Can high strength bolts be hot-dip galvanized?
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° |
Hi Dane,
So would you please tell , whats the difference between Hot Dip Galvanized Coating and Dacromet Coating?
As well as , is there any specific requirement for the Anchor Bolts of Grade 8.8 , that these should be HDG Coating, and the ones of the grade 10.9 should have a Dacromet coating ?
Thanks.
@Nikhil- the two procedures are completely different. Hot dip galvanizing is a dip spin process in molten zinc, where are Dacromet(now Geomet) is a zinc flake/aluminum compound with topcoats, sealers, etc. As for what coating is appropriate for which grade, we are unable to answer that, especially for the metric grades that we are less familiar with.
Is mechanical galvanizing per ASTM B695 permitted for ASTM F3125 Grade 1852 TC or is all galvanizing for ASTM F3125 Grade 1852 TC bolts not permitted?
@Jeremy- Mechanical galvanizing is permitted, however, it can only be applied by the bolt manufacturer or at their direction. An end user or supplier may not add the coating.
Can ASTM F3125 Grade 1852 TC bolts be hot-dip galvanized?
@Jeremy – No, hot dip galvanizing is not a permitted coating for TC bolts.
Can anybody tell me if a 3/8″ x 3 1/2″ galvanized bolt will hold 1000 lbs? It is rated at Grade A.
@Steven – it depends on how the load is applied. a UNC threaded machine bolt should have a minimum tensile of 4,650lbs and a minimum shear of approximately 2,800lbs. However, if the load is applied unevenly or if your bolt has different threads or some alternate geometry, your number may vary.
Hello. What standard can be used for the duration of hot galvanized materials in the kettle in different grades?
@Hosein- Apologies, but we are not understanding your question. If you can rephrase we are happy to help if we can.
Hello. We meet a problem when the bolt shaft diameter is shrinken by 0.2-0.4mm depended on sizes after HDG (compareed to black). Bolt is grade 8.8. Please let us know how it is possible for this case. Thanks a lots
@Viet- We think that it is common to undersize the male threads for HDG fasteners in other parts of the world. Here in the States we choose to oversize our female threads instead, so do not have any familiarity on the other method.
Hi,
I have question,for 8.8 grade HEX SOC HEAD CAP SCREW galvanizing and balcknening have same effect on the properties?
@Sreenivasachar- It will depend on the specifics of the processes. If the temperatures are low, the mechanical properties will likely not be affected. If the temperatures are high (i.e. high temp galvanizing), then care should be taken. We are less familiar with the blackening process, but the same principals would apply.
With grade 105 approaching 150ksi, do grade 105 and B7 rods need to be sandblasted prior to galvanizing?
@Brandon- We have heard of that being asked for, but it is not a requirement of either specification. Sandblasting does not eliminate the need for chemical pickling, that would still need to be done.
Can I galvanizing ASTM A193 Grade B7 bolt?
@Munit- Yes, A193 B7 bolts can be safely galvanized.
ASTM A354 BD high strength cap screw are not to be hot dipped galvanized –
1) What coating option can be used on these high strength bolts? Plain / Black finish rust over time.
@Khan- there are many types of alternative coatings in the marketplace, like Geomet, Magni, etc. We don’t have a lot of experience with these other coatings, so we are unable to make any recommendations for one vs the other.
Can you please explain to me that the SAE J429 Grade 8 bolt is the same of SAE 5140 grade 8.8?
Is there any possibility to use SAE 5140 Grade 8.8 as anchor rod in the place of Grade A490?
please give me the guidance.
@Bhoopathi- Bolts manufactured to grade 8.8 are not equivalent to SAE grade 8 nor to ASTM A490. Grade 8.8 fasteners have a minimum tensile strength of 830MPa, whereas SAE gr.8 and A490 bolts have a minimum tensile strength of about 1040MPa. You would need to get the project engineer’s approval to make that substitution. However, as noted above, 8.8 bolts are not as strong as grade 8 or A490 bolts.
Will Hot dip galvanized 2h nuts have the same HRB as the Black finished Nuts?
@Jason- The hardness requirements for A194 2H nuts is the same regardless of finish.
Can A325-TC bolt be galvanize?
@Darrel – A325?F1852 graded TC bolts are available mechanically galvanized, but not hot dip galvanized. A490/F2280 TC bolts are prohibited to be galvanized by either method.
We are unable to galvnize 8.8 grade bolt as we are facing problem of zinc flaking.
Tell me the exact process.
Temp range of zinc bath kettle,quenching water temp,
@Sourabh- You are able to find many of the details on the American Galvanizers Association website, as well as in the ASTM A153 and F2329 standards. We are unable to provide the specific details of our processes.
Nowdays,it is seen that Geomat coating are used on Gr 10.9 HSFG Bolt. What type of coating were done on Gr 10.9 HSFG Bolt in International major bridges made before 2005?
@R.K.- Sorry, but we are not familiar with what coating was used on international bridge jobs prior to 2005. We manufacture and supply bolts made per ASTM standards, so we are less familiar with the international standards.
Hello,
Could we use washer of 10.9 grade with ASTM A325 bolt? or there is compatibility issue and only washer of 8.8 grade with ASTM A325 bolt could be used?
Thanks
@Hisham- The washer required with A325 bolts is an ASTM F436 washer. A 10.9 or 8.8 washer may work for the application, but you would need to get the substitution approved by the project engineer.
Hi,
Can you please explain me that the SAE J429 Grade 8 bolt is the same of SAE 5140 grade 8.8?
Is there any possibility to use SAE 5140 Grade 8.8 as anchor rod in the place of Bs 3692?
I aware portland bolt is not manufacturing thi SAE 5140 GRade 8.8. However, please give me the guidance.
@Bhoopathi- I am sorry, but we are not familiar with SAE 5140 nor BS 3692, so we cannot speak to the requirements of those grades. Apologies.
Can Class 8.8, 10.9 and 12.9 steel bolts be hot dip galvanized?
@Kris- Class 8.8 bolts are ok to galvanize, but the 10.9 and 12.9 will be high enough in strength that they will run the risk of hydrogen embrittlement. We’d recommend against galvanizing them.
Hi sir ,
Based on which standard we can say that 10.9 is not possible to hot dip galvanized
@Mohamed- I do not have any published information to say that 10.9 cannot be galvanized, however, ASTM grades of similar strength (A490, A354 BD) either prohibit, or caution against hot dip galvanizing. We have chosen to use that same caution for 10.9 fasteners.
urgent answer pleas :
I do not know whether the documents in accordance with the standard screws made of weathering steel ( a325 type 3 ) can be hot-dip galvanizing. with regard to the impact of high temperatures in hot-dip galvanizing process whereby the Embrittlement .
tanks
@Behrouz- Normally speaking A325 type 1 or type 3 bolts can be galvanized without rick of hydrogen embrittlement. However, there is no benefit to galvanizing type 3 materials, since they are inherently corrosion resistant. If galvanized fasteners are required, type 1 will be commonly available and much less expensive than trying to galvanize type 3 bolts.
Dear Dane,
The ASTM A490 is on the project specification but this material is not available i can recommend to use the F10T .Are these two materials same characteristics for using to boiler structural beam?
Thank you
Wifredo Espino
@Wilfredo- I am sorry, but I do not know much about F10T bolts. They appear to be similar, but if you want to make a substitution, you will need to consult with the project engineer..
May I know value torque bolt gr 8.8 ( JIS ) hotdip galvanized , because the nut must oversize, so the value torque can’ t use same with bolt gr 8.8 black ( plain ), thx
@Dady- We do not have any torque values for metric grades, but I can tell you that galvanized torque values are typically about 25% higher than plain finish torque values.
Are there any changes to torque spec for galvanized B7 stud bolts? Thanks,
@Phil- The torque chart we have is applicable for your item as long as one end is restrained from rotating during tightening.
Dear Dane,
if hot dip galvanizing is not accepted by customer specification, which kind of coating is suggested for A193 Gr.B7 bolts at ambient temperature, indoor application but with occasional presence of water and/or petroleum?
@Matteo- There are many kinds of special and proprietary coatings available in the marketplace, but we are not familiar enough with the pros and cons of each to make any sort of recommendation.
Hi Dane,
Galvanized Bolts: Hot Dip vs. Mechanically galvanized.
Question: What is typically specified? Hot dip or Mechanically galvanized? What are the situations to use Mechanically galvanized. What are the precautions to use either of them. What is approximately the price difference.
Thanks,
Partha
@Partha- Hot dip galvanizing is much more common in our industry. We don’t do much with mechanical galvanizing, and I am not certain why it would be specified versus hot dip. Cost-wise, it will depend on the item and quantity. For us, hot dip galvanizing is less expensive, but it will depend on the specifics of your inquiry. For more technical information, you may want to contact the American Galvanizing Association.
If i use A490 ungalvanized as AISC, will it get Enviromental Hydrogent Embrittlement or not?
How to procure Dacromet in the bridge project.
@Ary- You may run the risk of environmental hydrogen embrittlement, it would depend on the atmosphere. You would need to consult an engineer familiar with the job to get that question properly answered. As for the Dacromet coating there are a few suppliers who specialize in having that coating applied. You would need to contact one of them to walk you through the process.
I can use Bolt grade 8.8 or 10.9 instead of ASTM A193 Grade B7 or let me know equivalent grade for ASTM A193 Grade B7.
@Gajendra- No, A193 B7 is a special grade used for high temperature applications. Bolt grades 8.8 and 10.9 are for general purpose applications. I am not aware of any high temperature equivalent to A193 B7.
Hi Dane help……. Im quoting on a large quote for Studs & Nuts A193 grd B8M class 1 and 2 they want it Hot dip Galvanised the bolts are imperial sizes and S.A. dont stock the taps to clean out the threads after plating.. my ques is do these bolts need to be stree relieved after plating? and what is the implications of Hot dip galvanising this ?
@Rayna- A193 B8M is stainless steel 316, so you cannot hot dip galvanize them. Stainless 316 is corrosion resistant by itself, so not only would galvanizing be a waste of time and money from a corrosion standpoint, but the zinc will not adhere to the steel.
Can high strength bolt F10T (JIS B1186) be hot dip galvanized or not?
Bolts F10T / Tensile strength : 1000 – 1200 N/mm2, HRC 27 – 38
Bolts F10T / Yield strength : 900 N/mm2 min.
Nut F10 : HRC 35 max.
Washer F35 : HRC 35 – 45
@Jakrin – We do not know if JIS B1186 specifically allows or prohibits hot dip galvanizing, but the rough ASTM equivalents that fall in that same strength range are not permitted to be galvanized due to hydrogen embrittlement concerns.
I want to obtain infromation about hydrogen embrittlement . First , in cleaning and pickling section of ISO 10684 : Unless otherwise agreed, parts heat treated or work hardened to a hardness of ≥ 320 HV shall be cleaning using an inhibited acid ,alkaline or mechanical process . Immersion time in the inhibited acid depends on the as –received surface condition and should be of minimum duration .
So ,
– Does still increases the risk of hydrogen embrittlement, immersed of material in an acid bath established between 5 and 10 percent after mechanical blasting ?
– What should be phosphorus and silicon ratio of used steel ?
– Must we need to use low rigidity or high resistant alloys ?
– What should be our material selection ? (High and low alloy chrome molybdenum steel ??? )
– What do we need to pay attention during manufacturing process of bolts ? So What do the risks posed by forging process for hydrogen embrittlement ??
In brief , can you give to general information about the risks posed of manufacturing process ,heat treatment and hot dip galvanizing proces for hydrogen embrittlement ??
@Gulsum – I am sorry, but your questions are beyond the range of our expertise. You will need to contact an engineer or metallurgist who specializes in hydrogen embrittlement.
dear sir, astm a193 gr.b7m & a194 gr.2hm studs nuts are hot dip galvanised with a range 150 to 200 microns coating.
min requirement is 70 microns. Does it make any effect in workability of products? iS IT ACCEPTABLE TECHNICALLY?
@Raj – You are correct, there is no maximum coating thickness in the F2329 galvanizing standard. The only issue with the coating being too thick is if the nut will not mate to the bolt. Otherwise, if the nut threads on, there is no downside to having more zinc thickness.
thank u for this excellent website.
high strength bolt A490 shouldnot be galvanized because of pickling but if i make blasting cleaning in stead of pickling, in this case A490 can be galvanized?
@Ibrahim – No, the ASTM A490 standard specifically prohibits any non-approved coating, regardless of how the bolts are prepared. At this time, galvanizing is still not permitted.
CAN WE USE 10.9 GRADE BOLT (ASTM 490) IN PLACE OF 8.8 GRADE BOLT (ASTM 325)IN STRUCTURE
@Naveed – You will need to consult an engineer for any substitutions like this, we cannot make those recommendations.
Please confirm as per code whether Hot dip galvanizing of SA 193 Gr.B16 bolts and SA 194 Gr. 4 nuts is recommended or not.
@Rahul – ASME SA193 B16 bolts are designed for high temperature service. Hot dip zinc is not designed for high temperature service, in fact ASTM A194 Appendix X2 cautions using zinc coated fasteners above 390F. So, it seems wasteful to use a high cost bolt like B16 when you aren’t getting the high temperature usage you are paying for. As for whether it is “per code”, I guess it depend on what code you are talking about. ASTM does not limit coatings on any A193 fasteners, and I am not familiar enough with other “codes” to know what may or may not be prohibited or recommended.
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!
@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.
Dane – since a ASTM A563 – DH nut can have a tensile strength up to approximately 171ksi, why is it not susceptible to hydrogen embrittlement?
@Guy- We are not certain why HBE is not an issue for A563DH nuts. Perhaps is it due to the tapping being performed after galvanizing, so the hydrogen does not get trapped? Or perhaps it is because the galvanized nuts are not typically used in super high tension applications, like A490 bolts (with plain nuts) would be? We are not certain, but can say that the issue has not ever been raised at any of the HBE discussions we’ve been a part of at ASTM meetings.
in the prep before galvanizing of A193 B7 is it better to acid etch the material or sandblast?
@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.
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?
@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.
Can we use 8.8 Grade instead of ASTM A193-B7 ? What is the differece between the two ?
@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.
Dear All,
Can we use A 193 B7 INSTEAD OF A 325.
@Sridhar – You will need to get any grade substitutions approved by the project engineer, we cannot make those kinds of recommendations.
Why bolts DIN933 DIN931 property class 10.9 can not be hot dip galvanized but bolts DIN6914 can?
@Andis Eglitis – I am sorry, but we are not familiar with the various DIN standards, so are unable to answer. Apologies.
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.
can we use Gr. 8.8 bolt in place of SA 193 Gr. B7
@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.
After galvanizing the ASTM A325 bolts, is the allowable tensile stress and allowable shear stress on bolts reduced?
Thanks
@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.
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.
@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.
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??
@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.
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.)
@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.
ASTM A325 High Strength bolts can be galvanized by either the hot-dip (A153 Class C) or mechanical (B695 Class 50) process.
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.
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.
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
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
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.
@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.
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.
For ASTM A193 B7 bolts we used to bake after galvanising, according to ASTM B633, that sends to B850
Follow link below…
https://www.nhml.com/newsletters/HydrogenEmbrittlement&ASTMB633.pdf
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.
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?