Dear Readers,
Generally the grades of
bolts are starts from 4.6 to 12.9. Also known as Ordinary bolt (Commercial
Grade), HT Bolt (High Tension) &
HSFG Bolts (High
Strength Friction Grip).
The bolt designation system is based on two numbers. The first number represents the ultimate
tensile strength of the bolt, 400 MPa, 800 MPa or 1200 MPa respectively. The
second number represents the point at which the bolt will permanently stretch. A 4.6 grade
bolt permanently
stretches at 60% of its ultimate, an 8.8
grade bolt
at 80% and a 12.9 grade bolt at 90 % of its
ultimate.
Technically
it can be quoted that the first number is the tensile
strength of the bolt material (N2 )/100.
The second number is =
1/100.(the ratio of the Proof (or Yield ) stress and the Tensile strength
expressed as a percentage = 100.[Yield (Proof stress) /Tensile strength] /100
For Example the tensile and proof strength of the steel for a 8.8 bolt is therefore calculated as follows
Tensile strength = 8 X 100 N/mm2 = 800 N/mm2
and Proof
strength = 0.8*800 * 100 /100 = 640 N/mm2 (generally
speaking 80% of 800N/mm2)
The
lowest grade is 4.6, commonly known as commercial grade. Next comes grade 8.8,
known as structural grade, and finally, the highest grade is 12.9, known as HT / HSFG bolt.
Available grade
of bolts are 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 & 12.9
Generally foundation bolts are made from 4.6 or 4.8 Grade
Hardness of bolts are as follows
