Austenitic steels machining

Austenitic steels have a number of advantages over their ferritic cousins. They are tougher and more ductile. They can be formed more easily by stretching or deep drawing. Because diffusion is slower in f.c.c. iron than in b.c.c. iron, they have better creep properties. And they are non-magnetic, which makes them ideal for instruments like electron microscopes and mass spectrometers. But one drawback is that austenitic steels work harden very rapidly, which makes them rather difficult to machine. 

Perhaps the most informative work has been that of Jacquet (16), Samuels (7,10), and Thomas sen and McCutcheon (52) on 70 30 brass Wulff (44), and Samuels and Wall work (36) on austenitic steels Samuels and Wallwork (37) on zinc and Vacher (23) on steel, copper and aluminum. Most of the measurements in the references quoted pertain to metallographic abrasion processes, but they also include standard machining operations (35,36,41,51), metallographic polishing (7,16,23,... 

P-BN tools work satisfactorily in hardened steel up to contact temperatures of 1000°C, since there is no chemical reaction between boron nitride and iron. This, however, also depends on the binding phase of the polycrystalline materials and can lead to adhesive wear [24, 25]. In hard steel, the main wear mechanism on the tool is abrasion by hard alloy carbide particles [26]. In the case of Co-based super alloy (Vitallium), the results on hard-BN tool wear are somewhat incongruous [27, 28], while Inconel 718 can be machined under proper selection of the cutting conditions [29]. Apparently, austenitic steels containing a high percentage of Co are difficult to cut by hard-BN tools, due to the formation of cobalt nitrides which leads to high tool wear [8]. 

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... 

From the manufacturing point of view, austenitic steels have a high formability and these commercial alloys are currently produced in various shapes at a conventional industrial level. Many nuclear products involve welded parts, and are typical assemblies of plates, sheets, bars, or tubes with a certain amount of machining. Nevertheless, to fulfill SFR achievements, it is also necessary to obtain industrial products that demonstrate the capability of steelmakers and manufacturers to meet new SFR design, code, and licensing requirements. Some challenging examples are given in this section. 

Depth-of-Gut Notching. Depth-of-cut notching (DOCN) is a localized wear process common when machining materials such as austenitic stainless steels or high temperature alloys. Notching is attributed to the chemical reaction of the tool material and the atmosphere, or to abrasion by the hard, sawtooth outer edge of the chip. DOCN may lead to tool fracture. 

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. 

Figure 2.17 Pitting at a free-machining austenitic stainless steel threaded valve throat. The valve controlled flow of a chlorine-containing, low-pH biocide.

All the stainless steels can be machined in the softened states, but they may present some problems unless the correct techniques are adopted. This is especially so with the austenitic grades where the extreme ductility minimises chip breaking and the work hardening may cause difficulties unless modest cuts are made. The free-cutting grades (those with high sulphur contents or selenium additions) are much easier to machine, but it must be remembered that they have somewhat reduced corrosion resistance, ductility and weldability compared to their normal counterparts. Detailed machining instructions are readily available from steel suppliers. 

Major uses of the ferritic steels have been on motor vehicles as trim and in domestic equipment such as cutlery and hollow ware, but use has also been made in refrigerators, washing machines and on sinks and similar fittings. Some types would no doubt find much wider application in the chemical field and other fields where their superior corrosion resistance would be a considerable advantage if it was not for the fact that the austenitic types have advantages (sometimes considerable) in fabrication. However, the availability of the low interstitial weldable types and the super ferritics is increasing in scope. 

The duplex range of stainless steels can be readily cast, wrought and machined. Problems can occur in welding, due to the need to keep the correct balance of ferrite and austenite in the weld area, but this can be overcome using the correct welding materials and procedures. 

Austenitic or duplex stainless steel pressure casing components may be hydrostatically tested with an additional amount of material on areas where machining to critical dimensions and tolerances is required. The additional amount of material shall not exceed 1 mm (0.040 in.) material stock or 5 percent of minimum allowable wall thickness, whichever is less. 

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