Aug 20 2012
Topics Covered
IntroductionChemical CompositionPhysical PropertiesMechanical PropertiesThermal PropertiesOther DesignationsFabrication and Heat Treatment Forging Annealing Stress Relieving Hardening MartemperingTempering
Introduction
Shock-resisting tool steels are designated as group S steels according to the AISI classification system. S1, S2, S5, S6, and S7 are the types of shock-resisting steels. The hardenability of group S steels can be controlled by varying their composition instead of adjusting the melting procedures and grain size. These steels can obtain optimum hardness at higher austenitizing temperatures. The tempering resistance of group S steels can be enhanced by the addition of silicon, which also forms a microstructure to resist distortion in tempered conditions.
The following section will discuss in detail about S1 shock-resisting tool steels.
Chemical Composition
The following table shows the chemical composition of S1 tool steels.
| Element |
Content (%) |
| C |
0.40-0.55 |
| Mn |
0.10-0.40 |
| Si |
0.15-1.20 |
| Cr |
1.00-1.80 |
| Ni |
0.3 |
| Mo |
0.5 |
| V |
0.15-0.30 |
| Cu |
0.25 |
| P |
0.03 |
| S |
0.03 |
| W |
1.50-3.00 |
Physical Properties
The physical properties of S1 tool steels are given in the following table.
| Properties |
Metric |
Imperial |
| Density (@20°C/68°F) |
7.89 g/cc |
0.285 lb/in3 |
Mechanical Properties
The mechanical properties of S1 tool steels are tabulated below.
| Properties |
Metric |
Imperial |
| Hardness, Rockwell C (air cooled from 899°C, 30 mins) |
54.5 |
54.5 |
| Hardness, Rockwell C (air cooled from 927°C, 30 mins) |
56 |
56 |
| Hardness, Rockwell C (air cooled from 982°C, 30 mins) |
56.5 |
56.5 |
| Modulus of elasticity |
207 GPa |
30000 ksi |
| Charpy impact (V-notch; oil quenched from 954°C; 149°C temper temperature) |
10.8 J |
8.00 ft-lb |
| Charpy impact (V-notch; oil quenched from 954°C; 625°C temper temperature) |
29.8 J |
22.0 ft-lb |
| Machinability (1% carbon steel) |
70.0 - 75.0% |
70.0 - 75.0% |
| Poisson's ratio |
0.27-0.30 |
0.27-0.30 |
Thermal Properties
The following table shows the thermal properties of S1 tool steels.
| Properties |
Conditions |
| T (°C) |
Treatment |
| Thermal expansion |
12.4 x 10-6/ºC |
20-100 |
- |
Other Designations
Other designations that are equivalent to S1 tool steels include:
- AFNOR 55 WC 20
- DIN 1.2550
- UNI 58 WCr 9 KU
- ASTM A681
- FED QQ-T-570
- SAE J437
- SAE J438
- UNS T41901
- ASTM A681
- DIN 1.2542
Fabrication and Heat Treatment
Forging
S1 tool steels are uniformly preheated at 1000°C (1832°F). These steels are then forged at temperatures ranging from 800 to 1000°C (1472 to 1832°F). This process is followed by cooling in a furnace to avoid stress.
Annealing
Annealing is carried out in S1 tool steels by uniformly heating the steels to 770°C (1418°F) followed by equalization. These steels are then cooled in the furnace.
Stress Relieving
S1 tools steels are heated up to 700°C (1292°F) before hardening, equalized, and then slowly cooled in order to remove stresses during heavy machining operations.
Hardening
S1 tools steels are preheated to 650°C (1202°F) and soaked. They are then continuously heated to the final temperature that ranges from 900 to 950°C (1652 to 1742°F). Finally, the steels are quenched in oil.
Martempering
Martempering is performed in the case of S1 tool steels as an alternative procedure for hardening. This is carried out in a salt bath equipment.
Tempering
S1 tool steels are tempered by uniformly heating the steels at the preferred tempering temperature followed by holding for an hour.