Magnetic particle testing uses magnetic flux and iron particles to detect surface and near-surface discontinuities in ferromagnetic materials. MT is a surface method — the symbol is placed below or above the reference line to indicate arrow-side or other-side examination per A2.4 §17.5.2 and §17.5.3. D1.1:2025 requires MT or PT for examining backgouged roots before welding the second side of CJP groove welds.
MT Symbol — Arrow Side (Below Reference Line)
Below ref line — arrow-side exam
Combined — weld then inspect
Method
How Magnetic Particle Testing Works
MT applies a magnetic field to the weld using a yoke, prods, or coil. Fine ferromagnetic particles — dry powder or wet suspension — are then applied to the surface. Where a discontinuity interrupts the magnetic flux, flux leakage occurs and the particles accumulate into a visible indication that outlines the discontinuity.
MT detects surface and near-surface discontinuities: cracks, incomplete fusion at the surface, laps, and seams. It is highly sensitive to linear discontinuities oriented perpendicular to the magnetic field. Because the examination inspects from a specific surface, MT has side significance. Per A2.4 §17.5.2, the MT letter placed below the reference line means arrow-side examination. Per §17.5.3, placement above means other-side examination.
Material limitation: MT only works on ferromagnetic materials — carbon steel, low-alloy steel, and ferritic stainless steel. It does not work on austenitic stainless steel, aluminum, copper, or any non-ferrous metal. For non-ferromagnetic materials, use PT (liquid penetrant testing) instead.
D1.1:2025
When D1.1 Requires MT
Backgouged roots (Clause 8.10): Before welding the second side of a CJP groove weld, the backgouged root must be examined by MT or PT to verify complete removal of unsound weld metal.
Intermediate stages: The engineer may specify MT at intermediate stages of completed CJP groove welds to catch surface discontinuities before they are buried by subsequent passes.
Repair welds: MT is commonly specified to examine the excavated cavity before re-welding and to examine the completed repair weld afterward.
MT is faster and cheaper than RT or UT for surface inspection. It provides immediate results in the field with portable equipment, making it the preferred surface method on ferromagnetic materials. For the same surface inspection on non-ferromagnetic materials, PT is used instead.
MT vs PT
Choosing Between MT and PT
Both MT and PT are surface examination methods with side significance. The key difference is material compatibility and detection depth. MT detects surface and near-surface discontinuities but only works on ferromagnetic materials. PT detects only surface-breaking discontinuities but works on any non-porous material — including austenitic stainless steel, aluminum, and nickel alloys.
On ferromagnetic materials, MT is preferred because it is faster (no dwell time required), can find near-surface flaws that PT would miss, and does not require the same level of surface cleaning. D1.1 allows either MT or PT for backgouged root examination — on carbon steel, MT is the practical choice.
CWI Exam Tip: MT is a surface method — the symbol goes below or above the reference line, not centered. This is the opposite of volumetric methods (RT, UT) which are centered. Remember: MT detects surface and near-surface discontinuities only, and it only works on ferromagnetic materials. If the exam question involves stainless steel or aluminum, the answer is PT, not MT.
Common Questions
MT Symbol FAQ
What does MT mean on a welding drawing?
MT on a welding drawing stands for magnetic particle testing, a surface nondestructive examination method defined in AWS A2.4:2020 Table A6. When you see MT on the reference line of an NDE symbol, it means the weld must be examined using magnetic flux and iron particles to detect surface and near-surface discontinuities. Unlike volumetric methods such as RT or UT, MT is a surface method with arrow-side and other-side significance. Per A2.4 section 17.5.2, the MT letter placed below the reference line indicates arrow-side examination. Per section 17.5.3, the MT letter placed above the reference line indicates other-side examination. MT only works on ferromagnetic materials such as carbon steel and low-alloy steel.
Why is the MT symbol placed below or above the reference line?
The MT symbol is placed below or above the reference line because magnetic particle testing is a surface method with side significance. The examination only inspects the surface it is applied to, so specifying which side matters. Per AWS A2.4:2020 section 17.5.2, the letter designation placed below the reference line means the examination is performed on the arrow side of the joint. Per section 17.5.3, the letter designation placed above the reference line means the examination is performed on the other side. This is different from volumetric methods like RT or UT, which are typically centered on the reference line because they penetrate the full joint thickness and side does not affect the result.
What does MT detect in welds?
Magnetic particle testing detects surface and near-surface discontinuities in ferromagnetic materials. When a magnetic field is applied to the weld and fine iron particles are distributed over the surface, discontinuities such as cracks, incomplete fusion at the surface, and laps create magnetic flux leakage that attracts the particles into visible indications. MT is highly sensitive to linear surface discontinuities, especially cracks oriented perpendicular to the magnetic field. It can also detect near-surface discontinuities slightly below the surface, but it does not detect deep internal defects the way volumetric methods like RT or UT do. MT only works on ferromagnetic materials such as carbon steel and low-alloy steel. It does not work on austenitic stainless steel, aluminum, or other non-ferrous metals.
When does D1.1 require magnetic particle testing?
D1.1:2025 specifies MT for several critical inspection points. Backgouged roots on CJP groove welds must be examined by MT or PT before welding the second side, per Clause 8.10. Intermediate stages of completed CJP groove welds may require MT when specified by the engineer. Repair welds often require MT examination of the excavated cavity before re-welding and of the completed repair afterward. MT is preferred over PT on ferromagnetic materials because it is faster, does not require surface-breaking for detection, and can find near-surface discontinuities that PT would miss. The engineer of record specifies MT on the drawing using the NDE symbol with appropriate side significance.