Plastic Mold Steels Comparison: 1.2311 vs 1.2312 vs 1.2738

1.2311, 1.2312, and 1.2738 belong to the same series: pre-hardened chromium-manganese-molybdenum plastic mold steels with a hardness of 280-325 HB, which can be directly machined without further heat treatment.The practical differences come down to two alloy additions. 1.2312 adds sulfur to make machining faster. 1.2738 adds about 1% nickel to raise toughness and hardenability.1.2311, without any additional alloys, remains the preferred steel for general mold making.

This article will provide a comprehensive comparison in terms of chemical composition, mechanical properties, heat treatment processes, processing performance, and applications.

Designations and Standards

1.2311, 1.2312, and 1.2378 are unique numerical designations under the DIN 17007 standard. Their corresponding designations are shown in the table below.

W-Nr	Corresponding Grade	Standard	USA Equivalent
1.2311	40CrMnMo7	EN 4957/DIN 17350	P20
1.2312	40CrMnMoS8-6	EN 4957/DIN 17350	P20+S
1.2738	40CrMnNiMo8-6-4	EN 4957/DIN 17350	P20+Ni

Chemical Composition Comparison

The three types of steel share a common basic chemical composition: approximately 0.40% carbon, 1.5% manganese, 1.9-2.0% chromium, and 0.2% molybdenum. The main differences lie in the sulfur and nickel content.

Grade	C	Si	Mn	Cr	Mo	P	S	Ni
1.2311	0.35-0.45	0.2-0.4	1.3-1.6	1.8-2.1	0.15-0.25	≤0.035	≤0.035	—
1.2312	0.35-0.45	0.3-0.5	1.4-1.6	1.8-2.0	0.15-0.25	≤0.03	0.05-0.1	—
1.2738	0.35-0.45	0.2-0.4	1.3-1.6	1.8-2.1	0.15-0.25	≤0.03	≤0.02	0.9-1.2

The sulfur in 1.2312 forms manganese sulfide inclusions: they break chips and protect the cutting edge,but they also limit how well the surface polishes and how evenly it etches.The low sulfur in 1.2738, combined with 1% nickel, keeps the steel clean and improves hardenability,which is what makes it the better grade for deep, highly polished cavities.

Heat Treatment Comparsion

These steel grades are purchased to avoid heat treatment: they have already been quenched and tempered at the factory, so they can be machined directly.

Here, we also need to understand their heat treatment process.

Process	1.2311	1.2312	1.2738
Soft annealing	~230 HB; 720-740°C, 2-4 h, furnace cool	~230 HB; 720-740°C, 2-4 h, furnace cool	≤ 235 HB; 720-740°C, 2-4 h, furnace cool
Stress relieving	≤ 580°C, 1-2 h, furnace cool	≤ 580°C, 1-2 h, furnace cool	≤ 580°C, 1-2 h, furnace cool
Hardening	~850°C, hold 1 h/25 mm; water, oil or air by section	840-860°C, oil; air for thin sections	840-860°C, air or warm bath 200-230°C
Tempering	per mill tempering curve; double temper	per tempering diagram to required hardness	per tempering diagram; min. 1 h/25 mm
Nitriding	suitable	suitable	suitable

Re-hardening is worth doing only when a cavity needs a specific hardness level after machining.If secondary quenching is required,it is recommended to use the mill’s tempering diagram for the grade.

Mechanical Properties Comparison

Grade	Hardness	Tensile strength	Toughness	Hardenability
1.2311	280-325 HBW	890-1030 Mpa	Medium	General
1.2312	280-325 HBW	890-1030 Mpa	Medium	General
1.2738	280-325 HBW	890-1030 Mpa	High	Excellent

1.2311 Mechanical Properties Typical Values

Grade	Tensile strength Rm	Yield strength Rp0.2	Elongation A	Charpy V-notch, 20°C	Hardness
1.2311	1014 MPa	854 MPa	11%	25 J	300 HBW

The table above shows typical mechanical properties for 1.2311. The data for 1.2312 and 1.2738 can be roughly referenced from those for 1.2311.

For 1.2312 and 1.2738, the key difference in practical use lies in their overall hardenability and ductility. 1.2738 has deeper hardenability due to the presence of nickel, while 1.2312 has slightly lower ductility due to its higher sulfide content.

Comparison:Machining, Polishing and Surface Work

Machining

If the parts are mainly machined and surface treatment is not required, 1.2312 steel is the best choice. Its manganese sulfides effectively break chips and reduce tool wear, resulting in excellent machinability. Therefore, it is faster to machine parts such as frames, ejector plates, and support plates with numerous drilling and milling features.

1.2311 and 1.2738 steels also have good machinability, but are not as fast as 1.2312 steel.

Polishing

1.2738 steel offers the best polishing performance. Its low sulfur content and inclusion-controlled melting process make it highly responsive to diamond plaster polishing,which is why it is the grade for high-gloss and transparent parts.

1.2311 steel also polishes well. However, 1.2312 steel is less suitable: its sulfides are prone to tearing during polishing, leaving pits.

Texturing and photoetching

1.2311 steel is particularly suitable for texturing, while 1.2738 steel achieves uniform pattern replication. Both are suitable for cavities with leather textures and etched logos.

1.2312 is not recommended for applications requiring high-quality etching.

Electrical Discharge Machining (EDM)

1.2311 and 1.2738 are suitable for electrical discharge machining (EDM). After EDM, the re-hardened white layer must be removed before polishing. EDM is not recommended for 1.2312.

Welding

1.2311 is suitable for GTAW welding, but requires preheating and post-heat treatment. 1.2738 can also be welded, but requires careful handling. 1.2312 has poor weldability due to its sulfur content.

Nitriding

All three respond to nitriding, which lifts surface hardness and wear resistance for molds that see abrasive or erosive resins.

How to Choose？1.2311，1.2312 or 1.2738?

Choose 1.2311 when the cavity will be textured or lightly polished, the mold is medium-size, and the section stays within the hardenability of a standard P20-type grade. It is the balanced default choice.

Choose 1.2312 when the part is a structural component with low surface quality requirements, and machining time is the main cost factor.

Choose 1.2738 when the mold is large, the plate is 100 mm or thicker, the part must reach a high-gloss or transparent finish, or requires secondary hardening within the cavity.

Application	1.2311	1.2312	1.2738
Injection mold cores and cavities	Excellent	Limited	Excellent
Holder blocks, frames, ejector plates	Good	Excellent	Good
Textured or etched surfaces	Excellent	Not recommended	Excellent
High-gloss or transparent parts	Good	Not recommended	Excellent
Large plates, sections ≥ 100 mm	Limited	Limited	Excellent
Short-run and prototype tooling	Good	Excellent	Workable

1.2311 is the economical default option, 1.2312 is usually similarly priced, while 1.2738 is significantly more expensive due to its nickel content. The point of comparison is to choose the cheapest grade that meets both surface and cross-sectional requirements.