If you have cast Heat Metal on a weapon you are holding (and willing to take the damage and try to hold onto it),  can you use your bonus action on a future turn when you hit another creature with that weapon to pump the damage onto the creature (since it is momentarily in contact with your weapon)?

That isn't what the spell is meant to do so...

I'd say no. You can't perform 2 actions at the same time (attack and trigger spell) and it makes the spell stronger than it was meant to be.

I can even give a viable explanation of it not working because heat transfer takes time. Like how you can click your fingers through a candle flame without getting burned. And an attack is not long enough contact, etc etc.

Quote from DxJxC >>

That isn't what the spell is meant to do so...

I'd say no. You can't perform 2 actions at the same time (attack and trigger spell) and it makes the spell stronger than it was meant to be.

I can even give a viable explanation of it not working because heat transfer takes time. Like how you can click your fingers through a candle flame without getting burned. And an attack is not long enough contact, etc etc.

 I agree with the 2 actions at the same time negating its viability.

As for real world explanations, it isnt a 1 to 1 comparison for magic spells, but as a counter example, if I had a knife that was red hot and I cut into someone with it, I imagine they would still feel that heat even if only momentary. Heat transfer does take time, yes, but as far as our perception is concerned it can be almost instantaneous. You don't need to be touching a pot of boiling water for more than a moment before you recoil your hand, because a small transfer of heat occurred and was registered by your skin.

Is the moment a hot blade cuts through skin enough time for it to burn and deal extra damage? Probably  Is it enough time to deal what is considered 2d8 fire damage in D&D 5e? I could see it being doubtful.

Overall I agree with you that it would be a stretch. The only reason I ask is because it seemed like a cool way to make use of a Forge Domain Cleric's fire immunity at 17th level

On a similar but separate note, what if a character is wearing plate armor and grapples another creature such that they are in contact with said armor as well? (if a normal grapple is not enough, perhaps consider if the creature was restrained due to use of the Grappler feat). Would pumping the 2d8 fire damage on each turn damage the grappled creature if it is arguably in contact with the caster's armor?

I feel this question is deeply semantic. Use your bonus action to heat the weapon, then use your attack action to attack with the red hot weapon. This is almost exactly what a paladin would do with the Searing Smile.

As both the attack and the spell take an action, they are not usually something you can do.

You also don't cause the damage automatically, it requires a bonus action to make it work, and that has to be on your subsequent turns.

So, you can cast it on your own weapon and take 2d8 fire damage, try to hold onto it, then in your next turn make an attack (with disadvantage because you're holding it), take 2d8 damage and deal 2d8 damage if you hit, and then have to try to hold onto it again. 

All told, it strikes me as a very poor tactic. Better to just cast it on an arrow head and then shoot them with it, dealing 2d8 damage every turn it's stuck into them as a bonus action. Or pick something of theirs and make it glow, so they take the damage.

Heat Metal is a pretty potent spell in the right circumstances.  It's meant to deny other people the use of their metal items, specifically their armor and weapons.  That can be devastating for a warrior type, especially for the ones who wear metal armor.  Taking off armor requires a lot of time.  Having to drop your weapons makes things petty difficult.

Using it on yourself?  It's really only viable at level 17 when you're immune to the damage, and at the Tier 4 level is casting Heat Metal on yourself worth doing?  It requires concentration, which limits your options a lot.  You have to keep using your Bonus action every round to cause the damage, and Fire damage is pretty common.  Most people will have good saves, resistances, or immunities of their own.  Being immune to the damage doesn't protect you from secondary effects so if you're wearing the armor you're going to have disadvantage on attack rolls and ability checks until the start of your next turn.  That's going to make grappling a lot harder and the skill checks to maintain it harder as well.

All that considered, if a player in my game wants to have their character pulls this stunt, I'll let them, and they can upcast Heat Metal for more damage if they like.  I don't see anything game breaking here.

RAW it's not clear if you can as the rules says you choose when to take a bonus action during your turn. 

Bonus Action: You choose when to take a bonus action during your turn, unless the bonus action’s timing is specified, and anything that deprives you of your ability to take actions also prevents you from taking a bonus action. 

RAI Jeremy Crawford says you can't during an attack. https://rpg.stackexchange.com/questions/72888/can-you-break-up-your-attack-action-for-a-bonus-action

@JeremyECrawford No general rule allows you to insert a bonus action between attacks in a single action. You can interrupt a multiple-attack action with a bonus action/reaction only if the trigger of the bonus action/reaction is an attack, rather than the action.

I can see so much attempted pc abuse with this. If the pc has resistance or immunity it becomes  insane. the armor grapple build. the weapon build. They both basically allow you to switch damage targets for free making a good ability better.  I understand the fun concept but this needs to be regulated so it doesn't get out of control.  

RAW, no. According to the RAI that Plaguescarred posted, also no.

Quote from Roscoeivan >>

I can see so much attempted pc abuse with this. If the pc has resistance or immunity it becomes  insane. the armor grapple build. the weapon build. They both basically allow you to switch damage targets for free making a good ability better.  I understand the fun concept but this needs to be regulated so it doesn't get out of control.  

 Its only really "insane" if you have immunity. Resistance alone still doesnt make it a worthwhile tactic. As far as I know, PCs cant get fire immunity before Tier 4 of play, and at that point I would say its fun, but not overpowered compared to their other options

I've considered this in the game I run because the Forge Cleric is Resistant to Fire Damage and has Heat Metal as one of her 'always ready' spells. As has been stated, it CAN be considered a viable tactic if the player is willing to take the damage. Heat Metal has no Save so you're GOING to take the 2D8 every turn regardless.

I would allow it (assuming they were willing to take the Action to cast the spell for the first turn) and against an enemy vulnerable to Fire damage I can see it working very well.

If only there was a spell you could cast to make your weapon deal additional fire damage on a hit.

JC is absolutely incorrect that Bonus Actions cannot be inserted in between the separate attacks of an Attack action, and nothing in the PHB says or suggests that they cannot, and ruling that way leads to some very undesirable outcomes for Rogues, Monks, TWF users, Rangers, Tavern Brawler, etc etc. But that isn't to say that a Bonus Action can be injected mid-Attack between the "hit" and "damage" steps (if indeed those even are seperate steps). 5E is only so granular, and there's not a good reason to think that "you hit" is big enough to inject whole Bonus Actions in the middle.

Always worth quoting the spell in these types of discussions. Heat Metal:

Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.

If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn't drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.

Do note that while Heat Metal can cause damage to anything "in physical contact" with the object when the spell is cast/bonus action is taken, its secondary "drop or disadvantage" effect only effects creatures "holding or wearing" the object. So for heated grapplers... yes, you could damage your grappled enemy with the spell, but be aware that the only disadvantage you'll be handing out is to yourself (unless you can fully mitigate and take 0 damage from it, either through Fire Immunity or some sort of complicated damage-moving feature like Arcane Ward). 

Interestingly, If you were being grappled by something which will do much more damage than 2d8 to you, casting heat metal on your armour/self (warforged) would mean that they have first to take a constitution save to avoid dropping you outright, then you can take an action to escape the grapple, during which they have disadvantage on the contested check. 2d8 fire damage might be a small price to pay for dealing 2d8 fire damage and having a good chance of escaping a grapple!

Quote from ThorukDuckSlayer >>

Interestingly, If you were being grappled by something which will do much more damage than 2d8 to you, casting heat metal on your armour/self (warforged) would mean that they have first to take a constitution save to avoid dropping you outright, then you can take an action to escape the grapple, during which they have disadvantage on the contested check. 2d8 fire damage might be a small price to pay for dealing 2d8 fire damage and having a good chance of escaping a grapple!

 Interestingly, if you aren't immune to the fire damage, you would also have disadvantage on the ability check to escape the grapple.

 I never understand these threads that occasionally come up that seem to think that heat metal is some sort of very bad and awkward buff spell with a self damage component rather than one of the most debilitating situational debuff spells for its level in the game.

Quote from WolfOfTheBees >>

 I never understand these threads that occasionally come up that seem to think that heat metal is some sort of very bad and awkward buff spell with a self damage component rather than one of the most debilitating situational debuff spells for its level in the game.

 I dont think anyone here thinks its a bad buff spell with a self damage component. Many of the comments in this thread alone have put forth that it can only be used effectively as a buff if you have substantial damage mitigation at your disposal (like fire immunity). 

The point of the questions wasn't "Heat Metal is a bad buff spell, but lets try to make it work" its more like "Hey, under the right conditions couldnt we turn this debuff spell into a buff spell and could it be effective?"   I dont think anyone disagrees that Heat Metal isn't meant to be used on oneself by its design, but the possibilities could be interesting if you find a way to make it work (such as in grappling). 

Quote from WolfOfTheBees >>

Quote from ThorukDuckSlayer >>

Interestingly, If you were being grappled by something which will do much more damage than 2d8 to you, casting heat metal on your armour/self (warforged) would mean that they have first to take a constitution save to avoid dropping you outright, then you can take an action to escape the grapple, during which they have disadvantage on the contested check. 2d8 fire damage might be a small price to pay for dealing 2d8 fire damage and having a good chance of escaping a grapple!

 Interestingly, if you aren't immune to the fire damage, you would also have disadvantage on the ability check to escape the grapple.

 Then, in that case, you both have disadvantage on your checks BUT there is also a possibility that they drop you without you needing to make a check in the first place. Its a situational application, but an interesting use of the spell in my opinion 

Haha yeah. There are much easier ways to do 2d8 extra damage on an attack then burning your own hand, giving yourself disadvantage, and breaking the rule system into pieces to justify doing so. Just like… I dunno, cast Hex or something and quit the shenanigans, Heat Metal is for targeting enemies’ equipment.