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N- max/m 62¢ch Cay Major professor Date 2/ ‘2 Cit/C7] C) 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution LIERARY Michigan State University PLACE DI RETURN BOX to romovo this checkout from your record. TO AVOID FINES Mum on or baton duo duo. DATE DUE DATE DUE DATE DUE lflf JL__ll MSU Is An Affirmative Action/Equal Opportunity Institution POLYVINYL PYRROLIDONE POLYMER QUENCHING OF SAE 1141 STEEL By Mandar Krishnaji Hingwe A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Metallurgy, Mechanics and Material Science 1990 1"- . 0 ‘ @0544 ABSTRACT POLYVINYL PYRROLIDONE POLYMER QUENCHING 0F SAE 1141 STEEL BY Mandar Krishnaji Hingwe Cooling curve analysis was carried out to find out the optimum range of Polyvinyl Pyrrolidone (PVP) concentration in the quenching media and the temperature of the quenching media that will produce the required as-quenched hardness in SAE 1141 steel, without quench cracking of the parts. Polymer concentrations used were 8%,11% and 14%. Polymer temperatures used were 600F,800F,1000F,1200F and 1400?. As quenched hardnesses, temperature difference between the surface and the core of the probe when the surface has already reached Ms temperature and the time required to reach Mstemperature were determined. Polymer concentration of 11% and polymer temperature in the range 1200F to 140°F is the best possible combination to produce the as-quenched actual surface hardness in the range Rc 44-53 in SAE 1141 steel without causing quench cracking of the parts. DEDICATED TO. MY BROTHER ANIL AND HIS WIFE ANURADHA ACKNOWLEDGEMENTS I wish to express my deepest gratitude to my advisor, Dr.K.N.Subramanian and my co-advisor, Dr.Michael Wisti for their suggestions and guidance in this work. Also, to Mr. Bill Reynolds and Mr. Charles Pierce for their encouragement, to Mr. Larry Schulze, Mr. Marvin Seymour, Mr. Anthony Meszaros and Mr. Brian Radford for their timely help in setting up the experimental set-up. and to Mr. David Schelter and Mr. John Spitzly for helping me to carry out the experiments. Finally, I would like to thank my family for their faith and encouragement. This project was funded by Atmosphere Annealing Inc., Lansing, Michigan. iv TABLE OF CONTENTS LIST or TABLES ....v" LIST or FIGURES ....y"[ 1. INTRODUCTION ...q 1.1 Advantages and disadvantages of ....2 Polymer quenchants 1.2 Quenching Mechanisam ....5 1.3 Cooling Curve Analysis ....9 1.4 Quenching Mechanisam of Polymer ....13 Quenchants 1.5 Effect of Polymer Concentrtion on ....]4 the Cooling Rate 1.6 Efect of Polymer Temperature on ....17 the Cooling Rate EXPERIMENTAL PROCEDURE ....25 2.1 Sample Preparation ....25 2.2 Quenching Medium ....3] 2.4 2.5 2.6 2.7 Procedure for Measurement of Polymer Concentration and Temperature Quenching Procedure Repeatability Test Cooling Curve Analysis Hardness Measurement EXPERIMENTAL RESULTS AND DISCUSSION 3.1 3.2 3.3 Repeatability Test Cooling Curve Analysis Meaurement of the Time Required to Reach Ms Temperature A T M 3 Measurement Hardness Measurement Thermal Arrests Quench Cracking CONCLUSIONS LIST OF REFERENCES vi ~34 ~37 ----46 ----46 ....47 ....49 ...49 ...53 °-111 ....]15 Table 1 Table 2 Table 3 Table 4 Table 5 LIST OF TABLES Chemical composition of ~°°°26 SAE 1141 steel Time versus 0/v Parquench 90 at ....36 80°F t 10F Repeatability test ....52 Flow rate measurement ....73 Thermal arrests observed during ....93' Polyvinyl Pyrrolidone polymer quenching of SAE 1141 steel vH Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 9 LIST OF FIGURES The three stages of cooling curve (6).... 7 Cooling curves for water, polymer 1 ....]2 and polymer 2 (6) Effect of polymer concentration on ....16 the maximum cooling rate (5) Effect of polymer concentration on ....19 the cooling rate at 300°C (5) Effect of polymer temperature on ....21 the maximum cooling rate (5) Effect of polymer temperature on ....24 the cooling rate at 300°C (5) Optical photomicrograph of the as ....28 received microstructure of hot rolled SAE 1141 steel (2% Nital) Optical photomicrograph of manganese ....3o sulfide inclusions in as received SAE 1141 steel (2% Nital) Schematic of the test specimen ....33 10 Photograph of the experimental set up....39 vfli Figure 11 Schematic of the agitation system ....4] Figure 12 Schematic of the fixture that holds ....43 the specimen in the salt bath furnace Figure 13 Schematic of the fixture that holds ....45 the specimen in the quenching tanks Figure 14 Repeatability test: Plot of the ....51 surface cooling curves for SAE 1141 steel specimens quenched in plain water maintaied at 110°F AzTest number 1 BzTest number 2 Figure 15 The surface cooling curves for SAE ....55 1141 steel specimens quenched in a solution with 8% polymer concentration and maintained at A:60°F 8:800F C:1000F D:120°F E:140°P Figure 16 The core cooling curves for SAE 1141 ....57 steel specimens quenched in a solution with 8% polymer concentration and maintained at A:60°F 8:800F Figure 23 Figure 24 Figure 25 concentrations and maintained at 80°F A:8% B:11% C:14% The surface cooling curves for ....72 SAE 1141 steel specimens quenched in a media with different polymer concentrations and maintained at 1000F A:8% B:ll% C:14% The surface cooling curves for ....74 SAE 1141 steel specimens quenched in a media with different polymer concentrations and maintained at 120°? A:8% B:11% C:14% The surface cooling curves for °°°°76 SAE 1141 steel specimens quenched in a media with different polymer X" Figure Figure Figure Figure Figure Figure 26 27 28 30 31 concentrations and maintained at 140°F A:8% B:11% C:14% Plots of the time required to reach M8 temperature versus polymer temperature for various polymer concentrations Plots of [‘A_T] Ms versus polymer temperature for different polymer concentrations Plots of the as-quenched actual surface hardness versus polymer temperature for various polymer concentrations Plots of the as-quenched surface hardness versus polymer temperature for various polymer concentrations Plots of the as-quenched half radius hardness versus polymer temperature for various polymer concentrations Plots of the as-quenched core hardness versus polymer temperature for various polymer concentrations .0008] '84 -87 ~89 91 -93 Figure 32 Test repeatability plots for SAE 1141°°°°96 Figure Figure Figure Figure 33 34 35 36 steel specimens quenched in a media with 11% polymer concentration and 0F maintained at 100 AzTest number 1 B:Test number 2 C:Test number 3 The surface and the core cooling ....po] curves for a stainless steel sample quenched in a media with 11% polymer concentration and maintained at 100°F The surface and the core cooling ....103 curves for a stinless steel sample quenched in a media with 14% polymer concentration and maintained at 120°F Optical photomicrograph of the --°-106 as-quenched microstructure of SAE 1141 . steel quenched in a media with 11% polymer concentration and maintained at 80°F (2% Nital) Optical photomicrograph of the ....103 as-quenched microstructure of SAE 1141 steel quenched in a media with 11% polymer concentration and maintained at 100°? (2% Nital) X“! Figure 37 Optical photomicrograph of the ....]10. as-quenched microstructure of SAE 1141 steel quenched in a media with 14% polymer concentration and maintained at 140°F.(2% Nital) XV 1 INTRODUCTION In the past, the principal quenching mediums used for heat treatment of ferrous and some nonferrous alloys were water, perhaps modified by some special additives such as inorganic salts and naturally occuring oils. With the discovery of petroleum oil, hydrocarbon products became a major medium for quenching. The term polymer quenchants is relatively new. It is only in the last thirty years that certain water soluble organic polymers have been found to be useful in modifying the cooling characteristics of water.Initially, insufficient basic understanding and experience with polymer quenchants resulted in operational problems and cracked components. Therefore heat treaters were reluctant to change the well-known conventional quenching mediums. But improvements in the polymer products and better understanding of their behaviour as quenching mediums led to progressive increase in their usage over the last decade. polymer quenchants are increasingly used in bulk quenching of ferrous materials and precipitation hardenable aluminum alloys and induction hardening (1). Polymer quenchants are expected to surpass oil in usage before turn of the century (2). Polyvinyl Alcohol (PVA), Polyalkaline Glycol (PAG), Polyvinyl Pyrrolidone (PVP) and Sodium Polyacrylate are the major polymer types listed in order of their historical use (2). Polyvinyl Pyrrolidone and Polyalkaline Glycol are the most widely used polymer quenchants. Sodium Polyacrylate and Polyvinyl Alcohol have specialized and limited uses. One of the important development is the introduction of substituted Oxazoline polymers such as Polythyloxazoline (PEO). Mixtures based on PVP and PEG are also being studied. Some of the most popular polymer quenchants used in the U.S.S.R are PVP, PAG, Sulfite Alkali and Polyacrylamide (1). Due to inherent viscosity of these polymers, water is always added for further handling and distribution. Polymer concentrates also contain corrosion inhibitors, defoamers and bactericides. The water level in as- supplied concentrate depends on the polymer type. 1.1 Advantages and Disadvantages of Polymer Quenchants As more alloy steels with greater hardenability came into use, aqueous quenchants like water, brine or caustic solutions were not only unsuitable because of excessive cracking and distortion, but high cooling rates provided by aqueous solutions were not necessary for the higher hardenability steels. Therefore conventional quenching oils and fast oils (they contain especially developed propritery additives that provide faster quenching effects.) were more widely used. However, the cooling power of even the‘fastest of the fast oils' was relatively slow compared to that of aqueous solutions. This resulted in a large gap between the cooling power of aqueous solutions and oil. Organic polymers appeared to be the best possible quenchants that could effectively fill this gap (3). By controlling the concentration, temperature and agitation of polymer solutions, a wide range of cooling rates can be obtained. Therefore, it is possible to process wide range of materials with varying hardenabilities giving better operational flexibility. There is significant reduction in distortion and residual stresses especially compared to aqueous solutions. The principal advantage of polymer quenchants over oil quenchants is the elimination of oil fire hazard. Polymer quenchants are invariably used with 75% of water. This is especially important when comparing quenching of voluminous components in oil and in polymer. This reduces cost of fire insurances. Polymer quenchants have almost double the specific heat of mineral quench oils. This results in higher productivity, since the temperature of the quenchant will rise only half of that of the same volume of oil, for a given charge weight (1). Use of polymer quenchants provides a much cleaner and safer working environment. There is no smoke or fumes as produced during oil quenching: Polymer quenchants are non-toxic. Simple water rinse is required to clean parts, baskets and fixtures. No detergents or water soluble materials are necessary. This reduces cost of post heat treating operations. In general,there is overall reduction in the cost of an end item using polymer quenchants (4). Even though polymer quenchants offer all these advantages, they are by no means the answer to all the heat treater’s problems. Polymer quenchants are not as‘forgiving' as oil quenchants. Careful contol over polymer concentration and temperature as well as the quench tank conditions is necessary for the successful applications of polymer quenchants. Cooling characteristics of polymer quenchants are altered by 011 residues, various organics, hard water ions and salt. Before making use of polymer quenchants in salt bath systems, the specific effect of salt should be determined and salt concentration must be maintained at a minimum level (5). Polymer quenchants have higher initial cost compared to water quenching. 1.2 Quenching Mechanism .The aim of quenching is to obtain high performance characteristics which will give an improved alloy in terms of yield strength, hardness and fatigue and wear resistance. However the quenching process must be properely controlled to avoid quench cracking and to limit distortion and internal stresses. The most useful method to determine the cooling power of a quenchant is by the basic cooling curve test. The test is performed by quenching specimens, with thermocouples embedded at various points, and monitoring the cooling process by using a temperature measuring device. Specimens can be made of austenitic stainless steel, silver or the same steel under investigation. A cooling curve illustrates the three stages of heat transfer when hot specimen is immersed in the cold quenching medium (Figure 1). Cooling curves are useful since the cooling rate is observed throughout the quench cycle and the cooling mechanism that occurs while Figure l The three stages of cooling curve (5) temperature C oStmo Tine l?ignir12 1 quenching an actual part during heat treating operations is recorded. Stage A:Vapor Blanket Stage This stage is characterized by the formation of an unbroken vapor blanket that surrounds the workpiece. It is known as Leidenfrost phenomenon (3). This stage exists as long as the heat supply from the metal surface exceeds the amount of heat needed to maintain the maximum vapor per unit of area. The vapor envelope acts as an insulator and cooling proceeds at a slow rate by conduction and principally by radiation through the vapor blanket.(3) Stage BzNucleate Boiling Stage This stage begins when the temperature of the metal surface drops down somewhat and the continuous vapor film collapses. This results in violent boiling of the quenching medium. This stage represents the fastest rate of heat removal from the metal, heat being removed as the latent heat of vaporization. Size and shape of the vapor bubbles are important in controlling the duration of stage B, as well as the c0oling rate developed within it (3). Stage CzLiquid cooling stage Stage C begins when the temperature of the metal surface is reduced to the boiling point or boiling range of the quenching medium. The cooling rate in this stage is slower than that developed in stage B. Below this temperature, boiling stops and slow cooling takes place mainly by conduction and convection. The temperature difference between the boiling point of the liquid and the bath temperature is a major factor influencing the rate of heat transfer in stage C. Viscosity of the quenchant also affects cooling rate in stage C (3). 1.3 Cooling Curve Analysis Several methods are available for characterizing and interpreting the cooling curves. They include visual inspection, calculation of various critical cooling rates, times and temperatures, integration of areas under the cooling curve or cooling rate curve, use of the Grossman quench severity factor, and most recently the use of quench factor analysis to relate cooling rate behavior to the physical properties of quenched pieces (6). The time required to complete stage A, the slope of stage B and the curvature of stage C are the three 10 most important criterions when cooling curves are analysed visually. From Figure 2 it can be concluded that water has the shortest time to complete stage A, the steepest slope of stage B and the greatest C stage curvature. It can be concluded from the figure that the order of quench severity is water > polymer 1 > polymer 2 (6). When high hardness and tensile strength are to be achieved, how fast the stage A ends, and how fast is the rate of cooling in stage B are important factors. The cooling rate in stage C is important in characterizing the ability of the quenching medium to reduce the cracking tendancy since the material undergoes rapid phase transformation in this region. The difference in the cooling rate between the center and the surface results in the development of transformation stresses which results in distortion and quench cracking. Cooling 0C is often recorded as a measure of a rate at 300 cooling rate in stage C and is valuable since 300°C is typically within the martensitic transformatoin range for many steels. Therefore an ideal quenchant should have the highest cooling rate in stage B and the slowest cooling rate in stage C. 11 Figure 2 Cooling curves for water, polymer 1 and polymer 2 (6) temperature 12 W2 — mu: Till Figure 2 13 1.4 Quenching Mechanism of Polymer Quenchants The quenching mechanism is more complicated in case of polymer quenchants. Stage A is delayed by formation of a polymer-rich film that encapsulates and stabilizes the vapor blanket. The formation of a polymer- rich film depends on the polymer structure. In case of PAS, the film is actually precipitated from solution at the inversion temperature which is below the boiling point of water. But polymers such as PVP and Polyacrylates have normal solubility in water. There is a localized evaporation occuring adjacent to the hot metal surface producing a polymer concentration gradient which increases towards the liquid gas interface. As the concentration increases, the chain entanglement among separate polymer molecules increases and a three dimensional entanglement network is formed. At this point, a "gel-like" polymer rich film is formed.This film extends towards the vapor blanket and encapsulates it. Stability of this film depends on polymer concentration, molecular weight, polymer temperature and level of agitation. When the pressure of the steam blanket becomes sufficient, the stable polymer-rich film breaks down at its weakest point and there is a transition from stage A 14 to stage B. The hot metal comes into contact with the solution and boiling spreads outwards from the point of initiation. During stage B, two processes occur simultaneously. The steam bubbles tend to escape from the hot metal surface and the surrounding polymer is trying to constrain their release, reproducing a stable film equivalent to stage A. This phenomenon is especially important in case of high concentration solutions. When the surface temperature of the metal falls below the boiling point of the quenchant, a thin polymer film deposits on the metal reducing the cooling rate in stage C. This is desirable to prevent distortion and cracking as rapid transformation to martensite occurs (5) . 1.5 Effect of Polymer Concentration on the Cooling Rate Figure 3 illustrates the effect of polymer concentration on the maximum cooling rate and the cooling rate at 3000 0 C, tested at a quenchant temperature of 30 C and agitated at 0.5 m/s. As the concentration increases, a stable and more thick film is formed around hot metal and temperature at which maximum rate occurs tends to be reduced. The maximum cooling rate is reduced linearly over its normal concentration range. There are 15 Figure 3 Effect of polymer concentration on the maximum cooling rate (5) Maximum cooling rate. °Cls as as 16 O \ O Polyacrylate \ \ ‘ \ (e) The effect of polymer concentration on maximum cooling rate 5 10 15 20 25 Polymer concentration. volume “6 , Figure 3 17 deviations from linearity for all polymers at concentrations below 5%. PVP has a lower maximum rate than PAG up to 10% concentration but above this level, the situation is reversed.(Figure 4). It is important to note that that 5% PVP has a linear cooling rate above 300°C and cools faster than pure water at 300°C; above this concentration, the rate continues to drop linearly. The rate at 300°C is actually lower for the PVP compared to PAG, for solution concentrations above 25% (5). 1.6 Effect of Polymer Temperature on the Cooling Rate Raising the temperature of the polymer solutions reduce the viscosity but there is an increased contribution to the latent heat of vaporization because temperature of the solution is near its boiling point. Therefore raising the quenchant temperature tends to stabilize stage A with reduction in temperature of the maximum rate (Figure 5). Raising the polymer temperature also reduces 0C. In case of PVP there is a small 0 cooling rate at 300 C for solution 0C and 50°C the effect on cooling rate at 300 temperatures up to 40°C but between 40 rate decreases dramatically by 65%. It is believed to be 18 Flgure 4 Effect of polymer concentration on the cooling rate at 300°C (5) Cooling rate at 300°C. °Cls 19 T I I I I (b) The effect of polymer .. concentration on cooling rate at 300°C \ \Polyacryllto .. 4‘ x. \ F ‘0 5 10 15 20 25 30 Polymer concentration, volume % Figure 4 20 Figure 5 Effect of polymer temperature on the maXImum cooling rate (5) Maximum cooling rate, “Ch: 8 8 180- 160' 140- 120- 100' i 20 " 21 la) The effect of polymer temperature on maximum cooling rate 1 l I l l l 10 20 ‘30 40 50 60 Polymer solution temperature. °C Figure 5 70 22 due to formation of a stable polymer-rich film after earlier high cooling rates with the nucleate boiling stage. In case of PAC, there is a small effect of polymer solution temperature on the cooling rate at 300°C (Figure 6) (5). Polymer quenchants are powerful tools when coupled with good engineering and shop practice. Polymer quenchants generaly exhibit faster cooling rates compared to oil through critical martensitic transformation range. Therefore polymer quenchants are used predominantly for applications requiring cooling rates intermediate between water and coventional oils. Polymer quenchants can not be used for heat treatment of certain materials like high carbon steels or alloy steels and also for components with surface defects and stress raisers (7). Utilization of improper polymer temperature, polymer concentration or incorrect immersion may result in-poor mechanical properties, severe distortion and quench cracking. Therefore it is essential to perform cooling curve analysis of polymer quenchants as a function of polymer concentration and polymer temperature before usig them in commercial heat treating. 23 Figure 6 Effect of polymer temperature on the cooling rate at 300°C (5) Cooling rate at 300°C, °Cls 24 100 I 1 1 T 1 x 90 - l bl The effect of polymer temperature . on cooling rate at 300°C 80 - . 70 - - 50 - so .. 4o - 30 - 2° - .\ \o . \‘\ d db Polyacrylate \ 4. 10 " ‘A ~ «I: '— -—0 OJ l L 1 l l P 0 10 20 30 40 50 60 70 Polymer solution temperature. °C Figure 6 2. EXPERIMENTAL PROCEDURE 2.1 Specimen Preparation Test specimens were cut from hot rolled SAE 1141 steel bars received from Lindell Drop Forge, Lansing. Exact chemical composition of the steel was analyzed using an Atomic Emission Spctrometer. (Table 1). As received microstructure is shown in Figures 7 and 8. 25 26 ' Table 1: Chemical composition of SAE 1141 steel C Mn P S Si Cu Ni 0.403 1.515 0.013 0.129 0.263 0.168 0.066 Cr Mo Al Cb V 0.078 0.014 0.0043 0.0004 0.0707 27 Figure 7 Optical photomicrograph of the as-received microstructure of hot rolled SAE 1141 steel (2% Nital) 28 Figure 7 29 Figure 8 Optical photomicrograph of manganese sulfide inclusions in as-received SAE 1141 steel (2% Nital) 3’0 Figure 8 31 The length of the specimen was 6 in. and the diameter was 1.2 in. The specimen was machined as illustrated in figure 9. The length of the specimen that was austenitized was 4.8 in. and was four times the diameter to avoid the end effects (8). Two 0.125 in. holes were drilled in the specimen to embed two thermocouples. The holes were 3 in. deep. One thermocouple was placed at the center of the specimen and other was located 0.050 in. away from the surface as illustrated in Figure 9. Two K-type 0.125 in. sheathed and ungrounded thermocouples were used. A protective cap was fitted over the specimen to protect the thermocouples from salt and the polymer. 2.2 Quenching Medium Parquench 90* is the trade name of the polymer used in the experiments. It is based on Polyvinyl Pyrrolidone polymer (9). It is manufactured by Park Chemicals of Detroit, Michigan. Polyvinyl Pyrrolidone (PVP) is derived from the polymerization of N Vinyl - 2 Pyrrolidone. It is characterized by its unusual complexing and colloidal properties and by its physical inertness (9). Parquench solutions have inherently low 32 Figure 9 Schematic of the test specimen All dimensions in inches. 33 lo— 1-2—-t1 Figure 9 34 foaming tendencies. They have'low drag out losses and are very safe to operate. (10). 2.3 Procedure for Measurement of Polymer Concentration and Temperature A 400 ml. beaker was filled with the polymer solution. The solution was allowed to stand at room temperature untill it clarified free of entrained air and /or sediments. The temperature was adjusted to 800 F using hot or cold water. A clean and dry shell cup was immersed in the solution so that the cup was fully below the liquid surface. The drainage time was measured by raising the cup rapidly but smoothly a few inches above the surface. A stopwatch was started the instant the the top edge of the cup emerged from the solution. The stopwatch was stopped when the continuous stream of the ‘polymer solution from the bottom of the cup was broken. Post dripping, if any, was ignored in timing the flow. The time for the cup to empty through capillary orifice at the bottom is a function of viscosity at a controlled temperature. The viscosity is correlated to the parquench concentration. The drainage time is correlated to the polymer concentration as given in Table 2 (11). * Parquench 90 is the registred trade mark of Park Chemicals, A Subsidiary of Whittaker Corporation, Detroit, Michigan 48204. 35 Parquench temperature was measured using a thermometer. 36 Table 2: Drainage time versus 0/v Parquench 90 at 80°F t 10F from reference (11) Drainage Time (seconds) Concentration (0/v) 11 9.0 11.6 ' 11.0 12.3 13.0 13.1 15.0 37 2.4 Quenching Procedure Experimental set up of the quenching system is as presented in Figure 10. The height of the quenching tanks was 23 in. and the diameter was 19 in. A mechanical agitator was placed in the quenching tanks as shown in Figure 11. A special fixture was designed to hold the specimen in the salt bath so that only part of the specimen would get austenitized (Figure 12). A steel plate of dimensions 16x4.5xo.250 in. was fitted inside the quenching tank. A rectangular slot of 15x 0.250 x 1.175 in. was cut on the plate. Specimen was placed on the plate during quenching. The plate was designed in such a way that only 4.8 in. length of the specimen would get quenched and the thermocouple would not be affected by the polymer.(Figure 13). Polymer concentrations used were 8%, 11% and 14%. 0 Polymer temperatures used were 60°F, 80 F, 100°F, 1200F and 140°F. Flow rate was measured for each combination of polymer temperature and polymer concentration in feet per second using a flowmeter. Each concentration was evaluated against the five temperatures. A particular concentration was prepared in each of the five quenching 38 Figure 10 Photograph of the experimental set up 39 Figure 10 40 Figure 11 Schematic of the agitation system All dimensions in inches 41 1 E::::' F ‘ STAND 16.0 1; ‘ 3—0 40 ' I .L ob l .4, U TUBE 23.0 N M l._.queu.cumc TANK .ir. - 1‘1 Figure 11 42 Figure 12 Schematic of the fixture that holds the specimen in the salt bath furnace All dimensions in inches 43 44 Figure 13 Schematic of the fixture that holds the specimen in the quenching tanks All dimensions in inches 45 T 8 LOT 15 x1.750x0. o T 25 l / . /,. i 7 /0.." L; 1 0-1L at 46 tanks. The temperature of the polymer solution in each ofthe five tanks was varied using individual temperature °F+5°F controllers. Specimens were austenitized at 1550 for 30 minutes. The austenitization and quenching process was monitored by a recording instrument(trade namezRustrak Ranger*). Temperature was recorded every 5/6 th of a second. The same instrument was used for plotting the cooling curves. After quenching was carried out, quenching tanks were emptied and thoroughly cleaned. Then, a fresh polymer concentration was used and the experiments were performed as explained above. 2.5 Repeatibility Test Before quenching the specimens in polymer, two 0F under the same samples were quenched in water at 110 experimental conditions the polymer would be tested, to check the repeatability of the experimental set up. Cooling curVes were analyzed for both the specimens and hardness measurements were made. 2.6 Cooling Curve Analysis Cooling curves were plotted for the surface and center thermocouple using a recording instrument. These curves were plotted for all the combinations of polymer * Rustrak Ranger is the registerd trade mark of Gulton Industries, Inc., RI 02818. 47 temperatures and polymer concentrations used. Cooling curves were plotted collectively for each concentration but at different temperatures to show the variation in cooling curves with increasing temperatures. Also cooling curves at the same polymer temperature but at different polymer concentrations were analyzed. The time required to reach Ms temperature was calculated for all the combinations of polymer concentrations and polymer temperatures used. Also, the temperature difference[.A T]Ms between the surface and the center of the sample was determined for all combinations of polymer temperatures and polymer concentrations used. It is often known as thermal gradient. 2.7 Hardness Measurement 'Approximately 0.010 in. flat was ground on the specimen and Rockwell hardness (Rc) was measured at a point whose temperature is monitored throughout the quenching process. (actual surface hardness). Approximately 0.750 in.thick section was cut out of the specimen at the same point and Rockwell hardness was measured at a distance of 0.050 in. away from the surface.(surface hardness). Also hardness was measured 48 at the half radius and at the center of the sample. 3 EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Repeatability Test To test the repeatability of the experimental set- up, two specimens were quenched from 1550°F in plain oF. The surface cooling curve water maintained at 110 plots for these two specimens are given in Figure 14. Cooling curves for both specimens are almost the same indicating the repeatability of the experimental_set-up. The actual surface hardness, hardness at a depth of 0.050 in., half radius hardness and core hardness were measured for both the specimens. Also, the temperature difference between suraface and core when surface has already reached Ms temperature[,A T ] Ms and time required to reach Ms temperature was determined for both the specimens. The results are listed in Table 3. The repeatability of the test is once again demonstrated by these results. 49 50 Figure 14 Repeatability test: Plot of the surface cooling curves for SAE 1141 steel specimens quenched in plain water maintaied at 110°F AzTest number 1 B:Test number 2 51 T 1.5«l e I P 1.40 e P a .:. t u 1' Lil» e 4 0.3+l e 9 .. 3.5-, r 9.4.» 9.2" XLI‘ifl 4.5 5.3 5.3 6.3 6.5 7.8 52 Table 3: Repeatability test Test Sample 1 Sample 2 Actual Surface Hardness Rc 57.0 Rc 56.0 Surface Hardness Rc 56.5 Rc 55.0 Half Radius Hardness Rc 49.0 Rc 49.0 Core Hardness Rc 47.0 Rc 46.0 Time Required to Reach 11.44 sec. 11.68 sec. Mé Temperature [ A '1‘] Ms 660 F 530 F 53 3.2 Cooling Curve Analysis Figures 15, 17 and 19 are the plots of surface cooling curves for SAE 1141 steel specimens quenched in media with 8%, 11% and 14% polymer concentrations. For each polymer concentration, the tests have been carried out at polymer temperatures of 60°F, 80°F, 100°F, 120°F and 1400 F. From these plots it can be seen that for each concentration, the cooling curves shift to the right and upwards as the polymer temperature is increased. For each polymer concentration, the vapor blanket stage is prolonged with increase in the polymer temperature. Thermal arrests were observed for cases with quenching media with , (a) 8% polymer concentration maintained at 1200F (b) 11% polymer concentration maintained at 100°F (c) 14% polymer concentration maintained at 1000F, 1200F and 140°F. Thermal arrests are addressed later in the discussion. Figures 16, 18 and 20 are the plots of core cooling curves fer SAE 1141 steel specimens quenched in media with 8%, 11% and 14% polymer concentrations respectively. For the same polymer concentration the 54 Figure 15 The surface cooling curves for SAE 1141 steel specimens quenched in a solution with 8% polymer concentration and maintained at A:60°F B:80°F c:100°F 0:120°F F:140°F 55 "I @09- O'Ififlfl'IO'UUGH Figure 15 56 Figure 16 The core cooling curves for SAE 1141 steel specimens quenched in a solution with 8% polymer concentration and maintained at A:60°F 8:800}? c:100°F D:120°F F:140°F 57 0.8‘ 0.60 "I ‘09- omawmmamIa—l is '. ’-d.’ 0.411 5:: sin 6;: 7:3 7:5. , 3.9 58 Figurel? The surface cooling curves for SAE 1141 steel specimens quenched in a solution with 11% polymer concntration and maintained at A:60°F B:30°F c:100°F 0:120°F F:140°p 59 is 8.3 lline s... C, 6.5 5.3 8.0 x 1.3613 Figure17 60 Figure 18 The core cooling curves for SAE 1141 steel specimens quenched in a solution with 11% polymer concentration and maintained at A:60°F B:80°F c:100°F 0:120°F F:140°F '61 r 106.1 I e l I P e 1‘ d t 11 l‘ O i O S l‘ 908 ‘ - A . r... 5.: 6.8 6.5 m 7.: 3.3 x Lifi Tin 11in: 62 Figure 19 The surface cooling curves for SAE 1141 steel specimens quenched in a solution with 14% polymer concentration and maintained at A:60°F S:30°F c:100°F D:120°F F:140°p 63 l ‘. ale-PCneatnFe ‘9 5 8.60 flhs 1" xLEflB Figure 19 64 Figure 20 The core cooling curves for SAE 1141 steel specimens quenched in a solution with 14% polymer concentration and maintained at A:600F B:80°F c:100°F 0:120°F E:1400F 65 1.6 ‘r 1.44 1.8 ‘ 0.8“ 8.6%) r-e can. onswmuo-olav—i 0.411 0.21. 8.8 x LEW 5:5 5.: sis 7.3 Figure 20 Tmeflhn 66 cooling curves shift to the right and upwards with increase in polymer temperature. Figures 21, 22, 23, 24 and 25 are the plots illustrating the effect of polymer concentration on the surface cooling curves when the quenching media is maintained at 60°F, 80°F, 100°F, 1200F and 140° F. From this figure it can be observed that for the same polymer temperature, the cooling curves shift to the right and upwards as the polymer concentration is increased. The effect of polymer concentration on the cooling rate at the same polymer temperature, and also the effect of polymer temperature on the cooling rate for the same polymer concentration can be seen from these plots. In general, with an increase in polymer temperature and polymer concentration, the vapor blanket stage was prolonged with decrease in the maximum cooling rate in stage B. For the same polymer concentration, higher polymer temperature results in formation of a more stable vapor barrier because increase in the polymer temperature contributes more to the latent heat of vaporization. This further reduces the maximum cooling rate that can be obtained in stage B. For the same polymer temperature, increase in the polymer concentration results in the formation of thicker and 67 Figure 21 The surface cooling curves for SAE 1141 steel specimens quenched in a media with different polymer concentrations and maintained at 60°F. A:8% B:1l% C: 14% 68 no mon- answer-eoroln—e 1.5 1.41 1.2% 1.34 3.34» 0.5« 0.44. 9.2‘ a: 9"" 5.: 5.3 6.5 7.3 79?. 3'9 x 1.33% I“ 111118 Figure 21 69 Figure 22 The surface cooling curves for SAE 1141 steel specimens quenched in a media with different polymer concentrations and maintained at 80°F A:8% B: 11% C:14% 7O T 3V i 0 I P 1.6 ‘l 8 P 2 4L a 1.. t '1! 3.34 8 d 008 1 8 9 9.5“ P 8.3 r 0.2‘ 0.8 - ‘ ~ 5.5 6.3 6.5 7.3 7"} O. x 1.1% It. 11118 Figure 22 71 Figure 23 The surface cooling curves for SAE 1141 steel specimens quenched in a media with different polymer concentrations and °F maintained at 100 A:8% B:11% C:14% 72 c l B \ 5 \ In \\\ . 3.1. l ‘-\--“ ' 3".-- x... 1 111.11. more. i ..v 4.. at no no 40 4. 4.. a . .. L .n a“ an an as a“ «1830.33.0ture .409 a! Figure 23 73 Figure 24 The surface cooling curves for SAE 1141 steel specimens quenched in a media with different polymer concentrations and maintained at 120°F A:8% B:11% C:14% 74 6.3 "he v < o h v a an ..m .8. 3 {w t t. 1 l. K a Te.PePature decor at Lia Figure 24 75 Figure 25 The surface cooling curves for SAE 1141 steel specimens quenched in a media with different polymer concentrations and maintained at 140°F A:8% B:11% C:14% 76 1.611 1.44 1.2 t 1.9 ‘ 0.31“ 0.61 "I GOO- OHSfiflflO'fl'OF’I 0.4i 0.241 3.0 5:: 6:8 6:5 7:: x 1.!4'03 more stable polymer film that surrounds the vapor blanket. This film prolongs stage A. At the begining of stage B, the polymer-rich film constrains the release of vapor bubbles from the surface of the sample forming a barrier similar to that in stage A. This minimizes the maximum cooling rate in stage B.(5). The rate of cooling was found to be decreased with increase in the polymer concentration and the polymer temperature. The rate of cooling in stage C is directly proportional to temperature difference between the bath temperature and the boiling point of the quenchant and inversly proportional to the viscosity of the quenchant. Rate of cooling in stage C was found to be decreased with increase in the polymer temperature and the polymer concentration. For the same polymer concentration, increase in the polymer temperature reduces the temperature difference between the actual bath temperature and the boiling point of the quenchant which results in the reduction of the cooling rate in stage C. However, increase in the polymer temperature reduces the viscosity of the polymer solution. This increases the flow rate (Table 4) results in increased cooling rate in stage C. 78 Table 4: Flow rate measurement Polymer Concentration Polymer Temperature Flow Rate 0/v °F F.P.S 8% 60 130 80 135 100 145 120 145 140 150 11% 60 110 80 120 100 125 120 ' 135 140 135 14% 60 115 80 120 100 130 120 135 140 145 79 The temperature difference between the polymer temperature and the boiling point of the polymer solution was found to govern the rate of cooling in stage C. 3.3 Measurement of Time Required to Reach Ms Temperature Figure 26 is the plot of the time required to reach Mstemperature versus the polymer temperature for the three different polymer concentrations. The Ms temperature was calculated using the formula Ms (0C) = 561-270 C -33Mn-17Ni-17Cr-17Mo.(12). Using the formula and Table l, the Ms temperature for SAE 1141 steel is °F. It can be seen that at the approximately 600 same polymer temperature, time required to reach MS temperature increases as the polymer concentration is increased. Also, at the same polymer temperature, time required to reach Ms temperature increased with an increase in the pelymer concentration. Rate of cooling in stage C, where austenite transforms into martensite, should be as slow as possible to minimize risk of quench cracking. 80 Figure 26 Plots of the time required to reach Ms temperature versus polymer temperature for various polymer concentrations. 0 Thermal Arrest mOZOOmm mg-d 81 80 70" 50— 40” 30" / / l l l l 1 1O 4O 60 80 100 120 140 POLYMER ‘I'EMPERATURE °F CONCENTRATION —"" 8'PERCENT "1— 11 PERCENT + 14 PERCENT Figure 26 160 82 3.4 [ A.T] Ms Measurement For this experiment,the thermal gradient is defined as the temperature difference between the surface and the core of the sample when the surface has already reached Mstemperature [ AT] MS. Figure 27 is the plot of [,3 T] M against the polymer temperature as a 8 function of the polymer concentration. For 8% polymer concentration, the thermal gradient decreased when the 0 0 polymer temperature was increased from 60 F to 120 F and started increasing again above 120°F. For 11% and 14% polymer concentrations, [ ‘A.T] MS decreased upto 100°F polymer temperature and then started increasing after 100°F polymer temperature. It can be seen that in general for any polymer temperature, the [ [5T] MS decreases as the polymer concentration is increased. Also, it can be seen that for the same polymer concentration, the thermal gradient varies depending on the polymer temperature and may decrease or increase depending on the combination of polymer temperature and polymer concentration used. 3.5 Hardness Measurement Figures 28, 29, 30 and 31 are plots of as-quenched 83 Figure 27 Plots of [ A_T] Ms versus polymer temperature for various polymer concentrations. 0 Thermal Arrest —D)DQ 1"):31'1'11-1 11 «2m 84 600 500 _ 400 ” 300 " 200 ' 100" 4O 60 80 100 120 140 160 POLYMER TEMPERATURE °F CONCENTRATION “" 8 PERCENT +11 PERCENT + 14 PERCENT Figure 27 85 actual surface hardness, as-quenched surface hardness, as-quenched half radius hardness and as-quenched core hardness respectivelly. The effect of polymer temperature on the as-quenched hardness for three different polymer concentrations can be seen in these polts. In general, it can be seen that for the same polymer temperature, the as- quenched hardness decreases with increase in the polymer temperature. Also, for the same polymer concentration, the as-quenched hardness decreases with increase in the polymer temperature. This is because of the fact that the rate of cooling in stage B decreases with increase in the polymer temperature and the polymer concentration. With reduction in the cooling rate in stage B, the percentage of non-martensitic transformation products in the structure increases with the resultant decrease in the as-quenched hardness.. Thermal arrests occured in media with 8% polymer 0 concentration maintained at 120 F, 11% polymer or, and 14% polymer °F, 120°F and 140°F. concentration maintained at 100 concentration maintained at 100 Therefore, the as-quenched hardness of the specimen quenched in a media with 8% polymer concentration 0F was below thw hardness value for the 0 maintained at 120 same media maintained at 140 F. Similarly, the as- 86 Figure 28 Plots of the as-quenched actual surface hardness versus polymer temperature for various polymer concentrations. 0 Thermal Arrest 87 50 55 r H a 50 ~ 0 N E 45 ~ S 3 _ C 40 K ‘3 35 - L .L. 30 r 25 I l l l l 40‘ 60 80 100 120 140 160 POLYMER TEMPERATURE °F CONCENTRATION "" 8 PERCENT +11 PERCENT + 14 PERCENT 0 Figure 28 88 Figure 29 Plots of the as-quenched surface hardness versus polymer temperature for various polymer concentrations. 0 Thermal Arrest .- ‘- rrmgxoom mmmzon> 89 60 55* 50* 40” I 35 30" 25 40 60 80 100 120 140 POLYMER TEMPERATURE °r= —"‘ 8 PERCENT CONCENTRATION + 11 PERCENT +14 PERCENT Figure 29 160 90 Figure 30 Plots of the as-quenched half radius hardness versus polymer temperature for various polymer concentrations. 0 Thermal Arrest rrmixoon mmmzom>1 91 60 50" 45” 40P 1 35 30 25 40 60 80 100 120 140 ROLYMER TEMPERATURE F CONCENTRATION ""_ 8 PERCENT "1“ 11 PERCENT +'14 PERCENT Figure 30 160 92 Figure 31 Plots of the as-quenched core hardness versus polymer temperature for various polymer concentrations. 0 Thermal Arrest rrmixoom mumzon>1 93 50 45 " 40” 1 35 25 l J 1 l 20 40 60 80 100 120 140 POLYMER TEMPERATURE “F ——- a PERCENT CONCENTRATION +11 PERCENT + 14 PERCENT Figure 31 160 94 quenched hardnesses of the specimens quenched in media 0 with 11% polymer concentration maintained at 120 F and 140°F polymer temperatures were more than that for the same quenching medium maintained at 1000 F. In contrary to 8% and 11% polymer concentrations where the thermal arrests occured only at a single polymer temperature, for 14% polymer concentration thermal arrests occured at 100°F, 120°F and 140°F. The as-quenched hardness dropped down gradually at 14% polymer concentration with increase in the polymer temperaure. 3.6 Thermal Arrests Thermal arrests were observed under the following conditions: (a) Quenching media with 8% polymer concentration and maintained at 120°F (b) Quenching media with 11% polymer concentration and maintained at 100°F (c) Quenching media with 14% polymer concentration and °F, 120°F and 140°F. maintained at 100 To check the repeatability of these arrests, three samples were quenched in a media with 11% polymer concentration and maintaied at 1000 F. As can be seen in Figure 32 thermal arrests are repeatable. Summary of 95 Figure 32 Test repeatability plots for SAE 1141 steel specimens quenched in a media with 11% polymer concentration and maintained at 100°F. A:Test number 1 B:Test number 2 C:Test number 3 96 T e l P e 1‘ a 1 II P e d e 9 F s. 5.5 6.3 s. 1: 1.34113 97 the polymer temperatures and polymer concentrations at which thermal arrests were observed are presented in Table 5. The table also lists the temperature at which arrests were observed and the duration of the arrests. Steven and Haynes (12) have given a formula for estimating bainitic transformation start temperature as BS(°C)- 830-27OC-9OMn-37Ni-7OCr-83M0. They also suggest the temperature for completion of bainitic transformation as Bf(°C)= Bs-120. Using these formulas and Table 1, BS for the SAE 1141 steel is approximately 1067°F and Bfis 0F. It can be seen from Table 5 that approximately 851 the arrests occur in the bainitic transformation range of SAE 1141 steel. 98 Table 5:Thermal arrests observed during Polyvinyl Pyrrolidone polymer quenching of SAE 1141 steel Polymer Polymer Arrest Arrest Concentration Temperature Temperature Duration O/v °F °F seconds 8% 120 917 02.538 11% 100 850 20.26 100 883 22.45 14% 100 863 17.64 120 874 22.02 140 (1) 992 08.94 (2) 914 05.50 99 The observed arrests could be attributed to either the decomposition of polymer or the exothermic reaction that occurs during the bainitic transformation. In order to check which one of these is the cause of the observed behavior, a 304 austenitic stainless steel specimens were quenched in a media with 11% °F, and in solution with 14% polymer concentration and maintained at 120°F concentration and maintained at 100 under the same experimental conditions, as for SAE 1141 steel, and cooling curves were Obtained. Conditions used in these tests were the ones that exhibited thermal arrests during quenching of SAE 1141 steel. During polymer quenching Of stainless steel, which does not transform during quenching did not exhibit thermal arrests (Figures 33 and 34). If thermal arrests would have been caused by the decomposition of the polymer, they would have occured in case of stainless steel specimens also. Therefore it can be concluded that the arrests observed in quenching of SAE 1141 steel were related to the bainitic phase transformation The amount of heat given out counterbalances the heat extracted by the quenchant. Further studies need to be carried out to verify this. 100 Figure 33 The surface and the core cooling curves for a stinless steel sample quenched in a media with 11% polymer concentration and 0 maintained at 100 F. 1'01 "3 “Oh- Oflflfifl'lfl‘fl-OH 102 Figure 34 The surface and the core cooling curves for a stinless steel sample quenched in a media with 14% polymer concentration and maintained at 120°F. 103 1 151 T e I P O P a t u T e d e 9 T w ‘ ' V ' ' 3 7 a.“ 6.8 7.: 1m? 5.: 5.4 5.. 5.: 6.0 a... ..4 .1“ m” X- Figure 34 104 Figure 35 is the photomicrograph Of the as- quenched microstructure of SAE 1141 steel quenched in a media with 11% polymer concentration and maintained at 80°F, a condition under which thermal arrest was not observed. The white etching constituent is untempered martensite and the dark etching constituent is bainite. The structure consists of approximately 10% bainite with 90% untempered martensite. The as-quenched actual surface hardness of this sample is Rc 57 and the as- quenched surfce hardness is Rc 53. Figure 36 is the photomicrograph of the as-quenched microstructure of SAE 1141 steel quenched in a media with 11% polymer °F, a condition under concentration and maintained at 100 which thermal arrest was observed. The structure consists of mainly bainite and approximately 15% untempered martensite. The as-quenched actual surface hardness of this sample is Rc 45 and the as-quenched surface hardness is Rc 38. Multiple arrests were observed during quenching of SAE 1141 steel in a media with 14% polymer concentration and maintaied at 1400 F. Figure 37 is the photomicrograph of the as- quenched structure of SAE 1141 steel quenched under such conditions. The structure consists of bainite, grain bondary ferrite, pearlite and traces of untempered 105 Figure 35 Optical photomicrograph of the as-quenched microstructure of SAE 1141 steel quenched in a media with 11% polymer concentration and 0 maintained at 80 F.(2% Nital) 106 Figure 35' 107 Figure 36 Optical photomicrograph of the as-quenched microstructure of SAE 1141 steel quenched in a media with 11% polymer concentration and 0 maintained at 100 F.(2% Nital) 108 36 igure 109 Figure 37 Optical'photomicrograph of the as-quenched microstructure of SAE 1141 steel quenched in a media with 14% polymer concentration and 0 maintained at 140 F.(2% Nital) 110 Figure 37 111 martensite. The as-quenched actual surface hardness of this sample is Rc 30 and the actual surface hardness is Rc 28.5. Therefore it can be concluded that the cooling curve analysis, photomicrographs and the as-quenched hardnesses are in good agrrement. 3.7 Quench Cracking When steel is quenched, two basic dimensional changes occur. First, there is the normal thermal contraction due to cooling and superimposed on this is the volumetric expansion as a result of martensitic transformation. If these two opposite stress patterns are not balanced, local stresses of high magnitude are set up within some regions of the hardened steel. When quenched, the surface always cools faster than the center and undergoes martensitic transformation first. Whether or not the surface will be set in tension relative to the center of the bar depends on the sign of the net volumetric change that occurs in the interior of the bar after the surface has hardened. If expansion in this region is larger than remaining thermal contraction, the surface will be placed in tension and the core will remain in compression. The reversed stress pattern is obtained when the thermal contraction in the 112 center of the bar exceeds the expansion during martensitic transformation. Quench cracks occur as a result of tensile stresses. The stress pattern depends on the relative cooling rate at the surface and at the center Of the bar. This is a function of both the bar size and the quench speed (13), and becomes especially important in through hardening alloy steels. When alloy steel is quenched, martensite forms first at the outermost surface, which is first to reach the Ms temperature. As the cooling proceeds, the center reaches Ms temperature. The expansion accompanying the newly formed martensite is restricted by the outer layer of the martensite formed earlier. This results in an internal stress pattern that places the surface in tension. Cracking occurs when enough martensite has formed to set up an internal stress that exceeds the ultimate strength of the as- quenched martensite at the outer surface.(14). In case of deep hardening steels, it is therefore necessary to reduce the [ A,T] Ms between the surface and the center of the sample so that cooling will progress uniformly from surface to the center. Any condition that tends to be a stress raiser will promote the formation of quench cracks. Sharp 113 changes in section, rectangular notches and keyways, and cold stamping marks and die marks, have been known to nucleate quench cracks. The quenching medium should be fast enough to produce the required microstructure and the as-quenched hardness but it should not be too severe to initiate cracks. The choice of the proper quenching medium is governed mostly by the hardenability Of the steel and the design of the part. In the present study, the objective is to find out the best possible range of the polymer concentration and the polymer temperature that would produce the slowest cooling rate to give the as-quenched actual surface hardness in the range of Rc 44-52 without causing quench cracking in the part. The quench cracks were found to nucleate at the die marks on rough forgings and at the point where thick and thin sections are adjecent to each other. These parts have been identified as the parts susceptible to quench cracking. In the commercial heat treating, a high value of the as-quenched hardness is the first indication of a severe quench. Historical data shows that as-quenched actual surface hardness in the range of Rc 53-60 is the first indication of the severe quench for SAE 1141 steel. From Figures 26 and 28 it can be concluded 114 that the 11% polymer concentration and the polymer °F to 140°F is the temperature in the range of 120 operating range that will produce the as-quenched actual surface hardness in the range Rc 44-52, with minimal risk of cracking. 4.CONCLUSIONS The cooling curve analysis was carried out for quenching media with polymer concentrations in the range of 8% to 14% and maintained in temperature range of 60°F to 140°F using SAE 1141 steel probe. The important results of the present investigation can be summarized as follows: 1. For the same polymer temperature, the surface and the core cooling curves shift to the right and upwards as the polymer concentration is increased. For the same polymer concentration, the surface and the core cooling curves shift to the right and upwards as the polymer temperature is increased. 2. Increase in the polymer temperature and the polymer concentration prolong the vapor blanket stage, with a decrease in the maximum cooling rate in stage B. 3. For the same polymer temperature, the time required to reach the MS temperature increases with an increase in the polymer concentration. For the same polymer concentration, the time required to reach Ms temperature increases with the increase in the polymer temperature. 4. The temperatre difference between the surface and the 115 116 core of the sample [ A.T] Ms depends on the polymer concentration polymer temperature and may increase or decrease depending on the particular combination of the two parameters. In general, for the same polymer concentration, the as-quenched actual surface hardness,the surface hardness,the half radius hardness and the core hardness decrease with an increase in the polymer temperature. Thermal arrests were observed during quenching of SAE 1141 steel in quenching media with (a) 8% polymer concentration and maintained at 120°F (b) 11% polymer concentration and maintained at 100°F (c) 14% polymer concentration and maintaied at 100°F, 120°F and 140°F. These arrests may result in a slack quench. 11% polymer concentration and polymer temperature in the range 120°F to 140°F is the best possible operating range to produce the required as-quenched actual surface hardness with reduction in susceptibility to quench cracking. It is very important to perform cooling curve analysis using a probe made up of the steel under consideration. 10. 11. LIST OF REFERENCES N.A.Hi1der "Polymer Quenchants - a Review", Heat Treatment of Metals,1, 15-26 (1986). R.W.Foreman and A.Meszaros "Polymer Quenching Update", Industrial Heating,§1, 22-24 and 29 (1984). H-E-Boyer and P-R-Cary._Quenchins_and_§ontrel_of Distgrtign, A.S.M.International, Metals Park, Ohio 44073. p 14-15,56-57 (1988). T.R.Croucher, "Applying Synthetic Quenchants to High Strength Alloy Heat Treatments", Sou; e Book 93 Heat Treat1ns1_1Ol1II1_2roduction_and_finglneering Practice, A.S.M.International, Metals Park, Ohio 44073. p 231 (1977). N.A.Hilder "The Behavior of Polymer Quenchants", Heat Treatment of Metals,z, 31-46 (1987). G.E.Totten, M.E.Dakins and R.W.Heins "Coling Curve Analysis of Synthetic Quenchants", Journal of Heat Treating,§, 87-95 (1988). R.T.Von Bergan "New Developments in Polymer Quenching Technology", Heat Treatment, Proceedings of the Intrnational Conference, Metals Society, London. 17.1 (1984). R.W.Heins and E.R.Muller "Characterization of Polymer Quenchants by Cooling Curve Analysis", Metals Progress, 122, 34, (1982). Kirk Oathemer, ' c , 3 rd ed, John Wiley and Sons Inc., 29, p 220, (1978). Parquench, Information Broucher, Park Chemicals, Detroit, MI 48204. Park Chemicals, Technical Bulletin no. G-3C, 2-4 (1980). 117 118 12. W.Steven and A.G.Haynes, Journal of the Iron and Steel Institute, 181, 349, (1956). 13. R-E.Reed Hill.2hxsisel_nerallurgx_£rinsinles Litten Educational Publishing Inc., 730-733 (1973). 14. Republig_Allgy_Steel" Republic Steel Corporation 312-317, (1968). RICHIGRN STATE UNIV. LIBRARIES IIIIII"111111111111IIIIIIIIIIIIIIIIIIIHI 31293006201036