Effect of temperature on an a325 bolt

[ucfmech]

My company uses ASTM a325 medium carbon bolts in the manufacturing of our components. Recently, a customer has been complaining of these bolts breaking on his part. I believe it is because of the high temperatures these bolts are being exposed to. These bolts are 1" in diameter and are possibly being heated to between 800 and 1200 degrees. Is there some way to corelate the ultimate strength of the bolt to the temperature it is being exposed to? I'm thinking there should be a chart somewhere, but I have yet to find it. Thanks in advance for any help.

 

[metengr]

ucfmech;
My recommendation is to send one or more of the failed bolts to a lab for proper metallurgical analysis of the failures. There can be several scenarios that can cause failure of these bolts - fatigue or tensile overload from loss of strength upon exposure to elevated temperature. Get the failure analysis completed and go from there.

 

[ucfmech]

We know these bolts are placed under a large torque, and the operating temperatures on his component are much higher than usual do to specific design requirements he had. Unfortunately, we don't have the time to send these to a lab, as he is runnning an asphalt plant, and every hour day that he is stopped from production, a significant amount of money is lost. So, we're trying to provide a quick answer to him as to waht he should do, and hopefully an easy one.

 

[metengr]

ucfmech;
If you are running at elevated service temperatures you need to switch to a stainless steel grade bolt - 304. Spec out ASTM A 193 Grade B8.

 

[TVP]

1. Definitely do as metengr suggests and have a proper failure analysis performed including metallurgical evaluation. This is the only way to have definitive answers.

2. In general, carbon steel fasteners such as those manufactured according to ASTM A 325 are limited to a maximum use temperature of 250-450 F (IFI Fastener Technology Handbook, NASA RP 1228, etc.). Depending on the exact grade and tempering temperature used, the change in strength vs. temperature will be in the following range:

at 800 F --> YS is ~ 70% of RT value, UTS is ~ 75%
at 1200 F --> YS is ~ 25% of RT value, UTS is ~ 35%

This is not the same as accounting for stress relaxation/creep, which depends on the specifics of the joint, but suffice it to say, any increase in temperature beyond 450 F, especially if it is continuous, will be extremely detrimental to these fasteners.