Tempered martensite

Austenite is slightly denser than martensite, and therefore, during the phase transformation upon quenching, there is a net volume increase. Consequently, relatively large pieces that are rapidly qnenched may crack as a result of internal stresses this becomes a problem especially when the carbon content is greater than about 0.5 wt%. 

Concept Check 10.5 Rank the following iron-carbon alloys and associated microstructures from the highest to the lowest tensile strength  

25 wt% C with spheroidite 0.25 wt% C with coarse pearlite 0.6 wt% C with fine pearlite 0.6 wt% C with coarse pearlite 

Concept Check 10.6 For a eutectoid steel, describe an isothermal heat treatment that would be required to produce a specimen having a hardness of 93 HRB. 

In the as-quenched state, martensite, in addition to being very hard, is so brittle that it cannot be used for most apphcations also, any internal stresses that may have been introduced during quenching have a weakening effect. The ductOity and toughness of martensite may be enhanced and these internal stresses reheved by a heat treatment known as tempering. 

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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. 

As with chemical etches, developing optimum conversion coatings requires assessment of the microstructure of the steel. Correlations have been found between the microstructure of the substrate material and the nature of the phosphate films formed. Aloru et al. demonstrated that the type of phosphate crystal formed varies with the orientation of the underlying steel crystal lattice [154]. Fig. 32 illustrates the different phosphate crystal morphologies that formed on two heat-treated surfaces. The fine flake structure formed on the tempered martensite surface promotes adhesion more effectively than the knobby protrusions formed on the cold-rolled steel. 

For steels that are most frequently used in the annealed or normalised condition the most important structural parameter that can be influenced by heat treatment is the grain size, although the extensive use of welding as a means of fabricating mild steels means that martensitic and tempered martensitic... 

Constituent properties of bainite, 23 280 of martensite, 23 280-281 of pearlite, 23 280 of tempered martensite, 23 281-282 Constrained geometry catalysts, 16 81 20 193... 

Temper designations, magnesium alloy, 75 347-348, 353-354 Tempered martensite, constituent properties of, 23 281-282 Tempering... 

A transverse cross-section through the fracture initiation site was examined by metallography. The fracture surface profile was found to be relatively flat and there was no crack branching. The microstructure showed dark-etching-tempered martensite. Further no plastic deformation was observed at the fracture initiation site. 

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...

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