28-03-2014, 11:50 AM
The Effect of Cryogenic Application on Surface Integrity in Manufacturing Process: A Review
ABSTRACT
The quality of work material’s surfaces after undergone various manufacturing processes is very important
in determining the functional performance of a component throughout the services. Application of coolant and
lubricant in manufacturing operations such as turning, milling, grinding, rolling, etc. has been proven to improve
the surface integrity of the work materials. In this review, application of cryogenic coolant in manufacturing
operations was investigated in terms of its effects on surface integrity of the work materials which includes
surface finish, microstructural changes, refinement of grain size, formation of white layer, residual stresses of
internal subsurface layer, and surface hardness. Cryogenic application is able to reduce the value of surface
roughness, allow for more comprehensive martensitic transformation, reduce the grain size, prevent the
formation of white layer at the subsurface, reduce the tensile residual stresses and increase compressive stresses
area, and finally increase the hardness of the work material. In conclusion, cryogenic application in a lot of
manufacturing processes has been determined to be able to enhance and improve the quality of the workpiece
surface, consequently, boost the functional performance of the components.
Introduction
Functional performance of a work material such as fatigue strength, corrosion rate, fracture toughness, and
tribological behavior (such as friction, wear and lubrication, and accuracy of dimensions) are highly dependent
on the surface properties. The integrity of the external surface topography (surface finish), and also the
microstructure, mechanical properties and residual stresses of internal subsurface layers are among the
properties of a machined surface that affecting the functional performance (Grzesik et al., 2010). Therefore,
surface integrity has been a subject of interest to many researchers in order to enhance the functional
performance of work materials.
Many techniques have been investigated for the purpose of improving the quality of surface integrity in
machining. Gentle machining is claimed to be able to enhance the surface integrity of machined surface
compared to abusive and conventional machining (Grzesik et al., 2010). Gentle machining can be defined as
machining in a “low stress conditions” which will result in little heat generated at the cutting zone. In order to
achieve the low stress conditions in machining, many attempts have been explored, and application of cutting
fluid is among one of them.
Cutting fluid applied during machining has a function as coolant and lubricant. Coolant is important to cool
the heat generation zone in machining process; meanwhile lubricant is used to minimize the friction between the
tool, chip, and workpiece interface. Methods of cutting fluid application include flood machining, near-dry
machining and also cryogenic machining. Cryogenic acts more as coolant to reduce the temperature generated in
machining process. Cryogenic coolant uses liquid gaseous such as liquid nitrogen (LN2) or liquid carbon dioxide
(CO2), which have very low melting temperature, to reduce the temperature at the cutting zone. Cryogenic
machining is more advantageous compared to the usage of conventional cutting fluid in term of environmental
friendly in such a way that the liquid gas used will evaporate into the air and become part of the atmosphere
(Nalbant and Yildiz, 2011). The evaporation of the gaseous also eliminate the cost of cutting fluid disposal
(Umbrello et al., 2012).
Surface finish / surface roughness:
Surface finish is one of the most important aspects that usually been analyzed because the affect it has on
product performance and life cycle, besides the residual stresses and the occurrence of surface and subsurface
micro cracks (Xavier et al., 2010). These aspects are very vital, especially when the product would be fit with
other parts or used under dynamic loading. It is also reflected the quality of surface integrity on work materials
(Yazid et al., 2011).
Some researchers have proven that the surface roughness measurement, Ra, gave some improvement when
machining with cryogenic application compared to dry and wet machining (Jerold and Kumar, 2011; Pu et al.,
2012; Ravi and Kumar, 2011; Umbrello et al., 2012). Smaller Ra measurements reflect better surface quality of
the work parts. Surface finish is largely influenced by the cutting force, tool wear and chip formation. Figure 1
shows improvement of surface roughness value in cryogenic machining compared to dry and wet machining.
The improved surface finish resulted in cryogenic machining is due to lower cutting temperature generated
during the machining process, hence lowering the cutting forces and tool wear (Kumar and Choudhury, 2008;
Ravi and Kumar, 2011; Zhang et al., 2012). Similar findings were reported by Dhar & Kamruzzaman (2007)
who also obtained a reduction in tool wear in turning AISI 4037 steel bar with cryogenic jet application.
Effective reduction of cutting temperature had been attained when the LN2 jet was aimed to the main cutting
edge, maintaining the tool’s sharpness (Ravi and Kumar, 2011) and hardness hence lessen the abrasion wear as
well as adhesion and diffusion wear (Dhar and Kamruzzaman, 2007).