I had several materials to choose from, basically, Y needs something that's comparable to his strength, and surprisingly, I have something on me that was even better than all the materials that Master Rain had stacked in his lab. Though most of the materials Master Rain had accumulated are of the celestial realm and are extremely precious, the materials were not best suited to make swords. What I had was better.
Neutron Steel, the Qi absorbing metal that once it comes in contact with Qi, it will turn thousands of times heavier.
I wouldn't have been able to melt that material before, but now with the Heart Flame, it's a whole different story, the Heart Flame not only does it possess superior heat to any other regular flame, but it also increases the user's resistance to flame. Such as when the Poison God showcased when he fought Zhang Tian.