The Effect of Hot-Mounting on the Microstructure of an As-Quenched Auto-Tempered Low-Carbon Martensitic Steel

Abstract

The effect of hot-mounting for metallographic studies of as-quenched low-carbon martensitic steels has been studied. Hot-mounting is typically carried out at 150–200 °C, i.e., a low-temperature tempering regime. Cold- and hot-mounted specimens from an as-quenched low-carbon auto-tempered steel were examined using a scanning electron microscope and their hardness levels were also compared. It was found that hot-mounting causes additional tempering that manifests as the appearance of new precipitates in those regions that are free of auto-tempered cementite. The observations were rationalized using DICTRA simulations to calculate the potential growth of cementite. Hot-mounting was also shown to cause a small but statistically significant increase in the hardness of the martensite.

Image Processing Tool Quantifying Auto-Tempered Carbides in As-Quenched Low Carbon Martensitic Steels

“…Due to the high M s temperature, the formation of martensite was followed by the precipitation of carbides. The precipitates have been previously identified as cementite [2]. This observation is also consistent with the observations of Speich [15], where only cementite precipitates form in quenched martensite with a carbon content <0.2 wt.%.…”

“…This study has shown that the custom image processing program was able to qualitatively estimate and compare the volume fractions of the auto-tempered carbides formed in as-quenched low carbon steels. To demonstrate how the image processing tool can be utilized in practice, a steel sample with the same chemical composition as that above was austenized and water-quenched at 1000 • C/s in a similar way to that described in [2]. The SEM micrograph of the resulting microstructure is shown in Figure 8a.…”

“…Cross-sections from the heat-treated cylindrical specimens were cut transverse to the long axes of the cylinders at the position of the control thermocouple. It was found previously [2] that hot mounting auto-tempered martensitic steel samples induces additional tempering. To prevent this, the samples were cold mounted with an epoxy resin (Struers EpoFix Resin, Struers ApS, Ballerup, Denmark) mixed with a hardener (Struers EpoFix, Struers ApS, Ballerup, Denmark), cured and left to harden for 24 h. The mounted samples were polished to a mirror finish using 0.04 µm colloidal silica, and subsequently etched with a 2% Nital solution.…”

“…High strength, excellent toughness, and good weldability are three steel properties that are highly desirable for mechanical applications [1]. Low-carbon (<0.2%) high-strength steels possess these three properties and, thus, are commonly used in structural applications [2]. For example, in the automobile industry, low-carbon high-strength structural steels offer the possibility of weight reduction while maintaining or increasing component strength [3].…”

“…Due to the small sizes of the carbides formed during auto-tempering, electron microscopy is the most optimal characterization technique to investigate the carbides [2]. By analyzing electron micrographs, it is possible to differentiate between regions of high and low auto-tempering.…”