Abstract
This study examined the microstructure and hardness of as-received and nitrided AISI 420 martensitic stainless steels. High temperature gas nitriding was employed to treat the steels at 1200°C for one hour and four hours using nitrogen gas, followed by furnace cooled. Chromium nitride and iron nitride were formed and concentrated at the outmost surface area of the steels since this region contained the highest concentration of nitrogen. The grain size enlarged at the interior region of the nitrided steels due to nitriding at temperature above the recrystallization temperature of the steel and followed by slow cooling. The nitrided steels produced higher surface hardness compared to as-received steel due to the presence of nitrogen and the precipitation of nitrides. Harder steel was produced when nitriding at four hours compared to one hour since more nitrogen permeated into the steel.
Highlights
AISI 420 martensitic stainless steel in the form of 20 mm diameter rod manufactured by Böhler Edelstahl GMBH & Co KG, Austria was selected for this study
The grain size of the nitrided steels enlarged after nitriding as shown in Figure 1b and 1c
It was inevitable to prevent the grain growth when it involved high process temperature such as High temperature gas nitriding (HTGN) followed by slow cooling to room temperature
Summary
Permeation of nitrogen in steel occurs through dissolution in austenite up to its solubility limit and this takes place according to reaction (1) [16]. Strong affinity between nitrogen and chromium at elevated temperature leads to formation of chromium nitrides. These precipitates are formed according to reaction (2) and (3) [16]. Even though nitrided austenitic stainless steels have established themself in many industrial applications, nitrided martensitic stainless steels remain a good choice for low temperature applications and mildly corrosive environments [12]. These steels have the highest strength among the stainless steels.
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