the thyristor for melting furnace  (or scr for melting furnace) soon burn, apart from the reason of SCR itself quality problem, the following conditions will also  lead to burn off thyristor

1:installation of thyristor  Equipment aging

2: circuit problem

3: bad cooling effictive ( water cooling  or air cooling)

4: thyristor install pressure (thyristor connected to heatsink ) is not enough

5: others