Tool steels oxidation

Tool Steels. Oxidation is a well-established process used for high-speed steel cutting tools. Increases in tool life of up to 100% are achieved, mostly due to a decrease in friction, because of the hard oxide coating and the ability of the porous oxide to entrap lubricant and draw it to the tool/workpiece interface. Steam oxidation of a finished tool is accomplished either by exposing it to steam at a temperature of about 565 °C (1050 °F) or by treating it in liquid sodium hydroxide and sodium nitrate salts at approximately 140 °C (285 °F) for 5 to 20 min. These treatments result in a black oxidized layer that is less than 5 p-m (0.2 mil) thick and will not peel, chip, or crack, even when the tool is bent or cut. Tool life improvements due to steam oxidation are listed in Table 5. 

Fig. 3.35 Oxidation rate at 535°C of I8Ni250 maraging steel compared with a generally-available tool steel. These tests were performed on 7 in (6-35 mm) cubes placed in refractory cubicles and exposed to still air for total times of 5, 25 and 100 h. The weight gain includes the scale formed during heating and cooling (after Reference 9)...

Powder Formation. Metallic powders can be formed by any number of techniques, including the reduction of corresponding oxides and salts, the thermal dissociation of metal compounds, electrolysis, atomization, gas-phase synthesis or decomposition, or mechanical attrition. The atomization method is the one most commonly used, because it can produce powders from alloys as well as from pure metals. In the atomization process, a molten metal is forced through an orifice and the stream is broken up with a jet of water or gas. The molten metal forms droplets to minimize the surface area, which solidify very rapidly. Currently, iron-nickel-molybdenum alloys, stainless steels, tool steels, nickel alloys, titanium alloys, and aluminum alloys, as well as many pure metals, are manufactured by atomization processes. 

Forging and extrusion do not involve the removal of metal by means of cutting tools. Many important shaping processes are based on cutting operations. Cutting tools are made of special steels (tool steels), hard metals, oxide ceramics, and diamond. 

Figure 6.25 Dynamic powder-wall friction coefficient versus the ratio of powder particle size to wall roughness, for three values of the ratio of particle hardness to wall hardness (Hp/H ). The powder is ferric oxide and the walls are (a) tungsten carbide, (b) hardened tool steel and (c) non-hardened tool steel. (From Ref. 29b.)...

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