Galvanizing High Strength Bolts

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.

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