Penetrant testing uses a liquid dye (visible red or fluorescent) that seeps into surface-breaking discontinuities by capillary action. PT is a surface method — the symbol has arrow-side and other-side significance. Symbol below the reference line means arrow side, above means other side, per A2.4 §17.5.2 / §17.5.3. PT works on any material, making it the go-to surface method for non-ferromagnetic metals.
PT Symbol — Arrow Side (Below Reference Line)
Below ref line — arrow-side exam
Combined — weld then inspect
Method
How Penetrant Testing Works
PT applies a liquid penetrant — either a visible red dye or a fluorescent penetrant — to the cleaned weld surface. The penetrant is drawn into any surface-breaking discontinuity by capillary action. After a specified dwell time, excess penetrant is removed from the surface and a developer (typically a white powder or spray) is applied. The developer draws the trapped penetrant back out of the discontinuity, creating a visible indication against the contrasting background.
Because PT relies on physical access to the surface, it is classified as a surface method. The examination is performed on a specific side of the joint, so the symbol has arrow-side and other-side significance. Per A2.4 §17.5.2, PT below the reference line means arrow side. Per §17.5.3, PT above the reference line means other side.
Material compatibility: PT works on any material — ferromagnetic, non-ferromagnetic, stainless steel, aluminum, nickel alloys, copper alloys, and titanium. This is the key advantage over MT, which is limited to ferromagnetic materials.
D1.1:2025
When D1.1 Specifies PT
Non-ferromagnetic materials: When surface examination is required on stainless steel, aluminum, or other non-ferromagnetic metals, PT is the only applicable surface method because MT cannot be used.
Completed welds: PT may be specified for examining completed welds where only surface-breaking discontinuities are of concern. The engineer of record places PT on the NDE symbol to require the examination.
Repair welds: After a weld defect is removed and repaired, PT can verify that the repair weld surface is free of cracks or other surface-breaking indications before the joint is accepted.
PT cannot replace RT or UT for detecting internal (subsurface) discontinuities. It is strictly a surface method. On ferromagnetic materials where both surface and near-surface detection are needed, MT is generally preferred over PT.
Limitations
What PT Cannot Detect
PT detects only discontinuities that break the surface. If a crack, porosity, or lack of fusion is entirely subsurface — even just below the surface — PT will not find it because the penetrant has no path to enter the discontinuity. For near-surface detection on ferromagnetic materials, MT is the appropriate method. For volumetric (full-thickness) detection, RT or UT is required.
Surface condition also matters: heavily oxidized, scaled, or painted surfaces can mask openings and produce unreliable results. The surface must be properly cleaned before PT application per the applicable procedure.
CWI Exam Tip: PT and MT are both surface methods with side significance (below = arrow side, above = other side). The key difference: PT works on ALL materials but only finds surface-breaking flaws. MT works only on ferromagnetic materials but finds both surface and near-surface flaws. If the question says "stainless steel" or "aluminum," PT is the answer — MT is not an option.
Common Questions
PT Symbol FAQ
What does PT mean on a welding drawing?
PT on a welding drawing stands for penetrant testing, a surface nondestructive examination method defined in AWS A2.4:2020 Table A6. Unlike volumetric methods such as RT or UT, PT has arrow-side and other-side significance. When you see PT below the reference line, the examination is required on the arrow side of the joint. When PT appears above the reference line, the examination is required on the other side. Per A2.4 sections 17.5.2 and 17.5.3, the placement follows the same side-significance rules as welding symbols.
What is the difference between PT and MT?
Both PT (penetrant testing) and MT (magnetic particle testing) are surface examination methods with arrow-side and other-side significance, but they differ in capability and material compatibility. PT works on any material including non-ferromagnetic metals such as stainless steel, aluminum, and nickel alloys, but it can only detect discontinuities that break the surface. MT works only on ferromagnetic materials such as carbon steel and low-alloy steel, but it can detect both surface-breaking and near-surface discontinuities. When the material is ferromagnetic, MT is generally preferred because of its ability to find near-surface flaws that PT would miss.
What does PT detect in welds?
PT detects surface-breaking discontinuities only. These include cracks, porosity that is open to the surface, lack of fusion at the weld surface, crater cracks, and laps or seams in the base metal. A liquid penetrant is applied to the surface and drawn into any opening by capillary action. After a dwell time, excess penetrant is removed and a developer is applied to draw the trapped penetrant back out, creating a visible indication. PT cannot detect subsurface discontinuities or discontinuities that do not break the surface, because the penetrant has no path to enter them.
When is PT used instead of MT?
PT is used instead of MT when the material being examined is non-ferromagnetic. MT requires a ferromagnetic material to establish the magnetic flux needed for detection, so it cannot be used on stainless steel, aluminum, nickel alloys, copper alloys, or titanium. On these materials, PT is the primary surface examination method. PT is also used on repair welds and for examining completed welds where only surface-breaking discontinuities are of concern. When the material is ferromagnetic, MT is generally preferred because it can also detect near-surface discontinuities that PT would miss.