The "Steinmetz transform" is most likely referring to the phasor method. For an introduction, see "Steinmetz and the Concept of Phasor: A Forgotten Story" by Araújo and and Tonidandel. (Caveat: the history of the method and characterizations of Heaviside and his and others' possible influence on Steinmetz are more accurately presented in the refs in the historical notes below.)
Historical notes:
Charles Proteus Steinmetz, the guru of Electric City, was a fascinating character and quite famous in his day--"In 1922, Thomas Edison came to visit Steinmetz. By then, Edison was nearly deaf, and Steinmetz tapped out a message on Edison’s knee in Morse Code. Edison beamed, and the two continued their silent conversation in front of bewildered reporters" from Charles Proteus Steinmetz, the Wizard of Schenectady. See also the two photo ops with Einstein and luminaries in the birth of telephony and radio in Professor Einstein Looks into Radio.
For any history buff, the PBS specials "Divine Discontent: Charles Proteus Steinmetz" and "Divine Discontent: Charles Proteus Steinmetz Extended Roundtable" are a must see.
For detail on the evolution of the method (and the slow introduction of imaginary numbers in the American and European curricula and possible influence of Heaviside), see Charles P. Steinmetz and the Development of Electrical Engineering Science by Kline. Certainly Heaviside influenced Stenmetz's associates E. J. Berg and J. R. Carson. According to Kline, "Berg's conversion to the Operational Calculus came sometime between 1912 and 1918." Berg mentions Heaviside in his lecture notes in 1916. Carson published "The Heaviside Operational Calculus" in Bell System Technical Journal in 1924. Heaviside used his operational/fractional calculus in his characterization of electric circuits, particularly in telegraphy. Both Carson and Berg are with Steinmetz and Einstein in the first photo op.
For a more accurate portrayal of Heaviside's relationships with others, see The Maxwellians by Hunt.