24-12-2012, 04:11 PM
Effect of rotational speed on the interface properties of friction-welded AISI 304L to 4340 steel
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Abstract
The aim of this study is to investigate experimentally the interface properties in terms of rotational speed in friction-welded AISI
304L to AISI 4340 alloy steel. Friction welding was conducted with five different rotational speeds using a direct-drive type friction
welding machine. Friction pressure, forging pressure, friction time and forging time are fixed. The integrity of joints was investigated
by scanning electron microscopy, while the mechanical properties assessments included microhardness and tensile tests. The experimental
results showed that the thickness of full plastic deformed zone (FPDZ) formed at interface reduce as a result of more mass
discarded from the welding interface with increase of the rotational speed. It was also observed that the width of the FPDZ has a
important effect on the tensile strength of friction-welded samples and the tensile strength increases with increase of the rotational
speed.
Introduction
The austenitic stainless steels are generally considered
the easy weldable of stainless steels [1]. Because of their
physical properties, their welding behavior may be considerably
different than those of the ferritic, martensitic, and
duplex stainless steels [2]. It has been generally noted that
the austenitic chrome-nickel stainless steels containing 12–
25% Cr and 8–25% Ni are the most widely used for corrosion
resistant applications and vessels, tubing, wire, medical
and dental devices materials [3–5]. The ability to join
austenitic steels itself and to other materials with conventional
fusion welding process such as gas tungsten, laser,
electron beam welding opens up the possibility to product
unexpected phase propagation and a series negative metallurgical
change such as delta ferrite phase, grain boundary
corrosion, strain corrosion and sigma phase occurs at the
welding interface. Therefore, extensive care and precautions
like pre and post heat treatment or quick welding
speeds are required [5–9]. And also the other joining problem
on these type steels with fusion welding that long waiting
time which lead to grain boundary corrosion between
grains and probably to be arisen chrome-carbide precipitate
at 450–850 C of some steels particularly consisting
Cr–Ni such as 18/8 [9,10].
Results and discussion
Microstructure characteristics in the interface zone of
friction welded joints
Microstructural evaluation of friction-welded joints
revealed four distinct zones across the specimens which
were identified as parent metal (PM), partial deformed
zone (PDZ), deformed zone (DZ) and transformed
and recrystallized fully plasticized deformed zone
(FPDZ) (see Fig. 3). Typical microstructures in various
regions of the weld across the interface are shown in
Fig. 4. It was observed that rotational speed influenced
the weld region geometry and width. The most microstructural
changes took place in the FPDZ and DZ region.
High rotational speed causes local heating at
interface to reach high temperature at short time.
Although this condition causes lower cooling rates and
wider heat affected zone (HAZ), high rotational speed
leads to narrower FPDZ due to a greater volume of
viscous material transferred out of at the interface. It
is well known that when pressure is used to bring joint
pair together by plastic deformation results in dynamic
recrystallization leading to a grain refinement in the central
region of the weld [9]. The effect of increasing rotational
speed over the friction welding joint is that both
temperature gradient and axial shortening increasing
as a result of more mass is transferred out of at the welding
interface (Figs. 5 and 6).
Microhardness distributions
Fig. 10 shows the microhardness distribution in the
direction perpendicular to the weld interface of
friction-welded joints. As can be seen from this figure,
almost the similar trend is observed in the microhardness
profiles of all samples. The hardness of the FPDZ
and PDZ increases with increasing rotational speed.
The increasing hardness in the welding interface can
be related directly to the microstructure formed in the
welding interface as a result of the increasing heat input
and plastic deformation. The plastic deformation causes
a decrease in the grain size which leads to hardening in
the region of the welding interface. The increase in the
hardness values in the PDZ of AISI 304L stainless steel
could be attributed to the work hardening of the austenitic
stainless steel. Although the work hardening is also
effective on hardening in AISI 4340 steel side, mostly
the hardening in this region is a direct result of the rapid
cooling from the welding temperature. This leads to a
full martensitic microstructure in the PDZ and FPDZ
(see Fig. 9(d) and (e). Additionally, the width of PDZ
varies according to rotational speed.
Conclusion
This study investigates joining performance of friction-
welded AISI 304L/AISI 4340 steel. Based on the results
of microstructure analysis, hardness and tensile
tests, the following conclusions were made.
1. Microstructural studies in detail for 304L/4340 friction-
welded joints it is revealed that there were four
different regions at the weld interface, which were
identified as parent metal (PM), partial deformed
zone (PDZ), deformed zone (DZ) and transformed
and recrystallized fully plasticized deformed zone
(FPDZ).
2. The higher microstructural changes took place in the
FPDZ and DZ region. The width of FPDZ region is
mainly affected by the rotational speed.