2. Objectives
• Discuss the purpose of the “back”
and “melt-thru” welds
• Determine the size of a backing
weld
• Determine the size of a melt-thru
weld
4. Back or Backing and
Melt-Thru Welds
• Frequently used with groove welds
to strengthen the weld
• Back welds differ from backing
welds by application sequence
– Back weld
• Groove weld made; backing is applied
– Backing weld
• Backing weld applied first, then groove
5. Back or Backing and
Melt-Thru Welds (cont’d.)
• Symbol for back and backing weld
is the same
– Melt-thru weld shown as solid
• When used with groove welds
– Symbols placed on side of reference
line
– Opposite groove weld symbol
6.
7.
8. Size of Backing
and Melt-Thru Welds
• Dimensions referring to backing
and back welds not applied
• Height of reinforcement dimension
applied to melt-thru symbol
– Placed to left
• When necessary, shown on detail
drawing
– Included in tail of welding symbol
9.
10. Contour and Finishing
• Contour symbols and finish
abbreviations added to symbols
• Mechanically finished convex
contour
– Convex contour symbol and finish
abbreviation added to symbol
11.
12. Selected Applications of
Back or Backing Symbols
• Multiple reference lines distinguish
back and backing welds
• Reference included in tail to specify
back or backing weld
– Except when multiple reference lines
used
13. Selected Applications of
Melt-Thru Symbols with
Groove Symbols
• Melt-thru symbol used with groove
weld
– Also used for flange welds
• Used in same manner for flange
and groove welds
– Has same significance
14.
15.
16.
17. Summary
• Back, backing and melt-thru welds
strengthen the weld
• Back and backing welds differ only
in application sequence
• Back and backing symbols identical
– Melt-thru symbol shown as solid
• Multiple reference lines distinguish
back and backing welds
