Comments on: Galvanizing High Strength Bolts

By: Sam Kinnis

Sam Kinnis — Fri, 19 Aug 2011 14:46:18 +0000

I am so thankful that I found you guys on Google. The table presented here has been instrumental in choosing the right steel to use for our project.

Thanks for providing accurate information regarding hydrogen embitterment. My boss is always concerned about that, and even though I am new to this, it is actually starting to make sense.

Sam Kinnis

By: Peter

Peter — Fri, 15 Jul 2011 08:03:23 +0000

Thanks for providing accurate information regarding hydrogen embrittlement. It’s always the main concern when it comes to steel. This table would surely be a great help in choosing which steel to use for our project.

Peter Simmons

By: Hasan Siddiqui

Hasan Siddiqui — Wed, 16 Mar 2011 23:28:41 +0000

The tensile strentgth of ASTM A193 Gr. B7 studs is 125 KSI (860 Mpa) and not 155 KSI as mentioned above by Adam Oakley. Therefore, the upper threshold of 150 KSI for hot-dip galvanizing will not be applicable to ASTM A193 Gr. B7 studs.

By: Dane McKinnon

Dane McKinnon — Mon, 04 Oct 2010 21:14:22 +0000

@Jason Hughes – You are correct in your observations. B7 fasteners that are hot dip galvanized should not be subjected to applications in excess of 390F unless approved by an engineer. Typically speaking, fasteners that are hot dip galvanized are usually being used in marine or outdoor applications, where temperatures are not usually that high. I think that what usually happens is that an engineer will specify B7 in a non-high temp application, like construction, because it is a common material and a known quantity. It was not really ever meant to be used outside in general purpose applications, but because of its availability and versatility, it gets spec’d in where other materials might be more suitable but harder to find. As far as the issues with liquid or solid embrittlement, I am not sure. We don’t usually have B7 embrittlement issues with our hot dip galvanizing because B7 does not fall into the high strength range of 150ksi tensile and above where embrittlement can be an issue. What I do not know is if embrittlement can be an issue at higher temperature applications as you approach and pass 390F. Our products are not typically used in those applications so I do not have any relevant experience. I also have to plead ignorance with respect to cadmium plating. It is not a plating type that we usually deal with, so I do not know what issues may be associated with cadmium.

By: Jason Hughes

Jason Hughes — Tue, 21 Sep 2010 13:56:49 +0000

I didn’t see any comments regarding solid or liquid metal embrittlement concerns with coated A 193 B7 studs used at temperatures within the allowable range for the bolt material. ASTM A 193 Appendix X2 indicates that coatings are not recommended particularly at temperatures above 1/2 the melting temperature of the coating. Based on this, a coated A 193 B7 stud no longer meets the full range of temperatures that are associated with the bolt material alone.

By: Cunha

Cunha — Wed, 18 Aug 2010 13:46:00 +0000

For ASTM A193 B7 bolts we used to bake after galvanising, according to ASTM B633, that sends to B850

Follow link below…

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

By: PIO RILLER

PIO RILLER — Mon, 16 Aug 2010 15:46:51 +0000

THANK YOU FOR THE INFORMATION, I WILL PASS IT ON TO MY BOSES, IT WILL HELP US IN DECIDING IF
THEY WILL GO THROUGH USING B7 HOT DIP GALV. OR TO USE ANOTHER TYPE OF BOLT.

By: Jonathan Waltner

Jonathan Waltner — Tue, 18 May 2010 17:58:20 +0000

@LZ – Thank you for your concern with regard to the accuracy of information on the topic of hydrogen embrittlement provided on our website. The information Adam provided previously is accurate, however his interpretation of the data may be a bit misleading. The fact that A193 grade B7 has no maximum tensile strength requirement provides no insurance that the actual strength of a given bolt (or lot of bolts) could exceed 150 ksi in ultimate tensile strength. However, the fact that the specification carries a maximum Rockwell C35 value in theory will limit the tensile strength to somewhere in the mid 150 ksi range. There is no magical line at 150 ksi tensile strength where if a bolt exceeds that value, it is automatically going to be susceptible to hydrogen embrittlement. On the contrary, you will see that based on a couple references I will present, the point at which hydrogen embrittlement may occur when a quenched and tempered medium carbon alloy fastener is galvanized appears to be at a much higher tensile strength than 150 ksi.

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.

By: LZ

LZ — Tue, 04 May 2010 17:34:05 +0000

Your chart is misleading with regard to the A193 B7 bolts in that it implies they can always be galvanized. As Adam Oakley pointed out, the tensile strength of this material can be well above the 150ksi range since there is no upper tensile limit in the specification. I am aware of one situation where a refinery galvanized some B7′s for a pipr flange with difficult access, and they ended up on the ground. The F1554 Grade 105 places an upside limit on the tensile strength just for this reason.

By: Adam Oakley

Adam Oakley — Thu, 09 Apr 2009 17:38:34 +0000

See this FAQ for details on corrosion protection options for high strength bolts.