DIN1.2355 | S7 Shock-Resisting Tool Steel

In the DIN standard, it is numbered as 1.2355, which is an impact-resistant tool steel, corresponding to S7 steel in the ASTM A681 standard.

With the characteristics of “high impact toughness + strong and toughness balance + easy processing and heat treatment”, 1.2355 steel has become the preferred material for molds and tools under impact load and medium and high temperature conditions, especially in complex molds in the automotive and machinery manufacturing industries.

1.2355 STEEL CHEMICAL COMPOSITION

Grade	C	Si	Mn	P	S	Cr	Mo	V
1.2355/50CrMoV13-15	0.45-0.55	0.2-0.8	0.5-0.9	≤0.03	≤0.02	3.0-3.5	1.3-1.7	0.15-0.35
S7	0.45-0.55	0.2-1.0	0.2-0.9	≤0.03	≤0.03	3.0-3.5	1.3-1.8	≤0.35

1.2355 STEEL RELATED STANDARD & EQUIVALENT GRADE

China: 5Cr3MnSiMo1V (GB/T 1299)

USA: S7(ASTM A681)

Europe: 50CrMoV13-15(DIN EN ISO 4957)

1.2355 STEEL FORGING

Preheating:Preheat in stages to 650-750°C, holding long enough to ensure uniform temperature from center to surface.
Initial Forging Temperature：1065-1150℃
Final Forging Temperature：above 925℃.Absolutely avoid forging below 870°C. Below this temperature, the plasticity of the material drops sharply, and forced forging is very likely to cause cracks (cold forging cracks).
Forging Ratio：above 4:1
Post-Forging Treatment:For 1.2355 steel, slow cooling is required after forging, Isothermal annealing is recommended

1.2355 STEEL HEAT TREATMENT

Pretreatment: Spheroidizing Annealing For 1.2355 Steel

1. Purpose: To obtain uniform and fine spheroidal carbide structure, reduce hardness, improve machinability, and prepare the structure for subsequent quenching.
2. Process:

Heating temperature: 845 – 900°C. The common range is 870 – 900°C.
Holding time: Ensure that the workpiece is heated through, usually 1-2 hours per inch of thickness.
Cooling:
–Furnace cooling (recommended): Slowly cool to 595°C at a rate not exceeding 15°C/hour. After that, it can be cooled slightly faster (air cooling) to room temperature.
–Isothermal annealing: After heating to the annealing temperature and holding, quickly cool to 705 – 720°C, and hold at this temperature for 4-6 hours (to ensure the completion of pearlite transformation), and then air cool to room temperature.

3. Expected hardness: After annealing, the hardness is usually around HB 192 – 212 (Brinell hardness) or HRC 15 – 18.

1.2355 Steel Final Heat Treatment: Quenching + Tempering

1. Preheating:
Very important! S7 steel has medium thermal conductivity and high alloy content. Direct heating to quenching temperature is prone to thermal stress and cracking.

Two preheatings are recommended:
–First preheating: 450 – 540°C. The holding time is at least equal to the quenching heating holding time.
–Second preheating: 760 – 815°C. The holding time is at least equal to the quenching heating holding time.
Purpose: Reduce thermal gradients, reduce thermal stress, prevent cracking, and ensure uniform temperature of subsequent austenitization.

2. Austenitizing (quenching heating):

Temperature range: 925 – 980°C. The most common range is 950 – 980°C.
Selection criteria:
–925 – 955°C: Get the highest toughness and good wear resistance combination, slightly lower hardness (HRC 55-58 after quenching and tempering). Suitable for the most severe impact applications.
–955 – 980°C: Get higher hardness (HRC 57-60 after quenching and tempering) and wear resistance, with slightly reduced toughness. Suitable for applications that require higher hardness and wear resistance while still requiring good toughness.
Holding time: Make sure the workpiece is fully heated through. Rule of thumb: 20 – 30 minutes per inch of thickness (minimum cross section). The time can be shortened by about 1/3 in a salt bath furnace. Avoid holding for too long to prevent coarsening of grains and decarburization.
Atmosphere Control: Controlled atmosphere (e.g. endothermic, nitrogen-based) or vacuum furnace is recommended to prevent decarburization and oxidation. Salt bath furnaces are also good options.

3. Quenching:

Cooling medium:
– Still or forced air cooling (preferred): S7 is an air-hardened steel that can be fully hardened with still or forced air (fan). This is the safest method with the least risk of deformation and cracking, especially for complex shapes. The cooling rate is sufficient to form martensite.
– Pressurized gas cooling (vacuum furnace): The effect is similar to forced air.
– Oil quenching: Only considered for very thick sections (>75mm / 3 inches), or when slightly higher hardness/wear resistance is required and the higher risk can be accepted. Use hot oil (50-80°C) to reduce stress and deformation. The risk of oil quenching is significantly higher than air cooling, and the risk of cracking is increased.
– Water quenching is not recommended: Very prone to cracking!
Cool to 50-65°C, not to room temperature, and then temper immediately.

4. Tempering:
Critical! S7 is a hard, highly stressed martensitic structure after quenching and is very brittle. Tempering must be done immediately! Delayed tempering (even for a few hours) may result in cracking.

Temperature range: 480 – 650°C. The specific temperature is determined by the final hardness and toughness required.
Secondary hardening phenomenon: S7 will have a secondary hardening peak around the tempering temperature of about 540°C. Tempering near this temperature will produce a higher hardness than tempering at a lower temperature (such as 480°C) or higher temperature (such as 595°C). This is valuable for applications that require both high hardness and good toughness.
Number of temperings: At least two times. Each holding time is 1-2 hours (calculated according to the largest cross-section, usually 1 hour per inch). Two temperings, sometimes even three times, are strongly recommended to ensure complete elimination of residual austenite and stabilize the structure and size.
Cooling: After each tempering, air cooling to room temperature must be performed before the next tempering. Slow cooling in the furnace is not allowed.
Relationship between hardness and tempering temperature (for reference, depending on quenching conditions and workpiece size):
-480°C: HRC 58-60
-510°C: HRC 57-59
-540°C: HRC 58-60 (secondary hardening peak) The most commonly used area, taking into account high hardness and good toughness
-565°C: HRC 56-58
-595°C: HRC 54-56
-620°C: HRC 52-54
-650°C: HRC 48-50