Martensite surface

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. 

Other diffusion coatings include manganising to produce austenitic or martensitic surface layers on steel. Mixed Mn/Cr diffusion coatings are readily produced by pack techniques. Carbide coatings based on Ti, V and Cr, singly or in combination, are mainly produced for their wear-resisting properties There are now several commercial boronising processes Con-... 

Improved wear resistance through the development of a hard martensitic surface... 

When a component at an austenitizing temperature is placed in a quenchant, eg, water or oil, the surface cools faster than the center. The formation of martensite is more favored for the surface. A main function of alloying elements, eg, Ni, Cr, and Mo, in steels is to retard the rate of decomposition of austenite to the relatively soft products. Whereas use of less expensive plain carbon steels is preferred, alloy steels may be requited for deep hardening. 

Because the time at high temperature is much less, austenite is produced, which is chemically inhomogeneous especially with undissolved carbides, and has a fine grain crystal size. The formation of the hard martensite requites more rapid cooling than for conventional hardening. Thus case hardening by heat treatment intrinsically requites that the surface region to be hardened be relatively thin and cooled rapidly. 

In some cases, the carbon profile may not provide the necessary hardness or other properties. For example, if the carbon content is too high, quenching to room temperature may not produce all martensite at the surface because the high carbon content places the martensite finish temperature, Mj below room temperature. This results in the presence of soft retained austenite, and a low surface hardness. Conversion to martensite by subzero cooling to below the temperature can increase the hardness (Fig. 6) (12). 

Fig. 6. (a) The effect of sub2ero cooling on the hardness gradient in a carburized and quenched 3312 steel where (e) is oil quenched from 925 to 20°C and ( ) is cooled to -195°C. The initial quench to 20°C does not convert all of the austenite to martensite because the high carbon content in the surface region lowers the temperature below 20°C. Subsequent cooling to -195°C converts most of the retained austenite to martensite, raising the hardness, (b) The... 

Fig. 30. Phosphoric acid etched A.314 steel surface showing extensive etching along martensite boundaries. This alloy contains the same constituents as A606, but has been given a different heat treatment 54. ...

On the other hand, the formation of the high pressure phase is preceded by the passage of the first plastic wave. Its shock front is a surface on which point, linear and two-dimensional defects, which become crystallization centers at super-critical pressures, are produced in abundance. Apparently, the phase transitions in shock waves are always similar in type to martensite transitions. The rapid transition of one type of lattice into another is facilitated by nondilTusion martensite rearrangements they are based on the cooperative motion of many atoms to small distances. ... 

Martensite is a hard, nonductile microconstituent formed when steel is heated above its critical temperature and cooled rapidly. In the case of steel of the composition conventionally used for rope wire, martensite can be formed if the wire surface is heated to a temperature near or somewhat in excess of 1400°F (760°C), and then cooled at a comparatively rapid rate. The presence of a martensite film at the surface of the outer wires of a rope that has been in service is evidence that sufficient frictional heat has been generated on the crown of the rope wires to momentarily raise the wire surface temperature to a point above the critical temperature range of the steel. The heated surface is then rapidly cooled by the adjacent cold metal within the wire and the rope structure, and an effective quenching results. 

Figure 4.13 Representation of a plate of martensite (b) formed from a rectangular block of the parent crystal (a). Notice that the plane surfaces A2B2C2D2 and AjB C D remain undistorted and unrotated during the transition.

Martensitic transformations involve a shape deformation that is an invariant-plane strain (simple shear plus a strain normal to the plane of shear). The elastic coherency-strain energy associated with the shape change is often minimized if the martensite forms as thin plates lying in the plane of shear. Such a morphology can be approximated by an oblate spheroid with semiaxes (r, r, c), with r c. The volume V and surface area S for an oblate spheroid are given by the relations... 

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