For the past two years, the Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute (WPI) in Massachusetts has been working on a one-of-a-kind research project aimed at better understanding the mechanical properties and microstructural features of steels that have been gas tempered and induction tempered.
This comparative, pre-competitive research aims to help heat treaters better understand the optimal technology they should use, especially since tempering requires a balance between specified hardness and tensile strength while increasing toughness and maintaining a uniform microstructure.
Ultimately, the findings from this induction and furnace tempering research project are expected to help industry, especially heat treaters in the automotive industry, reduce cycle time and process costs while maintaining or improving product performance of toughness and strength.
According to Richard D. Sisson Jr., George F. Fuller Professor of Mechanical Engineering and technical director of CHTE at WPI, this is the first time a project like this has been undertaken.
“Currently, widely available, detailed comparative data for induction and furnace tempering does not exist,” Sisson said. “This project will help identify the difference between furnace and induction tempering in properties and performance of steel.”
Longtime CHTE member Lesley Frame, Ph.D. at Thermatool Corp. of East Haven, Conn., echoed that thought.
“The industry has long been reliant on dated gas furnace tempering data, which does not provide a sufficient guide for developing induction heat-treating recipes,” Frame said.
Specifically, the project aims to answer:
- What are the equivalent conditions for furnace and induction heating (time and temperature)?
- When hardness values are equivalent, are other mechanical properties also equivalent?
- When hardness values are equivalent, are microstructural features equivalent?
Samples Tested – AISI 4140 Steel
For this study, AISI 4140 steel, a medium-carbon alloy steel that contains Mn, Cr and Mo, was selected for its high hardenability and because it is widely used in engineering applications.
All of the AISI 4140 steel was purchased from Peterson Steel Co. of Worcester, Mass., as 304.8-mm-long x 12.7-mm-diameter rods.
Before the tempering experiments, all the material samples were austenitized at 850°C (1562°F) and held for one hour in endothermic gas at 0.4% carbon potential, followed by quenching in oil at CHTE member Bodycote in Worcester. After quenching, the furnace tempering procedure was conducted at WPI, and induction tempering was conducted at Thermatool. The as-quenched rods were tempered in an atmosphere furnace and by induction at 250°C, 350°C, 450°C, 550°C and 650°C for times from 1 minute to 15 hours.
In order to determine the effect of tempering time-temperature cycles on microstructure and mechanical properties of AISI 4140 alloy steels, surface hardness, impact toughness, tensile strength, yield strength and microstructure were studied.
During the furnace tempering process, the temperature and time were measured by a thermocouple that was connected to the sample. For the induction tempering procedure, the thermal data was measured by both a thermocouple and a thermal camera that was connected to computer-recorded data. The measurement results are shown in Figure 1. The furnace tempering experiments only include the heating process, and the induction tempering experiments include heating and cooling processes.
The selected samples were mechanically polished and etched with 2% nital solution, and the microstructures were observed by optical microscopy and scanning electron microscopy (SEM).
The mechanical properties of the tempered samples were evaluated by hardness, impact toughness test and tensile test. The Rockwell C hardness was measured on the surface of each rod, and the reported average hardness was from measurements of over 40 locations on each rod. The impact toughness test and tensile test were machined and conducted at Westmoreland Mechanical Testing & Research Co. in Youngstown, Pa. Charpy V-notch test samples were machined and prepared according to ASTM E23-12c and tested at room temperature. Tensile-test samples were machined and prepared according to ASTM E8-13a with gauge length at 24 mm and tested at room temperature.
Findings to Date
The microstructure of tempered AISI 4140 at 250-650°C (482-1200°F) is shown in Figure 2. These scanning-electron micrographs reveal that AISI 4140 tempered at 250°C mainly consists of lath martensite and some carbides, which might contain some epsilon carbide (Fe2.4C). With the increasing tempering temperature, lath martensite transforms to ferrite and carbides. It can be observed that the sample tempered at 450°C (840°F) consists of lath martensite and ferrite and rod-shaped carbide. When the tempering temperature is increased to 650°C, some spheroidal-shaped carbides precipitate. These spheroidal carbides are cementite.
The average surface hardness of tempered AISI 4140 is presented in Figure 3. The data shows that the surface hardness of both furnace- and induction-tempered AISI 4140 decreases with the increasing tempering temperature and time. It also shows that surface hardness is more sensitive to tempering temperature than time for both furnace and induction tempering. A comparison of the hardness results of furnace and induction tempering shows that, at the same tempering temperature, the hardness of furnace tempering is lower than induction tempering due to the longer times. The hardness of an as-quenched sample is the highest at 53 HRC.
According to the Hollomon-Jaffe equation, Hp=T(C+log(t)), it is known that hardness is affected by tempering temperature and time. The constant C in the Hollomon-Jaffe equation is determined by statistical analysis of the fitted data. The constant C shown was selected based on the best r2 value. The experimental hardness data presented in Figure 3 are plotted with the Hollomon-Jaffe relationship. The results are shown in Figure 4. C equals 13 for furnace-tempered AISI 4140, and C equals 15 for induction-tempered AISI 4140. The hardness of AISI 4140 can be predicted with the Hollomon-Jaffe equation for different tempering processes.
Our key conclusions are as follows:
- Lath martensite transforms to ferrite and carbides during tempering. During this process, the shape and size of carbides also change.
- Surface hardness decreases with the increasing tempering temperature and time. This phenomenon is well modeled with Hollomon-Jaffe analysis.
- According to the experimental results of impact tests, a martensite embrittlement phenomenon occurs when AISI 4140 is tempered at 350°C (660°F).
- Comparing mechanical properties of induction and furnace tempering procedures, it is found that samples can reach the same hardness with high tempering temperature, short tempering time and low tempering temperature, long tempering time.
- Analysis of surface hardness experimental results with the Hollomon-Jaffe equation determines the constant C and predicts hardness, tensile strength and yield strength with different tempering temperature and time.
- Tempering of steels starts during heating to the tempering temperature. The highest temperature is the most important tempering parameter.
CHTE’s work on this comparative study continues. The center is committed to helping industry understand the technology they should use to ensure high product performance. In the coming months, CHTE will continue this research to also include the mechanical properties and fatigue performance at selected equivalent hardness.
- George Krauss, Steel: Heat Treatment and Processing Principles (2nd Edition), ASM International
- J.H. Hollomon, L.D. Jaffe, “Time-Temperature Relations in Tempering Steel,” Metals Technology, Vol 162, 1945, p. 223-249
About the CHTE Collaborative
The Center for Heat Treating Excellence (CHTE) is an alliance between industry and university researchers that addresses short- and long-term needs of the heat-treating industry. Membership is unique because members have a voice in selecting quality research projects that help them solve today’s business challenges. The member research process works as follows:
- Each research project has a focus group composed of members who provide an industrial perspective.
- Members submit and vote on proposed projects.
- Three to four projects are funded yearly.
- Members have royalty-free intellectual property rights to pre-competitive research.
- Members have the option of paying to sponsor proprietary projects.
- CHTE periodically does large-scale projects funded by the federal government or foundations. These projects keep members informed about leading-edge technology.
- Members are trained on all research technology and software updates.
Other projects that CHTE is currently working on include:
- Nondestructive Testing for Hardness and Case Depth
- Alloy Life Improvement
- Additive Manufacturing
- Gas Quench Steel Hardenability Standard
- Enhancements to CHTE software: CarbTool©, CarboNitrideTool© and NitrideTool©