Machining of High Strength Steels With Emphasis on Surface Integrity by air force machinability data center_12 pptx

tial stress concentration location. Furthermore, cra- ters and pits normally exhibit shallow depth-to-width ratios and are normally only present a problem from the ‘cosmetic appearance’. Cracks in the surface are normally classied as either ‘micro-’ , or ‘macro-cracks’ , with these cracks having depth-to-width ratios of >4, typically they can promote: • Reductions in: mechanical strength; fatigue life; plus creep resistance 87 , • Increases in the susceptibility to stress-corrosion 88 , • Probability increase in a surface material break-out and generation of debris, • Surface delamination and fatigue. Cracks may be considered as either separations, or narrow ruptures that interrupt the surface continuity and normally include sharp edges, severe directional changes, or both. Macro-cracks can usually be visu- ally inspected with the naked eye, conversely micro- cracks obviously require microscopic examination. Oen these cracks are complex metallurgical interac- tions which are exacerbated by an ‘abusive’ machine regime, leading to an unacceptable surface condition. A crack’s origin can be the result of several multifari- ous phenomena, typically they can be an inter-granu- lar attack that might be degraded by surface dissolu- tion, via chemical processes. Whenever preferential intergrannular attack takes place, it can additionally promote a grain boundary network of micro-cracks that can extend beneath the surface, tracing-out and following the underlying grain boundaries. Even mi- cro-cracks should not be ignored, as they can aect the component’s functional performance, because they act as a potential source for macoscopic crack fatigue. Hence, once a crack has been generated it cannot be successfully resealed, owing to subsequent contamina- tion and continuous chemical reactions. In fact, the process of fatigue failure (i.e. see Fig. 190 bottom-right for photomicrographs of a cranksha’s fatigue failure 87 ‘Creep’ , is: ‘e time-dependent plastic deformation of materials that occur under constant load at relatively high temperatures and low stresses’. 88 ‘Stress-corrosion cracking’ (SCC), is: A combined mechanical and chemical failure mechanism in which a non-cyclic tensile stress [below the yield strength] leads to the initiation and prop- agation of fracture in a relatively mild chemical environment’. mechanism) can be characterised by three discrete steps: 1. Crack initiation – where a minute crack forms at a particular site, such where a high stress concentra- tion occurs, 2. Crack propagation – during which time at which the crack incrementally advances with each stress cycle 89 , 3. Final failure – rapidly occurs, once the advanc- ing crack has reached a critical size being close to ‘speed of sound’: Mach 1 – and is a catastrophic failure mechanism. White-Layers e so-called ‘white-layers’ 90 that can appear when ‘abusive machining’ certain ferrous work-hardening materials, are a result of microstructural and metal- lurgical alterations to the machined sub-surface layers of a workpiece (Fig. 189c). is undesirable and un- wanted ‘white-layer’ condition is visually apparent (i.e. when a taper-section through the machined surface has been taken), as it resists standard etchants and the consequence is a visible ‘white-layer’ – when viewed under an optical microscope. 89 ‘Striations’ , (also known as ‘Beach-’ , or ‘Clamshell-marks’ – see Fig. 190bottom-right), are concentric ridges that expand away from the initial crack site(s), frequently appearing in a circular, or as a semi-circular radial pattern. NB is ‘striation eect’ is analogous to that of a stone be- ing dropped into a still pond – with the stone entry being the equivalent of the initial crack site, while the radial/circular waves generated, are akin to the cumulating concentric stress ridges – until they intersect with the pond’s bank [ie free-sur- face]. 90 ‘White-layers’ , are a metallurgically unstable sub-surfaces exhibiting a very hard localised state, with a supplementary heat-aected zone (HAZ) beneath it, which is soer than the overall bulk hardness of the workpiece’s matrix – hence, this metallurgical instability. ‘White-layers, can be classied de- pending upon whether it resulted from: mechanical; chemi- cal; or thermal events, which also directly relates to machined workpiece factors such as: strain; strain-rate; heating/cooling rates; plus environmental conditions. NB In the past, ‘white-layers’ were known by several terms, such as: ‘white-phases’; ‘white-etchings’; ‘hard-etchings’; etc. – depending upon the variety and type of ‘white-layering’ pro- duction.  Chapter  Figure 189. The inuence of the cutting edge’s condition on the resultant machined surface integrity . Machinability and Surface Integrity  In Fig. 189c, a ‘white-layer’ (i.e. for this ferrous drilled part, being a localised untempered martensitic phase of 63 H Rc 91 ) exists beneath the recast and rede- posited layer, in this case produced by a ‘dull’ drill’s cutting lips and margins. Due to the fact that the recast layer (i.e. heat-aected zone – HAZ) has a similar met- allurgy to that of the ‘white-layer’ , with the delineation of these ‘white-layers’ regions and their accompany- ing HAZ’s are not clearly dened. is latter HAZ is a complex metallurgical condition, comprising of some: untempered martensite (UTM); over-tempered mar- tensite (OTM), while beneath these layers, the bulk substrate material remains unaected. e thickness of these ‘white-layer’ zones is strongly inuenced by both the actual plastic deformation created here and, to a lesser degree, by the thermal inuence of the pas- sage of the tool’s edge over the machined surface as heat penetrates into the locality of the component’s surface. Probably the worst ‘abusive machining’ condi- tions that can exist, are when drilling holes in work- hardening materials having long length-to-diameter ratios (i.e. L/D ratios of >12:1) with inadequate cool- ant supply, creating high levels of friction, this condi- tion being exacerbated by an ineciency produced by a ‘dulled’ drill’s cutting lips. Virtually all tooling even the most sharp – the no- table exception here being monolithic faceted natu- ral diamond cutting edges, have a nite tip radius of ≈8 µm (i.e. see Fig. 184 – high-lighting the tool tip ‘rounding eect‘), this results in increased forces and tool wear, which can transform the surface metallurgy by thermo-mechanical generation. e case has al- ready been made concerning the fact that machining processes impart residual stresses into the surface lay- ers, as indicated in the schematically-represented mill- ing conditions shown in Fig. 190 and graphically, in Fig. 191 for a series of milling operations where preset ‘wear lands’ were generated on the cutter’s teeth prior to workpiece machining. is latter case (Fig. 191) of articially-inducing a controlled ‘wear land’ onto the face-milling cutter’s individual tooth (i.e. with the other teeth removed, hence, acting as ‘Fly-cutter‘), then aer 91 By way of comparison of this untempered martensitic ‘white- layer’ phase, a conventional high-speed steel (HSS) milling cutter’s teeth would have had a maximum hardness aer heat- treatment of 62 H Rc , which clearly signies the true local hard- ness of these ‘white-layers’. several milling passes plotting the residual stress levels from the surface and into the 4340 steel workpiece’s substrate under standardised cutting data (i.e the steel specimens having previously been quenched and tem- pered to a bulk hardness of 52 H Rc ). Hence, the eect of these dierent induced tool wear rates and their inuence in terms of their respective magnitudes and depths, can clearly be seen. Even when the cutting edge has ‘sharp tooth’ , a certain degree of tensile residual stress was apparent in the immediate surface region. Here, directly under this tensile stress zone, the stress concentration changed to one of compression (i.e. to a depth of ≈50 µm). As each milling cutter tooth ank became steadily more worn, the substrate compression layer also increased in magnitude, which could lead to considerable workpiece distortion, once the clamping forces had been released – particularly if only one-side of the part was milled (i.e. see Fig. 186b). If the forces involved in the machining process ex- ceed the ow stress, plastic deformation occurs and the structure is deformed. In the case o ductile materi- als, the plastic ow can create a range of degenerative surface topography characteristics, such as: burrs; laps; BUE residue; plus other unwanted debris deposits. If this deformation becomes severe as a result of exces- sive plastic ow, any grains adjacent to the surface may become fragmented to such an extent that little, or no metallic structure can be metallographically re- solved, therefore ‘white-layering’ will result. Normally, a ‘white-layer’ region extends to quite a small depth beneath the surface, in the region of 10 to 100 µm, de - pending upon the severity of the ‘abusive regime’ of surface generation. Considering Fig. 191 once again, as can be seen, the residual stress is indicated along the vertical axis, here instead, it is alternatively possible to superimpose a micro-hardness axis – see Fig. 191 circular inset graph. A note of care is required when changing the vertical axis from residual stress to that of micro-hardness, as they are two distinct quantita- tive values. As mentioned the hardness prole closely approximates that of the residual stress curve, however in the latter case, instead of tensile stress at the in the surface region, the sub-surface layer could equally be compressive in nature. ‘White-layers’ must be avoided under all occasions, because of the unstable metallurgical condition, com- pounded by the fact that the these regions act as po- tential stress-raisers for any critically-engineered com- ponent and can lead to premature failure, or at worse, catastrophic failure in-service.  Chapter  Figure 190. Typical fatigue characteristics within the component’s surface region, being inuenced by the mode of milling: up-cut or down-cut . Machinability and Surface Integrity  Figure 191. Comparison of the residual stresses in some milled surfaces, obtained with articially- induced tooth wear lands. [After: Field & Kahles, 1971] .  Chapter  Altered Material Layers So that an impression of the altered material layers (AMLs) that can occur for a diverse range of: surface and sub-surface topographical features; dierent met- allurgical processes; mechanical applications and uses; Table 13 has been constructed, to high-light their par- ticular inuence on functional performance. In the majority of cases given in Table 13, the inuence of these sub-surface defects tends to be of signicance, especially with respect to an ‘abusive regime’ produc- ing a machined ‘white-layer’. In some instances, the ‘al- tered material zone’ (AMZ), can aect component in- service performance in a variety of ways. For example, where thein-service tribological situations produce ei- ther re-deposited, or recast layers in the surface region, it has been known that such defects will inuence wear and aect reliability. is oen undetected sub-surface Table 13. The inuence of substrate features on function Surface integrity: sub-surface features Function: Metallurgy Deformation Deposits Stress UTM or WL OTM rev Aust IGA WL Plastic defn Burrs Cracks Tears and laps Tool frags Redp matl Res stress Wear            Strength             Chemical attack         Fatigue            Magnetism   Bearings          Seals          Friction       Forming          Bonding and adhesion         Key: : strong inuence on function; : some inuence on function; : possible inuence on function Abbreviations: UTM: untempered martensite; OTM: over-tempered martensite; Aust rev: austenitic reversion; IGA: intergranular attack; WL: white-layer; Plast defn: plastic deformation; Tool frags: tool fragments; Redp matl: re-deposited material; Res stress: residual stress. [After: Griths et al., 2001] . Machinability and Surface Integrity  condition degrades the functional performance, due to the fact that they are the product of hard, brittle and unstable layers, with tensile residual stresses present. ese factors, combined with an acute alteration to the bulk substrate, are likely to ‘spall’ (i.e. delaminate and break-away). Conversely, if a sub-surface feature pro- duces severe plastic deformation, evidence has shown in particular for the die and tool industry, that some dies benet from increased life due to enhanced abra- sion resistance. From Table 13, the design engineer can see that by simply selecting a production process without an inti- mate knowledge of how components are to be manu- factured will inevitably aect the subsequent part’s in-service application. Moreover, due regard must be given to the machined workpiece’s potential sub-sur- face state, as this condition will inexorably lead to problems in terms of potential impairment of its ser- vicing needs and reliability. Surface integrity Manipulation – Burnishing Par t’s for Surface Improvement Burnishing and in particular roller burnishing (Fig. 192) is a very fast production technique for improving both the nish and dimensional accuracy of either an internal, or external surface, by pressure rolling with- out removal of workpiece material. Roller burnishing is a cold-working process, that produces a ne surface texture by the application of the planetary rotation of hardened rolls over the previously machined bored, or turned surface (Fig. 192c). Moreover, unlike the pri- mary forming process of cold-rolling which normally produces large sectional changes, roller burnishing involves cold-working just the surface layers of the workpiece, to improve the surface structure. Roller burnishing tooling (Fig.192a) can be used for minute diameter adjustment down to 25 µm, allow - ing component dimensional accuracies of ±0.006 mm to be obtained. e action of roller burnishing causes plastic deformation of the workpiece’s previously ma- chined surface. At a given depth below the burnished surface, the material is elastically deformed and at- tempts to spring back. is action, gives rise to com- pressive stresses at the surface and tensile stresses in the elastically-deformed zone. is complex stress interaction increases the resistance of the material to fatigue failure, because any external forces must rstly overcome these residual stresses. e potential for cracking that can occur due to the interaction between the static and tensile stresses in the metal and a corrosive medium is termed ‘stress corrosion cracking’. During roller burnishing, these tensile stresses are eliminated when the burnising tool compresses the workpiece surface. Likewise, any pits, scratches and porosities in the surface, which might otherwise collect reactive substances and con- taminants, are eliminated, hence, roller burnishing in- creases the corrosion resistance of the material. Crystalline materials typied by their metal lattices, are never completely without aws. e atomic lattice will always contain built-in irregularities of various types. ese so-called atomic dislocations reduce the strength of the material, as less force is necessary to alter the atomic lattice. Dislocation motion of atoms is a complex subject, which goes beyond the scope of the present text, however, it can be said that upon the application of an external load (i.e. burnishing tool- ing), because the lattice is invariably not perfect, less force is necessary to defrom the structure. Here, an at- tempt is made to inhibit the movement of dislocations by means of diering hardening procedures. Cold- working increases the number of dislocations and one would expect the material to become soer, but in fact, the opposite eect transpires. is increased hardness takes place, because there are so many dislocations as a result of cold-working, that they prevent and restrict each other’s motion, as a result the surface hardens. is is what occurs in roller burnishing, as the material is displaced and the net result is that it becomes both harder and stronger – due to dislocation obstructions. By way of a cautionary note, both Rockwell and Brinell hardness testing methods cannot realistically obtain surface hardnesses readings satisfactorily, therefore it is recommended that the Knoop test (Fig. 192b) should be used, then converted with a suitable ‘hardness comparison chart’ – see the appropriate table in Appendix 12. is completes a brief synopsis of a discussion on certain aspects of both machinability and surface in- tegrity, which hopefully conveys the importance of the machining activities and the resulting machined sur- face condition. Considerably more space could have been devoted to a comprehensive review of these top- ics, but space was limited, this is the reason for a rea- sonably comprehensive list of references – for a more in-depth discriminating reading on these important machining and related issues.  Chapter  Figure 192. Roller burnishing improves the metallurgical properties of the previously machined surface. [Courtesy of Sand- vik Coromant] . 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In: Inuence of Metallurgy on Hole Making Opera- tions, ASM Pub. (Ohio), 145–158, 1978.  Chapter  . region extends to quite a small depth beneath the surface, in the region of 10 to 100 µm, de - pending upon the severity of the ‘abusive regime’ of surface generation. Considering Fig. 191 once. characteristics within the component’s surface region, being inuenced by the mode of milling: up-cut or down-cut . Machinability and Surface Integrity  Figure 191. Comparison of the residual. ‘white-layering’ pro- duction.  Chapter  Figure 189. The inuence of the cutting edge’s condition on the resultant machined surface integrity . Machinability and Surface Integrity  In Fig.