Davim J. Paulo (editor). Machining. Fundamentals and Recent Advances
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90 J. Rech et al.
Lapping on a Hardened Steel [100]
Lapping induces strong compressive residual stresses in the surface and leads to
great improvements in the fatigue strength of a martensitic stainless steel during
plane bending fatigue tests, compared to conventional grinding operation.
Honing on Hardened Steel and Hastelloy X [20,
97,
24]
The honing technique shifts the external residual stress profile towards compres-
sion. Rotation bending fatigue tests and pulsating push–pull fatigue tests have
shown a great improvement of the resistance compared to a shot peening opera-
tion. This improvement was even better in an aggressive atmosphere (water + sod-
ium chloride) than in air, which proves the improvement of the resistance against
stress corrosion.
Burnishing/Roller Burnishing of Hardened Steels [14,
27,
101,
102]
In roller burnishing operation, a hydrostatically borne ceramic ball rolls over the
component surface under high pressure. The roughness peaks are flattened and the
quality of the workpiece surface is improved. Roller burnishing delay the appearance
of cracks in grooves. It has been shown that this process is able to generate deep com-
pressive stresses in both the axial and circumferential directions. Hard roller burnish-
ing does not generate any white layers. The affected layer is very thick (~5 mm).
Figure 3.22. Residual stresses induced by belt finishing on a AISI52100 hardened steel
-1000
-800
-600
-400
-200
0
200
0 102030405060
Residual stresses (MPa)
Distance from the surface (µm)
Hard turning
Hard turning + Honing
Figure 3.23. Influence of the residual stresses induced by honing on a AISI52100 hardened
steel
Workpiece Surface Integrity 91
Note that the residual stresses in reamed surfaces have some similarities with
burnished surfaces. Reamed surface are compressive in nature. This is because of
the rubbing and plastic deformation of the material caused by the sizing section of
the reamer.
3.2.3.7 Hole Manufacturing
In contrast to the previous processes, machining processes such as drilling, ream-
ing, etc. are poorly investigated. Small holes are usually not considered as critical
surfaces in terms of fatigue resistance. Moreover, it is very difficult to investigate
the surface texture of holes and almost impossible to investigate the residual stress
fields, at least by X-ray diffraction. Problems with surface integrity of holes are
rather recent. Indeed, critical surfaces have been intensively investigated and holes
have been neglected. As a consequence, great improvements have been made on
some surfaces whereas holes have not experienced any benefit. In parallel, the
increase of the thermo-mechanical loadings supported by some parts such as
crankshafts with the power increase of motor engines, or by some parts from tur-
bine engines, have led to breakages. As a consequence, holes are starting to be the
focus of attention of researchers. After the statement about the poor knowledge on
surface integrity induced by drilling and reaming, some companies have decided
to apply super-finishing processes such as roller burnishing or honing to induce
compressive stresses and improve surface texture. This clearly shows how a lack
of knowledge leads to abusive production costs.
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4
Machining of Hard Materials
Wit Grzesik
Department of Manufacturing Engineering and Production Automation, Opole
University of Technology, P.O. Box 321, 45-271 Opole, Poland.
E-mail: w.grzesik@po.opole.pl
This chapter presents basic knowledge on the special kind of the machining process
in which a workpiece material hardened to 45–70 HRC hardness or more is ma-
chined with mixed ceramic or CBN tools. An extended comparison with finish
grinding, as well with other abrasive finishing processes, is carried out. Specific
cutting characteristics, including cutting forces, chip formation mechanisms and
tool wear modes with relevant interface temperatures are discussed in terms of
process conditions. Currently developing finite element (FE) and analytical model-
ling is overviewed. A complete characterization of surface integrity including geo-
metrical features of hard-machined surfaces, along with specific microstructural
alterations and process-induced residual stresses, is provided. Finally, the state of
the art of hard cutting technology is addressed for many cutting operations to show
how manufacturing chains can be effectively utilized and optimized in practice.
4.1 Basic Features of HM
4.1.1 Definition of Hard Machining
Basically, hard turning, which is the dominant machining operation performed on
hardened materials, is defined as the process of single-point cutting of part pieces
that have hardness values over 45 HRC but which are more typically in the 58−68
HRC range. The world-leading manufacturer of cutting tools, Sandvik Coromant,
defines hard materials as those with hardness of above 42 HRC up to 65 HRC.
Commonly, hard-machined materials include white/chilled cast irons, high-
speed steels, tool steels, bearing steels, heat-treatable steels and case-hardened
steels. Sometimes, Inconel, Hastelloy, Stellite and other exotic materials are clas-
sified as hard-turned materials.
98 W. Grzesik
As shown in Figure 4.1, values of 1 µm Rz (equivalently 0.1 µm Ra) in CBN
high-precision machining and correspondingly IT3 dimensional tolerance are
possible. However, for extremely tightly toleranced parts, hard turning can also
serve as an effective pre-finishing operation, followed by finishing grinding. Their
applications have spread over such leading industrial branches as the automotive,
roller bearing, hydraulic, and die and moulds sectors. Gear wheels, geared shafts,
bearing rings and other transmission parts are typically machined by turning,
while high-speed milling dominates the die and mould industry.
In general, hard turning can provide a relatively high accuracy for many hard
parts but sometimes important problems arise with surface integrity, especially
with undesirable patterns of residual stresses and the changes of subsurface micro-
structure, so-called white layer, which reduces the fatigue life of turned surfaces.
This problem will be discussed in the following sections.
4.1.2 Comparison with Grinding Operations
Traditionally, the finishing operations on machine parts in a highly tempered or
hardened state with hardness value in excess of 60 HRC are grinding processes,
but recently hard cutting operations using tools with geometrically defined cutting
edges have become increasingly capable of replacing them and guaranteeing com-
parable surface finish. Grinding and turning are machining operations so opposite
that their full substitution is not always easy or possible.
Some of the inherent differences between these machining processes are as fol-
lows [2]:
1. Hard turning is a much faster operation because it can be done in one setup
and pass under dry conditions.
2. Lathes offer more production flexibility.
3. Rough and finish operations can be performed with one clamping using
a CNC lathe.
Figure 4.1. Achievable surface roughness and ISO tolerance in hard turning [1]
Machining of Hard Materials 99
4. Multiple turning operations are easier to automate through tool changes on
turning centre or turning cell.
5. Since hard turning is done dry, there are no costs for coolant, its mainte-
nance or disposal.
In particular, the hard cutting process performed with ceramic or CBN tools can
often cut manufacturing costs, decrease production time (lead time), improve
overall product quality, offer greater flexibility and allow dry machining by elimi-
nating coolants (Figure 4.2).
There are many opportunities for substituting grinding by turning operations
when finish-machining of hardened ferrous materials. In general, hard turning
reduces both equipment cost and personal expenses because it can be performed in
one pass using one setup. On the other hand, as shown in Figure 4.3, the tool cost
for finish-turning a gear blank of approximately 62 HRC hardness with CBN cut-
ting material is almost 50% of the overall cost.
Figure 4.2. Criteria used in comparison between grinding and hard cutting operations [1,
3]
Figure 4.3. Cost comparison of turning versus grinding [4]