This is a great article! I'd like to implement this approach. Do you back the boring head off the ID wall before retracting, or do you retract the boring head straight out? How much material do you leave for the boring head after the drill? Do you have more information on this application/technique? Thank you for the support!

What ishardenabilityin Materials Science

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What’s more, as bores get longer, stepped and/or more numerous, important decisions come into play. For example, if the typical approach is to use one tool, send it in the bore, offset it, pull it out and repeat, you’re sacrificing much-needed rigidity at the back end of larger holes.

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Spade- and replaceable-tip drills are also common holemaking solutions. However, they do not have the same flexibility as the indexable insert type drills and should not be used off center for additional boring type operations. Their lead angles are limiting and reaching them or deep into a hole, over and over, introduces risk while accelerating insert wear on only one cutting edge. Plus, drills don’t control chip wash very well, as their design pushes chips backward, often scarring up the hole’s finish.

Hardenability definitionsteel

These common issues are the reason we have developed ways to make some of our world-class boring tools—traditionally used on milling machines—just as effective on a CNC lathe.

Indexable insert drills are nice, in that they can be used not only for their specified size but can also be offset to make a slightly smaller or larger hole; the tool itself doesn’t have adjustability, but you can shift the tailstock over itself to get a different final hole size. They can also be additionally offset and re-ran through the hole as a boring bar. That's not something that you can do when rotating on center as with a milling spindle. It’s one reason you rarely see a finish boring tool on a CNC lathe, unless it’s for off-axis work.

Figure 1. Multiplying factors for most common alloying elements (other than carbon) used to determine ideal diameter in steel. Data plotted from equation provided in ASTM A255-10 [2].In plain carbon steels, a smaller austenite grain size has been shown to markedly, and negatively, influence hardenability [3]. The generally accepted mechanism for this observation is that increasing grain size decreases the grain boundary surface area and, therefore, the number of inhomogeneous nucleation sites for ferrite and pearlite [1]. However, this grain-size effect can be confounding in both low alloy and microalloyed steels. In these steels, the amount of alloying still in precipitate form can be significant at common austenitizing temperatures. In general, having the alloying in solid-solution yields the greatest influence on diffusional transformations, increasing hardenability.

1. R.E. Reed-Hill and R. Abbaschian, Physical Metallurgy Principles, ed. 3, 1992. Standard Test Methods for Determining Hardenability of Steel, ASTM A255-10 (reapproved 2014), ASTM International, Oct. 2014. 2. M. A. Grossmann, Elements of Hardenability. American Society for Metals, 1952. 3. M. F. Ashby and K. E. Easterling, “A First Report on Diagrams for Grain Growth in Welds,” Acta Metallurgica, vol. 30, pp. 1969-1978, 1982. 4. B. Garbarz and F. B. Pickering, “Effect of Vanadium and Austenitising Temperature on Hardenability of (0.2-0.3)C-1.6Mn Steels With and Without Additions of Titanium, Aluminium, and Molybdenum,” Materials Science and Technology, vol. 4, pp. 117 126, 1988. 5. B. Garbarz and F. B. Pickering, “Effect of Austenite Grain Boundary Mobility on Hardenability of Steels Containing Vanadium,” Materials Science and Technology, vol. 4, pp. 967 975, 1988. 6. H. Adrian, “A Mechanism for Effect of Vanadium on Hardenability of Medium Carbon Manganese Steel,” Materials Science and Technology, vol. 15, no. 4, pp. 366 378, 1999. 7. H. Adrian and R. Staśko, “The Effect of Nitrogen and Vanadium on Hardenability of Medium Carbon 0.4% C and 1.8% Cr Steel,” Archives of Materials Science and Engineering, vol. 33, no. 2, pp. 69 74, 2008. 8. C. Fossaert, G. Rees, T. Maurickx, and H. Bhadeshia, “The Effect of Niobium on the Hardenability of Microalloyed Austenite,” Metallurgical and Materials Transactions A, vol. 26, no. 1, pp. 21 30, 1995.

When boring holes on CNC lathe becomes too time-intensive, it’s time to reconsider your approach. Think creatively, invest in your own success. As they say, there’s more than one way to skin a cat, and twin cutters have proven to be one of those other ways that have proven to be even more effective than traditional approaches.

Hardenability definitionEngineering

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Definehardenabilityof steel

This solid carbide bit cuts from the surface down leaving a smooth edge at the surface.

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Hardenability is often used synonymously with “end-quench” or “Jominy” hardenability, a reference to a standardized test that quantifies the hardness of a steel as a function of distance from an austenitized and water-quenched surface [1, 2]. This test has both beneficial and detrimental aspects. It is beneficial in that it allows the chemical composition variation of the raw material (e.g., bar or billet) to be characterized for quality control purposes with relative ease, but is detrimental in that it is limited in translating those results to components processed through a specific heat treatment process. As a result, for the remainder of this discussion, hardenability will be defined generally as the suppression of ferrite/pearlite formation upon cooling from austenite [1]. This definition encompasses those additional effects that are directly dependent on a specific heat treatment process.

Fraction of alloy carbides dissolved upon austenitizing may have a significant influence on hardenability, thus requiring careful monitoring of the starting microstructure as well as the chemistry. Each heat-treatment process should be evaluated to determine its acceptable hardenability range to remain a capable process. Influence of both variation in microstructure and chemical composition should be examined independently, if possible.

Hardenabilitycurve

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With a simple, short adaptor, our twin cutter rough boring heads can fit seamlessly into a lathe’s tooling package. The twin cutter series has a unique design feature allowing different roughing methods without the need to change components or use mechanical systems to adjust insert height.

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Hardenabilitytest

The addition of a twin cutter boring head to a shop’s tooling arsenal opens the door to multiple holemaking options. Once thought of only as light-duty, semi-finishing tools, today’s high-performance twin cutter boring heads have proven they can deliver in holemaking operations.

Figure 3. Multiplying factors for calculating the effect of vanadium on the hardenability of steel. Data from ASTM A255-10 [2] and Grossman [3].Figure 3 shows the influence of vanadium (V), a common microalloying element in steel, on hardenability as a function of austenitizing temperature (from Grossmann [3]) as well as data currently used in ASTM A255 for determining the hardenability of steels [2]. Although the current ASTM standard indicates austenitizing temperature has no dependence, Grossmann showed a clear effect. At V levels typical of modern medium carbon microalloyed steels (approx. 0.08 wt.%), the influence of austenitizing temperature may be significant. Since Grossman’s work, the influence of microalloying elements such as V and niobium (Nb) has been investigated in great detail, and the mechanisms found to be relatively complex [5-9]. Therefore, their influence on hardenability is mentioned briefly for awareness only.

Many heat-treating processes cannot tolerate appreciable variations in steel hardenability. For an established in-control process, deviations in chemical composition and starting microstructure may result in a variety of issues including quench cracking, out-of-spec hardness, low ductility, and excessive distortion. Consequently, a hardenability tolerance should be assessed and defined for all new processes to ensure incoming material is properly controlled without being over specified.

Jominy end quench test

Hardenabilityvs hardness

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ID boring is a common and time-consuming operation on CNC lathes. The process typically starts with a drill that creates an opening for a more rugged roughing tool to come in and rough out the bore.

For larger holes, this assembly also offers the flexibility to use extensions to build a “custom” or more rigid setup, a superior alternative to reaching long, skinny tools deep into holes. What’s more, if vibration continues to be a nagging issue, the twin cutter makes it possible to employ the industry-leading Smart Damper boring bar, which maintains damping near the tool.

Figure 2. Solubility of chromium carbide (Cr23C6) in austenite with nominal chemical compositions for AISI 4140, 5160, and 52100 indicated. Solubility product data from Ashby and Easterling [4].Figure 2 shows the solubility of chromium carbide (Cr23C6) in austenite. The nominal compositions for three commonly heat-treated alloys containing chromium (Cr) are indicated: AISI 4140, 5160, and 52100. The dashed line for each alloy represents the precipitation path for Cr23C6 in austenite assuming all Cr has precipitated as Cr23C6. This assumption is not likely accurate, but provides a worst-case scenario for discussion purposes. This data shows that a significant amount of Cr may still be in precipitate form in all three alloys unless austenitized at high temperature, where grain growth will occur. Steels that are hardened from a pearlite or spherodized starting microstructure will have substantially different hardening behavior depending on the carbide size and the austenitizing temperature. This emphasizes the importance of having good quality-control checks and purchase specifications in place to ensure issues do not occur.

Each heat-treatment process should be evaluated to determine its acceptable hardenability range to remain a capable process.

Eighty-degree diamond-shaped inserts provide two 90-degree cutting edges. This effectively doubles your feed rate instantly. If more cutting edges or speed would help and the negative six-degree lead angle is workable, you can switch to a square insert and have four effective cutting edges.

Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite.

Chemical effects on hardenability are typically calculated using chemical ideal diameter (DI) – which is defined as the diameter in which the center of a round bar contains 50 percent martensite when quenched from austenite [3]. Grossmann used brine for his quenchant, but ideal diameter typically assumes an “ideal” quench rate, which is infinite [3]. Figure 1 shows the effect of different alloying elements on hardenability through multiplying factors used to calculate DI. The danger with this multiplying factor comparison is that the data represent only one austenite grain size (ASTM grain size 7) and does not take into consideration effects related to the form of the alloying elements in the steel.