Precipitation steels

Euml] Eumann, M., Palm, M., Sauthoff, G., Iron-Rich Iron-Aluminium-Molybdenum Alloys with Strengthening Intermetallic mu Phase and R Phase Precipitates , Steel Res., 75(1), 62-73 (2004) (Experimental, Meehan. Prop., Phase Diagram,, , 48)... 

The Zond VD - 96 is intended for the detection of cracks, precipitates, inclusions etc. and corrosive defects with the preliminary estimation of their depth in various objects of magnetic and non - magnetic steels, non-ferrous and refractory metals and welds including coatings. 

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. 

Pentaerythritol may be nitrated by a batch process at 15.25°C using concentrated nitric acid in a stainless steel vessel equipped with an agitator and cooling coils to keep the reaction temperature at 15—25°C. The PETN is precipitated in a jacketed diluter by adding sufficient water to the solution to reduce the acid concentration to about 30%. The crystals are vacuum filtered and washed with water followed by washes with water containing a small amount of sodium carbonate and then cold water. The water-wet PETN is dissolved in acetone containing a small amount of sodium carbonate at 50°C and reprecipitated with water the yield is about 95%. Impurities include pentaerythritol trinitrate, dipentaerythritol hexanitrate, and tripentaerythritol acetonitrate. Pentaerythritol tetranitrate is shipped wet in water—alcohol in packing similar to that used for primary explosives. 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

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. 

The bulk polycondensation of (10) is normally carried out in evacuated, sealed vessels such as glass ampules or stainless steel Parr reactors, at temperatures between 160 and 220°C for 2—12 d (67). Two monomers with different substituents on each can be cocondensed to yield random copolymers. The by-product sdyl ether is readily removed under reduced pressure, and the polymer purified by precipitation from appropriate solvents. Catalysis of the polycondensation of (10) by phenoxide ion in particular, as well as by other species, has been reported to bring about complete polymerisation in 24—48 h at 150°C (68). Catalysis of the polycondensation of phosphoranimines that are similar to (10), but which yield P—O-substituted polymers (1), has also been described and appears promising for the synthesis of (1) with controlled stmctures (69,70). 

Mg yA1 2> or P-(MgAl). Thus aluminum occurs in magnesium alloys both in soHd solution and as the intermediary intermetaUic phase. The latter is clear white and in slight rehef in poHshed and etched samples. In as-cast alloys, the hard phase occurs in massive form, but when precipitated from sohd solution a lamellar stmcture is formed similar to peadite in steel. When produced by aging at low temperatures, it appears as fine particles. 

Fig. 10. Schematic of casting machine used to make microporous membranes by watervapor imbibition. A casting solution is deposited as a thin film on a moving stainless steel belt. The film passes through a series of humid and dry chambers, where the solvent evaporates from the solution, and water vapor is absorbed from the air. This precipitates the polymer, forming a microporous membrane that is taken up on a collection roU (25).

HydrometaHurgical Processes. The hydrometaHurgical treatments of oxide ores involve leaching with ammonia or with sulfuric acid. In the ammoniacal leaching process, the nickel oxide component of the ore first is reduced selectively. Then the ore is leached with ammonia which removes the nickel into solution, from which it is precipitated as nickel carbonate by heating. A nickel oxide product used in making steel is produced by roasting the carbonate. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Primary consumers for ferrous scrap are the iron and steel mills and foundries. Minor consumers iaclude ferroalloy producers, copper producers for use ia copper precipitation (see Recycling, nonferrous metals), and the chemical iadustry. The steel iadustry consumes about three-fourths of the total. Scrap consumption for ferroalloy production, copper precipitation, and the chemical iadustry total less than one million t. The United States is the leading exporter of ferrous scrap, exporting almost nine million t ia 1994, valued at about 1.3 biUioa. Total value of domestic scrap purchases and exports ia 1994 was 8 biUioa (2). 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

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