© Copyright by International OCSCO World Press. All rights reserved. 2008 VOLUME 29 ISSUE 1 July 2008 Research paper 31 of Achievements in Materials and Manufacturing Engineering of Achievements in Materials and Manufacturing Engineering Microstructure, properties and hot deformability of the new maraging steels S.J. Pawlak*, W. Zalecki Institute for Ferrous Metallurgy, ul. K. Miarki 12-14, 44-100 Gliwice, Poland * Corresponding author: E-mail address: spawlak@imz.pl Received 09.04.2008; published in revised form 01.07.2008 Materials AbstrAct Purpose: The effects of relevant metallurgical factors on the structure, fracture mode and properties of the high cobalt and cobalt free maraging steel has been studied. The aim was to better understand structure- property relations and enhance mechanical properties of the steels. To provide data needed for production and manufacturing technology, the high temperature deformability using physical simulation method was used. Design/methodology/approach: To study structure-property relation, broad range of the experimental techniques was used: quantitative metallography, X-ray diffraction phase analysis, transmission electron microscopy and SEM fractography. The flow properties in the range of hot working processes were determined by physical simulation approach, using Gleeble 3800 system. Findings: The cobalt-free maraging steel proved to be a valuable structural steel. At much higher fracture toughness it had only about 100 MPa lower yield stress, compared to that of high cobalt steel. Fracture surface morphologies were highly dependent on the steel grade and type of the mechanical test. The hot stress-strain characteristics were established for cobalt free maraging steel and compared to that of a stainless steel. Research limitations/implications: To fully evaluate potential field of applications, deeper comparative studies of the high cobalt and cobalt-free maraging steels are needed, particularly fracture modes and service properties of some parts. Practical implications: Very high mechanical properties and fracture toughness values obtained for the steels studied, make them suitable for advanced structural applications. The studies on the hot deformation behaviour of the steels are of practical value for the hot working process development. Originality/value: Detailed evaluation of the metallurgical purity, microstructure and fracture modes, allowed for better understanding of the microstructure-property relationships in selected high strength steels. The results obtained are of practical value for the development, production and manufacture of the high strength maraging steels with improved properties. Keywords: Metallic alloys; Maraging steels; Mechanical properties; Hot deformability; Microstructure 1. Introduction In the development of the structural materials for the advanced applications, as in the aircraft industry, there is a constant strive to increase both strength and toughness. The best known ultrahigh strength steel grades (UHSS) - those with yield stress > 1400 MPa - are the quenched and tempered medium carbon low alloy Cr-Ni-Mo steels, having total alloy content limited to few percent. The other steels in the UHSS category, are the high nickel maraging steels, with the standard 18Ni-9Co- 5Mo-Ti grade, often designated as MS250 [1-3]. High toughness and many technological advantages over the medium carbon low alloy steels, assured spreading of that steel, despite high price of the alloying elements. The salient step in the maraging steel development was introduction of the high cobalt AM100 (0.23C- 11.5Ni-13.4Co-1.5Mo) grade. In the aged condition this steel has the yield strength well over 1700 MPa, at very high toughness [4- 1. Introduction