HOT ROLLING MILL - BILLETS Split ends and cracking problem
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Several defects can occur during the metal rolling process. Surface defects such as impurities, scale, rust, or dirt are common if adequate surface preparation is not performed before rolling. More serious internal defects like edge cracks, center cracks, and wavy edges can result from improper material distribution in the final product. Other defects include alligatoring, where the material splits during rolling, and cracking or tearing from non-uniform heating of the workpiece in hot rolling. Process parameters like roll bending force, billet movement speed, roller clearance, and alloy composition can also influence defect formation.
HOT ROLLING MILL - BILLETS Split ends and cracking problem
- 1. BY: ANIL SHARMA, ROLL SHOP ENGINEER, AFRICAN FOUNDERY LTD. NIGERIA.
- 3. A wide variety of defects are possible in metal rolling manufacture. Surface defects commonly occur due to impurities in the material, scale, rust, or dirt. Adequate surface preparation prior to the metal rolling operation can help avoid these. Most serious internal defects are caused by improper material distribution in the final product. Defects such as edge cracks, center cracks, and wavy edges, are all common with this method of metal manufacturing.
- 4. Another interesting defect that can occur in flat rolling is alligatoring, where the work being rolled actually splits in two during the process. The two parts of the work material travel in opposite directions relative to their respective rolls.
- 5. In hot rolling, if the temperature of the workpiece is not uniform the flow of the material will occur more in the warmer parts and less in the cooler. If the temperature difference is great enough cracking and tearing can occur
- 6. • Symmetrical edge wave - the edges on both sides of the workpiece are "wavy" due to the material at the edges being longer than the material in the center. • Asymmetrical edge wave - one edge is "wavy" due to the material at one side being longer than the other side. • Center buckle - The center of the strip is "wavy" due to the strip in the center being longer than the strip at the edges. • Quarter buckle - This is a rare defect where the fibers are elongated in the quarter regions (the portion of the strip between the center and the edge). This is normally attributed to using excessive roll bending force since the bending force may not compensate for the roll deflection across the entire length of the roll.
- 7. Hot tearing is considered as a phenomenon linked to the inadequate compensation of solidification shrinkage by melt flow in the presence of thermal stresses, there are more factors that could be involved in the formation of cracks at supersolidus temperatures. Hot cracking susceptibility and the composition of an alloy-- has been established on many occasions. A large freezing range of an alloy promotes hot tearing because such an alloy spends a longer time in the vulnerable state in which thin liquid films exist between the dendrites. The liquid film distribution is determined by wetting of grain boundaries, i.E., By surface tension between liquid and solid phases. When the surface tension is low and wetting is good, the liquid tends to spread out over the grain boundary surface, which strongly reduces the dendrite coherency, weakens the mush, and may promote hot tearing.
- 8. . The reduction in sulfur content for minimizing the formation and intrusion of complex iron oxides were suggested The increase of rolling passes and the decrease of reduction ratio for homogeneous rolling of billets Reduce the billet movement speed. Check the clearance between the rollers
- 9. Check for the chemical analysis of the billet. Check for Manganese, Sulphur, Phos content. Also check if the metal is getting completely killed in the furnace. Insufficient degassing of the metal and high sulphur and phos contents are usually the main culprits for this problem.If sulphur and phos is high say about in the range of 0.140 and above cumulatively and nothing can be done about it owing to the raw material composition inthe furnace then increase manganese content in the metal. Also check if aluminium being used to kill the final metal is being used in a manner to make the killing effective or else use ferro aluminium as it ensures better killing as it negates the chances of manual errors in method of dipping into the furnace if melting is taking place in an induction furnace
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