Microstructure tempered martensite

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. 

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

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. 

Bainite, In a given steel, bainite microstructures are 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 applications, particulady those involving wear resistance, the hardness of martensite is desirable in spite of the accompanying britdeness, this microstructure is mainly important as starting material for tempered martensite structures, which have definitely superior properties for most demanding applications. 

This method is particularly suitable for steels of higher carbc content showing high hardenability, where it is required to prodiv a softer HAZ microstructure than martensite directly after weldii and without recourse to postweld tempering. 

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... 

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 microstructure of these steels has little effect on corrosion in the atmosphere or in neutral or alkaline solutions. In acids, however, microstructure can have a significant effect because the carbide morphology affects the hydrogen discharge reaction. Untempered martensitic and pearlitic structures generally show the lowest rates of attack in acids, whereas tempered martensite and spheroidized structures show much higher rates [2],... 

A. Abdollah-Zadeh, A. Salemi, and H. Assadi, "Mechanical behavior of CrMo steel with tempered martensite and ferrite-bainite-martensite microstructure". Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 483-84, 2008 pp. 325-328. 

In spite of their differences in chemical composition, precipitates, microstructures, etc., similarities in the creep behavior of austenitic stainless steels and tempered martensite-ferritic steels should be noticed such as ... 

Because their as-received condition microstructure differs strongly from that of tempered martensite-ferritic steels, the stress-strain behavior of austenitic stainless steels differs strongly from that of martensitic steels. During creep and cychc deformation with and without hold time, dislocation production and microstructure are observed, which lead to hardening instead of softening. As creep strain rates in martensitic steels are usually higher than in austenitic stainless steels, necking is... 

Figure 10.8 Metallographic views of 9Cr, llCr-ODS steel having duplex microstructure consisting of tempered martensite and residual-a ferrite.

S. Ohtsuka, T. Kaito, T. Tanno, Y. Yano, S. Koyama, K. Tanaka, Microstructure and high-temperature strength of high Cr ODS tempered martensitic, J. Nucl. Mater. 442 (2013) S89-S94. 

The effect of velocity on the corrosion rate of steels is also dependent on steel composition and microstructures. Steels with more homogeneously distributed carbides as in tempered martensite... 

The materials under study are uncoated and aluminized 12% chromium steels. The 12% chromium steel reported in this chapter was a ferritic alloy steel HCM12A, prepared by Sumitomo, Japan, of nominal composition 12.22 wt.% Cr, 1.88 wt.% W, 0.86 wt.% Cu, 0.53 wt.% Mn, 0.36 wt.% Mo, 0.35 wt.% Ni, 0.31 wt.% Si, 0.20 wt.% V, 0.10 wt.% C, 0.052 wt.% N, 0.05 wt.% Nb, 0.014 wt.% P, 0.001 wt.% S, 0.0008 wt.% A1 and Fe (balance). This is a so-called third generation ferritic steel for power plants, with improved weldability and creep strength compared to prior ferritic steel versions. It has a duplex microstructure of tempered martensite and 6-ferrite [2]. The uncoated steel samples were ground to 320 grit finish using SiC grinding paper. Prior to the experiments, the samples were washed ultrasonicaUy in acetone and then in ethanol. 

Briefly describe the microstructure for each of the following microconstituents that are found in steel alloys fine pearlite, coarse pearlite, spheroidite, bainite, martensite, and tempered martensite. 

The microstructure of tempered martensite consists of extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. This is similar to the microstructure of spheroidite except that the cementite particles are much, much smaller. An electron micrograph showing the microstructure of tempered martensite at a very high magnification is presented in Figure 10.33. 

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