S1 Tool Steel | 1.2550

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S1 tool steel is a shock-resisting material. Its primary characteristic is exceptional toughness, which makes it highly resistant to impact loading. Understanding its properties helps determine if it fits your manufacturing needs correctly.

1. Chemical Composition¹

The typical chemical composition (weight %) for S1 steel falls within these ranges:

2. S1 Tool Steel Properties

Understanding these S1 steel properties will help you determine its suitability for your specific factory applications:

2.1 Hardness

S1 tool steel can achieve a hardness between 40 and 63 HRC after heat treatment. However, for most practical applications requiring its signature shock resistance, S1 is typically used in a working hardness range of 50-56 HRC, often around 55 HRC. It’s important to remember that while higher hardness improves wear resistance, it generally reduces toughness.

2.2 Heat Treatment Response

S1 is an oil-hardening steel, capable of reaching its required hardness through oil quenching. For simpler or larger cross-sections, water or brine quenching may also be viable options. It exhibits a medium depth of hardening.

Post-hardening and tempering are critical steps. While a low tempering temperature (around 300°C / 570°F) yields maximum hardness, S1 offers a broad usable tempering range from approximately 175°C to 540°C (350°F to 1000°F). Compared to steels with higher alloy content, S1 tool steel will show a more pronounced decrease in hardness as tempering temperatures rise. Be aware that tempering below 315°C (600°F) can make the steel more susceptible to notch sensitivity.

2.3 Toughness and Shock Resistance

This is where S1 tool steel truly excels. It has very high, often the highest, toughness among tool steels, particularly when compared to many cold-worked grades. This exceptional toughness, a core S1 steel property, makes it ideal for applications involving repeated impacts, mechanical shocks, or sudden loads. The medium carbon content, combined with silicon and tungsten additions, is key to this characteristic.

2.4 Wear Resistance

S1 tool steel offers medium to low wear resistance. This is generally lower than cold-work tool steels like D2 or A2. The tungsten content does contribute some inherent wear resistance. For applications requiring enhanced surface wear resistance, nitriding can be considered. However, this process requires careful management, as the temperatures involved might reduce the core hardness and strength, potentially leading to plastic deformation in high-stress areas if not properly controlled.

2.5 Hot Hardness and Heat Resistance

S1 tool steel possesses low to medium hot hardness. It doesn’t maintain its hardness at elevated temperatures as effectively as high-speed steels. Nevertheless, thanks in part to its tungsten content, it offers a degree of heat resistance, allowing its use in moderately elevated-temperature service or some hot-working applications, generally up to temperatures around 540°C (1000°F).

2.6 Machinability

When properly annealed, S1 tool steel is regarded as having good machinability. Its rating is often cited as 90-95, compared to a 1% carbon steel standard of 100. Even in the hardened condition, machinability is considered fairly good. The machinability of S1 steel can be further improved by increasing the sulfur content.

2.7 Dimensional Stability

Oil quenching, the typical method for S1, generally results in small and predictable dimensional changes. While air-hardening steels like S7 offer superior dimensional stability, the dimensional stability performance of S1 tool steel is considered good in oil- or water-hardening grades. Water quenching causes severe thermal shock to S1 tool steel, increasing the risk of cracking and deformation, so it should be used with caution.

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3. S1 Tool Steel Heat Treatment

S1 tool steel is a shock-resisting steel. Proper heat treatment is critical for its performance. This guide outlines the standard heat treatment steps for S1 tool steel.

3.1 Preheating

Preheating is necessary for S1 tool steel before austenitizing. It reduces thermal shock, helps relieve machining stresses, and can improve furnace productivity.

Procedure: Heat to 1200-1250°F (650-677°C). Hold for 10-15 minutes, or until uniformly heated. Rapid heating from preheat to austenitizing temperature is preferred.

3.2 Austenitizing (Hardening)

This step transforms the steel’s structure to austenite, dissolving alloying elements for hardening.

Procedure: Heat to 1725-1750°F (940-954°C). A range of 1550-1700°F (845-925°C) is also cited.

Soaking Time: Soak for 1 hour per inch of the smallest cross-section (e.g., 30 minutes for 1/8 inch). Adhere to soaking times rigidly, as excessive time can be detrimental.

3.3 Quenching

Rapid cooling transforms austenite into martensite. S1 is primarily an oil-hardening steel.

Procedure: Quench in oil to 125-150°F (52-65°C). Temper immediately. Air hardening may be possible for S1, but can be limited by section size.

3.4 Tempering

Tempering is performed immediately after quenching to relieve stresses, increase toughness, and adjust hardness. It also transforms retained austenite and precipitates fine carbides.

Procedure:

• Multiple Tempers: Multiple tempering cycles are highly recommended for S1 tool steel.
• Soaking Time: Soak for 2 hours per inch of cross-section thickness for each temper.
• Temperature Range: The general tempering range for S1 is 350-1000°F (175-540°C).

1. Lower temperatures (e.g., 300-600°F / 149-316°C) yield higher hardness.
2. Higher temperatures reduce hardness but can maximize toughness.
3. Hardness typically ranges from about 58 HRC (as-quenched) down to around 41 HRC (tempered at 1000°F/540°C).

• Cooling: Cool slowly in still air.

4. Applications of S1 Tool Steel

The distinctive blend of toughness and shock resistance makes S1 tool steel a dependable material for a wide array of challenging industrial applications. If your manufacturing processes involve tools or components that are regularly subjected to heavy impacts or high-stress scenarios, S1 provides a robust and reliable material solution.

The following table outlines common S1 tool steel applications and highlights why it’s selected for these tasks:

5. Important Considerations for S1 Tool Steel Selection

While S1 tool steel delivers excellent performance characteristics for its intended applications, prospective users should be mindful of the following points:

• Availability: In today’s market, the widespread use of S1 has seen some decline, with grades like S5 or S7 tool steel becoming more commonly stocked by suppliers. This might influence the immediate availability of S1. However, for those specific, critical applications where maximum toughness is paramount to prevent cracking under severe impact conditions, S1 tool steel remains an exceptionally valuable and effective material choice.
• Heat Treatment Precision: Achieving the optimal balance of hardness and toughness in any S1 tool steel component is critically dependent on executing the correct heat treatment protocol. The tempering cycle, in particular, is vital for fine-tuning these properties to ensure maximum service life and performance from the tool or part. 

6. Equivalent grade

• DIN: 1.2550
• China GB: 6CrW2Si or 5CrW2Si

1. Roberts, G., Krauss, G., & Kennedy, R. (1998). Tool steels (5th ed.). ASM International. ↩︎

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