S7 Shock-Resisting Tool Steel
Complete technical specification for S7 shock-resisting tool steel, including Canadian stock sizing, heat treatment, machinability, common defects, and storage guidelines for fabrication and machining shops.
Material Overview & Common Uses
S7 is a shock-resisting tool steel (UNS T41907, AISI S7) known for its exceptional toughness and ability to withstand high impact loads at moderate hardness levels (typically 48–56 HRC). It offers a good balance of wear resistance and ductility, making it ideal for tools and dies subjected to repeated shock. Common applications include: shear blades, punches, chisels, forging dies, coining dies, master hubs, plastic injection molds with high impact requirements, and cold forming tools. In Canadian shops, S7 is frequently used for heavy-duty stamping dies and pneumatic tool components.
Core Technical Properties & Sizing
| Property | Value |
|---|---|
| Density | 0.283 lb/in³ (7.83 g/cm³) |
| Hardness (annealed) | 197–229 HB (typical) |
| Hardness (hardened & tempered) | 48–56 HRC (typical range) |
| Modulus of Elasticity | 30 × 10⁶ psi (207 GPa) |
| Thermal Conductivity | 24.1 W/m·K at 100°C |
| Coefficient of Thermal Expansion | 11.5 × 10⁻⁶ /°C (20–200°C) |
| Standard Canadian Stock Sizes (flat ground & precision ground) | Thickness: 1/8", 3/16", 1/4", 5/16", 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 2", 2-1/2", 3", 4" (up to 6" on special order). Width: 12", 18", 24", 36". Length: 12", 24", 36", 48", 72", 96" (random mill lengths up to 144"). |
| Round bar stock (cold drawn or turned & polished) | Diameters: 1/4" to 8" in 1/16" increments up to 1", then 1/8" increments to 4", then 1/4" increments to 8". Lengths: 12", 24", 36", 72", 144". |
| Tolerances (precision ground flat stock per ASTM A681) | Thickness: ±0.001" for thickness ≤ 1", ±0.002" for 1–2", ±0.003" for >2". Width: +0.005" / -0.000". Length: +1/8" / -0". Squareness: 0.002" per inch of width. |
| Heat treatment | Preheat at 1200–1300°F (650–700°C), austenitize at 1725–1775°F (940–970°C) for 30–45 minutes, oil quench or interrupted oil quench (martempering recommended for complex sections). Temper immediately at 400–1100°F (200–600°C) to achieve desired hardness. Double tempering is standard for critical tools. |
Machinability, Tooling & Feeds
S7 in the annealed condition (197–229 HB) has a machinability rating of approximately 70–80% relative to 1212 free-machining steel. It machines similarly to A2 or D2 but with slightly better chip control due to lower alloy content.
| Operation | Tooling | Speed | Feed | Depth of Cut |
|---|---|---|---|---|
| Turning | HSS | 80–120 SFM | 0.005–0.015 IPR | 0.050–0.150" |
| Milling | HSS | 60–100 SFM | 0.003–0.008 IPT | — |
| Drilling | HSS | 40–60 SFM | 0.002–0.005 IPR | — |
| Turning / Milling / Drilling | Carbide | Increase HSS speeds by 50–100% | — | — |
Use positive rake geometries and ample coolant (water-soluble oil at 5–10% concentration). Avoid heavy interrupted cuts in the annealed state; if required, reduce speeds by 30%.
Grinding: Use aluminum oxide wheels (46–60 grit, J–K hardness) for roughing, and 80–120 grit for finishing. Surface grinding: downfeed 0.001–0.003" per pass, crossfeed 0.050–0.100" per pass.
EDM: S7 is readily EDM’able; use copper or graphite electrodes with typical parameters. Wire EDM: use brass wire, 0.010–0.012" diameter, with a trim pass for best surface finish.
Common Defects & Quality Control
| Defect | Description | Quality Control Check |
|---|---|---|
| Decarburization | Surface carbon loss during hot rolling or heat treatment, visible as a soft skin. | Verify surface decarb depth via microhardness traverse or etch test (ASTM E1077). |
| Carbide Segregation | Banding in heavy sections leading to anisotropic properties. | Perform spark test or spectrometer analysis to confirm grade. |
| Quench Cracking | From too rapid cooling or inadequate tempering. | Inspect for surface cracks using magnetic particle inspection (ASTM E1444) or dye penetrant (ASTM E1417) on finished tools. |
| Grinding Burns | From excessive heat during grinding, causing re-hardening and cracking. | Check hardness with Rockwell C tester (annealed: 20–24 HRC typical; hardened: per spec). |
| Residual Stress Distortion | Especially in complex geometries. | Measure dimensional stability after rough machining, after heat treatment, and after finish grinding to ensure within tolerance. |
| Surface Defects / Dimensional Drift | Surface cracks, thickness variation. | For precision ground stock, verify thickness with micrometer at three points along length and width. |
Storage, Handling & Racking Guidelines
- Store S7 tool steel in a dry, indoor environment with relative humidity below 60% to prevent surface rust.
- For long-term storage, apply a light rust-preventive oil (e.g., LPS-3 or equivalent) and wrap in VCI paper.
- Racking: use heavy-duty cantilever racks for flat stock (max span 48" for 1/4" thick, 60" for 1/2" and thicker). Support bars should be at least 2" wide to prevent bending.
- Round bars: store in vertical racks with padded cradles or horizontal racks with V-grooves.
- Do not stack flat stock more than 6" high without interleaving with wood or plastic spacers to avoid scratching.
- For heat-treated S7, handle with clean gloves to avoid oil contamination.
- When moving heavy sections (>50 lbs), use overhead cranes with nylon slings or magnetic lifters. Never drop or strike S7 against hard surfaces as it can cause micro-cracking.
- Inventory management: label each piece with grade, size, heat number, and date received. Use FIFO rotation to avoid age-related surface degradation.