What the numbers mean
Grade 420 carries around 0.15 to 0.4% carbon and hardens to roughly 48–52 HRC. Grade 440C carries about 1% carbon and hardens to 58–60 HRC. More carbon means a harder, more wear-resistant edge, at the cost of slightly lower corrosion resistance and a higher material price.
What it changes in use
For a nail clipper used at home, 420 is perfectly adequate: it is tough, cheap to grind and easy to polish to a mirror. For a cuticle nipper or a lash tweezer used all day in a salon, 440C keeps its edge and its tip alignment far longer and is what professionals expect. Tweezers in particular benefit, because a 440C tip can be ground finer without rolling.
Corrosion
Neither grade is as corrosion-resistant as the austenitic 300-series used for non-cutting instruments. Both need proper passivation after grinding and should be dried after autoclaving. A titanium PVD coating (gold, black, rainbow) adds a corrosion barrier as well as colour, which is one reason coated nippers have become popular for salon brands.
What to specify
Say which grade you want on the purchase order and ask for the mill certificate with the first delivery. If a supplier cannot tell you the grade, that is the answer. At Zuol we grind clippers in 420 or 440C to order and nippers and tweezers in 440C as standard.