For mold base plates exceeding roughly 400 mm in thickness, 1.2738 (P20+Ni, GB 3Cr2NiMo) is the technically defensible specification over standard P20/1.2311 because the nickel addition hardens and toughens the core, which preserves uniform through-thickness hardness in large pre-hardened blocks where 1.2311 starts to lose property consistency [S1][S2][S3].
P20 in its 1.2311 form (40CrMnMo7) is delivered quenched and tempered at 280-325 HB (28.3-34.2 HRC, 890-1030 N/mm² tensile) and is recommended by suppliers for "small and medium-sized molds up to 400 mm thickness" with process temperatures below 250 °C [S2]. 1.2738 is harder than both 1.2311 and standard P20, and is positioned by mill sources for larger molds with longer service life [S1].
Chemistry and Designation Equivalence
The 1.2738 grade adds roughly 0.8-1.2 % Ni to that base, which is the metallurgical reason 1.2738 is sold as P20+Ni and cross-listed as DIN 1.2738, AISI P20+Ni, and GB 3Cr2NiMo [S3].
Nickel does not raise hardness by itself, but it refines the as-quenched microstructure and raises impact toughness, so a 1.2738 block can be pre-hardened to a higher and more uniform level (commonly 30-36 HRC through full plate thickness) without sacrificing fracture resistance. That is the property the marketplace pays for in mold base plates larger than 400 mm [S1][S3].
Hardness, Through-Thickness Uniformity and Size Limit
The 1.2311 datasheet quotes a delivery hardness of 280-325 HB across thicknesses up to 400 mm, with ultrasonic acceptance per EN 10228-3 SEP 1921 Table 3 type 1 quality class 3 group 3 class D.d and cleanliness K4≤20 (DIN 50602) / A≤1.5, B,C,D≤2 (ASTM E45 method A) [S2]. Beyond 400 mm, the through-thickness hardness of 1.2311 drifts because the quench section becomes too thick for the alloy's hardenability, so suppliers explicitly stop recommending it for "large molds" at that point [S2].
1.2738 is the grade mill sources direct buyers to once the section size crosses that threshold: it is harder than both 1.2311 and P20, with the higher core hardness maintained across the full block, which suits larger molds and longer production runs [S1]. The standard target delivery condition is pre-hardened, so neither grade requires a post-machining heat treatment for use as a casting mold frame or bolster [S2][S3].
Machining, Welding, Nitriding and Polishability
1.2311 is supplied in a free-machining, pre-hardened condition and is widely used for both inserts and bolster plates on small to mid-size tooling; the datasheet rates polishability as SPI:N3 / A-3, weldability with CET = 0.65 % per DIN EN 1011-2, and nitriding hardness of 700-850 HV1 [S2]. 1.2738 is also delivered pre-hardened, so both grades cut without a downstream quench and the associated distortion risk [S2][S3].
For higher texturing reliability on either grade, suppliers point to remelted variants such as XPM or XPM VICTORY ESR, and recommend vacuum-remelted stock when the part is a sand casting mold frame with deep pockets and stringent inclusion limits [S2]. Both 1.2311 and 1.2738 can be further surface-hardened by nitriding, chrome plating, or flame hardening if a wear skin is needed for high-pressure injection [S2].
Selection Criteria: 1.2738 vs P20/1.2311 Decision Map
The four decision criteria that actually separate the two grades for a large mold base plate are plate thickness, required core hardness, expected service life, and budget. Below 400 mm thickness, at modest production volumes, 1.2311 is the lower-cost, easier-to-machine choice and is what most commercial mold bases are built from [S2][S4]. Above 400 mm, or when the buyer needs the higher, more uniform through-thickness hardness that 1.2738 provides, the nickel grade is the correct call despite its price premium [S1][S3].
Concretely: 1.2311 is the right pick for small and medium plastic molds, die-casting mold frames, and standard mold bases up to 400 mm at process temperatures below 250 °C; 1.2738 is the right pick for large mold bases, hot-work mold frames, and high-cavity / long-run tooling where core hardness must hold across the full plate section [S1][S2][S3]. As one shop-floor rule summarized on Practical Machinist: "Commercial mold bases normally are sold in one of 3 grades of steel, #'s 1, 2 & 3. #1 is 1020, or similar, #2 is 4130 or similar, and #3 is P-20. #3 costs a lot more than #1, BTW. Only #3 is recommended to have cavities cut directly in it, since it is pre-hard" [S4].
Thermal and Mechanical Properties for Large Plate Design
The published 1.2311 thermal envelope is 34.2 W/(m·K) at 20 °C falling to 27.5 W/(m·K) at 500 °C, with thermal expansion of 12.6 × 10⁻⁶/K at 100 °C rising to 14.2 × 10⁻⁶/K at 500 °C, and an elastic modulus of 212 kN/mm² at 20 °C dropping to 175 kN/mm² at 500 °C [S2]. These numbers are the basis for distortion budgeting when a buyer sizes a mold base for a large-format tool and decides whether 1.2311's 280-325 HB can hold flatness over the full footprint [S2].
1.2738 inherits the same Cr-Mo backbone as 1.2311, so the thermal conductivity and expansion envelope are comparable, but the addition of nickel raises the through-thickness hardenability, which is what makes the larger 30-36 HRC range deliverable in blocks thicker than 400 mm without quenching cracks or heat-treatment distortion [S2][S3].
Sourcing, Standards and Traceability
Both grades should be ordered against a named international designation, not a trade name, so that the mill certificate is unambiguous. P20/1.2311 is covered by ASTM A681, DIN EN ISO 4957, and GB/T 1299 (3Cr2Mo); 1.2738 (P20+Ni) is cross-listed as AISI P20+Ni, DIN 1.2738, and GB 3Cr2NiMo [S2][S3]. Buyers should also specify ultrasonic testing per EN 10228-3 SEP 1921 Table 3 (1.2311 is typically supplied to quality class 3 group 3 class D.d), inclusion rating per DIN 50602 K4≤20 or ASTM E45 method A (A≤1.5, B/C/D≤2), and pre-hardened delivery at 280-325 HB for 1.2311 with the higher 30-36 HRC band called out explicitly for 1.2738 [S2].
For a working reference on tolerance-stack and distortion when machining large pockets in these plates, the 0.250 x 0.250 x 1.37 in square key length tolerance guide is a useful adjacent read, since the same pre-hardened-steel machining-dictates-finish-grind logic applies to large P20 and 1.2738 mold base plates [S2][S4]. For shops deciding between building a base from stock plate versus buying a commercial pre-machined base, the same Practical Machinist discussion also notes that suppliers such as Mold Base Industries and DME will ship finish-ground plates with pockets roughed in, which removes a large share of the in-house distortion risk on either grade [S4].