P20 vs 2738 – Composition, Heat Treatment, Properties, and Applications

Introduction

P20 and 2738 are two widely encountered steels in the plastics mold and tooling sector. Engineers, procurement managers, and manufacturing planners often must decide between them when specifying mold bases, cores, cavities, or inserts. Typical decision drivers include balancing machinability and cost against hardenability, wear resistance, and long‑term dimensional stability under cyclic thermal/mechanical loading.

The principal difference between the two grades is their alloying strategy and intended performance envelope: P20 is engineered as a prehardened, machinable mold steel with moderate strength and good polishability and weldability; 2738 is a more highly alloyed tool/mold steel optimized for greater hardenability, wear resistance and tempering stability. Because both are commonly used for plastic molds, they are often compared for tradeoffs among cost, post‑machining heat treatment, and lifecycle performance.

1. Standards and Designations

• P20
• Commonly referenced in North America as AISI/ASTM P20 (mold steel). Equivalent or similar designations appear in EN/ISO and national standards through proprietary names used by steel suppliers.
• Classified as a low‑to‑medium alloy tool/mold steel (commonly supplied prehardened).
• 2738
• Referenced in some supplier and national systems as "2738" (note: exact numbering systems vary by country and vendor). It is typically positioned as a higher‑alloy mold/tool steel grade.
• Classified as an alloy tool steel designed for molds—greater hardenability than P20.

Note: Always verify the exact standard/specification and supplier certificate for chemical composition and guaranteed mechanical properties, since numbering and equivalents vary by region and producer.

2. Chemical Composition and Alloying Strategy

The following table summarizes the typical alloying strategy for each grade. Entries are descriptive to reflect that exact amounts vary by standard and vendor; consult material certificates for precise percentages.

Element	P20 (typical role)	2738 (typical role)
C (Carbon)	Medium: balanced for machinability, strength, and polishability	Medium–higher: increases hardness potential and wear resistance
Mn (Manganese)	Moderate: deoxidation, tensile/impact support	Moderate: contributes to hardenability and strength
Si (Silicon)	Low–trace: deoxidation and strength	Low–trace: deoxidation and strength
P (Phosphorus)	Trace (controlled impurity)	Trace (controlled impurity)
S (Sulfur)	Trace (improves machinability when present as free‑cut grade)	Trace (usually minimized for toughness)
Cr (Chromium)	Moderate: improves hardenability, wear resistance, and tempering stability	Higher: stronger contribution to hardenability, wear resistance, and temper resistance
Ni (Nickel)	Low to none: not a major alloying element	May be present in small amounts for toughness in some variants
Mo (Molybdenum)	Small to moderate: increases hardenability and tempering resistance	Moderate–significant: improves hardenability and temper strength
V (Vanadium)	Low (microalloying): refines grain and improves wear resistance	Present in larger amounts in many 2738 variants to refine carbides and increase wear resistance
Nb/Ti/B (microalloying)	Typically low/absent; occasionally microalloyed for grain control	May be present in microamounts for grain refinement and control
N (Nitrogen)	Trace	Trace

How the alloying affects properties: - Carbon and alloying elements (Cr, Mo, V) control hardenability, hardness potential, and tempered stability—higher alloy content yields greater hardenability and better high‑temperature temper resistance but can reduce weldability and increase cost. - Microalloying elements (V, Nb, Ti) refine carbides and grain structure improving toughness and wear resistance for long life in molds. - Low sulfur and phosphorus are necessary to maintain toughness and fatigue life in mold steels.

3. Microstructure and Heat Treatment Response

Typical microstructures and heat‑treatment responses for P20 and 2738 differ due to their alloying:

• P20
• Typical supply condition: prehardened to a moderate hardness (often in the range useful for machining and polishing).
• Microstructure: tempered martensite or tempered bainite with relatively low carbide population; grain sizes controlled for polishability.

• Heat treatment response: P20 is responsive to quenching and tempering; when properly austenitized and tempered it achieves a balance of hardness and toughness suitable for many plastic mold applications. It is often used in a prehardened condition to avoid distortion associated with full hardening.

• 2738

• Typical supply condition: can be supplied prehardened or as annealed/normalized depending on use; designed to be successfully hardened to higher hardness levels.
• Microstructure: after quench & tempering, 2738 tends to show tempered martensite with a dispersed population of alloy carbides (Cr, Mo, V carbides) that improve wear resistance and temper resistance.
• Heat treatment response: higher alloy content increases hardenability and gives more stable temper resistance at elevated service temperatures. Proper austenitizing and quench parameters are crucial to avoid retained austenite and to optimize carbide distribution.

Effect of common processing routes: - Normalizing/refining cycles improve grain size and response to subsequent hardening. - Quench & temper produces the highest hardness and best wear resistance—2738 achieves higher final hardness for the same treatment due to alloy content. - Prehardened delivery (common for P20) reduces post‑machining heat treatment needs and distortion risks.

4. Mechanical Properties

Exact mechanical properties depend heavily on heat treatment and supplier guarantee. The table below gives comparative, qualitative descriptors for typical, application‑relevant properties.

Property	P20 (typical state)	2738 (typical state)
Tensile Strength	Moderate — adequate for most injection and compression molds	Higher — designed for greater load and wear resistance
Yield Strength	Moderate	Higher
Elongation (ductility)	Higher/Good — supports machining and polishing	Lower than P20 when hardened — tradeoff for hardness
Impact Toughness	Good (especially in prehardened or tempered condition)	Good to moderate — can be lower at higher hardness levels
Hardness (as‑supplied / hardened)	Moderate (machineable prehardened state)	Higher achievable hardness after quench & temper

Interpretation: - 2738 is generally capable of higher strength and hardness after heat treatment because of greater alloying and hardenability, which improves wear resistance but tends to reduce ductility and can complicate welding and repair. - P20 offers a better compromise between machinability, polishability and workable toughness, which is why it is commonly used for large, complex plastic molds that are finished and used without full hardening.

5. Weldability

Weldability depends on carbon content, alloying, and hardenability. Two commonly used empirical indices are the IIW carbon equivalent and the Pcm formula:

• IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Pcm formula (used for assessing preheating and weld procedure requirements): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Lower $CE_{IIW}$ and $P_{cm}$ values indicate easier weldability with lower risk of hydrogen‑induced cold cracking. - P20: with moderate carbon and alloying, P20 generally has better weldability and is often selected when repair welding or modification is expected. Preheating and careful post‑weld heat treatment should still be applied per supplier recommendations. - 2738: higher Cr/Mo/V and higher effective carbon equivalents increase hardenability, raising susceptibility to cracking without controlled preheat and PWHT. Welding 2738 requires more stringent procedures and often interpass temperature control and post‑weld tempering.

Recommendations: - Always request supplier welding guidelines and follow qualified welding procedures (preheat, interpass, PWHT, consumable selection). - When in doubt, laboratory weld trials and hardness checks in the HAZ are advisable.

6. Corrosion and Surface Protection

• Neither P20 nor 2738 are stainless steels; both require surface protection if corrosion resistance is a concern.
• Typical protections:
• Painting, plating (nickel, chrome), and proper surface finishes (polish, nitriding) to slow corrosion and improve wear life.
• Hard chrome or nitriding can increase surface hardness and improve wear/corrosion resistance where appropriate.
• PREN (pitting resistance equivalent number) is applicable only to stainless grades: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to P20 or 2738 because they are not stainless alloys. For molds exposed to corrosive or humid environments, consider stainless tool steels or apply protective surface treatments.

7. Fabrication, Machinability, and Formability

• Machinability
• P20: Generally easier to machine in prehardened condition; good surface finish and polishability. Lower cutting forces and longer tool life relative to higher‑alloy tool steels.
• 2738: More difficult to machine when hardened due to higher hardness and carbides; often machined in softer condition and finished after heat treatment with grinding/polishing.
• Formability/Bending
• Both grades are steels; however, bending or forming is limited compared with low‑carbon structural steels. Cold forming is generally not recommended for hardened parts; machining and EDM are preferred for complex features.
• Grinding/EDM/Polishing
• 2738's carbide content can make fine polishing more challenging but provides better wear life in service.
• P20 polishes well and is preferred where surface finish and mirror polish are priorities (e.g., optical or cosmetic plastic parts).

8. Typical Applications

P20	2738
General injection mold bases, large multi‑cavity molds, prototype and production molds where polishability and machining speed are priorities	Cores, cavities, and inserts requiring higher wear resistance and truncal life; tooling for abrasive plastics or long production runs
Mold plates, cavities where moderate hardness suffices and weld repairs may be needed	High‑duty mold components exposed to higher thermal cycles and abrasive wear
Die components for soft forming or light wear	Inserts or cores where post‑hardening and high temper resistance are required

Selection rationale: - Choose P20 when priority is ease of machining, polishability, lower cost, and when service conditions do not subject the mold to heavy abrasive wear or extreme thermal cycling. - Choose 2738 when parts must withstand higher wear, elevated tempering temperatures, or when a longer lifecycle justifies higher material and processing costs.

9. Cost and Availability

• Cost
• P20: typically lower material cost and lower processing cost (often supplied prehardened, reducing hardening and warpage control expenses).
• 2738: higher cost per kilogram due to greater alloy content and often requires controlled heat treatment and more elaborate finishing—raising total cost.
• Availability
• Both grades are commonly available from mold‑steel suppliers, but the specific product form (plates, bars, prehardened blocks) and lead time vary by region and stock levels.
• P20 tends to be more ubiquitous in stock sizes for mold bases; 2738 may require ordering in specific sizes or receive additional lead time for through‑hardening treatments.

10. Summary and Recommendation

Summary table (qualitative)

Attribute	P20	2738
Weldability	Good	Moderate → requires controlled procedures
Strength–Toughness balance	Moderate strength / good toughness	Higher strength / lower ductility at high hardness
Cost (material & processing)	Lower	Higher

Concluding recommendations: - Choose P20 if: - The application requires good machinability and polishability. - You need a cost‑effective prehardened mold plate that is easy to repair/weld. - Service conditions involve moderate wear and the highest hardness is not required.

• Choose 2738 if:
• The component requires higher hardenability, superior wear resistance, and temper stability for long production runs.
• You can accommodate more intensive heat treatment, stricter welding controls, and slightly higher material costs to gain extended tool life.

Final note: Material selection should be confirmed against the supplier’s certified chemical and mechanical data, intended heat treatment cycle, expected loading and environment, and predicted maintenance/repair strategy. For critical molds, perform a lifecycle cost analysis that includes machining time, heat treatment, surface finishing, expected number of cycles, and downtime risk to arrive at the most economical and reliable choice.