The Fundamental Rule
BS EN ISO 9606 1 is explicit:
- Butt welds do not qualify fillet welds.
- Fillet welds do not qualify butt welds.
They are separate weld types. However, Clause 5.4 e introduces a controlled mechanism whereby a butt weld can qualify fillet welds. This is achieved through a supplementary fillet weld test piece via a structured qualification route.
What the Standard Requires
Where a butt weld is used to qualify fillet welds, the welder must weld a supplementary fillet test piece that:
- Is a minimum of ten millimetres thick
- Is completed in a single layer
- Is welded in the PB position
- Is carried out for each process and filler material group
Once this supplementary test is completed successfully, the standard states, “The welder shall be qualified for all fillet welds as given for the butt weld qualification variables related to the range of qualification for fillet welds.” This sentence is critical because it links the fillet weld approval directly to the butt weld qualification variables.
What This Means in Practice
Consider a welder who completes:
- A butt weld in H L045 position on pipe
- A supplementary fillet weld test piece in PB
The butt weld establishes the qualification variables:
- Material group
- Thickness range
- Outside diameter range
- Welding process
- Filler material group
Once the supplementary fillet requirement is satisfied, those same qualification variables apply to fillet welds. The supplementary PB fillet test is the gateway that unlocks fillet approval. It is not the limiting factor.
Position Approval for Fillet Welds
The supplementary fillet test must be performed in PB as that is a mandatory test condition. However, once Clause 5.4 e is satisfied, the range of approval for fillet welds follows the butt weld qualification variables and the applicable positional tables. In practical certification body interpretation across industry, the fillet weld positional range mirrors the positional range derived from the butt weld qualification. For example: An H L045 butt weld is regarded as an all position pipe test. With a compliant supplementary fillet test, fillet weld approval extends in accordance with the butt weld positional coverage. This is how certification bodies apply the clause in practice.
Why the Supplementary Test Exists
A pipe butt weld demonstrates:
- Root fusion control
- Positional discipline
- Full penetration welding ability
What it does not directly demonstrate is:
- Fillet geometry control
- Throat formation
- Fillet root fusion at ninety degree configuration
The supplementary PB fillet test confirms that the welder can control these specific characteristics. Once that is demonstrated, the butt weld qualification variables govern the range.
Common Industry Mistake
Practical Advice for Engineering Companies
If your production includes both butt and fillet welds:
- Always include the supplementary fillet test at qualification stage
- Ensure the test piece thickness meets the minimum requirement
- Confirm the certificate clearly records fillet weld approval
Under ISO 3834 environments, auditors will expect the link between Clause 5.4 e and the range of approval to be technically justified and documented. Failure to apply this correctly can invalidate welder approval in high consequence sectors such as rail, pressure equipment, and structural fabrication.
Final Technical Summary
A welder who completes a butt weld qualification in H L045 and a compliant supplementary fillet weld test piece is qualified for fillet welds within the range of qualification established by the butt weld variables, in accordance with BS EN ISO 9606-1. The supplementary fillet test activates fillet weld approval, while the butt weld determines the range. Understanding that distinction is critical to compliant certification. Contact NECIT Services for expert welding inspection services for the UK engineering sector and ensure full safety and compliance within your welding processes.
