S7 Tool Steel - DIN 1.2357 - 50CrMoV13-1 - Shock Resisting

Where can S7 tool steel be used?

The S7 has exceptional toughness and strength and is often used for punches, chisels, plastic molds, compression molds, jackhammers, forge dies, tool holders, and aircraft landing gear components.

As an air hardening steel it has, in comparison to water and oil hardening steels, less risk of distortion or warping.

Is the S7 a stainless steel?

No, the S7 air hardening tool steel is not a stainless steel. A stainless steel has a minimum chrome content of 10.5%, this steel grade has a mass fraction of 3 - 3.5%.

Is the S7 tool steel corrosion resistant?

Though the S7 has some corrosion resistance due to other containing alloys it does not reach the resistance of actual stainless steel.

To enhance the corrosion resistance the surface can additionally be treated or coated by nitriding or black oxide. As for all steels, regular maintenance, applying a protective coating, and keeping the tools in dry and clean conditions can give them a longer life span. 

Is  S7 tool steel magnetizable?

As a ferrous material containing iron the S7 tool steel can be magnetized and is suitable for magnetic clamping.

Heat treatment of the S7 tool steel

Annealing the S7 tool steel

Heat the material uniformly to a temperature of 1508 - 1562°F (820 - 850°C), then slowly cool down to 1112°F (600°C) in the furnace at a rate of 50 - 68°F (10 - 20°C) and finish the process of by cooling the material further in air.

S7 tool steel stress relieving

To relieve machining stresses, for high accuracy during hardening, first rough machine the material, then heat it uniformly to a temperature of 1200 - 1250°F (649 - 677°C) and hold for 2 hours. Finish this off by cooling the parts in still air and finish machining.

Hardening S7 tool steel

Preheat the parts to 1200 - 1300°F (649 - 704°C), then raise the temperature to 1598 - 1652°F (870 - 900°C), hold for 2 hours and cool in still air. Quench and immediately temper after quenching.

Quenching S7 tool steel

As a classic air hardening steel the S7 is for most applications quenched in air. 

• Air (this method should be used for parts under 2 inches (50.8 mm) max. thickness)
• Dry lime or ash, for larger pieces and more uniform cooling
• Oil (temper as soon as a temperature of 120 - 150°F (49 - 66°C) has been reached)

S7 tool steel tempering

To use S7 for cold work applications, temper the parts at a temperature of 400 - 500°F (204 - 260°C), hold the temperature for 11/2 to 2 hours and then cool to ambient temperature and temper a 2nd time.

For hot work applications, heat the material to a temperature of 900 - 1000°F (482 - 538°C), then hold the temperature for 11/2 to 2 hours and cool to ambient temperature. As before it is recommended to temper the parts a second time.

Dimensional changes of the S7 tool steel

Stress concentrators, like sharp corners, can distort or break the parts. Long thin pieces might warp or bow. Parts with different thicknesses throughout may lead to different and uneven heat within the part due to the different cooling rate in the different thicknesses and may distort the parts.

S7 tool steel sub-zero treatment

Sub-zero treating the S7 tool steel will enhance its wear resistance, dimensional stability and the transformation of retained austenite to martensite.

After the first temper, cool the material to approx. -100°F (-73°C) and hold depending on size and thickness for 2 - 4 hours. Then let the parts warm up to room temperature and follow up with another tempering cycle to relieve any stresses from the sub-zero treatment and to optimize its mechanical properties.

S7 tool steel Electrical Discharge Machining (EDM)

EDM can be a very effective method to create intricate shapes, small details and complex geometries that may be challenging to normal machining due to its hardness and potential work hardening, especially after heat treatment of the St7 tool steel. 

Using the correct electrodes and machining parameters is important to achieve the needed surface finish.

Heat affected zones might require an additional heat treatment or tempering to lessen the localized changes in hardness and microstructure.

After the EDM process the recast layer, a thin white layer, should be removed.

S7 tool steel surface treatment

There are many different surface treatments, it depends on the desired finish and applications the S7 may be used for.

Following are a few examples of surface treatments that can be applied to the S7.

Nitriding the S7 tool steel

To enhance the wear resistance and fatigue life the S7 can be nitrided. Nitriding a metal introduces nitrogen into the material surface to harden the surface and give it a better wear resistance.

Black oxide (bluing) the S7 tool steel

Though primarily a superficial treatment it may enhance the corrosion resistance of the parts. It is often used as a finishing touch for tools or firearms as it lends the material surface a blue-black finish. 

PVD and CVD deposit for S7 tool steel

To enhance wear resistance and friction and with PVD enhanced corrosion resistance, Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) deposits a thin coating over the material surface to give it additional protection.

Shot peening / shot blasting S7 tool steel

Shot peening, an impact treatment, blasting multiple high velocity shots to the material surface leaving behind small indentations which replaces the tensile stress on the surface with a compressive layer. It solidifies the material and makes the surface more resistant which can prevent fatigue and stress corrosion failure and can also optimize the form and weight of the parts.

Note: Protective gear like goggles, masks, helmets, gloves and protective clothing should be worn and adequate ventilation or dust extraction should be provided.

Appropriate guards to prevent shot spillage should be in place, equipment regularly maintained and shot should be free from contaminants.

S7 tool steel machinability

In the annealed condition S7 is comparatively easy to machine. It can work harden which affects sharpness and also makes machining more challenging. 

Forging S7 tool steel

Heat the material uniformly to a temperature range of 1950 - 2050°F (1066 - 1121°C). Make sure the temperature does not drop below 1700°F (927°C), reheat as often as needed. Cool small forgings in lime, ashes, or other insulating materials. Large forgings should be placed in the furnace which is heated to approx. 1400°F (760°C) and soaked until the parts are uniformly heated through. Finish the process by shutting the furnace off and let the forgings cool down slowly.

Note: This is not an annealing. The forgings should be annealed after forging, for that please see “Annealing” above.

Welding S7 tool steel

To weld the S7 tool steel it is recommended to uniformly preheat the part to reduce the risk of cracking. The filler should be similar to the base material and heat input controlled to avoid distortion and excessive grain growth in the heat affected zone. To finish the process off it should be considered to give the parts a post weld heat treatment to restore its former/wanted properties.

S7 tool steel wear resistance

The wear resistance for the S7 tool steel is 3 on a scale where 1 is low and 6 is high.

S7 tool steel tensile strength

The tensile strength for the S7 tool steel is approx. 110.2 KSI (0.145KSI = 1MPa). This value is the result from a tensile test to show how much force is needed before the material starts to stretch or elongate before it breaks.

S7 tool steel working hardness

The working hardness for the S7 tool steel is 572 - 627 BHN ( 56 - 60 HRC). 

S7 tool steel specific heat capacity

The specific heat capacity of the S7 tool steel at room temperature is  0.460J/g-°C (0.110BTU/lb-°F). This value shows how much heat is needed  to heat 1lb of material by 1 Fahrenheit.

AISI S7 tool steel continuous TTT-diagram

The following diagram shows the micro changes over time at different temperatures which are important during heat treatment. They show the optimum conditions for the processes such as hardening, annealing and normalizing.

AISI S7 tool steel Isothermal TTT-diagram

The following diagram shows the structural changes at micro levels over time at a constant temperature. It shows at what temperatures the different phases, e.g., perlite, martensite and bainite start to form.

Properties of tool steel S7

S7 tool steel hot work

S7 can be used for hot work applications involving high impact and shock. For hot work tools, temper in the high range up to 1000°F (538°C) but make sure the working temperature of the tool does not exceed 1000°F (538°C).

S7 tool steel cold work

S7 can be used for cold work tools like shear blades, punches and chisels and other tools requiring high shock resistance.

S7 tool steel density

Typically the density of S7 tool steel is 0.282 lb/in3 (7.8g/cm3) at room temperature.

Thermal conductivity for the tool steel S7

The heat conductivity for the S7 is at 31.0 W/(m*K) (215 BTU/(h-ft*°F)) at room temperature.

S7 tool steel thermal expansion coefficient

This diagram shows how much the S7 tool steel might expand or contract when the temperatures change which can be very important when working with high temperatures or strong temperature changes.

Modulus of elasticity (Young’s Modulus) of tool steel S7

The relationship between stress and strain of steel is described in the modulus of elasticity (Young’s Modulus) and can be found in the table below with the according values and temperatures.

Is tool steel S7 a knife steel?

S7 may be used for knife making, though it does not offer the edge retention high carbon steels may offer. They would be suitable for applications needing to withstand high impact or shock as the S7 has an excellent shock resistance. With the appropriate machining and heat treatment this steel grade can be used for knives with specific tasks in mind.

S7 tool steel Conclusion

The S7 is used when high toughness and shock resistance is asked for, as for applications under dynamic loads.

The air hardening properties are advantageous as the material/parts do not distort or warp and they promote dimensional stability to this grade.   

S7 tool steel datasheet

As a S7 steel supplier we provide you with the datasheet for tool steel S7 in PDF format.

S7 tool steel alternatives or equivalent

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