An MPI test, or magnetic particle inspection, is a non-destructive testing method required after crane boom repair to verify that welded joints and repaired steel are free from surface and near-surface cracks, inclusions, and other defects. It is specifically mandated after welding high-grade crane steel because the heat and stress introduced during welding can create microscopic discontinuities that are invisible to the naked eye but capable of causing catastrophic failure under load. The sections below unpack how the process works, what it detects, and what it means for your crane’s certification.
How does magnetic particle inspection actually work on a crane boom?
Magnetic particle inspection works by magnetizing a section of steel and applying fine ferromagnetic particles to the surface. Where a crack or subsurface defect interrupts the magnetic field, the particles cluster visibly at that point, forming an indication that the inspector can identify and record. The process is non-destructive, meaning the crane boom is not damaged or altered during testing.
In practice, the inspector uses either a permanent magnet, an electromagnetic yoke, or a prod technique to introduce a magnetic field into the boom steel. A wet or dry suspension of iron particles, often fluorescent and viewed under ultraviolet light, is then applied to the test area. Any discontinuity in the steel distorts the magnetic flux and causes particle accumulation precisely at the location of the defect. The technique covers both the surface and material up to a few millimetres below it, making it far more reliable than a purely visual check for post-weld quality assurance.
What types of defects can an MPI test detect in crane steel?
MPI testing on crane steel can detect surface-breaking and near-surface defects including weld cracks, lack of fusion, porosity clusters, slag inclusions, cold laps, and fatigue cracks that have propagated under repeated loading. These are precisely the failure modes most likely to develop in heavily stressed boom sections after welding or in-service loading.
In the context of crane boom repair, the most critical defects to catch are:
- Weld toe cracks — hairline fractures forming at the boundary between the weld bead and the parent material, often caused by residual stress or rapid cooling
- Lack of fusion — areas where the weld metal has not fully bonded to the base steel, creating internal voids that reduce load-bearing capacity
- Slag inclusions — non-metallic particles trapped within the weld during solidification, which act as stress concentrators
- Fatigue cracks — propagating cracks resulting from cyclic loading during the crane’s operational life, often invisible until they reach a critical length
- Hydrogen-induced cracking — delayed cracking that can appear hours or even days after welding in high-strength steels, making post-weld inspection timing important
MPI does not detect defects deep within the material. For volumetric inspection of subsurface flaws beyond a few millimetres, ultrasonic testing or radiographic (X-ray) methods are used as complementary techniques.
Why is MPI specifically required after welding high-grade crane steel?
MPI is specifically required after welding high-grade crane steel because steels with yield strengths in the 960 and 1100 N/mm² range are significantly more susceptible to hydrogen-induced cracking and heat-affected zone embrittlement than standard structural steel. The higher the steel grade, the more precise the welding procedure must be and the more critical post-weld inspection becomes.
High-strength crane steels require tightly controlled preheat temperatures, inter-pass temperatures, and post-weld heat treatment. Even small deviations in these parameters can introduce microscopic cracks that only become apparent after the steel has fully cooled. Because these steels are used in the highest-stress regions of a crane boom, any undetected defect carries a disproportionate risk. A standard visual inspection cannot identify subsurface discontinuities, which is why 100% MPI coverage of all new welds is the industry standard for quality assurance in crane boom repair.
The consequence of skipping MPI on a repaired high-grade boom is not merely a compliance issue. An undetected crack in a load-bearing weld can propagate rapidly under operational loading, leading to sudden boom failure with serious safety and liability consequences.
What standards and regulations require MPI testing after crane boom repair?
MPI testing after crane boom repair is required under a combination of European machinery safety standards, crane-specific technical standards, and manufacturer welding procedure specifications. The primary regulatory framework in Europe is the Machinery Directive (now transitioning to the Machinery Regulation), which mandates that repaired lifting equipment must meet the same safety requirements as new equipment before being returned to service.
The most relevant standards include:
- EN 1090 — the European standard for the execution of steel structures, which specifies non-destructive testing requirements for structural welds based on execution class
- EN ISO 17638 — the specific standard governing magnetic particle testing of welds, defining technique, coverage, and acceptance criteria
- EN 13001 — the crane safety standard series, which sets design and verification requirements that repaired cranes must continue to satisfy
- Manufacturer Welding Procedure Specifications (WPS) — major crane manufacturers such as Liebherr, Manitowoc, and Tadano specify approved repair procedures that include mandatory NDT testing requirements
In practice, the specific NDT requirements for a boom repair are also defined within the Repair Plan and WPS prepared before work begins. These documents specify which weld joints require MPI, the inspection sequence, and the acceptance criteria against which indications are evaluated.
How does MPI testing affect crane re-certification after repair?
MPI testing is a mandatory step in the documentation package that supports crane re-certification after boom repair. Without a traceable MPI report confirming that all new welds have been inspected and accepted, a Notified Body or competent person cannot sign off on the crane’s CE conformity or issue a new load test certificate.
When a crane boom is repaired to European standards, the repair process must be documented end-to-end: material certificates, the Welding Procedure Specification, welder qualifications, dimensional measurements, photographic records, and the NDT results. The MPI report forms a core part of this technical file. A Notified Body reviewing the repair may also commission independent ultrasonic or radiographic testing on specific weld joints as additional verification, but the 100% MPI coverage of all new welds is the baseline requirement.
Importantly, when the repair is carried out correctly and all NDT criteria are met, the crane’s CE marking remains valid. The repaired boom is considered to have been restored to its original performance specification, and the crane can return to its full rated capacity. This is a significant commercial advantage over sourcing a replacement boom section, which involves long manufacturer lead times and considerably higher costs.
What should crane operators look for in an MPI-qualified repair provider?
Crane operators should look for a repair provider whose welding and NDT personnel hold current certifications from recognized bodies, whose process is governed by a documented Welding Procedure Specification, and who can provide a full traceability package including MPI reports, material certificates, and photographic records of every repair performed.
Specific qualifications and indicators to verify include:
- NDT Level 2 or Level 3 certified inspectors under EN ISO 9712 or an equivalent scheme — the person performing and signing off MPI results must hold a recognized personal certification
- Approved welding procedures for the specific steel grades being repaired, particularly for 960 and 1100 N/mm² steels, which require specialized high-density welding procedures
- Welder qualification certificates aligned to EN ISO 9606, demonstrating that the individuals performing the welds are qualified for the joint type and material
- Third-party Notified Body involvement for high-criticality repairs, where an independent body verifies the repair and supports re-certification
- ISO 9001 quality management certification, which ensures the entire repair process is governed by a quality system with documented controls
- Experience with the specific crane grades in question — repairing 1100 N/mm² boom steel is not a general welding task and requires a provider with a demonstrable track record
Operators should request copies of the WPS and the NDT inspection report as part of the standard repair deliverables. Any provider unwilling to supply this documentation should be treated with caution, as these records are essential for maintaining the crane’s certification and defending its operational safety record.
How Rusch Cranes approaches MPI testing in crane boom repair
Rusch Cranes integrates MPI testing as a non-negotiable step in every crane boom repair, not as an optional add-on. As one of only three companies in Europe qualified to repair telescopic booms made from 960 and 1100 N/mm² steel, Rusch applies 100% MPI coverage to all new welds on every repair performed, whether in their workshop in Medemblik or at a client site anywhere in the world. Their process delivers the full traceability package needed to keep your crane’s CE certification valid and your operations running.
- Every repair begins with a documented Welding Procedure Specification and Repair Plan specific to the boom and steel grade
- 100% visual inspection and 100% MPI are performed on all new welds upon completion
- Third-party Notified Body ultrasonic or X-ray testing is commissioned where required
- A complete quality file including material certificates, photographs, and NDT reports is delivered with every repair
- All repairs carry a one-year guarantee, and CE testing of the crane remains valid after repair
- Crane boom repair services are available worldwide; on-shore inspection services are available within the Netherlands
If your crane has sustained boom damage or is due for a crane boom repair, contact Rusch Cranes to discuss your situation and receive expert guidance on the repair process, NDT requirements, and re-certification pathway.
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