Development of nevvlovvnickel, cobalt free maraging steel K. T. Tharian, D. Sivakumar, R. Ganesan, P. Balakrishnan, and P. P. Sinha A new low Ni, Cofree maraging steel containing 12Ni-3·2Cr-5·1Mo-ITi (wt-%) has been developed. The optimum heat treat1~ent w~sfound to be s~lu~ion treatment. at 1098 Kfor 60 minfollowed by air cooling then aging at 753 Kfor 180 min. Detazled optzcal and transmZSSlOn electron mzcroscopy revealed that the solution treated steel transformed to lath martensite. The optim~sed steel a~hieved an ultimate tensile strength of 1700 MN m- 2 and a yield strength of 1660 MN m- 2 • The toughness measured zn terms of zmpact energy was found to be 38 J. Fractographic analysis carried out in the scanning electron microscope showed predominantly dimpled structures indicating ductilefailure. The transformation temperatures for this new steel were established using dilatometry. MSTj 1398 © 1991 The Institute of Metals. Manuscript received 7 January 1991; in final form 25 June 1991. The authors are in the Material Processing Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum, India. Introduction Maraging steel has long been considered an excellent structural material for a number of critical technological applications in, for example, the aerospace, nuclear, and defence industries. H,owever, such steels are costly and contain two strategic alloying elements: Ni and Co. There have been numerous studies on the substitution of these elements with other less expensive elements. 1 - S A significant achievement was the joint development of 180/0Ni, Co free maraging steel by Teledyne Vasc0 3 and Inco, USA. In this alloy considerable strengthening was achieved by the addition of up to 1'5% Ti. An equally important modifi- cation was reported by Asayama,4 wherein not only could Co be removed, but the Ni content was reduced to 14°/0. Significantly, the Asayama steel also contained up to 1.5°/0 Ti. It has been reported 6 - 8 that the toughness of Fe- Ni alloys strengthened by Ti deteriorates sharply at higher strength levels because of the formation of low temperature Ti rich metastable ordered phases NiTi (B2) or Ni 3 Ti (D0 3 ) during the initial stages of aging. Higher Ti contents can also impair toughness by grain boundary precipitation of TiC or. TiCN unless the C content is kept very low and thermomechanical processing is highly controlled. There- fore, any newly designed maraging steels must have a minimal Ti content. . Bas~d?n the results of previous work,9-18 the present InvestIgatIon was undertaken to develop a new low Ni, Co free maraging steel having an Fe-Ni-Cr matrix and a minim urn tensile strength ~ 1700 MN m - 2. This required the selection of an alloy system which could be transformed to martensite using a simple heat treatment and be subsequently strengthened by a precipitation mechanism. One of the earliest attempts in this direction was the development of an Fe-12Ni-5Cr maraging steel 9 and martensitic transformation in the Fe- Ni-Cr system has been reported by a number of workers. lO ,l1 Asayama 12 has already described the beneficial effect on the toughness of Fe- Ni steels of Cr in combination with Ni and in the present work the effects of 11'5-13'5°/oNi and 3-5°/oCr were investigated. Although of the various alloying elements considered to serve as precipitation hardener in the Fe- Ni system Ti has been reported 13 ,14 to be the most effective for the reasons given above, the Ti content was confined to a maximum of 1'00/0. Another alloying element beneficial to both strength and toughness is Mo. It has been observed that Mo rich zones precipitate during the initial stages of 1082 Materials Science and Technology December 1991 Vol. 7 aging in maraging steel and playa very significant role in overall strengthening and in maintaining the toughness of the stee1. 1S - 18 Because Mo is a known inhibitor of embrittlement due to intergranular segregation of impurities to grain boundaries and is also a potential precipitation hardener, its influence in the range" of 3-5°/0 was examined. Experimental procedure Five heats of different chemical composition were produced by double vacuum melting using VIM-VAR*. The VAR ingots (110 mm dia.) were homogenised at i373 K for 600 min and forged down to 35 mm thick slabs. The slabs were further reduced by rolling to 15-20 mm thick plates in a 2 high/4 high hot/cold rolling mill. The chemical compositions of the homogenised steels are given in Table I. Steels M3,. M4, and M5 were designed essentially to optimise the chemistry and M6 and M7 were used to study the respective effects of additions of up to 5°1o Cr and Mo on Fe-12Ni-I·OTi. To establish the heat treatment behaviour, alloys M3, M4, and M5 were subjected to solution treatment (ST) at various temperatures in the range 1023-1223 K, held at temperature for 60 min, then air cooled (AC). To investigate the effects of solution treatment on tensile properties, tests were carried out on steels M4 and M5 after applying the standard aging treatment (753 Kjl80 minjAC) for 18Ni- 8Co-5Mo maraging steel and the optimum solution treat- ment temperature was determined. The effects of variations of aging temperature (623-823 K) and time (30-420 min) on hardness were established for all the steels after solution treatment at the optimum temperature. Having then established the optimum solution treatment and aging cycle, th~ tensile strength was evaluated. Values of Charpy V-notch Impact energy were determined for steels M4 and M5. Optical metallography (OM) was performed on samples after mechanical polishing then etching in 10°10 ammonium persulphate and 100/0 nital solutions. To corroborate that ana.lysis, transmission electron microscopy (TEM) was carned out for steel M5. Thin foils for TEM were prepared using a combination of mechanical and chemical thinning processes followed by electropolishing with 90°/0 methanol-I 0°/0 perchloric acid solution. All the * VIM vacuum induction melting; VAR vacuum arc remelting.