[en] The work presents the results of studies of the effect of parameters of the welding thermal cycle on the structure and mechanical properties of the metal in the seam area. Metallographic investigations and mechanical tests of specimens after heating up to 900-1200 deg C have revealed that the size of grain increases with the heating temperature and with the duration of exposure at these temperatures. In the process of welding the 00N18K9M5 steel there are two regions in the thermal area. The regions sharply differ in properties from the basic metal: one region being characterized by enhanced resistance to brittle failure, and another one-with low resistance to brittle failure