Taking the recrystallization temperature as roughly 0.4 times the melting point (in absolute temperature), which of these counts as hot working? Take the melting points as 327 °C for lead, 3422 °C for tungsten and 1538 °C for iron.

Answer: A. Rolling lead at room temperature (25 °C)

Hot and cold working are distinguished not by whether the temperature is high or low but by whether working takes place above the recrystallization temperature. For lead the guide is 0.4 × (327 + 273) = 240 K, about −33 °C, so rolling at room temperature (25 °C) counts as hot working; recovery and recrystallization proceed during working and almost no work hardening remains. For tungsten the guide is 0.4 × 3695 ≈ 1478 K (about 1205 °C), so at 1000 °C, even though it glows red, it is cold working by this criterion (working at elevated temperatures below the recrystallization temperature is sometimes called warm working). Computing 0.4 × 3422 ≈ 1369 °C in °C is wrong. For iron the guide is 0.4 × 1811 ≈ 724 K (about 451 °C), so rolling at room temperature is cold working. Actual recrystallization temperatures vary with the amount of cold work and the purity.