Precipitate Volume Fraction Estimation in High Strength Microalloyed Steels

Abstract

Many attempts have been made in the past to estimate the volume fraction of small precipitates in steels, both theoretically and experimentally. Theoretical analysis can be very complicated, while experimental analysis can be either difficult or can yield numbers that are inaccurate. In this paper, a simple method is presented to estimate the precipitate volume fraction of nanosize microalloy carbides. The method is based on a combination of simple and well-established thermodynamic calculations for high-temperature precipitation and a simple experimental method for the determination of solutes remaining in solid solution in the steel. Upper bound and lower bound values, as well as the results obtained from an extraction replica method, are presented and discussed.Résumé -Dans le passé, il y a eu plusieurs tentatives d'estimation de la fraction volumique de petits précipités dans les aciers, tant théoriques qu'expérimentales. L'analyse théorique peut être très compliquée, alors que l'analyse expérimentale peut être difficile ou peut produire des nombres qui sont faux. Dans ce document, on présente une méthode simple d'estimation de la fraction volumique de précipité de carbures nanométriques de micro alliage. La méthode est basée sur une combinaison de calculs thermodynamiques simples et bien établis pour la précipitation à haute température et d'une méthode expérimentale simple pour la détermination de solutés qui restent en solution solide dans l'acier. On présente et discute des valeurs limites supérieures et inférieures, ainsi que des résultats obtenus à partir d'une méthode d'extraction de répliques.

Evolution of microstructure and precipitation state during thermomechanical processing of a X80 microalloyed steel

“…In particular, the results shown in Fig. 13c are very similar to the size distribution reported for fine Nb-Mo rich precipitates (essentially carbides) that start to form between 950 and 1050 • C in a X100 steel [41]. On the other hand, the constant value of mean particle size measured throughout hot rolling is in agreement with previous works [30] that found that the precipitate size can strongly decrease during hot rolling when long interpass times that allow particle coarsening at high temperatures are used, but mean size is roughly constant throughout rolling if a short interpass time as those used in present work is applied.…”

“…the existence of two distinct populations of precipitates: coarser cuboidal carbonitrides rich in TiN that form above 1150 • C and finer and more spherical NbC-based precipitates that form between 1100 • C and 900 • C [73,80], even though they measured a coarser mean particle size than in this work. On the other hand, previous studies on X100 steels describe fine precipitates having average sizes near 4-5 nm with a remarkable presence of Mo together with smaller amounts of other microalloying elements substituting Nb [33,41]. In particular, the results shown in Fig.…”

“…Consequently, the precipitation of fine Nb-Mo precipitates in the ferrite that are beneficial for strengthening is easier [16] and accordingly the importance of final cooling and coiling temperatures to enhance precipitation strengthening has been suggested [8]. In fact, it is usually assumed that fine Mo carbides or carbonitrides do not form in austenite and only precipitate during transformation or already in the ferrite phase [36,40,41]. The absence of Mo precipitates in austenite might be true for pure Mo carbonitrides formed with single Mo additions [42], but it has been observed that Ti and Nb carbonitrides or carbides precipitated in the austenite are significantly complex in nature and can also include a certain amount of Mo [39,[43][44][45][46].…”

Hydrogen Embrittlement as a Conspicuous Material Challenge─Comprehensive Review and Future Directions

“…It should also be noted that the effectiveness of the second-phase trapping approach is highly dependent on the service environment of a material. Due to the constraint of H trapping/storage content arising from the typically low second-phase volume fraction (e.g., below 1% for strengthening carbides in ferritic steels 872 ), such approach is expected to be most effective for the cases with only a limited H uptake from environments (e.g., during the pickling process). However, it should not be an ideal approach for materials operated in H-abundant atmosphere (e.g., H transport and storage) where continuous H ingress will occur and inevitably will saturate the H traps.…”

Microstructure design strategies to mitigate hydrogen embrittlement in metallic materials

“…In addition to the characteristics of the matrix phase, the effectiveness of the second‐phase trapping approach is also closely linked to the service environment of a material, particularly to the amount of H that enters into a material. Typically, the volume fraction of introduced precipitates in strong and ductile materials is quite low (e.g., below 1% in ferritic steels 102 ). This suggests a limited H storage volume caused by the trapping effect.…”