2.2. Heat Treatment

To vary the material properties, the investigated particles and impact plates were heat-treated in a standard vacuum furnace under varying conditions prior to the deformation processes. Table 3 gives an overview of the parameters used for heat treatment. Due to their larger dimensions and different initial microstructural states, the holding times for the impact plates have been adjusted for each step. In the table, the holding times for the microscopic particles are underlined and the holding times for the impact plates are highlighted in grey.

Heat treatments (micro: microscopic particles, macro: macroscopic samples).

Figure 4 exemplarily shows the microstructure after heat treatment for the macroscopic workpieces. The annealing treatment (SA) led to a finely distributed spherodized state of cementite within a ferritic matrix. Chromium carbides are also visible. The Q800 (800 °C) heat treatment let to fine martensitic microstructure (martensite needles) with fine carbides. The former austenite grain size can be estimated at about a diameter of 30 µm. Heating the material up to an austenitisation temperature of 950 °C (Q950) led to a dissolution of most of the iron carbides. Figure 4c) is showing a fully martensitic microstructure with a low retained austenite content. The needle martensite is coarser and the retained austenite is light optically visible. Due to the austenitizing in the austenite field above A_cm no globular cementite is present any more. The Q1050 (1050 °C) variant is showing a coarse needle martensite microstructure with retained austenite. Because of the high austenitizing temperature, all carbides of iron and chromium carbide type are solute and a pronounced grain grows has occurred. The prior austenite grain size can be estimated to approx. 100 µm diameter. The morphology has changed from fine needle martensite into a coarse needle and stringer martensite with chevron martensite being present in some areas. Figure 4e shows the microstructure of the QT (quenched and tempered) variant. The tempering at 180°C of the quenched martensitic microstructure with very fine dispersed non optical microscopy visible retained austenite was achieved. Additionally, fine dispersed spherical carbides can be seen in the microstructure. Along with the carbides in some instances very little nonmartensitic black etching microstructure constituents can be seen, that are identified as troostite.

Microstructure of the macroscopic samples for single-impact peening after the heat treatment (a) SA, (b) Q800, (c) Q950, (d) Q1050, (e) QT); etched 25–60 s in 3% alcoholic HNO₃.