I never found it to get harder, the way I play is not to make things as theyre ordered but in advance. I usually have 3-5 of each item laid out on the floor. When a customer comes in I give them the requested item, then go back and forge the replacement.
Also when you have the money to buy an ingot of the next metal, buy one and make a blade but dont attach it, wait til someone comes in asking for a polearm or dagger and then attach to what they want and replace