The Acciaio-class submarine was the fifth subclass of the 600 Series of coastal submarines built by the Regia Marina. They were completed during the early 1940s and saw service in World War II. The Acciaios were a development of the Adua and Perla designs, with some improvements, such as a lower conning tower to improve stability and reduce the silhouette. Of the 13 vessels in the class, six were built by CRDA at Monfalcone, four by OTO at Muggiano, and three by Cantieri Tosi in Taranto, the three main Italian shipyards for submarines. They were single-hulled with side tanks, and built to a Bernardis design, though the Tosi vessels had more powerful engines (at the expense of the two stern torpedo tubes), giving a surface speed of 14 knots. The word Acciaio means "steel", and all vessels in this class were named for metals and minerals.

Submarine Nichelio laid down on 01 July 1941 at Cantieri Odero Terni Orlando (O.T.O.), La Spezia, Italy. Launched on 12 April 1942 , commissioned on 30 July 1942 and was assigned to the Italian Navy.

Following Italy's declaration of war on Germany, between November 1943 and July 1944, Nichelio carried out 7 special missions on behalf of the Allies, details of these are lacking and are being researched. It is believed most involved the landing and/or retrieval of agents on German-held coastlines, probably Greece. On completion of the last of these missions she was allocated to the training of Italian Navy ASW units as well as the training of cadets from the Naval Academy, then temporarily located at Taranto. Submarine surrendered at armistice on September 1943.

As a result of the drawing of lots conducted by representatives of the naval command of Great Britain, the United States, France and the Soviet Union after the Second World War, the latter received 45 ships and auxiliary vessels from Italy, including a battleship, a light cruiser, three destroyers and destroyers, two submarines, 10 torpedo boats, three patrol boats, the same number of landing craft, a military transport, a tanker, four water tankers, 12 tugs and a training sailing vessel.

On 06 February 1949, the act on the transfer of submarines was signed. On 19 February 1949, both submarines due to the Soviet Union (Magea and Nichelio), accompanied by the tanker "Feolent", left Valona, and on February 26.02.1949, they arrived in Sevastopol.

Upon arrival in the Soviet Union, Nichelio first became known as I-42, and from 16 June 1949 - as S-42. Although the submarine was enlisted in the submarine brigade, but it was used only for training.

On March 1955 the submarine S-42 was decommissioned, excluded from the lists of the Black Sea Fleet and scrapped in Inkerman.