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Galvanizing High Strength Bolts

Question: Can high strength bolts be hot-dip galvanized?

Answer: Some high strength bolts can be galvanized while others cannot. In the construction fastener industry, typically the phrase “high strength” refers to bolts that have been quenched and tempered (heat treated) to develop the proper strength requirements of a given specification. Two separate issues are involved in determining whether or not a high strength bolt can be galvanized.

Hydrogen Embrittlement Concerns

The first issue involves a phenomenon called hydrogen embrittlement which may occur when atomic hydrogen is absorbed by the steel during the acid pickling process that takes place prior to galvanizing. This embrittlement can potentially lead to the loss or partial loss of ductility in the steel and consequently result in the premature failure of the fastener in the field.

According to specification ASTM A143 – Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement:

“In practice hydrogen embrittlement of galvanized steel is usually of concern only if the steel exceeds approximately 150 ksi (1100 MPa) in ultimate tensile strength.”

Additionally, section 7.2.2 of specification ASTM F2329 – Zinc Coating, Hot-Dip, Requirements for Application to Carbon and Alloy Steel Bolts, Screws, Washers, Nuts, and Special Threaded Fasteners reads as follows:

“For high strength fasteners (having a specified minimum product hardness of 33 HRC), there is a risk of internal hydrogen embrittlement.”

By reviewing Table 1 below, it is clear that specifications ASTM A490, ASTM A354 grade BD, and SAE J429 grade 8 are all potentially susceptible to hydrogen embrittlement and should therefore not be hot-dip galvanized. This is further reinforced by specific references found in both the ASTM A490 specification and the ASTM A354 specification. According to section 4.3 – Protective Coatings of specification ASTM A490:

“The bolts shall not be coated by hot-dip zinc coating, mechanical deposition, or electroplating with zinc or other metallic coatings.”

Similarly, Note 4 in section 4 of specification ASTM A354 reads as follows:

“Research conducted on bolts of similar material and manufacture indicates that hydrogen-stress cracking or stress cracking corrosion may occur on hot-dip galvanized Grade BD bolts.”

This information taken directly from the ASTM specifications supports the belief that hot-dip galvanizing of ASTM A490, ASTM A354 grade BD, and SAE J429 grade 8 should be avoided due to the risk of hydrogen embrittlement.

Which Bolt Grades Can Be Galvanized?

Grade Can I Galvanize? Raw material Nominal Size Minimum Yield Strength Minimum Tensile Strength Minimum Hardness
ASTM A325 Yes Medium Carbon Steel, Quenched and Tempered 1/2″ – 1″
1 1/8″ – 1 1/2″
92
81
120
105
C24
C19
ASTM A449 Yes 1/4″ – 1″
1 1/8″ – 1 1/2″
1 5/8″ – 3″
92
81
58
120
105
90
C25
C19
B91
SAE J429 Grade 5 Yes 1/4″ – 1″
1 1/8″ – 1 1/2″
92
81
120
105
C25
C19
ASTM A193 Grade B7 Yes Medium Carbon Alloy Steel, Quenched and Tempered 1/4″ – 2 1/2″
2 5/8″ – 4″
105
95
125
115
NA
ASTM A354 Grade BC Yes 1/4″ – 2 1/2″
2 5/8″ – 4″
109
94
125
115
C26
C22
ASTM F1554 Grade 105 Yes 1/4″ – 3″ 105 125 NA
ASTM A320 Grade L7 Yes 1/4″ – 2 1/2″ 105 125 NA
ASTM A490 No 1/2″ – 1 1/2″ 130 150 C33
ASTM A354 Grade BD No 1/4″ – 2 1/2″
2 5/8″ – 4″
130
115
150
140
C33
C31
SAE J429 Grade 8 No 1/4″ – 1 1/2″ 130 150 C3

Effect of Heat on Quenched and Tempered Fasteners

The second issue of concern when considering hot-dip galvanizing high strength fasteners is potentially reducing the mechanical strengths due to the introduction of heat during the hot-dip galvanizing process. Portland Bolt operates its hot-dip galvanizing tank at 840° Fahrenheit. Is this enough heat to potentially alter the strength of a quenched and tempered fastener? In theory, the amount of heat that a high strength fastener is exposed to during the hot-dip galvanizing process should not alter its mechanical properties.

It is obvious that the application of heat to a bolt that develops its strength through a heat treating process could adversely affect the strength of the fastener. The question is, how much heat is necessary to potentially change the mechanical properties? On page 4-4 of the Ninth Edition of the AISC Manual (American Institute of Steel Construction), the following statement occurs:

“Anchor bolt material that is quenched and tempered (heat treated) should not be welded or heated.”

However, it does not address the amount of heat that should be avoided. Welding obviously applies a tremendous amount of heat to the components being welded, whereas hot-dip galvanizing is performed at significantly lower temperatures. Another reference that addresses the heating of high strength bolts (which occurs during hot-dip galvanizing) can be found in the ASTM F1554 specification. Section 6.4.3 of the ASTM F1554 specification states:

“Hot bending performed on heat-treated bar stock shall not have the temperature come within 100°F (56°C) of the tempering (stress relieve) temperature of the heat-treat process at any location during hot bending and shall be allowed to air cool after bending.”

Although this statement refers to hot bending, it implies that any process (including galvanizing) that applies heat approaching or exceeding the tempering temperature of a high strength bolt may potentially alter the mechanical properties of the fastener and should therefore be avoided. However, galvanized bolts only remain in the zinc tank for a few minutes and even though the surface temperature may approach 840°, it is unlikely that the entire fastener is heated to that temperature. The most relevant reference found in any ASTM specification addressing the possibility of the hot-dip galvanizing process altering the mechanical properties of a high strength fastener is found in section 7.2.1.2 of the new hot-dip galvanizing specification ASTM F2329 which states:

“Testing for mechanical properties is not necessary if the galvanizing process is carried out at a lower temperature than the stress relief or tempering temperature of the fasteners.”

This implies that the mechanical properties have the potential to be altered only if high strength bolts are tempered at temperatures at or below the 840° temperature that the bolts are subjected to during hot-dip galvanizing. As Table 2 indicates below, the minimum tempering temperature for both ASTM A193 grade B7 and ASTM F1554 grade 105 is 1100° Fahrenheit. Therefore, it is not possible for the galvanizing temperatures to exceed the tempering temperature for these specifications. Although in theory it is possible for the other specifications to be tempered below the galvanizing temperature, recent records indicate that the minimum tempering temperature used for any of these grades listed in Table 2 that Portland Bolt has manufactured is 950°. Therefore, hot-dip galvanizing performed by Portland Bolt on any of the high strength bolt specifications listed in Table 2 will not adversely affect the strength of these fasteners.

Minimum Tempering Temperatures

ASTM Grade Minimum Tempering Temperature, Fahrenheit
A325 800°
A449 800°
A354 Grade BC 800°
A320 Grade L7 No Requirement
A193 Grade B7 1100°
F1554 Grade 105 1100°

Posted by Dane McKinnon

Phone: (503) 219-6991 Email: danem@portlandbolt.com
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The below content is submitted by readers and has not been researched or checked for accuracy. It is not endorsed in any way by Portland Bolt.

40 Responses to “Galvanizing High Strength Bolts”

  1. Dane McKinnon says:

    @Gilbert – There should be no issues with hot dip galvanizing A563DH nuts, it is done all the time. They are galvanized per ASTM A153 and F2329. However, you should be careful using galvanized nuts with A490s that are coated with something else. Galvanized nuts are tapped oversize to allow for the zinc coating thickness, they may not fit properly with bolts that are coated with something other than hot dipped zinc.

  2. Gilbert says:

    Thank you very much for the information! I am looking to use an A490 blot nut assembly and protection the the bolt with another protection than the hot-dip galvanization. But, is the hot-dip galvanization of an A563DH nut could cause hydrogen embrittlement problems? Do the nuts have to be galvanized in accordance with the ASTM A143?
    Thank you very much!

  3. Dane McKinnon says:

    @ToddAnderson – I can’t say if one method is better than the other, just different. Which method works better will likely depend on the item being galvanized – the type of steel, size, shape, capabilities of the galvanizer, etc.

  4. ToddAnderson says:

    in the prep before galvanizing of A193 B7 is it better to acid etch the material or sandblast?

  5. Dane McKinnon says:

    @Zam – A193 B7 is designed for high temperature and high pressure applications. Grade 8.8 is more for general purpose use, and would not perform well in high temperature/pressure applications. Depending on your use, any substitution would need to be approved by an engineer. A193 B7 is an alloy steel, heat treated to withstand high heat. Grade 8.8 can be a number of different steel chemistries, and is heat treated at a lower temperature for general purpose use.

  6. Dane McKinnon says:

    @Baw – ASTM specifically prohibits galvanizing on A490 bolts due to hydrogen embrittlement concerns. If the site consultant has rejected them, I do not know of any information you could use to overturn that. ASTM is very specific about not allowing HDG on A490s.

  7. Baw says:

    Hi, projects specs were referring to HDG A490 bolts which we already purchased; now site consultant has rejected those bolts for being HDG!
    consultant requested us to perform tensile strength on the galvanized bolts and results were found satisfactory.
    however; bolts are already installed, is there a way to get it approved?

  8. Zam says:

    Can we use 8.8 Grade instead of ASTM A193-B7 ? What is the differece between the two ?

  9. Dane McKinnon says:

    @Sridhar – You will need to get any grade substitutions approved by the project engineer, we cannot make those kinds of recommendations.

  10. Sridhar says:

    Dear All,

    Can we use A 193 B7 INSTEAD OF A 325.

  11. Dane McKinnon says:

    @Andis Eglitis – I am sorry, but we are not familiar with the various DIN standards, so are unable to answer. Apologies.

  12. Andis Eglitis says:

    Why bolts DIN933 DIN931 property class 10.9 can not be hot dip galvanized but bolts DIN6914 can?

  13. Clarke Johnston says:

    I found this very informative page while researching bolts, the galvanizing process, and hydrogen embrittlement. Did anyone at CalTrans or working on the new SF Bay Bridge project even do this level of inquiry? From one taxpayer who doesn’t trust the PR spin that CalTrans is putting out. No.

  14. Dane McKinnon says:

    @Hitender – That is a question you need to pose to your project engineer. A193 B7 bolts are rated for high temperature and high pressure applications, whereas grade 8.8 are not. Your engineer would need to sign off of any grade changes.

  15. Hitender Negi says:

    can we use Gr. 8.8 bolt in place of SA 193 Gr. B7

  16. Dane McKinnon says:

    @Arnold – the AISC Steel Construction Manual, 13th Edition, makes no distinction between plain and galvanized bolts in the allowable tensile and shear stress tables, so it is our interpretation that the values are not reduced.

  17. Arnold says:

    After galvanizing the ASTM A325 bolts, is the allowable tensile stress and allowable shear stress on bolts reduced?

    Thanks

  18. Dane McKinnon says:

    @Emad – In our galvanizing line, we typically get thicknesses ranging from 2-5 mils (0.002-0.005 in). With threaded fasteners, the risk of extra thick coating thicknesses would be that the thread fit tolerances would be too tight. Normal hot dip galvanized nuts are tapped a bit oversize in order to accommodate the thickness of the zinc on the threads. If the zinc thickness is very high, the nut would likely not fit onto the bolt. ASTM A563 gives the maximum overtapping tolerances for hex nuts, so additional overtapping is not allowed.

  19. emad says:

    Hi, Could you please give me some clarifications about the normal/common galvanizing thickness for bolts ? and what is the problems/risks of using high galvanization thickness ?
    Thanks.

  20. Dane McKinnon says:

    @Gulsum – Apologies, but I do not have that information. ASTM does not make any references to cooling baths after galvanizing. I am not familiar enough with ISO or any of the metric standard organizations to know if they make such a recommendation.

  21. Gulsum says:

    Thank you for response Dane.
    I have one more question.What should to be cooling water temperature for quality 10.9 bolts after galvanizing process?
    So Is there a standard about it??

  22. Dane McKinnon says:

    @Gulsum – Yes, boron steels can react differently when hot dip galvanized than non boron steels. Exactly how is dependent on the chemistry of the steel and the temperature of the zinc bath. Apologies, but we cannot answer this question precisely since we do not have any metallurgists on staff.

  23. Gulsum says:

    1)What should temperature galvanizing of boron steels be for not decrease of hardness?
    2)Is Grade 530 problem for temperature of galvanizing?
    (Quality of bolt is 10.9 and diameter of bolt is 20 metric.)

  24. Dane McKinnon says:

    Hot dip galvanized A325s are very common here in the US and would be roughly equivalent to 8.8 bolts in terms of chemistry and mechanical properties. If the galvanized coating is not necessary, another option (besides plain finish A325s) would be SAE J429 grade 5 bolts. These are also mechanically equivalent to 8.8 and A325, but are used more for general purpose applications. As for stainless, there are sizes and grades (A193 B8/B8M or F593) that are somewhat available, but they would not be nearly as strong as the heat treated carbon steel bolts, and they would be considerably more expensive.

  25. Carl Wang says:

    ASTM A325 High Strength bolts can be galvanized by either the hot-dip (A153 Class C) or mechanical (B695 Class 50) process.

  26. Steve Huppert says:

    Hi,

    Thanks for an excellent and informative website.

    We are an Australian firm engaged in design and manufacture of architectural tension fabric structures.
    We have designed several large architectural structures in Australia and overseas, including the USA. We are currently engaging some local (USA) fabricators but designing and specifying the structuture here (down-under) to AISC specs (as best we can).
    We commonly specify metric HD galvanised grade 8.8 bolts here in Aus but we are unsure of the specs for high strength galvanised bolts in the USA.
    I’ve checked several references on your website but I’m still not perfectly clear on what is ok and what is not.
    The client has queried the fact that we have specified HD gal finish (for outdoor corrosion protection) with ASTM A-325 bolts. The bolts are primarily in shear and friction grip type are not required, so the gal coating is not an issue in this regard.
    Can you please clarify what is available or approved within the range of high strength bolts.
    The client is suggesting the use of stainless steel as an alternative, but we’d like to be properly informed for future projects.

    Thanks.

  27. Sam Kinnis says:

    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

  28. Peter says:

    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

  29. Hasan Siddiqui says:

    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.

  30. Dane McKinnon says:

    @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.

  31. Jason Hughes says:

    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.

  32. Cunha says:

    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

  33. PIO RILLER says:

    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.

  34. Jonathan Waltner says:

    @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.

  35. LZ says:

    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.

  36. Adam Oakley says:

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

  37. Julie Blouin says:

    What is the safiest alternative (without modifying the strenth) to protect a A490 bolt from corrosion if those treatments (galvanazing and all)is not recommended?

  38. Bill King says:

    I agree with Adam on this issue. Grade 8 bolts should not be galvanized. I am not aware of anyone stocking galvanized grade 8 bolts. It may be possible the word “galvanize” used for grade 8 bolts actually refers to the older term “electro-galvanize”, which is actually standard zinc plating, not hot-dip galvanize. Zinc plating is fine for Grade 8 bolts.

  39. Adam Oakley says:

    @Michael Nietch: Thanks for your input regarding this topic. Galvanizing high strength bolts is a complicated subject and the discussion helps to better clarify the issues.

    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.

  40. Michael Nietch says:

    1. Have you excluded SAE J429 Grade 8 from “capable of galvanizing” list specifically because hardness minimum is >= 33 HRC and included A193 B7 because the specification doesn’t have a minimum? If yes, I question the reasoning since the maximum hardness of A193 B7 is 35 HRC at less than 4″ and therefore a A193 B7 and SAE J429 Gr. 8 fastener may have identical hardness.

    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?

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