Understanding the Backing Weld Symbol: A Comprehensive Guide

Welding symbols serve as a form of shorthand, conveying the necessary instructions from designers and engineers to welders. One critical symbol in this visual language of welding is the ‘Backing Weld Symbol‘. In this blog post, we will delve …

Permanent Backing-bar-weld-symbol

Welding symbols serve as a form of shorthand, conveying the necessary instructions from designers and engineers to welders. One critical symbol in this visual language of welding is the ‘Backing Weld Symbol‘. In this blog post, we will delve into the specifics of what the backing weld symbol represents, how it’s used, and why it’s so important.

What is a Backing Bar Weld?

Before we dive into the symbol itself, let’s first understand what a backing weld is. A backing weld or also called Weld Backing Bar is a type of support (either metal or ceramic) that’s used to support or reinforce a main weld. It’s often applied to the backside of a single groove weld to ensure complete joint penetration or to facilitate the welding process.

Backing Weld Symbol

Permanent Backing Welding Symbol
  • The backing symbol, positioned either above or below the reference line, signifies the use of a backing ring, strip, or a consumable insert.
  • To prevent confusion with a plug or slot weld, this symbol must always be used in conjunction with a groove weld symbol.
  • For a permanent backing bar, ‘MR’ is placed within the Backing weld Symbol as shown in above example.
  • If the backing is intended to be taken away after the welding process ( i.e., a removable Backing), an ‘R‘ is incorporated within the backing symbol. In AWS Backing Bar Symbol, a Backing Symbol Without ‘R’ means permanent Backing Bar to be used.
  • The type, material, and dimensions of the backing should be clearly detailed either in the tail of the welding symbol or directly on the drawing itself.
Backing weld symbol
Removable Backing Bar Welding Symbol

ISO 2553 Backing Bar Welding Symbol

In the ISO 2553 welding symbology, as depicted on the right side of the above figure, the backing weld symbols are defined as follows:

  • MR‘ signifies a removable backing.
  • M‘ indicates a permanent backing. Be aware that this is contrary to the AWS convention, where a backing symbol without an ‘R‘ signifies the use of a permanent backing bar. Careful attention is required when interpreting these symbols due to this difference.

Importance of Backing Bar Welding Symbol

The backing bar weld symbol is used to indicate that a piece of backing material is to be placed at the root of the joint to facilitate welding. This backing material (typically a metal bar) ensures proper penetration and fusion at the root of the joint and provides support for molten metal during the welding process.

See also  Fillet Weld Symbol

It’s important to note that the backing bar weld symbol should be used in conjunction with a groove weld symbol to avoid confusion with a plug or slot weld. Additional specifications for the backing bar (such as type, size, or material) can be provided in the tail of the welding symbol or directly on the drawing itself.

Conclusion

In conclusion, the backing bar weld symbol holds a crucial place in welding symbology, signifying the need for a backing bar in the welding process. Its placement, either above or below the reference line, indicates the side of the joint where the backing bar should be applied, providing clear instructions for welders.

Its correct use, in tandem with a groove weld symbol, ensures proper communication and prevents confusion with other weld types like plug or slot welds. The backing bar weld symbol underscores the importance of accurate and effective communication in the welding industry, enabling the production of safe, reliable, and high-quality welded structures.


Kumar is a highly experienced welding professional and author known for his expertise in welding symbols. With certifications including IWE, AWS-CWI, and ASNT NDT Level III.