1. Inventor's Certificate No. 1574649 SSSR, MKI C 21 D1/78, ?Method for nitriding parts,? Otkrytiya Izobret., No.30, p. 12 (1990).
2. Inventor's Certificate No. 1770445 SSSR, MKI C 21 D 1/78, ?Method for nitriding parts,? Otkrytiya Izobret., No. 39, p. 28 (1992).