Resulfurized 4140 (often called 4140+S or 4140 resul) carries an A-I-T machinability rating of 0.52-0.32 over the 150-425 BHN range, versus 0.36-0.08 for standard wrought 4140 in the 175-52 HRC band [S3]. That 0.52 ceiling puts resulfurized 4140 on par with low-carbon resulfurized carbon steels (1212/1213) on a chip-control basis, a roughly 44% step up from the unmodified grade [S3].
Standard 4140 is a chromium-molybdenum medium-carbon alloy: 0.38-0.43 C, 0.80-1.10 Cr, 0.15-0.25 Mo, 0.75-1.00 Mn, with sulfur capped at 0.040% as an impurity [S1]. Resulfurized variants push S to roughly 0.08-0.13% to generate manganese sulfide (MnS) inclusions that act as chip breakers and lubricate the tool-chip interface [S1][S3]. Engineers comparing these two specs need to read the data carefully, because the headline machinability number is only half the story.
Composition Delta: Same Family, Different Sulfur Ceiling
Standard 4140 keeps S at or below 0.040% to preserve transverse impact energy and forgeability [S1]. Resulfurized 4140 (e.g. The trade is mechanical: every MnS stringer is a stress concentrator lying in the rolling direction, so ductility and impact energy drop in the transverse (short-transverse) orientation versus the longitudinal.
Phosphorus is held to 0.035% max in standard 4140 for the same toughness reason [S1]. Manganese runs 0.75-1.00% in both variants, and it does double duty in the resulfurized grade: it ties up the added sulfur as MnS rather than letting FeS form low-melting grain-boundary networks that would cause hot shortness. Without that Mn buffer, the higher S would crack the billet during forging.
Machinability Numbers, Read Carefully
The A-I-T chart lists two separate rows for 4140 because the resulfurized version is no longer the same material as the wrought baseline [S3]. The relevant comparisons in the 150-425 BHN band:
Standard wrought 4140 (medium carbon, alloy): 0.36-0.08 across 175 BHN to 52 HRC, where the 0.08 floor is the heat-treated end of the curve, and tool life collapses at higher hardness [S3]. Resulfurized 4140: 0.52-0.32 over 150-425 BHN, so the lower-hardness end matches 4118/8620-class wrought alloys, and even at the high end (0.32) it is roughly 4x the rating of standard 4140 at 52 HRC [S3]. For comparison, leaded 41L40 sits at 0.60-0.36, and 12L14 (low-carbon leaded) tops the chart at 1.12-1.08 [S3].
For rough milling and drilling of pre-hardened mold bases in the 27-33 HRC range, this rating gap translates into real shop-floor outcomes: published HolderBloc data shows feed rates can typically be raised 30-50% versus standard 4140 at the same tool life, with corresponding cycle-time compression [S4]. The 0.32 lower bound on resulfurized 4140 still beats 0.36 upper bound on standard, meaning the resulfurized grade retains its advantage even as hardness climbs.
Toughness and Fatigue: Where the Sulfur Bill Comes Due
Manganese sulfide inclusions are mechanically soft relative to the ferrite-pearlite or tempered martensite matrix, and they elongate in the rolling direction. Under transverse or through-thickness loading they open as voids, so Charpy V-notch impact energy in the short-transverse orientation drops meaningfully versus longitudinal, and fatigue life under cyclic bending falls with it. The resulfurized grade is therefore specified with explicit exclusions. [S2]
Finkl's HolderBloc data sheet (DIN 1.2331) calls out exactly the boundary conditions: supplied pre-hardened to 27-33 HRC surface, suited for mold bases and holders with extensive machining, but not recommended for highly polished surfaces where the sulfides will reveal as fine directional scratches under diamond paste, and limited to holder/core sections 14" thick or less due to low through-hardenability [S4]. COREBLOC (P20 modified, 1.2312) is the recommended substitute for sections from 14" to 30" precisely because the same resulfurized approach is paired with a higher-Mn, higher-Cr mix for better hardenability [S4].
For dynamic-loaded components (crankshafts, axle shafts, drive shafts, landing gear, drill collars), the standard 4140 specification remains the default, because the AISI 4140 chemistry with 0.80-1.10% Cr and 0.15-0.25% Mo delivers the fatigue strength, deep hardenability, and reliable transverse impact energy that those duty cycles demand [S2][S1]. Sulfur in those parts is treated strictly as a residual to be minimized.
Selection Matrix: When to Pick Which
Decision criteria, lined up head-to-head for the same 28-32 HRC working hardness: [S1]
Cost per pound: resulfurized 4140 carries a 5-15% premium over standard 4140 in the same size, driven by sulfur control and inclusion-shape conditioning; leaded 41L40 typically prices higher still [S3]. Machinability rating (150-425 BHN): 0.52-0.32 resulfurized, 0.36-0.08 standard, 0.60-0.36 leaded 41L40 [S3]. Transverse impact toughness: standard 4140 is the reference, resulfurized is reduced, leaded 41L40 sits between with its own Pb-induced anisotropy. Polishability / surface finish: standard 4140 is required for any mirror or A1 SPI finish, resulfurized is explicitly excluded above diamond-paste levels [S4]. Through-thickness hardenability: standard 4140 deep-hardens well via Cr-Mo; resulfurized versions in the HolderBloc class are limited to roughly 14" section size and need COREBLOC or MD grades beyond that [S4].
Pick resulfurized 4140 when the dominant cost driver is CNC cycle time on a pre-hardened mold base, holder, or large fabricated block where no polishing is required and the loading is essentially static. Pick standard 4140 when the part sees cyclic or impact loading, needs through-thickness hardness, will be polished, or will be welded (preheat and PWHT still required, but the lower S base metal gives more headroom) [S1][S2]. Pick leaded 41L40 only when you need both machinability and tighter mechanical-property spread than resulfurized 4140, and you can accept the lead-content compliance overhead [S3].
Standards, Spec Writing, and Sourcing Anchors
Standard 4140 procurement typically references ASTM A29 (general alloy steel bars), ASTM A322 (hot-rolled alloy steel bars), or the older A4140 reference in the chemistry callout, with mechanical properties verified per ASTM A370 [S1]. Resulfurized 4140 for plastic-mold work is often called out as DIN 1.2331 or AISI 4140 mod, with the sulfur range, pre-hardness (27-33 HRC or 285-321 BHN), and ultrasonic cleanliness written explicitly into the purchase spec [S4].
Machinability ratings are dimensionless indices where B1112 steel at 100 BHN is the 1.00 reference, and 12L14 at 1.12-1.08 represents the practical ceiling for free-machining steels [S3]. The 0.52 upper bound for resulfurized 4140 sits between AISI 1212 (1.00) and standard 4140 (0.36) on that same axis, which is why shops that have only cut standard 4140 see a noticeable step-change in chip morphology and tool wear when they switch.
For tool wear economics and coating selection when running either grade at scale, the PVD TiN vs DLC tooling guide maps the same hardness range (27-33 HRC pre-hardened) onto modern carbide-grade and coating choices, and is the natural follow-on read for shops that have just qualified either 4140 variant. For shops also comparing structural alloy alternatives at the bar-stock stage, the 4140 vs 1045 hardenability discussion anchors the broader Cr-Mo decision tree, and the 4130/4140/4340 selection chart is a useful sanity check on when 4140 is actually the right Cr-Mo grade versus 4130 or 4340. Mold-shop buyers should also note that 4140 (both variants) sits below P20+Ni (DIN 1.2738) on through-hardenability and above it on as-quenched strength, so the P20+Ni vs 4140 mold-base guidance is the relevant cross-reference for any holder or cavity that needs polishing.
Trackable signals to watch over the next two quarters: (1) any revision to ASTM A29 sulfur limits for resulfurized 4140, since tighter MnS-shape control via calcium treatment is now common in mold-grade specifications; (2) shop-floor data on HolderBloc 1.2331 versus the newer COREBLOC 1.2312 P20 mod for sections above 14", since COREBLOC was specifically formulated to extend resulfurized machinability into the larger block sizes that standard 4140 used to dominate [S4].