Creation of extremely fine lath martensite combined with nano needle like structured carbides in ultra high strength (UHS) military Steel

Document Type : Research Paper

Authors

1 Plastic Deformation Dept., Central Metallurgical R&D Institute (CMRDI), 1st Elfelazaat St., El-Tabeen, 11722 Helwan, Egypt.

2 Plastic Deformation Dept., Central Metallurgical R&D Institute (CMRDI), 1st Elfelazaat St., El-Tabeen, 11722 Helwan, Egypt;

3 Department of Production Engineering and Mechanical Design, Faculty of Engineering, Tanta University, Tanta 31527, Egypt.

Abstract

The steel alloys of the current study contain 0.3% carbon with different amounts of Cr, and Mo, in addition to W. The alloys were cast as Y-blocks. Cast slabs with 150x100x35 mm dimensions were then subjected to 4-steps directional hot forging into 15 mm diameter round bars in the temperature range 1200-830 oC.

Austenite grain refinement was taken place by the 4-steps cross-sectional reductions, where it secures extreme fine lath martensite combined with crammed nano needle- like Cr23W2C6 carbides between the martensite laths.
A quenching tempering cycle was carried on the forged bars at 250 oC for 25 min. and then air cooled to modify the martensite laths and reorganize the nano needle-like carbides to modify the mechanical properties.

The optical microscopic investigation confirms extensive grain refining of the martensite laths. However, scanning electron microscope (SEM) photos at high magnifications reveal. The structure composed mainly of extreme fine laths martensite with nano needle-like structured carbides with 50 nm diameter crammed between the martensite laths. Little amounts of retained austenite (RA) phase was located tightening on the boundaries of martensite laths as white nano films. A blend of extreme-fine laths martensite and nano films of retained austenite (RA) phases in addition to the crammed nano needle-like structured carbides are working cooperatively for ultra-high strength creation. The tensile test specimens possessed continuous yielding, as the predominant microstructure contains a martensite phase. Strength increases continuously due to tungsten (W) addition up to 1.6%.

During tempering, some of the nano needle-like Cr21W2C6 carbides, which were crammed between the martensite laths have a chance to migrate out from the martensite laths. The migrated nano needle carbides favor to coagulate and stick to each other to form 3D nano bulk-size spheroidal shaped carbides sizing 200 nm and precipitating on the martensite laths surface.

Keywords

References

 

Journal of Ultrafine Grained and Nanostructured Materials
Volume 56, Issue 2
December 2023
Pages 173-183

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History

  • Receive Date: 20 September 2023
  • Revise Date: 11 December 2023
  • Accept Date: 14 December 2023

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