Austenitic iron

The austenitic iron—chromium—nickel alloys were developed in Germany around 1910 in a search for materials for use in pyrometer tubes. Further work led to the widely used versatile 18% chromium—8% nickel steels, the socaHed 18—8. 

A286 Alloy. A286 is an austenitic iron base alloy that has been used for years in aircraft engine applications. Its use for industrial gas turbines started about 1965, when technological advances made the production of sound ingots sufficient in size to produce these wheels possible. 

Fig. 3.44 Structure of typical spheroidal graphite austenitic iron...

The austenitic irons show excellent casting properties and good machin-ability, which, in combination with the good mechanical properties and good corrosion resistance, ensures wide use of these materials in many applications. 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

Similar curves determined in 50 Vo sodium hydroxide solution at 60°C show (Fig. 3.46) that the austenitic irons exhibit more noble active dissolution and also lower current densities in the active and passive regions than the ferritic irons the current densities in both regions decrease markedly with increasing nickel content (Fig. 3.47). 

In 3% sodium chloride solution at 60°C the austenitic irons again show superior characteristics to the ferritic. The breakdown potentials determined in this environment, which provide a relative measure of the resistance to attack in neutral chloride solutions, are generally more noble for the austenitic irons than for the ferritic (Table 3.47). This indicates that the austenitic irons should show better corrosion resistance in such environments. 

The more favourable electrochemical characteristics exhibited by the austenitic irons in this range of environments are reflected in the corrosion behaviour of the alloys discussed below. 

One of the outstanding properties of the austenitic irons is their resistance to graphitic corrosion or graphitisation . In some environments ferritic cast irons corrode in such a manner that the surface becomes covered with a layer of graphite. This compact graphite layer, being more noble than the matrix, markedly increases the rate of attack. The austenitic irons rarely form this... 

The austenitic irons have also been shown to exhibit better corrosion resistance than the ferritic irons in sea-water. Tests over long periods of time have shown that Ni-Resist irons of Types 1, 2 and 3 corrode at rates of 0 020 to 0-058 mmy in relatively quiet sea-water. Under similar conditions low alloy cast irons have shown corrosion rates ranging from 0-066 to 0-53 mmy" . The Ni-Resist irons maintain this superiority over a wide variety of conditions (Figs. 3.49 and 3.50) both in stationary and flowing sea-water. In a test lasting 740 days in sea-water moving at l-5m/s low... 

The austenitic irons can be usefully applied in handling very dilute solutions of sulphuric acid at ambient or moderately elevated temperatures under conditions which can be very corrosive to ordinary cast iron and carbon steel. Austenitic irons have also given satisfactory service in handling... 

The austenitic irons are superior to ordinary cast iron in their resistance to corrosion by a wide range of concentrations of hydrochloric acid at room temperature (Table 3.50). However, for practical uses where such factors as velocity, aeration and elevated temperatures have to be considered, the austenitic irons are mostly used in environments where the hydrochloric acid concentration is less than 0- 5%. Such environments occur in process streams encountered in the production and handling of chlorinated hydrocarbons, organic chlorides and chlorinated rubbers. 

The austenitic irons are also useful in some circumstances for handling organic acids such as dilute acetic, formic and oxalic acids, fatty acids and tar acids. They are more resistant to organic acids than unalloyed cast irons, e.g. in acetic acid the austenitic irons show corrosion rates 20-40 times lower than the ferritic iron (Table 3.51). 

The austenitic irons show poor resistance to solutions of nitric acid even when dilute and at low temperatures. 

The austenitic irons also show good corrosion resistance in caustic alkalis containing sulphides and mercaptans and have therefore proved useful materials for the construction of pumps, valves and piping in caustic soda regenerators in oil refineries. 

The individual characteristics and uses of the basic grades of the austenitic irons are given in Table 3.55. The major uses for these materials occur in the handling of fluids in the chemical and petroleum industries and also in the power industry and in many marine applications. The austenitic irons are also used in the food, soap and plastics industries where low corrosion rates are essential in order to avoid contamination of the product. Ni-Resist grades Type 2, 3 or 4 are generally used for such applications but the highly alloyed Type 4 Ni-Resist is preferred where low product contamination is of prime importance. 

AUSIOIA Type 1 Least expensive austenitic iron good corrosion resistance particularly in acidic media Pumps, valves, furnace components... 

Type 4 Type D-4 Best corrosion resistance and erosion resistance of the austenitic irons Castings for industrial furnaces used in food industry for low contamination of product... 

Cox and Gilbert have given a detailed account of the behaviour of the austenitic irons when exposed for 32 weeks to temperatures ranging from 320°C to 800°C, with particular reference to the effect of this exposure on... 

Austenitic irons are used for valve and pump bodies condenser water boxes in refineries, cryogenics, marine and electricity generation systems. In general austenitic irons have better corrosion resistance than unalloyed or low-alloy irons. 

Discaloy [Westinghouse], TM for an austenitic iron-base alloy containing nickel, chromium, and relatively small proportions of molybdenum, titanium, silicon, and manganese. This alloy is precipitation-hardened and was developed primarily to meet the need for improved gas-turbine disks, one of the most critical components of jet engines. 

Nickel has a very small effect on the anodic polarization behavior of iron, and hence, iron-nickel alloys are of minor significance as corrosion-resistant alloys. However, the addition of nickel to iron-chromium alloys (AISI 200 series) permits conversion of the latter as ferritic alloys to austenitic iron-chromium-nickel alloys (AISI 300 series). In... 

In many processes, carbon is present as the primary or secondary oxidant. Where carbon is the primary oxidant, the principal reaction is that of carburization or the dissolution of carbon into the metal matrix. The solubility of carbon in metals varies widely, being very low in Ni, Cu, Co, and ferritic iron but quite substantial in austenitic iron. Carburization of alloys, principally steels, is a common treatment for developing a hard and strong surface on components that are exposed to contact wear during service. The theory and techniques for this are clarified in the literature. ... 

Gra] Grabke, H.J., Iyer, S.K., Srinivasan, S.R., The Solubility of Nitrogen in Austenitic Iron-Manganese and Iron-Chromium Alloys , Z. Metallkd., 66, 286-292 (1975) (Experimental, Phase Relations, Themodyn., Caleulation, 13)... 

Mun2] Mundt, R., Hoffineister, H., The Isothermal 8-7 Transformation of Ferritic-Austenitic Iron-Chromium-Nickel Alloys , Arch. Eisenhuettenwes., 54(7), 291-294 (1983) (Experimental, Kinetics, 19)... 

Jol] Jolley, W., Effect of Mn andNi on hnpaet Properties of Fe and Fe-C Alloys , J. Iron Steel Inst, London, 206, 170-173 (1968) (Experimental, Morphology, Meehan. Prop., 16) [1968Zup] Zupp, R.R., Stevenson, D.A., Statistical Thermodynamics of Carbon in Ternary Austenitic Iron-Base Alloys , Trans.Metall. Soc. AIME, 242, 862—869 (1968) (Thermodyn., Calculation, Phase Relations, 35)... 

CP, coarse pearUte M, martensite A, austenite F, ferrite Can be produced from a malleable-iron base composition Copper can replace all or part of the nickel Such as Durion, Durichlor 51, Superchlor Such as Ni-Resist austenitic iron (ASTM A 436)... 

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