Traditional Japanese katanas are indeed manufactured using a labor-intensive process of repeatedly forging and folding tamahagane steel to refine its microstructure and manage carbon distribution.
The retrieved papers consistently confirm that traditional Japanese katanas (nihonto) are produced using tamahagane steel subjected to repeated forge-fold cycles. Papers [7] and [8] specifically analyze the microstructural evolution during these successive folding and forging operations, validating the traditional manufacturing technique described in the claim.