When Yang Guo defeated GWM in the Quan Zhen temple with his heavy iron sword, Yang Guo considered the sword as "cheating", "unfair", "invincible" and cannot perform his martial arts in full potential, so he definately gave up on using his iron sword because of the unfairness
WHAT, if Yang Guo fights with Sweeper Monk with his Heavy Iron Sword?? Who will win???