V-rich MC Research Articles

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness. Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M 7C 3 types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M 2C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M 6C developing at the expense of the fine M 7C 3 carbides previously formed.

Evolution of Carbides during Aging of a Spray-Formed Chromium-Containing Tool Steel

The evolution of carbides during aging of a spray-formed chromium-containing tool steel was studied. In the as-spray-formed steel, there are two prominent types of carbides: the V-rich proeutectoid MC and the Fe-rich M3C in lower bainite. Evolution of the carbides during aging can be described as follows. While the proeutectoid MC remains unchanged, a portion of the M3C dissolves into the bainitic ferrite matrix, and another portion of it is transformed into Cr-rich M7C3. In addition, fine alloyed carbides, such as M7C3, MC, Cr-rich M23C6, and Mo-rich M6C, precipitate from the matrix consisting of bainitic ferrite, martensite, and retained austenite.

Influences of Co addition and austenitizing temperature on secondary hardening and impact fracture behavior in P/M high speed steels of W–Mo–Cr–V(–Co) system

The secondary hardening and fracture behavior in P/M high speed steels of W–Mo–Cr–V(–Co) system has been investigated in terms of Co addition and austenitizing temperature. Austenitizing was conducted at 1100 and 1175 °C of relatively low and high temperatures, respectively. Tempering was performed in the range of 500–600 °C. Coarse primary carbides retained after heat treatment were V-rich MC and W–Mo-rich M 6C types. Since the dissolution of W–Mo-rich M 6C with relatively low stability was more promoted with increasing austenitizing temperature, the alloy content of the W and Mo in the matrix was enhanced. In turn, it gives a significant influence on the precipitation of fine secondary alloy carbides, that is, secondary hardening during tempering. The major secondary carbides were W–Mo–Cr-rich M 2C type. The peak hardness was observed in the tempering range of 500–540 °C, depending on Co addition and austenitizing temperature. With an increase in austenitizing temperature, the aging deceleration was observed. This phenomenon may be attributed to the increased content of W and Mo in the matrix, both diffusing slowly in the matrix and inhibiting growth of M 2C carbide, as compared to Cr. In addition, the aging acceleration occurred in the Co bearing alloy, promoting the precipitation of M 2C carbides, as well as the overall increase in hardness. In general, the impact toughness was decreased with an increase in hardness. In addition, the impact toughness to hardness balance was lowered in the Co bearing alloy.

Characterization of carbides at different boundaries of 9Cr-steel

A combination of structural and analytical investigation by transmission electron microscopy was carried out to characterize carbides formed at two types of boundaries in strain-relieved 9Cr-steel, prior-austenite grain boundary (PAGB) and small-angle grain boundary of subgrain boundary (martensite-lath boundary). Structural characterization by selected-area electron diffraction pattern revealed the presence of fine V-rich MC- and coarse Cr-base M23C6-type carbides at prior-austenite grain boundaries, while fine Mo-base M2C- and coarse M23C6-type carbides at small-angle grain boundaries. Analytical investigation, spectral and mapping analysis, by energy dispersive X-ray spectroscopy (EDS) showed the elemental composition and distribution of carbides at both boundaries. The composition of M23C6-type carbides at both boundaries were almost the same. Even though the processes of M23C6-type carbide formation are expected to be different from each other, the origin of similarities in compositions is ascribed to being the stable phase of M23C6-type carbides. The formation mechanisms of these carbides at different boundaries are also proposed.

Identification of the Precipitates by TEM and EDS in X20CrMoV12.1 After Long-Term Service at Elevated Temperature

The crystal structures, morphologies, and compositions of carbides in a martensitic pipe steel of type X20CrMoV12.1 after long-term service at 550 °C have been carefully investigated using transmission electron microscopy (TEM) and dispersive x-ray spectrometry (EDS). Most of carbides are M23C6. The M23C6 particles in prior austenite grains and in martensite lath boundaries are extensively coarsened. Most of the finer carbides within the matrix are also identified as M23C6, and no other type of carbide was found except for some tiny plate-like V-rich carbides identified as MC. The V and O mass fractions in the M23C6 carbides have variable morphologies and locations, which might be due to their coarsening process. They are coarsening at different rates even after long-term exposure at elevated temperature. The formation of V-rich MC during the long-term service contributes to hardness maintenance and is good for microstructural stability.

The postweld heat-treatment response of simulated coarse-grained heat-affected zones in a new ferritic steel

The tempering behavior of simulated coarse-grained (CG) heat-affected zones (HAZs) in two ferritic alloy steels, 2.25Cr-1Mo and HCM2S, was investigated. The hardness of HCM2S was found to be stable at longer times and higher temperatures than the 2.25Cr-1Mo steel, even though the “as-welded” hardnesses were approximately equal. Both materials reached a peak secondary hardness after tempering for 5 hours at 575 °C. The increase in hardness of the 2.25Cr-1Mo steel was due to precipitation of Fe-rich M3C carbides within the prior-austenite grains, whereas the secondary hardening in HCM2S was due to a fine dispersion of intragranular, W-rich carbides. The HCM2S steel retained its hardness at longer times and higher temperatures than 2.25Cr-1Mo steel, because of the precipitation of intragranular, W-rich carbides and V-rich MC carbides that stabilized the lath structure. This study shows that HCM2S should not be heat treated in the same way as 2.25Cr-1Mo steel and also provides a basis for defining the postweld heat treatment (PWHT) of HCM2S.

Evolution of secondary phases 12% Cr steel during quenching and tempering

Abstract Secondary phase evolution in the 12 wt.% Cr steel austenitized at 1100 and 1330°C and subsequently tempered at 750°C was investigated. To describe the microstructures and identify the secondary phases, methods of light microscopy and transmission electron microscopy including electron diffraction and microanalysis were used. It was found that oil quenching from 1100°C leads to formation of autotempered martensite containing intragranular (Fe4Cr)3C and MN particles. After oil quenching from 1330°C the microstructure consists of martensite, δ-ferrite, intragranular MN particles, and M23C6 particles precipitating at the γ/δ interfaces. In microstructures of tempered states Cr-rich M23C6, Ti-rich MN, and V-rich MC particles were found. It was shown that quenching temperature and/or appearance of δ-ferrite in the microstructure do not significantly affect the evolution of secondary phases during tempering.