Abstract
Standard heat treatment of martensitic stainless steel consists of quenching and tempering. However, this results in high strength and hardness, while Charpy impact toughness shows lower values and a large deviation in its values. Therefore, a modified heat treatment of 0.1C-13Cr-3Ni martensitic stainless steel (PK993/1CH13N3) with intercritical annealing between Ac1 and Ac3 was introduced before tempering to study its effect on the microstructure and mechanical properties (yield strength, tensile strength, hardness and Charpy impact toughness). The temperatures of intercritical annealing were 740, 760, 780 and 800 °C. ThermoCalc was used for thermodynamic calculations. Microstructure characterization was performed on an optical and scanning electron microscope, while XRD was used for the determination of retained austenite. Results show that intercritical annealing improves impact toughness and lowers deviation of its values. This can be attributed to the dissolution of the thin carbide film along prior austenite grain boundaries and prevention of its re-occurrence during tempering. On the other hand, lower carbon concentration in martensite that was quenching from the intercritical region resulted in lower strength and hardness. Intercritical annealing refines the martensitic microstructure creating a lamellar morphology.
Highlights
The quasi binary phase diagram with the variation of the Ni in Figure 1a shows that both Ae1 and Ae3 points are lowered with the increase of Ni
The austenite that was formed at intercritical annealing contains less carbon, chromium, molybdenum and vanadium due to the stability of M23 C6 carbides
The conventional (Q + T) heat treatment results in a formation of a carbide film on the prior austenite grain boundaries that has a detrimental effect on the impact toughness and causes a large deviation in its values
Summary
Stainless steels contain at least 11% Cr. Because of the passivating chromium oxide film formation they exhibit good corrosion resistance and are widely used in the industrial sector, from the automotive and aerospace industries to food and material processing lines [1]. Stainless steels are classified into different categories according to their microstructures, namely ferritic, austenitic, duplex, martensitic and precipitation hardening. The different microstructures of stainless steels possess different properties that have been extensively studied [2,3,4,5]. Stainless steel microstructures depend mainly on the chemical composition, especially chromium and nickel [6,7]. Chromium increases the stability of ferrite, while nickel is the main austenitic stabilizer. Both elements substantially improve hardenability that can lead to martensite formation
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