TITLE:
A NOVEL ETCHING PROCESS FOR DETERMINING THE SIZE OF
AUSTENITE GRAIN IN LOW CARBON MICRO ALLOYED HIGH
STRENGTH STEELS.
FIELD OF INVENTION:
This invention relates to a novel etching process for determining the size of
austenite grain in low carbon micro alloyed high strength steel.
BACKGROUND OF THE INVENTION:
An austenite grain size influences the kinetics of phase transformation
during the cooling cycle. Thus accurate determination of the prior austenite
grain size of materials becomes of great interest. In this application, an
optimized method of revealing prior austenite grain size in low carbon
microalloyed high strength steels, just by using a single etchant, has been
discussed.
Prior austenite grain sizes are often required due to its great importance in
resultant microstructure and mechanical properties in steel. Thus the
accurate determination of the prior-austenite grain size (PAGS) of materials
becomes of the greatest interest. The procedures for the measuring of grain
size are well established and are reliable. The process of revealing prior

austenite grain boundaries depend on the chemical composition of the steel,
the prior heat treatment that the steel undergoes and some not so well-
identified factors.
Methods of delineating original austenite grains are chemical etching,
oxidation, thermal etching of grain boundaries and McQuaid-Ehn testing. In
recent years several investigations have been carried out in order to
determine PAGs in micro alloyed steels. This has resulted into the
development of various techniques to reveal the austenite grain boundaries
that can be categorized as following:
1) Chemical etchants are widely used etching reagents mainly based on
saturated aqueous picric acid in association with a wetting agent,
which appear to give best results in quenched and tempered steels.
2) PAGs can be revealed by the formation of proeutectoid ferrite or
cementite on the grain boundaries. For eutectoid steel, the austenite
grain boundaries can be outlined with pearlite. Further, the
carburization method by McQuaid and Ehn consists of decorating the
austenite grain boundaries by the proeutectoid cementite network.
3) Oxidation etching consists in heating a polished surface in an
oxidizing atmosphere to reveal grain boundaries by oxide
accumulation or by grain boundary decarburization.
4) Thermal etching method consists of heating up to the austenitizing
temperature of a polished surface in vacuum or in an inert atmosphere.

Grooves form at the grain boundaries by surface tension effects and
mass transport mechanism to define the grain boundaries.
However, there are disadvantages with thermal etching and McQuaid-Ehn
methods in that they require long furnace holding times, which may result in
austenite grain growth, and the results may misrepresent the prior austenite
grain (PAG) size. Thus, the chemical reagent etching technique has proven
to be a superior method because of its speed as well as reliability in
determining PAG size.
The etchants are extremely sensitive to the chemical composition along with
the prior heat treatment that the steels undergo. Thus, a single etchant is
generally not sufficient to reveal a range of grain sizes despite the fact that
the chemical composition of the steel is the same. Hence, it becomes time
consuming and cumbersome to reveal austenite grains if different etchants
are used for different heat treatments while the chemical composition of the
steel remains the same. Usually, higher the carbon, phosphorous and
niobium levels, better the etching response and it essentially becomes even
more difficult while the C and P contents are low.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a novel etching process for
determinating the size of Austenite grain in low carbon micro alloyed high
strength steel;

Another object of this invention is to propose a technique for the
determination of austenite grain size which is of great importance and has
large impact on the microstructure and metallurgical application.
Further object of this invention is to propose a process for ascertaining the
austenite grain size which influences the kinetics of phase transformation
during the cooling cycle;
Still further object of this invention is to propose an optimized method for
revealing austenite grain size in low carbon microalloyed high strength
steels by using a single etchant.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a novel etching process for
determining the size of austeinite grain in low carbon micro alloyed high
strength steel comprising:
subjecting the low carbon, low phosphorous microalloyed high strength steel
to the step of thermal cycling;
tempering the samples at around 500°C in a tube furnace to facilitate the
revealing of austenite grain;
grinding & polishing the tempered samples;
ascertaining the grain boundaries using an etchant, using the sample in hot
etchant and cleaning the sample with water.

DETAILED DESCRIPTION OF THE INVENTION:
In the present study a low carbon, low phosphorous microalloyed high
strength steel (C-0.05-0.06, Mn-1.55-1.65, S-0.002-0.003, P-0.007-0.008,
Si-0.11-0.13, Nb-0.029-0.035, Mo-0.20-0.25, A1-0.022-0.028, N-0.004-
0.005 and Ti-0.01-0.015) is used to reveal PAGS in the simulated heat
affected zone (HAZ). The PAG in the HAZ of a weldment controls the
resultant grain size after phase transformation and the mechanical properties.
Thus, it is essential to determine the austenite grain size in the HAZ.
After several trials with various etchants in different etching conditions, the
PAGs in the range of-3-55 micron have been distinctly revealed using just a
single etchant in optimized etching conditions. This is an exceptionally
unique etchant since till date no single etchant has been successful in
revealing a wide range of austenite grain sizes ranging from as small as ~3
micron to as large as -55 micron in low carbon steels.
Etching procedure- The thermally cycled samples were cut and tempered at
500°C in a tube furnace in the presence of argon for 24 hours to allow the
segregation of elements like P and S on the grain boundary to improve the
response of the grain boundaries to etching and to facilitate the revealing of
austenite grain boundaries. Subsequently, the through-thickness section of
the quenched and tempered samples was ground and polished using the
conventional metallographic technique. The austenite grain boundaries were
revealed using equal volumes of 4 percent of sodium dodecylbenzene

sulfonate in distilled water in the presence of 1-4 drops of Triton, a surface
active reagent and 100 ml of supersaturated aqueous picric acid solution.
After immersing the sample in the hot etchant, the same is thoroughly
cleaned under running water and subsequently dried and observed under
optical microscope. The time of etching is gradually increased and after
every few seconds the revelation of austenite grain boundaries are observed.
The process is repeated until the boundaries are revealed. However, extreme
attention must be paid towards the optimization of time. An excess of couple
of seconds may result over etching of the samples. The optimized
temperature and time for etching were between 62-68°C and 60-200s,
respectively. The etchant is quite temperature sensitive and thus the
temperature band obtained in the study is quite narrow to get satisfactory
result. This etchant could reveal the austenite grain size in the range -3-55
urn, which has till date not been observed using several etchants that are
used for revealing austenite grains. Figs 1 and 2 show the prior austenite
grain sizes revealed by the etchant for the thermal cycles of 10-1000°C/s
heating rate leading to 950-1350°C austenitizing temperatures, followed by
cooling rate of 2500-4500°C.

WE CLAIM:
1. A novel etching process for determining the size of austeinite grain in low
carbon micro alloyed high strength steel comprising:
subjecting the low carbon, low phosphorous microalloyed high strength steel
to the step of thermal cycling;
tempering the samples at around 500°C in a tube furnace to facilitate the
revealing of austenite grain;
grinding & polishing the tempered samples;
ascertaining the grain boundaries using an etchant, using the sample in hot
etchant and cleaning the sample with water.
2. The process as claimed in claim 1, wherein the samples are tempered at
500°C in a furnace in presence of argon for 24 hours to allow segregation of
elements like P & S.
3. The process as claimed in claim 1, wherein the said etchant comprises of
sodium dodecylbenzene sulfonate in distilled water in presence of a surface
active reagent and an aqueous acid solution.
4. The process as claimed in claim 3, wherein the surface active reagent is
Triton.
5. The method as claimed in claim 3, wherein the acid is aqueous picric acid
solution.

6. The process as claimed in claim 1, wherein the time of etching is
increased gradually after every few seconds till the austenite grain
boundaries are revealed.
7. The process as claimed in claim 1, wherein the optimized temperature and
time for etching is between 62-68°C & 60-200 seconds.

A novel etching process for determining the size of austeinite grain in low
carbon micro alloyed high strength steel comprising: subjecting the low
carbon, low phosphorous microalloyed high strength steel to the step of
thermal cycling; tempering the samples at around 500°C in a tube furnace to
facilitate the revealing of austenite grain; grinding & polishing the tempered
samples; ascertaining the grain boundaries using a hot etchant, and
subsequently cleaning the sample with water.