Kiln shell cracks on the down hill side of the 2nd tyre at the welding joint. The shell thicness under the tyre is 65mm and adjacent tothat is 38mm and crack appeared at that welding point of size 2200mm in circumference. The kiln ID is 4300mm and the temperature at that time was 360c. The tyre migeration is 13~18mm. 

Kindly help me what is the reason?