The fracture

composition

and microscopic structure in cracked position of the failure welded condensation tube have been analysed. It is believed that the main causes lending to cracking of the welded condensation tube in service are joint effect of the following factors: the carbide precipitated in grain boundaries

resulting in local in-tergranular embrittlement; the groove in melting line area

leading to stress concentration; and the stress applied to welded tube in course of production and utilization.