In Part 1 of our series on head markings, we talked about manufacturers’ head markings and how they allow the part to be traced back to the manufacturer. We then talked about fastener standards and how head markings can identify a part as a grade or class from a particular standard. We then went on to talk specifically about SAE J429 and Grade 2, Grade 5, and Grade 8. These three grades are probably the most common grades for inch fasteners that we here at Wilson-Garner deal with. However, there are plenty of other standards out there with their own grades and head markings. Let’s look at a couple more today.
ASTM A307 Carbon Steel Bolts and Studs, 60,000 Psi Tensile Strength is a specification that creates three grades of bolts and studs: Grade A, Grade B, and Grade C. Grade C has been replaced by a different standard, so we will ignore it here. The spec covers diameters ¼ inch through 4 inches. The grades are again differentiated by tensile strength, although the spec also differentiates by intended use.
Here is a look at what a head marking for Grade A would look like:
Grade A bolts and studs have a minimum tensile strength of 60ksi and are intended for general applications. If you are like me and have trouble remembering numbers, you will appreciate this head marking, because it includes the number of the spec. Grade A bolts and studs are considered to be low strength, having a tensile strength roughly equivalent to a SAE J429 Grade 2. It is generally not necessary to heat treat Grade A parts in order to meet the minimum tensile requirements.
Here is the head marking for a Grade B:
Yep, this is pretty much what we expected. Once again, the number of the spec is right there on the head if you need it, which is nice. Grade B bolts and studs have a tensile strength between 60ksi and 100ksi. Their intended use is in flanged joints in piping systems with cast iron flanges. The major difference between Grade B and Grade A is that Grade B has minimum and maximum tensile requirements set by the spec, while Grade A only has a minimum tensile requirement.
ASTM A193 Alloy Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications is a specification that creates a rather large number of grades for inch-series bolts, screws, and studs up to seven inches in diameter. As the title would lead you to believe, this spec deals with fasteners intended for use in high temperature or high-pressure environments. The spec provides guidelines for six grades for parts made of ferritic (carbon or alloy) steels and more than twenty grades for parts made of austenitic stainless steels. There is also a version of the standard, ASTM A193M, which lays out guidelines for metric-series fasteners.
For the sake of brevity, I am only going to spotlight the most common grade that Wilson-Garner manufactures, Grade B7. Grade B7 parts are made from alloy steel. While there is a metric version of Grade B7 that can be found in ASTM A193M, I am referring to the inch-series version.
Here we see the grade marked into the head of the bolt. You may have noticed that this standard requires only the grade to be marked, while the previous standard we covered, ASTM A307, required the number of the standard along with the grade. The first standard we talked about, SAE J429, used symbols to designate the grade. The more you look at these things, the more you’ll notice that each standard, even standards written by the same organizations, tend to handle head markings a little differently.
Grade B7 bolts have a minimum tensile strength of 125ksi for diameters 2-1/2 inches and under, 115ksi for diameters over 2-1/2 inches through 4 inches, and 100ksi for diameters over 4 inches through 7 inches. These minimum tensile strengths are roughly equivalent to the requirements for SAE J429 Grade 5. Therefore, these parts are considered to be medium strength. Grade B7 parts are required to be heat-treated, quenched, and tempered.
ASTM A354 Quenched and Tempered Alloy Steel Bolts, Studs, and Other Externally Threaded Fasteners lays out the guidelines for two grades, BC and BD. All parts in this standard must be made from alloy steel and be heat-treated, quenched, and tempered. This spec covers parts up to 4 inches in diameter.
Let’s look at the head marking for Grade BC:
Again, we simply have the grade itself marked. Grade BC fasteners have a minimum tensile strength of 125ksi for diameters ¼ inch through 2-1/2 inches. The minimum tensile strength is 115ksi for diameters over 2-1/2 inches. Generally, Grade BC parts are considered to be medium strength fasteners.
Here is the head marking for Grade BD:
No surprises here. Grade BD parts have a minimum tensile strength of 150ksi for diameters ¼ inch through 2-1/2 inches and 140ksi for diameters over 2-1/2 inches. Grade BD parts are considered to be high strength. The mechanical properties of Grade BD fasteners in diameters ¼ inch through 2-1/2 inches are almost identical to those of SAE J429 Grade 8.
Grade BD vs Grade 8
The mechanical properties are so similar, in fact, that I’ve got one more head marking to show you:
Whoa! Are those Grade BD AND Grade 8 markings? That’s right, the mechanical properties of BD are so close to Grade 8 that ASTM A354 allows you the option to include both grade identifiers if you wish. Therefore, the same fastener can qualify and be marked for both Grade BD and Grade 8. Of course, this begs the question: Why do ASTM Grade BD and SAE J429 Grade 8 both exist? Well, SAE J429 only covers diameters up to 1-1/2 inches, and ASTM A354 covers parts up to 4 inches. So ASTM A354 allows for larger fasteners to be covered in the same standard as smaller ones. Other than size restrictions, is there any reason to specify one standard over the other one? I honestly don’t know. If anyone out there does, please leave a comment and let me know.
We have now covered every major grade specification for inch-series fasteners that we make here at Wilson-Garner. Be sure to check out Part 3 of our series on head markings, where I cover the metric side of things.
In addition to our Head Marking Series, we’ve got plenty of other articles covering fastener basics. Here’s a sampling.
Proof Load, Yield Strength, and Tensile Strength of Fasteners
The Basics of External Threads
The Difference Between 2A and 3A Threads
Cold Heading Vs Hot Forging Vs Machining
Or, check out the full list of articles.