General principles for processing stainless steel
Introduction Austenite types are the most common and therefore the most commonly processed stainless steels, such as grades 304 (1.4301) and 316 (1.4401). They are characterized by high work hardening rate and poor chip breaking performance during processing. The important issues affecting the successful processing of these steels are described below. Machine and mold rigidity When machining stainless steel, it is important to ensure that there are no stalls or friction caused by machine vibration or tool chatter. The machine must be "sufficient" and capable of performing the deep cuts required for austenitic stainless steel machining without reducing the set feed rate or surface speed. Small training or "hobby" lathes and milling machines for machining mild steel, brass, etc. cannot be sufficient to successfully process stainless steel. The machine should not excessively vibrate in the base, drive and gearbox or on the cutting tool or its mount. Avoid overhanging the tool handle from the tool box. The distance between the tool tip and the toolbox support should be as short as possible, and the cross section of the tool holder should be as large as possible. This can also help dissipate heat from the cutting surface. The shank of the supporting cylindrical milling cutter should be as short as possible. Knife holder support should be as close as possible to the end of the cutter to provide maximum support. It's not uncommon to have a "shrill" phenomenon when cutting metal, but it may indicate that the tool may be worn and needs to be replaced. Tool material High speed steel (HSS) (forged or sintered) or hard alloy tools can be used to process stainless steel. High speed steel Either tungsten or molybdenum HSS can be used. These are particularly useful in machining operations involving high feed and low speed machining operations where variable cutting edge stresses are generated due to complex tool shapes. Tungsten type (such as T15) can be used for its good abrasion resistance and red hardness. Molybdenum HSS is more widely used, and M42 can be used in applications such as milling cutters, where a good combination of hardness and strength is required at lower cutting speeds. M42 has a higher hardness than ordinary M2, but its toughness may not be as good as M2. If the tool is easy to fall off, use a harder grade, such as M2, M10. If the tool is burning, use a higher red hardness grade, such as M42, T15. If the tool is worn, use a more wear-resistant grade, such as T15 Cemented carbide Cemented carbide is commonly used for machining stainless steels, where higher speeds or higher feed rates are required than those produced using high-speed steel. Disposable or brazed inserts can be used (lower cutting speeds can be tolerated) and they consist of tungsten carbide or a mixture of tungsten with other metal carbides, including titanium, niobium and chromium. Carbides are combined with cobalt. "Straight" tungsten carbide grades are used for austenitic and duplex stainless steels, while "complex" carbide grades are used for martensite and ferrite series. Coated carbides also have the added advantage of improving abrasion and fracture resistance. As a result, they have higher cutting speeds than uncoated carbide tools. The wide variety of cemented carbide tools commonly available means that machining tests are required to obtain the best machining characteristics for a particular situation. Tool geometry and clarity When machining stainless steel, the cutting tool must be sharpened. It is important to carefully sand and grind the tool face to provide an accurate and sharp face angle. This helps to optimize: Tool life, accuracy and tolerance productivity between grindings are reduced: Tool breakage power requirements should be re-sharpened as soon as the cutting quality deteriorates. In order to obtain the necessary tool geometry accuracy, mechanical grinding using a hand-ground, glass-free grinding wheel is preferred over manual grinding. The correct tool geometry is important to minimize chip buildup on the tool face. Chip accumulation can also lead to increased demand for machine power and poor surface finish on the machined surface. The back angle of the tool must be flat. The concave flank may cause the tool to crack or be damaged by reducing the support of the cutting edge. Where possible, because austenitic stainless steel is prone to long spiral turning, it is easy to wrap around tools and tool holders. Therefore, a chip curler or breaker should be installed on the tool surface. These are easily tangled in the tool, and removal is difficult and time consuming. In extreme cases, the tool may get stuck by entangled turning. Lubrication and cooling When processing stainless steel, cutting fluid must be used. This is due to the combined effects of deep cutting and high feed rates to overcome the effects of work hardening, and the low thermal conductivity of austenitic stainless steel, which limits the flow of heat away from the work surface. Overheating of the stainless steel surface (characterized by the formation of heat-stained colors) during processing can impair corrosion resistance and must be avoided. If pickled, this surface can be used to restore the corrosion resistance of the finished part. Overheating can also cause deformations that are difficult to compensate or correct. The lubrication provided by the cutting fluid also helps reduce tool wear and flush away machining chips. Generally, cooling is more important than lubrication and cutting speed is faster, so high cutting fluid flow rates are often used when machining stainless steels. Either mineral oil or water-soluble emulsified oil can be used. Mineral oil is more suitable for severe machining with heavy loads at low speeds or with HSS tools. Emulsified oils are used to process at higher speeds using carbide tools. mineral oil Sulfurized, chlorinated or thiochlorinated mineral oils can be mixed with up to 10% fatty oils to process non-free processing grades. Paraffin is used to dilute these oils. The oil / paraffin ratio is 1/5 for high speed and light feeds, and 1/1 for slower and heavier feed processing. If excessive wear occurs, consider using a larger diluent. If the cutting edge burns easily, consider reducing the dilution. EC These oils are diluted with water and provide better cooling than paraffin-diluted mineral oils. If extreme pressure (EP) emulsified oils are used, more stringent machining operations can be supported. It is important to dilute by refueling in water rather than in water to form the correct emulsion form with proper lubrication and cooling properties. After processing, all traces of cutting fluid on the surface should be removed so that the stainless steel surface can be self-passivated. In some cases, acid passivation should be considered.