"Creatures" in D&D have the ability to resist magic that objects do not. Constructs have this feature as well. So they aren't objects; the animating force gives them this ability.
I'd be tempted to allow a saving throw each turn to shrug off the effect. Like a con save on cast and each turn. On success the hest metal fails.
The damage from heat metal occurs at initial casting, and when the bonus action occurs. This, to me, represents continuous damage, not from an instant of contact. As an example, heat metal on a sword doesn't grant it fire damage.
But a point of fire damage on an attack seems reasonable, and full damage if grappling is involved.
I'd be tempted to allow a saving throw each turn to shrug off the effect. Like a con save on cast and each turn. On success the hest metal fails.
The damage from heat metal occurs at initial casting, and when the bonus action occurs. This, to me, represents continuous damage, not from an instant of contact. As an example, heat metal on a sword doesn't grant it fire damage.
But a point of fire damage on an attack seems reasonable, and full damage if grappling is involved.