Controlling Welding Distortion in Large-Span Non-Standard Cranes
Key points for controlling welding deformation in large-span non-standard crane main girders: preset camber of 20–40 mm, longitudinal shrinkage of 0.3–1 mm/m, and transverse shrinkage of 0.5–2 mm/m. The welding sequence follows the principle of butt welds before fillet welds and center before edges, using backstep welding in segments of 300–500 mm. Preheating is required when plate thickness exceeds 25 mm, at temperatures between 50–200 °C. Kelude Heavy Industry employs a combined approach of symmetrical welding, flame straightening, and vibratory stress relief in non-standard main girder fabrication, keeping welding distortion within the allowable limits of ISO 4301.
The main girders of large-span non-standard cranes typically use a box girder cross-section, with lengths ranging from 22.5 to 40 m, web plate thicknesses of 6–12 mm, and flange plate thicknesses of 10–25 mm. During welding, localized heating and cooling cause shrinkage in the weld zone, leading to longitudinal shrinkage, transverse shrinkage, and angular distortion in the main girder. For non-standard cranes—given their large spans and non-standard cross-sectional dimensions—controlling welding distortion is significantly more challenging than for standard cranes. Improper control can result in insufficient or excessive camber, out-of-tolerance side bow, or web plate buckling, all of which directly affect the crane's operational performance and safety. This article systematically covers the mechanisms behind welding distortion, control measures, and stress-relief techniques.
Welding Distortion Mechanisms and Classification
The root cause of welding distortion in box girder main girders is the shrinkage stress generated as the weld metal cools. During welding, the weld zone is heated to melting temperature (approximately 1,500 °C) while the surrounding base metal remains cooler, creating a non-uniform temperature field. As the weld cools and shrinks, it is constrained by the cooler surrounding base metal, producing residual tensile stresses. When these residual stresses exceed the yield strength of the base metal, plastic deformation occurs. For large-span main girders, welding distortion is classified by direction: longitudinal shrinkage (length reduction of approximately 0.3–1 mm/m), transverse shrinkage (width reduction of approximately 0.5–2 mm/m), and angular distortion (flange plate bending of 0.5–2 degrees).
Controlling welding distortion in non-standard crane main girders requires a welding procedure tailored to the specific structural configuration. For box girder sections, the four longitudinal fillet welds (upper and lower flange-to-web welds) are the primary welds, and the transverse shrinkage of each weld accumulates. If these four welds are executed sequentially without controlling the order, the main girder will develop noticeable bending—the first welds cause the girder to bow toward that side. Kelude Heavy Industry uses computer simulation during fabrication to predict distortion amounts and applies pre-set counter-deformation during assembly to offset welding distortion.
Counter-Deformation and Welding Sequence Control
Counter-deformation involves pre-setting the main girder with a deformation opposite to the expected welding distortion during assembly, so that the two cancel out after welding. For large-span box girders, the preset camber is typically 20–40 mm (controlled at 1/1,500 to 1/1,000 of the span), achieved by adjusting the height of the assembly jig. The preset side bow is typically 5–10 mm. The counter-deformation value must be determined based on the volume of deposited weld metal, plate thickness, number of weld passes, and heat input.
Welding sequence is a critical process control for managing distortion. The recommended sequence is: (1) complete the butt welds for plate splicing during cutting; (2) weld the web-to-lower-flange fillet welds; (3) weld the longitudinal stiffener-to-web welds; (4) weld the transverse stiffener welds (using intermittent welding); and (5) weld the web-to-upper-flange fillet welds (last). Each weld pass uses the backstep technique with segments of 300–500 mm, skipping between segments. At least two welders work symmetrically from the center toward both ends on each weld. Welding speed is maintained at 20–35 cm/min, with uniform travel to avoid localized overheating.
Preheating, Post-Heating, and Stress Relief
Preheating reduces the cooling rate after welding, minimizes the formation of hardened microstructures, and lowers residual welding stresses. Per ISO 4301 welding specifications: no preheating is required for plate thicknesses below 25 mm; preheat to 50–100 °C for 25–40 mm; 100–150 °C for 40–60 mm; and 150–200 °C for thicknesses above 60 mm. The preheat zone extends 100 mm on each side of the weld. Non-standard crane main girder plates typically range from 6–25 mm, so preheating is generally unnecessary; however, during winter construction (ambient temperatures below 0 °C), preheating to 30–50 °C is recommended even for plates under 25 mm.
Post-heating for hydrogen removal and vibratory stress relief are effective methods for eliminating residual welding stresses. Post-heating is performed at 200–300 °C, with holding time calculated at 1–2 minutes per millimeter of plate thickness. Vibratory stress relief applies alternating loads to the main girder via an exciter, allowing residual stresses to release and redistribute. Kelude Heavy Industry prioritizes vibratory stress relief in non-standard main girder fabrication due to its high efficiency and low energy consumption, achieving a 40%–60% reduction in residual stress after treatment. For extra-large main girders exceeding 30 m in length, flame straightening can also be used, but heating temperature must be strictly controlled to not exceed 650 °C (the steel transformation temperature).
Comparison of Welding Deformation Control Methods
| control measures | principle | control effectiveness | application scope |
|---|---|---|---|
| counter-deformation method | preset counter-deformation to offset welding distortion | camber Deviationless than5mm | all box-type/I-section beams |
| symmetrical welding | offset by simultaneous welding on both sides Deformation | bending deformation reduced by 60%–80% | two-person operationbox girder |
| back-step welding | 300~500mmsegmented intermittent welding | reduced transverse shrinkage30%~50% | long weld seams (greater than 2 m) |
| Flame straightening | localized heating shrinkage deformation zone | after straightening, deviation less than 2 mm/m | Weldingsecondary straightening after |
| vibratory stress relief | apply alternating loads to release stress | residual stress reduced by 40%–60% | main girders less than 40 m in length |
| Preheating and post-heating reduce cooling rate and eliminate hardened microstructures | crack occurrence rate reduced by 80% or more | plate thickness greater than25mmor low-temperature environment |
| longitudinal shrinkage per meter of weld seam: longitudinal shrinkage 0.3–1 mm. For large-span girders (30 m), the total shrinkage of four long weld seams can reach 10–40 mm, and cutting allowance must be reserved. | transverse shrinkage per fillet weld pass: transverse shrinkage 0.5–2 mm. The total shrinkage of four fillet welds is 2–8 mm, affecting the main girder width dimension. | Angular distortion: flange plate welding angular distortion 0.5–2 degrees. Controlled via counter-deformation jig or pre-bent flange plates, with deviation less than 0.5 degrees. |
| camber control non-standard camber is L/1000–L/1500. After welding, camber deviation must be less than or equal to 5 mm. | Weldingmaterial Q355B base metal matched with ER50-6 welding wire or E5015 electrode. Welding consumable strength is not higher than the base metal, and heat input is controlled at 15–25 k J/cm. | Inspection Standard Weld Seamquality per GB/T 11345Class IIAcceptance.Main Girder Deflectionquality per ISO 4301 Crane Design Standard-2008in5.5section measurement. |
Non-Standard Main Girder Welding Procedure Design Workflow
Kelude Heavy Industry follows a standardized workflow for welding procedure design on large-span, non-standard crane main girders:
Step 1: Welding Procedure Qualification — PQR (Welding Procedure Qualification Record) is performed in accordance with GB/T 50661 to determine welding parameters and filler materials.
Step 2: Distortion Prediction — Finite element analysis or empirical formulas are used to predict distortion, establishing the required pre-set camber and welding sequence.
Step 3: Assembly Jig Setup — The jig bed is adjusted to the calculated pre-set camber and side bow values.
Step 4: Trial Weld Verification — Test plates of identical thickness are welded to validate welding parameters and pre-set distortion values.
Step 5: Production Welding — Welding is executed in the prescribed sequence, using symmetrical and back-step techniques.
Step 6: Post-Weld Inspection — Ultrasonic Testing (UT) combined with coordinate measuring machine (CMM) verification of distortion.
Design standards referenced include ISO 4301 Crane Design Standard, GB/T 50661-2011 Code for Welding of Steel Structures, and JB/T 10560-2006 Technical Conditions for Crane Welding. Kelude Heavy Industry strictly enforces welding procedure qualification, and all non-standard main girder welding procedures are validated through witnessed testing. For more on main girder structural design, refer to The Complete Guide to Overhead Crane Main Girder Design and FEA. For further details on non-standard welding procedures, see Crane Main Girder Welding Procedure and Weld Quality Control Guide.
Frequently Asked Questions on Main Girder Welding
Q: What should be done if post-weld camber deviation on a non-standard main girder exceeds tolerance?
A: If camber is insufficient (less than L/2000 or showing deflection), flame straightening is applied to the center of the bottom flange plate. The heat-induced contraction restores the girder's camber. Heating width must not exceed one-third of the flange plate width, with temperature controlled between 600–650°C and monitored using a spot thermometer. If camber is excessive (greater than L/800), heat is applied to the center of the top flange plate instead. Flame straightening must be performed in stages, with each pass limited to a maximum correction of 5mm. The girder must be allowed to cool naturally after straightening — water quenching is strictly prohibited. All Kelude Heavy Industry flame straightening technicians hold the qualifications required under GB/T 50661.
Q: Is welding deformation control or post-weld straightening the better approach?
A: Welding deformation control (process prevention) is far superior to post-weld straightening. Process prevention keeps distortion within predictable limits, preserves mechanical properties, and improves efficiency. Post-weld straightening — whether by flame or mechanical means — introduces secondary residual stresses and localized changes to material properties, making it a last-resort corrective measure. Under Kelude Heavy Industry quality standards, the first-pass weld acceptance rate for main girders must be 95% or higher, meaning over 95% of girders require no post-weld straightening. The key to effective welding deformation control lies in thorough procedure qualification and accurate pre-set distortion calculations before welding begins.
Q: How do you correct web plate wave distortion on large-span box girders after welding?
A: Web plate wave distortion results from buckling of thin plates caused by transverse weld shrinkage, typically occurring where web thickness is small (6–8mm) and stiffener spacing is large (greater than 1.5m). Preventive actions include limiting stiffener spacing to 1.2–1.5m and maintaining a web thickness-to-span ratio of at least 1/200. For correction, flame heating is applied to the convex areas of the wave, working outward from the crest of each ripple. Each heating spot should be 30–50mm in diameter. After straightening, web plate flatness deviation must not exceed twice the plate thickness.
Q: What precautions are required for winter welding?
A: When ambient temperature falls below 0°C, the following measures are mandatory: ① Preheating — even plates under 25mm thick must be preheated to 30–50°C; ② Wind protection — a windbreak shelter must be erected around the welding area, keeping wind speed below 2m/s; ③ Controlled cooling — the weld seam area must be covered with insulating blankets for a minimum of 1 hour after welding; ④ Electrode management — electrodes must be baked and stored in holding ovens per manufacturer instructions before use, and welding wire must be free of oil and rust. Per GB/T 50661, structural steel welding is prohibited when ambient temperature falls below -10°C.
