This study based on dilatometry test and microstructural observations showed a different phase transformation path during conventional heat treatment of AISI D2 tool steel instead of the traditionally expected fully martensitic transformation. Transformation started with carbide precipitation and continued with allotriomorphic ferrite, Widmanstätten ferrite, acicular ferrite, and finally bainite formation. This microstructure showed microhardness value of 730 HV, which is very high for conventionally cooled samples. They are not expected to have gone under martensitic transformation. The results indicated that bainitic transformation take place even at temperatures near 173 K and to obtain fully martensitic microstructure, cooling rates in the range of 50K.s -1 must be utilized.
The influences of friction stir welding process parameters on microstructure evolution and mechanical properties of lap welded 5456 aluminum alloy plates with different thickness and temper conditions were investigated. The upper plate was 5mm thick, cold rolled aluminum alloy 5456-T321 and the lower plate was 2.5mm annealed sheet (5456-O). Four different pin geometries (conical thread pin, cylindrical-conical thread pin, stepped conical thread pin, and Flared Triflute pin tool) and two rotational speeds (600 and 800rpm) were used to produce the joints. Microstructures and microhardness values in the weld nugget (WN), thermomechanically affected zone (TMAZ), and the heat affected zone (HAZ) were examined and correlated with selected processing conditions. Specifically, the influence of tool geometry on the flow of the plasticized material in the nugget zone, extent of hooking defect, and mechanical properties (microhardness) of the FSW joints were documented and quantified.It was found that weld joints made by using the stepped conical thread pin tool produced a homogeneous microstructure with finer grain size (5.4 µm) and higher microhardness levels than the other tools. The optimum processing conditions resulting in sound and defect free joints with highest mechanical properties were obtained with the stepped conical thread pin and 600rpm rotational speed.The evolution of the microhardness in each region is characterized and related to processing conditions.
Abstract:Martensitic transformation of AISI D2 tool steel continuously cooled from 1303 K to the cryogenic temperature of 173 K is investigated by dilatometry using 10 Ks -1 or 50 Ks -1 cooling rates. A 'typical' expansion takes place from the s M temperature and reaches a maximum at 325 K. However, an atypical behavior is observed below this temperature implying the activation of further martensitic transformation. A modification to existing equations is proposed, which allows for more accurate description of the kinetics of martensitic transformation. Scanning electron microscopic studies indicated the presence of plate and lath martensite for both cooling rates. Carbide precipitation takes place at the rate of 10 Ks -1 before the start of martensitic transformation while it was not observed when the 50 Ks -1 rate was used. Transmission electron microscopic studies revealed that the microstructure also contains a significant amount of nano twined martensite.
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