Effect of Heat Treatments on Microstructural Evolution and Tensile Properties of 15Cr12MoVWN Ferritic/Martensitic Steel

Abstract

In this study, the phase transformation temperature of 15Cr12MoVWN ferritic/martensitic steel was determined by differential scanning calorimetry to provide a theoretical basis for the design of a heat treatment process. An orthogonal design experiment was performed to investigate the relationship between microstructure and heat treatment parameters, i.e., normalizing temperature, cooling method and tempering temperature by evaluating the room-temperature and elevated-temperature tensile properties, and the optimum heat treatment parameters were determined. It is shown that the optimized heat treatment process was composed of normalizing at 1050 °C followed by air cooling to room temperature and tempering at 700 °C. Under the optimum heat treatment condition, the room-temperature tensile properties were 1014 MPa (UTS), 810.5 MPa (YS) and 18.8% (elongation), while the values are 577.5 MPa (UTS), 469 MPa (YS) and 39.8% (elongation) tested at 550 °C. The microstructural examination shows that the strengthening contributions from microstructural factors were the martensitic lath width, dislocations, M23C6, MX and grain boundaries of prior austenite grain (PAG) in a descending order. The main factors influencing the tensile strength of 15Cr12MoVWN steel were the martensitic lath width and dislocations.

A Review: Ferritic-Martensitic Steels – Treatment, Structure and Mechanical Properties

“…The martensitic lath width and dislocations were the main microstructural factors influencing the tensile strength of 15Cr12MoVWN steel [96]. The strengthening contribution from M 23 C 6 and MX was higher than that from grain boundaries of PAG and was the other important factor for strengthening.…”

“…The influence of HT on structure and tensile properties of 15Cr12MoVWN (HT9 type) FM steel was determined in [96]. HT regeimes were as follows: Austenitizing at 1000, 1050, and 1080 °C, tempering at 650, 700, and 760 °C with next water quenching (WQ), oil quenching or AC.…”

“…Generally, the sizes of М 23 С 6 precipitates are 60…200 nm, while МХ -≤ 50 nm [109][110][111][112][113]. The mean grain size according to [86, 96,114] is ~ 20 µm. But if the Eurofer 97 and T91 steels are supplied as a hot rolled plates, they present a fine structure with a PAG size in the ASTM range 10…11.5 (6.7…11 µm) for the 14 mm plate and ASTM 10.5…11 (8…9.4 µm) for the 25 mm thickness plate [92,115,116].…”

“…The microstructures of 15Cr12MoVWN steel after austenitizing at 1000, 1050, and 1080 °C, tempering at 650, 700, and 760 °C with next WQ, oil quenching or AC is investigated in [96]. It is observed that 15Cr12MoVWN steel has a fully martensitic structure without formation of δ-ferrite after all HT regimes.…”

The Microstructure and Mechanical Properties of Ferritic-Martensitic Steel EP-823 after High-Temperature Thermomechanical Treatment

“…The efficiency of a nuclear reactor and the possibility of increasing its operating temperature are determined, among other things, by the long-term strength properties of its structural materials. Both long-term and short-term strength properties of ferritic-martensitic steels are controlled by the elemental composition, microstructure features (dimensions of tempered martensite packets, volume fraction of ferrite, dislocation density), composition, dimensions and spatial distribution of carbide (carbonitride) phases [4][5][6][7][8][9][10][11]. Steels of this class are most thoroughly investigated in the structural state after normalization and tempering; this processing is referred to here as a traditional heat treatment (THT).…”