From Houghtalings Handbook of Useful Information ©1889
The largest steam hammer in the world has been made at Pittsburgh, Penn. It is used for forging steel plates of enormous thickness and size, is ten feet square, and strikes a blow of 200 tons. The anvil block upon which its fearful impact decast upon the foundation it now rests on and it took the molten contents of six cupolas to make the block. When it was cast the Pittsburgh Fire Department sent six steam engines to the scene and kept them there for a week, or until all danger that the fearful mass of molten iron might burst its bonds or by its exploding gases fire the neighborhood, had ended. It took nearly six months for the anvil to cool so that the sand mold might be dug away, and even then workmen could not touch it with their hands.
Further research shows that the largest steam hammer was built in 1891 by Bethlehem Iron Works. I don't know if this is the same one that is mentioned in Houghtalings Handbook but there is at least a 4 year difference between the two.