- When tool steels contain a combination of more than 7% molybdenum, tungsten and vanadium, and more than 0.60% carbon, they are referred to as high speed steels. This term is descriptive of their ability to cut metals at 'high speeds'.
- Additions of 5-10% Mo effectively maximize the hardness and toughness of high-speed steels and maintain these properties at the high temperatures generated when cutting metals.
- Molybdenum provides another advantage: at high temperature, steels soften and become embrittled if the primary carbides of iron and chromium grow rapidly in size.
- Molybdenum, especially in combination with vanadium, minimizes this by causing the carbides to reform as tiny secondary carbides which are more stable at high temperatures.
- The largest use of high-speed steels is in the manufacture of various cutting tools: drills, milling cutters, gear cutters, saw blades, etc.
- The useful cutting characteristics of high-speed steel have been further extended by applying thin, but extremely hard, titanium carbide coatings which reduce friction and increase wear resistance, thereby increasing cutting speed and tool life.
Heat Treatment of High Speed Steel
I. Proper heat treatment is as critical to the success of the cutting tool as material selection itself.
II. The object of the heat treating or hardening operation is to transform a fully annealed high-speed tool steel consisting mainly of ferrite (iron) and alloy carbides into a hardened and tempered martensitic structure having carbides that provide the cutting tool properties.
III. The heat treatment process can be divided into four primary areas, preheating, austenitizing, quenching, and tempering.
IV. Preheating: It performs three important functions. The first of these is to reduce thermal shock, which always results when a cold tool is placed into a warm or hot furnace. The second major benefit of preheating is to increase equipment productivity by decreasing the amount of time required in the high-heat furnace. Thirdly, if the high-heat furnace is not neutral to the surface of the tool or part, preheating will reduce the amount of carburization and decarburization that would result if no preheat were employed. It is heated from 790 to 850 degree Celsius in preheat.
V. Austenititzing: Here it is heated at 1150 to 1250 degree Celsius for just 2 to 3 minutes. Here it attends temperature above recrystallization temperature and for austensitic structure. Lower the temperature, lower the hardeness of steel and vice a versa.
VI. Quenching: Cooling is done in two steps that is one is done known as primary quenching in which the metal is dipped in molten sault bath maintained at 350 to 450 degree Celsius. After that is air cooled at room temperature to attend ambient conditions. By this process we get austensitic- martensitic formation to steel giving hardness to it.
VII. Tempering: Now due to quick quenching the metal has developed internal stresses, and to relieve the stress tempering is done. Multiple tempering is done for 2 to 3 hours at 540 degree Celsius for better stress reliving and hardness improving.