The precipitation-hardened steels were developed in 1950s century for the purpose of combining good mechanical properties with corrosion resistance at high temperatures. Precipitation hardening consists of two steps. The first step is solution treatment where steel is heated to a temperature sufficient for the dissolution of any present precipitates and obtaining a homogenous austenite microstructure. The second step is artificial aging at high temperatures where numerous fine precipitates are formed which cause an improvement of mechanical properties of steel.
The purpose of diploma work was to research the effect of conditions of precipitation hardening on microstructure and hardness of 17-4 PH stainless steel. Hardness measurements were taken on a Rockwell hardness tester. The microstructure was examined on a light and on a scanning electron microscope. Steel, solution treated at 1080 °C and air quenched to room temperature, had slightly higher hardness (35,6 HRC) than steel which was solution treated at 1000 °C and air quenched to room temperature (34,9 HRC). Steel aged for 4 hours had the highest hardness at temperature 400 °C. The highest hardness was measured (46,6 HRC) on steel, solution treated at 1080 °C and aged for 1 hour at 480 °C. The hardness of steel aged at 700 °C and double aged steel did not improve compared to the hardness of solution treated and quenched only steel. The microstructure of quenched steel consisted of lath martensite, the microstructure of steel aged at 480 °C consisted of tempered martensite and copper-rich precipitates while the microstructure of steel aged at 700 °C consisted of martensite as well ferrite. NbC precipitates were observed in the microstructure in all cases.
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