Stainless steels precipitation hardness

In appHcations as hard surface cleaners of stainless steel boilers and process equipment, glycoHc acid and formic acid mixtures are particularly advantageous because of effective removal of operational and preoperational deposits, absence of chlorides, low corrosion, freedom from organic Hon precipitations, economy, and volatile decomposition products. Ammoniated glycoHc acid Hi mixture with citric acid shows exceUent dissolution of the oxides and salts and the corrosion rates are low. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

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. 

The hardness of precipitated sigma phase in stainless steels seems to vary with the composition. In Type 446 plain chromium steel it is about 9GPa (Guimaraes and Mei, 2004) whereas in Type 316 high Cr, Ni steel it is about 17GPa (Ohmura et al., 2006). Since they are ordered phases that do... 

Hardness of Precipitation-Hardening Austenitic Stainless Steels Machinability Rating of Wrought Coppers and Copper Alloys Hardness of Wrought Aluminum Alloys Hardness of Wrought Titanium Alloys at Room Temperature... 

It should be noted that a different way to produce stainless steels with high hardness is by precipitation hardening. Such steels have a low carbon content and contain in addition to chromium a few wt% of Ni and Cu. The hardening is caused hy Cu precipitates. Others use precipitation hardening hy intermetallic phases such as NiTi, TiAl, or NiAl. 

At higher temperatures, agents which cause hardness precipitate as the temperature rises, depending on the hardness of the cold water. The deposits formed on the surfaces of materials by this process can lead to the formation of electrochemical cells, especially if the surfaces are not coated uniformly, with the consequence of increased local corrosive attack. However, even if the surfaces are coated uniformly, pitting phenomena may occur due to the concentration of anions, especially chloride. This should be remembered, for example, in the case of heat exchanger pipes made of stainless steel. 

It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on Austenitic stainless steel by means low temperature nitriding and/or carburizing in a number of different media (salt bath, gas or plasma), each medium having its own strengths and weaknesses (Bell, 2002). In order to retain the corrosion resistance of austenitic stainless steel, these processes are typacally conducted at temperatures below 450 °C and 500 °C, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-phase or expanded austenite (Sun et al, 1999 Li, 2001 Li, et al., 2002 Christiansen, 2006). 

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