https://hal.archives-ouvertes.fr/hal-01715082/document

The effect of the addition of alloying elements on carbide precipitation and mechanical properties in 5% chromium martensitic steels

Indexable insert tools for machining operations are in service exposed to high temperatures and cyclic mechanical loads. Secondary hardening steels such as hot-work steels are commonly used for tools subjected to thermal exposure. However, these steels, highly alloyed with strong carbide forming elements as Cr, V and Mo, are generally difficult to machine and machining represents a large fraction of the production cost of a tool. Thus, the present study concerns the development of a new steel with improved machinability and meeting the requirements for high-temperature properties. Softening resistance of the THG2000 and QRO90 tool steels, commonly used in hot-work applications, and a newly developed tool steel MCG2006 with lower alloying content of carbide forming elements, was investigated by tempering and isothermal fatigue testing. Mechanisms of high-temperature softening of the tested tool steels were discussed with respect to their microstructure and high-temperature mechanical properties. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis. No difference in softening behaviour was found among the QRO90 and MCG2006 regarding hot hardness measurements. The THG2000 indicated some stabilization of the hardness between 450 and 550 °C and a considerable hardness decrease at higher temperatures. The short-time cyclic softening in isothermal fatigue was controlled by dislocation rearrangement and annihilation. The alloying composition of the steels presently tested had no influence on the dislocation density decrease. The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance. The QRO90 with greater molybdenum and lower chromium contents than in the THG2000 show the best resistance to softening among the tested grades at all temperatures. The MCG2006, leaner alloyed with the carbide forming elements and alloyed with 4 wt% nickel, has better temper resistance than THG2000 at higher temperatures and longer tempering times.

High-temperature properties and microstructural stability of hot-work tool steels

In this work, SiC particles were added to the molten pool during WAAM of a high strength low alloy steel. The introduction of these high melting point particles promoted grain refinement, and the precipitation of Fe3C due to SiC dissociation. The microstructural evolution was studied by optical and electron microscopy techniques and high energy synchrotron X-ray diffraction. Additionally, mechanical testing and hardness profiles were obtained for the SiC-containing and SiC-free parts. An improvement in the mechanical strength of the SiC-added WAAM parts was observed, which was attributed to the refined grain structure and finely dispersed Fe3C.

In-situ strengthening of a high strength low alloy steel during Wire and Arc Additive Manufacturing (WAAM)

The effect of silicon on precipitation and decomposition behaviors of M6C carbide in a Ni–Mo–Cr superalloy

The effects of annealing temperature and cooling rate on carbide precipitation behavior in H13 hot-work tool steel

In comparison with the conventional AISI H11 tool steel, which contains approximately 1 wt.% silicon, the modified steel AISI H11 (∼0.35 wt.% silicon) exhibits improved tensile and fatigue properties at 550 °C – the estimated tool surface temperature during the high-pressure injection of aluminium alloys. The effect of silicon on the stability of secondary carbides was studied using transmission electron microscopy and small-angle neutron scattering. Silicon has a considerable influence on the precipitation of secondary carbides. A higher volume fraction and density of small particles were observed in the low-silicon-grade steel, both after heat treatment and after fatigue testing. The final discussion focuses on the influence of silicon in the precipitation sequence. It is concluded that silicon has a detrimental effect as it shifts the secondary hardening peak towards lower tempering temperatures.

https://hal.archives-ouvertes.fr/hal-01715084/document

N Mebarki

Papers

Influence of silicon content on the precipitation of secondary carbides and fatigue properties of a 5%Cr tempered martensitic steel