Hardness tempered martensite

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. 

High-alloyed steels with a martensitic microstructure show machining results, which heavily depend on the workmaterial hardness and thus on the applied heat treatment. However, hardened and tempered martensitic stainless steels can be machined relatively well with suitable cutting parameters, tool materials, and coating systems, respectively. The dominant failure modes when using coated carbide tools for cutting hardened... 

In contrast to the pearlite structure, which is lamellar (Figure 2.15), tempered martensite contains the carbide particles as a spheroidal dispersed phase. While the tempered martensite is soft and tough, the parent martensite is hard and abrasion resistant. 

To summarize, the relative hardness of the various phases discussed thus far (Brinell hardness values in parentheses) martensite (300-700) > tempered martensite (300-450) > bainite ca. 400) > fine pearlite (100-300) > coarse pearlite (100-220) > spheroidite (90-180). The hardness and brittleness of cementite is much greater than ferrite, whereas the latter has significantly greater ductility. 

Martensite at the bottom on the diagram is the metastable form of ferrite in which the excess carbon contained in the larger austenite interstities is trapped in a distorted body-centered tetragonal (bet) lattice instead of the stable bcc lattice. Martensite (named for the German metallurgist, Adolf Martens) is any crystalline structure that is formed by a diffusionless, i.e., displacive transformation and this term will be used to describe similar transformations in other systems. The martensite formed from austenitic Fe is very hard and brittle but can be converted to tempered martensite by heat treatment. 

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.

Tempered martensite a-Ferrite + Fe3C Very small FesC spherelike particles in an a-ferrite matrix Strong not as hard as martensite, but much more ductile than martensite... 

Briefly explain why the hardness of tempered martensite diminishes with tempering time (at constant temperature) and with increasing temperature (at constant tempering time). 

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