Is the Iron Rune local or global in a weapon?

Local: in a martial weapon it gives "% increased Physical Damage" to that weapon (the game's stat local_physical_damage_+%), so it shows in the weapon's damage.

Iron Rune vs Elemental Runes

Which weapon rune adds more damage, worked out from the rune and the weapon · Path of Exile 2✓ Checked against game files · 0.5.5

Iron wins above75avg physical hit vs Perfect Glacial Rune

Iron wins above77avg physical hit vs Perfect Desert Rune

Iron wins above85avg physical hit vs Perfect Storm Rune

An Iron Rune raises a weapon's local physical damage by a percentage; a Desert, Glacial or Storm Rune adds flat elemental damage. The Perfect Iron Rune (+20%) adds more once the weapon's physical damage per hit, before % increases, averages more than 75 against the Perfect Glacial Rune, 77 against the Perfect Desert Rune, 85 against the Perfect Storm Rune (at 20% quality); attack speed doesn't change that. Counted over 173 weapon bases, the rune that adds the most on the plain base is Perfect Storm Rune on 146 of 173, Perfect Iron Rune on 27 of 173; on a perfect rare it is Greater Tempered Rune on 163 of 173, Perfect Iron Rune on 10 of 173, because the rare's % increases multiply flat physical damage too. Patch 0.5.5, from the game files.

Break-even physical hit for every pair of tiers

Average physical damage per hit of the weapon (base plus flat added, before % increases) above which the Iron Rune in the column adds more than the rune in the row. Below it, the elemental rune adds more.

Flat elemental or chaos rune	Lesser Iron Rune (+14%)	Iron Rune (+16%)	Greater Iron Rune (+18%)	Perfect Iron Rune (+20%)
Lesser Desert Runeavg 5.0	30	26	23	21
Desert Runeavg 9.0	54	47	42	38
Greater Desert Runeavg 14.5	86	76	67	60
Perfect Desert Runeavg 18.5	110	96	86	77
Lesser Glacial Runeavg 4.0	24	21	19	17
Glacial Runeavg 8.0	48	42	37	33
Greater Glacial Runeavg 12.0	71	63	56	50
Perfect Glacial Runeavg 18.0	107	94	83	75
Lesser Storm Runeavg 5.5	33	29	25	23
Storm Runeavg 10.5	62	55	49	44
Greater Storm Runeavg 15.5	92	81	72	65
Perfect Storm Runeavg 20.5	122	107	95	85

Best rune for each weapon type

Weapon	Top plain base	Top perfect rare
bow	Obliterator Bow: Perfect Iron Rune +23.6	Obliterator Bow: Greater Tempered Rune +59.0
crossbow	Trarthan Cannon: Perfect Iron Rune +32.2	Desolate Crossbow: Greater Tempered Rune +85.5
quarterstaff	Dreaming Quarterstaff: Perfect Iron Rune +42.0	Dreaming Quarterstaff: Perfect Iron Rune +88.3
spear	Flying Spear: Perfect Storm Rune +32.8	Flying Spear: Greater Tempered Rune +92.3
one hand mace	Molten Hammer: Perfect Iron Rune +32.6	Molten Hammer: Greater Tempered Rune +84.6
two hand mace	Massive Greathammer: Perfect Iron Rune +36.8	Massive Greathammer: Perfect Iron Rune +72.6
talisman	Thunder Talisman: Perfect Iron Rune +34.5	Thunder Talisman: Greater Tempered Rune +74.7

DPS added by one socket, 20% quality. Every weapon base page has the full table for that base.

How it is worked out

In a martial weapon the Iron Rune's stat is local_physical_damage_+% and the elemental runes' are local added fire, cold or lightning damage (game files: SoulCoreStats). Per hit, the Iron Rune adds P × r × 1.2 (P = physical hit before increases, r = its percentage, 1.2 = 20% quality), the elemental rune adds its average roll. Both are multiplied by the same attacks per second, so the break-even is P = average ÷ r ÷ 1.2.

Assumptions

Weapon tooltip damage only: elemental and physical damage scale differently with your passives, gear and skill.
Flat physical runes (Tempered) are multiplied by the weapon's % increases, so they have no fixed break-even; the per-base tables include them.
The game rounds physical damage per hit; the break-even ignores that rounding (a point either way).
Bonded rune effects and soul cores without a damage line are not compared.