Solution: Material samples made of different microstructure and hardness will be fabricated and tested. Paired data studies statistically analyzing dimensional distortion will be performed on coupons of similar size and process.
Assumptions
The material samples are assumed to fully represent their population. For example, a group of normalized material samples is assumed to represent all normalized material.
The change in coupon gap width is assumed to represent the relative dimensional distortion of an actual gear.
Methods and Procedures
This section contains the details of coupon manufacturing and processing. Standard distortion coupons were manufactured for each population as shown in Figure 1. The dimensions of the coupon were proportional to the gear being developed and are shown in Figure 3.
Figure 3--Detailed specimen drawing (units = inches).
Coupons from each population were machined, stress relieved, carburized, and hardened together. The coupons were randomly located in the carburization furnace and quench basket. A summary of the populations is shown in Table 1.
A sample size of 10 was used with one additional sample used for metallurgical evaluation. The samples’ letter designations will be used from this point on to identify the populations. The raw material requiring hardening and tempering was heat treated in-house as listed in Table 2 and Table 3 prior to any machining.
The normalizing process was done per the AMS specification prior to receiving the material. The manufacturing and inspection stages of the coupons are listed in Table 4.
The coupons were carburized and hardened using a cycle common to the actual gear (see Table 5).
Dimensional measurements. A Zeiss Prismo scanning coordinate measuring machine (brass tag #253685) was used to perform all measurements. The outside diameter, inside diameter, and the gap width of every coupon was measured before carburizing, after carburizing, and after hardening. Each time, the outside diameter and etched face were scanned and set as reference. The gap width was measured at a constant radius of 0.7000 inches from the reference center. All measurements were taken in a plane 0.1500 inches (half overall length) from the reference face using a 0.054" diameter probe. The coupons were soaked in mineral spirits, wiped dry and rinsed with alcohol before each measurement. The cleaned coupons were placed in the CMM room 24 hours before measurement to thermally soak and stabilize. The CMM room temperature is held at 69°F +/– 2°F. The actual measurements are contained in Appendices A–D. A sample inspection report is in Appendix D.
Findings
This section contains all of the data and findings collected during the study. Data collected includes characterization of the pre-carburization microstructure and dimensional measurements.
Pre-carburization microstructure. To document the pre-carburized material, an extra coupon was manufactured from each population for metallurgical evaluation. The evaluation was performed after finish machining and before carburizing. The chemistry of Sample E is not reported in Table 7. The sample was lost during the metallurgical evaluation process. The hardness (see Table 6), chemistry (see Table 7), and microstructure (see Figures 4–12) were evaluated on the etched face side of the coupon in two radial locations, center and near the outer diameter.
Figure 4 (left)--Sample A center microstructure 100X, 5% nital etch (Ref. 10).
Figure 5 (center)--Sample A near OD microstructure 100X, 5% nital etch (Ref. 10).
Figure 6 (right)--Sample B center microstructure 100X, 5% nital etch (Ref. 10).
Figure 7 (left)--Sample B near OD microstructure 100X, 5% nital etch (Ref. 10).
Figure 8 (center)--Sample C center microstructure 100X, 5% nital etch (Ref. 10).
Figure 9 (right)--Sample C near OD microstructure 100X, 5% nital etch (Ref. 10).
Figure 10 (left)--Sample D center microstructure 100X, 5% nital etch (Ref. 10).
Figure 11 (center)--Sample D OD microstructure 100X, 5% nital etch (Ref. 10).
Figure 12 right)--Sample E center microstructure 100X, 5% nital etch (Ref. 11).
Dimensional measurements and descriptive statistics. Measurements were recorded before carburization, after carburization, and after hardening. Details of the measurement method are in the Methods and Procedures section. Serial numbers B5 and B6 were lost during carburization and serial number C10 was scrapped during manufacturing (see Appendix B). Descriptive statistics of the pre-carburization, post-carburization, and post-hardening measurements are listed in Tables 8–10.
The actual measurements are shown in Figure 13. Descriptive statistics of the paired difference between pre-carburization and post-hardening are listed in Table 11. The mean paired difference values are shown in Figure 13.
Figure 13--Gap width measurements after each processing step of carburizing and hardening.
Paired difference. The pre-carburization and post-hardening gap measurements were paired to enable a relative comparison. The gap width difference is reported as initial minus final. Thus, a change resulting in an increased gap width is reported as a negative value.
Graphical checks of the gap width change data were performed and shown in Figures 14 and 15. The checks include an individual data plot and a normal probability plot. Notice that the paired difference measurements were sorted based on each population’s mean value and plotted.
Figure 14--Data plot of gap width paired distance.
Figure 15--Probability of paired gap width distance.
An analysis of variance (ANOVA) was performed to compare the five population means (see Table 12).
Formally, the data analysis is stated as:
H0: µA = µB = µC = µD = µE
H1: µA, µB, µC, µD and µE are not all equal
a = 0.05
The F statistic is greater than the F critical value. Therefore, the null hypothesis is rejected, and the alternate accepted. The ANOVA identifies if difference is present between any of the mean values. A multiple comparison procedure is required to determine in what way they are not equal. A Fisher’s Least Square Difference (LSD) test was performed at an individual a = 0.05 to determine which of the population means were significantly different from each other. The results are listed in Table 13.
An upper value equal to or greater than zero indicates that the population is significantly less (greater distortion) than the population subtracted from. A summary of the LSD results is listed in Table 14.
Discussion
The gap width change was used to indicate differences in dimensional distortion during carburizing and hardening between five different raw material mechanical and thermal processes. Statistical analysis of the gap width change provides the following:
1.) Coupons manufactured from hardened and tempered barstock and forgings at 1,725°F had the greatest gap width change.
2.) Coupons manufactured from normalized barstock had the smallest gap width change. Coupons manufactured from normalized forgings had significantly more gap width change than those made from normalized barstock.
3.) Coupons manufactured from barstock hardened and tempered at 1,725°F had significantly more gap width change than those made from barstock hardened and tempered at 1,525°F.
4.) Coupons manufactured from barstock hardened and tempered at 1,550°F had no significant difference in gap width change than those made from normalized barstock.
The barstock used to manufacture coupons was from a common heat lot. Also, the forgings used were from a common heat lot. Additional heat lots could change the mean and/or scatter gap width change. It is recommended that future studies include multiple heat lots of materials. It is further recommended that future studies include residual stress measurements prior to carburization and hardening.
Based on the results of this study, hardened and tempered barstock at 1,550°F was selected. The surface integrity of the shaped gear and spline teeth improved greatly. Pre- and post-heat treat data collected from actual gears showed no change to the heat treat distortion.
References
1. French, H. The Quenching of Steels. American Society for Steel Treating, Cleveland, OH, 1930.
2. INFAC. “Distortion of aerospace precision gears Part I: The effects of processing variables prior to carburizing,” Report number TR-94-F-004, IIT Research Institute, Sept. 1994.
3. Evard, M. E-mail containing photos from Evard to Blake, 4/14/2005.
4. Metcut Research Associates Inc. Machining Data Handbook, 3rd Edition, Vol. 2, Zimmerman & Son, Newport, KY, 1980.
5. Mott, R. Machine Elements in Mechanical Design, Bell & Howell Company, Columbus, OH, 1985.
6. Verzahntechnik Lorenz. Gear Cutting Tools, G. Braurn, Ettlingen, Germany, 1980.
7. Cluff, B. Gear Process Dynamics, American Pfauter. Malloy Lithographing Inc., Ann Arbor, MI, 1993.
8. Mullins, J.D. Ductile iron data for design engineers, Rio Tinto & Titanium Inc., Montreal, Canada, 1990.
9. Woldman, N. “Microstructure versus Machinability of Alloy Gear Steels,” American Gear Manufacturers Association, Presented at the AGMA September 20, 1937, Semi-Annual Meeting at Lake Wawasee, Indiana.
10. Weinrich, K. “Short Form Metallurgical Routine Test,” Rolls Royce lab report number 03MTS-0712, Feb. 24, 2003.
11. Ballard, C. “Short Form Metallurgical Routine Test,” Rolls Royce lab report number 03MTS-00407, Feb. 25, 2003.
See the attached PDF for the Appendices.
This article is printed with permission of the copyright holder, The American Gear Manufacturers Association. Statements presented in this paper are those of the author and may not represent the position or opinion of The American Gear Manufacturers Association.
Gregory Blake is a senior specialist, mechanical engineer at Rolls Royce. He is responsible for technical support of gear system design and manufacturing, especially in the process development of gears.
