SOME kinds of chrome‐molybdenum steels widely employed in the construction of aeroplanes, the chrome‐manganese‐silicon steels introduced comparatively recently and even some carbon steels with a higher content than 0·2 per cent. carbon show minute cracks after welding with oxy‐acctylene, even when the actual operation of welding is carried out faultlessly. This tendency is described as “weld‐crackability,” cracking occurring at high temperatures. At first, micro‐fractures form, causing in turn the formation of cracks that are visible at a temperature still high enough to cover their surface with a dark bluish flux. These cracks occur not only in the weld itself but also in the whole of the region affected by the thermic influence of the flame. It is characteristic that welding by an electric are does not produce this phenomenon.