Rafael Agnelli Mesquita

High-Speed Steels Produced by Conventional Casting, Spray Forming and Powder Metallurgy

Powder metallurgy (PM) and spray forming (SF) have been reported as important alternative routes for tool steel production. The ability to promote refined and more uniform microstructures is their main advantages, leading to improved properties and larger isotropy. While PM application is a completely established technology, the SF process may be considered as a not totally explored field. Therefore, the present work aimed to study the potential of both processes, focusing at high-speed steel (HSS) production. AISI M3:2 highspeed steel was produced by conventional casting, spray forming and powder metallurgy. Conventional ingots and a 400 mm diameter SF billet were rolled to small diameter bars, with cross section around 110 mm. The PM material was evaluated in the as-HIPed condition, in comparative diameters. Large diameter HSS bars are used mainly in cutting tools, but are also applied in cold work tooling when high wear resistance is required. In the present characterisation, microstructures and bend test analysis were used, both in transverse and longitudinal directions. The results show that the as-HIPed PM material presents finer and more uniform carbide distribution, leading to a complete isotropy and higher toughness than conventional steel. In the SF material, carbides are also finer, have good distribution and the isotropy is considerably higher than that for conventional HSS.

Effect of Cooling Rate During Quenching on the Toughness of High Speed Steels

High speed steels are usually employed in cutting tools and forming dies. Heat treatment is an important step during the tool manufacturing process, being responsible for most of the final properties, mainly hardness and toughness. In this context, an especial interest relies on the effect of hardening variables on mechanical properties and tool performance. The present paper aimed to study the effect of cooling rate during the quenching process of standard designation AISI M2 high speed steel, under typical industrial conditions. The experiments were carried out in an industrial vacuum furnace, with high pressure nitrogen quenching. Several cooling rates were obtained by variation of the nitrogen pressure and by the use of test specimens with different dimensions. Toughness results were mainly evaluated through static bend test. Low cooling rates were shown to decrease material toughness and large parts presented a decrease in mechanical properties from surface to core regions. Carbide precipitation on grain boundaries are pointed as the main explanation for all these effects.

M3C and M7C3 Carbide Precipitation in Modified H11 Tool Steel

Recent modifications in chemical composition have been applied commercially to high alloy tool steels, using different combinations of Cr, Si and Mo contents. Several reports have been published in the literature about the effects of such modifications on mechanical properties and tool performance, but only a few of these studies were concerned with the effects on secondary carbide formation. In previous papers, improvements in toughness and tempering resistance that were found in a 5% Cr tool steel (type H11 with lower Si contents) have been attributed to particular distributions of Cr-rich M7C3 particles. Although M7C3 carbides have been studied extensively in low alloy steels, some important differences have now been observed by the present authors for high alloy tool steels, especially regarding the effects of Si and Cr. The present work is concerned with the formation of Cr-rich M7C3 as well as Fe-rich M3C particles in modified H11 tool steels, discussing the precipitation sequence and particle distributions developed during tempering within the martensite microstructure. By means of transmission electron microscopy, the effect of Si on M3C cementite formation has been found to be responsible for a substantial change in the distribution of the M7C3 carbide phase, leading to a concentration of these particles at high energy interfaces in interlath and interpackage regions.

Secondary Carbide Precipitation in Low Silicon Hot Work Tool Steels

A reduction from 1.0 to 0.3%Si has recently been shown to improve mechanical properties of H11-type hot work tool steels. The present paper shows that an important improvement in toughness can be explained by the effect of Si content on the precipitation sequence of secondary carbides during tempering after quenching. Carbide particle distributions were observed and identified by electron microscopy, allowing to relate the effect of Si on mechanical properties directly to its effect on cementite and subsequent alloy carbide formation during high temperature tempering.