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Today, I´ll be talking about surface integrity by turning and grinding and I will identify the subsequent impacts of surface integrity on fatigue life.
I shall divide my talk in the three following parts.
First, I´ll tell you about turning and grinding.
Next, I´d like to show you the basic differences of surface integrity by turning and grinding.
And finally, I want to speak about identify the subsequent impacts of surface integrity on fatigue life.
What is turning?
This operation is one of the most basic machining processes. That is, the part is rotated while a single point cutting tool is moved parallel to the axis of rotation. The starting material is generally a workpiece generated by other processes such as casting, forging, extrusion, or drawing.
What is hard turning?
Hard turning is a turning done on materials with a Rockwell C hardness greater than 45. It is typically performed after the workpiece is heat treated. The process is intended to replace or limit traditional grinding operations. Hard turning is appropriate for parts requiring roundness accuracy of 0.5-12 micrometres, and/or surface roughness of Rz 0.8–7.0 micrometres. It is used for gears, injection pump components, hydraulic components, among other applications.
What is polishing?
Polishing is the process of creating a smooth and shiny surface When an unpolished surface is magnified thousands of times, it usually looks like peaks and valleys. By repeated abrasion, those "peaks" are worn down until they are flat or just small "peaks".
Hard turning and grinding are competitive finishing processes for the manufacture of precision mechanical components.
What are differences between turning and grinding?
The different geometrical features of cutting edges and grains used in hard turning and grinding create different surface structures.
A turned surface shows much wider and more regular feed marks than those of a ground one.
Grinding can achieve very smooth surfaces of less than 0.05 μm Ra.
The peaks and valleys of the turned and ground surfaces also differ in depth and occurrence.
The most significant difference are in residual stress by fresh hard turning and grinding.
Hard turning may induce a relatively deep maximum compressive residual stress in the subsurface, while grinding produces maximum compressive residual stress at the surface.
A worn tool or grinding wheel induces tensile residual stresses on the surface.
Furthermore, deep compressive residual stresses are much more beneficial to fatigue life.

Specimen preparation:
A material of the specimens was a carbon tool steel AISI 52100. Work specimens of were cut from a 76.2 mm diameter solid bar at 19.05 mm thickness.

Heat treatment consisted of austenizing at a temperature of 815ºC for 2 hours, followed by quenching in an oil bath for at 65ºC for 15 minutes. Finally, tempering was conducted at 176ºC for 2 hours which produced a final hardness of 61-62 HRC.
The test specimens were then machined by both turning and grinding processes as shown in these Tables.The turning process was conducted using a CNC BRIDGEPORT lathe with a constant cutting velocity. A turning tool was used fresh round CBN cutting tool inserts. The grinding process was conducted using a vitrified Al2O3 wheel and ample coolant.

Surface structure and roughness
The ground and feed marks are clearly seen on both surfaces Optical images of the as turned and as ground and as polished surfaces are shown in the Figure. You can see, that grinding wheel creates many random irregular bonded abrasives and therefore the surface has a random distribution of grinding marks. While hard turning the feed marks have a uniform distribution.
The roughness was measured using a stylus profilometer. The average surface roughness, Ra is shown in this Table. The roughness Ra between 0.06 μm ~ 0.07 μm of the polished surfaces is smooth enough to eliminate or at least minimize the roughness effects.

Subsurface microstructure
Optical microscope images of the cross-sections of the test specimen are shown in this Figure.
Both turned and ground surfaces are free of thermal damage, while grinding temperature has a much deeper excursion in the subsurface.
In the picture you can see, it shows that the subsurface has two different zones characterized by a strain hardened zone in near surface and a thermally affected zone in the subsurface.
The different appearance of the two zones is just due to their different resistance to etching as a result of different grain deformation and size.