Applications · Hardened and Heat-Treated Steel
Hardened Steel Machining: Cutting Material, Edge Geometry and Cutting Data Above 45 HRC
Above roughly 45 HRC the workpiece stops being a soft material that wears a tool out and becomes a hard material that destroys one. The part is close enough in hardness to the cutting material that the two are competing, the heat the cut generates cannot escape through the workpiece, and the failure that follows is edge collapse rather than gradual wear. What decides the result is the cutting material — coated carbide at the lower hardness bands, CBN or ceramic at the top — together with an edge geometry and a depth of cut chosen for a surface that is already hard and often not round. Hard machining is usually a finishing operation competing with grinding rather than a bulk removal operation, and the cutting speeds published below for hardened turning, milling and drilling run far below the figures for soft steel, for reasons the failure modes explain.
Failure Modes
Three failure modes that decide how you cut hardened steel
Hard machining fails differently from ordinary machining. The tool does not wear down gently; it stops being hard enough at the temperature the cut reaches, and then it stops working. Reading the three modes below decides the cutting material, the edge preparation and how the part is held before anything is bought.
Failure mode 01
The heat has nowhere to go
What actually fails
Hardened steel conducts heat poorly, so instead of leaving with the chip the way it does on soft steel, a large part of the cutting heat stays at the edge. The edge softens, deforms plastically and then fails, and the workpiece keeps a thin heat-affected layer that is softer than the material underneath and harder to hold measured. The insert looks as if it has been pushed out of shape rather than worn, which is exactly what has happened.
What to look for in the tool
Look for a cutting material that holds its hardness at the temperature the cut actually reaches, and for an edge prepared for it: a honed edge with a controlled land and a negative rake makes the edge strong enough to survive the load, where a sharp positive edge on soft steel would collapse. Take a depth of cut large enough to remove the skin the previous pass left rather than skimming it, and choose whether to use coolant rather than letting it arrive in bursts.
Failure mode 02
The hard case and the soft core
What actually fails
A case-hardened or induction-hardened part is not one material. It has a hard shell over a core that is far softer and tougher, and the depth to which the shell extends is not always what the drawing implies. Cutting into the transition asks the edge to absorb a load that changes under it, and because the part distorts as it is quenched, the hardened surface is often out of round before the first cut is taken. The cut therefore enters and leaves the material on every revolution even when the operation is nominally continuous.
What to look for in the tool
Look at where the finished surface actually sits relative to the case, and plan the depth so the finishing pass cuts one material condition rather than straddling two. Treat an out-of-round hardened surface as an interrupted cut and say so when you order, because it is the interrupted case that decides between a hard grade and a tough one. Measuring the case depth on the actual part before quoting costs less than a scrapped batch.
Failure mode 03
Rigidity decides whether any of it works
What actually fails
Hard turning transfers the precision burden to the machine, the holder and the part. The cutting force is lower than in soft steel, but so is the tolerance, and a holder that deflects a few microns or a workpiece that rings under the cut will show up directly in the measured result. No cutting material compensates for that, and a harder insert on a flexing setup fails sooner rather than later.
What to look for in the tool
Look at the setup before the grade: the shortest possible overhang, the most rigid holder that fits, and a part that is supported rather than cantilevered. Where the machine or the part cannot be made rigid, the answer is a lighter depth of cut with a tougher cutting material rather than a harder one, and often two finishing passes where one was planned.
One boundary is worth stating plainly: hardened here means steel that has been through-hardened or case-hardened above roughly 45 HRC, together with quenched and tempered wear plate. Steel in the normalised or annealed condition is cut as ordinary carbon and alloy steel, even when the same part number appears on both. If the drawing carries a hardness callout rather than a material specification, that callout decides which cutting data applies — above roughly 45 HRC the figures here, below it the carbon and alloy steel data. The ISO H group itself spans 40 to 60 HRC, and a row in the table is published at the hardness that row was tested at, so a published figure can sit either side of the 45 HRC line.
Two cases sit outside these figures. Hardened stainless steel is cut on the stainless steel data rather than these numbers, and hardened tool steel at the top of the hardness range is usually ground rather than cut, because the operation stops being economic before it stops being possible.
Tool Matrix
Which tool family for which hardened steel operation
Turning leads the matrix here, because hard turning is the operation that most often replaces grinding outright. Milling and drilling follow, and both are used at the softer end of the hardened range far more than at the top of it.
The dividing line that runs through all three blocks is hardness band rather than operation. A given cutting material earns its place within a band, and the same band in a different operation takes the same cutting material with a different edge preparation.
Turning
Hard turning is a finishing operation in which the cutting material does the work that abrasive grinding would otherwise do. The choice is between coated carbide and the superhard materials, and it is set by hardness, continuity of cut and the tolerance on the drawing.
CBN inserts for the hardest surfaces
Cubic boron nitride, second only to diamond in hardness and stable at the temperatures hard turning produces. Chosen for through-hardened surfaces at the top of the range, where a coated carbide edge would deform before it wore.
Coated carbide inserts for the lower hardness bands
Coated carbide grades with hard substrates and a honed edge preparation, for continuous hard turning from around 40 HRC upward, and tougher grades where the hardened surface is not round and the cut is interrupted.
All turning shapes and cutting materials
The ISO turning shapes available in hard-turning grades and in superhard materials, with the note that belongs to each, so a shape can be settled before the cutting material is chosen.
Milling
Hard milling is a finishing and semi-finishing operation, used where a hardened pocket or profile has to be cut without going back to a grinder. It is less forgiving than hard turning, because the cut enters and leaves the material on every tooth.
Milling inserts for hardened steel
Wear-resistant grades for continuous passes in the lower hardness bands, and tougher grades where the cut is interrupted or where the hardened skin is uneven, since it is the interruption that decides the grade rather than the average hardness.
Solid carbide end mills for hardened steel
Coated solid carbide with a strengthened edge for finishing hardened pockets, profiles and edges, taking the place of a grinding operation where the geometry is difficult to reach and the depth of material to remove is small.
All milling and holemaking ranges
Insert milling, solid end mills, drills and threading tools in one place, grouped by the operation they are bought for, so the hardened part can be planned as a whole rather than tool by tool.
Drilling, threading and workholding
Holes and threads in hardened material are the operations most likely to end a part, because the tool is enclosed by the workpiece and a broken tool is rarely recoverable. The hardness band decides whether the operation is routine or whether the hole should be put in before heat treatment.
Drill bits for hardened steel
Indexable and solid carbide drills with internal coolant for the lower hardness bands, and grades intended for quenched and tempered plate where the surface is hard but uniform. At the top of the range, a hole planned before heat treatment is usually the cheaper answer.
Threading tools for hardened and high-strength material
Thread milling and carbide taps for threads in hardened and high-strength parts, where an HSS tap has neither the hardness nor the edge strength to survive the cut and where a broken tap inside a finished part is the outcome to avoid.
Tool holders and boring bars for hard turning
Rigid holders and boring bars with the shortest practical overhang. Hard turning to a tolerance is mostly a stiffness problem, and the holder is where stiffness is cheapest to buy.
Cutting Data
Published cutting speeds for turning, milling and drilling hardened steel
The table gives the surface speed published for hardened steel in each operation, drawn from the same source data as the cutting speed reference so the two cannot disagree. The figures are low compared with soft steel, and that is the honest picture: in hard machining the edge survives by keeping the heat in the chip, and even then only within a narrow window.
Hardness band matters more than operation here. A 55 to 60 HRC surface runs at a fraction of the speed of a 40 HRC one, in every operation, and quenched and tempered wear plate behaves as a separate case because its hardness is uniform through the thickness rather than concentrated in a case.
| Hardness band | Operation | Tool family | Cutting speed (m/min) | Cutting speed (SFM) | Basis of the range | Source (as recorded) |
|---|---|---|---|---|---|---|
| Hardened steel 44 to 48 HRC | Turning | coated indexable carbide, hard-turning grades | 10-90 | 30-290 | published across the hard-turning grades the source lists for this band | KCHS25S 10-90; KCU25S 10-60 |
| Hardened steel 48 to 55 HRC | Turning | coated indexable carbide, hard-turning grades | 10-90 | 30-290 | published for the band as a single row | H2 row |
| Hardened steel 55 to 60 HRC | Turning | coated indexable carbide, hard-turning grades | 10-80 | 30-262 | published for the band as a single row | H3 row, SFM min from source, max converted |
| Quenched and tempered wear plate, 40 to 50 HRC | Turning | tough coated carbide | 70-130 | 230-427 | the three figures are the published machining positions across the plate | Vc 130-90-70 m/min; SFM converted |
| Hardened steel under 54 HRC | Milling | coated indexable carbide | 30-40 | 98-131 | published as a single row for the band | p.11; SFM converted |
| Hardened steel 45 to 50 HRC | Milling | coated solid carbide | 53-69 | 175-225 | the published column for this band, converted from surface feet per minute | OSG VG446 col.5 = 175-225 SFM (45-50 HRC, side milling); SFM converted |
| Tool steels and high-strength alloys, 40 to 50 HRC | Milling | HSS-Co | 5-10 | 16-32 | published as one column with the hardness band printed above it | OSG p.1411 col.2, 16-32 SFM (hardness 40-50 HRC printed above this column) |
| Hardened steel under 54 HRC | Drilling | indexable coated carbide | 20-50 | 66-164 | published as a single row for the band | p.144; SFM converted |
| Quenched and tempered wear plate, 400 to 500 grade | Drilling | solid carbide, internal coolant | 40-80 | 131-262 | three published plate grades; the range spans all three | Hardox 400 50-70; 450 40-60; 500 40-60; SFM converted |
| Quenched and tempered wear plate, 370 to 540 HBW | Drilling | HSS, HSS-E and HSS-Co drills | 3-12 | 10-39 | published as a range with one grade recommended by the steel supplier | HSS-Co 8% recommended by SSAB; SFM converted |
Sources: recommended cutting speeds are taken from the published application data of tool makers — Kennametal TopSwiss MBS and inserts application data, Ingersoll IMC Cutting Data and Parameters handbook, OSG VGM7 / VG446 / HSS-Co datasheets, Guhring RF 100 series, Uddeholm Unimax cutting data and SSAB Hardox machining recommendations. The reference recorded for every row — as it is logged in our data, which is a document page, a column, a document number, or the grade breakdown the document itself prints — is carried in the last column of the table, so each figure can be taken back to what it came from without asking us for it. Values published only as surface feet per minute were converted at 1 m/min = 3.28084 SFM and are marked as converted. The rows in the table are the extremes of the published range for each hardness band and operation, with no interpolation between them. ISO P / M / K / N / S / H group letters follow ISO 513:2012.
Read the row that matches your hardness band and operation, and start at the bottom of the window. The upper figure assumes a rigid setup, a first-choice cutting material and a continuous cut, and a hard-turning job rarely satisfies all three at once. Where the cut is interrupted, expect to stay in the lower half of the window and to accept a shorter tool life rather than to find a faster setting.
The table gives surface speed only. Feed and depth of cut are properties of the insert geometry and the machine rather than of the hardness band, and a fixed figure printed here would be wrong for most readers. Thread cutting follows the same rule: the published figures that govern a thread in hardened material come with the specific tap or thread mill, and the practical answer is often to cut the thread before heat treatment. Send us the hardness band and the thread callout and we come back with that figure rather than a general one.
From surface speed to spindle speed
- Spindle speed in metric: n = (vc x 1000) / (pi x D), with vc in m/min and D the diameter in millimetres.
- Spindle speed in imperial: n = (SFM x 3.82) / D, with D the diameter in inches.
Worked from the table above
- Turning a 60 mm hardened shaft at 55 HRC: at 60 m/min, n = (60 x 1000) / (3.14159 x 60) = 318 rpm.
- Finishing a 40 mm bore at 48 HRC: at 80 m/min, n = (80 x 1000) / (3.14159 x 40) = 637 rpm.
- Milling a hardened pocket at 250 SFM: with a 12 mm cutter, n = (250 x 3.82) / 0.472 = 2,021 rpm.
- The top of the hardness range: a 55 to 60 HRC surface turned at 40 m/min gives n = (40 x 1000) / (3.14159 x 60) = 212 rpm on a 60 mm diameter, which is the point at which the operation is closer to grinding than to turning.
Put it on the machine
The same arithmetic is on the cutting speed reference, with drill and tap diameters from every series rather than the hardened steel rows above, so a figure read here can be turned into a spindle setting for any tool in the job.
Where the table gives a window rather than a figure, start at the bottom of it on an unfamiliar setup and move up while the chip breaks cleanly and the finish holds. The machine, the holder and the part decide how far up the window you can go; the table only says where the window is.
Data and Service
Where the figures come from
Every figure here carries its origin, and the three kinds of source are kept apart: published standards named by number and year, industry figures named by the document they were read from, and customer feedback written up as what it is.
Published standards
The ISO P / M / K / N / S / H application groups used to describe the tooling come from ISO 513:2012, which fixes what each letter covers. Where a figure comes from a standard, it is named with the standard and the year it was issued, per row.
Tool makers and industry data
The cutting-speed ranges in the table above are the values printed by the tool makers our data is taken from. They are used as published, at the extremes of the published window, and every row carries its document and page or column reference in our source file so any figure can be checked before it is used on a job.
Trial feedback
Where a customer tells us what a grade actually did on their machine, it is recorded as an attributed note rather than reworked into an illustrative story. We do not publish references we cannot point to.
What comes with a quotation
Two things we do go beyond the table. Both are actions rather than figures, and both are visible in a quotation before anything is ordered.
A starting cutting speed with the quotation
Send the hardness specification or the case depth, the operation, and the machine it runs on. The quotation comes back with a starting surface speed and a feed range for the insert family we propose. Treat it as the first setting rather than a guarantee of the result: the figure is decided by your setup, the holder and the part, and it is meant to be adjusted from the chip and the finish.
Trial quantities to prove it on the machine
The honest answer to "will this grade run on my hardened steel" is a trial order, not a data sheet. Small quantities and mixed orders are welcome, and charges and freight are confirmed against the actual enquiry rather than published as a fixed number.
How we handle a tooling enquiry from first question to delivery
Questions Buyers Ask
Hardened steel questions we are asked before quoting
The changeover is a band rather than a line, and it moves with the continuity of the cut. Coated carbide with a honed edge covers hard turning from around 40 HRC upward and is often the right answer into the low fifties on a continuous cut. Above roughly 55 HRC, or wherever the cut is interrupted on a very hard surface, CBN is the material the job is asking for. Tell us the hardness, the depth of cut and whether the cut is continuous and we will say which side of the band we would quote.
The published windows in the table above run from 10 m/min at the top of the hardness range to 130 m/min on quenched and tempered wear plate. Start at the bottom of the window for your band, and treat the upper figure as a target for a rigid setup, a first-choice cutting material and a continuous cut rather than as a setting. On a 60 mm shaft at 55 HRC, 60 m/min is only 318 rpm, which is the practical picture of hard turning.
Because the edge reached a temperature at which it was no longer hard enough to cut, and the load then pushed it out of shape. This is the characteristic failure of hard machining: the heat cannot leave through the workpiece, so it stays at the edge. A cutting material that holds hardness at higher temperature, a honed edge preparation instead of a sharp one, and a larger depth of cut that removes the skin rather than skimming it are the three levers, in that order.
It depends on the cutting material and on the machine, and the one strategy to avoid is coolant arriving at the edge in bursts, which thermally shocks a hard, brittle edge. Many hard-turning operations in the low bands run dry by design, so the heat leaves with the chip, and the higher bands often run with a directed, continuous flow rather than a flood. Tell us the band and the cutting material and we will be specific rather than general.
Often yes, below the top of the range and where the material to be removed is small. Hard milling is a finishing and semi-finishing operation: it earns its place on profiles and pockets that are awkward to reach with a wheel, and it does not replace grinding for bulk removal or where the surface finish requirement is at the extreme. The deciding factor is usually how much material is left on the surface rather than how hard the part is.
Treat it as an interrupted cut and say so when you order, because that single fact moves the answer from a wear-resistant grade to a tougher one. Distortion during quenching is normal, so the first pass often enters and leaves the material on every revolution. Where the out-of-roundness is large, a roughing pass that cuts air as well as metal is cheaper than a finishing grade used up on a surface that is not yet continuous.
The rule that matters is to take a depth that removes the layer the previous pass altered rather than skimming it, because a light pass on a hardened surface rides on the hardest skin and dulls the edge without producing a clean cut. Feed and depth are otherwise set by the insert geometry and by the rigidity of the setup, so we give the starting figures with a quotation against the actual part rather than publishing one number here.
Yes. Send the hardness specification or the case depth, the operation and the machine, and we come back with the cutting material, the edge preparation and a starting surface speed for it. Where a part is case-hardened rather than through-hardened, say so: the case depth decides whether the finishing pass stays inside the case, and that decides the depth of cut before it decides anything about the tool.
Send the hardness, the operation and the setup — get the cutting material and a starting speed
Tell us the hardness band or the case depth, what the part is and how it is held. You get the cutting material and edge preparation we would quote, the holder for the setup, and a starting surface speed for the combination, rather than a catalogue to search through.