Medium-alloy steels

The physical and mechanical properties of steel depend on its microstmcture, that is, the nature, distribution, and amounts of its metaHographic constituents as distinct from its chemical composition. The amount and distribution of iron and iron carbide determine most of the properties, although most plain carbon steels also contain manganese, siUcon, phosphoms, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper. These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is tme even of medium alloy steels, which may contain considerable amounts of nickel, chromium, and molybdenum. 

Carbon Steels and Low—Medium Alloy Steels. Plain carbon steels, the most common cutting tool materials of the nineteenth century, were replaced by low—medium alloy steels at the turn of that century because of the need for increased machining productivity in many appHcations. Low—medium carbon steels have since then been largely superseded by other tool materials, except for some low speed appHcations. 

Low—medium alloy steels contain elements such as Mo and Cr for hardenabiHty, and W and Mo for wear resistance (Table 4) (7,16,17) (see Steel). These alloy steels, however, lose their hardness rapidly when heated above 150—340°C (see Fig. 3). Furthermore, because of the low volume fraction of hard, refractory carbide phase present in these alloys, their abrasion resistance is limited. Hence, low—medium alloy steels are used in relatively inexpensive tools for certain low speed cutting appHcations where the heat generated is not high enough to reduce their hardness significantly. 

Table 4. Compositions of Carbon and Low-Medium Alloy Steels, Wt...

Grinding Abrasion. The suitable alloys range from austenitic manganese steel (which once dominated the field) through hardenable carbon and medium alloy steels to the abrasion-resistant cast irons. 

Materials of Construction - Mostly low and medium alloy steels. 

In practice, tubes,with inside diameter from 65 mm (Stone and Webster) to 120 mm [Lummus, Selas) and 7 to 5 mm thick are employed for naphtha steam cracking Tbe gases flow within these tubes at a linear velocity of about 300 m/s. The pressure drop recorded between the furnace inlet and outlet may reach 0.4 to 0.7.10s Pa in these conditions. The tubes are fabricated by extrusion or centrifugal casting Cast tubes are made of medium alloy steels (25 to 30 per cent chromium and 20 to 30 per cent nickel) containing 0.5 per cent carbon. If they are positioned vertically, their sagging strength allows for metal skin temperatures of 1000 to 1100 C. 

Medium-alloy steels have only slightly higher corrosion fatigue strength than carbon steels. 

Heat treatment does not improve corrosion fatigue strength of either carbon or medium-alloy steels residual stresses are deleterious. 

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