S1 Tool Steel | 1.2550

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, tempering is a critical step. 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.