During a welding procedure qualification for structural components of mobile lifting equipment, assessors found that the tensile specimen fractured at the weld, with white spots visible on the weld fracture surface. In response to this quality defect, the welding procedure was judged as unqualified. Combining production practice, we analyzed the causes of white spot formation in the weld and formulated targeted process measures, ensuring the manufacturing quality of this type of lifting equipment structural component.

 

1. Original Welding Procedure

 

The structure of the tensile specimen for this type of lifting equipment structural component is shown in the accompanying figure.

 

 

 

Tensile Specimen

 

The joint groove of the tensile specimen was machined by water-cooled cutting into a double-sided "X" groove. The welding method was horizontal-position CO2 mixed gas shielded welding, the base metal was SM570 steel with good weldability, and the filler material was GB ER50-6 welding wire. The original welding procedure is shown in Table 1.

 

Pre-welding preparation included: grinding to remove oil, rust and other debris from the area to be welded and within 10–20 mm on both sides; checking whether the CO2 gas pressure met requirements; checking whether the welding torch contact tip was in good condition and cleaning spatter deposited on the torch end; checking the surface quality of the welding wire to ensure uniform copper plating, no rust, and no obvious bending or kinks in the overall shape.

 

2. Causes and Hazards of White Spots in Welds

 

(1) Causes of white spots

 

During welding of the tensile specimen, hydrogen-containing substances in the arc zone are ionized at high temperature, decomposing into hydrogen ions that can dissolve in the liquid metal. After these hydrogen ions dissolve in the liquid molten pool, at the instant the weld metal crystallizes, the solubility of the dissolved hydrogen suddenly decreases and the hydrogen cannot escape from the molten pool in time, thus forming hydrogen porosity in the weld.

 

Analysis showed that the main cause of high hydrogen content in the weld was excessive moisture in the arc zone. During welding, this moisture decomposes into hydrogen ions, which then form hydrogen porosity in the weld. The moisture in the arc zone mainly comes from two sources: first, humid shielding gas or a humid welding environment, causing excessive moisture in the weld; second, the use of water-cooled cooling during the cutting of the specimen groove, causing excessive moisture adsorption on the surface of the base metal groove.

 

(2) Hazards of white spots

 

The round or elliptical white dots visible to the naked eye on the weld fracture surface are a form in which hydrogen exists in the weld, indicating that hydrogen porosity was generated during welding. Such hydrogen porosity is a fine crack defect in the weld and can generate considerable internal stress. During service of the weldment, the internal stress of the hydrogen porosity interacts with external stress, promoting crack propagation and enlargement, which can easily cause sudden early fracture of the weld, i.e., hydrogen embrittlement. Hydrogen embrittlement is one of the most harmful types of weld defects. For mobile lifting equipment, ensuring that no white spots appear in the welds of structural components is an important part of ensuring safe use.

 

3. Tests to Eliminate Weld White Spots

 

Common methods for eliminating hydrogen in welds include pre-welding removal of rust and oil, preheating dehydration treatment, and post-welding hydrogen elimination treatment.

 

Before conducting the welding test to eliminate weld white spots, the following two preparatory steps must be completed: first, use pipeline gas supply at the welding site and ensure that the pressure-reducing preheating device at the pipeline outlet is in normal condition; second, perform dehydration cleaning of the area to be welded.

 

After the above preparatory work was completed, welding tests were carried out on the tensile specimens. The main differences among the tests were the groove fabrication method and the process measures. The test results showed that the main cause of white spots in the weld was that the coolant used during groove cutting entered the surface layer of the base metal. The test methods and verification results for the tensile specimens are shown in Table 2.

 

4. Process Improvement Measures

 

The comparative test results for the tensile specimen joint grooves verified the correctness of the inference. After water-cooled cutting of the groove, we added a process measure, namely preheating the tensile specimen to 100–150°C with a propane flame before welding. Upon re-qualification, the weld was acceptable. The improved welding procedure was applied to actual workpiece production, and inspection showed that the weld quality was acceptable, demonstrating good results from the process improvement.