Alloy Steel Class 12.9 Socket Screws | 1200 MPa Tensile · 39–44 HRC
Class 12.9 is the highest commonly supplied metric property class: quenched-and-tempered 42CrMo-type alloy steel at 1200 MPa nominal tensile, 1100 MPa minimum yield and 39–44 HRC. It serves moulds, hydraulics and power heads where envelope is tight and high preload is required, and ships in black oxide as standard. The trade-off is toughness: 12.9 is brittle, dislikes impact and is the most sensitive class to delayed fracture, so hydrogen baking after plating is mandatory. In corrosive service switch to A4-80 stainless or a lower class with a Delta-Tone coating.
Properties & Specifications
| Tensile Strength | 1200 MPa (nominal) |
| Yield Strength | 1100 MPa (minimum) |
| Hardness | 39 – 44 HRC |
| Base Material | 42CrMo and similar alloy steel |
| Heat Treatment | Quenched and tempered — mandatory |
| Fracture Toughness | Brittle — not for impact loads |
| Delayed Fracture | Highest sensitivity of the common classes; hydrogen bake required after plating |
| Relative Price | ≈ 1.5 – 1.8 × class 8.8 |
| Standard Finish | Black oxide (no corrosion protection claim) |
Material Advantages
Highest capacity per diameter: inside mould pockets and hydraulic blocks where the hole cannot grow, 12.9 is the only way to reach high preload from a small thread size
39–44 HRC holds the internal hex against the key, so repeated make-and-break service does not round the drive
Black oxide adds no electroplating hydrogen, which is the safer default finish at this class
Legible class stamp and batch traceability suit mould shops that manage spares part by part
Preload can be costed precisely against the torque table so joint stiffness is fully used
Applications
Cavity fixing, clamp plates and taper-pin retention on injection and die-casting moulds
Hydraulic manifold blocks and high-pressure line flanges
Power heads, spindles and tool-holding components
Heavy bolted joints in gearboxes and speed reducers
Precision structures on automation equipment (predominantly static load)
Not recommended: impact duty, cryogenic impact, corrosive service, or on-site re-plating
Grade Selection
| Class / Grade | Service temperature | Notes |
|---|---|---|
| 10.9 | ≤ 300°C（达克罗） | The better default for most joints: enough strength, better toughness, about 0.7–0.8× the cost |
| 12.9 | 发黑件随基材；达克罗 ≤ 300°C | 1200 MPa / 39–44 HRC, reserved for high-preload static duty |
| 12.9 + Delta-Tone | ≤ 300°C，盐雾 500–1000 h+ | Corrosion protection without downgrading the class; no electroplating, no hydrogen |
| A4-80（316） | 耐氯优于 A2；无 12.9 级强度 | The corrosive-service route; re-check the load against an 800 MPa class |
Frequently Asked Questions
Class 12.9 is the highest commonly available alloy-steel class: 1200 MPa tensile and HRC 39–44, used for high-load mould, hydraulic and power-head applications. The finish is black oxide, so it is not recommended for corrosive environments.
The harder the class, the more brittle it is. Class 12.9 has limited fracture toughness, dislikes impact, and quenched-and-tempered alloy steel is the most sensitive to delayed fracture, so hydrogen must be driven off after plating. Where impact or low temperature is present, go to a larger diameter in 10.9 instead of forcing 12.9 into a small hole.
It can be processed, but the risk is high: pickling and plating introduce hydrogen, and without baking it is a delayed-fracture time bomb. For corrosion protection at 12.9, Delta-Tone is preferred (no electrolytic process, no hydrogen, 500–1000 h+ salt spray). If plating is unavoidable, write the 190–220°C bake into the contract.
A class 10 nut, or a Type 2 heavy class 8 nut, following the equal-or-one-below rule. Note that a DIN985 nylon insert caps out at 120°C and loses gripping force after 5–10 reuses; for hot or maintenance-free joints specify a DIN980 all-metal lock nut or a threadlocker.
Get Grade Selection & Quotation
Submit your duty parameters (temperature, pressure, media, speed) — engineers reply with grade selection and quotation within 24 hours.