Martensite tempering

Fig. 7.30 Pitting potential versus factor, Cr + 3.3 Mo + 1 6 N. Steels were austenite, martensite, tempered martensite, or ferrite. Composition range 0-29 wt% Cr, 0-20 wt% Ni, 0.3-4 wt% Mo, 0.01-0.5 wt% N, and 0-0.3 wt% Nb. Redrawn from Ref 47...

Vod] Vodarek, V., Pahuta, R, Precipitation of e (Ti)-Carbide During the First Stage of Martensite Tempering , Met. Mater. (Czech.), 29(3), 151-159 (1991) (Abstract)... 

Low magnification imaging (xlOOOO-20000) can be used to provide information on the general microstructure. For instance Fig. 9.4 from Burke et alP shows a representative micrograph from a low-aUoy forging steel. In this case, the mixed tempered martensite-tempered bainite structure with extensive carbide precipitation is visible. 

Transmission electron image obtained from an unirradiated A508 Gr4N steel. Note the mixed tempered martensite-tempered bainite microstructure. Several M3C and M7C3 carbides are labelled from reference 12. 

The martensitic tempering steel X4CrNiMol6-5-l (DIN-Mat. No. 1.4418) has proved resistant to stress corrosion cracking in stress corrosion cracking tests in seawater up to 333 K (60 °C) and stress loads of up to 90% of the 0.2% yield point, but did show pitting corrosion [117]. 

Figure 10.32 Hardness (at room temperature) as a function of carbon concentration for plain carbon martensitic, tempered martensitic [tempered at 371°C (700°F)], and pearlitic steels.

FigMte 10.33 Electron micrograph of tempered martensite. Tempering was carried out at 594°C (HOOT). The small particles are the cementite phase the matrix phase is a-ferrite. 9300X. 

Pearlite (coarse and fine) Bainite Spheroidite Martensite Tempered martensite... 

Microstnicture Uniform martensite tempered to a degree consistent with the hardness range with flne, well-distributed carbides, if any. 

Mittemeijer E J, Cheng L, der Schaaf P J V, Brakman C M and Korevaar B M 1988 Analysis of nonisothermal transformation kinetics tempering of iron-carbon and iron-nitrogen martensites Metall. Trans. A 19 925... 

Steels develop exceUent combinations of strength and toughness if heat-treated by quenching to martensite, foUowed by tempering (see Steel). 

Ba.inite. In a given steel, bainite microstmctures ate generally found to be both harder and tougher than peadite, although less hard than martensite. Bainite properties generally improve as the transformation temperature decreases. Lower bainite compares favorably with tempered martensite at the same hardness and can exceed it in toughness. Upper bainite, on the other hand, may be somewhat deficient in toughness as compared to fine peadite of the same hardness (33). 

Although for some appHcations, particulady those involving wear resistance, the hardness of martensite is desirable in spite of the accompanying bntdeness, this microstmcture is mainly important as starting matenal for tempered martensite stmctures, which have definitely supenor properties for most demanding appHcations. 

Fig. 19. Properties of tempered martensite (1). FuUy heat-treated miscellaneous analyses, low ahoy steels 0.30—0.50% carbon. To convert MPa to psi,...

Fig. 20. Transforniation diagram for quenching and tempering martensite. The product is tempered martensite.

Austempering. Lower bainite is generally as strong as and somewhat more ductile than tempered martensite. Austempering, which is an isothermal heat treatment that results in lower bainite, offers an alternative heat treatment for obtaining optimum strength and ductility if the specimens are sufficiently small. 

Fig. 11.10. Changes during the tempering of martensite. There is a large driving force trying to moke the martensite transform to the equilibrium phases of or + Fe3C. Increasing the temperature gives the atoms more thermal energy, allowing the transformation to take place.

The final note is that pearlite and bainite only form from undercooled y. They never form from martensite. The TTT diagram eannot therefore be used to tell us anything about the rate of tempering in martensite. 

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