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S7 STANDARD VALUES

Composition - Chemical analysis:

C

0.45 – 0.55

Si

0.2 – 1

Mn

0.2 – 0.9

P

0 – 0.03

S

0 – 0.03

Cr

3 – 3.5

Mo

1.3 – 1.8

V

0 – 0.35
Chemical name:
50CrMoV13-1
Working hardness:
56 HRC - 60 HRC
Delivery condition:
max. 225 HB
Chemical name:
50CrMoV13-1
Working hardness:
56 HRC - 60 HRC
Delivery condition:
max. 225 HB
s7 tool steel physical properties
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S7 PHYSICAL PROPERTIES

S7 is an air hardening tool steel. The medium carbon content combined with moderate amounts of chromium and molybdenum provide the excellent properties, which make this steel grade suitable for applications where shock resistance, easy machining as well as medium hot work properties are important.

The excellent resistance to deformation during air hardening as well as the easiness of heat treatment are further advantages of this material. As its properties are well balanced the S7 is suitable for both hot and cold work, though is more often used for cold work.

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%.
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.
As a ferrous material containing iron the S7 tool steel can be magnetized and is suitable for magnetic clamping.
The wear resistance for the S7 tool steel is 3 on a scale where 1 is low and 6 is high.

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

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S7 TECHNICAL PROPERTIES

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

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

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.

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 conductivity diagram chart
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.
s7 tool steel thermal expansion coefficient diagram chart

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.

S7 tool steel specific heat capacity chart

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.

Modulus of Elasticity

Value

At a temperature of

28600  KSI

68°F 

27800  KSI

390°F 

25700  KSI

750°F 

 

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s7 tool steel procedures
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S7 PROCEDURE

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.
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.

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)

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.

s7 tool steel tempering chart diagram
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.

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.

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.

s7 tool steel continuous ztu-diagram ttt-chart structural changes

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.

s7 tool steel isothermal time temperature transformation (TTT) diagram
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S7 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.
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.
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.
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, 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.

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S7 PROCESSING

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

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.

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.
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.

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.

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S7 APPLICATION OPTIONS

This tool steel is a general purpose air hardening grade having high impact and shock resistance. It has also a good resistance to softening at moderrately high temperatures. This combination of properties makes it suitable for many hot-work and cold-work applications.

• Cutting tools
• Cold forming and bending dies
• Plastic molding dies
• Shear blades
• Chisels
• Rivet sets
• Punches
• Driver bolts

s7 tool steel application options
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S7 CONCLUSION

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.

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S7 ALTERNATIVES

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S7 DATASHEET

Download the technical data sheet in PDF format here.

download the data sheet of s7 tool steel
Disclaimer

The data shown here has been compiled with the greatest diligence and is regularly updated with regard to the correctness and completeness of its content. The content is indicative only and should not be taken as a warranty of specific properties of the product described or a warranty of suitability for a particular purpose. All information presented is given in good faith and no liability will be accepted for actions taken by third parties in reliance on this information. ABRAMS Industries reserves the right to change or amend the information given here in full or parts without prior notice.