Fishplate welding process and quality control
- What are the application differences between flash butt welding and gas pressure welding in fishplate welding?
Flash butt welding heats the rail contact surface through high - current flash effect, followed by upsetting for welding. It suits rails below 60kg/m, with a 3 - minute welding time per joint, but prone to oxidation and incomplete fusion. Gas pressure welding uses high - temperature flames to heat the rail end to plastic state and then applies upsetting force for metallurgical bonding. It offers high - toughness joints with fewer defects, ideal for high - speed rail, but requires 8 - 10 minutes per joint. A conventional railway reduced joint fatigue crack rate from 12% to 3% by switching from flash butt welding to gas pressure welding.
- How do welding current and upsetting pressure affect fishplate welding quality?
In flash butt welding, excessive current (over 8000A) causes rail overburning, reducing joint strength; insufficient current (below 6000A) leads to poor fusion. Upsetting pressure should be 40 - 60MPa-too low (20MPa) leaves gaps, while too high (80MPa) extrudes excessive metal. A construction site with 7000A current and 30MPa pressure had numerous lack - of - penetration defects; adjusting to 7500A and 50MPa achieved 100% qualification.
- What are the criteria for ultrasonic flaw detection of fishplate welding defects?
Using type A ultrasonic flaw detectors at 2.5MHz with 45°/60° angle probes, defects with amplitude exceeding the evaluation line (20% of full - screen height) need recording, and those exceeding the quantitative line (50%) are 不合格. For example, a 2mm - equivalent pore with 60% amplitude was rejected, preventing operational failures.
- What is the impact of "post - heat treatment aging" on the performance of welded joints?
Post - welding heat treatment aging (tempering at 550 - 650℃ for 2 - 3 hours) relieves residual stress and refines grain structure. Untreated joints may have residual stress >300MPa, prone to delayed cracks; aged joints reduce stress to <100MPa, increasing impact toughness by 25% and fatigue life by 40%. A railway project with unaged joints experienced multiple cracks within six months, resolved after corrective treatment.
- What are the requirements for controlling the "hardness gradient" of fishplate welded joints?
Ideal joints should have a gradient: high hardness in the weld zone (HV300 - 350), gradual change in the heat - affected zone, and original hardness in the base metal (HV200 - 250). Excessive weld hardness (>HV400) reduces toughness, while too low (<HV250) weakens strength. Controlling heat input and post - weld treatment achieves the desired gradient. A factory's improper parameters led to HV450 weld hardness and brittle fractures; after adjustment, the hardness met standards.