ASTM A193 is a specification for alloy and stainless steel fasteners for high temperature service, high pressure service, or both. Within the A193 specification there are multiple grades of bolts made from either alloy or stainless steel. The stainless steel grades, such as B8 and B8M, can be ordered as either Class 1 (carbide solution treated) or Class 2 (carbide solution treated and strain hardened). To understand the peculiarities of strain hardened material, you need to understand the strain hardening process, and the different ways in which class 2 bolts can be manufactured.
What is Strain Hardening?
As stated in the ASTM A193 specification, strain hardening is achieved by “reducing oversized bars or wire to the desired final size by cold drawing or other process.” In essence, stainless steel material of a larger diameter than is required for the finished round bar is drawn down through a “compression” process. Because the material is under a huge amount of tension and pressure during this process, the strain reduces the material down to a smaller diameter, while at the same time work hardening it to a higher strength.
Because of the way in which Class 2 material is strain hardened, there are some potential issues involved with producing headed bolts to this specification. There are three different ways in which a headed bolt can be made from raw steel.
Cold Forming
The first method is cold forming. This is the method used by most mass production fastener manufacturers. In this method, raw steel, usually coiled wire stock, is cold worked by a bolt making machine that is set up to make the same part over and over in mass production runs of thousands of bolts. No heat is applied during the manufacture of this style of fastener, thus, it is cold formed. Because there is no heat involved in the manufacture, Class 2 bolts can be made in this process without problem. In fact, common sizes of bolts are available from overseas manufacturers (imports) that will meet this specification. The only potential problems are with nonstandard sizes or jobs that require the bolts to be domestically manufactured (Buy America Act or similar). In the case of nonstandard sizes, overseas manufacturers usually have lead times of several months or more to manufacture a nonstandard size fastener under this specification and ship it to the U.S. In the case of domestic bolts, there are only a limited number of cold formed fastener manufacturers in the U.S. and all of them are mass production facilities that will not even consider a special production run of bolts unless it is for a quantity of several thousand bolts, if not more.
Hot Forging
The second way in which headed bolts can be manufactured is hot forging. This is how Portland Bolt manufactures headed bolts. In this process the raw steel is heated, usually through electrical induction, until the end of the steel bar is red hot. At that point the heated end is placed into an upsetter that forges the now malleable steel into a hex head (or other shape if required). Because of the heating involved in the forging process, the mechanical properties of the steel in the heated portion can be altered. In low carbon steels this is not an issue. With higher strength alloy steels, the mechanical properties are restored through a heat treating process after hot forging is complete. However, in the case of A193 Class 2 material, headed bolts CANNOT be manufactured by the hot forging process. This is because when Class 2 material is heated prior to forging, the heat relieves that tension of the material in the heated area. Consequently, that portion of the bolt would no longer be strain hardened. There is no process that exists to restore this portion of the bolt to a strain hardened state after it has been heated.
Machining from Bar Stock
The third possible way to manufacture a bolt to this specification is a rarely used form of bolt manufacture, machining from bar stock. There are also potential problems with manufacturing a Class 2 bolt this way as well. The A193 specification states that in the case of large diameter bars, the strain hardening will not occur evenly throughout the material. Specifically, “…plastic deformation will occur principally in the outer regions of the bar so that the increased strength and hardness due to strain hardening is achieved predominantly near the surface of the bar.” Because of this, if a bolt is machined from larger diameter strain hardened material, the finished bolt would have the correct strength and hardness in the bolt head, but the body (which was in the center of the original, larger bar) would most likely have a much lower strength and hardness because the strain hardening process did not penetrate the center of the material. Consequently, the finished bolt would not meet the requirements of A193 Class 2. Additionally, strain hardened round bar is typically not readily available in diameters larger than 1-1/2″.
This FAQ should help explain some of the peculiarities, technical challenges, and limitations of the types of bolts that can be manufactured to this specification. If you have a requirement for this type of fastener, or any other fastener specification, and have questions or concerns, please contact our knowledgeable sales staff.
We were asked to do stud B8M class 2 which require both carbide solution and strain hardened. Do we still need to do carbide solution if the raw mat has already been done solution annealed?
@Nipol- You would only need to re-carbide solution anneal the studs if a heating operation was performed (like forging or welding) that could adversely affect the material. If you are simply threading or cutting the material, there is no need to re-solution anneal.
Will there be a problem if you use Class 1 bolts instead of Class 2?
@Marvin – Class 2 bolts are much stronger than class 1 bolts, so if the design calls for class 2, and you substitute class 1, then yes, you may have a problem. You should consult with the project engineer to see if this change is acceptable.
With the machining bar stock method, after finished machining does a bolt still need to do the “carbide solution treated”?
@Nipolw- It depends on the grade of A193 you are making. The “A” grades, that is the grades that end in “A” like B8MA require carbide solution treating regardless of the manufacturing method used. With more common grades, like B8 and B8M, you shouldn’t have to carbide solution anneal if you machined the bolt out of bar.
FYI – Regarding machining Class 2 bolts from larger diameter bar, most of the strain hardening can be put back into the shank by flat rolling and if you thread roll, that by itself can increase the strength enough. Combined with shear and tensile tests to evaluate the amount of reduction required, you can re-establish the properties. Unfortunately, you can do the math to determine the required amount of reduction but it’s really not accurate. Anyway, this was the Old School way of handling it.
Derek, thanks for your help and explanation.