Follow link below…

http://www.nhml.com/newsletters/HydrogenEmbrittlement&ASTMB633.pdf

According to the National Physical Laboratory, in their Guide to Good Practice in Corrosion Control, they state the following:

“There is no hard-and-fast limit for the strength level above which problems will be experienced, as this will be a function of the amount of hydrogen in the steel, the applied stress, the severity of the stress concentration and the composition and microstructure of the steel. As a rough guide hydrogen embrittlement is unlikely for modern steels with yield strengths below 600 MPa (87ksi), and is likely to become a major problem above 1000 MPa (145ksi).”

I have converted the MPa values to relative ksi and added them above.

High strength alloy steels with a 1000 MPa yield strength would likely correlate to a tensile strength in the range of 165 ksi to 170 ksi. Therefore, it is the opinion of the National Physical Laboratory that hydrogen embrittlement may begin to occur at tensile strengths significantly higher than 150 ksi.

Another reference on this subject can be found in the Research Council on Structural Connections’ Specification for Structural Joints Using ASTM A325 or A490 Bolts (June 30, 2004). This document states the following:

“Steels in the 200 ksi and higher tensile-strength range are subject to embrittlement if hydrogen is permitted to remain in the steel and the steel is subjected to high tensile stress. The minimum tensile strength of ASTM A325 bolts is 105 ksi or 120 ksi, depending upon the diameter, and maximum hardness limits result in production tensile strengths well below the critical range. The maximum tensile strength for ASTM A490 bolts was set at 170 ksi to provide a little more than a ten-percent margin below 200 ksi”

(Note: since 2004, the maximum tensile strength requirement for A490 bolts has been moved to 173 ksi).

This RCSC and National Physical Laboratory data both imply that embrittlement has the potential to occur at tensile strength levels significantly higher than 150 ksi.

The final argument backing our stance that A193 grade B7 bolts can indeed be galvanized is that ASTM does not restrict this practice within the specification. ASTM makes it very clear that hot-dip galvanizing should not be performed on A490 bolts and on A354 grade BD bolts, but does not restrict this practice on any other common ASTM fastener specification including A354 grade BC which has an identical minimum tensile strength as A193 grade B7 (125 ksi) but an even higher yield (109 ksi vs. 105 ksi) and a higher maximum Rockwell hardness requirement (C36 vs. C35).

In summary, these supporting documents combined with the fact that ASTM does not restrict the practice of hot-dip galvanizing within the A193 specification leads us to believe that there is no potential for hydrogen embrittlement when galvanizing A193 grade B7 bolts.

1. Hardness is a rough estimate of tensile. Generally speaking a 34 Rockwell equals 150 ksi tensile. The chart above doesn’t include A193 B7 hardness, since it has no minimum. There is slight overlap between the minimum Grade 8 hardness and maximum A193 B7 hardness. Technically a B7 bolt could have a tensile in the 155 ksi range. In that case, hot-dip galvanizing would not be recommended. The upper threshold for hot-dip galvanizing is material with 150 ksi tensile. B7 bolts will occasionally be above 150 ksi, but are typically well below it, whereas Grade 8 bolts have a minimum tensile of 150 ksi so are always above it. I’m not aware of a SAE standard that addresses hot-dip galvanizing, since they are fasteners designed for automotive and OEM applications. Neither ASTM A354 grade BD or A490, which are both 150 ksi minimum tensile bolts and are chemically and mechanically identical to SAE grade 8, allow galvanizing. It is best to analyze this issue based on tensile and tempering temperatures, rather than hardness.

2. I would question anyone supplying galvanized grade 8 bolts. Because of the reasons stated above the mechanical properties of these bolts are greatly compromised through the galvanizing preparation process. The reason galvanized B7 bolts are difficult to find is more application than function. They are typically used for high temperature-high pressure designs.

2. Why is it easy to find stock galvanized SAE J429 Grade 8 bolts (most suppliers) and impossible to find stock galvanized A193 B7 bolts?