Relieving Residual Stress in Weld Seams: Crane Heat Treatment Logic
📋 Key Summary
After a steel structure is welded, the weld seam holds a hidden force — welding residual stress. It is the internal stress trapped inside the weld as it cools and contracts. Invisible under normal conditions, it can crack the structure when combined with the working stress of hoisting. Heat treatment for stress relief uses controlled heating and slow cooling to release this trapped force. This article explains where residual stress comes from and how to eliminate it.
📌 Core Logic
Welding locks stress into the structure; heat treatment releases it.
Heat to the recrystallization temperature, hold, and cool slowly — the stress is relieved.
After welding a main girder, the weld seam looks smooth and the structure appears solid. But beneath the surface, the weld holds a hidden force — welding residual stress.
Here is how it forms: during welding, the weld metal is heated to high temperatures and expands. As it cools, it contracts, but the surrounding base metal resists that contraction, trapping stress inside the weld. This residual stress stays dormant under normal conditions, but when combined with the working stress of hoisting, it can crack the structure.
Heat treatment for stress relief is designed to release that trapped force. Here is how it works.
Where Welding Residual Stress Comes From: Contraction Restrained During Cooling
Residual stress originates from the thermal expansion and contraction inherent to welding.
During welding, the weld zone is heated to high temperatures, and the metal expands. The surrounding base metal, still cool, restrains that expansion, compressing the weld metal.
During cooling, the weld metal contracts, but the base metal again resists, placing the weld metal under tension. This tension leaves tensile stress locked in the weld — that is residual stress.
The magnitude of residual stress depends on the welding procedure, restraint level, and material. The stronger the restraint and the faster the cooling, the higher the residual stress. ISO 23857, Welding Quality Requirements for Crane Steel Structures, sets requirements for welding quality control.
Why Residual Stress Is Dangerous: It Combines with Working Stress
Residual stress is dangerous because it combines with working stress.
During crane hoisting, the main girder experiences bending stress — that is working stress. If tensile residual stress is also present in the weld, the two stresses add up at the same location, and the local stress can exceed the material's yield limit.
The result is premature fatigue and early weld cracking. Many weld cracks in cranes start at points of residual stress concentration.
Stress relief is therefore not an optional step — it removes the internal threat so that working stress acts alone, without accumulation or overload. Kelude treats stress relief as a mandatory process for critical welds.
How Stress Relief Works: Heat, Hold, and Cool Slowly
Heat treatment for stress relief follows three steps: heating, holding, and slow cooling.
Heating raises the workpiece to a specific temperature, typically near the recrystallization temperature of the metal. At this temperature, internal stresses relax.
Holding maintains that temperature for a set period, allowing stress to fully release.
Slow cooling prevents new thermal stress from forming — cooling too quickly would reintroduce it. The cooling phase after stress relief must therefore be gradual and controlled.
Once these three steps are complete, residual stress drops significantly. Kelude determines the temperature, duration, and cooling rate based on the material and thickness of the welded component. FEM 1.001, Crane Design Standard, specifies requirements for stress relief treatment.
Stress Relief Methods: Whole, Local, and Vibratory
Three common methods are used for stress relief.
Thermal stress relief of the entire assembly involves placing the whole structural component in a furnace and heating and cooling it uniformly. This method delivers the best results but is limited by furnace size — large components may not fit.
Local heat treatment applies heat to the weld area only, using heating tapes or induction heating. It suits large components that cannot fit in a furnace, but the heating zone and temperature gradient must be carefully controlled.
Vibratory stress relief uses an exciter to vibrate the structure, releasing stress through mechanical energy. It requires no heating and suits large structures, though it is less thorough than thermal stress relief.
The choice among these methods depends on structural dimensions and stress requirements. Kelude applies stress relief to critical welds per the design specification, using local or vibratory methods for large components.
Common Mistakes in Stress Relief Treatment
The first mistake is skipping stress relief where it is required. Critical welds, thick plates, and highly restrained joints carry high residual stress; leaving them untreated creates hidden risks. Which welds require stress relief must be determined per the specification.
The second mistake is using an incorrect stress relief procedure. Insufficient temperature, inadequate holding time, or rapid cooling leaves stress incompletely released — the treatment is wasted. Temperature, time, and cooling rate must follow the specification.
The third mistake is performing stress relief without verification. Without inspection, there is no way to confirm the treatment was effective. Kelude performs a Stress Test after stress relief or reviews process records to ensure stress has genuinely been reduced.
Comparison of Three Stress Relief Methods
| Method | Principle | Effect | Applicability |
|---|---|---|---|
| Overall Stress Relieving | Overall Heating and Slow Cooling | Optimal | Furnace-Capable Small Components |
| LocalHeat treatment | Weld SeamLocal Heating | Good | Local Treatment of Large ComponentsWeld Seam |
| vibratory stress relief | vibrationReliefStress | Moderate | Large-Scalestructure |
Quick Reference of Standard Clauses for Heat Treatment Stress Relief
| Standard | Key Clause Points | Relationship with Stress Relief |
|---|---|---|
| ISO 23857 | welding quality requirements | Stress Relief Processrequirements |
| FEM 1.001 Crane Design Standard | crane design specification | Stress Relief Treatmentrequirements |
| GB/T 28264 Safety Monitoring and Management System | safety monitoringTraceability Record | Stress Relief Record Traceability |
FAQ: Heat Treatment for Stress Relief in Crane Welds
Q: Which weld seams require stress relief?
A: Thick-plate welds, highly restrained joints, and critical load-bearing welds all require stress relief. The thicker the steel plate and the greater the welding restraint, the higher the residual stress—and the greater the need for treatment. Main girder load-bearing welds and critical node joints typically require stress relief as well. The specific welds to be treated are determined by the welding procedure and design requirements.
Q: Vibratory stress relief vs. thermal stress relief—which is better?
A: Thermal stress relief (either overall or localized) provides more thorough results, achieving more complete residual stress release, and is the preferred method. Vibratory stress relief suits large structures that cannot fit into a furnace; it is convenient and cost-effective, but less thorough than thermal treatment. In short: thermal stress relief delivers superior results, while vibratory stress relief offers greater convenience. The choice depends on structural dimensions and stress requirements.
Q: How do you verify that stress has actually been reduced after treatment?
A: Verification relies on process records and stress testing. The treatment temperature, holding time, and cooling speed must be documented per the procedure to confirm compliance. Stress testing can also be performed on critical areas to measure actual residual stress values. The key is combining process record review with on-site measurement at critical points—confirming the stress has genuinely been relieved, not just going through the motions.
Stress relief is an integral part of welding quality. For more on welding procedures, refer to our Complete Guide to Crane Main Girder Welding Procedures and Weld Quality Control.
Residual stress in weld seams is an invisible internal defect. Kelude Heavy Industry applies a three-step stress relief process—heating, holding, and controlled cooling—to release the tension locked inside the weld, allowing the structure to perform at its best without being compromised by internal stress.
