Hydrogen embrittlement is a metal’s loss of ductility and reduction of load bearing capability due to the absorption of hydrogen atoms or molecules by the metal. The result of hydrogen embrittlement is that components crack and fracture at stresses less than the yield strength of the metal.
Causes leading to hydrogen absorption
Hydrogen can enter and diffuse through steel even at room temperature. This can occur during various manufacturing and assembly operations or operational use – anywhere that the metal comes into contact with atomic or molecular hydrogen.
In our standard process for which there is a possibility of absorption of hydrogen is electroplating. During electroplating, hydrogen is produced at the surface of the metal being coated. Electroplating is used to deposit zinc or cadmium on the socket set screws for corrosion protection of the steel.
Hydrogen absorption can also occur when a socket set screw is in service if the steel is exposed to acids or if corrosion of the steel occurs.
Requirements for failure due to hydrogen embrittlement
There are three requirements for failure due to hydrogen embrittlement:
- A susceptible material.
- Exposure to an environment that contains hydrogen.
- The presence of tensile stress on the component.
High-strength steels with a hardness of R/C 32 or greater are the alloys most vulnerable to hydrogen embrittlement. Although electroplated socket set screws in alloy steel fall in that category, they are designed to be used in a compression application deeming tensile stress absent, thereby making socket set screws an exception to the rule.
Prevention of hydrogen embrittlement
Baking socket set screws after electroplating at 375° +/- 25°F within 4 hours after the electroplating process for a period of 23 hours to allow the hydrogen to diffuse out of the metal.
For applications where socket set screws are used in a tensile situation, special attention to the hardness must be taken into consideration to avoid fracture due to hydrogen embrittlement. Alloy socket set screws have a standard hardness of R/C 45-53. This level of hardness does not allow any ductility in a tensile situation. It’s just too hard. By lowering the hardness will allow the part to be more ductile. That being said, our electroplated socket set screws are always baked within four hours after they are plated for a minimum of 23 hours.
Finally, there is no test for hydrogen embrittlement for socket set screws. The reason is that socket set screws are designed for a compression application making hydrogen embrittlement not applicable. The reason for this is when the socket set screws are put in compression the hydrogen molecules (if present) will not be activated. The hydrogen will only be activated in a tensile application and socket set screws are not designed to be put in a tensile application because of the high hardness.
Ring gages used for gaging of externally threaded parts, aid in establishing physical limits for Maximum Material size and thread function. In addition, ring gages can help detect a lack of roundness or surface discontinuities on the threads.
A Go ring gage simultaneously checks the thread elements of form, lead and pitch diameter in a cumulative manner. It is manufactured to the high limit of the thread tolerance allowing for clearance of the minor diameter and major diameter of the screw. A Not Go gage is produced to the low limit of the thread tolerance, and ensures that the size is larger than the minimum pitch diameter of the screw.
When using a Go gage, a correctly sized screw thread will pass completely through the gage with little or no effort, whereas an oversized screw thread will not pass through the gage. Conversely, a part must stop in a Not Go gage within three turns. If it passes through the Not Go gage, it is considered undersized and is unacceptable. Because ring gages tend to have frequent contact with work pieces and are exposed to the everyday shop environment, it is important that they maintain a strict calibration schedule.
Thread setting plugs used to calibrate ring gages can be found in “Truncated” or “HiLo” designs. With the truncated setting plug version, the ring gage is adjusted to fit snugly on the truncated section of the plug, then carefully screwed onto the full form section. A properly fitting ring gage will have little or no difference in the feel while passing from truncated to full form. Reliability on the accuracy of human judgment or good feel can be a concern using truncated setting plugs.
The HiLo setting plug version removes most of the uncertainty by allowing the setting of ring gages while taking the judgement factor out of calibrations. The “Lo” end of the setting plug member represents the low tolerance limit while the “Hi” end of the plug member represents the high tolerance limit.
Adjusting the ring gage so it will accept the Lo or front of the plug with a drag without threading more than one and one half turns onto the Hi or back section of the setting plug, will achieve the correct setting. Areas the HiLo setting plug examines are worn flanks, angle checks and bellmouthing conditions.
Keeping ring gages clean and free of debris is very important. Oil, chips..ect. can aversely affect the performance of the ring gage not only during calibration, but also in use inspecting screw threads. Using bristle tube brushes frequently, which relate in size to the major diameter of the ring gage and compressed air, will help keep it from “loading” up with debris while in use.
Set screws are designed to provide torsional holding power in a compressive force application. The tighter they are applied, the more holding and less prone to vibration they become. If excessive torque is used during these applications, reaming or cracking of the socket can occur. Hence there is a need to perform torque testing to standard specifications to achieve the recommended torque per ASTM F880 which is used for stainless and ASTM F912 is used for alloy.
High hardness values are required for alloy set screws during heat treatment. It is imperative that a balanced heat treating is achieved that will result in strength of the socket. Too hard and the socket could crack, too soft and the socket may ream under the stress of the hex or spline bit used during testing.
Set screws should never be used under Tensile applications. Using as a stud or with a nut will place tensile stress on the set screw which opens the door to cracking, fracture, and failure due to the high hardness range to which it is heat treated.
A typical torque test fixture for testing socket set screws would involve a torque wrench with the proper socket sized bit, applying force in a vertical manner to a test piece installed in a threaded testing block. A backing screw is inserted in the opposite end of the block to provide a hard stopping point for the set screw being tested. Once the test reaches the minimum torque recommended by the specification, the set screw is considered acceptable.
Since this is a destructive form of testing, approval is based on acceptance sampling of the entire lot and not performed at 100 percent.
Tensile strength is the ability of a material to resist tearing. It’s defined as the amount of force that can be applied on the material before it tears apart.
Yield strength is the amount of stress a piece of steel must undergo in order to permanently deform.
Tensile strength is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking. Whereas, Yield strength is the stress a material can withstand without permanent deformation or a point at which it will no longer return to its original dimensions (by 0.2% in length).
Elongation stands for a mechanical property of metal. The degree to which a material is supposed to be delivered, stretched, or compressed prior to rupture. It is a point among tensile strength and yield strength and is presented as a percentage of the original length.
Proof load is defined as the maximum tensile force that can be applied to a bolt or screw that will not result in plastic deformation. The material must remain in its elastic region when loaded up to its proof load. Proof load is typically between 85-95% of the yield strength.
Self-locking fasteners allow reusability while preventing the problems of accidental disassembly when pre-load is lost.