Pure copper (typically >99.9%) is a great electrical conductor, and in some applications its strength is important as well. Copper grain growth closely follows Feltham’s observation, i.e.

D² - D₀² = K₀t exp(-H₀/kT).

For this problem, ignore temperature dependence and use D² - D₀² = constant · t.

And in annealed pure copper, nearly all of its strength is based on grain size, that is, intrinsic strength is approximately zero. Assume temperature across the wire is uniform because, well, 2 it’s copper wire, the Biot number is bound to be really small - though feel free to check this for relatively small heat transfer coefficient values if you’re so inclined.
After annealing a 3mm wire with initial grain size 30 μm at 600°C (in vacuum) for 0.5, 1 and 2 hours respectively, grain size is observed to be 40, 48 and 61 μm respectively. What do you expect the grain size to be after 4 hours?