Effect of the Thermo-Mechanical Processing on the Impact Toughness of a 12% Cr Martensitic Steel with Co, Cu, W, Mo and Ta Doping

Abstract

This paper presents the results of an experimental investigation of a 12% Cr steel where a significant increase in Charpy impact toughness and a slight decrease in ductile-brittle transition temperature (DBTT) from 70 °C to 65 °C were obtained through thermo-mechanical processing, including interim hot forging at 1050 °C with long-term annealing at 1000 °C, as compared with conventional heat treatment. At lower temperatures ranging from −20 °C to 25 °C, the value of impact toughness comprised ~40 J cm−2 in the present 12% Cr steel subjected to thermo-mechanical processing. The amount of δ-ferrite decreased to 3.8%, whereas the size of prior austenite grains did not change and comprised about 40–50 μm. The boundaries between δ-ferrite and martensitic laths were decorated by continuous chains of Cr- and W-rich carbides. M23C6 carbides also precipitated along the boundaries of prior austenite grains, packets, blocks and martensitic laths. Thermo-mechanical processing increased the mean size of M23C6 carbides and decreased their number particle densities along the lath boundaries. Moreover, the precipitation of a high number of non-equilibrium V-rich MX particles was induced by hot forging and long-term normalizing at 1000 °C for 24 h.

Thermo-Mechanical Processing as Method Decreasing Delta-Ferrite and Improving the Impact Toughness of the Novel 12% Cr Steels with Low N and High B Contents

“…As it was shown in Ref. [33], additional non-equilibrium V-rich MX phase with a mean size of 30 ± 5 nm precipitated after the TMP (Figure 7d, Table 6). The average chemical composition of this phase was 75% V, 20% Nb, and 5% Cr (Figure 7d).…”

“…As it was mentioned in [33], the significant increment in entire Charpy energy curve for the 12% Cr steel via the thermo-mechanical processing was accompanied with the structural changes in (a) the delta-ferrite amount, (b) the dispersion of M 23 C 6 carbides and (c) an additional precipitation of nanoscale MX carbonitrides. However, such thermo-mechanical processing provides a slight decrease in the DBTT from 70 • C (after conventional heat treatment) to 65 • C [33]. This led to the modification of the thermo-mechanical processing via decreasing the temperature of homogenization annealing from 1200 • C to 1150 • C as it was in a conventional heat treatment.…”

“…Equilibrium phase diagrams were evaluated via Thermo-Calc software (Version 5.0.4 75, Thermo-Calc software AB, Stockholm, Sweden, 2010). Analysis of the mechanical properties and structural characterization technique was considered in detail in [7,8,10,[18][19][20][21][31][32][33]36].…”

“…The average chemical composition of this phase was 75% V, 20% Nb, and 5% Cr (Figure 7d). Since this phase was non-equilibrium and could not be estimated via Thermo-Calc software, their volume fraction was calculated using TEM images in dark field [33] as follows:…”

“…In the study [33], we reported on the positive impact of thermo-mechanical processing on the decrease in delta-ferrite to 3.8% (vs. 10% as it was after conventional heat treatment) and increase in impact toughness of the 12% Cr steel with the low N and high B contents. As it was mentioned in [33], the significant increment in entire Charpy energy curve for the 12% Cr steel via the thermo-mechanical processing was accompanied with the structural changes in (a) the delta-ferrite amount, (b) the dispersion of M 23 C 6 carbides and (c) an additional precipitation of nanoscale MX carbonitrides. However, such thermo-mechanical processing provides a slight decrease in the DBTT from 70 • C (after conventional heat treatment) to 65 • C [33].…”