Steels free-cutting

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. 

The presence of small quantities of S in steels has little effect on the initial scaling rates in air, but may be detrimental to long-term scale adhesion. Sulphur has, however, been shown to be detrimental to breakaway oxidation in CO/CO2 environments. However, sulphur has been shown to reduce the total uptake of carbon in the steel under CO/C02 and reduce the scale thickening rate. In this context, free-cutting steels were found to oxidise at a significantly lower rate, as did steels subjected to pretreatment in H2S. 

ISO 683-1, Heat-treatable steels, alloy steels and free-cutting steels — Part 1 Direct-hardening unalloyed and lo w-alloyed wrought steel in form of different black products. 

The carbon conient of steels usually dues ntn exceed 1,7 >. In addition to carbon, j lmn einhon. steels contain small amounts of silicon, manganese, phosphorus, and sulfur derived from Ihe raw materials and fuel used in the steelmaking process. Within limitations, silicon and manganese are beneficial anti often are purposely added, mainly because they are dcoxidants. Except in free-cutting steels, where sulfur is purposely added, sulfur and phosphorus are deleterious and their content is kept as low as... 

Sulfur. This element is detrimental to surface quality, but bcnelicial to machinability, particularly in low-carbon and low-manganese steels. Sulfur decreases transverse ductility and impact resistance, but has only a small effect on longitudinal properties. As sulfur content increases, weldability decreases. Sulfur is added to the extent of 0 2-0.4 7 in free-cutting steels to improve machinabilily. 

M 40 mild free cutting steel, low tensile steel, non-ferrous metals and light alloys turning, parting off, particularly on automatic machines... 

Some sources of information must be examined carefully to determine whether the composition quoted is typical or maximum permitted , whether it represents ladle analysis or product analysis, and even whether the steel is a free-cutting grade. It is not unknown for typical values to be used for some elements and maximum values for others, in the same table ... 

Free-cutting or free-machining grades of steel normally contain up to 0.2% of sulphur, although up to 0.5% is possible. Other elements such as lead and selenium may also be present. In these cases special precautions are necessary and are outlined in Chapter 4. 

Cutting Temperature, Fig. 10 Temperature distribution during orthogonal cutting (Work Free-cutting mild steel, Cutting speed 75 f/min, Width of cut 0.25 in., Workpiece temperature 611 °C)... 

Table 3.1-U Standard free-cutting (re-sulfurized) carbon steel compositions applicable to semi-finished products for forging, hot-roUed and cold-finished bars, and seamless tuhing [1.80]...

The carbon content of machine steels is increased to 0.35-0.45%. Addition of sulfur (0.2%) gives so called free cutting steels with very good qualities for drilling, turning and mUling. 

Free-cutting, leaded, resulphurized steel high-speed, automatic machining... 

The gas nitriding for improvement of corrosion resistance of plain carbon steels and low-alloy steels can be carried out for shorter times at elevated temp>eratures (Minkevich, 1965). The purpose is to obtain on the steel surface non-etched nitrided layer without pores and thickness about 0.015 - 0.030 mm. In Table 1 are given the conditions of this process for some plain carbon steels and free-cutting steels. 

Table 1. Gas nitriding process conditions for improvement of the corrosion resistance for plain carbon steels and free-cutting steels.

Martensitic and austenitic free cutting steels Quenched 450 640-840 0.54-0.70 11... 

C1018 (Standard) C1018 (Soft) 316 Stainless steel Brass (Free Cutting) A360 A380... 

Machinability index for a material is expressed as a percentage based on the relative ease of machining a material with respect to free cutting mild steel which is 100 per cent and taken as the standard. 

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