What does magnetic particle testing find that ultrasonic testing misses on a boom weld?

When a crane boom weld fails in the field, the consequences go well beyond a repair bill. Projects stall, loads stay grounded, and the safety of everyone on site is at risk. That is why boom weld inspection is never a one-size-fits-all exercise. Two of the most widely used non-destructive testing methods in crane maintenance are magnetic particle testing (MT) and ultrasonic testing (UT) — and while both fall under the umbrella of crane NDT, they do not detect the same things. Understanding where each method excels, and where it falls short, helps you make smarter decisions about how your booms are inspected and maintained.

The short answer to the question in the title is this: magnetic particle testing finds surface and near-surface cracks that ultrasonic testing can miss entirely, especially in the weld toe area and heat-affected zones. But the full picture is more nuanced than that, and it has real implications for how inspection protocols should be structured on high-grade steel booms.

How each method reads a boom weld differently

Both MT and UT are proven non-destructive testing techniques, but they work on completely different physical principles. Magnetic particle testing works by magnetizing the steel in and around the weld, then applying fine ferromagnetic particles to the surface. Any discontinuity — a crack, a lap, a lack of fusion at the surface — disrupts the magnetic field and causes the particles to cluster visibly at the defect location. It is a direct, visual method that gives the inspector an immediate read on surface condition.

Ultrasonic testing, by contrast, sends high-frequency sound waves into the material and listens for echoes. When a sound wave hits a void, inclusion, or crack inside the weld, it reflects back to a transducer, and the inspector reads the signal on a screen. UT is powerful for finding volumetric defects buried deep inside the weld — things you would never see from the surface. The trade-off is that interpreting the signals requires significant skill, and the method has known limitations close to the surface of the material.

Surface and near-surface defects MT catches first

Magnetic particle testing is the go-to method for surface crack detection on boom welds, and for good reason. The weld toe — the point where the weld metal meets the base material — is one of the highest-stress locations on any crane boom. Fatigue cracks almost always initiate here, and they start small. MT can detect a crack that is just fractions of a millimeter wide at the surface, long before it has grown deep enough for UT to detect it reliably.

The types of crane boom weld defects that MT catches most effectively include:

Fatigue cracks at the weld toe and weld root
Hot cracks and cold cracks that open to the surface during or after welding
Lack of fusion defects where the weld metal has not bonded properly at the surface
Grinding cracks introduced during post-weld dressing
Hydrogen-induced cracking in high-strength steel, which often appears at or near the surface

This last point is especially relevant for booms made from high-grade steel in the 960 and 1100 N/mm² range. These materials are more susceptible to hydrogen cracking, and because the cracks can appear hours or even days after welding, a timely MT inspection after the weld has cooled is a critical part of any responsible repair process.

Where ultrasonic testing loses resolution near the weld surface

UT is exceptionally good at finding internal defects — porosity clusters, slag inclusions, and cracks that run through the body of the weld. But it has a structural limitation near the surface of the material known as the dead zone or near-surface resolution limit. This is the region directly beneath the transducer where the initial pulse and its echo overlap, making it difficult or impossible to distinguish a real defect signal from background noise.

In practical terms, this means UT can struggle to reliably detect defects that sit within the first few millimeters of the weld surface — precisely the zone where fatigue cracks in crane booms tend to originate. The exact depth of this dead zone depends on the equipment, the transducer frequency, and the skill of the operator, but it is a known and documented limitation of the method.

There is also a geometry challenge. Crane boom welds are often made in tight sections with complex profiles. Curved surfaces, weld cap geometry, and restricted probe access can all reduce UT’s effectiveness in ways that do not affect MT at all. For a straight, flat plate weld in a controlled workshop setting, UT performs very well. On an actual crane boom in the field, conditions are rarely that clean.

Why boom weld inspections require both NDT methods

The conclusion is straightforward: MT and UT are complementary, not interchangeable. Relying on one method alone leaves a gap in your inspection coverage that a defect could slip through undetected.

A robust boom weld inspection protocol uses both methods in sequence:

MT first — to screen the entire weld surface and heat-affected zone for any cracks or surface-breaking defects. This is fast, cost-effective, and highly sensitive to the defects most likely to cause in-service failures on crane booms.
UT second — to investigate the internal volume of the weld, confirm the depth of any surface defects found by MT, and identify any subsurface inclusions or voids that MT cannot reach.

When a third-party Notified Body is involved in verifying a repair, both methods are often required as part of the certification process. This is not bureaucratic overlap — it reflects the reality that no single NDT technique covers the full defect spectrum in a high-stakes structural weld. On booms manufactured from ultra-high-strength steels, where the consequences of a missed crack are severe, using both methods is simply responsible engineering practice.

If MT finds something, UT helps characterize it. If UT finds something, MT confirms whether it breaks the surface. Together, they give inspectors and engineers the full picture needed to make a confident, well-informed decision about whether a weld is fit for service or requires repair.

How Rusch Cranes helps with boom weld inspection and repair

We carry out 100% visual inspection and 100% MPI (Magnetic Particle Inspection) on all new welds as a standard part of every crane boom repair we perform. When the repair scope or the client’s certification requirements call for it, we also bring in a third-party Notified Body to conduct ultrasonic or X-ray testing. This means every repair we deliver has been verified by the right combination of NDT methods for the specific weld, material grade, and application involved.

Here is what that means in practice for our clients:

Repairs on booms up to 1100 N/mm² steel, backed by a full Welding Procedure Specification and Repair Plan
Complete photographic documentation of every defect and every repair performed
CE testing validity maintained after repair, with a 1-year guarantee on all work
Access to both on-site repair and workshop-based repair in Medemblik, Netherlands
Global deployment for boom repairs — we handle all logistics, visas, and customs paperwork

If you have a boom weld that needs inspection, repair, or a second opinion, get in touch with our team and we will help you figure out the right next step.

Frequently Asked Questions

How do I know whether my crane boom weld needs MT, UT, or both?

The decision depends on the repair scope, the steel grade, and what you are trying to verify. As a baseline, MT should always be performed on boom welds because surface and near-surface fatigue cracks are the most common failure mode. UT should be added whenever the weld is in a structurally critical location, the boom is made from high-strength steel (such as 960 or 1100 N/mm²), or a Notified Body is involved in certifying the repair. When in doubt, using both methods is the responsible choice — the cost difference is small compared to the risk of a missed defect.

Can MT be performed on a boom that is still assembled on the crane, or does it need to be disassembled first?

MT can often be performed in situ, which is one of its practical advantages for field inspections. The inspector needs adequate access to the weld area to apply the magnetic yoke and the ferromagnetic particles, and the surface must be clean and free of paint or coatings that would mask a crack indication. In some cases, localized surface preparation — such as grinding back a paint layer — is needed before MT can give reliable results. Your NDT technician will assess access and surface condition before starting.

How soon after a boom weld repair should MT inspection be carried out?

For standard structural steels, MT is typically performed once the weld has cooled to ambient temperature. However, for high-strength steels in the 960–1100 N/mm² range, hydrogen-induced cracking can develop hours or even days after welding as hydrogen diffuses through the heat-affected zone. For these materials, a delayed MT inspection — often performed 24 to 48 hours after welding — is considered best practice to ensure any hydrogen cracks that have developed are caught before the boom is returned to service.

What qualifications should an NDT technician have to inspect crane boom welds?

NDT technicians inspecting structural crane welds should hold a recognized certification such as PCN (Personnel Certification in Non-Destructive Testing) or EN ISO 9712 at Level 2 or higher in the relevant method — MT or UT. For UT on complex weld geometries or high-strength steels, Level 2 certification with documented experience on similar components is the minimum you should expect. When a third-party Notified Body is conducting the inspection for certification purposes, their personnel qualifications are part of the accreditation they operate under, which gives you an additional layer of assurance.

What happens if MT finds an indication but UT cannot confirm it — is the weld still safe?

An MT indication that UT cannot confirm should not be dismissed — it should be investigated further rather than assumed to be a false positive. MT is highly sensitive to surface discontinuities, and a visible particle cluster is a meaningful signal even if UT does not show a corresponding subsurface echo. The next step is typically a closer visual examination, surface grinding to characterize the indication, or re-inspection with a different UT probe configuration. A qualified welding engineer or NDT Level 3 specialist should be consulted before the boom is returned to service.

Are there any boom weld scenarios where MT alone is sufficient, without needing UT?

MT alone may be sufficient for routine in-service inspection of lower-risk welds on standard-grade steel booms, where the goal is to screen for fatigue cracking between scheduled maintenance intervals. However, for any repair weld — particularly on high-strength steels or structurally critical sections — MT alone is generally not enough to satisfy engineering sign-off or third-party certification requirements. UT is needed to confirm that the internal volume of the weld is sound and to characterize the depth of any surface defects found by MT.

What common mistakes should be avoided when setting up a boom weld inspection protocol?

The most common mistake is treating NDT as a box-ticking exercise rather than a defect-finding process — for example, performing MT on a painted surface without adequate preparation, or using UT without verifying that the probe configuration is suited to the weld geometry and material thickness. Another frequent error is skipping the delayed MT inspection on high-strength steels, which risks missing hydrogen cracks that develop after the initial inspection. Finally, relying on a single NDT method for a high-stakes repair weld, rather than combining MT and UT, leaves a documented gap in your inspection coverage that could have serious consequences if a defect is missed.

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