1. If austenite undergoes no change in the impact test than the impact toughness decreases monotonically with decreasing temperatures. 2. The martensite forming during the test increases the work of formation of the crack but reduces the work of its propagation. The curves of the impact toughness have peaks. The less the samples are deformed during the impact tests, the less martensite is formed, and the less distinct the peaks. 3. When the samples contain martensite before the tests then the impact toughness decreases because of the values of Ap and, with large amounts of martensite, Af.