S1 Tool Steel Heat Treatment Guide

S1 tool steel is a shock-resistant tungsten-chromium-vanadium alloy steel designed for applications demanding high toughness and sustained impact stress. It is widely used in pneumatic tools, chisels, punches, shear blades, and forming dies. The steel contains approximately 0.50% carbon, 2.50% tungsten, and 1.50% chromium. It offers a balanced combination of strength, wear resistance, and hardenability, with moderate resistance to softening at elevated temperatures. Its greatest advantage lies in its exceptionally high transverse strength and outstanding resistance to chipping under impact loads. 

At Aobo Steel, we supply S1 in the tempered condition. Our customers’ heat-treatment processes for S1 tool steel include stress-relief and hardening treatments. The hardening process comprises four steps: preheating, austenitizing, quenching, and tempering. This article focuses primarily on these steps.

A Quick Checklist for S1 Tool Steel Heat Treatment

Stress Relief Treatment

Stress-relief treatment serves as a preparatory step for S1 components after rough machining or cold working, aiming to minimize deformation and cracking risks during subsequent hardening processes. Internal stresses generated by machining, if not eliminated prior to heat treatment, can cause severe microstructural distortion or uncontrolled dimensional distortion when heated to high hardening temperatures.

Process Procedure:

• Heating Temperature: Heat the workpiece uniformly to 650°C to 675°C (1200°F to 1250°F).
• Soak Time: Calculate based on cross-sectional thickness: 1 hour per 25 mm (1 inch), with a minimum soak time of 1 hour.
• Cooling Method: Slow air cooling (air quench).

Preheating

To prevent thermal shock and excessive deformation, it is strictly prohibited to place cold S1 workpieces directly into a furnace at the austenitizing temperature.

Process Procedure:

• Heating Temperature: Slowly raise the temperature to approximately 650°C (1200°F).
• Soaking Requirement: Maintain at this temperature until the workpiece cross-section is uniformly heated.

For S1 workpieces with complex geometries or significant cross-sectional variations, preheating is critical to ensure synchronized temperature rise between the core and surface during the final heating stage. This reduces thermal stress and prevents cracking.

Austenitizing (Hardening)

Austenitizing is the core hardening process, transforming the steel microstructure into austenite by heating and promoting carbide dissolution into the matrix. The austenitizing heating temperature for S1 tool steel must be neither too high nor too low.

Process Procedure:

• Heating Temperature: 900°C to 955°C (1650°F to 1750°F).
• Hold Time: Hold for 15 to 45 minutes. The exact duration depends on the workpiece’s cross-sectional thickness to ensure complete through-hardening of the entire section.
• Atmosphere Control: Due to S1’s moderate decarburization resistance, heating must occur in a controlled neutral atmosphere, vacuum environment, or neutral salt bath to prevent surface oxidation and decarburization.

Quenching

S1 is oil-hardening steel. Although water quenching is occasionally used for large sections or simple shapes, we strongly recommend oil quenching to achieve optimal toughness and minimize the risk of cracking.

Process Procedure:

• Quenching Medium: Use preheated and circulating quenching oil.
• Operating Procedure: Quench the workpiece in oil until its temperature drops to 50°C to 70°C (120°F to 160°F). At this point, the S1 workpiece should feel warm to the touch.

STRICTLY PROHIBITED: Allowing the workpiece to cool to room temperature after quenching before tempering. Significant microstructural stresses are present in the quenched microstructure. Failure to temper immediately renders the S1 workpiece highly susceptible to spontaneous brittle fracture.

Tempering

Tempering should be performed immediately after quenching to enhance the toughness and ductility of S1 material. At the microstructural level, the quenched microstructure is transformed into tempered martensite. The tempering temperature can be adjusted according to the required S1 hardness.

Process Procedure:

• Soak Time: Calculated based on section thickness, a minimum soak time of 2 hours is required for every 25 mm (1 inch).
• Secondary Tempering: We strongly recommend secondary tempering to effectively eliminate unstable retained austenite, thereby optimizing yield strength and impact properties, and addressing deformation or cracking issues that may occur during subsequent use of S1.
• Cooling Requirements: Between primary and secondary tempering, the workpiece must be cooled to room temperature.

The following table provides approximate hardness values for S1 tool steel after oil quenching at 900–955°C (1650–1750°F).

For applications requiring maximum toughness, such as chisels and heavy-duty punches, tempering is typically performed between 400°F and 600°F. For hot-working applications, higher tempering temperatures may be necessary, but hardness will decrease significantly.

Common Issues and Solutions in S1 Heat Treatment

1. Quenching cracks. 

This defect primarily arises from two causes: first, delayed tempering—failure to temper promptly after quenching, resulting in prolonged exposure at room temperature and generating stresses exceeding the material’s strength; second, stress concentration points on S1 workpieces, such as sharp corners, stamp marks, or rough machining. To address these issues, immediate tempering should be implemented—transferring the workpiece to the tempering furnace while it retains residual heat (approximately 50–70°C). Concurrently, during the design and machining stages, rounded corners should be employed at critical junctions, and surfaces must be maintained to a high surface finish.

2. Low hardness. 

The primary causes of this defect include: surface carbon loss (decarburization) due to improper atmosphere during austenitization; insufficient heating temperature or holding time during austenitization; and inadequate quenching severity caused by excessively high oil temperature or insufficient agitation during quenching.

3. Deformation. 

Deformation typically results from thermal shock caused by uneven expansion due to excessive heating rates or from residual stresses generated during rough machining that are released during heating. Preventive measures include: Preheating to 650°C prior to hardening; Performing stress relief treatment on rough-machined parts before quenching; and providing physical support for long or complex parts to prevent sagging.

4. Soft layer (decarbonization). 

This defect occurs when oxygen or moisture in the furnace reacts with surface carbon, producing carbon dioxide or carbon monoxide gases that escape, thereby reducing the carbon content. You can test it with a file. If it easily leaves scratches on the surface, it indicates the presence of a soft layer. To prevent such issues, the following measures can be taken: Use vacuum furnaces or protective atmosphere furnaces filled with inert gases (such as nitrogen or argon) to isolate oxygen; When using a salt bath furnace for heating, the brine must be periodically calibrated to ensure it remains neutral and does not react with carbon; Prior to heat treatment S1, allow sufficient tolerance to ensure that this decarburized layer can be completely removed through precision grinding after heat treatment.

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