Carbon Steel Class 8.8 Bolts & Nuts | 800 MPa Tensile · 640 MPa Yield · 22–32 HRC
Class 8.8 is the broadest-reaching fastener grade, made from 35# / 45# medium-carbon steel with 800 MPa nominal tensile strength, 640 MPa yield and 22–32 HRC hardness. Its fracture toughness is good and quenching and tempering is not mandatory. Priced as the baseline (1.0), it is the default class for machinery, steel structures and automotive assembly. Our 8.8 bolts and nuts come off the same batch, M6–M24 are held in stock, and each shipment carries a mill certificate plus tensile and hardness reports.
Properties & Specifications
| Tensile Strength | 800 MPa (nominal) |
| Yield Strength | 640 MPa |
| Hardness | 22 – 32 HRC |
| Base Material | Medium-carbon steel 35# / 45# |
| Heat Treatment | Not mandatory at this class |
| Fracture Toughness | Good |
| Relative Price | 1.0 (baseline) |
| Nut Counterpart | Class 8 (pair equal or one below the bolt) |
| Service Temperature | Set by the finish: electro-zinc ≤ 80°C · HDG ≤ 250°C · Delta-Tone ≤ 300°C |
Material Advantages
Best strength-to-cost balance: most static joints run on class 8.8 without paying for strength you do not use
Good toughness means low susceptibility to brittle and delayed fracture, and it tolerates a wider range of plating processes than classes above 10.9
A mature domestic supply base gives high stock coverage for M6–M24, keeping lead times predictable for small and mixed-container orders
Easy pairing: class 8 DIN934 nuts, DIN985 lock nuts and DIN6923 flange nuts are all available in the matching class
A clear upgrade path: re-check to class 10.9 for alternating load or vibration rather than forcing 8.8 to survive it
Applications
Steel structures — platforms, walkways, guard rails and equipment frames
Machine assembly — holding-down and housing joints on general machine tools and conveyors
Automotive and farm machinery — chassis accessories, bracketry and cover fixings
Bridges and construction — ordinary bolted nodes and embedded fittings
Pipe supports — clamps, brackets and back-iron fixings
Tooling and jig bases — clamp plates and locators away from heavy load
Grade Selection
| Class / Grade | Service temperature | Notes |
|---|---|---|
| 4.8 | ≤ 80°C（电镀锌） | Light duty such as furniture and brackets; looks like 8.8, so check the head mark on receipt |
| 8.8 | ≤ 80°C（电镀锌）/ ≤ 250°C（热浸锌）/ ≤ 300°C（达克罗） | 35# / 45# medium-carbon steel, the workhorse high-strength class |
| A2-70 | 含氯水环境 > 60°C 有应力腐蚀风险 | Corrosion-resistant alternative at 700 MPa, below class 8.8 |
| 10.9 | ≤ 300°C（达克罗） | The upgrade when 8.8 preload is insufficient or fatigue-limited; about 1.2–1.4× the price |
Frequently Asked Questions
Typically 35# / 45# medium-carbon steel. Quenching and tempering is not mandatory at this class, toughness is good and price is the baseline (1.0), which is why it dominates machinery, steel structures and automotive use.
Read the head mark. Passing 4.8 off as 8.8 is one of the most common misuses, and the 4.8 stamp is plainly visible on the head, so check marks part by part on receipt. A proper supplier also states class and measured properties on the batch certificate.
For static load it is fine. Under continuous vibration do two things: install to the torque table and audit with a calibrated wrench (insufficient preload is behind almost every fatigue failure), and replace the plain nut with a DIN985 nylon insert lock nut or a serrated flange solution — stepping up to 10.9 if the duty demands it.
The class itself sets no temperature ceiling; the finish does. Electro-plated zinc is limited to about 80°C, hot-dip galvanising to 250°C (higher for short excursions) and Delta-Tone to 300°C. Tell us the duty temperature and we will re-select the coating and material.
Get Grade Selection & Quotation
Submit your duty parameters (temperature, pressure, media, speed) — engineers reply with grade selection and quotation within 24 hours.