Maraging steels based on iron–nickel martensite constitute a very important family of high-strength steels, which distinguishes itself by demonstrating an unparalleled combination of excellent fabricability and high strength and fracture toughness after heat treatment. Heat treatment of these steels has now been perfected to ensure consistently high levels of strength, ductility, and toughness in a variety of product shapes and sizes. Cobalt-free variants have been commercialized as part of efforts to save production costs. Further knowledge has been generated on 18% nickel maraging steels regarding phases precipitating during aging, thermal embrittlement, thermal cycling and austenite reversion/retention and their effect on mechanical properties. The paper reviews the progress made over the last one and half decades.

Progress in understanding the metallurgy of 18% nickel maraging steels

The present work reports effect of different heat treatment processes on the microstructure and hardness of C 250 grade maraging steel. Different heat treatment processes have been carried out and the resultant microstructure and hardeness have been characterized. Various structural characterization techniques, such as, optical microscopy, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, etc. have been adopted and the microstructural features thus evaluated have been correlated with the mechanical property, viz. hardness. Presence of Ni3(Ti,Mo) particles in lath martensitic matrix are revealed in solution treated and aged alloy. It is found that conventional maraging treatment consisting of solution treatment at 850 °C, followed by forced air cooling and finally ageing at 500 °C gives the best hardening response. Grain refinement techniques are not much effective for the improvement of hardness of maraging steel in the age hardened condition.

Study on Precipitation Hardening of C 250 Grade Maraging Steel

Among the various grades of commercially available 18 wt. % nickel maraging steels, the one with nominal 0.2% proof strength in the range 1700-1750 MPa is the most commonly used and is distinguished by an excellent combination of high strength and high fracture toughness. The main alloying elements are nickel, cobalt, molybdenum and titanium. The first three of these are present at relatively high concentrations in the chemical composition. The high cost of these metals leads to a high cost of production and this becomes a deterrent to extensive use of the steel. In the present study, an attempt was made to produce the steel by pegging the levels of these alloying elements in the lower half of the specified range. The objective was to save on the raw material cost, while still conforming to the specification. The steel so produced could not, however, attain the specified tensile properties after final heat treatment. The observed behavior is explained based on the role played by the different alloying elements in driving the precipitation hardening reaction.

/pdf/significance-of-alloying-element-levels-in-realizing-the-30exqhp6dx.pdf

Significance of Alloying Element Levels in Realizing the Specified Tensile Properties in 18 Wt % Nickel Maraging Steel

Improvement of fracture toughness and tensile ductility of massive rings made of 18 % nickel maraging steel Verbesserung der Bruchzähigkeit und Duktilität massiver Ringe aus 18%-Nickel-Maraging-Stahl

The hot ductility behaviour of a material exhibits a good correlation with its heat affected zone (HAZ) hot cracking sensitivity. This study was aimed at evaluating the HAZ hot cracking susceptibility of 18%Ni maraging steel and exploring the relationship between hot ductility and microstructure of the HAZ in an 18%Ni maraging steel. Optical metallography and scanning electron microscopy (SEM) investigations revealed that the ductility loss was accompanied by fracture transition from ductile transgranular mode to brittle intergranular mode. Fracture specimens were examined using SEM with an attached energy dispersive X-ray spectrometer. The SEM fractographs indicate that the ductile transgranular dimple fracture (i.e. coarse) surface was the titanium rich inclusion precipitated at the grain boundary, which promotes microvoid nucleation, growth, and coalescence and leads to localisation deformation until final fracture. Results of hot ductility tests also indicate that the titanium rich inclusion ...

Hot ductility behaviour of 18%Ni maraging steel

The thermal embrittlement behaviour of 18Ni cobalt free (250 grade) maraging steel (T250) compared with that of 18Ni–8Co–5Mo (250 grade) maraging steel (M250) was studied for different heat treatme...

T. Tharian

Papers

Thermal embrittlement in 18Ni cobalt free and 18Ni-8Co-5Mo maraging steels