Introduction

GB/T 3098.8 specifies the mechanical properties, materials, testing, identification marks, and markings for fasteners used in bolted connections operating within the temperature range of -200℃ to +700℃. This standard applies to bolts, screws, studs, and nuts made from austenitic stainless steels, steels, and nickel alloys, ensuring reliability in extreme temperature environments such as cryogenic applications and high-temperature industrial settings.

The standard emphasizes compliance with related documents like GB/T 3098.6, GB/T 3098.15, and DIN EN 10269 for material performance and heat treatment conditions. It provides guidelines for mechanical performance grades, inspection methods, and compatible material pairings to achieve full load-bearing capacity in fastener assemblies.

Propriétés mécaniques

For austenitic steel fasteners capable of operating at minimum temperatures down to -200℃, Table 1 outlines the steel groups and performance grades. The material properties must conform to GB/T 3098.6 and GB/T 3098.15.

Note: Temperatures above these limits have no impact on usage. Below these, performance tests are required based on conditions.
^un Copper content ≤1% (per GB/T 3098.6 and GB/T 3098.15).
^b For headed bolts.
^c For studs.

For steel and nickel alloy fasteners suitable for high temperatures up to +700℃, Table 2 details the applicable operating temperatures per DIN EN 10269. Material properties must align with DIN EN 10269 Table 4 for heat treatment states. Reference data for 0.2% proof stress at elevated temperatures, creep rupture strength, and relaxation properties are provided in DIN EN 10269 Tables 5, C.1, and D.1.

Note:
^un Upper temperature limit for yield and tensile strength.
^b Upper temperature limit for creep and rupture strength.
Conditions per DIN EN 10269 Table 4: +N (normalized); +NT (normalized and tempered); +QT (quenched and tempered); +WW (warm worked); +AT (solution annealed); +P (precipitation hardened).
^d For nuts only.
^et Abbreviation V per DIN EN 10269, material X22CrMoV12-1, 0.2% proof stress R_p0.2 ≥ 600 N/mm² (+QT1).
^f Abbreviation VH per DIN EN 10269, material X22CrMoV12-1, 0.2% proof stress R_p0.2 ≥ 600 N/mm² (+QT2).
^g See also VdTÜV material sheet WB 490.

Load-Bearing Capacity of Bolted Connections

Exigences générales

A bolt-nut assembly is considered to have full load-bearing capacity if it meets the following:
– Both bolt and nut are full load-bearing, with nut height not less than Type 1 for coarse threads or Type 2 for fine threads.
– Appropriate thread tolerances, e.g., 6H/6g.
– Nut material tensile strength at least 70% of bolt material tensile strength.
– Rational material matching as per Table 3.
Note: If load-bearing sleeves are used, prefer the same material as the bolt.

Rational Matching of Bolt and Nut Materials

Note: ● Preferred match; ○ Allowed match; – Not recommended.
^un Proof stress considers test mandrel hardness higher than matching bolt, but greater than minimum tensile strength of highest grade bolt.
^b Elevated proof stress for identification of full load-bearing nuts per GB/T 3098.15.

Minimum Tensile Loads for Coarse Thread Bolts (Table 4a)

Full Load-Bearing Bolts

Bolts are full load-bearing if they achieve minimum tensile loads per Tables 4a or 5a, with fracture occurring in the free threaded length or unthreaded shank per GB/T 3098.1 or GB/T 3098.6.

Foire aux questions (FAQ)

What materials are recommended for fasteners at -200℃?

Austenitic steels such as A4L, A4, and A5 with performance grades 70 or 80, as per Table 1, ensuring compliance with GB/T 3098.6 and GB/T 3098.15 for cryogenic conditions.

How do I select matching bolt and nut materials for high-temperature applications?

Refer to Table 3 for rational pairings. Preferred matches (●) ensure optimal performance; ensure nut tensile strength is at least 70% of the bolt’s and appropriate thread tolerances like 6H/6g.

What tests are required for fasteners below the specified minimum temperatures?

Performance tests based on specific usage conditions, including impact and tensile testing, to verify integrity beyond the standard’s limits.

Are there specific heat treatment requirements for nickel alloy fasteners?

Yes, conditions like +AT+P for grades such as SB and SD per DIN EN 10269 Table 4, with hardness ranges specified in Table 2.

How is full load-bearing capacity verified in assemblies?

Through tensile testing achieving minimum loads in Tables 4a/5a for bolts and proof loads in 4b/5b for nuts, with fracture location checks per relevant GB/T standards.

What is the significance of elevated proof stress for austenitic nuts?

It allows identification of full load-bearing nuts, especially for small-batch production, compensating for material inhomogeneities from cold working.