The influence of tempering on cleavage fracture stress and transition behaviour of bainitic 2.25Cr1Mo steel

Abstract

The effect of tempering on the mechanical properties and on the cleavage fracture stress of bainitic 2.25Cr1Mo steel was studied in a variety of structures with the Hollomon-Jaffe parameter M ranging between 18.23 x 10 3 and 21.70 x 10J. The rapid decrease in yield stress and UTS was observed up to value M = 20 x 10 3 . For values M > 20 x 10 3 the reduction of yield stress and UTS slows down.The cleavage fracture stress continuously decreases with increasing value of M. Concurrently, the changes in dislocation density and in carbide size, morphology and distribution were observed. Various theories of cleavage origin of tempered bainitic microstructures are discussed. The transition temperatures, determined from impact Charpy test, reach minimum at M = 20 x 10 3 . Using Orowan's theory of initiation of the brittle fracture in steels, the minimum of transition temperatures may be explained by the different reduction rates of cleavage fracture stress and yield stress with value of M. EinfluB derAnlaBbedingungen auf Spaltbruchfestigkeit und Obergangsverhalten des bainitischen 2.25Cr1 Mo-Stahls. Die GefUgeanderungen, mechanischen Eigenschaften und die Spaltbruchfestigkeit des bainitischen Stahles 2.25Cr1Mo wurden nach dem Anlassen untersucht. Die AnlaBbedingung wurde mit Hilfe des Hollomon-Jaffe-Parameters definiert, dessen Wert im Bereich von 18.23 x 10 3 bis 21.7 x 10 3 variierte. Ein schneller Abfall der Streckgrenze und der Zugfestigkeit wurde bis zu dem Wert von M = 20 x 10 3 beobachtet. Fur groBere M-Werte verlangsamt slch diese Abnahme. Dagegen tallt die Spaltbruchfestigkeit mit zunehmendem AnlaBparameter M kontinuierlich und fast linear. Parallel dazu wurden Veranderungen der Versetzungsdichte, CarbidgrOBe, Carbidform und -verteilung verfolgt. Verschiedene Theorien uber MikroriBbiidung und -ausbreitung werden diskutiert. Die im Charpytest ermittelten Ubergangstemperaturen erreichen bei M = 20 x 10 3 ihr Minimum. Dies konnts mit der Theorie von Orowan tur Sprodbruchseinleitunq als eine Foige der unterschiedlichen Abnahmegeschwindigkeit von Spaltbruchfestigkeit und Streckgrenze mit zunehmendem AnlaBparameter erklart werden.

The Susceptibility of Local Parameters to Steel Microstructure Evaluated Using Charpy Type Specimen

“…2: the lower brittleness transition temperature, b y , and comparably lower general yield force, FGY, at this temperature are observed for the microstructure with coarse carbides. The above mentioned findings correspond well to upper nose temper embrittlement phenomena analysed elsewhere [33] and are well supported by fractographic observations of bend specimens:…”

Effects of phosphorus grain boundary segregation and hardness on the ductile-to-brittle transition for a 2.25Cr1Mo steel

“…As is well known [4][5][6] , 2.25Cr1Mo steel is an important high strength low-alloy steel in the power and petrochemical industries. The phosphorus segregation to prior austenite grain boundaries during service of the steel between 350 and 600 o C is of concern in engineering applications.…”

Dependence of ductile-to-brittle transition temperature on phosphorus grain boundary segregation for a 2.25Cr1Mo steel

“…Since 2.25Cr1Mo steel is one of main Cr-Mo low-alloy steels and is widely employed in the power and petrochemical industries for boilers, piping, and chemical reaction vessels [8][9][10], in the present work a relationship between phosphorus grain boundary segregation and DBTT for this steel has been established experimentally using Auger electron spectroscopy (AES) in conjunction with Charpy impact tests. Based on this relationship, predictions of DBTT have been made through predicting the grain boundary segregation of phosphorus.…”