Stainless martensitic

The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

C 0.15 stainless steels 400 Series basic martensitic alloy hardenable by heat acids gaining import-ance in food industry roofing, siding, blades on... 

Fig. 5. Metastable Fe—Ni—Cr "temary"-pliase diagram where C content is 0.1 wt % and for alloys cooled rapidly from 1000°C showing the locations of austenitic, duplex, ferritic, and martensitic stainless steels with respect to the metastable-phase boundaries. For carbon contents higher than 0.1 wt %, martensite lines occur at lower ahoy contents (43). A is duplex stainless steel, eg. Type 329, 327 B, ferritic stainless steels, eg. Type 446 C, 5 ferrite + martensite D, martensitic stainless steels, eg. Type 410 E, ferrite + martensite F, ferrite + pearlite G, high nickel ahoys, eg, ahoy 800 H,...

Table 12. Martensitic or Hardenable Grades of Stainless Steel...

Martensitic Stainless Steels. The martensitic stainless steels have somewhat higher carbon contents than the ferritic grades for the equivalent chromium level and are therefore subject to the austenite—martensite transformation on heating and quenching. These steels can be hardened significantly. The higher carbon martensitic types, eg, 420 and 440, are typical cutiery compositions, whereas the lower carbon grades are used for special tools, dies, and machine parts and equipment subject to combined abrasion and mild corrosion. 

The body-centered-cuhic (bcc) metals and alloys are normally classified as undesirable for low temperature construction. This class includes Fe, the martensitic steels (low carbon and the 400-series stainless steels). Mo, and Nb. If not brittle at room temperature, these materials exhibit a ductile-to-brittle transition at low temperatures. Cold working of some steels, in particular, can induce the austenite-to-martensite transition. 

There are three groups of stainless alloys (I) martensitic, (2) ferritic, and (3) austenitic. 

Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, fresh water, and organic exposures). 

TABLE 28-9 Standard Wrought Martensitic Stainless Steels ... 

Figure 2.21 Orange oxides weeping from a tight seam in a martensitic (440) stainless steel surgical scissors. (Magnification 15x.)...

Hardenable stainless steels usually contain up to 0.6% carbon. This is added in order to change the Fe-Cr phase diagram. As Fig. 12.7 shows, carbon expands the y field so that an alloy of Fe-15% Cr, 0.6% C lies inside the y field at 1000°C. This steel can be quenched to give martensite and the martensite can be tempered to give a fine dispersion of alloy carbides. 

C) 370/656X brittleness after exposure to temperatures between about 700 to 1. OSO-F. stainless steels. chromium stainless steels, over 13% Cr and any 400 Series martensitic chromium stainless steels low in carbon content (high Cr/C ratio). complex chromium compound, possibly a chromium-phosphorus compound. chromium steels at temperatures above about 700 F (370 C) keep carbon up in martensitic chromium steels and limit Cr to 13% max. 

Martensitic stainless steels are of limited use in chemical environments because of their inferior corrosion... 

Wilde, B. E., Mechanism of Cracking of High-strength Martensitic Stainless Steel in NaCl Solutions , Corrosion, 27, 326 (1971)... 

Szklarska-Smialowska, Z., Electron Microprobe Study of the Effect of Sulphide Inclusions on the Nucleation of Corrosion Pits in Stainless Steels , Br. Corros. J., S, 159 (1970) Weinstein, M. and Speirs, K., Mechanisms of Chloride-activated Pitting Corrosion of Martensitic Stainless Steels , J. Electrochem. Soc., 117, 256 (1970)... 

Most of the martensitic stainless steels are based on 11-5 to 13-5% chromium and those from BS 970 and BS 1449 are shown in Table 3.11. 

Martensitic stainless steels are usually used in the softened (tempered at or above 650°C) or in the fully hardened condition (tempered at or below 250°C) so that there is no substantial reduction in corrosion resistance resulting from carbide precipitation. However, the hard soldering of knife blades can result in carbide precipitation and pitting of the blade at the area adjacent to the handle, and care must be taken in the soldering process to avoid this danger. 

Times to failure for various stainless steels tested in MgClj have been shown to increase with increasing proportions of martensite present Perhaps the role of martensite under anodic dissolution conditions is comparable to that of ferrite in duplex stainless steels where the enhanced dissolution of one phase prevents crack initiation in the other. There is, of course, another aspect of martensitic transformation that should be mentioned, i.e. the transformation of austenite to martensite either in the bulk material or at a growing crack tip that can give increased susceptibility to... 

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