Steel parts after manufacture will not have desired properties like wear resistance, tensile strength and surface and core hardness. To attain these, heat-treatment processes like case hardening (CH) or through hardening (TH) were carried out in a sealed-quench furnace and a rotary furnace.
The microstructure of the steel part influences the hardness. The required microstructure is fine tempered martensite (FTM), and the quench media has a very important role in achieving this. Using different grades of steels, the trials were carried out in Savsol Q001 oil and Polyquench-GN polymer. Finally, comparative trials were also carried out in order to determine the suitability of new quench media. In terms of time and energy savings, polymer was found to be a better quench media to get the required FTM microstructure, which gave the improved, desired properties.
Heat treatment can be defined as “a process in which steels or alloys are acted upon thermally so as to change their structures and properties in the desired direction.” There are a variety of heat treatments – such as annealing, normalizing, softening, case and through hardening, etc. – to convert surface materials chemically and physically, ranging in thickness from a few microns to substantial depths in order to impart enhanced hardness and wear-resistance properties.
Steel is our most important engineering material, and it responds well to heat treatments. Fundamentally, all steels are alloys of iron and carbon. Iron is the primary metal used to make a variety of steels, and carbon is the principal ingredient in most of the steels.
In this study, CH (hardening the surface of low-carbon steel at 900-1000˚C/1652-1832˚F for 2-6 hours) and TH (achieved by austenizing and quenching with a carbon-neutral atmosphere at 830-870˚C/1526-1598˚F for 2-3 hours) trials were performed. Quench processes were carried out in different media – oil and polymer – and comparative studies were also carried out on different types of steel parts with the aim of determining the better quench media and improving the metallurgical properties of a steel.
The heat-treatment process, irrespective of technique (CH or TH), involves heating, quenching and tempering. Heating is a process in which the steel part is heated to a temperature at which it changes from a ferrite crystal structure to austenite.
Simultaneously, carburizing is performed to impart carbon content, which enhances the hardness, wear and tensile properties. It is carried out by exposing the parts to a carbon-rich atmosphere, which is created by utilizing carbonaceous gases like methanol, LPG, etc. During carburizing, various chemical reactions occur in the furnace. Methanol on the hot surfaces breaks into carbon monoxide (CO) and hydrogen. The formed CO decomposes to nascent carbon, while LPG changes to low-quality hydrocarbons and nascent carbon. Finally, methane formation occurs, and it also decomposes to nascent carbon. This process is followed by carbon diffusion, and it depends on the Fick’s law of diffusion.
Quenching is a process of rapidly cooling steel parts in oil or polymer from austenizing temperature. The quenchants used in this work are oil (Savsol Q001) and polymer (PolyQuench-GN). Tempering is a process in which the quenched steel part is heated below the critical temperature (150-500˚C/300-932˚F) for a specified time to impart toughness.
The goal of heat treatment is the transformation of microstructure. The metallic materials consist of crystals or grains. When they are heated (austenized), the grains get larger in size, and the carbon goes into solution. This red-hot steel is austenite, and when it is rapidly cooled, the grains transform. The result is a hard, brittle, supersaturated solution called martensite. To reduce brittleness and to impart toughness, tempering is carried out to convert (as-quenched) martensite into FTM.
Materials and Methods
Some of the properties of the oil and polymer solutions were checked. Most importantly, the cooling characteristics of Savsol Q001 were determined by ivf smart-quench method. The other properties – such as concentration by refractometer, specific gravity by hydrometer and pH by pH strips – were maintained around 10%, 1.05-1.07 and 9.9 respectively for the polymer solution. Before comparative analysis, a few trials were carried out on CH and TH processes in both the oil and polymer quenchants to learn about the features of the media. All the oil-quench trials were carried out in a sealed-quench furnace and polymer-quench trials in a rotary furnace.
Using oil, the first CH trial was carried out on a 32-tooth 20MnCr5 gear. The process cycle is shown in Fig. 1, and the tempering cycle was carried out at 150˚C (300˚F) for 90 minutes. The third trial was successful in attaining the property requirements.
The last step of the process is metallography, which involves sampling the heat-treated steel part, molding, grinding, polishing and etching. The same piece was checked for microstructure and case depth with a metallurgical microscope and hardness using a Rockwell machine. Utilizing a conversion chart, tensile strength was also noted.
Similarly, TH trials were carried out on a 1541 shaft propeller. The temperature maintained in the process cycle was 850˚C (1562˚F) for 90 minutes and 450˚C (842˚F) for 90 minutes in tempering. The third trial was successful.
Using polymer, the first CH trial was carried out on an AISI 1018 bearing cup. The temperature in the process cycle was 930˚C (1706˚F) for 150 minutes, and the tempering cycle was 150˚C (300˚F) for 90 minutes. Success was achieved after five trials.
Similarly, TH trials were carried out on a propeller shaft. The temperature maintained in the process cycle was 850˚C (1562˚F) for 90 minutes, and the tempering cycle was 600˚C (1112˚F) for 90 minutes. The second trial saw success.
Comparative studies on the CH process were carried out by varying the time and maintaining constant gas-flow rates and temperature – 930˚C (1706˚F) for hardening and 150˚C (300˚F) for tempering. The components studied include a 32-tooth gear, En353 driveshaft, En36 rollers, MS bolts, AISI 1018 washers and bearing cups.
Comparative studies on the TH process were carried out by varying time and maintaining constant gas-flow rates and temperature – 850˚C (1562˚F) for hardening and 450˚C (842˚F) for tempering. The components studied include a propeller shaft, 15B25 screws, C48 break pin, En48 clips and En31 piston pins. In the first trial, even though the temperature was constant, the hardness obtained in the polymer quench was higher than in oil. So, the temperature was decreased by 10˚C and properties were checked in the other four trials.
Results and Discussion
In oil quench, the three trials of the CH process on the 32-tooth gear demonstrates that the case depth increases with the increasing carburizing time. The optimum temperature required was found to be 930˚C (1706˚F) and the carburizing time 195 minutes (Fig. 2) with the FTM structure of the successful trial shown in Fig. 3.
On the other hand, the three trials of the TH process on the propeller shaft demonstrate that tempering temperature is indirectly proportional to hardness. The optimum tempering temperature (Fig. 4) was found to be 500˚C (932˚F). The FTM of the successful trial is shown in Fig. 5.
In the polymer quench, the five trials of the CH process on the bearing cup show that the CH process is mainly about carburizing time, and case depth is directly proportional to carburizing time. The optimum temperature was found to be 930˚C (1706˚F) and carburizing time 210 minutes (Fig. 6). The FTM structure of the successful trial is shown in Fig. 7.
Similarly, the two trials of the TH process on the propeller shaft explain that the hardness increases as the tempering temperature decreases, and the optimum temperature was found to be 550˚C (1022˚F).
Comparative studies were carried out to determine the effect of quench media on the properties of a steel part. The result of the CH process on the 32-tooth gear and driveshaft showed that the time requirement is lower in polymer quench when compared to oil quench because of its quench severity. With the polymer quench, some distortions were observed on gear teeth because of the repetitive retort movements. The polymer-quench severity produced cracks on the drive shaft because the shafts are long, slender, thin-walled components. The CH trials on rollers, bolts, washers and bearing cups show that the polymer quench is better in terms of time and energy because of the shortened time requirement for polymer quench due to its high quench severity (Fig. 8).
The first trial of the TH process on a propeller shaft was carried out by maintaining a constant temperature of 850˚C (1562˚F) for both quench media. Both were successful in achieving requirements, but the core hardness obtained was lower in the oil quench compared with polymer because of its lower quench severity. So, the line of oil in the graph coincided with polymer (Fig 9a). The other trials on screws, brake pins, clips and piston pins were carried out by decreasing the temperature by 10˚C in polymer quench. Best-case scenario, even with the lower temperature, the core hardness obtained was higher in polymer because of its high cooling rate (Fig. 9).
The following conclusions have been drawn from the experimental results and this analysis.
1. PolyQuench-GN quenchant is better in terms of time and energy savings because the cooling rate of the polymer solution is higher than oil.
2. For a given temperature, case depth increases with the increasing carburizing time.
3. An increase in the tempering temperature decreases the hardness of the steel parts.
4. In comparative analysis, polymer quenchants were better in attaining required case depth, tensile strength and core hardness within a shorter time compared to oil.
5. Polymer quenchants are not suitable for crack-sensitive steels.
6. PolyQuench-GN has no negative impact on environmental conditions, especially during disposal activities.
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For more information: Contact Prof. S.C. Maidargi, Dept. of Chemical Engineering; Dayananda Sagar Institutions, Shavige Malleshwara Hills, Kumaraswamy Layout, Bangalore - 560 078, Karnataka, India; tel: (080)-26662226; fax: (080)-26660789; e-mail: firstname.lastname@example.org.