GB/T 3098.24-2020: High-Temp SS & Ni Alloy Fasteners

Introduction to the GB/T 3098.24-2020 Standard

GB/T 3098.24-2020 specifies the mechanical properties of bolts, screws, studs, and nuts made from stainless steels and nickel alloys intended for high-temperature service. This standard is part of the broader GB/T 3098 series on fasteners and focuses on materials that maintain structural integrity under elevated temperatures, such as those encountered in aerospace, power generation, and petrochemical industries. It ensures that these fasteners exhibit reliable performance in terms of strength, ductility, and corrosion resistance when exposed to temperatures exceeding ambient conditions.

The standard categorizes materials into martensitic stainless steels, austenitic precipitation-hardening stainless steels, and nickel alloys, each tailored for specific high-temperature applications. Key aspects include chemical composition limits, heat treatment regimes, mechanical testing requirements, and guidelines for pairing bolts with nuts to prevent issues like galling or corrosion. Compliance with this standard is crucial for engineers and manufacturers to select appropriate fasteners that withstand thermal stresses, oxidation, and creep without compromising safety or functionality.

In practice, this standard aligns with international norms such as ISO 3506, providing a framework for quality assurance in fastener production. It emphasizes the importance of material selection based on operating environments, where factors like creep resistance and thermal expansion play pivotal roles. For instance, nickel alloys like Alloy 718 are preferred for their superior high-temperature strength, while martensitic grades offer cost-effective solutions for moderate temperatures. The document also references appendices for domestic material equivalents and guidelines on selecting stainless steels or nickel alloys as per GB/T 3098.25.

Understanding this standard requires knowledge of fastener mechanics, including stress-strain behavior at high temperatures. It mandates testing at ambient conditions (10°C to 35°C) but recommends additional high-temperature evaluations for critical applications. This ensures fasteners meet minimum tensile strength, proof stress, and elongation criteria, preventing failures in service. Manufacturers must adhere to specified heat treatments to achieve desired microstructures, such as martensite for hardness or austenite for ductility. Overall, GB/T 3098.24-2020 promotes reliability in high-temperature fastening systems, reducing risks associated with material degradation over time.

Furthermore, the standard addresses surface treatments to mitigate galling, a common issue with stainless and nickel alloys due to their low thermal conductivity and high friction coefficients. Lubrication is recommended to achieve consistent torque-tension relationships, enhancing assembly efficiency. By optimizing chemical compositions and processing, this standard facilitates the production of fasteners that perform under demanding conditions, contributing to advancements in engineering design and material science.

Symbols and Designations

The following symbols apply to this document, providing precise definitions for mechanical and dimensional parameters essential in evaluating fastener performance. These notations ensure consistency in testing and specification, allowing engineers to accurately assess properties like strength and elongation under load.

• A: Actual elongation after fracture of the fastener, in millimeters (mm).
• A_s,nom: Nominal stress cross-sectional area of the thread, in square millimeters (mm²).
• A_T: Actual high-temperature elongation after fracture of the fastener, in millimeters (mm).
• b: Thread length, in millimeters (mm).
• D: Nominal diameter of internal thread, in millimeters (mm).
• D₂: Basic pitch diameter of internal thread, in millimeters (mm).
• d: Nominal diameter of external thread, in millimeters (mm).
• d_h: Hole diameter in tensile test fixture or nut proof load test fixture for external thread fasteners, in millimeters (mm).
• d_s: Shank diameter without thread, in millimeters (mm).
• d₁: Basic minor diameter of external thread, in millimeters (mm).
• d₂: Basic pitch diameter of external thread, in millimeters (mm).
• d₃: Minor diameter of external thread (for stress area calculation), in millimeters (mm).
• F_mf: Ultimate tensile load, in newtons (N).
• F_mf,T: High-temperature ultimate tensile load, in newtons (N).
• F_n,T: High-temperature ultimate stripping load for nuts, in newtons (N).
• F_p: Proof load for nuts, in newtons (N).
• F_pf: Actual load at 0.2% plastic extension of the fastener, in newtons (N).
• F_pf,T: Actual high-temperature load at 0.2% plastic extension of the fastener, in newtons (N).
• H: Original triangle height of thread, in millimeters (mm).
• h: Thickness of nut proof load test fixture, in millimeters (mm).
• L₀: Total length of fastener before loading, in millimeters (mm).
• L₁: Total length of fastener after fracture, in millimeters (mm).
• L₂: Grip length before tensile test, in millimeters (mm).
• l: Nominal length of external thread fastener, in millimeters (mm).
• l₁: Total length of stud, in millimeters (mm).
• l_th: Length of unengaged thread in test fixture for fastener, in millimeters (mm).
• m: Nut height, in millimeters (mm).
• P: Pitch, in millimeters (mm).
• R_mf: Actual tensile strength of fastener, in megapascals (MPa).
• R_mf,T: Actual high-temperature tensile strength of fastener, in megapascals (MPa).
• R_n,T: High-temperature ultimate stripping strength for nuts, in megapascals (MPa).
• R_pf: Actual stress at 0.2% plastic extension of fastener, in megapascals (MPa).
• R_pf,T: Actual high-temperature stress at 0.2% plastic extension of fastener, in megapascals (MPa).
• S_p: Proof stress, in megapascals (MPa).

These symbols are integral to calculations in mechanical testing, such as determining tensile strength (R_mf = F_mf / A_s,nom) and proof stress. They facilitate precise communication in design specifications, ensuring fasteners are evaluated consistently across manufacturing and application phases. For high-temperature scenarios, symbols like R_mf,T and F_pf,T account for thermal effects on material behavior, such as reduced yield strength due to elevated temperatures. Proper use of these designations prevents misinterpretation, enhancing safety in engineering applications.

In addition, understanding these symbols aids in compliance with related standards, where dimensional parameters like d and P influence thread strength and load distribution. For example, the nominal stress area A_s,nom is calculated using formulas involving d₂ and d₃, critical for predicting failure modes under tension.

Marking System

All stainless steels and nickel alloys specified in this part fall into three distinct categories: martensitic stainless steels (CH0, CH1, CH2, V, VH, VW), austenitic precipitation-hardening stainless steels (SD), and nickel alloys (SB and 718). This marking system provides a standardized way to identify material grades, ensuring traceability and appropriate selection for high-temperature applications.

Martensitic grades like CH0 (e.g., X20Cr13) are marked for their hardenability through heat treatment, offering good strength at moderate temperatures. The V, VH, and VW designations indicate varying proof stress levels, with VH requiring R_pf ≥ 700 MPa for enhanced performance. Austenitic SD marks denote precipitation-hardened alloys like X6NiCrTiMoVB25-15-2, known for their corrosion resistance and strength retention up to 650°C. Nickel alloys SB (NiCr20TiAl) and 718 (NiCr19NbMo) are marked for superior creep resistance, ideal for temperatures up to 800°C and 700°C, respectively.

Marking ensures compatibility in assemblies, preventing mismatches that could lead to failures. For lubricated fasteners, “Lu” is appended (e.g., SD Lu) to indicate surface treatments for reduced galling. This system aligns with ISO standards, facilitating global trade and quality control in fastener manufacturing.

Detailed marking includes material code, heat treatment state (e.g., +QT for quenched and tempered), and performance class, allowing quick verification during inspection. Proper marking is essential for inventory management and regulatory compliance in industries like turbine manufacturing.

Materials and Processing

Chemical Composition

Tables 1 through 3 specify the chemical composition limits for stainless steels and nickel alloys used in fasteners. These limits are evaluated per relevant national standards, with domestic equivalents in Appendix A. Unless agreed otherwise, the manufacturer selects the composition within the group.

GB/T 3098.25 provides guidelines for selecting suitable alloys. Compositions are given as mass fractions (%), with maximum values unless ranges or minima are noted.

The chemical compositions are designed to optimize properties like corrosion resistance, strength, and high-temperature stability. For example, high Cr content in martensitic steels enhances oxidation resistance, while Nb in Alloy 718 stabilizes against creep. Strict control of elements like P and S minimizes embrittlement. Manufacturers must verify compositions through spectroscopic analysis to ensure compliance, as deviations can lead to reduced performance in service. This section underscores the importance of material purity for long-term reliability in high-temperature environments.

Heat Treatment

Fasteners manufactured under this standard must undergo heat treatment to achieve mechanical properties as specified in Chapter 7. Heat treatment regimes are detailed in Table 4, with minimum tempering temperatures for martensitic steels selected accordingly. Holding times not specified are chosen by the manufacturer, considering required properties and service temperatures.

Process flow: For SD, SB, and 718, solution treatment (AT) is required, preferably after forming. For high-strength external threads (R_mf ≥1100 MPa), AT may be on raw material by agreement. Heat treatment for cold-headed or hot-forged fasteners occurs post-forming. For machined fasteners, it can be on raw material or finished product, with threading possible before or after treatment.

Heat treatment optimizes microstructure for desired properties, such as hardening martensitic steels or precipitating phases in nickel alloys for strength. Incorrect treatment can cause brittleness or reduced corrosion resistance. Manufacturers must monitor temperatures and cooling rates to achieve uniform properties, with post-treatment inspections ensuring compliance.

Surface Finish

Unless specified otherwise, fasteners shall be cleaned and polished. Lubrication is recommended to prevent galling during assembly, especially under high torque or speed. Factors increasing galling risk include thread damage and high preloads.

Note 1: Parameters like high tightening speed elevate galling risk. Note 2: No national standards specify surface defects or torque-clamping force for these alloys.

Surface treatments provide controlled torque-tension, marked with “Lu” (e.g., SD Lu). Special requirements by agreement.

Surface finish is critical for performance, reducing friction and enhancing corrosion resistance. Polishing removes oxides, while lubrication ensures reliable preloads. In high-temperature service, coatings must withstand thermal degradation.

Bolt and Nut Pairing Design

Bolts, screws, studs, and nuts shall pair per Table 5. Nuts match same code fasteners (e.g., CH0 bolt with CH0 nut). Different materials possible if consulting experts, considering corrosion and galling.

When clamped parts differ from fastener material, use isolation to avoid galvanic corrosion.

Pairing ensures load distribution and compatibility, minimizing risks like stripping. Expert consultation is key for non-standard pairs.

High-Temperature Environment Resistance

Materials are suited for environments where creep strength determines sizing and oxidation occurs at high temperatures. SD, SB, and 718 also resist moist corrosion.

Resistance to oxidation and scaling is achieved through alloying, with Cr forming protective oxides. Creep resistance is vital for long-term loads at elevated temperatures.

In applications like gas turbines, these materials maintain integrity under thermal cycling, preventing failures from fatigue or embrittlement.

Fastener Operating Temperatures

Chapter 7 properties are tested at 10°C~35°C. High-temperature use reduces properties. Recommended maximum temperatures in Table 6, but may be lower based on conditions.

For specific applications, conduct high-temperature tensile, creep, or relaxation tests per Chapter 10, simulating assembly conditions.

These temperatures guide design, considering factors like oxidation and creep. Testing ensures performance in actual service.

Mechanical Properties of Fasteners

Bolts, Screws, and Studs

When tested per Chapter 9, mechanical properties at ambient temperature shall meet Tables 7-11, applicable during manufacturing or on finished products.

These properties ensure fasteners withstand tensile loads without excessive deformation. For example, high R_mf in 718 suits demanding applications. Hardness ranges prevent brittleness while maintaining strength.

Pähkinät

Mechanical properties for nuts are specified similarly, focusing on proof loads and stripping strength at high temperatures. They must match bolt properties to avoid weak links in assemblies.

Testing Methods

Testing per Chapter 9 includes tensile tests for R_mf and R_pf, hardness measurements, and high-temperature evaluations per Chapter 10 for creep and relaxation. Methods ensure accurate assessment of properties under simulated conditions.