Grade 8 bolts are medium carbon alloy steel(fully austenitized), quenched and tempered at a minimum temperature of 800 degrees Farenheit. They are then heat-treated to achive a hardness of 33 to 39 on the Rockwell C scale. Grade 8 bolts should not be exposed to temperatures above 450°F or below -50°F. Expect a reduction in yield strength at elevated temperatures.
SAE J429 - indicates: The mechanical properties are for ambient temperature of approximately 20 °C (68 °F). Performance testing is desirable in applications where the operating temperature exceeds 260 °C (500 °F).
ASTM A325 and SAE J429 grade 8 are not the same fastener. A325 bolts are typically manufactured from a medium carbon steel (These bolts are designated by type, denoting chemical composition, as follows: Type 1—medium carbon, carbon boron, or medium carbon alloy steel; and Type 3—weathering steel.) while SAE grade 8 bolts are made from a medium carbon alloy steel. Grade 8 bolts are significantly stronger than an A325.
boo1 is correct, ASTM A 325 and SAE J429 grade 8 are not the same fasteners, but they are similar, meaning steel fasteners that are used in the quenched and tempered condition. The references listed in that previous thread do not make a distinction between the various alloys and property classes (grades in SAE parlance) of typical quenched and tempered steels when suggesting basic temperature limits. Neither does ISO 898-1 Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — Coarse thread and fine pitch thread, which was just updated in 2009 and now includes the following notes about temperature limits:
NOTE 1 Fasteners conforming to the requirements of this part of ISO 898 are used in applications ranging from ?50 °C to +150 °C. Users are advised to consult an experienced fastener metallurgist for temperatures outside the range of ?50 °C to +150 °C and up to a maximum temperature of +300 °C when determining appropriate choices for a given application.
NOTE 2 Information for the selection and application of steels for use at lower and elevated temperatures is given, for example, in EN 10269, ASTM F2281 and in ASTM A 320/A 320M.
While ASTM A 325 and SAE J429 fasteners may have different mechanical properties (120 ksi min ultimate tensile strength vs. 150 ksi), they most certainly can be the same material. ASTM A 325 states that the material "shall be plain carbon steel, carbon boron steel, alloy steel or alloy boron steel at the manufacturer’s option." SAE J 429 states that medium carbon alloy steel is to be used, then also states "When agreed to by manufacturer and purchaser, medium carbon steel may be used for products 7/16 inch in nominal diameter and smaller and medium carbon boron steel may be used for products 1 inch in nominal diameter and smaller." Both standards refer to alloy steel in the exact same way (SAE J429 by way of reference to SAE J411, ASTM A 325 with a footnote that reads the same as section 3.2 of SAE J411). Materials for both standards are quenched and tempered martensite that will have very similar elevated temperature behavior. The most common "alloy steel" used for Grade 8 is SAE 4037, which only has 0.25 % Mo and doesn't have significantly different elevated temperature properties compared to plain carbon steels.
Regarding maximum allowable temperature, here is some information. SAE J429 states "Users should recognize the difference in stress relaxation characteristics of various steels between the tempering temperature range of 340 °C (650 °F), minimum, specified for Grade 8.2 and 427 °C (800 °F), minimum, specified for Grade 8, when considering bolts and screws that may be exposed to such temperature range. The data available on elevated temperature properties of Grade 8.2 indicates that performance testing is desirable in applications where the operating temperature exceeds 260 °C (500 °F) (as may also be the case with Grade 8 fasteners)."
ASM Handbook Volume 1 states "For service temperatures of 200 to 370 °C (400 to 700 °F), specific bolt steels are recommended (Table 5) because relaxation is an influencing factor at these temperatures. Although other steels will fulfill requirements for the tabulated conditions, those listed are the commonly used grades. Only medium-carbon alloy steels are recommended; in all instances, they should be quenched and tempered."
The alloys in Table 5 are 1038, 4037, 4140 and 4145.
ISO 898-1 (a metric fastener standard with materials very similar to SAE J429) states "Continuous operating at elevated service temperature may result in significant stress relaxation. Typically 100 h service at 300 °C will result in a permanent reduction in excess of 25 % of the initial clamping load in the bolt due to decrease in yield stress."
Besides the differences in creep strength (or "stress relaxation") that CoryPad points out among the various material options acceptable for a Gr.8 fastener, you should also consider the fact that these values are likely based on an acceptance rate of about 90%. Which means that possibly 10% of a sample of these bolts won't meet that requirement. So a conservative FoS may be in order for your analysis.
Also, be sure to take into account any additional fastener strains due to CTE mismatch between the fastener and the clamped material stack.
