It is well known that maraging steel is one of ultra-high strength steel. This steel is strengthened by fine precipitates in the martensitic matrix. The general precipitates are intermetallic compounds such as NiAl. It is also known that Ti addition leads to enhancing the strength by Ni3Ti precipitation but Ti addition may cause TiN precipitation which may deteriorate fatigue lives. Therefore Ti added maraging steels have not been used widely . To overcome the dilemma for higher strength of the steel, recently a maraging steel strengthened by not only NiAl but also carbides such as M2X are developed and used, which can obtain over 2000 MPa (300 ksi) tensile strength at room temperature without TiN precipitation risk. In this study, the influence of Al content on mechanical properties and microstructure of the developed steel were investigated to understand the effect of Al on strengthening mechanisms.
The base composition of the experimental steel is 0.2C-9Ni-15Co-2.5Cr-1.4Mo (mass%), and three steels changing Al content of 1.0, 1.25 and 1.5 mass% were melted by 50kg vacuum induction furnace. The obtained ingots were forged to bars of 22mm in diameter after homogenized at 1473K for 86.4ks. The forged bars were conducted of the following heat treatment sequence, solution treatment at 1173K for 3.6ks / air cooling, sub-zero treatment at 77K for 3.6ks and aging treatment at 723 - 823 K for 3.6 – 32.4 ks. Subsequently the microstructure was observed by optical microscope and SEM, EBSD, and Atom Probe Analysis. Then tensile and fracture toughness tests were carried out.
Consequently, it was confirmed that Al content attributes to increase amount of NiAl precipitation, which was corresponding to the hardness of about 7HV by increment of 0.1 mass% Al. In contrary, increasing Al content makes the block sizes in martensite structure large and it declines the toughness drastically.