Hai,
We are one of the Automotive component suppliers. We had a complaint of screw head(low carbon, cold forged,bought-out from sub vendor) breakage in one of our model. Currently we are perfoming Hydrogen De embrittlement process for the screw to overcome the above said problem, but our customer asks for some type of verification test to ensure that de embrittlement has been done on screws and checking to be carried-out daily i.e., for every lots. Now we are conducting destructive test to find out the torque at which screw head breaks. Can anyone specify en easier test method for ensuring embrittlement is done on screws without breaking the part.
regards,
Guru Prasad
Prasad:
I have been experiencing the same problem from a customer's perspective. We had a batch of screws suddenly start breaking after days of installation. This has required me to learn about the problem.
The other replies are correct about the hydrogen atoms collecting in the screw substrate. According to my research, the atoms begin to find places where they can combine into Hydrogen (H2) mollecules. This increases pressure and then can cause screw failure. The hydrogen will collect preferentially to areas of higher strain. For example, screw heads are never perfectly perpendicular to the screw shaft, or the hole they are being screwed into are not perfectly perpendicular to the surface under the head. This will put a torque on the head, increasing the strain on one side of the screw. The hydrogen pressure builds up there, and that will probably be the site of the initial crack. A lot of this is theory, of course, since there is no way to measure the hydrogen level in the screw. Also, be aware that hydrogen embrittlement may cause screw failure days, weeks or even months after plating.
The standard test for hydrogen embrittlement is to have a plate with holes in it. You put a standard washer under the head, and torque the screw down to some value you think is right. Then you wait 24 hours to see if any heads come off. This is, of course, a destructive test, so you do it on a sample of a batch of screws.
We have come up with a test that is a little tougher on the screw, and which I believe is a good accelerator for screw failure. We make a bar with screw holes drilled and tapped. The surface where the head will seat is machined down at a 3deg. angle from perpendicular to the hole. This causes a reliable strain in the screw. Then we screw it down with a washer, at the torque with which it will be screwed in service. We wait for 24 or 48 hours, then we torque each screw until it breaks. The break force will tell us whether we have a good batch. Batches with bad hydrogen embrittlement will break with almost no torque. Some of the heads just fall off. Good batches will have high torque. The level that is good will be found by testing several batches with known good and bad HE.
I have found that the critical factors for making a good screw are:
1. Minimize the time between the wet baths and the oven. It is best to put the parts directly in the hot oven when they come out of the last bath, but probably before chromate, as the color will be poor if you chromate before bake.
2. The literature that I read indicates much longer baking time. We were having problems at 4 hours of baking, but found the parts much stronger after 8 hours.
3. High hardness screws have the problem worse.
4. Carburized screws have the probelm worse.
Good Luck.
Mike
