M3*40 Carbon Steel Black Oxide Hexagon Socket Screws

At M3*40 (3mm diameter, 40mm length), this screw handles fine-pitch assembly tasks in small mechanical components, motor housings, sensor brackets, and medical device frames. 
Carbon steel construction with Grade 8.8 heat treatment achieves a minimum 800 MPa tensile strength — adequate for the vast majority of light-to-medium duty precision applications.
The black oxide finish provides a non-reflective surface appropriate for optical instruments and electronics, while also offering basic corrosion resistance sufficient for indoor, dry operating environments. 
When combined with the correct hex key or Allen driver, these screws enable precise torque application without risk of head deformation.
Compliant with GB/T 70.1, ISO 4762, DIN 912, and ASME B18.3. Grades available from 4.8 to 12.9; materials include alloy steel and stainless steel 304/316. OEM/ODM customization accepted.