Carbide transformations in Cr−Mo steels in the process of long-term aging and operation

Abstract

Chromium-molybdenum steels are widely used in machine-building as a material for pipes for different purposes. They operate at elevated temperatures for long durations and undergo structural changes that can worsen their mechanical properties. For this reason, it is interesting to study their structural changes as a function of the temperature and time of aging or operation in order to determine the regular features of the redistribution of the alloying elements in the process of phase transformations. The given work is devoted to carbide transformations in steels with different contents of chromium and molybdenum in the process of aging after heat treatment and in the process of long-term operation in industrial installations.The nature of carbide reactions as a function of the content of the alloying elements, the regimes of heat treatment, and long-term operation of Cr-Mo-V steels have been considered in [1,2].The present work is devoted to the results of a study of Cr-Mo steels (Table 1 ).These steels are commonly used after a heat treatment that consists of quenching or nonJaalizing followed by tempering in a temperature range of 700 -750~ or after annealing.We studied the carbide transformations in steels after aging at 450 -600~ Before the aging the specimens were subjected to a preliminary heat treatment using various regimes. Specimens 20 x 20 • 80 mm in size after a hold at 960-1000~ were cooled in water, in air, and in the furnace. The use of different cooling media made it possible to imitate the possible range of cooling rates of industrial parts. The study after a long-term operation was conducted for specimens cut from parts that have operated for various times.The carbide transformations were studied by the method of phase analysis. The carbides were isolated from the matrix by an electrochemical method in an electrolyte based on methanol. The obtained TABLE 2 powders were studied by the method of x-ray diffraction analysis in cobalt K,~-radiation. The amount and the composition of the carbides I5 were determined by a chemical analysis of the 100 precipitates. 1000 Steel 12MKh. In this steel the formed carbides can be of three types, namely, cementite M3C based on iron, carbide of M2C type based on molybdenum, and a composite cubic carbide of M23C 6 type. The proportion of the carbides in the steel varies with the aging conditions and the rate of preliminary cooling from the y-region, which can be seen clearly from the data of Table 2.The type of molybdenum carbides segregating in the aging process depends on the cooling rate in the preliminary heat treatment. After a rapid cooling the main result of the

Short‐term Creep Behavior of an X 37 Cr Mo V 5‐1 Hot‐work Tool Steel with almost Bainitic and fully Martensitic Microstructures

“…Especially, less coarsening resistant M 3(Fe,Cr) C carbides coarsen and coalesce stronger in tempered bainite than in tempered martensite (Figure 4a and 6a). Both microstructures show an increase in the number density of M 7(Cr,Fe) C 3 carbides, probably by transformation from M 3(Fe(Cr)) C type carbides 25–27. Additionally, an appreciable amount of M 6(Mo,Fe) C carbides is formed in tempered bainite, which is not present in tempered martensite in this order of magnitude.…”

Evolution of Carbide Precipitates in 2.25Cr-1Mo Steel during Long-Term Service in a Power Plant

“…[2] Carbide transformations in Cr-Mo steels with varying contents of chromium and molybdenum have been studied in order to determine the carbide reaction sequence. [3] Softening due to aging was clearly observed in annealed 2.25Cr-1Mo steel in the temperature range 697 to 977 K (424 to 704°C). [4] However, most of these previous studies are based on simulated aging processes to limit the aging times to those deemed reasonable for laboratory experiments.…”