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“Never apply Electro Galvanizing process to grade 12.9 fastener, if you would like to, apply it on fasteners grade up to 10.9” I have heard this form my senior at seminar when I was first-year employee to this business.
This is a big lesson for us because if we apply Electro Galvanizing process on 12.9 fasteners, fasteners head will tear off the bolt or even cause something scary called “Delayed fracture” that will occur later on after fastened. I will separate this content into 2 parts.
For this part, I will explain about “What is the cause of hydrogen embrittlement?”
What is Hydrogen Embrittlement?
When fasteners are Electro-galvanized (As we called, plating) for rust prevention purpose, but plating process always including pickling process to remove rust and scale from the metal surface.
Hydrogen gas generated during the pickling process, this gas will get into metal material causing material became losing its flexibility and finally, became brittle. This phenomenon is called, Hydrogen Embrittlement.
What is delayed fracture?
Delayed fracture is a phenomenon when fastener under certain tensile load is suddenly break without any caution after certain period of time. In other words, there will be no problem at the time when fastener is fastened, but after several hours or days, the fastener head will pop out like a bullet and break.
In the past, Hydrogen Embrittlement was not widely known. There are many applications where Electroplated 12.9 fasteners widely used in the industry. Sometimes, there are cases that fasteners broke during machine operation or fall off the truck while they are in transport.
The delayed fracture is likely to occur with fasteners with a strength class 10.9 or higher because when the stress that applied to the fasteners exceed 1,000Mpa, it will have a possibility of Hydrogen Embrittlement.
In other words, if fasteners with a 4.8 strength class, it be free of Hydrogen Embrittlement risk because of tightening torque are not as high as 10.9 or 12.9 fasteners.
Baking treatment is a solution to prevent Hydrogen Embrittlement
The cause of steel became brittle and got delayed fracture is from Hydrogen Embrittlement entered the steel. The solution for this problem is to heat the plated material at 190 C° for 2-4 hours to remove the Hydrogen. This process called, Baking.
However, even with this baking treatment, delayed fracture still remains for some reason. So it is common that do not plate 12.9 fasteners for safety reason.
Next chapter, we will go deeper into the reason why metal become brittle when hydrogen entered the material.