S7 steel technical overview

S7 steel technical overview: S7 steel is a shock-resisting tool steel. Key characteristics include its high toughness and air hardening, which makes it quite stable during heat treatment. It’s a versatile grade often used for medium cold-work tools and dies, plastic molding dies, and shear blades. After proper heat treatment, you can expect a working hardness of around 56-58 HRC.

1. S7 steel chemical composition

Element	Carbon (C)	Manganese (Mn)	Silicon (Si)	Chromium (Cr)	Molybdenum (Mo)
Content (%)	0.45-0.55	0.20-0.80	0.20-1.00	3.00-3.50	1.30-1.80

2. Heat treatment

• Preheating: Heat the S7 steel to a temperature of 1250° F (677° C) and hold for 10-15 minutes until it is thoroughly soaked. We suggest using one furnace for preheating and another set to the austenitizing temperature to speed up the process and save energy. For delicate parts, the preheat step would be beneficial.
• Austenitizing (Hardening): Heat the preheated S7 steel to an austenitizing temperature of 1725° F (940° C)73 or 1600° F (871° C). Soak for one hour per inch (25mm) of the smallest cross section for parts in excess of 1″ (25mm) thick. For smaller parts, specific soak times are recommended, such as 30 minutes for 1/8″ (3.175mm), 40 minutes for 1/4″ (6.350mm), 45-50 minutes for 1/2″ (12.70mm), and 50-55 minutes for 3/4″ (19.05mm)75. Before timing the soak cycle, ensure the part reaches the same temperature as the furnace.
• Quenching: Quench the S7 steel in oil to 150° F (66° C) or air cool. Air-hardening steels like S7 can not achieve full hardness in air cooling when their certain cross-section size is beyond 2.5″ (63mm).
• Tempering: Temper S7 material immediately after quenching. Typically, S7 steel is double tempered. Soak at the tempering temperature for 2 hours per inch (25 mm) of thickness for each temper. Air cool S7 to room temperature. The normal tempering range for S7 is 400–450° F (204–232° C). The first temper is at 450° F (230° C), and the second temper is at 425° F (220° C). The best working range is obtained by tempering at 450° F (230° C), leading to a 58 Rc hardness.

3. Hardness

The hardness of S7 steel varies depending on its condition and heat treatment:

Condition/Treatment	Hardness	Notes
Annealed Condition	~230 HB (maximum)
Hardened (Air Quenched)	59–60 HRC
	56–58 HRC	Fully hardens up to 2.5 inches cross-section
	60 HRC	Maximum obtainable hardness
Working Hardness	56–58 HRC	Normal recommended range
	58 Rc	Obtained by tempering at 450° F (230° C)
Case Hardened (Surface)	Up to 64 Rc	After initial heat treatment

4. S7 steel applications

4.1 High Shock Resistance (Toughness): This is the most significant characteristic of S7 steel. Due to its ability to withstand sudden and repeated impacts without fracturing, it is ideally suited for:

• Punches: Used for creating holes in materials through forceful impact.
• Blanking dies: Employed to cut out shapes from sheet materials under significant impact loads.
• Gripper dies: Components in machinery that need to firmly hold materials, often experiencing shocks.
• Cold forming dies: Tools used to shape metal at room temperature through impact or high pressure.
• Shears: Blades designed to cut materials with a shearing action, which can involve substantial impact forces.
• Shear blades: Specifically, the cutting elements in shearing machinery.
• Other impact-related applications include rivet sets, cold coining dies, cold header punches, knock-out pins, chisels, forming dies, and rock drills.

4.2 Versatility: S7 steel’s combination of properties makes it a versatile material suitable for various tooling applications. This allows it to be used in:

• Medium cold-work tools and dies: Applications that require a balance of wear resistance and toughness, operating at or near room temperature.
• Plastic molding dies: Tooling used in injection molding of plastics, where durability and resistance to wear and tear are important.
• Medium hot-work dies: Applications involving elevated temperatures (up to 1000° F or 540° C) where the steel needs to maintain hardness and strength under moderate heat and stress.

4.3 Stability in Heat Treatment: S7 is an air-hardening steel, which means it hardens when cooled in air. Compared to water or oil quenching, it reduces the risk of cracking and distortion during the hardening process, making it suitable for intricate or complex tool designs.

4.4 Good Machinability: This is beneficial for manufacturing complex tools and dies.

4.5 Moderate Wear Resistance: While its wear resistance is not as high as that of some other tool steels (e.g., O6), it is still better than that of some common structural steels like 4140/4150. This allows it to be used in applications where some wear is expected but high toughness is the primary requirement.

5. Compare S7 with D2 steel

S7 and D2 are tool steels commonly used in die-making, but they possess distinct properties that make them suitable for different applications. Here’s a comparison of the two:

Feature	S7 Steel	D2 Steel
Classification	Shock-Resisting Tool Steel	High-Carbon, High-Chromium Cold-Work Tool Steel
Toughness	Excellent (Highest shock resistance)	Moderate (Lower than S7)
Wear Resistance	Good (Less than D2)	Excellent (Standard for abrasion resistance)
Hardening Method	Air-hardening	Air-hardening
Dimensional Stability	Good (0.001 in./in. expansion)	Excellent (0.0005 in./in. movement)
Machinability	Good (Rating of 70)	Fair (Rating of 45)
Primary Use	Impact-resistant applications, versatile tooling	High wear resistance, long production runs

Essentially, S7 is chosen when toughness and shock resistance are paramount, even if it means sacrificing some wear resistance. D2 is selected when high wear resistance and dimensional stability are critical for long production runs, even if it means a lower resistance to impact.

6. FAQs

6.1 Is S7 a good steel?

S7 is a good steel, particularly when the application demands excellent shock resistance and toughness combined with good working hardness and ease of heat treatment.

6.2 Is S7 tool steel brittle?

S7 is not considered brittle because its high toughness and shock resistance characterize it.