The effect of additional external cooling on the formation of the weld during pipe welding.

Abstract

The paper presents an analytical description of real-time thermal field arising in industrial welding of a pipe wall taking account of possible accelerated cooling of the seam after welding. Considered was the case when the surface opposite to the welding bath is cooled with a spray of air or air-water mixture under certain pressure. A concept of characteristic cooling time is introduced depending on heat exchange coefficient at the cooled surface of the pipe. Calculations were performed for various heat exchange coefficient values ranging from 1•10–3 (no cooling) to 6•104 W/(m2•K) (cooling with air-water mixture under pressure). Results show that even cooling with an air spray results in sufficient increase of cooling rate, and in case of cooling with an air-water mixture it increases much greater. Namely, the time to finish crystallization becomes shorter, and cooling rate in the temperature range of austenite transformation increases. There are a number of factors favouring the greater impact toughness of the welded pipe under accelerated cooling of the internal pipe surface: accelerated crystallization resulting in grain size refinement; accelerated cooling of gamma phase resulting in more disperse ferrite-pearlite (or even bainite) structure, which may have also resulted in an increase of impact toughness; all these structure changes do nevertheless result in martensite formation