Objectives of Heat Treatments:
Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.
Heat Treatment: is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics.
Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material. Steels are heat treated for one of the following reasons:
In heat treatment mechanical properties are altered by:
1. Changing the size of the grains of what it is composed or by
2. Changing its micro constituents
Purpose of heat treatment:
1. To Improve Machinability
2. To Relieve Internal Stresses
3. To Change or Refine Grain Size
4. To Improve Mechanical Properties
5. To Improve Resistance to wear, heat & corrosion
6. To Produce a hard surface on a ductile interior
7. To improve magnetic & electrical properties
Different process of heat treatment:
1. Annealing is generally used to soften the steel
2. Normalizing is used to eliminate coarse grain structure obtained during forging rolling and stamping and produce fine grains.
3. Hardening is done to develop high hardness to resist wear and enable to cut other metals.
4. The hardness produced by hardening depending upon the carbon content of steel.
Steel containing less than 0.15% C does not respond to hardening treatment.
5. Tempering is done to reduce internal stresses and reduce some of the hardness produced during.
Almost all metals and alloys respond to some form of heat treatment, in the broadest sense of the definition. The response of various metals and alloys are not equal practically all steels respond to one or more type of heat treatment. This is the major reason why steels account for over 80% of total metal production. Many non ferrous metals/alloys namely Aluminium, copper, nickel, magnesium and titanium alloys can be strengthened to various degrees by specially designed heat treatments. But not to the same degree and not by the same techniques as steel.
Annealing: It is the process of heating a metal which is in a metastable or distorted structural state to a temperature which will remove the instability or distortion and then cooling (usually at a slow rate) to the room temperature.
1. Inducing a complete structure
2. Refining and homogenizing the structure
3. Reducing hardness
4. Improving Machinability
5. Producing desired micro structure
6. Removing residual stresses
7. Removing gages
8. Improving mechanical, Physical, Electrical and Magnetic properties.
9. Improving cold working characteristics for facilitating further cold work.
Stress relieving:
1. Stress relief annealing eliminates stresses produced by casting, grinding, machining, cold working, welding etc.,
2. Stress relief annealing equally well to ferrous and non ferrous metals
3. Stress relieving does not affect the metallurgical structure of the castings
4. The temp required for stress relief of a casting varies from 0.3 MP to 0.4 MP. Where MP is melting point of the cast metal or alloy.
Process annealing:
1. Process annealing is applied to remove the effects of cold work to soften and permit further cold work as in sheet and wire industries.
2. Ferrous alloys are heated to temp close to but below the lower limit of the transformation range (550o – 650oC) are held at that temp and then cooled in air in order to soften the alloy further cold working as is wire drawing.
3. Process annealing does not involve any phase change.
4. Process annealing is generally carried out is either batch type or continues furnace.
Spheroidise Annealing:
It involves subjecting steel to a selected temperature cycle, usually with in or near the transformation range in order to produce a spheroidal or globular form of carbide in steel.
Purpose:
1. Improves Machinability.
2. Facilitates a subsequent cold working operation.
3. Obtains a desired structure for subsequent heat treatment.
4. Prevents cracking of steel during cold forming operations.
Method: Heating to a temperature above the lower critical line (730 – 770oC) with subsequent holding at this temperature followed by slow cooling at a rate of 25 to 30oC/hour to 600oC.
Full Annealing:
1. Heating the steel to proper annealing – temperature is the austenitic zone.
2. Holding the steel at that temperature for a definite period of time depending upon its thickness or diameter so that it becomes completely austenitic
3. Cooling very slowly the steel object through the transformations range till the object acquires a low temperature.
Normalizing: Heating the steel to about 40 – 50oC above its upper critical temperature (i.e. A3 & ACM) and if necessary, holding it at that temperature for a short time and then cooling is still air at room temperature. Normalizing produces micro structure consisting of ferrite and pearlite for hypo eutectoid (i.e. upto 0.8%) fc) steels.
Purpose of Normalizing:
1. Produces a uniform structure
2. Refines grain size of steel
3. Reduced internal stresses
4. Produces a harden and stronger steel then full annealing
5. Eliminates the carbide network at the grain boundaries of hypereutectoid steels in general improves engineering properties
The degree of hardness produced in steel depends upon:
1. Composition of steel (0.35 to 0.5% of C steels)
2. Nature and properties of quenching medium
3. Quenching temperature
4. Size of objective to be quenched
5. Homogeneity of austenite
6. Degree of agitation
7. Rate of cooling
Quenching Medium types:
1. 5 – 10% caustic soda
2. 5 – 20% brine (Nacl)
3. Cold water
4. Warm water
5. Mineral oil
6. Animal oil
7. Vegetable oil
Tempering: Quench hardening produces structure martensite and retained austenite.
1. The martensite formed in quench hardened steel is exceedingly brittle hard and highly stressed; the cracking and distortion of the hardened article is liable to occur after quenching for this reason; the use of the steel is this condition is inadvisable except in cusses where extreme hardness is required.
2. Quench hardened steel besides containing martensite has some retained austerite too. Retained austerite is also an unstable phase and as it changes with time dimensions may alter.
3. It is therefore necessary to return towards equilibrium after quench hardening by heating (hardened) steel to a temperature below the lower critical temperature (A2) is called tempering.
Process:
1. Heating hardened steel below the lower critical temperature
2. Holding is at that temperature for 3 to 5 min. for each mm of thickness or dia.
3. Cooling the steel either rapidly or slowly (water, oil, air)
4. Types in tempering:
Low temperature tempering:
1. This tempering carried out in the temperature range from 150 to 250oC.
Internal stresses are relieved
2. Low temperature tempering is applied to cutting tools of carbon steels and low alloy steel and to the components that are surface hardened and carbonized.
Medium temperature tempering:
1. This tempering carried out at the temperature range from 350oC to 450oC
2. Develops troostite structure
3. Hardness and strength of steel decrease
4. % of elongation and ductility increase
5. This tempering is applied to objects such as coil springs, laminated springs hammers and chisels etc.
High temperature tempering:
1. This tempering is carried at the temperature range from 500oC to 650oC
2. Eliminates internal stresses completely
3. Develops a sorbite struct
Materials and treatments for mould parts at a “glance”: Service requirement, fabrication requirement & economical requirement of various products are different from one another.
These input requirements are to be clearly understood before the selection of right material. There are some common properties required in all parts such as good strength, less dimensional changes during Heat Resistance treatments, good Machinability, good polishability etc.,
Softening: is done to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size or change the electromagnetic properties of the steel. Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed, such as in a cold-rolling operation or wiredrawing. Annealing-full Process, spheroid zing, normalizing and tempering — austempering, martempering are the principal ways by which steel is softened.
Hardening: Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened.
Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques.
Material Modification: Heat treatment is used to modify properties of materials in addition to hardening and softening. These processes modify the behaviour of the steels in a beneficial manner to maximize service life, e.g., stress relieving, or strength properties, e.g., cryogenic treatment, or some other desirable properties, e.g., spring aging.
The main difference between full annealing and normalizing is that fully annealed parts are uniform in softness (and mach inability) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part. This may not be desirable if further machining is desired, since it makes the machining job somewhat unpredictable. In such a case it is better to do full annealing.
Process Annealing is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite region, line A1on the diagram. This temperature is about 727 ºC (1341 ºF) so heating it to about 700 ºC (1292 ºF) should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase throughout the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.
Spheroidization: is an annealing process used for high carbon steels (Carbon > 0.6%) that will be machined or cold formed subsequently. For tool and alloy steels heat to 750 to 800 ºC (1382-1472 ºF) and hold for several hours followed by slow cooling.
All these methods result in a structure in which all the Cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix. This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.
Direct hardening:
Hardness is a function of the Carbon content of the steel. Hardening of steel requires a change in structure from the body-centred cubic structure found at room temperature to the face-centred cubic structure found in the Austenitic region. The steel is heated to Austenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched it would form Austenite and Pearlite which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite, which is extremely soft.
Hardenibility, which is a measure of the depth of full hardness achieved, is related to the type and amount of alloying elements. Different alloys, which have the same amount of Carbon content, will achieve the same amount of maximum hardness; however, the depth of full hardness will vary with the different alloys. The reason to alloy steels is not to increase their strength, but increase their hardenibility — the ease with which full hardness can be achieved throughout the material.
Usually when hot steel is quenched, most of the cooling happens at the surface, as does the hardening. This propagates into the depth of the material. Alloying helps in the hardening and by determining the right alloy one can achieve the desired properties for the particular application. Such alloying also helps in reducing the need for a rapid quench cooling — thereby eliminates distortions and potential cracking. In addition, thick sections can be hardened fully.
Quench Media: Quenching is the act of rapidly cooling the hot steel to harden the steel.
Water: Quenching can be done by plunging the hot steel in water. The water adjacent to the hot steel vaporizes, and there is no direct contact of the water with the steel. This slows down cooling until the bubbles break and allow water contact with the hot steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots.
Water is a good rapid quenching medium, provided good agitation is done. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking.
Salt Water: Salt water is a more rapid quench medium than plain water because the bubbles are broken easily and allow for rapid cooling of the part. However, salt water is even more corrosive than plain water, and hence must be rinsed off immediately.
Oil: is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of Martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard.
Polymer quench: that will produce a cooling rate in between water and oil. The cooling rate can be altered by varying the components in the mixture-as these are composed of water and some glycol polymers. Polymer quenches are capable of producing repeatable results with less corrosion than water and less of a fire hazard than oil. But, these repeatable results are possible only with constant monitoring of the chemistry. Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100% Martensite at room temperature. However high carbon and high alloy steels have retained Austenite at room temperature. To eliminate retained Austenite, the quench temperature has to be lowered. This is the reason to use cryogenic quenching.
Tempering: is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of Martensite. This brittleness is removed by tempering. Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability. Tempering is not to be confused with tempers on rolled stock-these tempers are an indication of the degree of cold work performed.
The mechanism of tempering depends on the steel and the tempering temperature. The prevalent Martensite is a somewhat unstable structure. When heated, the Carbon atoms diffuse from Martensite to form a carbide precipitate and the concurrent formation of Ferrite and Cementite, which is the stable form. Tool steels for example, lose about 2 to 4 points of hardness on the Rockwell C scale. Even though a little strength is sacrificed, toughness (as measured by impact strength) is increased substantially. Springs and such parts need to be much tougher — these are tempered to a much lower hardness.
Tempering is done immediately after quench hardening. When the steel cools to about 40 ºC (104 ºF) after quenching, it is ready to be tempered. The part is reheated to a temperature of 150 to 400 ºC (302 to 752 ºF). In this region a softer and tougher structure Troostitic is formed. Alternatively, the steel can be heated to a temperature of 400 to 700 ºC (752 to 1292 ºF) that results in a softer structure known as Sorbitic. This has less strength than Troostitic but more ductility and toughness.
Tool steel: is normally delivered in the soft annealed condition. This is to make the material easy to machine with cutting tools and to give it a microstructure suitable for hardening.
The microstructure consists of a soft matrix in which carbides are embedded. In carbon steel, these carbides consist of iron carbide, while in the alloyed steel they are chromium (Cr), tungsten (W), molybdenum (Mo) or vanadium (V) carbides, depending on the composition of the steel. Carbides are compounds of carbon and these alloying elements and are characterized by very high hardness.
Higher carbide content means higher resistance to wear. In alloy steels, it is important that the carbides are evenly distributed. Other alloying elements are also used in tool steel, such as cobalt (Co) and nickel (Ni), but these do not form carbides. Cobalt is normally used to improve red hardness in high speed steels, nickel to improve through-hardening properties.
Hardening and tempering: When a tool is hardened, many factors influence the result.
Some theoretical aspects: In soft annealed tool steel, most of the alloying elements are bound up with carbon in carbides. In addition to these there are the alloying elements cobalt and nickel, which do not form carbides but are instead dissolved in the matrix. When the steel is heated for hardening, the basic idea is to dissolve the carbides to such a degree that the matrix acquires an alloying content that gives the hardening effect—without becoming coarse grained and brittle.
Note that the carbides are partially dissolved. This means that the matrix becomes alloyed with carbon and carbide-forming elements. When the steel is heated to the hardening temperature (Austenitizing temperature), the carbides are partially dissolved, and the matrix is also altered. It is transformed from ferrite to austenite. This means that the iron atoms change their position in the atomic lattice and make room for atoms of carbon and alloying elements. The carbon and alloying elements from the carbides are dissolved in the matrix. If the steel is quenched sufficiently rapid in the hardening process, the carbon atoms do not have time to reposition themselves to allow the reforming of ferrite from austenite, i.e. as in annealing.
Instead, they are fixed in positions where they really do not have enough room, and the result is high micro stresses that can be defined as increased hardness. This hard structure is called marten site. Thus, marten site can be seen as a forced solution of carbon in ferrite. When steel is hardened, the matrix is not completely converted into marten site. Some austenite is always left and is called “retained austenite”. The amount increases with increasing alloying content, higher hardening temperature and longer soaking times. After quenching, the steel has a microstructure consisting of marten site, retained austenite and carbides. This structure contains inherent stresses that can easily cause cracking. But this can be prevented by reheating the steel to a certain temperature, reducing the stresses and transforming the retained austenite to an extent that depends upon the reheating temperature. This reheating after hardening is called tempering. Hardening of a tool steel should always be followed immediately by tempering. It should be noted that tempering at low temperatures only affects the marten site, while tempering at high temperature also affects the retained austenite. After one tempering at high temperature, the microstructure consists of tempered marten site, newly formed Marten site, some retained austenite and carbides. Precipitated secondary (newly formed) carbides and newly formed marten site can increase hardness during high-temperature tempering. Typical of this is the so called secondary hardening of e.g. high speed steel and high alloyed tool steels. Tool steel should always be double tempered. The second tempering takes care of the newly formed marten site formed after the first tempering. Three tempers are recommended for high speed steel with high carbon content.
How hardening and tempering is done in practice: Due to hardening must be taken into consideration when a tool is rough-machined. Rough machining causes local heating and mechanical working of the steel, which gives rise to inherent stresses. This is not serious on a symmetrical part of simple design, but can be significant in asymmetrical machining, for example of one half of a die casting die. Here, stress relieving is always recommended.
Stress relieving: This treatment is done after rough machining and entails heating to 550– 650°C (1020–1200°F). The material should be heated until it has achieved uniform temperature all the way through and then cooled slowly, for example in a furnace. The idea behind stress relieving is that the yield strength of the material at the elevated temperature is so low that the material cannot resist the inherent stresses. The yield strength is exceeded and these stresses are released, resulting in a greater or lesser degree of distortion. The correct work sequence is: rough machining, stress relieving and semi finish machining.
Heating to hardening temperature: The fundamental rule for heating to hardening temperature is that it should take place slowly. This minimizes distortion. In vacuum furnaces and furnaces with controlled protective gas atmosphere, the heat is increased gradually. When molten salt baths are used, preheating is employed, whereas heating is automatically slow in a muffle furnace when steel is packed in cast-iron chips. It is important that the tools are protected against oxidation and decarburization. The best protection is provided by a vacuum furnace, where the surface of the steel remains unaffected. Furnaces with a controlled protective gas atmosphere or salt baths also provide good protection. If an electric muffle furnace is used, the tool can be protected by packing it in spent charcoal or cast iron chips. It should be observed that these packing materials can have a carburizing effect if the steels have low carbon content, such as conventional hot Vacuum furnace, Salt bath furnace, Batch type furnace with a controlled atmosphere. Wrapping in stainless steel foil also provides good protection when heating in a muffle furnace. Decarburization results in low surface hardness and a risk of cracking. Carburization results in a harder surface layer, which can have negative effects.
Holding time at hardening temperature: It is not possible to state exact recommendations briefly to cover all heating situations. Factors such as furnace type, furnace rating, temperature level, the weight of the charge in relation to the size of the furnace etc., must be taken into consideration in each case. We can, however, give one recommendation that is valid in virtually all situations. When the steel has reached hardening temperature through its entire thickness, hold at this temperature for 30 minutes.
An exception to this rule is for thin parts heated in salt baths at high temperature, or high speed steel. Here the entire period of immersion is often only a few minutes.
Quenching: The choice between a fast and slow quenching rate is usually a compromise; to get the best microstructure and tool performance, the quenching rate should be rapid; to minimize distortion, a slow quenching rate is recommended.
Slow quenching results in less temperature difference between the surface and core of a part, and sections of different thickness will have a more uniform cooling rate. This is of great importance when quenching through the marten site range, below the Ms Temperature. Marten site formation leads to an increase in volume and stresses in the material. This is also the reason why quenching should be interrupted before room temperature has been reached, normally at 50–70°C (120–160°F).
However, if the quenching rate is too slow, especially with heavier cross-sections, undesirable transformations in the microstructure can take place, risking a poor tool performance. Water is used as a quenching medium for unalloyed steels. 8–10% sodium chloride (salt) or soda should be added to the water in order to achieve optimum cooling efficiency. Water hardening can often cause problems in the form of distortion and quench cracks.
Oil hardening is safer, but hardening in air or mar tempering is best of all. Oil should be used for low alloyed steels. The oil should be of good quality, and preferably of the rapid quenching type. It should be kept clean and must be changed after a certain period of use. Hardening oils should have a temperature of 50–70°C (140–160°F) to give the best cooling efficiency. Lower temperatures mean higher viscosity, i.e. the oil is thicker.
Hardening in oil: is not the safest way to quench steel, in view of the risks of distortion and hardening cracks. These risks can be reduced by means of mar tempering. In this process, the material is quenched in two steps. First it is cooled from hardening temperature in a salt bath whose temperature is just above the MS temperature. It is kept there until the temperature has equalized between the surface and the core, after which the tool can be allowed to cool freely in air down through the marten site transformation range. When mar tempering oil hardening steels, it should also be kept in mind that the material transforms relatively rapid and should not be kept too long at the mar tempering bath temperature. This can lead to excessive binate transformation and the risk of low hardness.
High alloy steels can be hardened in oil, a mar tempering bath or air. The advantages and disadvantages of the different methods can be discussed.
Oil gives a good finish and high hardness, but it also maximizes the risk of excessive distortion or cracking. In the case of thick parts, quenching in oil is often the only way to achieve maximum hardness. Mar tempering in salt bath produces a good finish, high hardness and less risk of excessive distortion or cracking. For certain types of steel, the temperature of the salt bath is normally kept at about 500°C (930°F). This temperature ensures a relatively mild thermal shock, but a sufficient cooling rate to avoid phase transformations. Full marten site transformation has, in many cases, time to occur when the steel is cooled in air from the mar tempering bath temperature. However, if the dimensions are big, it is often necessary to use a forced quenching rate depending of the harden ability of the steel.
Air quenching: entails the least risk of excessive distortion. A tendency onwards lower hardness is noticeable at greater thicknesses. One disadvantage is a poorer finish.
Some oxidation takes place when the material comes into contact with air and cools slowly from the high hardening temperatures. The choice of quenching medium must be made from job to job, but a general recommendation could perhaps be made as follows: A mar tempering bath is the safest in most cases. Air is used when dimensional stability is crucial. Oil should be avoided and used only when it is necessary to achieve satisfactory hardness in heavy sections.
Tempering: The material should be tempered immediately after quenching. Quenching should be stopped at a temperature of 50–70°C (120–160°F) and tempering should be done at once. If this is not possible, the material must be kept warm, e.g. in a special “hot cabinet”, waiting tempering.
The choice of tempering temperature is often determined by experience. However, certain guidelines can be drawn and the following factors can be taken into consideration:
Hardness
Toughness
Dimension change.
If maximum hardness is desired, temper at about 200°C (390°F), but never lower than 180°C (360°F). High speed steel is normally tempered at about 20°C (36°F) above the peak of the secondary hardening temperature. If a lower hardness is desired, this means a higher tempering temperature. Reduced hardness does not always mean increased toughness, as is evident from the toughness values in our product brochures. Avoid tempering within temperature ranges that reduce toughness. If dimensional stability is also an Convection type tempering furnace important consideration, the choice of tempering temperature must often be a compromise. If possible, however, priority should be given to toughness.
How many tempers are required?
Two tempers are recommended for tool steel and three are considered necessary for high speed steel with a high carbon content, e.g. over 1%. Two tempers are always recommended. If the basic rule in quenching is followed—to interrupt at 50–70°C (120–160°F)—then a certain amount of austenite remains untransformed when the material is to be tempered. When the material cools after tempering, most of the austenite is transformed to marten site. It is untendered. A second tempering gives the material optimum toughness at the hardness in question. The same line of reasoning can be applied with regard to retained austenite in high speed steel. In this case, however, the retained austenite is highly alloyed and slows transforming. During tempering, some diffusion takes place in the austenite, secondary carbides are precipitated, and the austenite becomes lower alloyed and is more easily transformed to marten site when it cools after tempering. Here, several tempering can be beneficial in driving the transformation of the retained austenite further to marten site.
Holding times in connection with tempering: Here also, one should avoid all complicated formulae and rules of thumb, and adopt the following recommendation: After the tool is heated through, hold the material for at least 2 hours at full temperature each time.
Dimensional and shape stability: Distortion during the hardening and tempering of tool steel. When a piece of tool steel is hardened and tempered, some war page or distortion normally occurs. This distortion is usually greater at high temperature. This is well known, and it is normal practice to leave some machining allowance on the tool prior to hardening. This makes it possible to adjust the tool to the correct dimensions after hardening and tempering by grinding, for example. How does distortion take place? The cause is stresses in the material. These stresses can be divided into:
Machining stresses: This type of stress is generated during machining operations such as turning, milling and grinding. (For example, such stresses are formed to a greater extent during cold forming operations such as blanking, bending and drawing.) If stresses have built up in a part, they will be released during heating. Heating reduces strength, releasing stresses through local distortion. This can lead to overall distortion. In order to reduce this distortion while heating during the hardening process, a stress relieving operation can be carried out prior to the hardening operation.
Thermal stresses: These stresses are created when a piece is heated. They increase if heating takes place rapidly or unevenly. The volume of the steel is increased by heating. Uneven heating can result in local variations in volume growth, leading to stresses and distortion.
As an alternative with large or complex parts, heating can be done in preheating stages in order to equalize the temperature in the component. Very powerful stresses arise during quenching. As a general rule, the slower that quenching can be done, the less distortion will occur due to thermal stresses. It is important that the quenching medium is applied as uniformly as possible. This is especially valid when forced air or protective gas atmosphere (as in vacuum furnaces) is used. Otherwise temperature differences in the tool can lead to significant distortion.
Transformation stresses: This type of stress arises when the microstructure of the steel is transformed. This is because the three microstructures in question—ferrite, austenite and marten site—have different densities, i.e. volumes. The greatest effect is caused by transformation from austenite to marten site. This causes a volume increase. Excessively rapid and uneven quenching can also cause local marten site formation and thereby volume increases locally in a piece and give rise to stresses in this section. These stresses can lead to distortion and, in some cases, quenching cracks.
HOW CAN DISTORTION BE REDUCED?
Distortion can be minimized by:
1. keeping the design simple and symmetrical
2. eliminating machining stresses by stress relieving after rough machining
3. heating slowly during hardening
4. using a suitable grade of steel
5. quenching the piece as slowly as possible, but quick enough to obtain a correct microstructure in the steel
6. tempering at a suitable temperature.
7. Some words of advice to tool designers
CHOICE OF STEEL: Choose air-hardening steels for complex tools.
DESIGN: Avoid:
• Sharp corners
• Notch effects
• Large differences in section thicknesses.
These are often causes of quench cracks, especially if the material is cooled down too far or allowed to stand untendered.
Sub-zero treatment: Tools requiring maximum dimensional stability in service can be sub-zero treated as follows: Immediately after quenching, the tool should be sub-zero treated to –70 to –80°C (–95 to –110°F), soaking time 1–3 hours, followed by tempering. The sub-zero treatment leads to a reduction of retained austenite content. This, in turn, will result in a hardness increase of 1–2 HRC in comparison to not sub-zero treated tools if low temperature tempering is used. For high temperature tempered tools there will be little or no hardness increase and when referencing the normal tempering curves, a 25 to 50°C (45 to 90°F) lower tempering temperature should be chosen to achieve the required hardness. Tools that are high temperature tempered, even without a sub-zero treatment, will normally have a low retained austenite content and in most cases, a sufficient dimensional stability. However, for high demands on dimensional stability in service it is also recommended to use a sub-zero treatment in combination with high temperature tempering. For the highest requirements on dimensional stability, sub-zero treatment in liquid nitrogen is recommended after quenching and after each tempering.
Case hardening –diffusion treatment: The Carbon content in the steel determines whether it can be directly hardened. If the Carbon content is low (less than 0.25% for example) then an alternate means exists to increase the Carbon content of the surface. The part then can be heat-treated by either quenching in liquid or cooling in still air depending on the properties desired. Note that this method will only allow hardening on the surface, but not in the core, because the high carbon content is only on the surface. This is sometimes very desirable because it allows for a hard surface with good wear properties (as on gear teeth), but has a tough core that will perform well under impact loading.
Carburising: is a process of adding Carbon to the surface. This is done by exposing the part to a Carbon rich atmosphere at an elevated temperature and allows diffusion to transfer the Carbon atoms into steel. This diffusion will work only if the steel has low carbon content, because diffusion works on the differential of concentration principle. If, for example the steel had high carbon content to begin with, and is heated in a carbon free furnace, such as air, the carbon will tend to diffuse out of the steel resulting in Decarburisation. Pack Carburising: Parts are packed in a high carbon medium such as carbon powder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 ºC (1652 ºF). At this temperature CO gas is produced which is a strong reducing agent. The reduction reaction occurs on the surface of the steel releasing Carbon, which is then diffused into the surface due to the high temperature. When enough Carbon is absorbed inside the part (based on experience and theoretical calculations based on diffusion theory), the parts are removed and can be subject to the normal hardening methods. The Carbon on the surface is 0.7% to 1.2% depending on process conditions. The hardness achieved is 60 - 65 RC. The depth of the case ranges from about 0.1 mm (0.004 in) up to 1.5 mm (0.060 in). Some of the problems with pack carburising are that the process is difficult to control as far as temperature uniformity is concerned, and the heating is inefficient. Gas Carburising is conceptually the same as pack carburising, except that Carbon Monoxide (CO) gas is supplied to a heated furnace and the reduction reaction of deposition of carbon takes place on the surface of the part. This process overcomes most of the problems of pack carburising. The temperature diffusion is as good as it can be with a furnace. The only concern is to safely contain the CO gas. Liquid Carburising: The steel parts are immersed in a molten carbon rich bath. In the past, such baths have cyanide (CN) as the main component. However, safety concerns have led to non-toxic baths that achieve the same result.
Nitriding: The purpose of nitriding is to increase the surface hardness of the steel and improve its wear properties. This treatment takes place in a medium (gas or salt) which gives off nitrogen. During nitriding, nitrogen diffuses into the steel and forms hard, wear resistant nitrides. This results in an inter metallic surface layer with good wearing and frictional properties.
Nitriding is done in gas at about 510°C (950°F) and in salt or gas at about 570°C (1060°F) or as ion nitriding, normally at around 500°C (930°F). The process therefore requires steels that are resistant to tempering in order not to reduce the core strength.
Nitriding is a process of diffusing Nitrogen into the surface of steel. The Nitrogen forms Nitrides with elements such as Aluminium, Chromium, Molybdenum, and Vanadium. The parts are heat-treated and tempered before nitriding. The parts are then cleaned and heated in a furnace in an atmosphere of dissociated Ammonia (containing N and H) for 10 to 40 hours at 500-625 ºC (932 - 1157 ºF). Nitrogen diffuses into the steel and forms nitride alloys, and goes to a depth of up to 0.65 mm (0.025 in). The case is very hard and distortion is low. No further heat treatment is required; in fact, further heat treatment can crack the hard case. Since the case is thin, surface grinding is not recommended. This can restrict the use of nitriding to surfaces that require a very smooth finish. The obtained hardness depends the amount of alloying elements present. The highest hardness obtained by the nitriding process ranges from VH 1000 to 1300.
Nitriding Operation: Widely used method is Gas nitriding. In the nitriding process, nitrogen is introduced to the steel by passing ammonia gas through a muffle furnace containing the steel to be nitride. The ammonia is purchased in tanks as a liquid and introduced in to the furnace as a gas at slightly greater than atmospheric pressure. With the nitriding furnace operating at 490-5500C the ammonia (NH3) gas partially dissociates in to a nitrogen & hydrogen gas mixture. The dissociation of ammonia is shown by the following equation. 2NH3 – 2N + 3H2
The atomic nitrogen diffuses in to steel forming nitrides, thus producing a hard ware resistant surface free from scale containing the minimum of distortion. However only a portion of the nitrogen reacts with the hot steel, for the greater part remains inert and passes out of the container with the liberated hydrogen. Since the life of atomic nitrogen is short, it is necessary to replenish it continuously by supplying fresh ammonia to the steel surface. It is thus necessary to circulate ammonia in such a way that the atomic nitrogen produced through dissociation is continuously. The case depth specified in nitriding is usually a very shallow one but requires from 18 to 90 hrs. To obtain nitriding at 515oC for 16 hrs. Result in about 0.010” case depth, 48 hrs. Result in about 0.020” case depth, 90 hrs. Result in about 0.030” case depth.
Carbo nitriding: process is most suitable for low carbon and low carbon alloy steels. In this process, both Carbon and Nitrogen are diffused into the surface. The parts are heated in an atmosphere of hydrocarbon (such as methane or propane) mixed with Ammonia (NH3). The process is a mix of Carburising and Nitriding. Carburising involves high temperatures (around 900 ºC, 1652 ºF) and Nitriding involves much lower temperatures (around 600 ºC, 1112 ºF). Carbo nitriding is done at temperatures of 760 - 870 ºC (1400 - 1598 ºF), which is higher than the transformation temperatures of steel that is the region of the face-centred Austenite. It is then quenched in a natural gas (Oxygen free) atmosphere. These quenches are less drastic than water or oil-thus less distortion. However this process is not suitable for high precision parts due to the distortions that are inherent. The hardness achieved is similar to carburising (60 - 65 RC) but not as high as Nitriding (70 RC). The case depth is from 0.1 to 0.75 mm (0.004 to 0.030 in). The case is rich in Nitrides as well as Martensite. Tempering is necessary to reduce the brittleness
Selective hardening: Carbon steels that have minimum carbon content of 0.4%, or alloy steels with a lower carbon content (hardenable stainless steels with only 0.1% Carbon), can be selectively hardenened in specific regions by applying heat and quench only to those regions. Parts that benefit by flame hardening include gear teeth, bushings etc. These techniques are best suited for medium carbon steels with a carbon content ranging from 0.4 to 0.6%.
Flame hardening: A high intensity oxy-acetylene flame is applied to the selective region. The temperature is raised high enough to be in the region of Austenite transformation. The "right" temperature is determined by the operator based on experience by watching the colour of the steel. The overall heat transfer is limited by the torch and thus the interior never reaches the high temperature. The heated region is quenched to achieve the desired hardness. Tempering can be done to eliminate brittleness. They don’t change the chemical composition of the steel. They are essentially shallow hardening methods. Selected areas of surface of steel are heated into the austenite range and then quenched to form martensite
The depth of hardening can be increased by increasing the heating time. As much as 6.3 mm (0.25 in) of depth can be achieved. In addition, large parts, which will not normally fit in a furnace, can be heat-treated. Depth of the hardened zone may be controlled by an adjustment of the flame intensity, heating time or speed of travel.
Four methods are in general use for flame hardening:-
1. Stationery: Both torch and works are stationery. This method is used for the spot hardening of small parts such as valve stems and open and wrenches.
2. Spinning: the torch is stationery while the work rotates; this method is used to harden parts of circular cross sections such as precision gears, pulleys and similar components.
3. Progressive: The torch moves over a stationery work piece. This is used for hardening of large parts, such as the way of Lathe, but is also adaptable to the treatment of teeth of large gears.
4. Progressive spinning: The torch moves over a rotating work piece is used to surface harden long circular parts such as shafts and rolls.
Induction hardening: In Induction hardening, the steel part is placed inside an electrical coil which has alternating current through it. This energizes the steel part and heats it up. Depending on the frequency and amperage, the rate of heating as well as the depth of heating can be controlled. Hence, this is well suited for surface heat treatment. Five basic designs of work coils for use with high frequencies units and heat patterns developed by each are (a) simple solenoid for external heating (b) a coil tube used internally for heating bores (c) a die-plate type of coil designed to provide high current densities in a narrow bond for scanning applications. (d) A single turn coil for scanning a rotating surface provided with a contoured half turn that will aid in heating the fillet and (e) a pancake coil for spot heating. When high frequency a.c. current passes through the work coil a high frequency magnetic field is set up. The magnetic field induces high frequency eddy currents and hysteresis current in the metal. Heating results from the resistances of the metal to the passage of these currents. The high frequency induced currents tend to travel at the surface of the metal. This is known as skin effect. It is possible to heat a shallow layer of the steel without heating the interior. It tends to flow interior by conduction. Heating is an important factor in controlling the depth of the hardened zone. The surface layer is heated practically instantaneously to a depth which is inversely proportional to the square root of the frequency. The range of frequency commonly used between 10,000 and 50,000 Hz. An advantage of induction hardening is the ability to fit the equipment directly into the production line and use relatively unskilled labour since the operation is practically automatic. The surface remains free from scale. Among disadvantages are the cost of equipment, the fact that small quantities or irregular – shaped parts cannot be handled economically and high maintenance cost. Typical parts that have been induction hardened are piston rods pump shafts, spur gears and cams.
Ion nitriding: This is a new nitriding technology. The method can be summarized as follows: The part to be nitrided is placed in a process chamber filled with gas, mainly nitrogen. The part forms the cathode and the shell of the chamber the anode in an electric circuit. When the circuit is closed, the gas is ionized and the part is subjected to ion bombardment. The gas serves both as heating and nitriding medium. The advantages of ion nitriding include a low process temperature and a hard, tough surface layer. The depth of diffusion is of the same order as with gas nitriding.
Plasma (Ion) Nitriding: Plasma nitriding is a modern technique of surface hardening metallic components to improve their service lives. The nitriding processes based on solid, liquid and gas treatments have been traditionally used which, however, suffer from several drawbacks. Plasma nitriding overcomes them and, moreover, it is environmental-friendly which makes it even more attractive. Because of its advantages, it is gradually replacing the conventional processes in the industry. The conventional nitriding process is carried out in a solid or a liquid or a gaseous medium. However, these processes suffer from some serious drawbacks, e.g. 1) the treated layers are not of good quality (generally porous); 2) their growth are non uniform; 3) time-consuming; 4) the consumables are high; 5) they are environmentally not friendly. Basically, plasma nitriding is a glow discharge process in a mixture of nitrogen and hydrogen gases. The apparatus consists of a vacuum vessel, a gas handling system and a high voltage power supply. First, the vacuum vessel is evacuated using a mechanical pump to a base pressure of 10-1 torrs. Then a nitrogen and hydrogen mixture is introduced in to the vacuum chamber through a leak valve and filled up to a few torrs. A high voltage is established between the grounded vessel and the sample (to be nitride) which is at the cathode potential. (The glow discharge for nitriding falls in the abnormal region of the current-voltage characteristic). By adjusting the high voltage, it is possible to control the current density on the cathode which in turn controls the temperature needed for nitriding. The hardness of the nitride layer depends on the alloying constituents of the steel sample. The diffused nitrogen reacts with the alloying elements and forms stable nitrides, which is the reason for increased hardness. More the stability of the nitride compound, more the hardness it will provide. In terms of stability, nitrides may be ordered as follows: Fe, Mn, W, Mo, Cr, V, Ti, Al. Thus alloy steel containing a high percentage of aluminium will provide maximum hardness, followed by Ti, V, Cr and the rest, in that order. The plastics industry has been one of the sectors which have used nitriding extensively as a surface hardening process. Four different types of steels, namely EN24,P20, OHNS, and EN8 which are commonly used in making plastics dies, moulds and other parts were selected. The duration of treatment was kept at 13 hours. Out of the four, P20 shows the best microhardness profile followed by EN24, OHNS and EN8. On comparing with the literature data of gas nitriding for P20, it is found that plasma nitriding provides more case depth and surface hardness for the same duration of treatment. To put it in another way, for the same case depth, it takes a factor of three less time (with more surface hardness) compared to the best available conventional process, i.e. gas nitriding.
The main advantages of plasma nitriding over conventional methods can be listed as follows:
1. Reduced cycle time (up to a factor of four, compared to gas nitriding)
2. Controlled growth of the surface layer.
3. Elimination of white layer is possible.
4. Reduced distortion.
5. No need for finish grinding.
6. Pore free surface layer is possible.
7. Mechanical masks can be used instead of copper plating.
8. The ability to provide uniform cooling.
Cryogenics, or deep freezing is done to make sure there is no retained Austenite during quenching. When steel is at the hardening temperature, there is a solid solution of Carbon and Iron, known as Austenite. The amount of Martensite formed at quenching is a function of the lowest temperature encountered. At any given temperature of quenching there is a certain amount of Martensite and the balance is untransformed Austenite. This untransformed austenite is very brittle and can cause loss of strength or hardness, dimensional instability, or cracking.
Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100 % Martensite at room temperature. However, high carbon and high alloy steels have retained Austenite at room temperature. To eliminate retained Austenite, the temperature has to be lowered.
In Cryogenic treatment the material is subject to deep freeze temperatures of as low as -185°C (-301°F), but usually -75°C (-103°F) is sufficient. The Austenite is unstable at this temperature, and the whole structures become Martensite. This is the reason to use Cryogenic treatment.
Summary of Benefits:
1. Conversion of retained austenite (soft) into marten site (hard)
2. Increased strength, toughness, stability and durability
3. Increased density of the steel structure
4. Lower coefficient of friction
5. Decreased residual stresses and brittleness
6. Significantly improved abrasive wear resistance
Testing of mechanical properties: When the steel is hardened and tempered, its strength is affected, so let us take a closer look at how these properties are measured. Hardness testing is the most popular way to check the results of hardening. Hardness is usually the property that is specified when a tool is hardened. It is easy to test hardness. The material is not destroyed and the apparatus is relatively inexpensive. The most common methods are Rockwell C (HRC), Vickers (HV) and Brinell (HBW). We shouldn’t entirely forget the old expression “file-hard”. In order to check whether hardness is satisfactory, for example above 60 HRC, a file of good quality can provide a good indication.
Rockwell hardness testing: a conical diamond is first pressed with a force F0, and then with a force F0+F1 against a specimen of the material whose hardness is to be determined. After unloading to F0, the increase (e) of the depth of the impression caused by F1 is determined. The depth of penetration (e) is converted into a hardness number (HRC) which is read directly from a scale on the tester dial or read-out.
Vickers hardness testing: a pyramid-shaped diamond with a square base and a peak angle of 136° is pressed under a load F against the material whose hardness is to be determined. After unloading, the diagonals d1 and d2 of the impression are measured and the hardness number (HV) is read off a table.
When the test results are reported, Vickers hardness is indicated with the letters HV and a suffix indicating the mass that exerted the load and (when required) the loading period, as illustrated by the following example: HV 30/20 = Vickers hardness determined with a load of 30 kgf exerted for 20 seconds.
Brinell hardness testing: a ball is pressed against the material whose hardness is to be determined. After unloading, two measurements of the diameter of the impression are taken at 90° to each other (d1 and d2) and the HB value is read off a table, from the average of d1 and d2.
When the test results are reported, Brinell hardness is indicated with the letters HB and a suffix indicating ball diameter, the mass with which the load was exerted and (when required) the loading period, as illustrated by the following example: HB 5/750/15 = Brinell hardness determined with 5 mm ball and under load of 750 kgf exerted for 15 seconds.
SURFACE TREATMENT PROCESS
Chrome plating: Hard chromium plating can improve the wear resistance and corrosion resistance of a tool. Hard chromium plating is done electrolytic ally. The thickness of the plating is normally between 0,001and 0,1 mm (0,00004–0,004 inch). It can be difficult to obtain a uniform surface layer, especially on complex tools, since projecting corners and edges may receive a thicker deposit than large flat surfaces or the holes. If the chromium layer is damaged, the exposed steel may corrode rapidly. During the chromium plating process, hydrogen absorption can cause a brittle surface layer. This nuisance can be eliminated by tempering immediately after plating at 180°C (360°F) for 4 hours. This technique, as applied to molds, is common practice in the industry and offers molders the following advantages: Chrome plate has a low co-efficient of friction, and thus facilities the flow of plastic in the mold, and also the removal of the molded article. Chrome plate has excellent resistance to abrasion, and thus tends to protect the mold from wear. Chrome plate, on a previously well-polished surface, provides a bright smooth finish, which is duplicated on the molded article. Chrome plate protects the surface of the steel mold against rusting during shut downs and periods of storage.
Nickel plating: In molds for some processes requiring plated surfaces to facilitate mold release, as in sheet molding compound processing using low-shrink resins, evaluation is going on in the concept of nickel plating. Costs are comparable to chrome and the process is more flexible than chrome plating (i.e. minor alternations can be made and the tool nickel plated while still in the press). Nickel-plating, however, unlike chrome plating, is not applicable to large parts. Nickel usually needs more time in the mold making process, but in parts involving a deep draw, it can do a better job than chrome.
Finding the correct working hardness for the mould: The chosen working hardness for the mould and the heat treatment method used to achieve it affect a lot of properties. Properties such as toughness, compression strength, wear and corrosion resistance. Generally it can be said that increased hardness results in better resistance against wear, pressure and indentation and that decreased hardness leads to better toughness. A normal working hardness for through-hardening steel is 48–60 Rockwell C. The optimum working hardness used depends on the chosen steel, the mould size, layout and shape of the cavities, the molding process, plastic material etc. For recommended steel grades and working hardness related to various plastic materials and processes, A substantial part of the total tooling cost is that incurred in the making of the mould. It is therefore of great importance that the mould making process should be as straight-forward as possible.
How to deal with dimensional changes -It is true that some dimensional changes are inevitable during hardening. But it is also possible to limit and control these changes to a certain extent. For instance by slow, uniform heating to the austenitizing temperature, by using a temperature that is not too high and a suitable quenching medium.
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