The Influence of Lath, Block and Prior Austenite Grain (PAG) Size on the Tensile, Creep and Fatigue Properties of Novel Maraging Steel

Abstract

The influence of martensitic microstructure and prior austenite grain (PAG) size on the mechanical properties of novel maraging steel was studied. This was achieved by looking at two different martensitic structures with PAG sizes of approximately 40 µm and 80 µm, produced by hot rolling to different reductions. Two ageing heat-treatments were considered: both heat-treatments consisted of austenisation at 960 °C, then aging at 560 °C for 5 h, but while one was rapidly cooled the other was slow cooled and then extended aged at 480 °C for 64 h. It is shown that for the shorter ageing treatment the smaller PAG size resulted in significant improvements in strength (increase of more than 150 MPa), ductility (four times increase), creep life (almost four times increase in creep life) and fatigue life (almost doubled). Whereas, the extended aged sample showed similar changes in the fatigue life, elongation and hardness it displayed yet showed no difference in tensile strength and creep. These results display the complexity of microstructural contributions to mechanical properties in maraging steels.

The Influence of Segregation of Mn on the Recrystallization Behavior of C-Mn Steels

“…In comparing the fully recrystallized homogenized sample (30 seconds) and segregated sample (300 seconds), it can be seen that the mode and largest grains are nearly 50 pct greater in the segregated sample, and this is likely to have implications for the final product as the prior austenite grain size affects the transformed microstructure. For example, this grade of steel is used for forged automotive components where the prior austenite grain size affects the martensite packet and lath size, which in turn influence the mechanical properties [20,21] : a larger mode packet and lath size reduces the yield strength of the material, while a larger coarse (or bimodal) martensite packet and prior austenite grain size will cause large scatter in toughness. [22,23] To obtain such a distinct difference between the homogenized and segregated sample distributions after 30-seconds-hold means that a significant difference in the global Rf time for the material must be present.…”

A novel ultra-high strength maraging steel with balanced ductility and creep resistance achieved by nanoscale β-NiAl and Laves phase precipitates

“…In order to provide a material solution for low pressure turbine shaft applications in jet engines, a novel maraging steel with ultra-high strength and good resistance to creep at temperatures up to 500 °C has recently been developed [8,9]. Unlike the traditional 18Ni maraging steels that mainly utilise Ni3Ti precipitates to provide strengthening, the new alloy contains much lower amount of Ni (around 7 wt.%).…”

The Effect of a Two-Stage Heat-Treatment on the Microstructural and Mechanical Properties of a Maraging Steel

“…The lowest austenitisation treatment has more block boundaries than for the highest austenitisation treatment. This is probably due to the smaller PAG size, since a relationship between block and PAG sizes has been found in previous work [ 19 ]. The precipitates formed during austenitisation could act as nucleation points for martensite or may stop the progress of martensite as it forms, but there is no clear evidence for this based on these maps.…”

“…In previous work on this alloy, the influence of martensitic microstructure have been studied [ 19 ], and the phases and precipitation in the alloy have been studied by transmission electron microscopy (TEM) and atom probe tomography (APT) [ 8 ]. In a previous work a model [ 9 ] was presented to relate the precipitate volume fraction and size from small angle neutron scattering (SANS) measurements with the strength of three similar alloys one of which (F1E) is the one studied in this work.…”