Grade Black Oxide Double-End Studs 

Grade 8.8 black oxide double-end studs are medium-high strength threaded fasteners featuring threading on both ends with an unthreaded shank center. They combine reliable mechanical performance (800 MPa tensile strength), economical corrosion protection via a thin Fe3O4 conversion coating, and versatile installation for industrial machinery, automotive, and structural applications.

1. Grade 8.8 Classification

Per ISO 898-1, the "8.8" designation means nominal tensile strength of 800 MPa and yield ratio of 0.8 (yield strength 640 MPa). Manufactured from quenched-and-tempered medium carbon alloy steel, Grade 8.8 offers an excellent strength-to-cost balance.

Table 1 — Key Mechanical Properties (ISO 898-1)

2. Black Oxide Surface Treatment

Black oxide (Fe3O4) is a chemical conversion coating 0.5-1.5 micrometers thick, formed by immersion in a hot alkaline nitrate bath at 140-150C. Unlike electroplating, it causes no dimensional change and no hydrogen embrittlement. When sealed with rust-preventive oil, it provides up to 96 hours of salt spray resistance.

Table 2 — Black Oxide Coating Properties

3. Double-End Stud Design

A double-end stud has a tap end (screwed into a tapped hole) and a nut end (accepts a nut), separated by an unthreaded shank. The tap end length is typically 1.0-1.5d and the nut end 2.0-3.0d, where d is the nominal diameter.