Investigation Of Heat Treating Of Railroad Wheels And Its Effect On Braking Using Finite Element Analysis

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1 Investigation Of Heat Treating Of Railroad Wheels And Its Effect On Braking Using Finite Element Analysis Kexiu Wang Richard Pilon Griffin Wheel Company Abstract This paper studies the heat treatment process of a 36 freight car wheel manufactured by Griffin Wheel Company. Ideal and non-ideal heat treatment processing and the effect on the residual stress after on-tread braking are evaluated. Two models are developed to simulate both the heat treatment process and on-tread braking using ANSYS. The residual stress due to the heat treatment is evaluated and applied as prestress in the braking simulation model. Both nonlinear thermal and structural simulations are conducted. Introduction Railroad Wheel Freight car wheels, as shown in Figure 1, have the toughest job among all the components of a freight car. The wheel must support the weight of the car and steer it on the rails. The static load on a 36 wheel under a 286,000-loaded freight car is lbs. The wheel must withstand tremendous amounts of abuse from extreme thermal and mechanical stresses caused by such factors as brake shoe friction and high dynamic loading. Figure 1 - The CH36 Freight Car Wheel In railroad service the wheel acts as a brake drum in addition to supporting lateral and vertical mechanical loads. When brake shoes are applied to the wheel tread, the tread surface is heated due to friction. Severe thermal input into a wheel occurs when a loaded train descends a grade for an extended period of time. Also, the failure of the brake mechanism may keep the brake locked on the tread of the wheels. On these occasions, the wheel rim is heated to a high temperature for an extended period of time. The steel on the

2 tread surface and in the rim under the tread surface gets hotter and tries to expand. At the same time, the expansion of the hotter part of rim is constrained by the colder body of the wheel rim and plate and is in compression. The steel in the hotter part of the wheel rim will have a reduced yield strength and the compressive thermal stress will be higher than the yield strength. Therefore, the hotter part of the wheel rim will yield. However, during the wheel cooling, the situation is reversed. The tread surface and top part of the rim get cooler and shrink first. They are constrained by hotter lower part of rim and are in tension. After cooling and shrinking, the material in the top wheel rim could now be in tension due to the locally yielded material in the rim. As the result, the residual stress could be altered to tensile from compressive that is generated during the heat treatment. Thermal fatigue due to repeated brake applications may initiate and propagate thermal-cracks. The presence of a thermal crack in a wheel with tensile residual stress in the rim may lead to failure by brittle mode rim fracture initiated from the thermal crack. Therefore, it is important to know the residual stress change in a wheel. Manufacturing Process The wheels are manufactured by casting. During the wheel manufacturing process after casting, they are reheated to 1500 o F to remove undesired residual stress that remains in the wheels after casting. Once the wheels are homogenized, rims are quenched with water spray on the tread surface using the equipment as shown in Figure 2. Rim quenching wheel increases the strength level of the steel, improves wear resistance, and induces desirable compressive residual hoop stress in the rim. When the water spray quenches the hot wheel rim, the outer wheel rim cools and shrinks. However, the inner rim is still hot and has reduced yield strength due to the high temperature. The inner rim and the upper plate are in compression caused by the colder, outer rim and yielding occurs. Following quenching, the wheels are placed in a tempering furnace at 950 o F temperature for two hours to reduce the levels of residual stress. During this phase, the inner yielded rim and upper plate are constrained by the cooler outer rim. This results in the lower part of the rim and the plate starting in tension while the outer portion of the rim is in the compression. Then the wheels are exposed to ambient conditions as they cool to room temperature. This heat-treatment results in the beneficial circumferential residual compressive stresses in the wheel rim. These compressive stresses are known to help prevent the formation of rim fatigue cracks in railroad service and thus are important to wheel safety. Figure 2 - The Wheel Quenching Equipment During the non-ideal quench process, the water is sprayed on the back plate and hub of the wheel. To compare between normal quench and non-ideal quench of Griffin Wheel CH36 fright car wheels during the heat treatment operation, a thermal stress analysis was performed on a CH36 wheel, using the commercial finite element program ANSYS Revision 5.7. The analysis simulated the thermal manufacturing process of quenching followed by time in a tempering furnace. The heat-treated steel wheels are classified into three classes by Association of American Railroads (AAR) based on the chemical compositions. Class A are those with % carbon, Class B % carbon, and Class C % carbon. Currently, manufactured steel wheels are dominated by the Class C type. Therefore, all material properties used in the analysis are based on Class C steel.

3 Method For the heat treatment simulation, a decoupled thermo-mechanical analysis was performed to estimate the as-manufactured residual stress state in a Griffin CH36 fright car wheel for both the ideal and non-ideal process. The analysis includes two parts, a nonlinear transient thermal analysis and a nonlinear static structural analysis with creep effects. The thermal analysis determines the temperature distribution that varies over time. The thermal analysis includes the five time spans of pre-quench, quench, pre-draw, draw, and post-draw. To simulate the five time spans, a time history of the loading was generated using the concept of the load step within ANSYS. The temperature distribution of the wheel, which is calculated in the thermal analysis, is used as an input load for the structural analysis. Previous work by Kuhlman, Sehitoglu, and Gallagher (1988) indicates creep as an important consideration in such simulations. The creep effect is included in the structural analysis to represent a stress relaxation phenomenon that occurs in a stressed body held at elevated temperature for an extended period of time. Similar to the heat treatment simulation, the braking simulation was also divided into two parts. A nonlinear elasto-plastic structural analysis was conducted for the thermal stress computation. For the thermal analysis, the analysis includes two time spans of heating and cooling. The heating time span represents brake application and the heat is generated due to the friction from the brake shoe. Again, the time history of the temperature distribution are used as the input load for the nonlinear static elasto-plastic structural analysis to evaluate the change of the residual stress of the wheel due to the braking. The residual stress from the heat treatment simulation is applied to the model of braking simulation as initial stress. By varying the heating time, the brake application time at which the residual stress on the tread surface is reversed from compression to tension can be found. Analysis Heat Treatment Simulation Mesh Generation Due to the axisymmetric nature of both the geometry and load, a two-dimensional axisymmetric model of the wheel cross-section was constructed, as shown in Figure 3 using four-node 2-D isoparametric elements. The model contains 1826 nodes and 1695 quadrilateral elements. The model is oriented in space with the wheel radius aligned with global X and the axis of the wheel aligned with global Y. The origin of the global coordinate system is at the center of the back hub. The same finite element mesh was used for both the thermal and structural analyses. However, different ANSYS element types were used: PLANE 55 for thermal analysis and PLANE 42 for structural analysis.

4 Figure 3 - The Finite Element Mesh of the Wheel Material Properties An isotropic material was used for the analysis. All material properties were based on the previous work by Kuhlman, Sehitoglu, and Gallagher (1988), which is the result of testing data for an untreated cast steel. A creep material model was used for the structural analysis. The creep strain rate is dependent on the local values of temperature and stress and determined from the following equation, T = σ e 1 ε& ( ) Hr where σ: effective stress, psi T: temperature, o F The thermal and mechanical material properties are listed in Table 1 and Table 2, respectively. The values of specific heat, thermal conductivity, modulus of elasticity, yield stress, and tangent modulus were input as a function of the temperature. Because there are not enough temperature points from the test data, a curve fitting was made to fill up temperature gaps. Table 1. Thermal Material Properties-Heat Treatment Temperature, o F Specific Heat, BTU/(lb* o F) Thermal Conductivity, BTU/(hr*in* o F) Density, lb/in

5 Table 2. Mechanical Material Properties-Heat Treatment Temperature o F Modulus of Elasticity, msi Yield Stress, ksi Tangent Modulus, msi Poisson s Ratio 0.3 Coefficient of Thermal 9.44E-6 Expansion, in/in- o F Boundary Conditions The nonlinear transient thermal analysis process is divided into five load steps. The initial temperature of the wheel is 1500 o F. For the ideal heat treatment, two constant values of the film coefficient were used. One, from wheel to water, was applied along the surface of the tread. The other, from wheel to air, was applied on all unquenched surfaces. The boundary condition is shown in Table 3. For the non-ideal heat treatment, the only difference is in step 2, the quenching step. Another film coefficient was applied on the back plate and hub. It is assumed to equal one fourth of the tread film coefficient. For the ideal and nonideal heat treatment processes, the boundary conditions of the quenching step are graphically shown in Figure 4 and Figure 5 respectively. Figure 4 - The Boundary Conditions of the Ideal Quenching Step

6 Figure 5 - The Boundary Conditions of the Non-Ideal Quenching Step The transient temperature field determined during the thermal analysis becomes the time-dependent loading for the structural analysis. The nodal temperatures of each sub-step calculated from the transient thermal analysis were applied as body loads for structural analysis. A single constraint of the global Y degree of freedom at the front left corner of the wheel hub was applied. Table 3. Boundary Condition of Thermal Analysis-Ideal Heat Treatment Step Process Duration Film Coefficient, BTU/(hr*in 2 * o F) Bulk Temperature, o F Tread Non-Tread 1 pre-quench 2min 3.47E E quench 4min E pre-draw 4min 3.47E E draw 2hr 3.47E E cooling 14hr 3.47E E Braking Simulation Mesh Generation The same finite element mesh used in heat treatment simulation was used for braking simulation. However, a PLANE 182 element type was used for structural analysis. Material Properties Because the wheels are heat-treated, the different parts of the wheel will have different material properties. In thermal analysis, there are two different sets of thermal material properties. In addition to the untreated cast steel material properties used in the heat treatment simulation, another set of heat treated material properties is applied in the wheel rim area and listed in Table 4. The two regions applied with different thermal material properties are shown in Figure 6 and Figure 7. The heat-treated material is represented by

7 the number 2. For the structural analysis, three different sets of mechanical material properties are applied. The first material represents the steel that is least affected by the heat treatment, which includes the wheel hub and lower plate. The material number 3 is the material in heat treated area including the upper rim and the middle plate for the non-ideal treated wheel. The second material is between the first and the third materials. All material property data are based on the experimental data for Class C steel. All material property values except Poisson s ratio were input as a function of temperature. The three materials use the same Thermal Coefficient of Expansion listed in Table 4 due to the unavailable data for the first and second material. A bilinear, kinematic hardening, elasto-plasticity material model was used for the structural analysis and the plasticity mechanical properties can be found in Table 5, Table 6 and Table 7 respectively for the material numbered 1, 2, and 3. A value of 0.3 was used for Poisson s ratio. The three different material areas are graphically presented in Figure 8 and Figure 9. Figure 6 - Regions with the Different Thermal Material Properties of Ideal Heat Treated Wheel Figure 7 - Regions with the Different Thermal Material Properties of Non-Ideal Heat Treated Wheel

8 Figure 8 - Regions with the Different Mechanical Material Properties of Ideal Heat Treated Wheel Figure 9 - Regions with the Different Mechanical Material Properties of Non-Ideal Heat Treated Wheel Boundary Conditions For the thermal analysis, the heating region as specified in AAR S660, an AAR standard for the freight car wheel design, is over a distance of 3-3/8 inches, centered on a line 3-7/16 inches from the back rim face.

9 Table 4. Thermal Material Properties-Braking Material 2 Temperature, o F Specific Heat, BTU/lb- o F Thermal Conductivity, BTU/hr-in- o F Thermal Coefficient of Expansion, µin/(in* o F) Density, lb/in Table 5. Plasticity Material Properties-Mat 1 Temperature, degree o F Yield Stress, ksi Tangent Modulus, msi Table 6. Plasticity Material Properties-Mat 2 Temperature, degree o F Yield Stress, ksi Tangent Modulus, msi Table 7. Plasticity Material Properties-Mat 3 Temperature, degree o F Yield Stress, ksi Tangent Modulus, msi The power input of 45HP, which is 10HP higher than the thermal load specified in AAR S660, in the form of heat flux in ANSYS was applied on the tread surface of the wheel during the heating step, which represents braking. A film coefficient from wheel to air is the same as in the heat treatment simulation and

10 equal to 3.47E-2 BTU/(hr*in 2 * o F). The bulk temperature is 70 o F. The film coefficient is applied along all other surfaces except for the heating region. At time zero, the model was initialized to a temperature of 70 o F. The same convection values were applied on all surfaces during the cooling step for a period of 10 hours. The loading for the structural analysis are the temperatures calculated for each sub-step during the transient heat transfer analysis. At time zero, a residual stress resulting from quenching was applied as a prestress. A constraint of the global Y degree of freedom was applied at the lower left corner of the model. This boundary condition restrains the model from rigid body translation. Analysis Results & Discussion Heat Treatment Simulation The temperature distribution through the wheel at the end of quench for both ideal and non-ideal heat treatment are shown in Figure 10 and Figure 11, respectively. For the ideal heat treatment, the low temperature area is located in the rim area only. However, for the non-ideal heat treatment, the wheel also achieves a low temperature area around the middle wheel plate and at the back hub. The minimum temperature occurs on the tread surface and is approximately 425 o F. Figure 12 and Figure 13 show the residual hoop stress distribution in the wheels under both ideal and non-ideal heat treatment processes. By comparison of the two results, the stress distributions of the wheel are significantly different between the ideal and non-ideal heat treatment. For the wheel with the non-ideal heat treatment, a higher tensile stress is obtained in the rim-plate fillet area. The whole rim-plate fillet area is in tension even in the back rim area in which the wheel with ideal heat treatment has circumferential compressive stress. Their stress results from the fact that the rim-plate fillet area is sandwiched by two colder areas, the rim and plate, during the quenching phase. Similarly, another high-tension area occurs in the wheel hub due to the colder plate area of the non-ideal heat treatment. The wheel plate is in compression for the non-ideal heat treatment while it is under tension for the ideal heat treatment. Figure 10 - The Temperature Distribution at the End of Ideal Quenching

11 Figure 11 - The Temperature Distribution at the End of Non-Ideal Quenching Figure 12 - The Residual Hoop Stress Distribution after the Ideal Heat Treatment

12 Figure 13 - The Residual Hoop Stress Distribution after the Non-Ideal Heat Treatment Braking Simulation The values of the temperature and hoop stress at the tape line located inch from the back rim face, at which the wheel diameter is measured, are taken and presented. The analysis results from three different heating durations, 20min, 57min, and 70min, are included, which represent the three typical residual stress states in the upper rim, compression, stress-free, and tension. The time-varying temperatures at the tape line are shown in Figure 14. The time-varying hoop stresses at the tape line are shown in Figure 15. As shown in Figure 14 and Figure 15, both temperature and residual hoop stress on the tread surface are basically identical for the ideal and non-ideal heat treatment. At the beginning of heating, the temperature on the tread surface increases rapidly, which causes the tread surface to expand. Then, the compressive stress on the tread surface rises sharply. As the wheel rim is heated and the temperature starts to increase, the temperature difference between the tread surface and rim area decreases. The compressive hoop stress on the tread surface decreases after reaching the maximum compressive stress near 55ksi and returns to the initial stress level as shown in Figure 15. This phenomenon is independent of heating duration. The time after heating that the compressive stress decreases depends on the thermal material properties. While increasing the heating duration, the compressive stress on the tread surface will exceed the yield stress due to the decreased yield strength with increased temperature. During the cooling phase, the tread surface is cooling first and the temperature is decreasing rapidly. The tread surface is shrinking and subjected to a tensile load that further decreases the compressive stress. Reversal of the residual stress state on the tread surface from the compression to tension is shown in the curve for 70min heating.

13 Figure 14 - The Variation of Temperature with Time Figure 15 - The Variation of the Residual Hoop Stress with Time Figure 16 displays the variation of the residual hoop stress at the tape with the heating time. It shows a bilinear relationship between residual hoop stress and heating time. Again, there is no significant difference between the ideal and non-ideal heat treatments. For the 45HP heat input, the residual hoop stress on the tread surface does not change if the heating time is below 50min. The high temperature yield stress of the steel determines how long a wheel can stand the brake heating and still hold a constant residual hoop stress. For the heating duration over 50 minutes, the residual hoop stress on the tread surface increases rapidly with increasing time. A neutral residual stress state of the wheel rim occurs at 57 minutes for a 45HP heat input.

14 Figure 16 - The Variation of the Residual Hoop Stress with Brake Heating Time The residual hoop stress distributions for both ideal and non-ideal heat treatments with a heating time of 70 minutes are shown in Figure 17 and Figure 18. The stress distribution does not change for most of the wheel except in the rim area with the brake heating and dominated by the residual stress due to the heat treatment. The residual hoop stress distributions for brake heating times less than 50 minutes are similar to those from the heat treatment. Figure 17 - The Residual Hoop Stress Distribution with 70min Brake Heating - Ideal Heat Treatment

15 Figure 18 - The Residual Hoop Stress Distribution with 70min Brake Heating Non Ideal Heat Treatment Conclusions 1. The non-ideal heat treatment process significantly changes the residual stress distribution of the wheel after the completion of heat treatment and achieves a higher tensile stress in the rim-plate fillet area. 2. The non-ideal heat treatment process does not significantly affect the residual hoop stress level on the tread surface after brake heating. 3. The residual hoop stress on the tread surface is independent of heating time if the heating time is less than 50 minutes with a 45HP heat input. 4. The residual hoop stress state on the tread surface is reversed from compression to tension if the heating duration is longer than 57min with 45HP heat input. References C. Kuhlman, H. Sehitoglu, and M. Gallagher, 1988, The Significance of Material Properties on Stresses Developed During Quenching of Railroad Wheels, Proceedings of the 1988 Joint ASME/IEEE Railroad Conference, The American Society of Mechanical Engineers, 345 East 47 th Street, New York, NY 10017