Machining of High Strength Steels With Emphasis on Surface Integrity by air force machinability data center_11 pot

the machining operation, but it also provides the user with validated and relevant data analysis. ese posi- tive benets enable tool designers and users alike, to design and develop advanced cutting tools and to un- dertake ecient and optimised machining operations. Beyond the positive advantages of tool optimisation, simulation can signicantly reduce tooling develop- ment costs and lead times to bring a newly-developed product to market. e role of machining simulation is likely to rapidly grow, as more tooling and produc- tion engineers become aquainted with these soware packages. Figure 184. The insert’s cutting edge: illustrating the ‘rounding eect’ (exaggerated) or, a manufacturer’s ‘edge preparation’ and the material ow conditions that arise as a result . Machinability and Surface Integrity  7.10 Surface Integrity of Machined Components – Introduction Previously in Section 7.5 concerning machined sur- face texture, the discussion was principally concerned with the resultant surface topography where the topo- graphical information was valid, but disguised the fact that potential sub-surface material layers might have compromised and altered the machined component. e concept of the overall functional performance of a surface and its accompanying sub-surface condition was recognised by Field and Kahles (1971), where they used the term ‘Surface Integrity’ to describe its poten- tial state. e overall concept of surface integrity ant its various generating mechanisms in conjunction with the production process is known as the ‘unit event’ 81 . is unit event has now been reclassied into ve discrete generating mechanisms: chemical, mechani- cal, mechano-thermal, thermo-mechanical and ther- mal – the order they are listed reects their respective power density per unit area. For example, increases in the power density from the chemical end of the series, results in an augmented level of thermal energy enter- ing the surface leading to greater thermal damage and poorer part surface integrity. e chemical mechanism is dominant across all classes of production process to some degree and that surfaces react with their imme- diate environment, via absorbates, oxidation, etc., as illustrated in Fig. 185 – more will be said on these ef- 81 ‘Unit event’ , is a complex interrelated series of reactions with the potential for distinct zones to be present within the sur- face vicinity, including a: Chemically aected layer (CAL) – resulting from chemical surface changes by the production process, or from post- production exposure to a local environment, Mechanically aected layer (MAL) – this may be due to factors such as material bulk transportation: deposits; laps; folds and plastic deformation, Heat aected layer (HAL) – principally concerned with factors such as: phase transformation; thermal cracking and retempering, Stress aected layer (SAL) – is in the main, the result of residual stresses being a combination of the above. (Field and Kahles, 1971) – – – – fects when discussing the machined surface condition in the following section. .. Residual Stresses in Machined Surfaces A machined surface is the product of either ‘abusive’ , or ‘gentle’ machining regimes, these being the direct result of the cutting process and its chosen machining data. us, machining being a complex relationship of many interrealated factors, aects the outcome of the production process – see Fig. 144. Here, a simplistic schematic diagram attempts to show the complexity of a machining operation, with the surface integrity grouping indicating for a turning operation the fol- lowing features: • Surface condition – surface texture and its associ- ated roundness, • Micro-structural changes – micro-cracks, disloca- tions and ssures, etc., • Surface displacement – bulk transportation of ma- terial and residual stresses, • Surface/sub-surface micro hardness – plastic de- formation and localised residual stress layers. Machined surfaces are even more complex than seem- ingly at rst glance, as their performance can be in- uenced by either external layers (chemical transfor- mations and plastic deformations) and internal layers (metallurgical transformations and residual stresses). By way of example, the anisotropic – periodic – longitudinally turned surface illustrated in Fig. 185, is aected by the cutting insert’s tool tip geometry and the regularity of the cusps (i.e. peaks and val- leys) – the surface topography being dominated by the pre-selected feedrate. A series of other micro-tech- nological features can also occur, these oen being superimposed onto the machined surface, typically the result of: tool wear, vibrational inuences and to a lesser extent, machine tool-induced errors. In the circumferential direction the ‘Lay’ is both periodic and regular, albeit this round generated surface by the turning operation, will probably have some form of harmonic eects present: departures-from-round- ness characteristics (i.e. a combination of harmonic inuences present). e exposed sterile surface (Fig. 185), is the result of highly localised temperatures and transients, which when turned the machined surface will be instantaneously oxidised and adsorb contami-  Chapter  Figure 185. The cross-section of an anisotropic (i.e. periodic) surface, illustrating surface contaminants (oxides and adsorbates), together with some sub-surface plastic deformation (the residual stress zone) and an unaected substrate . Machinability and Surface Integrity  nants. e outermost adsorbate layer is oen termed the ‘Beilby layer’ 82 : ≈1 µm in thickness and consisting of many complex factors. Notably, this ‘layer’ would more than likely have hydrocarbons present and wa- ter vapour, that originated in the coolant, or the at- mospheric environment, respectively. Underneath this metallic surface for work-hardening materials, there is normally a plastically-strained region that has usually been metallurgically altered. e depth of this strain-hardened layer will vary somewhat, but it is in the region of 10 µm, its actual thickness is dependent upon the amount of plastic deformation induced by the tool’s passage over the surface and is inuenced by the metallic substrate’s composition. e plastic defor- mation and work-hardening depths 83 , can penetrate to fractions of a millimetre this is particular true, if a ‘wiper-insert‘, or roller burnishing tools is employed to purposely create this localised hardened region to the component’s surface. Residual Stress Deformations For any residual stresses acting within a body (i.e. component), they will occur without any external forces, or moments. Internal forces form a system that is currecntly in a state of equilibrium and if portions are removed – by machining, the equibrium status is normally disturbed, resulting in potential component deformation. is eect of machining distortion is well-known to practising industrial engineers, when, for example, machining just one side of a thin compo- nent, this operation will cause a partial release of local residual stresses causing it to bend and bow. If either a casting, or forging has not been heat-treated for stress relief and its needs asymmetrical machining (i.e. on one side only), it is likely to deform aer unclamping restraint from its work-holding device on the machine tool. In an attempt to minimise this distortion created by residual stress release, an experienced machinist will release the clamping forces aer roughing cuts so that 82 ‘Beilby layer’ , on the machined surface is ‘practically amor- phous’ – this condition being proposed by Sir George Beilby around the beginning of the 20 th century. 83 As an approximation, the depth of hardness penetration is ap- proximately 50% to that produced by residual stress penetra- tion, whereas the observational plastic deformation is about 50% greater than this penetration. the stressed surfaces are equalised, prior to reclamp- ing and taking a nish pass. If this unclamping and then re-clamping activity is not possible, components clamped in-situ on the machine tool are occasionally vibrated at their natural frequency, to minimise these induced residual stresses. Component deformation is roughly proportional to the removed cross-section of workpiece material. Any further nishing is usually concerned with just a light cut to minimise any detri- mental eects resulting from residual stresses by a pre- vious production processing operation, or route. e release of internal residual stresses must not be confused with the input of such stresses by machin- ing, as indicated in Fig. 186b. e machining process generates residual stresses by plastic deformation (Fig. 187a), or from localised metallurgical transforma- tions. In Fig. 186a, the residual stress eects inuence a range of mechanical and physical properties of the workpiece material, such as: • Deformation – this point has been alluded to above and can create problem with small workpiece cross- sections, • Static strength – is aected by the yeild point of the workpiece material, which in turn, is inuenced by the presence of residual stresses, • Dynamic strength – of the part in-service can oen have its fatigue strength and life aected by the in- uence of residual stresses present, • Chemical resistance – if certain metals are sub- jected to induced residual stresses on exposure to atmosphere over a period of time, then stress corro- sion may occur, • Magnetism – residual stresses present, can aect a component’s magnetic properties, creating distur- bances of the crystalline structure. Taper-Sectioning and Micro-Hardness Assessment So that an improvement of metallographical inspection of a sectioned machined surface can be made without unduly aecting any form of surface distortion, ‘taper- sectioning’ has oen been utilised. A tapered-section (Fig. 187b), allows such sub-surface features as: phase transformations; plastic ow zones; localised cracking; bulk transportation and redeposit of material; to be in- vestigated which would otherwise have been missed, if only prolometry (i.e. surface topography assessment) had been undertaken. As its name implies, a taper-section overcomes the limitation of perpendicular sectioning. By taking an  Chapter  angular planar slice through the components cross- section, this modied cut angle enhances the substrate magnication, without unduly distorting exposed sur- face features – giving greater discretion when observ- ing, or testing the surface topography. In Fig. 187b, an 11° sectional cut improves surface discrimination by increasing the vertical section magnication by around ve times. e taper-section angle (TSA) will thus be 79°, with the vertical magnication being ob- tained from the following expression: TSM = secant (TSA) Where: TSM = taper-section magnication, TSA = taper-section angle. Oen, the exposed sub-surface feature of interest that has been plastically deformed, or mechanically altered is in the main quite small, somewhat less than 0.1 mm in width. If a micro-hardness indentor such as either Figure 186. The eects of residual stress and deformations of a workpiece by machining. [After: Brinksmeier et al., 1982] . Machinability and Surface Integrity  the Vickers 84 , or the Knoop 85 is utilised (Fig. 187c) to establish hardness readings in the vicinity of this re- sidual stress zone, then more indentations are possible using the Knoop, rather than the Vickers indentor, giv- ing, more discrimination to the ‘foot-printing’ assess- ment. A note of caution here when originally attempt- ing to take the taper-section, is that it is quite possible to metallurgical alter the sub-surface features, if when taking the section too much heat is induced when cut- ting it from the parent component. is comment is also a valid statement for the subsequent grinding and polishing of the removed taper-section, prior to metal- lographical/hardness assessment. Surface Condition – Being Affected by Cutting Speed Prior to discussing the surface and sub-surface modi- cations to the machined part – shortly to follow, it is worth taking a closer look at the series of photo- micrograph images shown in Fig. 188. Here, a group of identical metallurgical composition ferrous work- pieces was machined, but at various cutting speeds. It can be demonstrated that the role played in aect- ing the machined surface condition, is signicantly inuenced by the cutting speed, with its accompany- ing amplication of induced temperature eects as ‘speeds’ are increased. Moreover, it can also be said, 84 ‘Vickers indentor’ , has a square-based dymond pyramid with and indentor included angle of 136°. Its indentation is dened as: ‘e load divided by the surface area of the indentation’. e Vickers hardness [i.e. penetration] number (VPN), may be determined from the following expression: VPN = 2Psin(θ/2)/L 2 Where: P = applied load (kg), L = average length of diagonals (mm), θ = angle between opposite faces of diamond (136°). 85 ‘Knoop indentor’ , has complex facets to its diamond indentor, having angle of 130° (Short diagonal) and 172.5° (Long diago- nal), respectively. is facet geometric indentor arrangement (i.e. having a diagonal ratio of 7:1), leaves a signicantly nar- rower and longer surface indentation, to that of the Vickers – mentioned in Footnote 84. us, the Knoop hardness number (KHN) has been dened by the National Bureau of Standards (USA), as: ‘e applied load divided by the unrecovered pro- jected area of the indentation’. e following expression relates to the Knoop’s surface indentation: KHN = P/A p = P/L 2 C Where: P = applied load (kg), A p = unrecovered projected area of indentation (mm 2 ), L = length of long diagonal (mm), C = constant – supplied by indentor manufacturer. that a material’s properties are dependent on the strain rate, with the type and magnitude of tool wear chang- ing according to the cutting speeds, so simplistically speaking: • Low cutting speeds – wear is normally character- ised by attrition (i.e. mechanical removal of surface layers), • High cutting speeds – here, attrition gives way to diusion type wear and ‘Fick’s laws’ dominate the cutting regime. NB Such ‘broad classications’ of tool wear mech- anisms occurring, aects the type of: surface pro- duced; chip formation and strain behaviour. In some interesting trials undertaken by Watson and Murphy (1979) – which highlight the disguised nature of the underlying factors in surface integrity investi- gations. In this practically-based experimental work, they used a cemented carbide insert on an alloy steel (Fig. 188). It was found that the feedrate and D OC have only marginal eects on the sub-surface damage to a machined workpiece, with the cutting speed being the most inuential in this situation. is fact has been established in Fig. 188, when a range of similar work- piece specimens was machined with the only variable being the cutting speed, as follows: •   Photomicrograph a – the machined specimen was machined at a very low cutting speed (2.6 m min –1 ) e chip formation was discontinuous and the sur- face shows an alternating eect of both chip forma- tion and fracture, with some evidence of deposited residual BUE. Here, the surface topography is the result of complex interactions by various eects, such as changes in shear angle in the contact area between the tool and chip, plus ‘straining’ causing increases in the chip thickness. ese phenomena produce a variety of conditions, from strain-to- cracking and visually introduces an irregular and an alternating surface topography, •   Photomicrographs a to d – cutting speeds in the range from 11 to 59 m min –1 , generate a continuous chip formation. It is evident from these photomi- crographs (b, c and d), that the surface texture was gradually improving as the cutting speed increased, although even at 59 m min –1 , there was some indi- cation of debris from re-deposited BUE here (i.e. in ‘d’), •   Photomicrograph e – once the ‘optimum’ cutting speed had been reached (112 m min –1 – for this ce-  Chapter  Figure 187. The tribological action of machining and its aect on induced residual stresses and the micro- hardness ‘foot-printing’ technique . Machinability and Surface Integrity  mented carbide insert grade), the surface texture appears to be in the main, ‘good’ , with only isolated areas of the topography exhibiting marginal work- piece side-ow eects, •   Photomicrograph  f  – when the cutting speed was increased to 212 m min –1 , then in these trials, greater cutting insert wear-rate occurred and was attributed to appreciable carbide edge breakdown, although the surface topography indicated that an excellent surface texture was present. e machined surfaces produced at the lower range of cutting speeds indicated in Figs. 188 a to d, shows evi- dence of some re-deposited BUE material to greater- or-lesser extent: having broken away from original ‘BUE mass’ , then being re-deposited over several adjacent machined feed cusps (i.e. see Fig. 28a, fully- appreciate this eect). To obtain a better and deeper understanding of these machined surface and sub- surface eects at the extreme conditions of either very low, or high cutting speeds: Figs. 188 a and f, respec- Figure 188. Some photomi- crographs of component surfaces machined at dierent cutting speeds – otherwise with identical cutting data – illustrating the surface, but not sub-surface steel’s condition. [Source: Watson & Murphy, 1979] .  Chapter  tively, the following comments can be made. When longitudinal taper-sections were taken through these specimens’ cross-sections, the ground, polished and etched surfaces reveal their true substrate damage. In the case of Fig. 188a, BUE was presents on the sur- face, moreover, there was a cutting/fracture sequence indicated with conrmation of work-hardening hav- ing ‘layered scales’ of with cracks and crevices beneath them. Conversely, the test specimen machined at high cutting speed (Fig. 188f), there is some verication of a ‘white-layer’ formation – which is a complex metal- lurgical phenomena found in certain ‘abused’ ferrous workpiece situations – more will be said on this condi- tion shortly. In fact, the ‘good’ machined surface to- pography disguises the fact that an underlying ‘white- layer’ condition was present, having a local recorded hardness of 860 H VPN . By way of comparison, if this same alloy steel composition had received a ‘conven- tional’ hardness heat-treatment process: heated and water-quenched from 1200°C, then the bulk hardness would only be approximately 700 H VPN – see Appendix 12 for Hardness Comparison Tables. From these examples of cutting speed investigative results and the previously mentioned discussion, it is evident that the ‘optimum’ machined surface texture is obtained when the cutting speed is closely aligned to that of the tooling manufacturer’s recommenda- tions, so here in this case it is ≈112 m min –1 , with a correspondingly ‘good’ surface topography/integ- rity. If the cutting speeds had been employed at the ‘higher’ cutting data (i.e. 212 m min –1 ), then one could have been fooled into accepting this apparently ‘im- proved’ surface topography. Nevertheless, underlying this machined surface would be an unstable sub-sur- face condition, which if used in a stressed and critical in-service environment, it might potentially fail, by a reduced fatigue-life – this is why the topic of surface integrity is so important in today’s climate of potential industrial litigation, when component failure occurs! Surface Cracks and White-Layers If any cracks are present at the free surface which ex- tends into the material’s substrate, they are potential sites for premature component failure – for highly stressed in-service components. It has been reported in the ndings of industrial enquiries into the UK railway industry of late, that despite these railroad tracks being precision machined and then occasion- ally inspected by non-destructive (NDT) 86 techniques – according to the maintenance schedule, instances have occurred when these rails and particular on high-speed banked corners – have delaminated. is catastrophic rail delamination has caused several pas- senger trains to lose contact with the rails and crash, resulting in signicant loss of life. Hence, the method of machining – ‘abusive’ – can contribute poor surface integrity and to the susceptibility of these machined surfaces to prematurely fail. In the case of milling op- erations, it has been recognised for a number of years that up-cut milling – alternatively termed ‘conventional milling’ (Fig. 190a), can introduce a surface tensile re- sidual stress into the surface layers of a milled work- piece. If this machined component is then subjected to both an arduous and potentially fatigue-inducing environment, then the cyclical nature of continuous stressing followed by its immediate stress release, can initiate surface crack sites causing them to open-up, which could result in premature part failure. Con- versely, an identical machined component that has been ‘down-cut’ – otherwise termed ‘climb-milling’ (Fig. 190b), will induce surface compressive residual stresses. is surface layer with its residual stress com- pression, has invariably been shown to remain closed and thus, avoiding crack propagation and growth, when machined under identical cutting data and en- vironmental circumstances. Moreover, for many years, it has been recommended that for CNC milling appli- cations ‘climb-milling’ not only generates this favour- able machined surface compressive stress eect, but is a more ecient cutting process and as a result, draws less spindle power. In Appendix 13a and b, two useful ‘nomographs, are given to determine either the cutting data (Appendix 13a) this is related to the workpiece’s diameter and, a diagram (Appendix 13b) to obtain the spindle power from the anticipated chip area, respec- tively. In a machined surface, both craters and pits do not pose too great a fatigue problem, as they cannot achieve the ‘critical radius’ (i.e see Footnote 67) neces- sary to instigate a site for crack initiation at a poten- 86 ‘Non-destructive testing’ (NDT), is a range of ‘non-invasive’ sub-surface inspection testing techniques, typically: Eddy- current testing, Ultrasonics tests, X-ray investigation, etc., that can, in many cases be automated for the detection of otherwise hidden aws in the component(s). Machinability and Surface Integrity  . preparation’ and the material ow conditions that arise as a result . Machinability and Surface Integrity  7.10 Surface Integrity of Machined Components – Introduction Previously in Section. sub -surface condition was recognised by Field and Kahles (1971), where they used the term Surface Integrity to describe its poten- tial state. e overall concept of surface integrity ant. that potential sub -surface material layers might have compromised and altered the machined component. e concept of the overall functional performance of a surface and its accompanying sub-surface