stainless. Surface corrosion protection is typically required where moisture or corrosive environments are present. Common protections include:
• Painting, plating, or local coatings (PVD/CVD) on finished surfaces.
• Galvanizing is not typical for tooling; rather nitriding or surface coatings (TiN, CrN) are used to increase wear resistance and reduce corrosion-pitting initiation.
• NAK80: Martensitic stainless steel providing passivity and improved resistance to corrosion compared with P20 when polished and maintained. For stainless characterization, corrosion indices such as PREN (pitting resistance equivalent number) are commonly used for austenitic and duplex grades: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Note: PREN is less directly applicable to martensitic stainless mold steels like NAK80, but the formula illustrates how Cr/Mo/N content correlates to pitting resistance. NAK80’s chromium content and surface finish are the main contributors to its superior resistance to staining and corrosion in service.
• Practical consequence: For high-gloss plastic parts and molds operating in humid or corrosive environments, NAK80 reduces staining and maintenance needs; P20 requires protective strategies (coatings, controlled environment) to maintain surface quality.

7. Fabrication, Machinability, and Formability

• Machinability:
• P20: Good machinability in the prehardened state; widely used because it machines easily to tight tolerances and EDMs well. Carbide tooling and standard coolant practices suffice.
• NAK80: Generally good machinability for stainless tool steels, but work-hardening tendencies and stainless-alloy behavior require optimized tool geometry, cutting speeds, and coolant. Surface finish achievable is excellent with proper tooling.
• Formability and bending: Both are tool steels—limited ductility relative to mild steels. Forming is typically performed before final hardening/tempering. Avoid heavy forming after hardening.
• Surface finishing and polishing:
• This is a key differentiator. NAK80 polishes to a mirror finish more readily and retains a high-gloss appearance due to its stainless matrix and fine carbide distribution. P20 can be polished to a good finish but is more susceptible to staining, oxidation, and requires additional coatings or maintenance to preserve high-gloss finishes.

8. Typical Applications

P20 — Typical Uses	NAK80 — Typical Uses
General-purpose injection mold cavities and cores for non-critical surface appearance	High-gloss injection mold cavities for optical, medical, and consumer parts
Large mold bases and structural components where cost and machinability dominate	Corrosion-prone tooling (humid environments) or molds requiring long-term polish retention
Prototype and low-to-medium volume production where expedient machining and EDM are needed	Multi-compoent molds where surface finish and part aesthetics are priorities
Tooling requiring subsequent nitriding or PVD coatings for wear resistance	Precision molds where stainless performance reduces maintenance and staining

Selection rationale: - Choose P20 for balance of cost, availability, and machinability when final part appearance is not critical or when coatings/nitriding can be applied. - Choose NAK80 for high-gloss parts, reduced staining, and where corrosion resistance in the tool is required to maintain appearance and reduce maintenance downtime.

9. Cost and Availability

• Cost:
• P20 is typically less expensive per kilogram than NAK80 due to lower alloy content and broad commodity supply. It is a cost-effective choice for large molds and where budget constraints exist.
• NAK80 commands a premium due to higher alloy content (Cr, Ni) and processing for stainless properties; expect higher material costs and sometimes higher processing costs for heat treatment and finishing.
• Availability:
• P20 is widely available in prehardened plates, blocks, and bars from many suppliers and is common in mold shops.
• NAK80 is widely produced but availability in very large plate sizes or non-standard dimensions may be more constrained than P20; lead times can be longer depending on thickness and finish requirements.

10. Summary and Recommendation

Summary table (qualitative):

Attribute	P20	NAK80
Weldability	Good with controls; higher CE requires care	Challenging; stainless weld procedure and PWHT often needed
Strength–Toughness balance	Strong and tough in prehardened state	Good balance but optimized for polish and corrosion resistance
Cost	Lower (more economical)	Higher (premium stainless alloy)

Recommendations: - Choose P20 if: - Budget and rapid machining/EDM turnaround are priorities. - The part does not require a high-gloss finish or is protected/coated. - Large mold bases or components with heavy machining are required. - Choose NAK80 if: - High surface finish, polish retention, and corrosion resistance are critical (optics, medical, high-gloss consumer parts). - You want reduced tool maintenance for staining-prone materials or humid production environments. - The premium cost is justified by reduced rework, longer polish life, or improved cosmetic quality of molded parts.

Final note: Material selection should be validated by comparing supplier datasheets, running sample polish tests under expected process conditions, and accounting for welding/repair procedures and surface treatments. When surface finish or corrosion resistance drives part acceptance, trial tooling with the final material and polish regimen is strongly recommended.