Effects of cryogenic and stress relief treatments on temper carbide precipitation in AISI D2 tool steel

The effects of cryogenic and stress relief treatments on temper carbide precipitation in the cold work tool steel AISI D2 were studied. For the cryogenic treatment the temperature was −196°C and the holding time was 2, 24 or 30 h. The stress relief heat treatment was carried at 130°C/90 min, when applied. All specimens were compared to a standard thermal cycle. Specimens were studied using metallographic characterisation, X-ray diffraction and thermoelectric power measurements. The metallographic characterisation used SEM (scanning electron microscopy) and SEM-FEG (SEM with field emission gun), besides OM (optical microscopy). No variation in the secondary carbides (micrometre sized) precipitation was found. The temper secondary carbides (nanosized) were found to be more finely dispersed in the matrix of the specimens with cryogenic treatment and without stress relief. The refinement of the temper secondary carbides was attributed to a possible in situ carbide precipitation during tempering.

Tool wear evaluations in friction stir processing of commercial titanium Ti–6Al–4V

This research addresses the application of friction stir welding (FWS) of titanium alloy Ti–6Al–4V. Friction stir welding is a recent process, developed in the 1990s for aluminum joining; this joining process is being increasingly applied in many industries from basic materials, such as steel alloys, to high performance alloys, such as titanium. It is a process in great development and has its economic advantages when compared to conventional welding. For high performance alloys such as titanium, a major problem to overcome is the construction of tools that can withstand the extreme process environment. In the literature, the possibilities approached are only few tungsten alloys. Early experiments with tools made of cemented carbide (WC) showed optimistic results consistent with the literature. It was initially thought that WC tools may be an option to the FSW process since it is possible to improve the wear resistance of the tool. The metallographic analysis of the welds did not show primary defects of voids (tunneling) or similar internal defects due to processing, only defects related to tool wear which can cause loss of weld quality. The severe tool wear caused loss of surface quality and inclusions of fragments inside the joining, which should be corrected or mitigated by means of coating techniques on tool, or the replacement of cemented carbide with tungsten alloys, as found in the literature.