14.9 and 14.8U fasteners have similar tensile strength. Below is a table showing the requirements of two bolts.

	14.9 (JIS B 1051 1976)	14.8U (VDA 235-205)
Tensile Strength Mpa	1373-1569	1400-1570
Yield Strength Mpa	>1236	>1120
Hardness HRC	40-49	42-48
Breaking Elongation	>7%	>10%

 From the table, we can see the main differnces are at yield strength and breaking elongation.

14.9 must have higher yield strength, and 14.8U must have greater elongation.

Sun Through did tensile tests on M12x90mm bolts, one in 14.8U grade and the other in 14.9 grade. Same material, only different way of heat treatment. And below is the stress-strain graphs for them.

By observing the graphs, we can note that:

1. The 14.8U bolt broke at around 4.4mm displacement, while 14.9 broke at around 3.8mm displacement. 14.8U does have higher toughness than 14.9.

2. Ultimate tensile stress of 14.8U happened at around 2.7mm displacement, and it kept streching after the tensile point. 14.9 broke just around the tensile point which was around 3.8mm displacement. This means in real situation, people have more time to observe when the 14.8U bolts are reaching to their limit, and 14.9 tends to be a sudden failure. 

3. 14.8U broke at 4.4 displacement, and the bolt length is 90mm. It seems that the breaking elongation can be derived by 4.4/90 = 4.9%, and it did not meet the requirments. However, the M12 bolts are partially threaded, and the threaded area was the only part that deformed and caused displacemeny. The thread length of M12 bolts are 36mm. Here the corrected elongation for 14.8U bolts is 12.2%, and 10.6% for 14.9 bolts. Both of them meets their respective requirements. 

Feel free to contact Sun Through if you have feedbacks or questions on high strength fasteners.