Report Heat Treatment Eng Lab 3

1.0 TITLE

:

HEAT TREATMENT

2.0 OBJECTIVE

: 1.

To learn and conduct the methods of heat treatment for engineering material (metal/alloys).

3.0 APPARATUS

:

Electric furnace and specimen (mild/low carbon steel)

4.0 THEORY :

Heat treatment is a method used

stenite will be formed. Austenite is transformed from Pearlite. Austenite will transform into ferrite and cementite during the cooling process. Soft and ductile structure will be formed in slow cooling. However, rapid cooling will result in a hard but brittle structure.

5.0 PROCEDURES:

A. Experiment procedure: i)

S1 - Full Annealing -

Full annealing is the process of slowly raising the temperature about 50 ºC (122 ºF) above the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC (122 ºF) into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon). It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate of about 20 ºC/hr (36 ºF/hr) in a furnace to about 50 ºC (122 ºF) into the Ferrite-Cementite range. At this point, it can be cooled in room temperature air with natural convection.

The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.

Process Annealing is used to treat work-hardened parts made out of lowCarbon 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 through out 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. ii)

S2 - Normalizing The heating procedure is similar to full annealing process, but the carbon steel is air-cooled.

Normalizing is the process of raising the temperature to over 60 º C (140 ºF), above line A3 or line ACMfully into the Austenite range. It is held at this temperature to fully convert the structure into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite. The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling. The main difference between full annealing and normalizing is that fully annealed parts are uniform in softness (and machinablilty) 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. iii)

S3 - Quenching the upper critical point (refer to %carbon content), followed by rapid cooling in water or oil. In materials science, quenching is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby increase toughness of both alloys and plastics (produced through polymerization). In metallurgy, it is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel andmanganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.

Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous metal or "metallic glass". Quenching metals is a progression; the first step is heating, i.e. heating it to the required temperature. Second step is soaking. Soaking can be done by air (air furnace), or a bath or in a vacuum. The soaking time in air furnaces should be 1 to 2 minutes for each millimeter of cross-section. For a bath the time can range a little higher within a vacuum, soak is generally similar to in air. The recommended time allotment in salt or lead baths is 0 to 6 minutes. Uneven heating or overheating should be avoided at all cost. Most materials are heated from anywhere to 815 to 900 °C (1,500 to 1,650 °F). The next item on the progression list is the cooling of the part. Water is one of the most efficient quenching media where maximum hardness is acquired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, whale oil, cottonseed oil and mineral oils are used. These often tend to oxidize and form a sludge, which consequently lowers the efficiency. The uenching velocity (cooling rate) of oil is much less than water. Intermediate rates between water and oil can be obtained with water containing 10-30% UCON from DOW a substance with an inverse solubility which therefore deposits on the object to slow the rate of cooling. Quenching can also be accomplished using inert gases. Most commonly, nitrogen is used at pressures greater than atmosphereic pressure ranging up to 20 bar absolute. Helium is also used because of its greater thermal capacity than nitrogen.Alternatively argon can be used however its density requires significantly more horsepower to move it and its thermal capacity is less than the alternatives. To minimize distortion, long cylindrical workpieces are quenched vertically; flat workpieces are quenched on edge; and thick sections should enter the bath first. iv)

S4 - Tempering The quenched carbon steel is heated again to a temperature which is bel

B. Specimen Preparation: 1. Prepare specimens with the dimension of 20mm (diameter) and 100mm length. 2. Mark the specimens and then start the heat treatment according to their procedures. 3. Once the heat treatment is done, clean the specimens.

6.0 QUESTIONS 1. State the purpose of full annealing, normalizing, quenching and tempering heat treatment. 

The purpose of full annealing is to reduce the hardness of a material.



The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining. The purpose of quenching steel after heating is hardening, i.e. to produce a hardened microstructure over the full cross section of the workpiece. The purpose of tempering is to reduce the brittleness imparted by hardening and to produce definite physical properties within the steel.

 

2. State the effects of cooling process to the specimen structure for each heat treatment. 

Reverting the effect of cold work by process annealing eases further deformation. Heating allows recovery and recrystallization but is usually limited to avoid excessive grain growth and oxidation.

3. Draw the specimen microstructure after subjecting to the heat treatment accordingly.

7.0 DISCUSSION As can been seen from the results of this experiment, steel can have a wide range of properties depending on its heat treatment state. Annealing reforms the grain structure and produces the softest and most ductile material. Austenitizing involves heating steel hot enough to form austentite. This changes the crystal structure, and allows pearlite and bainite to reform into different phases. Quenching involves quickly cooling a material to form martensite, which is an extremely strong but brittle phase of steel. Tempering quenched steel gives enough energy for the carbon atoms to diffuse, which decreases the strength but increases the ductility. This subject is extremely important for any engineer as the heat treatment condition of the steel greatly influences its material properties. If a hard, brittle object such as a cutting tool were required, one would want to quench the steel from the austenite region. Alternatively, if a soft and easy to machine material is desired, annealing before cutting operations is ideal. How microstructures influence mechanical properties – microstructure has thick bands of Pearlite – minimal phase boundaries compared to other possible microstructures, which results in more ductility and lower hardness. Thicker bands of pearlite form when steel is cooled very slowly. – microstructure has fine bands of Pearlite – more phase boundaries than annealed, but not as much as other methods, gives more hardness and less ductility. Thinner bands of pearlite form when steel is air-cooled. – microstructure is martensite, a super-carbon-saturated steel solution – extremely hard and brittle, has almost no ductility. Successful formation of martensite depends on how rapidly the steel is cooled. – Microstructure has extremely small spheres of cementite in a matrix of martensite – very fine structure has a very large amount of phase boundaries, can be very hard but less brittle than as quenched. Material properties depend on size of cementite spheres which vary based on tempering temperature and time.

8.0 CONCLUSION: This experiment demonstrated some key differences in material properties of several heat treatments for 4140 steel. Annealing creates a soft and ductile material that is easy to machine and perform other operations on. Quenching creates a very hard and brittle material that has extremely high yield and tensile strengths. Tempering quenched steel softens the steel and provides a tradeoff between the hard and strong quenched steel and the soft annealed steel. Engineers are benefited by an understanding of heat treatment because they are better able to predict how a material will perform when loaded if they know the heat treatment state of that material. Thus the results of this experiment, and others like it available in literature, are very important for design and manufacturing engineers.