Convergence problem in 2-d axisymmetric elasto-plastic formulation

Dear all,

I am trying to solve the thermal ratcheting of a hollow cylinder using elasto-plastic formulation.

I am applying the moving thermal gradient in axial direction. Our FEM code(Newton Raphson) is converging for first increment but after that it is diverging. I have used 8 noded serendipity element with 9 gauss point.

Anyone  can please help me that why it is happening?

Thanks

Raj