That's almost ten inches thick!!! In fact, we're talking 9.843 inches (rounded off to the nearest thousandth of an inch) thick... In other words: 9.843" X 3.937" X 3.937" correct???
I mean that's what my conversion comes out to be according to what gaydonat gave in millimeters (250mm X 100mm X 100mm).
First and foremost, what is the base metal composition because without knowing this, one can only speculate on what the root cause of the cracking is!!! No one in here has the ability to assist you in finding out the cause of your problem if you do not include complete, and specific details.
Another question I have is where did you come up with the preheat temperature of 200 degrees C??? Also, can you describe in detail how you are measuring this, and where on the Base metal are you measuring the temps???
Everybody else is giving you some excellent advice yet without more specific details, we can only guess some of the potential causes for the cracking you are experiencing.
Please include as many details as possible. ;)
Respectfully,
Henry