High temperature intergranular crack growth in martensitic 214Cr-1mo steel

doi:10.1016/0001-6160(88)90018-1

Acta Metallurgica

Volume 36, Issue 2, February 1988, Pages 425-439

https://doi.org/10.1016/0001-6160(88)90018-1 Get rights and content

Abstract

Micromechanisms of high-temperature crack growth in martensitic $214 Cr-1Mo$ steel have been studied in vacuum, under static loading at 500°C. Detailed metallographic and fractographic measurements have been combined with Scanning Auger Microscopy and crack growth resistance curves to characterise the micro-mechanisms of failure. At low stress intensities, less than 48–55 MPam^$12$, the mode of crack growth is high-temperature, brittle intergranular fracture (HTBIGF) and is controlled by the dynamic segregation of sulphur to crack-tip regions. Crack advance appears to occur by discrete jumps when a critical concentration of sulphur is achieved over the jump-distance. At high stress intensities, greater than 48–55 MPam^$12$, the mode of fracture changes to intergranular microvoid coalescence (IGMVC), and is controlled by the distribution of sulphides. Of crucial importance are the relatively fine sulphides that reprecipitate from solid solution during the austenitising treatment. The transition from HTBIGF to IGMVC is observed to occur over a critical range of stress intensity, and is accompanied by a steeper dependence of crack growth rate on stress-intensity. Both mechanisms of crack growth are encouraged by an increase in final austenitising temperature for a constant grain-size and scale of microstructure, through changes in the nature of sulphur in solution and small reprecipitated sulphides.

Résumé

Nous avons étudié, sous chargement statique à 500 C sous vide, les micromécanismes de la croissance des fissures à haute température dans un acier martensitique à $214 %$ de chrome et 1% de molybdène. Pour caractériser les micromécanismes de rupture, nous avons combiné des mesures de métallographie et de fractographie précises avec de la microscopie Auger par balayage et des courbes de résistance à la croissance des fissures. Pour de faibles contraintes, inférieures à 48–55 MPam^$12$, le mode de croissance des fissures est la rupture intergranulaire fragile à haute température: celle-ci est contrôlée par la ségrégation dynamique du soufre vers les régions en tête de fissures. Les fissures semblent avancer par sauts distincts lorsqu'une concentrations critique de soufre est atteinte sur la distance de saut. Pour de fortes contraintes, supérieures à 48–55 MPam^$12$, le mode de rupture évolue vers la coalescence intergranulaire des microcavités: il est régi par la distribution des sulfures. Les sulfures de relativement petite taille, qui reprécipitent à partir de la solution solide pendent le traitement d'austénisation, sont d'une importance cruciale. La transition du premier mode vers le second a lieu dans un domaine critique de contraintes; elle est associée à une influence plus marquée de l'intensité de la contrainte sur la vitesse de croissance des fissures. Les deux mécanismes de croissance des fissures sont aidés par un accroissement de la température finale d'austénisation pour une taille de grains et une gamme de microstructures constantes, grâce à des modifications de la nature du soufre en solution et dans les petits sulfures reprécipités.

Zusammenfassung

Die Mikromechanismen des Hochtemperatur-Riβwachstums in dem martensitischen Stahl $214 Cr-1Mo$ wurden im Vakuum unter statischer Belastung bei 500°C untersucht. Dazu wurden ausführliche metallografische und fraktografische Messungen mit Raster-Augermikroskopie und den Widerstandskurven des Riβwachstrums kombiniert. Bei niedrigen Spannungsintensitäten, kleiner als 48–55 MPam^$12$, tritt als Riβwachstumsmode der intergranulare Sprödbruch bei hoher Temperatur auf. Diese Mode wird gesteuert von der dynamischen Segregation des Schwefels im Bereich der Riβspitze. Der Riβfortschritt scheint in Sprüngen abzulaufen, die einsetzen, wenn eine kritische Schwefelkonzentration innerhalb der Sprungstrecke erreicht worden ist. Bei hohen Spannungsintensitäten, gröβer als 48–55 MPam^$12$, wechselt die Mode zum intergranularen Zusammenwachsen von Mikroporen; diese wird von der Sulfidverteilung gesteuert. Von kritischer Wichtigkeit sind die relativ feinen Sulfide, die sich während der Austenitisierungsbehandlung aus dem Mischkristall wieder ausscheiden. In einem kritischen Bereich der Spannungsintensität wird der Übergang zwischen diesen beiden Moden beobachtet. Dieser Übergang wird von einer stärkeren Abhängigkeit der Riβwachstumsrate von der Spannungsintensität begleitet. Beide Mechanismen des Riβwachstums werden bei sonst konstanter Korngröβe und MikroStruktur durch eine höhere Temperatur bei der letzten Austenitisierungsbehandlung verstärkt, weil sich die Verteilung des Schwefels in Lösung und in den wieder ausgeschiedenen Sulfiden verändert.

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^†: Previous address: Department of Metallurgy and Materials Science, University of Cambridge, Cambridge, England.

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High temperature intergranular crack growth in martensitic 214Cr-1mo steel

Abstract

Résumé

Zusammenfassung

Acta metall.

Acta metall.

Mater. Sci. Engng

Acta metall.

Acta metall.

I. J. Press. Vess. Piping

Acta metall.

Acta metall.

Acta metall.

Mater. Sci. Technol.

Fracture Control of Engineering Structures—ECF6

Metal. Tech.

High temperature intergranular crack growth in martensitic $214 Cr-1mo$ steel