High speed steels
High Speed Steels (HSS) have been given this name due to the fact that these steels may be operated as cutting tools at much higher speeds that are possible with plain carbon tool steel. High speed steels cutting tools operate at cutting speed 2 to 3 times higher than for High carbon steels. At higher cutting speeds, sufficient heat may be developed during the cutting process. This heat causes the cutting edge of the tool to reach a high heat (red heat).
This heat softens the carbon tool steel and thus the tool will not work efficiently for a longer period. These steels have the property of retaining their hardness even when heated to red heat. High hardness at elevated temperatures is developed by addition of elements such as tungsten, chromium vanadium to high carbon steels. These steel are generally used for making lathe cutting tools, planner cutting tools, shaper cutting tools, slotting cutting tools, drills, reamers, broaches, milling cutter and punches. There are four general types of high speed steels used in machine shop.
1. High speed steel (18:4:1)
High speed steels (HSS) are most commonly operated as cutting tools at much higher speed i.e. twice or thrice where as tool steel. It is the most common kind of cutting tool. It contains 18% tungsten, 4% chromium and 1 % vanadium, 0.8 carbon and remaining iron. It is considered to be one of the best of all purpose tool steels. This brand of high speed steel is used for machining operations on steel and non-ferrous materials. This is generally used for lathe, planer and shaper tools, drills, millings cutters, punches etc.
2. Molybdenum based high speed steel
It contains 6% Mo, 6% W, 4% Cr, 2% V, 0.8% C and remaining Fe. It has excellent toughness and cutting ability. Molybdenum high speed steels are cheaper than other types of steels and are particularly used for drilling and tapping tools. These steels are also used for making rough cutting tools, lathe tools and various kinds of milling cutters.
3. Cobalt based high speed steel
It contains 1 to 12% Co, 20% W, 4% Cr, 2% V, 0.8 carbon and remaining iron. This is also known as super high speed steel, because cutting tool made of this steel can be operated at much higher speeds in comparison to high speed steel of 18:4:1 kind. In this steel, cobalt is added from 2 to 15 per cent in order to increase the cutting efficiency especially at high temperature. Cobalt high speed steel generally contains 20% W, 4% Cr, 2% V and 12% Co and remaining Fe. Since the cost of this steel is more, therefore, it is principally used for making cutting tools for heavy operations which impose high pressure and temperature on the tool.
It is extensively used for making high production tools of heavy work for high production lathe, planer, shaper, milling and boring machine.
4. Vanadium High Speed Steel
Generally, this steel contains more than 1% V and 0.70% C. This steel possesses better abrasive resistance in comparison to normal HSS type steel. It is preferred for machining materials which are highly difficult to machine by conventional means. These steels cutting tools are close competitors of carbides cutting tools such as drills, reamers, milling cutters etc. In addition to having heat resistance properties of high speed steels possesses desirable properties of high hardness, high compressive strength and outstanding wear resistance.
Availability of steel in market
Steel are available in market in various rolled forms like sheets, plates, strips, rods, beams, channels, angles, tees etc. The common structural shapes for market form of supply are reflected in Fig. 4.5. Steels are generally identified, coded and designated according to suitable standards by a group of symbols indicating the important characteristics.
Reference Introduction to basic Manufacturing Processes and Workshop Technology by Rajender Singh.
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