April 2004 - Forging Industry Association · 2016. 5. 16. · increase of the Ac1 and Ac3. The data...

MT-SRC-004-012

EFFECTS OF HEATING RATE AND PRIOR MICROSTRUCTURE

ON SAE 1541 FORGING STEEL

Submitted to:

Forging Industry Eductional and Research Foundation 25 W. Prospect Avenue, Suite 300

Cleveland, OH 44115-1000

Submitted by:

A.Shutts Advanced Steel Processing and Products Research Center

Colorado School of Mines Golden, Colorado 80401

April 2004

TABLE OF CONTENTS

Page INTRODUCTION 3 INDUSTRIAL RELEVANCE 3 PREVIOUS RESEARCH 3 MATERIAL 4 EXPERIMENTAL DESIGN 4 RESULTS 8 DISCUSSION 15 POSSIBLE FUTURE WORK 15 CONCLUSIONS 15 ACKNOWLEDGEMENTS 16 REFERENCES 16

2

EFFECTS OF HEATING RATE AND PRIOR MICROSTRUCTURE ON SAE 1541 FORGING STEEL: by Adam

Shutts

Introduction

This project was initiated in May 2003 as summer research for Adam Shutts, an undergraduate student in

Metallurgical and Materials Engineering. The purpose of this project was to investigate the effects of heating rate on

1541 steel as pertinent to forging related processes. Heating rates, varying over several orders of magnitude, were

applied to two vastly different prior microstructures: as quenched martensite and coarse pearlite. These heating rates

were applied and measured using a Gleeble 1500 thermo-mechanical testing machine. Each specimen provides data

consisting of prior austenite grain size, presence of undissolved carbides and retained ferrite, and hardness. This project

is linked to Jason Coryell’s project on induction heating which makes use of the same material in applications of

induction heating.

Industrial Relevance

This project was identified in response to research objectives set forth by the FIA Technical Committee in the

October 30, 2001 document entitled “Forging Industry Educational and Research Foundation Magnet School Student

Project Ideas.” The two titles selected were “Develop a matrix of where phase change occurs at different heating rates

for a given alloy” and “Quick normalizing techniques.” These projects both involve issues related to microstructural

evolution during heating.

Industrial use of rapid austenitization processes is becoming more prevalent due to economic benefits versus

conventional furnaces. Quantitative knowledge is needed to refine the processes. For some induction heated forgings,

practices should avoid undissolved carbides and soft microstructures in the final product. The austenite grain size is

also of importance to the final structure/properties of the forging (4).

The two prior microstructures selected, coarse pearlite and as quenched martensite, were chosen to represent

two extremes in the microstructural spectrum, particularly related to expected carbide dissolution and austenite grain

growth kinetics. Comparison of these two microstructures should demonstrate the range of effects of prior

microstructure in rapid heating.

Previous Research

K.C. Smith has used similar laboratory practices at CSM in past programs. Smith examined the heating

response of a 1070 steel using similar procedures as planned for the present research. Dilatometry provided a

temperature associated with the phase changes. An inverse relationship between heating rate and prior austenite grain

size was reported. Limits on heating rate were defined for lamellar cementite dissolution in both microstructures used

by Smith (2). Other pertinent work has also been done at CSM. T.J. Favenyesi performed induction case hardening of

1541 steel as applied to fatigue response. The heating rate used by Favenyesi corresponds with the higher heating rates

being planned for this project (3).

3

Similar studies were reported by Feuerstein and Smith in 1953. Heating rate, prior microstructure, and alloy

were examined. Feuerstein and Smith used different degrees of tempering on their quenched steels and measured the

increase of the Ac1 and Ac3. The data show that the critical temperatures increase with heating rate, in some cases as

much at 120° C. Coarser microstructures exacerbate this effect. Annealed 4130 showed an increase of 120° C while

4130 tempered at 400° F showed an increase of only 65°C (5). Another researcher, Chen, studied the effect of heating

rate on the Ac1 as applied to laser surface hardening. With heating rates approaching 1000°/sec Chen reported an

increase of 47°C (6).

Extensive work has also been done at the Heinz-Joachim Spies Freiberg University of Mining and Technology

in the Institute of Materials Engineering at Freiberg, Germany by Johannes Rodel. This work focused on austenite

formation in spherodized microstructures, which were surface hardened through the use of laser or electron beams.

Extensive mathematical modeling was used to predict the austenite formation kinetics. Rodel indicates that rapid

heating raises the austenite finish temperature. He also makes the assertion that diffusion controls the rate at which the

austenite may form. Alloy elements, mainly carbon, must be given adequate time to diffuse into the austenite in order

for it to form completely. Rodel’s heating rate of 1000 K/s matches the highest heating rate applied in the present study.

One difference between Rodel’s work and the work done here at CSM is the material evaluated. Rodel’s work and

much of the other work found on laser hardening was performed on a variety of steels from plain carbon to high alloy

compositions with chromium and molybdenum (7).

Material

The steel being used is SAE 1541 and was received from North Star Steel in 1.259-inch diameter 4-foot long

bars. The steel was hot rolled, consisting roughly of a 10% ferrite- 90% pearlite microstructure. The composition of the

as-received steel is listed in Table 1 (4).

TABLE 1 – Chemical Composition Of Selected Steel (4)

Steel C Mn P S Si Cu Ni Cr Mo V Al Ti Nb 1541 0.39 1.444 0.01 0.024 0.27 0.119 0.05 0.09 0.01 0.005 0.024 0.003 0.001

Experimental Procedure

Bandsaw Services reduced the 1.259-inch diameter bars into .25-inch square rods 12 inches in length as is

illustrated in Figure 1a. The steel was then heat treated in the CSM physical metallurgy lab to attain quenched

martensite and coarse pearlite microstructures. The pearlite samples were held at 900°C for 35 minutes and then were

allowed to furnace cool to room temperature in approximately 12 hours (Fig. 2a). The martensite samples were held at

900°C for 35 minutes and then quenched in oil (Fig. 2b). The samples were ground to 3mm diameter cylinders and cut

into 70mm lengths as is illustrated in Figure 1b.

4

A) B)

Figure 1 Schematic cross-section of machining operations; (A) Bandsaw reduction of 1.259 in. samples to .25

in. square rods. (B) Final form for Gleeble test, after grinding; 3mmx70mm.

5

B

Figure 2 Lig10

A)

10 µm

)

20 µm

ht optical micrographs of 1541 steel with 2% nital etch, (A) Pearlitic microstructure taken at 00x (B) Martensitic microstructure taken at 500x.

6

The samples were then heat treated in the Gleeble tester to simulate heating as applied in forging or final heat

treatment. Copper jaws of the prescribed size were already available to hold and cool the ends of the sample from

Smith’s previous work. Temperature monitoring and current delivery were executed with a type-K thermocouple

welded to the center of the sample. An attempt was made to use dilatometry to measure expansion and contraction

during heating in order to deduce the transformation temperature. This attempt was not successful due to small cross-

sections and a dilatometer malfunction, which was discovered much later. Heating rates tested ranged from

1.0°C/second to 1000°C/second, incremented by orders of magnitude. Each of these five rates was carried out to 4

different maximum temperatures ranging between 900°C-1200°C. After heating, the samples were quenched by a

direct spray of argon gas to the center of the sample. Cooling rates of approximately 200°C/sec were attained by this

method and martensite was formed throughout the cross section, where austenitized. Sample Gleeble data can be seen

in Figure 3.

0 40 80 120

Time (sec)0

400

800

1200

Tem

pera

ture

(C)

Target temperature profileActual temperature profile

Figure 3 Sample heating and cooling curve for Gleeble sample heated at 10 C/s to 1000 C and argon gas

quenched.

After the samples were resistance heated and quenched, metallography was performed. The samples were

cross-sectioned and mounted in Bakelite, ground with SiC paper to 600 grit, and polished with six and one micron

diamond solution. The prior austenite grain size was evaluated using an etchant composed of 100ml picric acid, 2ml

Teepol, and two drops of hydrochloric acid. This mixture was gently boiled using a hot plate and samples were

7

immersed for 35 seconds and then back polished with a cotton ball. These two steps were repeated until the austenite

grain boundaries were easily visible. Statistical grain counting methods were then employed by placing circles of

known circumference over a micrograph at 100 and 200x. Intersections with grain boundaries were counted for three

cross-sections and the magnification was incorporated in the calculation of the average austenite grain size (4).

Hardness profiles were taken from the center of the samples out to the edge with a Vickers Microhardness tester.

Conversion to Rockwell C was presented in another display on the tester. The presence of undissolved carbides and

retained ferrite was assessed by light microscopy after a 2% nital etch for 35 seconds.

Results

The results of this experiment are summarized in Tables 2 and 3 and Figures 5 through 11. The data show the

presence of “retained ferrite” and undissolved lamellar carbides (i.e. “ghost pearlite”) at heating rates between 100°/s

and 1000°/s for maximum temperatures under 1100°C with a coarse pearlite starting microstructure. These phenomena

provide clear evidence of incomplete austenitization under these conditions. Examples of “retained ferrite” are shown

in Figure 7. Hardness tests on these areas as well as “ghost pearlite” regions show a marked decrease in hardness as

presented in Table 2, with values around 50 HRC compared to about 58 HRC in the fully austenitized regions. The

number of these zones is observed to increase as heating rate is increased and maximum temperature is decreased.

Figure 4 shows the variables of heating rate and maximum temperature plotted against each other. This figure denotes

the “boundary” that was defined for complete austenitization in this process. A high presence is defined here as

retained ferrite and undissolved carbide being visible in every view field below 500x magnification, medium presence is

defined as easily resolvable at 500x magnification though not a dominant structure; low presence is only discernable at

1000x magnification with extensive searching required. Figures 8 and 9 show a completely austenitized version of this

starting microstructure (i.e. uniformly martensitic) as a contrast to Figure 7.

The martensitic starting microstructure provided a fully austenitized microstructure in all instances examined

in this work. From the micrographs presented in Figures 10 through 12, large martensite packets are observed after

slow heating rates with high maximum temperatures. There is significant “alignment” and much greater length to the

martensite laths in these samples. At high heating rates and lower maximum temperature, a very fine microstructure is

found, and prior austenite grains were not easily resolved.

Prior austenite grain size for all samples tested is quantified in Table 3. There is an inverse relationship

between heating rate and prior austenite grain size. Higher maximum temperatures also resulted in larger austenite

grain size. Plots of the prior austenite grain size versus maximum temperature are shown in Figures 5 and 6 for the

applied heating rates. The trends pertaining to heating rate and maximum temperature are clearly apparent in these

figures. In most instances, the martensitic starting microstructure was associated with somewhat finer austenite grains

after subsequent heating, in comparison to the pearlitic starting microstructure.

8

800 900 1000 1100 1200Maximum Temperatures (C)

Hea

ting

Rat

e (C

/s)

1

10

100

1000

Legend: Presesnce of Retained Ferrite and "Ghost Pearlite"

High

Medium

Low

None

Boundary of Complete Austenitization

Figure 4 Plot of maximum temperature vs. heating rate, denoting the boundary for complete austenitization of the pearlite starting microstructure.

9

TABLE 2 – Hardness Data Matrix for Pearlitic Starting Microstructure

Maximum Temperatures

Heating Rates 900 C 1000 C 1100 C 1200 C

1000 C/Sec

Avg. HRC: 56.2 Retained ferrite:

48.3 HRC Ghost Pearlite:

50.2 HRC

Avg. HRC: 56.1 Retained ferrite:

51.2 HRC Ghost Pearlite:

51.3 HRC

Avg. HRC: 58.5

Avg. HRC: 58.2

100 C/Sec

Avg. HRC: 56.3Retained ferrite:

49.3 HRC Ghost pearlite:

50.4 HRC

Avg. HRC: 58.4

Avg. HRC: 58.2

Avg. HRC: 58.6

10 C/Sec

Avg. HRC: 56.5

Avg. HRC: 57.7

Avg. HRC 57.0

Avg. HRC: 57.8

1 C/Sec

Avg. HRC: 59.1

Avg. HRC: 57.4

Avg. HRC: 58.2

Avg. HRC: 57.3

TABLE 3- Prior Austenite Grain Size Matrix For Martensitic and Pearlitic Prior Microstructures (Average Diameter)

Maximum Temperature

Heating Rates 900 C 1000 C 1100 C 1200 C

1000 C/Sec

Martensite PAGS: 3.8 µm

Pearlite PAGS: 7.0 µm







100 C/Sec

Martensite PAGS:4.7 µm


Martensite PAGS: 13.8µm






10 C/Sec









1 C/Sec


Pearlite PAGS: 11.2µm







10

900 1000 1100 1200

Maximum Temperature (C)0

20

40

60

80

Ave

rage

Dia

met

er o

f Prio

r Aus

teni

te G

rain

Siz

e(m

icro

met

ers)

1000 C/s100 C/s10 C/s1 C/s

Incr

easi

ng H

eatin

g R

ate

Figure 5 Prior austenite grain size plotted against maximum temperature for applied heating rates. Starting

microstructure: martensite.

900 1000 1100 1200

Maximum Temperature (C)0

20

40

60

80

100

Aver

age

Dia

met

er o

f Prio

r Aus

teni

te G

rain

Siz

e(m

icro

met

ers)

1000 C/s100 C/s10 C/s1 C/s

Incr

easi

ng H

eatin

g R

ate

Figure 6 Prior austenite grain size plotted against maximum temperature for applied heating rates. Starting microstructure: pearlite.

11

Figure 7 Pearlite heated at 1000°C/sec to a max T of 900°C and immediately quenched by Ar gas. 500x magnification. Arrow denotes retained ferrite.

Figure 8 Pearlite heated at 1000°C/sec to a max T of 1200°C and immediately quenched by Ar gas. 500x

magnification. Complete austenitization and small prior austenite grains are present.

12

Figure 9 Pearlite heated at 1°C/sec to a max T of 900°C and immediately quenched by Ar gas. 500x magnification. Complete austenitization and large prior austenite grain size are evident.

Figure 10 Martensite heated at 1000°C/sec to a max T of 900°C and immediately quenched by Ar gas. 500x magnification. Small prior austenite grain size is evident.

13

Figure 11 Martensite heated at 1000°C/sec to a max T of 1200°C and immediately quenched by Ar gas. 500x

magnification. Large prior austenite grain size is evident.

Figure 12 Martensite heated at 1°C/sec to a max T of 1200°C and immediately quenched by Ar gas. 500x

magnification. Large prior austenite grain size is evident.

14

Discussion

Austenite nucleates at interfaces. In the case of pearlite this means that nucleation occurs at ferrite-carbide

boundaries. The austenite then grows into a ferrite or pearlite grain. If insufficient time is allowed for this growth to

occur there will be retained ferrite. Similarly, if austenite formed by a massive transformation without substantial

carbon enrichment during heating, ferrite formation during cooling might occur readily. Such “retained ferrite” can be

seen clearly in Figure 7. Austenite is usually more carbon enriched than the ferrite it replaces. The source of this

carbon is carbides. In the case of pearlite these carbides are in the form of lamellae (Fig. 2a). Regions of retained

ferrite or undissolved carbide exhibit much lower hardness as can be seen in Table 2. These “soft spots” are a clear

disadvantage in a microstructure that is supposed to be hardened. Incomplete austenitization would be undesired for

either forging or final heat treatment. If the entire microstructure is not austenitic, as in some of the samples run in this

study, then the part might not be forged homogenously and the final product may contain uneven residual stresses.

When extremely high heating rates are involved the austenitization temperature may be increased. On the

other hand, rapid heating and lower temperatures provide finer austenite grains. While recrystallization occurs during

and after forging, a finer initial grain size is likely to carry through to the final as-forged microstructure (6). As can be

seen in Table 3 there are clear relationships between prior austenite grain size and microstructure/processing variables

examined. Growth of austenite requires time; this pertains to the heating rate evolved. A sample heated at 1 degree per

second allows a much greater time for growth than a sample heated at 1000 degrees per second. This trend is

discernable in the data by the larger grain sizes at lower rates.

The tendency for smaller prior austenite grains to for in the martensitic starting microstructure might be

explained by the greater amount of nucleation sites associated with this microstructure. Nucleation of austenite occurs

at interfaces. As can be seen in Figure 2 pearlite has far fewer interfaces than martensite. A greater amount of

nucleation sites would correspond to greater numbers of grains and thereby a smaller average grain size.

Possible Future Work

Future work might consider alloy steel and microalloyed steels. In such steels, carbide dissolution during

heating prior to forging may have particular relevance. Also with a functional dilatometer available, further testing may

be done to quantify the temperatures at which austenitization occurs for the different heating rates employed. Some

interesting mechanistic questions remain – i.e. whether the ferrite was truly “retained” during heating, or may have

transformed from low-carbon austenite.

Conclusion

This study revealed several important results that are relevant to the forging and heat-treating industry. This

study systematically studied the heating response of steel in relation to heating rate and maximum temperature (i.e.

processing “boundaries”). Prior austenite grain size was measured and related to heating rate and maximum

temperature. Retained ferrite and undissolved carbide lamellae were found and limits where they exist were roughly

determined. The influence of this phenomenon was also confirmed through hardness testing, indicating a major

influence on microstructure and properties.

15

16

Acknowledgements

The support of the Forging Industry Educations and Research Foundation, and the Advanced Steel Processing

and Products Research Center are appreciated, along with the contributions of Drs. Speer, Matlock, Van Tyne, and

Jason Coryell.

References

1. Smith, K.C., Van Tyne, C.J., and Robino, C.V., “Effect of Heating Rate, Composition, and Prior

Microstructure on Austenite Formation in Medium Carbon Steels”, ASPPRC Bar and Forging Report, Section 5.0, September 1, 2002.

2. Smith, K.C., A Microstructural Analysis of Rapid Austenite Formation in Induction Hardened Steels, M.S.

thesis, Colorado School of Mines, Golden, CO, February, 2001. 3. Favenyesi, T.J, The Effect Of Prior Microstructure On The Induction Hardening Response And Fatigue

Performance Of Medium-Carbon Steels, M.S. thesis, Colorado School of Mines, Golden, CO, September 1, 1998.

4. Coryell, J.J., Matlock, D.K., and Speer, J.G., “Effects of Induction Processing on the Mechanical Properties of

Steels With Controlled Microstructures”, ASPPRC Bar and Forging Report, Section 5.0, March 1, 2003. 5. Feuerstein and Smith, “Elevation of Critical Temperatures in Steel by High Heating Rates”, Transactions of the

ASM, October 1953. 6. Chin-Cheng Chen, Chun-Ju Tao, Lih-Tyan Shyu, “Eutectic Temperature of Carbon Steels During Laser

Surface Hardening”, Journal of Materials Research, Vol.11, No. 2, 1997.

7. Rodel, J.; Spies, H.-J. “Calculation of Temperature Fields and Austenite Formation During Electron Beam Hardening of Steels.” In M. Jeandin, T.S. Sudarshan, editor, SMT 8, Proc. of 8th. Int. Conf. on Surface Modification Technologies, Nice, 26-28.9.1994, pages 638-650. The Institute of Materials, (1995).

8. Personal Communication, Dr. Chester VanTyne, Colorado School of Mines, 2003-2004.

April 2004 - Forging Industry Association · 2016. 5. 16. · increase of the Ac1 and Ac3. The data...

Documents

Transcript of April 2004 - Forging Industry Association · 2016. 5. 16. · increase of the Ac1 and Ac3. The data...