1.1730, 1.2311, and 1.2312 sit in the same mold-build conversation but solve different problems: 1.1730 (DIN C45W, ~640 MPa, 0.45 C, 0.30 Si, 0.70 Mn) is a non-alloy tool steel that cannot be hardened and is easy to machine, used for frames and simple structural parts of a mold base [S3].
1.2311 (P20 equivalent, ~1050 MPa) and 1.2312 (P20+S, ~1050 MPa) are both pre-hardened low-alloy tool steels at 28-32 HRC with 0.40 C, 1.50 Mn, 1.90 Cr, 0.20 Mo, but 1.2312 adds ~0.07% S to break chips cleanly during deep-pocketing CNC work [S3]. The hard call is matching the steel to the cavity-versus-base split on the SPI mold class.
Composition and tensile-strength snapshot
1.1730 is a plain carbon steel, no Cr or Mo, with a delivery tensile strength around 640 MPa per DME's plate/frame reference list [S3]. Its job is to act as the cheap A/B/retainer plate, support pillars, and clamp slots where the casting mold frame absorbs clamping force but never sees a polished cavity surface [S3][S4].
1.2311 (40CrMnMo7) carries 0.40 C, 0.40 Si, 1.50 Mn, 1.90 Cr, 0.20 Mo and ships pre-hardened to about 1050 MPa, suitable for carbon steel mold frames and cavities that do not need a through-hardening cycle [S3]. 1.2312 (40CrMnMoS7) matches 1.2311's chemistry but adds 0.07 S, which forms MnS stringers that act as chip breakers; the trade-off is a small reduction in transverse toughness versus the S-free grade [S3].
Hardness, polishability, and machinability
1.1730 ships in the annealed condition, typically 12-18 HRC, and is rarely heat-treated; it machines freely but cannot reach cavity-grade hardness [S4]. 1.2311 lands at 28-32 HRC, polishes to a medium-grade finish, and is the default choice for pre-hardened mold bases that need a usable cavity surface [S4][S5].
1.2312 holds the same 28-32 HRC window as 1.2311 but accepts higher feed rates and deeper cuts; this free-machining behavior makes it the workhorse for thick plates and pockets on large mold base assemblies where cycle-time on the mill matters [S3][S7]. Note that 1.2312 is generally not specified for mirror-finish cavities because the MnS inclusions can show up under SPI-A1 class polish [S3].
Selection criteria across the SPI mold class
The Deep Mould mold-class table maps steel choice to shot count, and 1.2311/1.2312 dominate the higher classes: Level 101 (over 1 million shots) and Level 102 (500,000-1 million shots) both call for 1.2311 or 1.2312 mold bases at HRC 28-32 minimum, with cavity/core in H13/1.2343/1.2344 heat-treated to HRC 50-54 [S5]. Level 103 (under 500,000 shots) drops the base to 45# (the AISI 1050 / DIN 1.1730 equivalent) at about HRC 16, and Level 104 (under 10,000 shots) can run 1.1730 or P20 even for the cavity [S5].
Two practical rules follow: specify 1.2312 wherever the plate will be deep-pocketed or screw-rail machined (MnS improves chip flow); specify 1.1730 wherever the plate is a non-cavity structural face that needs maximum weldability and minimum cost. Where the SPI class demands a high-end cavity surface, the reference shifts to P20+Ni (1.2738) at 33-37 HRC, harder than both 1.2311 and 1.2312 because of its nickel addition [S1][S2].
Decision matrix for the procurement engineer
Three criteria drive the call: required surface hardness, polish class, and CNC cycle time. On hardness, 1.2738 wins (33-37 HRC), 1.2311/1.2312 tie (28-32 HRC), and 1.1730 trails (12-18 HRC annealed) [S1][S2][S4]. On machinability in delivered condition, 1.2312 leads due to the S addition, 1.1730 is the next easiest (low hardness), 1.2311 sits in the middle, and 1.2738 is the slowest [S3][S7].
On polishability, 1.2311 and 1.2738 are preferred for cavity faces, 1.2312 is acceptable for textured or non-cosmetic surfaces, and 1.1730 is restricted to structural frames where polish is irrelevant [S3][S4]. For a base assembly where a 1.2311 cavity plate mates to 1.1730 backing plates, the common practice is to verify that the bolt circles, ejector housings, and leader pins land in plates with at least 28 HRC; 1.1730 retainer or clamp plates below this level can fret under repeated cycling [S3][S5].
Failure modes and limits in service
1.1730 plates show wear and brinelling at ejector-pin bores and sprue bushings if the local hardness is not raised by nitriding or local induction hardening; this is the most common revision request on a Level 103 build that underperformed [S4][S5]. 1.2311 can suffer from weld-cracking during repair if the pre-hardened HRC climbs above the recommended 32 HRC; the same risk applies to any post-build flame-hardening attempt on alloy steel P20 plates [S3].
1.2312's MnS stringers, the same feature that aids machinability, can act as fatigue initiation sites under cyclic ejector loading and are the reason most shops prohibit 1.2312 for high-stress moving plates or for any plate that receives a subsequent nitriding treatment (the S interferes with the nitrided case) [S3]. For plastic grades with abrasive fillers (glass-filled PA, PC, POM), specifying 1.2311 or upgrading to 1.2738 is the safer call; 1.1730 in the cavity will pit within a few thousand shots in those service conditions [S1][S5].
Standards crosswalk and sourcing
The DIN 1.2311 designation maps to AISI P20 and to JIS equivalent grades used widely in Asian tooling, and the AISI/JIS/DIN cross-reference is the practical way to keep an international spec unambiguous on a mold base drawing [S4]. 1.2312 is the DIN P20+S variant; 1.1730 maps to AISI 1050 / JIS S50C, a medium-carbon steel sold in the annealed condition [S3][S4].
For procurement, DME's plate-and-frame reference is a useful shortcut: 1.1730 is flagged as "not to be hardened and easy to machine, for molds and simple structural components," 1.2311 as a "low alloy pre-heat treated tool and cavity steel ... supreme surface appearance after polishing," and 1.2312 as a "low alloy pre-heat treated tool steel ... excellent machinability ... for mould plates under high stress" [S3]. Match that to the SPI class and the cavity-versus-base role, and the steel call becomes a checklist instead of a debate.
Verify the mill certificate on every 1.2311/1.2312 plate for actual hardness (not just tensile), confirm S content on 1.2312 if polish is in scope, and confirm that 1.1730 retainer plates are not being misapplied as cavity plates on a Level 102+ build [S3][S5].
Background reading: LiDAR for Industry 4.0: spec-driven adoption, mechanical vs solid state, and where the.