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MODELING AND EXPERIMENTAL STUDY OF QUENCHING PROCESS FOR
AISI 4340 AERONAUTICAL STEEL UNDER DIFFERENT
COOLING CONDITIONS
M. Sedighi and M. M. Salek
Department of Mechanical Engineering, Iran University of Science and Technology,
Tehran, Iran
Contact: [email protected]
Received April 2006, Accepted January 2008
No. 06-CSME-17, E.I.C. Accession 2936
ABSTRACT
In this paper, the hardness prediction and microstructures of AISI 4340 high strength alloy
under different cooling conditions are studied. The heat-transfer coefficient of specimens
quenched in water, oil, and air are very different. These different conditions were used in a code
to obtain cooling curves. Through the superimposition of the cooling curves on CCT diagrams,
the microstructural features and hardness were predicted. The results show that the
microstructure and hardness of the steel are significantly affected by quenching media. The
existent phases and hardness obtained from both the experiments and the modeling are in good
agreement. Finally, the cracking is reported, during water rapid cooling condition.
MODELISATION ET ETUDE EXPERIMENTALE DU PROCEDE DE
TREMPAGE D'UN ACIERAERONAUTIQUE AISI 4340 SOUS
DIFFERENTES CONDITIONS DE REFROIDISSEMENT
RESUME
Dans cet article, la prediction de durete ainsi que la microstructure d'un alliage a haute
resistance AISI 4340 dans differentes conditions de refroidissement sont etudiees. Les
coefficients de transfert de chaleur des specimens trempes dans l'eau, l'huile et l'air sont tres
differents. Ces differentes conditions etaient utilisees dans un code pour obtenir des courbes de
refroidissement. A travers la super-imposition des courbes de refroidissement sur un diagramme
de courbes de transformation en refroidissement continu, les caracteristiques de la microstructure
et la durete ont ete predites. Les resultats montrent que la microstructure et la durete de l' acier
sont affectees de fayon significative par Ie milieu de trempage. Les phases existantes et la durete
obtenues a partir d'experiences et de modelisations concordent. Finalement, la fissuration est
rapportee, durant Ie refroidissement rapide dans I' eau.
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1. INTRODUCTION
Quenching is one of the most common manufacturing processes widely used to improve
mechanical properties of steels. Effective control of cooling rate during the quenching of steels is
necessary for better mechanical properties, and also to prevent of cracking. In this process, the
surface of the part will be subjected to cooling by immersion rapidly into a fluid, normally by
either water or specially formulated oil.
The main purpose of quenching is to produce the martensitic phase; however steels still need
to be tempered after quenching to reduce the residual stresses without changing their basic
structures. Due to the demand for hardened steel parts, an understanding of the distortion
mechanism and residual stress development is necessary.
The kinetics of phase transformation can be modeled through a set of complex interactions
between thermal processes, mechanical properties, and phase transformation effects (1,2,3).
Numerical modeling of the various parts during the quenching process is usually employed as an
efficient tool to predict the strength of materials. In order to predict microstructures reliably, an
increased accuracy of mathematical modeling is therefore needed. Through the years, many
models have appeared in the literature; focusing, with varying detail or complexity, on the
physical phenomena involved in quenching (4,5).
Mathematical models of quenching process have been employed for nearly 30 years. A lot of
these studies are mainly concentrated on the heat transfer phenomena. The models are basically
aimed to predict the hardness and other mechanical properties (4,6,7,8). Buchmayr and Kirkaldy
(9) used continuous cooling transformation (CCT) diagrams and an isotropic hardening model to
predict the microstructure and to estimate residual stresses in a quenched cylinder specimen
made of low-alloy steel. Recently, many methods in modeling of the quenching in order to
predict temperatures, microstructure, and residual stresses have been studied (5). A finite element
commercial code has also been upgraded by the author to predict phase changes and residual
stresses in steels (1).
The mechanical properties of high strength steel have been subjects of researches. AISI 4340
aeronautical steel is used as a heavy duty steel in transmission in fixed and rotary wing airplanes.
For this steel, Lee and Su (10) experimentally studied the effect of tempering on the mechanical
properties of 4340. In the present work, the modeling and experimental results of 4340
quenching process have been compared.
In this work, A dedicated software for modeling of quenching process, AC3 (11), has been
employed to determine how the quenching material around the part influences heat transfer,
micro-structural properties and thus hardness. This program has many uses including:
computerized design of heat treatment processes and material selection, optimization of existing
treatment cycles, and reconstruction of the influences that an out of control process may have.
The process development in AC3 can be carried out in hours instead of weeks in a real
experiment and the final work made in the furnace will be closed to those predicted by the
software. In spite of a large number of applications, AC3 has some limitations such as modelling
of the quenching process in higher alloy steels.
2. THEORY AND EXPERIMENT
2.1. Different Cooling Regimes
In hardening process, the austenitization of carbon steel
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accomplished by heating the
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material into the appropriate temperature range, typically to 850°C for 4340 steel. Then, it is
rapidly cooled by immersion in water, oil, salt bath, or high pressure gas.
The quenchant media can be employed with agitation to cause different temperature fields
inside the specimen, which generated different types of phase transformation and variety of
microstructures, hardness, undesirable deformations, and residual stress distribution. Houghton
(12) showed a comparison of severity of quenching in air, oil, water and brine at different
conditions where the data were based on still water taken as unity.
Quenchant performance has been under investigation by many authors. Hernandez-Morales
(13) studied different types of quenching media. He presented a comparison between cooling in
air, oil, water, and brine in the range of 100-800'c .
100000
~ 10000
~
~
1000
8..
100
~
10
l::i"
~
~
«i
d!
o
1000
500
Temr:erature
'(J
Fig. 1: Estimated heat transfer coefficient vs. surface
temperature for water, oil, and air (13)
The extent and depth of hardening are a function of many variables such as cooling rate and
chemical composition of steel. To harden fully, some steels must be cooled very fast while some
can be quenched slowly.
2.2. Kinetics of Transformation
When a steel specimen cools from a high temperature, austenite transforms to ferrite, pearlite,
bainite, or martensite in the process of phase transformation. The kinetics of transformation is
complex and is generally modeled by using means of the time-temperature-transformation (TTT)
diagram which presents isothermal transformation behavior. Cooling curve can not be
superimposed on TTT diagram, but it can be superimposed on a continuous cooling
transformation (CCT) diagram which can be extracted from the TTT diagram data. Normally in
FEM, the element temperature-history from the thermal analysis is used to calculate the phase
fraction transformed for each element in each time step (14).
The diffusional transformation from austenite into ferrite, pearlite and bainite has two stages:
incubation and growth. In non-isothermal transformation the incubation period is said to be over
when the Scheil sum S reaches unity (15):
S=J~=f~=l
o ;reT)
;r(T;)
(1)
i=1
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where :r(T;) is the temperature-dependent isothermal incubation time and M; is the time
increment at step i. For the growth period, the relationship between the transformed volume
fraction and time has been proposed by Johnson and Mehl (14):
y = 1- exp(-bt n )
(2)
where y is the volume fraction, t is the time, and b and n are temperature-dependent constants
that can be obtained from the TTT diagram.
For martensitic transformation, the growth is modeled by Koistinen-Marburger equation (16):
y
= 1- exp[-a(M s -
T)]
(3)
where M s is the martensite start temperature and the coefficient a = 1.1 x 10-2 K- 1 for most
steels.
Thus, for each element, first of all the Scheil nucleation sum is calculated using equation (1),
with different Scheil values used for pearlite and bainite transformation as appropriate. If the
Scheil sum reaches unity, the parameters n and b from equation (2) are calculated from the TTT
diagram, and then this equation and equation (3) are used to compute the transformed phase
fractions, taking into account the available austenite and the temperature-dependent maximum
amounts of each phase.
2.3. Material and Experiment Details
AISI 4340 steel is one of the medium carbon steels with O.4%C, mainly for aeronautical and
high strength applications (17). The chemical composition ofAISI 4340 is shown in Table 1.
Table 1. Chemical composition
ofAISI4340
%C
%Mn
%Cr
%Mo
0.4
0.7
0.8
0.25
Three specimens of 20mm diameter x 100mm long 4340 cylindrical steel are used in the
experiments. The specimens were heated in furnace at 850 C for 30 min and subsequently
quenched vertically in air, oil, and water container without agitation. Because of decarburization
during the quenching, the specimens were cut by a wire cut into two 50 mm long cylindrical
parts and the hardness was measured in a few points at the middle of each cross section.
Tracing microstructural evolution to calculate the hardness is the most difficult and time
consuming part of the quenching analysis. This job was done and already discussed by the author
(1). Here the method used by AC3 has been explained in detail. AC3 models the hardness
variation of a given steel by calculating a cooling curve and superimposing it on a calculated
CCT diagram for the specified composition of the steel. It uses one dimensional analysis and
divides the specified section into 20 nodes, each one having a CCT diagram and a cooling curve.
Then it defines the CCT diagram by identifying strategic datum points. More than 600 CCT
diagrams (not including high alloy steels) were considered and statistical regression equations
0
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were formulated and used in evaluating the datum points. The cooling curves then will be
calculated, using thermal conductivity, heat capacity, quenchant temperature, and surface heat
transfer coefficient. Microstructure at a given point in the part is calculated mathematically by
superimposing the calculated cooling curve onto the calculated CCT diagram for that point. The
program divides the cooling curve into 54 equal intervals and evaluates the state of
transformation for each temperature/time point, determining whether the point is in a
transformation region and if so evaluating the percentage of transformation to each of ferrite,
pearlite, bianite, and etc. Then it calculates the highest percentage region through which the
curve has passed and takes it as the current state of transformation at the end of the step. The
metallurgical structure develops at each interval during the quench and the resulting structure at
ambient temperature will be presented. Finally AC3 calculates hardness based on the chemical
composition and cooling rate. The total hardness of a mixed microstructure is calculated using a
linear summation of the hardness of each microstructure.
3. RESULTS AND DISCUSSION
Table 2 shows the variations in the hardness in quenching with water, oil, and air without
agitation which have been obtained from modeling and experiments. The hardness values are
related to the surface of the specimen after quenching in both software and experiments. To
avoid the end effect the measurements have been done at the middle of the cylinder surface.
"AC3 and experiments
Table 2. Quenched WI"th d"ffi
1 erent quenc hant WI"thout agI"tati on In
Quenching
Hardness (RC)
Hardness
(RC)
experiment
material
AC3
water
56.0
54.0
54.0
oil
50.0
42.0
36.5
air
The CCT diagrams related to quenching analysis are presented in figures 2, 3, and 4. Due to
rapid cooling process in water and oil a full martensitic structure are formed.
1999
ac
T
E
M
P 599
E
R
A
T
U
R
E
1
19
199
1999
19999
TIME SECONDS
199999
Fig. 2: CCT diagram (AC3), water quenched
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1000 . . . - - - - - - - - - - - - - - - - - - - - - - - ,
°C
T
E
M
Fs
Ps
~ 500
R
A
T
U
R
Ms
lVIf
E
10
1
100
1000
10000
100000
TIME SECONDS
Fig. 3: CCT diagram (AC3), oil quenched
1000
°C
. = :.: ,;,;,::.
r---~
T
.
~
E
M
P 500
E
R
Ms
A
T
U
R
E
Mf
0
1
10
100
1000
10000
100000
TIME SECONDS
Fig. 4: CCT diagram (AC3), air quenched
According to the CCT diagrams, the cooling curves are crossing CCT diagrams differently
depending on the different cooling speeds. The cooling curves for water quenching at the surface
and center of the cylinder are quite different which can cause formation of high stress and
cracking (figures 2 and 8). The cracking occurring during quenching depends on not only items
such as size, shape, and chemical composition of the part, but also quenchant characteristics.
During the quenching, the temperature at the surface of the specimen falls more rapidly than the
interior's temperature. So the outer part of the specimen transforms to martensite; however, the
interior is still austenite. When austenite transforms, the increase in volume associated with this
transformation in the interior, induces tensile stresses in the exterior and when it exceeds the
failure stress, the cracking occurs. This statement is not always true, depending mi the
comparisons with water, oil quenching reduces temperature gradients and internal stresses due to
moderate time differences in phase volume dilatation at surface and center. This fact can be seen
in figures 2 and 3 for cooling curves at surface and center of the parts. Distortion and cracking
tendencies can be minimized in oil quenching while both water and oil can produce fully
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martensitic structure. The experimental results of water and oil quenching micrographs have
been shown in figures 5 and 6. Water quenching micrograph shows finer needle-like martensite
structure than oil quenching.
~}i,m,
L.-'J
Fig. 5 - Micrograph of the 4340 steel
specimen quenched in water
':~~ni
W
Fig. 6 - Micrograph of the 4340 steel
specimen quenched in oil
"5,!ifti
.0
Fig. 7 - Micrograph of the 4340 steel
specimen quenched in air
In air cooling condition, the transfonned structure presents a combination of martensite and
bainite. The cooling curve at the surface and core ofthe specimen are close together (figure 4). It
causes a unifonn micro-structure from surface to the core, as verified by the modeling results.
The modeling result proves this fact. From experimental result, figure 7 presents a feather shape
bainitic structure. While the modeling result shows about 85% bainite and 15% martensite
microstructure. This difference can be also seen in modeling and experimental hardness test
(table 2). There might be some islands of martensite within the bainitic structure. Distinguishing
these two from each other, especially when the volume fraction of martensite is low, is difficult.
These may imply somewhat an agreement between experimental result and that of the
simulation.
Regarding the difference between hardness values from experiment and simulation in oil
quenching, it seems that the software can not fully take into account the effect of alloying
elements on the martensite hardness. This could be an explanation for the minor difference
between experimental result and simulation (50 to 54 RC).
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4. CONCLUSION
Modeling of quenching process for AISI aeronautical steel under different cooling condition
has been studied. It shows that prediction of hardness and microstructure could be essential and
reliable step prior to any manufacturing process involving quenching process. The modeling
result for water and oil quenching has good agreement with experimental result in prediction of
microstructure and hardness. Some minor differences were observed in the prediction of exact
micro-structure for the air cooling.
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