SummaryConventional low-alloy steels at strengths above 100 ton/in2 have been successfully used in various aerospace applications, but laboratory test results, service-performance data and problems in production indicated a need for steels which possessed improved mechanical properties, and which were simpler to machine and fabricate. The 18% Ni-Co-Mo maraging steels were developed in 1961 with the aim of fulfilling this need, and since that date several thousand tons of the steels have been produced and used in all common shapes and forms. In comparison with low-alloy steels of equivalent strength, 18% nickel maraging steels have greater resistance to fracture at highly-stressed notches, defects or cracks, and their susceptibility to hydrogen embrittlement/stress-corrosion failure is generally far superior. The un-notched rotating bend fatigue properties of machined and mar aged specimens of 18% Ni-Co-Mo maraging steels tend to be superior in the low-cycle range and similar in the low-stress range to the properties of low-alloy steels of comparable strength. The fatigue properties of unnotched, in contrast to those of notched, specimens are affected by heat-treatment/machining procedure. Experience gained in production of components has proved that the 18% nickel maraging steels are extremely simple to heat treat, to machine, to cold form and to weld. Exploitation of these advantages can frequently offset the higher initial cost of maraging steel. For example, an analysis of production costs for a complex aircraft component has demonstrated that the use of maraging steel enabled the component to be made by forging and machining at 5-20% lower cost than with a conventional low-alloy steel, despite the fact that initial material costs were five times greater. 18% Ni-Co-Mo maraging steels retain high strength at temperatures up to 350-400°C, and have given good service in short-duration heating cycles at much higher temperatures. Maraging steels may be surface hardened by various methods, and gears and dies treated by simultaneous nitriding and maraging have given excellent performance.The tremendous technological efforts which have been made in the past five years have yielded the majority of data and experience necessary to ensure successful use of the 18% Ni-Co-Mo maraging steels in the aerospace industry.
The Static and Low Cycle Fatigue Strength of Notched and Fatigue-Cracked Specimens of Low Alloy Steels