The interrelation was established between the observed low values of impact energy and features of structure formation in HAZ in submerged arc welding of pipes of low-alloy steels X80, X90, K70 (occurrence of brittle precipitates with high content of the main alloying elements on the boundaries of large grains). It is shown that for thermal cycles with maximum heating temperatures 900 °C ≤ Tmax ≤ 1350 °C and cooling rate ω8/5 = const (in the range of 800–500 °С) stability of austenite increases with decreasing Tmax and the median of the temperature interval between the beginning and the end of the austenite transformation decreases with increasing cooling rate.