Deep cryogenic treatment applied in high-speed steel yielded promising results, and it was found that tempering played a vital role. This paper systematically studied the effect of tempering temperatures on the microstructural composition (residual austenite, martensite, and carbides) and mechanical properties (Vickers hardness and impact toughness) of AISI M35 high-speed steel subjected to deep cryogenic treatment. An increase in the tempering temperatures facilitated the transformation of residual austenite and the formation of martensite blocks corresponding to a decrease in the number of dislocations. Moreover, the carbide precipitation (secondary carbides and nanoscale carbides) began at a tempering temperature of 350 °C, increased at 450 °C, and reached its maximum at 550 °C. The fracture mechanism on the micro-level could be interpreted as follows: cracks occurred in the carbides with larger sizes (primary carbides and large secondary carbides) and at the carbide/matrix interface, and small secondary carbides decohered at the interface, forming microvoids and facilitating plastic deformation. In addition, the specimen tempered at 150 °C exhibited the highest hardness of 880.4 HV 1 due to the highest number of dislocations. The impact toughness of the sample tempered at 550 °C was the best, namely 2.50 MJ m −2 , due to an increase in the number of martensite block boundaries and the more homogeneous carbide precipitation. 

Microstructural variations and mechanical properties of deep cryogenic treated AISI M35 high-speed steel tempered at various temperatures

In this study, the effect of cryogenic treatment on the microstructure, especially the retained austenite (RA) evolution behavior, and mechanical properties of 55Cr17Mo1VN plastic die steel was investigated. The quenching microstructure consisted of martensite, RA, and undissolved carbides. After the cryogenic treatment, the RA volume fraction decreased from 44.7 to 35.4%, while the hardness increased from 39.0 to 54.6 HRC because of the increase in the dislocation density. Besides, the “new martensite” phase broke the blocky RA phase during the cryogenic treatment, and the two phases keep a strict K–S or N–W orientation relationship, which improved the stability of the steel. Interestingly, in the cryogenically treated sample, the RA decomposed completely. On the other hand, the untreated sample retained 20.7% of its RA phase after the tempering process. The cryogenic treatment decreased the thermal stability of the RA phase during the tempering process, which can be attributed to the release of the hydrostatic pressure and the generation of a large number of precipitates from the RA phase. As a result of precipitation strengthening, fine grain strengthening, and dislocation strengthening, the cryogenically treated sample showed excellent tensile strength (∼2241 MPa) and high hardness (∼56.2 HRC).

Effect of cryogenic treatment on microstructure evolution and mechanical properties of high nitrogen plastic die steel

Investigation of cryogenic friction and wear properties of Invar 36 alloy against Si3N4 ceramic balls

Effect of Sub-zero Treatment on the Microstructural Characteristics and Mechanical Properties of Maraging Stainless Steel

In the work, high nitrogen γ-Fe based alloy was fabricated by N2 pressure sintering in which the diffused N content could be up to 0.521wt%. Microstructure and mechanical properties were systematically studied. The increased activity of N atom with the added N2 pressure accelerated the generation of MnSiN2, which had an orthorhombic structure of an orientation relation of (11¯1)γ-Fe//(121¯)MnSiN2 with γ-Fe. HRTEM map displayed the incoherent interface of duplex phase on account of a large difference of the crystal plane distance between (11¯1)γ-Fe (d11¯1 = 0.2082 nm) and (121¯)MnSiN2 (d121¯ = 0.2428 nm). Besides, due to the solute N atoms in γ-Fe, an ordered structure with a superlattice structure of selective electron diffraction spots was formed. The elevated nanohardness for austenite grains implied the solution strengthening mechanism of N. The EBSD diagrams showed that the grain sizes of austenite are 9.81, 7.27 and 6.22um respectively, indicating that the formation and precipitation of MnSiN2 hindered the grain growth and led to the finer grain structure with the increased pressure of 0.05Mpa, 0.5Mpa and 1Mpa. The combination of solution strengthening and fine crystal reinforcing was the main reinforcement mechanism which enhanced the hardness austenite grains and wear resistance. 

Microstructure evolution and wear resistance of nitrogen-strengthened γ-Fe based alloy fabricated by N2 pressure sintering

Transition in wear behavior and mechanical properties of novel high nitrogen martensitic steel in cryogenic temperature regimes

Supersonic intake experiences various types of shock-shock and shock boundary layer interaction patterns along the inlet and isolator sections. Also, the flow path consists of small, protruded objects which cannot be avoided from the design point of view. Such protrusions induce not only the aerodynamic and thermal load on the surface of the aerodynamic bodies. In the present work, computational analysis has been performed at Mach number 2.0 to investigate the shock wave reflection pattern generated by a set of triangular-type protrusions placed along the scramjet isolator and its effect on the performance parameters. The isolator is a constant area duct between the inlet section and the combustion chamber of the scramjet intake. For the computational analysis, a duct was considered with constant length and height in which two wedges were kept at in the middle of the duct facing each other, then the flow was passed over it. The oblique shocks generated due to the presence of the wedges, the shock type has been determined past the wedges whether the deflection contributes to Mach reflection or Regular reflection. Using theoretical analysis, the pressure deflection diagram has been obtained by varying turn angles with a constant freestream Mach number and correlated with the data obtained from numerical simulations. The comparison of shock polar suggests a small transition region between Mach reflection and regular reflection phenomenon. It was also observed that the flow gets detached before the maximum deflection angle due to the incoming boundary layer. 

Aishwarya Upadhye

Papers

Transition in wear behavior and mechanical properties of novel high nitrogen martensitic steel in cryogenic temperature regimes

Computational and Analytical investigation of the effect of protrusion generated shock reflections on intake flow performance