A mill in Europe that fought persistent quality problems for 15 years. While rolling smaller size quenched rebar, specifically 8mm quenched and tempered bar, the finished product would exhibit a slight wavy wrinkle that customers were slowly starting to reject in the marketplace. The customer was convinced that all parameters had been checked and that there was no possible solution to the problem. Russula was contacted by the mill management through another supplier to do a performance audit. The goal of the audit was to determine the cause of the rolling defect that had been plaguing the operators for over a decade.
Russula followed a methodical approach in order to eliminate the potential sources for this type of defect, one by one. On-site, Russula reviewed the mill in operation against a list of best rolling practices. The starting point of any mill analysis is to review tension. The tension was fully removed and the balance of stock line to line was optimized as best as possible. All loopers were set correctly and tension free. All guiding inside the block was reviewed and determined not to be a potential source of the defects and the stand to stand tension condition was verified while rolling. The alignment through all pinch rolls was checked and determined to be adequate for the rolling conditions; the guide bore to groove was adjusted to prevent any issues that were caused when closing on the head end.
The application was thoroughly reviewed and the distribution of water was set up according to Russula’s best operating practices. All the pinch rolls were reviewed to ensure that torque, pressure and lead speeds were not excessive, which could induce stresses during rolling, leading to product deformation. After fully completing this entire analysis, Russula and the customer rolled bars and determined the defect to still be present.
Russula carefully analyzed the switch that deviates the product from one line to the other. Engineers observed that the angle the product was being bent through, combined with the bore diameter, allowed the product to bend in the shear cavity, inserting stresses into the already quenched product. Russula proposed several trials with the customer to determine if this was indeed the source of the problem. The test result proved conclusively that the switch was, in fact, the source of the wrinkled bar phenomenon that had plagued the mill for so many years.
Subsequently, Russula redesigned the switch and shear guides, while also altering the code for the switch itself. The combination of these changes completely eliminated the problem for the customer, and they are now, for the first time ever, producing a truly straight product.
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