1. Describes in basic terms, the following heat-treatment processes: - tempering - annealing - normalizing - hardening2. Explains, indicates why the processes in the above objectives are used3. States how low-carbon steels can be given a hard case4. States why low-carbon steels are sometimes given a hard case5. Describes in basic terms a suitable heat-treatment process for common steels, given the properties required
Various types of heat treatment processes are used tochange the following properties or conditions of thesteel:• Improve the toughness• Increase the hardness• Increase the ductility• Improve the machinability• Refine the grain structure• Remove the residual stresses• Improve the wear resistance
The Hardening ProcessesSteels can be heat treated to high hardness and strength levels. Thereasons for doing this are obvious. Structural components subjectedto high operating stress need the high strength of a hardenedstructure. Similarly, tools such as dies, knives, cutting devices, andforming devices need a hardened structure to resist wear anddeformation.QuenchingIn this process steels which contain sufficient carbon, and perhapsother alloying elements, are cooled sufficiently rapidly from abovethe transformation temperature to produce Martensite.There is a range of quenching media of varying severity, water orbrine being the most severe, through oil and synthetic products to airwhich is the least severe.
TemperingAfter quenching the steel is hard, brittle and internally stressed. Beforeuse, it is usually necessary to reduce these stresses and increasetoughness by tempering. There will also be a reduction in hardnessand the selection of tempering temperature dictates the finalproperties.As a rule of thumb, within the tempering range for a particular steel,the higher the tempering temperature the lower the final hardness butthe greater the toughness.Note:In general terms, if steel is heated until it glows red and isquenched in clean water immediately, it becomes very hard butalso brittle. This means it is likely to break or snap if put undergreat pressure. On the other hand, if the red hot steel is allowed tocool slowly, the resulting steel will be easier to cut, shape and fileas it will be relatively soft.
Main objectives of2. Hardening > To increase the hardness of the metal, so that it can resist wear > To enable it to cut other metals, ie to make it suitable for cutting tools2. Tempering > To reduce brittleness of the hardened steels and thus to increase its ductility > To remove the internal stresses caused by rapid cooling of steel > To make steel sufficiently tough to resist shock and fatigue
Type of work Tempering Tempering Temperature,oC colourScrapers & lathe tools for 220 Light straws orbrass pale yellowHammer faces, planers for 230 Straw or darksteel, screwing dies for brass strawMoulding, and planing cutters 255 Yellowish brownfor haedwood, punches and or browndiesGold chisels and sets for steel, 280 Purplecold chisels for cast iron,chisels for woodCold chisels for wrought iron 290 VioletScrew drivers 295 BlueSprings 300 Dark blue
Example 1: Hardening and Tempering ProcessIf a mild steel or silver steel screw driver blade has beenmanufactured at some point it will have to be ‘’hardened’ to prevent itwearing down when used.
1. The screw driver blade is heated, slowly at first, warming up the whole blade. Then the heat is concentrated on the area at the end of the blade. This gradually becomes ‘red’ hot.
2. The screw driver blade is removed quickly from the brazing heart, with blacksmiths tongs and plunged into clean, cold water. Steam boils off from the water as the steel cools rapidly. At this stage the blade is very hard but brittle and will break easily.
3. The screw driver blade is cleaned with emery cloth and heated again on the brazing hearth. Heat is concentrated at the end of the steel blade. The steel must be watched very carefully as it changes colour quite quickly. A blue line of heat will appear near the end of the blade and it travels towards the tip as the temperature rises along the blade. When the line of blue reaches the tip the brazing torch is turned off. The blue indicates the correct temperature of ‘tempering’.
4. The screw driver blade is placed on a steel surface, such as an anvil face. This conducts the heat away and allows slow cooling of the screw driver blade. When cold, the blade should be tough and hard wearing and unlikely to break or snap. This is due to the tempering process.
Example 2: Case Hardening of Mild SteelCase hardening is a simple method of hardening steel. It is lesscomplex than hardening and tempering.This techniques is used for steels with a low carbon content.Carbon is added to the outer surface of the steel, to a depth ofapproximately 0.03mm.One advantage of this method of hardening steel is that the innercore is left untouched and so still processes properties such asflexibility and is still relatively soft.
1. The steel is heated to red heat. It may only be necessary to harden one part of the steel and so heat can be concentrated in this area.2. The steel is removed from the brazing hearth with blacksmiths tongs and plunged into case hardening compound and allowed to cool a little. The case hardening compound is high in carbon.
3. The steel is heated again to a red colour, removed from the brazing hearth and plunged into cold, clean water. The steel rod should now have a hardened outer surface and a flexible, soft interior. The process can be repeated to increase the depth of the hardened surface.
The Softening ProcessesHardening is reversible. If a hardened tool needs to be remachined, itmay be softened by heat treatment to return it to its machinablecondition. Most steels weld better in their soft state than in theirhardened state; softening may be used to aid weldability.Annealing is a heat process whereby a metal is heated to a specifictemperature /colour and then allowed to cool slowly. This softens themetal which means it can be cut and shaped more easily. Mild steel,is heated to a red heat and allowed to cool slowly. However, metalssuch as aluminium will melt if heated for too long.Aluminium can be annealed but care must be taken whilst heating.The flame should be held at a distance to the aluminium so that itgives a generalised heating to the metal.
A ‘trick of the trade’ is to rub soap on to the surface of the aluminiumand then heat it on the brazing hearth. It takes only a short time for thesoap to turn black. The brazing torch should be turned off immediatelyand the aluminium allowed to cool slowly. It is now annealed andshould be very soft and malleable.You anneal metals to relieve internal stresses, soften them, makethem more ductile, and refine their grain structures.
Normalizing is a type of heat treatment applicable toferrous metals only. It differs from annealing in that themetal is heated to a higher temperature and then removedfrom the furnace for air cooling. The purpose ofnormalizing is to remove the internal stresses induced byheat treating, welding, casting, forging, forming, ormachining. Stress, if not controlled, leads to metal failure;therefore, before hardening steel, you should normalize itfirst to ensure the maximum desired results. Normalizedsteels are harder and stronger than annealed steels.
The animation above shows that an annealed metal is usually softerand can be deformed more easily than metals that are not annealed.The animation above shows that metals that have not been annealedare very difficult to deform.
Annealed metals are relatively soft and can be cut and shaped moreeasily. They bend easily when pressure is applied. As a rule they areheated and allowed to cool slowly.Hardened metals are difficult to cut and shape. They are verydifficult if not impossible to bend. As a rule they are heated andcooled very quickly by quenching in clean, cold water.
It should be noted that not all steels will respond to all heat treatmentprocesses, the following table summaries the response, or otherwise,to the different processes. Anneal Normalise Harden Tempe rLow Carbon <0.3% yes yes no noMedium Carbon 0.3-0.5% yes yes yes yesHigh Carbon >0.5% yes yes yes yesLow Alloy yes yes yes yesMedium Alloy yes yes yes yesHigh Alloy yes maybe yes yesTool Steels yes no yes yesStainless Steel (Austenitic eg 304, yes no no no306)Stainless Steels (Ferritic eg 405, 430 yes no no no442)Stainless Steels (Martensitic eg 410, yes no yes yes440)
Approximate Soaking Periods for Hardening, Annealing,and Normalizing Steel
Conclusion• The process of heat treating is the method by which metals are heated and cooled in a series of specific operations that never allow the metal to reach the molten state.• The purpose of heat treating is to make a metal more useful by changing or restoring its mechanical properties.• Through heat treating, we can make a metal harder, stronger, and more resistant to impact. Also, heat treating can make a metal softer and more ductile.• The one disadvantage is that no heat-treating procedure can produce all of these characteristics in one operation. Some properties are improved at the expense of others; for example, hardening a metal may make it brittle.
STAGES OF HEAT TREATMENTHeat treating is accomplished in three major stages:Stage lHeating the metal slowly to ensure a uniform temperatureStage 2Soaking (holding) the metal at a given temperature for a given timeand cooling the metal to room temperatureStage 3Cooling the metal to room temperature