You may wish to read Radmir Pleiner's book, The Celtic Sword (1993). (free, legal download, if you have an academia.edu account.)

Northern European ('celtic') swords were frequently made using a technique called 'pattern welding', in which bars of iron are twisted together to form complex decorative patterns on the blade. This seems to be distinct from the technological processes used further south around the Mediterranean.

There's ongoing debate about whether or not this process was merely cosmetic, or added extra strength to the blade. The traditional view has been that twisting hard (phosphoric) and soft (low carbon) iron together created a blade that was hard yet flexible. Recent laboratory tests have found that the process doesn't add significant strength or flexibility, however, which suggests it was more about showcasing the smith's skill or else visually demonstrating the quality of the weapon's materials than it was improving the physical performance of the blade. (free version of the article linked above.)

It's generally accepted that Romans copied a lot of their weaponry from the 'celtic' people they met as they expanded northward, but there have been few studies of the metallurgy of Roman swords, and I don't believe it's yet well known whether Roman smiths adopted pattern welding techniques. The practice did continue outside the empire, however, and shows up across the northern provinces in late antiquity as the spatha (a longer, N. European style sword) begins to replace the gladius. 90% of the spathas found in England from the 5-7th centuries were made using pattern welding, continuing the tradition begun 1000 years before.