Heat treatment method for prolonging service life of heavy drill rod made of 42CrMo material

The invention discloses a heat treatment method for prolonging the service life of a heavy drill rod made of a 42CrMo material, which is characterized in that a national standard 42CrMo raw material added with a V element is adopted to manufacture the drill rod, and a heat treatment mode of quenching, tempering with a sodium nitrate solution and secondary tempering with the sodium nitrate solution at the tail part of the drill rod is adopted. Quenching, heating to 850-870 DEG C, keeping the temperature after uniform temperature, discharging from the furnace, and cooling with water; tempering with a sodium nitrate solution, namely putting the whole drill rod quenched to the room temperature into the sodium nitrate solution heated to 400 +/-5 DEG C for heat preservation, discharging from the furnace, and performing air cooling; and carrying out secondary tempering on the tail part of the drill rod by using a sodium nitrate solution, vertically putting 20-30mm of the tail part of the drill rod into a sodium nitr