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. In many cases, low alloy steels like ASTM A572g50 or F1554g55 are called “high strength”. There are no issues galvanizing those low alloy grades. However, for the purposes of this FAQ, we are only discussing quench and tempered fasteners. 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 F3125/A490 specification and the ASTM A354 specification.
According to F3125 Annex A1 and Table A1.1 – Permitted Coatings, for grade A490 bolts both mechanical galvanizing per B695 and hot dip galvanizing per F2329 are “Not Qualified” meaning that those two coatings are currently prohibited on A490 grade fasteners.
Additionally, A354 in Section 4.3.5, Note 4 references ISO TR 20491, “Fundamentals of Hydrogen Embrittlement in Steel Fasteners”. Although this is well short of a specific prohibition, it is clear that the authors of A354 want to make sure that the user fully understands the potential pitfalls of hot dip galvanizing bolts of this grade.
This information taken directly from the ASTM specifications supports our belief that hot-dip galvanizing of ASTM A354 grade BD and SAE J429 grade 8, whereas not specifically prohibited, should be avoided due to the risk of hydrogen embrittlement. Additionally, F3125/A490 bolts are specifically prohibited from being coated with any non approved metallic coating.
| Grade | Can I Galvanize? | Raw material | Nominal Size | Minimum Yield Strength | Minimum Tensile Strength | Minimum Hardness |
|---|---|---|---|---|---|---|
| ASTM F1554 Grade 55 | Yes | Low Alloy Steel | 1⁄2" - 4" | 55 | 75 | - |
| ASTM F3125/A325 | Yes | Medium Carbon Steel, Quenched and Tempered |
1⁄2" - 11⁄2" | 92 | 120 | C25 |
| ASTM A449 | Yes | 1⁄4" - 1" 11⁄8" - 11⁄2" 15⁄8" - 3" |
92 81 58 |
120 105 90 |
C25 C19 B91 |
|
| SAE J429 Grade 5 | Yes | 1⁄4" - 1" 11⁄8" - 11⁄2" |
92 81 |
120 105 |
C25 C19 |
|
| ASTM A193 Grade B7 | Yes | Medium Carbon Alloy Steel, Quenched and Tempered |
1⁄4" - 21⁄2" 25⁄8" - 4" |
105 95 |
125 115 |
NA |
| ASTM A354 Grade BC | Yes | 1⁄4" - 21⁄2" 25⁄8" - 4" |
109 99 |
125 115 |
C26 C22 |
|
| ASTM F1554 Grade 105 | Yes | 1⁄2" - 3" | 105 | 125 | NA | |
| ASTM A320 Grade L7 | Yes | 1⁄4" - 21⁄2" | 105 | 125 | NA | |
| ASTM F3125/A490 | No | 1⁄2" - 11⁄2" | 130 | 150 | C33 | |
| ASTM A354 Grade BD | No | 1⁄2" - 4" | 130 | 150 | C33 | |
| SAE J429 Grade 8 | No | 1⁄4" - 11⁄2" | 130 | 150 | C33 |
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 6.5.3 of the ASTM F1554 specification states:
“The maximum hot bending temperature for heat treated anchor bolts shall be less than….1000F for grade 105. Anchor bolts 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° |
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.
@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:
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:
(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.
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?
See this FAQ for details on corrosion protection options for high strength bolts.
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.
@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.
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?