What is hydrogen cracking and can it affect a repaired crane boom weld?

Welding a crane boom is not like welding a garden gate. The steels involved are extraordinarily strong, the stresses the boom will face in service are enormous, and the consequences of a weld failure are severe. One of the most serious risks in this kind of work is hydrogen cracking — a form of weld cracking that can develop silently, hours or even days after the welding is done, and that is particularly dangerous in the high-strength steel used to build modern mobile crane booms.

Understanding hydrogen-induced cracking matters for anyone responsible for a crane fleet, because it explains why not every repair shop is qualified to touch a crane boom weld — and why the procedures used during crane boom repair are every bit as important as the skill of the welder holding the torch.

How hydrogen enters a weld in the first place

Hydrogen gets into a weld through moisture. It sounds almost too simple, but that is the core of the problem. Water vapour in the air, moisture on the base metal, damp welding consumables, rust, grease, paint residue — all of these can introduce hydrogen into the molten weld pool during the welding process.

At high temperatures, hydrogen dissolves readily into liquid steel. As the weld cools and solidifies, the steel can no longer hold that hydrogen in solution. The gas tries to escape, but if the cooling rate is fast — which it often is with thick, high-strength sections — the hydrogen becomes trapped in the microstructure of the weld and the surrounding heat-affected zone. That trapped hydrogen is the starting point for delayed cracking.

This is why workshop conditions matter so much. Temperature, humidity, the storage of consumables, and the cleanliness of the base material are not just housekeeping details — they are direct variables in whether hydrogen cracking occurs.

Why high-strength steel booms are especially vulnerable

Not all steels respond to trapped hydrogen in the same way. Mild steel is relatively tolerant of it. High-strength steels, particularly those used in modern telescopic crane booms, are far more sensitive.

Mobile crane booms are manufactured from steels with yield strengths of 960 N/mm² and even 1100 N/mm². These grades achieve their strength through specific alloying and heat treatment processes, and that same microstructure makes them susceptible to hydrogen embrittlement. The higher the strength of the steel, the lower the threshold of hydrogen concentration needed to initiate a crack.

There are three conditions that must be present simultaneously for hydrogen cracking to occur:

A susceptible microstructure (present in high-strength steel almost by definition)
A sufficient concentration of diffusible hydrogen in the weld zone
Residual tensile stress in or around the weld

Crane booms tick all three boxes. The steel is highly alloyed, the welds are subject to significant residual stress from the thermal cycle, and any moisture contamination during welding introduces hydrogen. This combination is why welding defects in crane boom repairs can be so consequential — and why the term delayed cracking is used. The crack does not always appear immediately. It can develop over time as hydrogen diffuses through the steel under stress.

When cracking appears — and why it’s often missed

The delayed nature of hydrogen cracking is what makes it genuinely dangerous. Unlike hot cracking, which occurs while the weld is still cooling and is often visible immediately, hydrogen-induced cracking can appear anywhere from a few hours to several days after welding is complete. A visual inspection carried out right after welding may show nothing at all.

The cracks themselves are typically fine and tight, often located in the heat-affected zone just beside the weld rather than in the weld metal itself. They can be subsurface, meaning they are entirely invisible to the naked eye. A boom that looks perfectly repaired may already be compromised.

This is why the timing and method of post-weld inspection is so important. A visual check immediately after welding is not sufficient for high-strength steel. Inspection needs to be delayed to allow any hydrogen cracking to manifest, and it needs to use methods capable of detecting fine, subsurface defects — specifically Magnetic Particle Inspection (MPI), and in some cases ultrasonic or radiographic testing.

Skipping or shortcutting this inspection stage is one of the most common ways that weld cracking in crane booms goes undetected until it becomes a structural failure.

How specialist repair procedures prevent hydrogen cracking

Preventing hydrogen cracking is not a matter of being more careful in a general sense. It requires a structured set of procedural controls applied at every stage of the repair, from preparation through to post-weld inspection.

The most important controls include:

Preheat and interpass temperature control — Heating the base metal before welding slows the cooling rate, giving hydrogen more time to escape from the weld zone rather than becoming trapped. Maintaining minimum interpass temperatures throughout the weld keeps this benefit consistent across multi-pass welds.
Low-hydrogen consumables, stored and handled correctly — Electrodes and wire must be kept dry and used within strict time limits after removal from sealed packaging. Even brief exposure to humid air can reintroduce moisture.
Thorough surface preparation — All rust, paint, grease, and moisture must be removed from the weld area before work begins. This is not optional; it is a direct line to hydrogen content in the finished weld.
Post-weld heat treatment where required — For the highest-strength grades, controlled post-weld heating can help drive residual hydrogen out of the steel and reduce residual stress.
Delayed, method-appropriate inspection — MPI performed at the right time after welding, not immediately, ensures that any cracking that has developed is detected before the boom returns to service.

All of this needs to be documented in a Welding Procedure Specification (WPS) — a formal document that defines exactly how each weld must be made, including preheat temperatures, heat input limits, consumable specifications, and inspection requirements. A WPS for 960 or 1100 grade steel is not something that can be improvised. It needs to be developed, tested, and qualified before any repair work begins.

This level of procedural discipline is what separates a genuinely safe crane boom repair from one that looks fine on the surface but carries hidden risk. The crane boom repair process demands it — and operators should always ask to see the WPS before authorizing any repair on a high-strength steel boom.

How Rusch Cranes helps prevent hydrogen cracking in boom repairs

We have been developing and refining welding procedures for high-strength crane boom steel since 1991, and we were among the first companies in Europe to establish qualified procedures for 960 and 1100 grade steel. Every repair we carry out follows a documented WPS and a formal Repair Plan prepared specifically for the boom in question.

Our process controls hydrogen cracking risk at every stage:

Full surface preparation and material strength verification before welding begins
Controlled preheat and interpass temperatures maintained throughout
Low-hydrogen consumables managed under strict storage conditions
100% visual inspection and 100% MPI on all new welds after an appropriate delay period
Third-party ultrasonic or X-ray testing arranged where required
CE certification remains valid after repair, backed by a one-year guarantee

We carry out boom repairs at our workshop in Medemblik, the Netherlands, and we deploy repair teams internationally at short notice for on-site work anywhere in the world. If you want to discuss a boom repair or have questions about the procedures involved, get in touch with our team directly.

Frequently Asked Questions

How do I know if a repair shop is actually qualified to weld a high-strength crane boom?

Ask to see their Welding Procedure Specification (WPS) for the specific steel grade of your boom — a qualified shop will have a documented, tested, and formally approved WPS for 960 or 1100 N/mm² steel, not a generic welding procedure. You should also ask whether their welders are certified to the relevant standard (such as EN ISO 9606) and whether they carry out post-weld MPI as a standard part of the process. If a shop cannot produce a qualified WPS for your boom’s steel grade, that is a clear disqualifying factor, regardless of how experienced their welders appear to be.

Can hydrogen cracking occur even if the weld looks perfect visually?

Yes — and this is precisely what makes it so dangerous. Hydrogen-induced cracks are typically very fine and tight, often located in the heat-affected zone just beside the weld rather than in the weld bead itself, and they can be entirely subsurface. A visual inspection, even a thorough one carried out by an experienced welder, will not reliably detect them. This is why Magnetic Particle Inspection (MPI) performed after an appropriate delay period is essential for any weld on high-strength crane boom steel.

How long should you wait after welding before carrying out the post-weld inspection?

For high-strength steels in the 960–1100 N/mm² range, a minimum delay of 24 to 48 hours is generally recommended before performing MPI, though the exact waiting period should be specified in the Welding Procedure Specification for the material in question. This delay is necessary because hydrogen cracking is a time-dependent process — cracks can continue to initiate and propagate for hours or days after the weld has cooled. Inspecting too early risks missing cracks that have not yet fully developed, giving a false sense of security.

What happens to a crane's CE certification if the boom has been repaired?

A crane boom repair can affect the machine’s CE certification if it is not carried out in accordance with the original manufacturer’s specifications and documented welding procedures. However, when a repair is performed by a qualified specialist using a formally approved WPS and full post-weld inspection, CE certification can and should be maintained. Always confirm with the repair contractor before work begins that they will provide the necessary documentation to support continued CE compliance — and be cautious of any shop that cannot clearly explain how they will handle this.

Is it ever safe to carry out a crane boom weld repair on-site rather than in a controlled workshop environment?

On-site repairs are sometimes operationally necessary, but they introduce additional risk factors that must be actively managed — particularly wind, ambient temperature, humidity, and the inability to use some workshop-based equipment. A qualified repair team carrying out on-site work should use windbreaks and environmental enclosures, apply preheat with calibrated equipment, and follow the same WPS they would use in a workshop setting. If site conditions cannot be controlled to the standard required by the WPS, the repair should not proceed until they can be.

Can a crane boom that has already developed hydrogen cracking be repaired, or does it need to be replaced?

In many cases, a boom with confirmed hydrogen cracking can be repaired, but the scope and viability depend on the location, extent, and depth of the cracking, as well as the structural integrity of the surrounding material. A thorough assessment — typically involving ultrasonic testing to characterise the full extent of the damage — is needed before any repair decision is made. Attempting to weld over an existing crack without fully understanding its extent is one of the most common and serious mistakes in boom repair, and it can result in a structurally compromised repair that passes visual inspection but fails under load.

What common mistakes do crane operators or fleet managers make when arranging a boom repair?

The most common mistakes are choosing a repair contractor based on cost or availability alone without verifying their welding qualifications for high-strength steel, and allowing a boom to return to service without confirmed post-weld MPI results. Another frequent issue is failing to disclose the full history of the boom — previous repairs, known impacts, or operating conditions — which can affect how the repair is planned and what inspection methods are appropriate. Treating a crane boom repair like a general fabrication job, rather than a safety-critical structural intervention with its own procedural requirements, is the underlying error that most of these mistakes share.

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