Abstract
Continuous heating transformation (CHT) diagrams and continuous cooling transformation (CCT) diagrams of precipitation-hardening steels have the drawback that important information on the dissolution and precipitation of Cu-rich phases during continuous heating and cooling are missing. This work uses a comparison of different techniques, namely dilatometry and differential scanning calorimetry for the in situ analysis of the so far neglected dissolution and precipitation of Cu-rich phases during continuous heating and cooling to overcome these drawbacks. Compared to dilatometry, DSC is much more sensitive to phase transformation affecting small volume fractions, like precipitation. Thus, the important solvus temperature for the dissolution of Cu-rich phases was revealed from DSC and integrated into the CHT diagram. Moreover, DSC reveals that during continuous cooling from solution treatment, premature Cu-rich phases may form depending on cooling rate. Those quench-induced precipitates were analysed for a broad range of cooling rates and imaged for microstructural analysis using optical microscopy, scanning electron microscopy and transmission electron microscopy. This information substantially improves the CCT diagram.
Highlights
Precipitation-hardening stainless steels are iron– chromium–nickel alloys containing precipitationhardening elements such as aluminium, titanium, niobium, and copper (e.g. [1,2,3])
The precipitation reaction is seen as a weak slope discontinuity, indicating a relative shortening of the sample with respect to the expansion of the martensitic matrix at about 500 °C and it is hard to determine the characteristic temperatures of precipitation start and end from this dilatometer curve
Precipitation during continuous cooling from solution temperature is well seen in differential scanning calorimetry (DSC), while it is not detected in dilatometry
Summary
Precipitation-hardening stainless steels are iron– chromium–nickel alloys containing precipitationhardening elements such as aluminium, titanium, niobium, and copper (e.g. [1,2,3]). [1,2,3]) The strength of such precipitation-hardening steels can be increased substantially by an age-hardening process, resulting in a microstructure with fine precipitates that hinder dislocation gliding and, thereby, increase the strength. The alloying elements need to be kept in solid solution, resulting in supersaturation of the base material. This condition is far from equilibrium and is, unstable. From this unstable supersaturated solid solution during the final ageing treatment, a high number density of nano-scale precipitates grows, which hinder the dislocation movement and, thereby, increase the strength. A core/shell structure with Ni surrounding fine Cu precipitates is reported to occur within the sequence of Cuprecipitation [9]
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