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I need help to write a nice introduction for experiment 6 please ( no hands write ) typing

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HEAT TREATMENT OF STEELS EXPERIMENT 6 EXPERIMENT 6 HEAT TREATMENT OF STEELS THEORY The Effect of Cooling Rate One of the most convenient methods for controlling the properties of a given steel, i.e., a steel whose composition is already fixed, consists of austenizing the steel and ten cooling to room temperature at some predetermined rate. A variation of cooling rates allows for a wide spectrum of properties that can be induced in the steel product. This dependence of the steel's properties on the rate of cooling is merely a reflection of the more basic dependence of three structural factors on the cooling rate, viz., (1) the steel's eut TIME LDG SCALE Figure 28. CCT Diagram for a 1080 Steel 67 EXPERIMENT6 HEAT TREATMENT OF STEEL final microstructure, (2) thermal strain remaining in the steel after the cooling step and (3) the final grain size of the steel. (1) Microstructure: Referring to the Continuous Cooling Transformation (CCT) curve for a 1080 steel (cf. Figure 28), note that all possible cooling rates have been divided into three separate ranges. In the .Yast" range are included all rates from the extremely fast rate (almost a vertical line) to the cooling rate labeled CR which is a line tangent to the curve representing the start of the eutectoid reaction. Since all rates in this range enter directly into the martensite region without first going through the eutectoid region, 100% martensite is produced (assuming that the steels are quenched through M). In the slow range, the range bounded by the extremely slow rate (almost a horizontal line) and the cooling rate labeled CRP, the tangent to the curve for the finish of the eutectoid or reaction, belong all rates that cause the formation of 100% pearlite. Note that all of these rates pass completely through the region of the CCT curve representing the eutectoid reaction. The intermediate range, bounded by CRw and CRp includes all cooling rates that produce mixtures of pearlite and martensite, the relative amount of each depending on how close the particular cooling rate approaches either CR or FAST SLOW RATE OF COOLING Figure 29. Hardenability Curve The sequence of resulting microstructures as one passes from the very fast to the very slow rates, namely, pure martensite, a martensite-pearlite mixture, and then pure pearlite, is nicely demonstrated in Figure 29, a graph of the stee's final 68 EXPERIMENT HEAT TREATMENT OF STEELS hardness-vs-cooling rate (called a "hardenability" curve). Note that the hardnesses in the fast" range are all very high, as should be expected for the martensite microstructure, and those in the "slow" range are all relatively low, as expected for the pearlite microstructure In the intermediate range, the large hardness variation reflects the fact that, in the mixture, the martensite/pearlite concentration runs the entire gamut from all martensite to all (2) Thermal Strain: Although only pure martensite is formed by using any one of the cooling rates in the "fast" range, note that the R hardnesses, while all very high, do vary to a degree, increasing slightly as the rate of cooling increases. This rise in the hardness is due to the greater thermal strains remaining in the steel after the cooling step. When a piece of steel is rapidly cooled (quenched) by placing it into a cold water bath, the portion of steel near the surface cools more rapidly than the portion near the center Because of the specific volume decrease resulting from a drop of temperature, the outside portion of the steel tends to contract while the inside portion remains, for some time, in a more "expanded state. This results in the appearance of "thermal shear stresses between the inside and outside portions. In an attempt to diminish these stresses andthus lower the potential energy, a certain amount of flow (slip) occurs within the steel's lattice structure. Eventually, the inside portion cools down to the temperature of the bath causing the contraction of the inside portion to the same specific volume as the outside, but because the steel has already been strained and distorted in an attempt to alleviate the previous situation, this only causes greater stresses and more distortion. The end result is a harder, stronger but also more brittle steel- the same effects that cold-working of the steel would have produced. As faster cooling rates are used, therefore, the greater becomes the discrepancy between inside and outside temperatures, and, consequently the greater are the residual thermal stresses and subsequent hardness of the steel (3) Grain size: In the "slow range, the hardness again varies somewhat with cooling rate even though the only microstructure possible in this range is pure pearlite. The reason for this can be found in a variation of the pearlite grain size. Since longer times and higher temperatures encourage grain growth, the slower cooling rates in this range tend to produce a coarser (larger-grained) form of pearlite while the faster rates produce a finer form. Since a larger grain size invariably causes a material to be softer, it follows that the hardness should decrease with decreasing cooling rate. EXPERIMENT 6 HEAT TREATMEN Hardenability may be defined as the ease with which a steel can be made to achieve its maximum hardness, or equivalently, the ease with which a steel can be made to form pure martensite. Obviously, this property can be directly related to the position of the "C" part of the CCT diagram (cf. Figure 28). If the C-curve for a particular steel specimen lies well to the left in the diagram, the extent of the "fast" range is severely limited and only extremely fast rates of cooling can result in pure martensite formation. This specimen, therefore, would not be a very hardenable steel. If the C-curve lies well to the right, on the other hand, the "Tast" range is much larger, and hence slower cooling rates may be used for the production of martensite. This indicates a more hardenable steel. It stands to reason then that the factors that control the lateral position of the C-curve also control the property of hardenability. Each of the following tends to delay the eutectoid reaction, i.e., move the C-arve to the right and thereby increase hardenability: (1) alloying, (2) increase of carbon content and (3) larger grain size. For example, a 4340 (alloy) steel is more hardenable than a 1040 (iron and carbon only) steel and a 1 080 (0.8% C) steel is more hardenable than a 1060 (0.6% C) steel. Since the hardness-vs-cooling rate curve (Figure 28) is related to the CCT diagram, the relative hardenability of a steel can be determined from this curve as well. As hardenability increases, the size of the "Tast" range, or the region in which the R hardnesses remain very high, also increases. The Jominy End-Quench Test One obvious method for obtaining the hardenability curve of a particular steel Figure 28) is to austenize many small samples of the steel, cool each at a different rate and measure the final hardness of each sample. A far more efficient method is the Jominy End-Quench test. The steel specimen, basically in the shape of a cylinder 1 inch in diameter and 4 inches long, is brought to equilibrium in the austenite range. The cylinder is then removed from the furnace, one end is quenched with water and the other is left to air-cool. By the time the whole sample has reached room temperature, the various circular cross-sections of the bar have been subjected to a complete spectrum of cooling rates from the very fast (water-quenched) to fairly slow (air-cooled). Hardness readings are then taken along the length of the sample and the hardenability curve produced (cf. Figure 30). The abscissa is normally left as "distance from quenched end" instead of converting it to 70 EXPERIMENT 6 EAT TREATMENT OF STEE a "cooling rate" axis. Figure 30. The Jominy End-Quench Test Figure 31. Some Experimental Hardenability Curves Several hardenability curves which were obtained in this fashion are shown in Figure 31. Note the effect of alloying and carbon content. (For example, compare the three alloy-steel curves with the three 10xx curves in Figure 31(a) and note the trend in the four 40xx curves in Figure 31(b).) For the two 1060 samples in Figure 31 (a), grain sizes of 2 and 8 are indicated. (The higher the number, the smaller the grain.) Note that the #2 sample is more hardenable than the #8 sample, as expected, since the large grain 71 EXPERIMENT6 HEAT TREATMEN STEELS size delays the eutectoid reaction and pushes the C-curve to the right. It should be noted at the same time, however, that the hardness of the #2 sample is also greater than that of the #8 sample at any given cross-section. This is not expected, because a smaller grain size should cause a greater hardness. This surprising result is explainable by the simple fact that these numbers refer to the original grain size before the heat treatment and not the grain size of the final product on which these hardness readings were taken. The fact of the matter is that a #8 grain-sized austenite steel will form, after heat treatment, a larger-grained product than will a #2 grain-sized steel that undergoes exactly the same heat treatment. The reason for this will not be gone into here. EXPERIMENT AND PROCEDURE Jominy End-Quench Test Each group is to perform a jominy end-quench test on the steel sample assigned to it. To save time, these samples have already been placed into a furnace set at some temperature in the austenite range (circa 1500'F or 816'C). Itmay be assumed that they have been in the furnace long enough to insure that complete austenization has taken place. Wheel the movable quenching tank to the vicinity of the furnace. Turn on the water source for the tank and, by using the rotameter in this line, adjust the water flow rate to 165 gallons per hour This should provide a spout of water about 2.5 inches high in the quenching tank. The next step, the removal of the jominy bar from the furnace and its subsequent placement into the quenching tank, is the most critical part of the experiment. It is important that the bar be in place in the tank within 5 seconds after its removal from the furnace. The protective gloves and the large set of tongs are to be used for this purpose. Goggles should also be worm. If the bar sticks to the furnace floor, only a slight amount of force need be used to dislodge it Once the bar has been inserted into its slot in the quenching tank, it should immediately be centered over the water stream by laterally moving the top of the jominy bar with the tongs. Because the bar fits fairly closely into the slot, this adjustment is only a slight one. After a ten-minute lapse, the jominy bar may be removed from the quenching tank 72 EXPERIMENT 6 AT TREATME F STEELS and immersed completely in a water bath. The tongs should also be used for this step because the bar is still fairly hot The steel bar will, by this time, have formed a loosely-adhering oxide scale on its outside surface. Most of this scale may be removed by simply rubbing it off. A gentle filing or rapping with another jominy bar should take care of the remainder To prepare the sample for the Rockwell hardness tester, use the grinding belt to form a narrow flat strip about 1/8 to 1/4 inch in width along the bar's circular surface. It is not necessary to exert very much force downward while the sample is in contact with the belt, but the bar should be held firmly and as steady as possible. Before mounting the jominy bar on the magnetic holder (which replaces the anvil of the Rockwell hardness tester), first crank the handle at the right end of the holder and note the resulting movement of the carriage. Mount the jominy bar on the holder with the flanged end to the left and the ground strip facing upward. The plane of the ground strip on which the hardness readings are to be taken should be as level as possible. Move the carriage toward the left until the quenched end of the jominy bar is directly under the penetrator. Note that, whenever the handle reaches an almost upward position, it tends to click into place. Hardness readings should be taken only when the handle is in this position. One complete turn of the handle between these positions moves the jominy bar exactly one-sixteenth inch. Move the bar slightly to the right in preparation for the first hardness reading. The penetrator should be within 1/16 from the end of the bar. Starting at this position, take sixteen hardness readings on the Rockwe "O-sale 1/16" apart, followed by eight readings 118" apart. Only the first two inches of the bar need be tested. Record these results on the Data and Calculated Results sheet. Because of the hardness of these steels, the indentations will be slight and the closeness of the test sites will present no problem as far as local strain-hardening is concerned On the same graph, plot (EXCEL or Quattro Pro) two hardenability curves complete with experimental points, one for the sample assigned to your group, and the other for a sample that was end-quenched by one other group. The data taken by all groups will be recorded on the blackboard for this purpose. This information should also be copied onto the Data and Calculated Results sheet In the Discussion section of your report, first comment on your results, eg., whether or not the hardenability curve came out as 73

Answer 1

Introduction:

Steels can be heat treated to produce a great variety of microstructures and properties. Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state. In heat treatment, the processing is most often entirely thermal and modifies only structure.

The effect of cooling rate is very important because due to various cooling rate different microstructure is obtained. Annealing uses furnace cooling, Normalizing uses air cooling and hardening uses quenching. If slow cooling we get coarse grains and for faster cooling rate fine grains will be formed.

Hardenability is one of the most significant term used in heat treatment. The ease with which Martensite can be formed through out the component is called as hardenability.The ability for steel to form martensite at different cooling rates. Hardenability represents the ease of martensite formation or the necessary cooling rate, as it is easier to cool slowly

The most widely used hardenability test is Jomney quench end test. The Jominy End-Quench test determines hardenability of any variety of steel, and is straightforward due to its simplicity and minimization of variables. The size and shape of the sample are standardized as well as the quench process, so the extent of martensite formation can be compared quantitatively between different steels. And since steels have similar thermal conductivity, the distance from the quenched end correlates to a certain cooling rate. Knowing this, the hardness of a metal cooled at a given cooling rate can be accurately predicted from the Jominy test results this test using the standard procedure and specimen we are heating the sample to austenitic temperature and quenching is done. Two sides are flatten and hardness is measured and graph is plotted with distance from quench end verses hardness value.