Martensite microstructure

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. 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

A distinguishing feature of the soft-mode martensitic transformation is the anomalous pre-transformation bdiaviour that is observed in a range of physieal and microstructural... 

THE MICROSTRUCTURE AND MARTENSITIC TRANSFORMATION IN A (POTENTIALLY) SHAPE MEMORY Ni-AI-Ti-B ALLOY... 

Fig.2. Electron microstructure of the sample hot deformed by 25% a) matrix p phase, b) 7R martensite...

The characteristic temperatures of the martensitic transformation, as well as the (Af -Mf) temperature range were strongly influenced by the microstructure of the alloy. Applied predeformation lowered both and temperature range of transformation while the ageing at 510 K that followed decreased the width of the transformation. 

Fig. 20.47 Light micrograph showing the microstructure of a martensitic steel (x 550)...

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

Fig. 20.50 Some of the microstructures observed during the tempering of martensite...

Fig. 20.51 Effect of tempering temperature on the strength and microstructure of martensite...

J.M. Ball and R.D. James. Theory for the microstructure of martensite and applications. In Proceedings of the International Conference on Martensitic Transformations, pages 65-76, Monterey, CA, 1993. Monterey Institute for Advanced Studies. 

Plate 4.2 Microstructure of a wedge-type diffusion couple in which the y-Nb4N . % formed martensitically from b-NbN, x- Anodically oxidized. 

A relative decrease in Tafel slope has been observed with steel subjected to thermal treatments leading to different microstructures [274]. In this case an effect can indeed be expected because of the possible formation of different phases or different proportions (martensite vs ferrite) of phases (Fig. 10). An effect of the size of grains has been observed on the Tafel slope. 

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. 

A preliminary approach to the selection of the stainless steel for a specific application is to classify the various types according to the alloy content, microstructure, and major characteristic. Table 3 outlines the information according to the classes of stainless steels-austenitic, martensitic, and ferritic. Table 4 presents characteristics and typical applications of various types of stainless steel while Table 5 indicates resistance of stainless steels to oxidation in air. 

Microstructure after CVD Ferrite + Cementite Martensite +10% Retained Austenite + Carbide... 

Microstructure after Hardening Martensite + 20% Retained Austenite Tempered Microstructure + Carbide... 

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