The document summarizes an experiment that investigated the effect of austenitizing temperature and holding time on grain size and hardness in 4140 steel. Samples were heat treated at 900°C, 1000°C, and 1100°C for 1, 4, and 9 hours, followed by water quenching. Testing found that grain size increased and hardness decreased with higher temperature and longer time, consistent with standard grain growth models. Activation energy calculations found grain growth was driven by carbon diffusion in austenite. Overall, the study demonstrated that austenitizing conditions significantly influence prior austenite grain size and resultant hardness in 4140 steel.

1. The influence of temperature on the Austenite grain size and microstructure of
4140 steel
Rui Zhang and Richard D. Sisson, Jr
Center for Heat Treating Excellence, Worcester Polytechnic Institute, Worcester, Massachusetts, 01609, U.S.A
Abstract
Grain growth during heat treatment can affect mechanical
properties. A large grain size can result in a lower strength and
susceptibility to brittle failure. In order to control the prior
Austenite grain size, the effect of Austenitizing temperatures
and holding times on the grain size and hardness in 4140 steel
was experimentally investigated. Samples were heat treated at
900℃, 1000℃ and 1100℃, and held for 1, 4, and 9 hours. After
austenitizing, samples were cooled in the furnace to 850℃
before they were quenched in water at room temperature. Each
sample was cut, mounted and polished. Rockwell hardness and
micro-hardness tests were performed on each sample. A Picric
etch was used for grain size analysis. E112 Standard Test
Methods for Determining Average Grain Size was followed to
measure the grain size. It was found that the prior Austenite
grain size increased with temperature and time according to the
standard grain growth model, it was also found that the as-
quenched hardness decreased with an increase in grain size.
Introduction
Hypoeutectoid steel refers steels with less than 0.8% C which
can contain ferrite and pearlite microstructure. With an increase
of carbon, the portion of carbide increases which cause an
increase of hardness, strength and a decrease of ductility and
weldability.
The experimental results [1]
have shown a significant influence
of austenitizing temperature and time on grain size and hardness
changes, for hypoeutectoid steels. In this experiment, RSt 37-2
steel which has a carbon concentration less than 0.17% was
utilized. Its austenite temperature starts at 760℃, and austenite
transformation end is 895℃. The result of measured austenite
grain size have shown that the smallest grain size is obtained by
austenitizing temperature close to 895 ℃ with short
austenitizing time. It is also demonstrated that the temperature
increase over 895℃ and increase of Austenitizing time have a
significant influence on the grain growth. The results of
measured hardness have shown that the highest values
correspond to the sample with the smallest grain size. With
grain growth the hardness decreases.[1]
Experimental procedure and analysis
Sample preparation
All the samples were subjected to the following laboratory
preparation and tests including heat treating, cutting, mounting,
and polishing, hardness and micro-hardness testing, etching,
metallography, grain size as well as decarburization inspection.
Heat treatment (Austenitizing)
The main goal of the Austenitizing process is to remove the
inhomogeneity in the structure and to form a uniform alloy
concentration. The influence of the austenitizing process
largely depends on the heat treatment parameters, including:
heating rate, austenitizing temperature, holding time and
cooling rate.
Table 1: Chemical composition for AISI 4140 steel
4140 %Cr %Mo %C %Mn %P %S %Si %Fe
0.90 0.169 0.40 0.93 0.012 0.020 0.251 96.7
Samples from Bar stock of AISI 4140 were heat treated in air
at three different austenitizing temperatures 900℃, 1000℃ and
1100℃, each temperature has three holding times 1, 4, and 9
hours. After austenitizing, all the samples were cooled in the
furnace for approximately 30 minutes to 850℃ before they were
quenched in water (23℃ stagnant) at room temperature. The
goal of this experiment is to determine the influence of different
temperatures, different austenitizing holding time and their
interactions on the changes of the prior Austenite grain size and
as-quenched hardness.
Table 2: Design of experiment
Austenitizing
temperature ℃
900 1000 1100
Holding time (H) 1 4 9 1 4 9 1 4 9
Type of test -Rockwell hardness test HRC and Vickers hardness test
-Determination of the grain size by ASTM E112 method

2. Fig. 1 Heat Treating process--900℃, 1000℃, and 1100℃
Time and Temperature
Rockwell hardness
Table 3: Average As-Quenched Rockwell hardness at the surface
9H 4H 1H
1100℃ 28.2 29.55 39.05
1000℃ 46.9 50.9 52.4
900℃ 47.7 51.52 56.9
Fig. 2 Rockwell Hardness
According to the hardness profile, we observe that with the
temperature increase, the hardness decrease, at the same time,
with the holding time increases, the hardness decreases, so we
conclude, a higher temperature with a longer furnace holding
time results in a lower hardness value.
Vickers hardness
Vickers hardness is one of the most popular micro-hardness test
methods, The Vickers hardness (HV) number is obtained by
dividing the applied load in kilogram-force by the surface area
of the indentation. The area of the indentation produced from
the Vickers square-based pyramidal diamond is determined by
the distance between the two diagonals of the indentation. The
unit of hardness given by the test is known as the Vickers
Pyramid number (HV) or Diamond Pyramid Hardness (DPH).
The test load of this experiment is 200gf
Table 4 Vickers Hardness
9H 4H 1H
1000℃ 606.4 591 537.6
900℃ 606.5 600 545.7
Fig. 3 Vickers Hardness
According to the micro-hardness profile, we can see a good
correlation among the hardness, temperature and the holding
time, and from the curve, we can observe a decrease of its
micro-hardness with the increasing temperature and holding
time.
Fig. 4. Vickers hardness profile for 1000℃, the decrease near
the surface is a result of decarburization
Fig. 5. Vickers hardness profile for 900℃, the decrease near
the surface is a result of decarburization
Fig. 6 1100℃ 9H 4140 decarburization surface

3. According to the micro-hardness profile from the specimen
(decarburization) surface perpendicular to the core, we observe
the decarburization surface has the lowest micro-hardness, and
its micro-hardness increase with the increase of its depth. The
microstructure of the 1100℃ heat treatment revealed excessive
decarburization throughout the sample.
Microstructure analysis
To observe the microstructure, the most commonly used etchant
for steel is 2% Nital [2]
. Martensite is formed in carbon steels
by the rapid cooling (quenching) of Austenite.
Fig. 7 As received 4140 steel
Fig. 8 900℃ 1H 4140 water quench from 850℃
Fig. 9 1000℃ 4H 4140 water quench from 850℃
Fig. 10 1100℃ 9H 4140 water quench from 850℃
Fig. 7 optical photomicrograph showing a ferrite and pearlite
structure of as received 4140 steel.
Fig.8, 9 and 10 presents the Martensitic microstructure of 4140
with heat treatment at 900℃ holding time 1H, at 1000℃
holding time 4H and at 1100℃ holding time 9H, respectively.
Acicular Martensite is seen in these microstructure.
Grain size analysis
For many materials grain size can be directly related to their
mechanical properties. Although grain size is actually a
3-dimentional property, it is measured from a 2-dimensional
cross section of the material. Picric Acid etchant was utilized to
reveal the prior Austenite grain size of 4140 samples. After
etching, Planimetric (or Jeffries’) Procedure was used for the
Determination of the Average Grain Size. [3]
Fig. 11 4140 steel (as received) with picric etch

4. Fig. 12 900 ℃ 4H 4140 water quench from 850℃
Fig. 13 1000 ℃ 1H 4140 water quench from 850℃
Fig. 14 1100 ℃ 9H 4140 water quench from 850℃
Table 5 Grain size measurement
Grain Growth kinetics
Grain Growth kinetics: Dm
- D0
m
=Kt and K= K0 𝑒
−𝑄
𝑅𝑇 , D is the
final average grain diameter and D0 is the initial grain diameter
at t = 0, m is the time exponent for grain growth and K is rate
constant. Both m and K depend on the material. where m=2 for
the diffusion controlled grain growth, t is the holding time, K
and D0 can be determined and used to find the activation energy
for grain growth. [4] [5]
Fig. 15 Average grain diameter versus temperature at
900 ℃, 1000℃ and 1100℃
Fig. 16 Average grain diameter squared versus temperature at
900 ℃, 1000℃ and 1100℃
Sample Number
ASTM grain size
number G
D average diameter
(mm)
900 1 H 4140 6.59 0.0318
900 4H 4140 6.28 0.0378
900 9H 4140 5.72 0.0449
1000 1H 4140 4.68 0.0635
1000 4H 4140 4.35 0.0755
1000 9H 4140 3.8 0.0898
1100 1H 4140 3.87 0.0898
1100 4H 4140 3.6 0.1068
1100 9H 4140 3 0.127

5. Fig. 17 Log K versus 1/T
According to Fig.16, we can see at 900℃, K=0.0001,
D0
2
=0.0009, at 1000 ℃, K=0.0005, D0
2
=0.0036, and at
1100℃, K=0.001, D0
2
=0.0072. We can observe an increase in
both initial grain size D0, as well as the constant K with the
increasing temperature.
K=K0 𝑒
−𝑄
𝑅𝑇
Log K= log K0 +
1
𝑇
. (
−𝑄
2.3𝑅
)
A plot of Log K versus 1/T is presented in Fig. 17.
The slope of the line was determined to be -8119.9. The
activation energy Q was calculated to be 155.3KJ/mol. The
activation energy is similar to the activation energy for the
diffusion of carbon in Austenite [6]
. And we conclude that the
diffusion of carbon in Austenite may be responsible for the
grain growth.
SEM analysis
Fig. 18 4140 900℃ 1h 5000X with picric etch (left)
Fig. 19 4140 900℃ 1h 20,000X with picric etch (right)
Fig. 20 4140 1100℃ 1h 5000X with picric etch (left)
Fig. 21 4140 1100℃ 1h 20,000X with picric etch (right)
Fig. 22 4140 1100℃ 4H 1500X Nital + Picric etch (left)
Fig. 23 4140 1100℃ 4H 20000X Nital + Picric etch (right)
Fig. 24 4140 900℃ 4H 10000X Nital + Picrical etch (left)
Fig. 25 4140 900℃ 4H 14000X Nital + Picrical etch (right)
According to the SEM images, with EDS, nitrides and carbides
are detected within the grain boundaries, a further study will be
performed to determine the chemical composition of the
particles at the grain boundary.
Conclusions
A higher Austenitizing temperature with a longer furnace
holding time result in a larger grain size as well as a lower
hardness.
References
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parameters on the grain size of hypoeutectoid steel." 7th
International Scientific Conference RIM 2009
Development and modernization of production. Hrvatska
znanstvena bibliografija i MZOS-Svibor, 2009.
[2] Maity, Joydeep, and Dipak Kumar Mondai. "Isothermal
grain growth of austenite in hypoeutectoid and
hypereutectoid plain carbon steels." Journal of Iron and
Steel Research, International 17.7 (2010): 38-43.
[3] ASTM E-112. (2010). Standard test methods for
determining average grain size
[4] Xu, Yaowen, et al. "Prediction model for the austenite
grain growth in a hot rolled dual phase steel." Materials &
Design 36 (2012): 275-278.
[5] Khzouz, Erik. "Grain Growth Kinetics in Steels." A Major
Qualifying Project Report Submitted to the Faculty of the
WORCESTER POLYTECHNIC INSTITUTE Project
Number: RDS 21381 (2011).
[6] Lee, Yun Tack, et al. "Temperature-dependent growth of
carbon nanotubes by pyrolysis of ferrocene and acetylene
in the range between 700 and 1000 C." Chemical physics
letters 372.5 (2003): 853-859.