Head to the east part of the Smelting Chamber and search for the locked door #1. Choose the recently obtained Remote Electric Charge and aim (hold the left trigger) at the engine above the door #2. Press RB to send an impulse and activate the engine for a short while.
Note that now you can use the newly unblocked passage. Repeat the same process after reaching the second locked door #1. Approach the door and open it to get back to the Assembly Line. Use the Grapnel Gun #2 and get inside the ventilation shaft.
Head to the neighbouring room and note that 4 new enemies have appeared here. Carefully exit the shaft. The game will suggest you check out the new offensive mode of the Electrical Charge (activated by pressing the left trigger and B at the same time) #1, but it's much better to stick with traditional methods. Stun the two first enemies with a surprise attack #2 and afterwards take care of the two other Thugs.
Now you can head east and then turn north, using the door or jumping through the window on your way. Your objective is reaching the main room of the Loading Bay #1. Head to its middle part and listen out to another conversation with Harley Quinn #2.
You will have to solve a puzzle in order to gain access into Joker's office. Approach the generator found in the middle part of the room #1 and equip the Remote Electric Charge. Choose the negative charge (RB) #2 and note that it sets the big hook into motion.
Keep on sending charges to give the hook momentum, causing it to hit the door leading into Joker's office #1. It's not the end, as now you have to pull out the door. Once again aim at the generator #2, but this time use the negative charge by pressing the right trigger. It should cause the hook to tear out the door, creating a passage for you.
Now you can approach the place where the door used to be and use the Grapnel Gun #1. Unfortunately, as you try to climb up, you will be thrown down by Mister Hammer #2 and therefore forced to fight.
As the battle starts, focus on eliminating the "regular" enemies first #1. Remember about having to counterattack often, thanks to which you shouldn't lose too much energy here. At the same time try to dodge Mister Hammer's attacks #2. Normally he's rather slow, but occasionally he can perform a long jump and slam his hammer, be sure to quickly react and dodge it.
Fight until you get rid of the normal Thugs and afterwards move your attention to Hammer. Choose the Remote Electric Charge and send a charge towards the boss #1 by pressing the left trigger and B at the same time. Quickly approach the stunned Mister Hammer and start rhythmically pressing X to land some quick hits #2.
One such action should be enough to eliminate the boss #1. If it won't do, you will have to repeat the whole cycle - stun Mister Hammer and quickly approach him. After you're done with him, you can attempt to reach Joker's office for the second time #2. This time you will succeed and a cutscene will start. You will receive the next objective - finding Mister Freeze's hideout - only after "leaving" the Steel Mill.