development of Magnetic field on kiln shell while replacement

Dear Sir,

In our recent kiln shutdown we hav planned to replace  part of shell ..When shell was cut and put new shell it was observed that there is magnetic field near new shell & old shell up to distance of 200 mm on both sides of joints .

could you tell me the reason of magnetic field developed and which is  not allowing welding with rectifier .so, we put AC welding with . normal electrodes and could  get succeed .

pl let me know the reason of such a high magnetic field up +20 to -20 gauss.

SC Sharma

DCM Shriram