S7 Tool Steel Heat Treatment Guide

S7 tool steel is classified as an impact-resistant tool steel by the American Iron and Steel Institute (AISI). Optimal performance is achieved only through proper heat treatment. This guide systematically outlines the core steps and critical considerations for efficiently heat treating S7 tool steel, based on established metallurgical principles and our extensive experience in production and processing. The S7 tool steel heat treatment typically involves four stages: preheating, austenitizing, quenching, and tempering.

1. Preheating 

The preheating process is a critical initial step for nearly all tool steels, including S7 tool steel. It pre-adjusts the material’s molecular structure, helping to eliminate stresses generated during machining or material cutting. The preheating temperature for S7 tool steel is 1200°F (650°C), where it is held for 10 to 15 minutes until uniformly heated. Caution must be exercised to avoid prolonged exposure at this temperature, as it may disrupt the molecular structure and adversely affect subsequent heat treatment cycles. For complex or large-volume workpieces, a preliminary preheat at a lower temperature, such as 1200°F (650 °C), may be performed to mitigate thermal shock risks further.

2. Austenitizing

Austenitizing is the process of heating the steel to a temperature where its structure transforms from a ferrite-pearlite aggregate to austenite, dissolving a significant portion of its carbides. The recommended austenitizing temperature for S7 tool steel is 1725°F (940°C). The soak time should be calculated based on the smallest cross-section of the part. For parts exceeding 1 inch (25mm) thickness, a general rule is one hour per inch (25mm). For smaller cross-sections:

• 1/8 inch (3.175mm): 30 minutes
• 1/4 inch (6.350mm): 40 minutes
• 1/2 inch (12.70mm): 45-50 minutes
• 3/4 inch (19.05mm): 50-55 minutes 

Ensuring the part reaches the same temperature as the furnace, visually confirmed by a similar color, is essential before starting the soak time.

3. Quenching

Quenching refers to the process of rapidly cooling austenitic steel to transform the austenite into martensite, a hard matrix structure. S7 tool steel is an air-quenchable tool steel that can be quenched in air. Air quenching is the slowest quenching method, offering the advantage of reducing thermal shock and internal stresses.

Although S7 steel possesses air-quenchable properties, dimensional limitations exist. When cross-sectional dimensions exceed approximately 2.5 inches (63 mm), air cooling alone may fail to achieve full hardening, as the material volume significantly reduces the quenching rate. In such cases, oil quenching should be considered for safer and more efficient hardening.

After quenching, parts must be cooled to hand-safe temperatures (125-150°F or 52-65°C). At this stage, the steel is unstable and highly susceptible to cracking. Residual austenite may stabilize, complicating subsequent transformations. Therefore, parts must be immediately transferred for tempering. If straightening is required, it should be performed while the material remains above 400°F (205°C).

4. Tempering

Tempering is the reheating process following quenching, aimed at enhancing toughness and relieving internal stresses while contributing to the desired final hardness. As mentioned in the previous quenching step, tempering must be performed immediately after the part cools to 125-150°F (52-65°C) post-quenching, which is critical for preventing cracking.

For S7 steel, the recommended initial tempering temperature is 450°F (230°C). If secondary tempering is performed, the temperature should be reduced to 425°F (220°C). The secondary tempering temperature is typically 25°F (14°C) lower than the initial temper. Each tempering cycle should be maintained for 2 hours per inch (25 mm) of cross-section. Undertempering must be strictly avoided.

Relationship Between Hardness and Tempering Temperature of S7 Steel¹

Tempering Temperature	Rockwell C Hardness
As quenched	62
300°F / 150°C	59
400°F / 205°C	58
600°F / 315°C	55
800°F / 425°C	52
1000°F / 540°C	50
1200°F / 650°C	41

A typical application for S7 is in cold swaging dies, where it is typically used at a working hardness of 56–60 HRC. The tempering temperatures recommended (450°F / 230°C and 425°F / 220°C) are consistent with achieving hardness levels within this range from an as-quenched hardness of approximately 60 HRC.

For most tool steels, multiple tempering cycles (typically two or even three) are strongly recommended to reduce retained austenite, refine the grain structure, enhance wear resistance, and relieve stresses. Between tempering intervals, the workpiece should cool completely to room temperature to minimize retained austenite. Note that consecutive tempering at the same temperature is ineffective.

Tempering induces dimensional changes. When S7 steel is tempered at 500°F (260°C) per specification, it may expand approximately 0.0006 in./in. (0.01524 mm/25 mm) along the grain direction and about 0.0002 in./in. (0.00508 mm/25 mm) perpendicular to the grain direction. For critical dimensions, it is recommended to consult the dedicated tempering dimensional change chart for S7 steel. The following table shows the dimensional change curves of S7 steel at different tempering temperatures.

Effective heat treatment of S7 tool steel directly determines its final properties and service life, thereby saving production time and costs. Insufficient soaking time or improper tempering during heat treatment will significantly shorten tool life and compromise performance. Strict adherence to preheating, austenitizing, quenching, and tempering procedures, coupled with close attention to factors like deformation control, ensures S7 tool steel achieves its maximum performance potential.

1. Bryson, William E. Heat Treatment, Selection, and Application of Tool Steels. 2nd ed., Hanser Publications, 2005, p. 197. ↩︎