High Temperature Fasteners: Why Alloy Selection Matters More Than Strength Grade

A property class 10.9 bolt has a minimum yield strength of 940 MPa under ISO 898-1. An A286 stud made to ASTM A453 Grade 660, Classes A through C, have a lower specified minimum yield strength of 585 MPa. Both are room-temperature specification minimums and do not provide a meaningful basis for evaluating or comparing fastener performance in an exhaust joint operating at 600°C.

ISO 898-1 establishes the mechanical properties of carbon and alloy steel fasteners at room temperature and acknowledges that these properties may not be maintained under elevated-temperature operating conditions.

Fasteners in turbocharged exhaust systems commonly experience operating temperatures ranging approximately 450 to 600 degrees C, significantly exceeding the temperature range for which conventional room-temperature mechanical properties are specified. Consequently, material selection for these applications requires a more comprehensive evaluation of elevated-temperature performance characteristics.

A286, Nimonic 80A, and Inconel 718 are among the alloys commonly used for these applications, and their elevated-temperature properties make them strong candidates where quenched-and-tempered steels may fall short. However, these materials also present greater manufacturing challenges.

MacLean-Fogg uses warm forming at its Chesterfield Township, Michigan, operation to produce specialty-alloy bolts and studs that can be difficult to manufacture by conventional cold forming.

Table of Contents

• Why Standard Bolts Can Struggle at High Temperatures

• Where Warm Forming Fits
• What Determines Clamp Load Retention at High Temperatures?
• Choosing an Alloy for High-Temperature Service
• Manufacturing High Temperature Bolts at MacLean-Fogg
• Selecting the Right Fastener for a Hot Joint
• Frequently Asked Questions

Key Takeaways

• ISO 898-1 specifies property class 8.8 and 10.9 bolts at room temperature and advises consulting an experienced fastener metallurgist for service outside minus 50 to plus 150 degrees C, up to a maximum of 300 degrees C.

• Fasteners in turbocharged exhaust systems commonly run at about 450 to 600 degrees C fastener metal temperature, so alloy selection matters more than room-temperature strength grade.
• Clamp load retention depends on elevated-temperature strength, stress relaxation and creep, differential thermal expansion, and a fastener that keeps its strength can still lose preload.
• A286, Nimonic 80A, and Inconel 718 get their high-temperature capability from the alloy and its heat treatment, and warm forming is how MacLean-Fogg makes these hard-to-form materials into bolts and studs.

Why Standard Fasteners Can Struggle at High Temperatures

Property classes 8.8 and 10.9 reach their specified mechanical properties through quenching and tempering, and those properties change as operating temperature rises, particularly during prolonged exposure. ISO 898-1 specifies them at room temperature and advises consulting an experienced fastener metallurgist for service outside the range of -50 to +300 degrees C. An informative annex in the earlier ISO 898-1:1988 edition gives approximate elevated-temperature values for property class 10.9.