Austenite-martensite

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. 

In contrast to this the austenite - martensite transition temperature depends on the concentration of vacancies. The configurations A and E show a transition temperature of 10 K, while in C and D the bcc structure already occurs at 100 K. 

After a full transformation cycle, i.e. one austenite - martensite and one martensite - austenite transition, we find a striking difference between the perfect configurations and those with vacancies Only the systems with defects regain the initial cubic form of the simulation box. 

The transition temperatures which we find for the austenite - martensite transition in simulations without vacancies, are much to low. The introduction of vacancies lead to much more physical results, while the martensite - austenite transition is not affected by vacancies. From this we conclude that vacancies lower the energy barrier which the system has to overcome during the transition. The reason for this might be a weakening of long-range elastic couplings in the lattice. 

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diifusionless transformations (austenite -> martensite), continuous precipitation (austenite -> pearlite) and discontinuous precipitation (austenite -> bainite and tempering of martensite) are fairly common in other alloy systems. 

Figure 3.6 Shape-memory alloys transform from (a) a partially ordered, high-temperature austenitic phase to (b) a mixed austenite-martensite low-temperature state to (c) an ordered mixed-phase state under deformation.

If the SMA is sufficiently close to Tm, an imposed stress is sufficient to cause pressure-induced austenite —> martensite phase transitions in selected grains of the alloy, relieving the stress through pseudo-elastic deformation of the softer martensite grains. Similarly, if the original austenite-shaped alloy is brought below Tm to convert it to malleable martensite form, many deformations of macroscopic shape leave the martensitic atoms close to their... 

Metallurgy was one of the first fields where material scientists worked toward developing new alloys for different applications. During the first years, a large number of studies were carried out on the austenite-martensite-cementite phases achieved during the phase transformations of the iron-carbon alloy, which is the foundation for steel production, later the development of stainless steel, and other important alloys for industry, construction, and other fields was produced. 

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. 

We can thus make an absolute measurement of the austenite content of the steel by direct comparison of the integrated intensity of an austenite line with the integrated intensity of a martensite line. By comparing several pairs of austenite-martensite lines, we can obtain several independent values of the austenite content. 

Our intention in the remainder of this section is to build up a picture of some of the various interfaces that are present in martensitic systems. Our approach will be to consider microstructural elements of increasing complexity, beginning first with the case of the simple austenite-martensite interface and culminating in the investigation of martensitic wedges within the host austenite. In all of these cases, the primary theoretical engine in our analysis will be the compatibility conditions and their outcome as typified by eqns (10.62) and (10.63). 

Austenite-Martensite Interfaces. As a first exercise in the machinery set fortli above, we pose the question of whether or not a simple interface can exist between a cubic crystal and its tetragonal offspring. Without loss of generality, this situation can be represented via the condition... 

In addition to this constraint, we must also satisfy compatibility across the austenite-martensite interface which in light of the deformation gradients specified... 

Fig. 7.30 Pitting potential versus factor, Cr + 3.3 Mo + 1 6 N. Steels were austenite, martensite, tempered martensite, or ferrite. Composition range 0-29 wt% Cr, 0-20 wt% Ni, 0.3-4 wt% Mo, 0.01-0.5 wt% N, and 0-0.3 wt% Nb. Redrawn from Ref 47...

Stainless steels can be divided into four categories, based on their microstructure ferritic, austenitic, martensitic and austenitic-ferritic (duplex). Only specific grades of austenic and duplex stainless steel are currently used in concrete, although also a ferritic type with 12% chromium has been proposed [5-9]. In some countries also clad bars, i. e. bars with a carbon-steel core and an external layer of stainless steel are used. 

Low-temperature structural steels include low-, medium-, and high-strength steels. These steels possess a ferritic, austenitic, martensitic, or mixed structure, depending upon their chemical composition. Differences in phase structure account for differences in weldability and properties at temperatures down to 4 K. 

Nitrogen increases the absolute coefficient of the thermoelectromotive force [1999Kap] in the stability domain of austenite. This tendency increases sharply near the austenite-martensite transition. 

Job] Laue method. X-ray diffractometry quenched from 1228°C 3.09 mass% Cr and 1.51 mass% C austenite, martensite... 

Joh] Johnson, K.A., Wayman, C.M., The Crystallography of the Austenite-Martensite Transformation in an Fe-Cr-C Alloy , Acta Crystallogr., 16(6), 480-485 (1963) (Crys. Strueture, Experimental, Theory, 16)... 

I97OC0I, 1991Achl, 1991Ach2] studied the influenee of thermal and meehanical history, microstruc-ture, composition on electrical and magnetic properties (eoereive foree, magnetization) of alloys at the austenite martensite transformation. 

If the deformation process were continuous at any one location, a maximum load would occur at the normal strain, but this strain is exceeded because of the elastic loading and interrupted nature of the deformation. The true stress-strain curve therefore does not represent the strain hardening properties of one particular region of the bar in the same sense as at higher temperatures, but rather represents the upper envelope of flow stress for various austenite-martensite structures. 

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