Manganese sulfide (MnS) has long been the dominant machinability enhancing additive due to its ability to provide improvement in machining of PM materials, especially in turning and drilling of iron-copper-carbon materials. A newly developed machinability enhancer, SM4, has shown its capability in replacing MnS without corrosion and stain concern.

Powder metallurgy (PM) can provide significant benefits in minimizing material and energy waste based on its features as a near-net shape technology compared to other manufacturing technologies. The powder manufacturing processes are simple and have flexible capability in formulating alloying elements and additives into the base metal to achieve desired material properties including machinability. Since machining of PM components is becoming more prevalent as tolerances and features become more demanding, adding machining additives is a common practice to achieve improved productivity and tool life.

As a conventional machinability enhancer, manganese sulfide (MnS) has been recognized as an outstanding additive to improve the machining of PM materials, especially for Fe-Cu-C materials. At a 0.5% addition, it works well in almost any machining operation. The principal of MnS in improving the machinability is a combination of solid lubricant, chip breaker and tool protector. In commercially available machining additives, few of them could provide such combined features as seen with MnS. The drawbacks of using MnS are that it tends to easily cause stains on part surfaces and greatly decreases corrosion resistance of component. In many applications, the corrosion resistance and appearance of PM components are important since end customers and users generally consider rust and stains as defects. In these situations, MnS is either used at a low level or not used at all.

For decades, many research and development efforts have been undertaken exploring similar or better machining additives to replace MnS in order to alleviate the stain and rust concerns. Additives such as hexagonal boron nitride, calcium fluoride and magnesium silicates such as talc, enstatite, etc., are commercially used in PM materials for machinability improvement. Although they can provide certain levels of improvement for some PM material systems, none of them can be widely utilized as effective as MnS.

Recent development has realized several new machining additives commercially used in PM materials, resulting in similar or better machinability improvement for low-alloyed and sinter-hardened steels compared to MnS. The commercial additive named SM3 is a recent answer to the application without the concerns found with MnS. This additive is proprietarily formulated as engineered composites based on tribological concepts. During machining, it is considered to provide tool protection through a favorable transfer film generated by friction heat between the tool and work piece. It exhibits superior performances as compared to MnS in terms of better productivity and longer tool life in low-alloyed, sinter-hardened and heat treated steels with lower addition levels. For Fe-Cu-C steels, it can also provide better machinability improvement than MnS in turning operation. For drilling operation, it does improve the machinability of the Fe-Cu-C steels but still has not reached the level of performance as MnS.

This article appeared in the May/June issue of Metal Powder Report.

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