Describe the proper heat treating procedures for other tool steels. Safety The following procedures are suggested for a safe heat treating operation. When lighting the furnace, obey the instructions that have been provided by the manufacturer.
When quench tanks are not being used, always cover them. Use a nonflammable absorbent to clean leaks and oil spills. This should be done immediately. If possible, keep tools, baskets, jigs, and work areas free from oil contamination. Before breaks and before moving on to the next task, wash your hands thoroughly. If any skin trouble is shown or suspected, report to your instructor and get medical help.
Make sure there is good ventilation in the work area. Be on the lookout for contamination from pieces of carburized metal. Do not take oil-soaked clothes or equipment to areas where there are food or beverages. Do not take food or beverages where oils are either being used or stored. Procedure The first important thing to know when heat treating a steel is its hardening temperature. To begin the process: 1. Preheat the furnace to degrees Fahrenheit.
Previous: Chapter 5: Surface Grinder. Heat treatment is defined as an operation involving the heating and cooling of a metal or an alloy in the solid-state to obtain certain desirable properties without change composition. The process of heat treatment is carried out to change the grain size , to modify the structure of the material, and to relieve the stresses set up the material after hot or cold working.
Heat treatment consists of heating the metal near or above its critical temperature, held for a particular time at that finally cooling the metal in some medium which may be air, water, brine, or molten salts. The heat treatment process includes annealing, case hardening, tempering, normalizing and quenching, nitriding, cyaniding, etc. Annealing is one of the most important processes of heat treatment. It is one of the most widely used operations in the heat treatment of iron and steel and is defined as the softening process.
The main aim of annealing is to make steel more ductile and malleable and to remove internal stresses. This process makes the steel soft so that it can be easily machined. It is held at this temperature for a definite period of time depending on the type of furnace and nature of work.
The steel is then allowed to cool inside the furnace constantly. Normalizing: The main aim of normalizing is to remove the internal stresses developed after the cold working process.
Normalizing is the process of heat treatment carried out to restore the structure of normal condition. It is held at this temperature for a short duration. The steel is then allowed cool in still air at room temperature, which is known as air quenching. Hardening: The main aim of the hardening process is to make steel hard tough. It is the opposite process of annealing. The steel is heated above its critical temperature range.
It is held at that temperature for a definite period of time. The steel is then rapidly cooled in a medium of quenching. The quenching medium is selected according to the degree of hardness desired. The air, water, bring, oils and molten salts are used as quenching mediums.
A thin section such knife blades are cooled in air. Water is widely used medium but it results in the formation of bubbles on the surface of the metal. Hence brine solution is used to prevent this. Oil is used when there is a risk of distortion on cracks and is suitable for alloy steels. The molten salts are used to cool thin section to obtain crack-free and impact-resistant products. It is applied for chisels, sledgehammer, hand hammer, centre punches, taps, dies, milling cutters, knife blades and gears.
Tempering: When the hardening process hardens a steel specimen, it becomes brittle and has high residual stress. It is an operation used to modify the properties of steel hardened by quenching for the purpose of increasing its usefulness. Tempering or draw results in a reduction of brittleness and removal of internal strains caused during hardening.
This causes complete recrystallization in steel to form New grain structure. This will release the internal stresses previously the strip in the steel and improve the machinability. It consists of heating the Steel temperature at or near the critical point holding there for a suitable time and then allowing it cools slowly in the Furnace itself. This courses wipes out all traces of the previous structure and define the crystalline structure in addition to the softening of the metal.
It also removes internal stresses. Normalizing is a heat treatment process similar to annealing in which the Steel is heated to about 50 degree Celsius above the upper critical temperature followed by air cooling.
This heat treatment process is usually carried for low and medium carbon steel as well as alloy steel to make the grain structure more uniform and relieve the internal stresses. It consists of heating Steel components to the temperature within or above its critical range. Held at this temperature for a considerable time to ensure thorough penetration of heat at this temperature well inside the component and then allowed to cool separately by quenching in water oil or brine solution.
The strength and hardness of the Steel are increased but makes it more brittle since ductility is reduced.
This heat treatment process carried out for steel parts which have been already hardened, in order to reduce bitterness and unequal stress develop as a result of hardening. This process reduces brittleness and hardness but improves the tensile strength of Steel.
It increases the toughness of Steel at the expenses of loss of some hardness. Almost all the cutting tools need a hard cutting edge while at the same time they are required to be tough and strong so as not to break when subjected to shock or fatigue.
Sampling is done by reheating the hardened Steel to some temperature below the lower critical temperature and then quenching in oil or salt bath. Reheating of Steel during tempering temperature decreases the hardness Somewhat But improve the toughness.
Nitriding is a process of surface hardening in which nitrogen gas is used to obtain a hard surface for the Steel. In this process, the Steel parts are heated in an atmosphere of ammonia NH 3 for a prolonged period and then cooled slowly.
During this process, when Ammonia comes in contact with steel is diffuses into nascent hydrogen and nascent nitrogen.
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