Small-diameter heavy-wall seamless steel tubes occasionally develop transverse cracks on their surface during service, and there are numerous root causes for this defect. A detailed analysis is provided as follows. When small-diameter heavy-wall seamless tubes undergo hollow drawing with a small deformation amount, additional tensile stress fields emerge on both the inner and outer surfaces of the tube.
Due to the discrepancy in deformation degree, the outer surface undergoes greater elongation than the inner layer, generating extra tensile stress on both sides of the tube wall. When such additional tensile stress reaches a high magnitude, the superposition of basic forming stress and extra stress during drawing makes the tube prone to cracking.
The depth of grinding removal on small-diameter heavy-wall seamless steel tubes shall not exceed the negative tolerance of the nominal wall thickness; exceeding this limit will trigger transverse cracks inside the tube wall.
Under corresponding mechanical conditions, any factor that reduces the plastic deformation capacity of seamless tubes during processing will raise the risk of internal transverse cracking. Hence, controlling heat treatment quality and eliminating caustic embrittlement are critical in the production of small-diameter heavy-wall seamless steel tubes. In hollow drawing, apart from circumferential additional stress, axial additional stress also exists. Longitudinal cracks are induced by the extra axial tensile stress generated during hollow drawing.
Severe high-temperature corrosion occurred after the heating furnace was switched to long-flame coal with high sulfur content. This passage analyzes the mechanisms and primary causes of high-temperature sulfur corrosion and high-temperature chlorine corrosion of superheater tubes under high temperatures and highly oxidizing atmospheres. Combined with practical experience of switching to low-sulfur Datong pine pellets instead of Yangquan coal, upgrades were carried out on the centralized combustion system.
The grinding removal depth of small-diameter heavy-wall seamless steel tubes must not exceed the negative wall thickness tolerance, and the residual wall thickness after grinding shall not be less than the minimum allowable thickness specified for the tube.
Various measures are available to prevent structural deformation and collapse caused by rapid temperature rise in the furnace during accidents, with tailored solutions adopted for different scenarios. Common methods include thermal insulation and using refractory materials to isolate the steel frame from direct flame impingement. These measures slow down heat transfer and delay the temperature rise and strength degradation of steel frames. All these methods follow the same fundamental principle. Fire retardant coatings represent an effective fire protection technology developed in recent years.
