PRACTICAL APPLICATION OF INTENSIVE ...

Proceedings of 19th ASM Heat Treating Society Conference, 1-4 November 1999, Cincinnati, Ohio

PRACTICAL APPLICATION OF INTENSIVE QUENCHING TECHNOLOGY FOR STEEL PARTS AND REAL TIME QUENCH TANK MAPPING Michael A. Aronov, Nikolai I. Kobasko and Joseph A. Powell IQ Technologies, Inc. Akron, Ohio John Young Alpha Steel Treating Co. Livonia, Michigan

1.

IntensiQuenchSM Process Theory

Introduction In 1997-1998, Edison Material Technology Center (EMTEC), Kettering, Ohio, funded a team of Ohio heat treating companies, parts manufacturers and Case Western University to conduct demonstrations of intensive quenching for steel parts. Phase I of the EMTEC project promoted the transfer of the intensive quench technology from Ukraine to the American market. Several project members provided steel parts for intensive quenching. To validate the benefits of the intensive quenching process, known as IntensiQuenchSM, the Team conducted extensive experimental studies at the Akron Steel Treating Co. laboratory facility. Summaries of the results of the Phase I demonstration experiments are presented in References 1 and 2. Since these results were very promising, and additional heat treaters were expressing an interest in the intensive quenching process, EMTEC funded a second Phase of demonstration experiments. In this paper, we discuss the following:  



The theory of the IntensiQuenchSM process and the computer model that simulates this process. Results of the IntensiQuenchSM process demonstration tests conducted during 1997-1999 under the EMTEC project as well as the experimental data previously obtained by Dr. Kobasko and his colleagues in the Ukraine and Russia. The results of testing of a quench tank mapping system for in situ, real time data collection from multiple sites in the actual quench tank.

In Western heat treating practice several different types of quenching techniques are used, including direct quenching, time quenching, selective quenching, etc. Likewise several types of quenching media are employed: oil, water, polymer/water, inert gas, salt, air, etc. The selection of technique and of media are based on the effectiveness of the quenching process with the materials, parts, and quenching objectives; all with a view of balancing high hardness with acceptable distortion. In all cases, the quenching process is controlled to prevent a high cooling rate when the material is in the martensite range. This rule is used on the belief that high tensile residual stress, distortion, and the possibility of cracking can be avoided. Extensive research conducted in the Ukraine by Dr. Kobasko has shown that this procedure is not always necessary or correct. His studies showed that a very high rate of uniform cooling (usually in plain water) within the martensite range would actually prevent quench cracking. This phenomenon was discovered in laboratory experiments, that were confirmed by computer simulation (References 3 and 4), and further substantiated by a large number of field tests using a variety of steel parts (References 5 and 6). Figure 1 shows the experimental data that were obtained for a cylindrical specimen with diameter of 6 mm (0.25”) made of low alloy steel when quench cooling rate was compared to the probability of part cracking. The bell-shape curve clearly illustrates the effect of the cooling rate within the martensitic range on crack formation: the probability of quench cracking is low for both slow and very rapid cooling within the martensite range. Why does the IntensiQuenchSM process minimize cracking and distortion? Imaging a steel part with different cross-sectional thickness (Figure 2). During conventional quenching, the martensite forms first in the

Crack Formation Probability, %

75

50

25

200

400

600

800

Cooling Rate, C/s Figure 1

Effect of Cooling Rate on the Probability of Cracking

M

A A a)

M b)

Figure 2

Martensite Formation During Quenching a) Conventional quenching, b) Intensive quenching

=0

A

A

A

A

A

A

A

A

A

A

A

A

A

-

M

-

M

M

M

M

M

+

M M

M

M

A

M

A

M M

Figure 3

Stresses

+

1- Cooling from austenite to martensite temperature 2- Martensite formation on the surface 3- Martensite formation in the core

0

-

Surface Stress Conditions

Time

1

3

2 Figure 4

Surface Stresses vs. Time

thinner section of the part since this area cools faster and reaches the martensite range earlier than the thicker section thinner section of the part since this area cools faster and reaches the martensite range earlier than the thicker section below (Figure 2a). The martensite specific volume is greater than the specific volume of the remaining austenite. Therefore, the thin section expands in volume while the thick section of the part continues contracting due to cooling. At the interface between the martensite and the austenite phases there are stresses resulting in the distortion and possible part cracking. Now imagine that the same steel part is cooled very rapidly and uniformly. In this instance, the martensite forms simultaneously over the entire part surface creating a strong, hardened “shell” (Figure 2b). Findings revealed that this uniform, hardened “shell” contains high compressive stresses resulting in lower distortion and lower probability of part cracking. Why does the IntensiQuenchSM process create surface compressive stresses? Imagine a cylindrical steel part. Assume that the part’s surface layer consists of a set of “segments” joined together by “springs” to form an elastic ring (Figure 3). When the whole steel part is heated above Ac3 (the austenitizing temperature before quenching) there is neither tension nor compression in the “springs” and no stresses between the “segments” (=0, see Figure 3a). During the first split seconds of quenching, the surface layer cools rapidly resulting in the contraction of the above “segments” and the “springs” expand simulating the development of tensile thermal stresses (see Figure 3b). When the surface layer cools to the martensite formation temperature Ms, the austenite phase in the surface “segments” transforms into the martensite phase (see Figure 3c). Since the martensite specific volume is greater than the austenite one, the shell’s segments expand or swell causing the “springs” to contract. The contraction of the “springs” between the “segments” simulates the development of surface compressive stresses. As the part continues to cool, the martensite starts forming in the part core resulting in an increase in volume or core “swelling” (see Figure 3d). Due to the core expansion the distance between the surface layer segments increases resulting in the expansion of the “springs.” The spring expansion reflects the reduction of the compressive stresses on the part surface. Figure 4 presents the abovedescribed dynamics of the surface stress conditions during intensive quenching. The key element of the IntensiQuenchSM is to interrupt rapid cooling of the steel specimen when compressive stresses on the surface are at maximum. The specimen is then held at the start of the martensite forming temperature for the remaining quench time. The martensite phase advance ceases resulting in no further

core swelling, and these compressive stresses are maintained. Stress relaxation during the isothermal hold reduces slightly the final compressive stress. Numerous laboratory and field experiments have shown that the strength of the final part is directly related to the external rate of heat transfer. Higher heat transfer rate results in higher strength. Hence, water jets or highspeed uniform flow over the part during cooling will increase strength. Both higher surface compressive stresses and increased strength help to eliminate quench cracking and enhance the durability (service life) of machine parts and tools (References 7 and 8). The optimum hardened depth, corresponding to the maximum surface compressive stress, is a function of the part dimensions. For best results, the steel and its hardenability should be tailored to the part geometry to ensure that hardening occurs to the optimum depth. Use of the IntensiQuenchSM computer model makes reaching the optimum depth a certainty. For a steel with controllable hardenability, the intensive quenching method will provide an optimum combination of high surface compressive stress; highstrength, wear-resistance; quenched layer of optimum depth; and relatively soft but properly strengthened core (Reference 6). This combination is ideal for applications requiring high strength and resistance to static, dynamic, or cyclic loads. It was also demonstrated experimentally that by applying the IntensiQuenchSM process, the desired properties of the part could be obtained using less expensive steels (steels containing two or three times less alloying elements than conventional alloy grades). 2.

IntensiQuenchSM Process Computer Model

IntensiQuenchSM process can be adapted to almost any steel part. The process begins by analyzing the thermal and stress profiles within the part during quenching using a finite element approach. A computer model was developed to conduct this analysis (Reference 4). This model includes a non-linear transient heat conduction equation and a set of equations for the theory of thermoplastic-plastic flow with kinematics strengthening under the appropriate boundary conditions on the parts’ surface. An iterative technique is used to solve the above system of equations. At each time and space step, the calculation results are compared with the thermokinetic diagram of the supercooled austenite transformation, and new thermophysical and mechanical characteristics for the next step are chosen depending on the structural components. This analysis seeks the point along the cooling curve where surface stresses are maximized. The calculation results are as following:  Temperature field  Material phase composition  Components of the stress and strain tensors



Intensities of current (while cooling) and residual stress and strain Numerous laboratory and field experiments have been used in the validation of this model. A similar software package was developed on the collaborative research program managed by the National Center for Manufacturing Science (Reference 9). However, in contrast to the computer model developed in Reference 4, this software does not calculate heat transfer boundary conditions on the part surface. An accurate evaluation of these conditions is usually the key element in such calculations. 3. IntensiQuenchSM Process Experimental Validation The IntensiQuenchSM process has been validated by hundreds of laboratory and field experiments. The examples set forth below are but a small part of the intensive quench method validation. The team performed all tests in our experimental quenching system described in Reference 2. The experimental quenching system was designed only for demonstration purposes. While it can rapidly cool a variety of different parts, we did not adjust the quenching unit to any specific part and therefore we did not provide the optimum cooling rate and uniformity for a particular part. Therefore, the results presented are not the “best possible” in terms of the hardness obtained and depth of hardened layer, value of the residual compressive stresses, etc. Nevertheless, these results definitely show significant improvements of the part mechanical properties due to the intensive quenching process and no cracking.

3.1 Mechanical Property Improvements. Table 1 shows the improvement of mechanical properties for different steels (Reference 10). Test samples were taken from the core of the 50mm diameter specimens. As seen from the table, all mechanical properties improved due to the intensive quenching process, especially the impact strength. This characteristic increased by 2-3 times. IQ Technologies conducted a limited statistical study with three types of truck wheel bolts. The parts were provided by Alpha Steel Treating Co. of Detroit, Michigan. The bolts were made of 4140 and 1340 steels. The goal of this demonstration study was to show that the bolts would not have any cracks after intensive quenching in water and the part mechanical properties would be the same or better compared to the same bolts that were quenched in oil in accordance with the current practice. We intensively quenched 20 bolts of each type shown above. The same number and type of bolts were quenched in oil in production conditions. Fifteen bolts from each of the sample were subjected to the tensile strength test conducted at the Alpha Steel Treating laboratory facility. Two bolts from each sample were cut to measure the core hardness, and three bolts from each sample were subjected to the impact strength test. This test was conducted by ? University of ?… In our experiments, we used the same thermal cycles that are used in real production conditions (Table 2). We heated the parts to austenitizing temperature in a box type atmosphere furnace in batches of 10 bolts per batch. Then we intensively water quenched the bolts one by one in our experimental quenching system.

Table 1. Mechanical Properties at the Core of Conventional (oil) and Intensively Quenched Steels

Steel

Quench

Tensile Strength, MPa

Yield Strength, MPa

Elongation, %

Reduction in Area, %

Impact Strength, J/sm2

Hardness, HRB

40X

In oil

780

575

21

64

113

217

(5140)

Intensive

860

695

17

65

168

269

35XM

In oil

960

775

14

53

54

285

(4130)

Intensive

970

820

17

65

150

285

25X1M

In oil

755

630

18

74

70

229

(4118)

Intensive

920

820

15

68

170

285

Table 3 shows the final results for bolts intensively water quenched and for those that were quenched in oil. As seen from the table, the tensile strength for ? bolts improved by 5-8%, for ? bolts this parameter increased by ?%. For ? bolts there was no change in this characteristic. The impact strength Table 4 from Reference 2 presents the improvement in the torque-tofailure for 4140 steel hand tool sockets. As seen from the table, the socket average value of the torque-to-failure obtained after intensive quenching was 32% greater than after quenching in oil (the current practice). The part surface hardness did not appreciably change. The significant improvement in the part mechanical properties is mainly due to the increase of the fully hardened depth in the steel parts that were intensively quenched. IQ Technologies intensively quenched four kingpins (pin steering knuckles) made of 4140 steel provided by Meritor, Inc. of Troy, Michigan. The pin diameter was 45.6 mm, the pin length was 264 mm. The purpose of this demonstration was to show that the kingpins would not crack due to rapid cooling, would have greater hardened depth and that there would be

Table 2

compressive stresses on the part surface after intensive quenching.

After being heated in the atmosphere box furnace and intensively water quenched, the parts were shipped back to Meritor for the further metallurgical analysis. No quench cracks were detected. Table 5 shows the results of micro hardness measurements for the intensively quenched pins and induction hardened pins (the current technology). As seen from the table, IntensiQuenchSM process provides enhanced hardenability that means part designer can reduce alloy content and the end part cost. The intensively quenched and tempered king pin and five standard production pins were subjected to the bend test. The ultimate strength of the intensively quenched pin was 414,398N (93,140lb) which is considerably greater than the ultimate strength of the standard production king pins: 296,040N-324,142N (66,470lb-72,780lb).

Bolt Thermal Cycle

Bolt Type Steel 4140 4140 1340

Heating Cycle

Size 1.1254.62 1.1253.375 M22123mm

Quenching Soak at 1600oF for 1 hour

Table 3

Properties

Core Hardness, RHC Tensile Strength, lb/sq.inch

Bolt Mechanical Properties

4140 1.1254.62 Oil Intensive Quench Quench 36.6 39.0 67,531

68,827

Table 4 Socket Average Torque to Failure Quench

Torque to Failure, ft/lb

Hardness, HRC

Intensive

164.2

45

In oil

124.0

46

Tempering Soak at 980oF for 1 hr Soak at 1000oF for 1 hr Soak at 930oF for 1 hr

Bolt Type 4140 1.1253.375 1340 M22123mm Oil Intensive Oil Intensive Quench Quench Quench Quench 36.2 39.0 36.6 38.5 -

-

84,464

90,854

Table 5 Pin Hardness Distribution, HRC Quench Induction hardening Intensive quenching

Relative Depth R

0.75R

0.5R

0.25R

60

35

32

30

59

52

52

48

3.2 Compressive Residual Stresses. The ability of intensive quenching to create high surface compressive stresses was demonstrated first by calculations (Reference 6). These calculations frankly did not convince some members of metallurgical community. However, the results of experimental studies helped to overcome their objections. Authors of Reference 12 measured the residual stress distribution in 38.1 mm in diameter cylindrical samples of carbon steel AISI/SAE 1045 and a “eutectoid alloyed steel.” The carbon steel samples had a quenched layer of optimum depth with the characteristic “U”-shaped hardness distribution along the diameter. The samples made of alloy steel were through hardened to 64 HRC. High surface compressive stresses formed in the carbon steel samples, at levels corresponded very well with calculated data (Figure 5a and 5b). However, tensile stresses formed at the surface of through hardened alloy steel samples (Figure 6) due to a not enough cooling rate for this size of the steel specimen. The measured distribution of axial residual stresses in the 1045 steel bar (Figure 5a) shows a compressive stress of –430 MPa near the surface of cylindrical specimen and a tensile stress of 500 MPa at the center. The calculated values of these two parameters were –440 MPa and 450 MPa accordingly. There also is agreement between calculated and measured experimental data for the through hardened alloy steel samples. For example, measured values of the axial residual tensile stress at the part surface after quenching were 220 MPa vs. 200 MPa calculated value of the same characteristic. Thus the computer model accurately predicts part stress conditions during IntensiQuenchSM process. Meritor, Inc. measured residual surface stresses for the above king pins. Compressive residual stresses were detected on the surface layer of the intensively quenched pins: for a depth of 0.4 mm (0.017”), the longitudinal stress was –563 MPa (81.5ksi), the transverse stress was – 1,291 MPa. 3.3 Service Life Cycle Improvements. Both the improvements in part mechanical properties and the presence of residual compressive stresses on the part surface due to the IntensiQuenchSM process result in the Table 6

significant increase of the product service life cycle. Table 5 shows the improvement in service life for different steel parts. Parts that were intensively water quenched were made of plain carbon steels. Parts that were quenched in oil were made of alloy steel. For example, heavy truck semi-axles were made of steel AISI/SAE 4340 and AISI/SAE 1045. Alloy 4340 steel semi-axles were quenched in oil in accordance to the existing technology. Plain carbon steel semi-axles were intensively quenched in water. The quenched semi-axles were subjected to lifetime cyclic load testing. As seen from the table, the service time of the intensively quenched parts increased by about 7 fold. 4.

Real Time Quench Tank Mapping

By definition heat treat processes rely upon the controlled heating and cooling of the parts to be treated. Many quality specifications detail the required degree of heating uniformity required for the heating chamber of the furnace, e.g., ASTM 6875 H (formerly known as Mil-H6875 H). Some quality specifications will specify a quench cooling rate and back up the requirement with metallographic examination of the final martensitic grain structure. Perhaps because of the lack of high speed computer data logging equipment, most all heat treating specifications contain no requirement for “certifying” quench uniformity, an important component of the heat treating process. Intensive quenching works with a combination of high cooling rates (three times higher than that of agitated oil) and uniformity of part cooling. Successful application of IntensiQuenchSM processes will require a method of quantifying and proving actual quench intensity and uniformity. In order to measure the cooling rates of various quench methods, Alpha Cybernetics Inc, an affiliate of IQ Technologies is developing a high-speed data logger system for in situ quench tank mapping. The methodology is similar to a temperature uniformity survey for the heat zone of a furnace that uses multiple thermocouples in the corners and the center of the work

Parts’ Lifetime Improvement

Steel Part

Type of Steel

Life Time Improvement, Times

Reference

Truck half-axles Shaft Punches Dies Concrete Breaker Point

AISI 1045 AISI 1045 High Speed Steel AISI 52100 AISI 1078

7.6 8.0 1.5-2.0 1.5-2.0 From 1 hr (in oil) to no failure

13 14 15 16 14

zone coupled to a data logger/recorder. The real difference is the rate at which data is collected. For a furnace uniformity survey, the data is collected at approximately two-minute intervals. For quenching, the data must be collected at the rate of 100 readings per second or more. The experimental quench tank mapping system tests were conducted with three MgO, type-K thermocouples embedded in the geometric center of three stainless steel ball bearings of 25mm (one inch) in diameter, and the thermocouple sheath welded to each ball. The area of the between the thermocouple junction and the hole in the ball bearing was filled with a powdered metal to transmit the temperature data more uniformly from the surface of the ball to the thermocouple. Cleveland Electric Labs then certified the thermocouples to AMS 2750C and ASTM E220 standards. The three thermocouples were bent at 90 degrees approximately 125mm (five inches) from the ball end, tied together and oriented relative to each other as shown in Figure 7. The thermocouples would then “map” an area of temperature in three points on a circle approximately 250mm (10inches) in diameter.

stored on the disk of a PC and transferred to a charting software program. The thermocouples were heated to 1,550oF in the box retort furnace at Akron Steel Treating’s experimental facilities. The thermocouples were then quickly removed from the furnace and quenched into the different quench media: still oil, (clockwise) agitated oil, (reverse) agitated oil, still water and agitated water. An examination of the charted data shows the different slope of the cooling curves for different quenching media. The slope of the cooling curve “maps” characterizes the cooling rates for still oil, agitated oil, , still water and agitated water. As expected in such a small area, the cooling uniformity is good between the three balls in tests. The quench tank mapping system will be expanded to log eight points of data simultaneously in the 6000 gallon intensive water quench tank. The full-scale production intensive quenching system was described in Reference 2. This quenching system is currently being modified. Two additional props and two motors are being added to provide more intensive quenchant agitation. The quenchant supply manifolds that connect the props with the tank were also redesigned to provide more uniform flow distribution around the load. We will conduct a series of tests to evaluate the uniformity of part cooling in the full-scale intensive quenching system and make the necessary trim adjustments to the cooling baffles and prop speed controls. IQ Technologies progress on this may be reviewed on the web site: www.IntesiveQuench.com. Conclusion Numerous laboratory and field experiments with variety of steel parts have proved the following benefits of the intensive quenching technology: Acknowledgement

Figure 7 Multi-Thermocouple Probe The three MgO thermocouple leads were connected at a cold junction and then to the high-speed data logging board. The board is capable of capturing 32,000 readings per second. The actual rate used for the tests in the experimental quench tank was 100 per second per thermocouple. The data logging board digitizes the data and transfers it to the PC’s hard drive. (Unfortunately the higher the number of readings, the higher the data logger unit’s susceptibility to ambient “electrical” noise. More work is needed to reject the noise present in a heat treating shop environment.) Once logged the data was

The authors wish to acknowledge the Edison Material Technology Center of Kettering, Ohio for funding of this research and Meritor, Inc. of Troy, Michigan, Alpha Steel Treating Co. of Livonia, Michigan and American Punch Co. of Cleveland, Ohio for providing the steel parts and for conducting the metallurgical analysis. References 1. Aronov, M. A., Kobasko, N. I., Wallace, and J. F., Schwam, D., “Intensive Quenching Technology for Steel Parts,” Proceeding of The 1998 Heat Treating Conference, Chicago, 1998. 2. Aronov, M. A., Kobasko, N. I., Powell, J.A., Wallace,

3.

4.

5.

6. 7.

8.

and J. F., Schwam, D., “Practical Application of the Intensive Quenching Technology for Steel Parts,” Industrial Heating Magazine, April 1999, pp. 59-63. Kobasko, N. I. and Prokhorenko, N. I., “Quenching Cooling Rate Effect on Crack Formation of 45 Steel,” in Metallovedenie and Termicheskaya Obrabotka Metallov (in Russian), No. 2, 1964, pp. 53-54. Kobasko, N. I. and Morganyuk, V. S., “Numerical Study of Phase Changes, Current and Residual Stresses in Quenching Parts of Complex Configuration,” Proceedings of the 4th International Congress on Heat Treatment of Materials, Berlin, Germany, 1985, Vol. 1, pp., 465-486. Kobasko, N. I., “Method of Overcoming SelfDeformation and Cracking During Quenching of Metal Parts,” Metallovedenie and Termicheskay Obrabotka Metallov (in Russian), No. 4, 1975, pp. 1216. Kobasko, N. I. “Intensive Steel Quenching Methods. Theory and Technology of Quenching”, SpringerVerlag, New York, N.Y., 1992, pp. 367-389. Tensi, H. M., Kobasko, N. I., and Morganyuk, V. S., “Specific Features of Using Intensive Methods of Quenching for the Strengthening Parts of Complex Configuration,” Proceedings of International Heat Treating Conference: Equipment and Processes, April 1994, Schaumburg, Illinois, pp. 239-241. Kobasko, N. I. “Peculiarity of the Process of Quenching Carburized Steel Parts,” Proceedings of the Second International Conference on Carburizing and Nitriding With Atmospheres, December 1995,

Cleveland, Ohio, pp. 173-180. 9. “Predictive Model amd Methodology for Heat Treatment Distortion”, Phase 1 Project Summary Report, National Center for Manufacturing Science report No. 0383RE97, September 30, 1997. 10. Muhina, M. P., Kobasko, N. I., Gordejeva, L. V., “Hardening of Structural Steels in Chloride Quench Media”, Metalovedenie I Termicheskaya Obrabotka Metallov (in Russian), September 1989, pp. 32-36. 11. Mei Daming, “Intensive Quenching Method for Preventing Quench Cracking”, Proceedings of the 7th International Congress on Heat Treatment and Technology of Surface Coating”, Moscow, Russia, 1990, Vol.2, pp. 62-71. 12. Hernandez Morales et al., Residual Stress Measurements in Forced Convective Quenched Steel Bars by Means of Neutron Diffraction”, Proceedings of the 2nd International Conference on Quenching and the Control of Distortion, ASM, 1996, pp. 203-214. 13. Kobasko, N. I., “Steel Quenching in Liquid Media Under Pressure”, (in Russian), Naukova Dumka, Kiev, 1980, 206 pages. 14. Kern, R.F., “Intense Quenching”, Heat Treating, September 1986, pp. 19-23. 15. Lisovoy, V.A., Kobasko, N. I., Kindrachuk, M.V., Khalatov, A.A., “Stamp Quenching Method”, USSR Patent #4176788/31-02, 01.06.1987. 16. Kobasko, N.I., “Technical Aspects of Quenching”, (in Russian), MiTOM, April 1991, pp. 2-8.