Grain boundary in steel

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Henry Bessemer, the great Victorian ironmaster and the first person to mass-produce mild steel, was nearly bankrupted by this. When he changed his suppliers of iron ore, his steel began to crack in service. The new ore contained phosphorus, which we now know segregates badly to grain boundaries. Modern steels must contain less than =0.05% phosphorus as a result. 

Fig. 1.8(a) Intergranular precipitation of chromium carbide particles in a sensitised austenitic stainless steel and the consequent chromium-depleted zones adjacent to the grain boundaries, (b) variation of the chromium content across a grain boundary in a sensitised austenitic stainless steel (l8Cr) and (c) intergranular corrosion of a sensitised austenitic stainless steel... 

Intergranular corrosion is encountered in many metal systems, often associated with the presence of precipitates at grain boundaries. In the case of stainless steels, one widely accepted theory states that the precipitation of chromium carbides leads to a chromium-denuded zone which undergoes rapid corrosion. 

Figure 1.9. FIM micrograph showing boron segregation to a grain boundary in austenitic 316L stainless steel, together with atom probe composition profiles for boron (Karlsson and Norden 1988).

Figure 1.10. This shows a 3DAP atom map across a grain boundary in a neutron irradiated 15KkMFA Russian pressure vessel steel weld. It reveals significant levels of P, Mn, Mo and C segregation, which correlate with poor mechanical properties exhibited by these materials.

Figure 5.37. Average sputter profile from ten grain boundaries in temper brittle rotor steel using 600 eV argon ions. (After Seah 1980, Crown Copyright.)...

Etching of stainless steel represents the most typical case of the application of electrolytic etching. This technique has been used on die 18/8 stainless steels in order to obtain a reliable and rapid measure of their sensitization to intergranular corrosion after a thermal treatment at 600 to 900°C. Several organic electrolytes such as oxalic acid (1) and tartaric acid (2) as well as inorganic electrolytes such as chromic acid (3) and sodium cyanide (4) have been employed. All these electrolytes attack preferentially the grain boundaries in which a second phase has precipitated. The composition of this phase responsible for the etching is not always known. 

Solutions used for electrolytic etching of stainless steels may be divided into two classes (a) Electrolytes which readily etch grain boundaries in the form of grooves regardless of the presence or absence of intergranular precipitates. Nitric acid is of this type [ J. Electrochem. Soc., 106, 161 (1959)]. ... 

Fig. 18 Cr-depletion profile across a grain boundary in a sensitized stainless steel. (Ref [61], from Corrosion of Stainless Steels A. John Sedriks, copyright John Wiley. Sons. Reprinted by permission of John Wiley Sons, Inc.)...

The objective of FP-5 project PISA is to better understand the role of phosphorus in the embrittlement process of RPV steels and improve the predictability of the impact it can have on embrittlement. Further work is still needed to understand, in particular, the segregation mechanisms of phosphorus to internal grain boundaries in RPV steels, as a result of exposure to irradiation or elevated temperatures, and the subsequent brittle intergranular failure of the material. 

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