Effect of hot isostatic pressing temperature on the properties of inert gas atomized maraging steel

Prealloyed 250-grade maraging steel powder produced by the rotating electrode process was fully consolidated by hot isostatic pressing (HIP) at 1100 and 1200°C. The strength following aging (3 h at 480°C) equalled that of wrought material; however, ductility was negligible. This lack of ductility in the powder metallurgy product was traced to titanium segregation which occurred at the powder surface during powder production. The formation of a titanium intermetallic at the prior particle boundaries during aging caused failure at low plastic strains. Altered aging treatments successfully broke up the embrittling film and resulted in a significant ductility recovery for the HIP material. Analysis of the fracture process indicates that further ductility gains are possible by reducing the titanium content, refining the particle size, and optimizing the thermal cycles.

Ductility in hot isostatically pressed 250-grade maraging steel

: The report presents the results and accomplishments of the first year of a three year research program investigating the production of solid bars from rapidly quenched liquid metals. Steam atomization was used to prepare coarse metal powders of a maraging steel VM 300, a nickel base alloy IN100 and cobalt base alloys. This atomization process can be used successfully with cobalt base alloys but does not appear to be suitable for the nickel base alloys. Improvments will have to be made to control the chemistry and the purity of the maraging steel coarse powders produced by steam atomization. Billets and solid bars were produced by hot isostatic compaction, or by extrusion of coarse powders. The bond strength between metal powder particles prior to cold or hot working of the compacted bars is strongly dependent upon the presence of impurities such as oxygen, hydrogen and carbon. Hot isostatic compaction followed by extrusion gave the best results in terms of mechanical properties and homogeneity of the billets. The influence of cooling environments and cooling rates on the secondary dendrite arm spacing and the extent of microsegregation within the coarse powder particles was studied in detail. The mechanical properties and the microstructures of the different alloys produced by the powder process are reported and compared to the properties of the commercially cast and wrought alloys having the same chemistry.

Merton C Flemings

Papers

Structure and Property Control through Rapid Quenching of Liquid Metals