Workpiece Materials & Metallurgy for Machinists

materials · metallurgy · steel · aluminum · titanium · heat-treatment

Knowing your workpiece material is half the battle in machining. The material determines your tooling selection, speeds and feeds, coolant strategy, chip behavior, and how the part will perform in service. This article covers the materials you will encounter most often, how heat treatment changes them, and what to expect when cutting each one.

ISO Material Classification (P/M/K/N/S/H)

Insert manufacturers use the ISO system to classify workpiece materials into six groups. Every carbide insert grade is designed for one or more of these groups. When a catalog says an insert is "P20-P30," it means it is optimized for medium to roughing cuts in steel.

Group	Color	Materials	Key Characteristics
P	Blue	Carbon steel, alloy steel, ferritic/martensitic stainless	Long chips, moderate abrasion, good machinability
M	Yellow	Austenitic stainless, duplex stainless	Work hardening, gummy, built-up edge tendency
K	Red	Cast iron, CGI (compacted graphite iron)	Short/discontinuous chips, abrasive, dusty
N	Green	Aluminum, copper, brass, non-ferrous	Soft, long chips, built-up edge at low speeds
S	Orange	Heat-resistant superalloys (Inconel, titanium, cobalt)	Work hardening, high cutting forces, poor thermal conductivity
H	Grey	Hardened steel (>45 HRC)	Extremely abrasive, high heat, requires CBN or ceramic

When selecting inserts, match the ISO group first, then narrow by the application number (lower = finishing, higher = roughing).

Carbon and Alloy Steels (ISO P)

AISI 1018 (Low-Carbon Steel)

Composition: 0.18% carbon, low manganese
Hardness: 126 HB (annealed), carburized surfaces can reach 55-62 HRC
Machinability rating: 78% (relative to 1212 free-machining steel at 100%)
What to expect: Soft and gummy. Tends to produce long, stringy chips that wrap around tooling. Wants to build up on the cutting edge at lower speeds. Run higher surface speeds (500+ SFM with carbide) and use a positive-rake insert to get clean cuts. Chip breakers help significantly.
Common uses: Shafts, pins, fixtures, case-hardened gears. The cheap general-purpose steel.
Tip: If 1018 is giving you grief with finish, try 12L14 (leaded free-machining steel) instead. It machines like butter. Just check if lead-free is required.

AISI 1045 (Medium-Carbon Steel)

Composition: 0.45% carbon
Hardness: 163 HB (annealed), can be through-hardened to 50-55 HRC
Machinability rating: 57%
What to expect: Much better chip formation than 1018. Breaks chips more readily and produces a better surface finish. Still reasonably easy to machine in the annealed condition. Harder on tools when hardened -- expect 30-40% reduction in tool life versus annealed.
Common uses: Shafts, bolts, gears, hydraulic cylinders. The go-to medium-carbon steel when you need more strength than 1018 without going to alloy steel.
Tip: If the print calls for a specific hardness (e.g., 28-32 HRC), it will need heat treatment after rough machining. Leave stock for finish machining after heat treat because the part will distort.

AISI 4140 (Chrome-Moly Alloy Steel)

Composition: 0.40% carbon, 0.80-1.10% chromium, 0.15-0.25% molybdenum
Hardness: 197 HB (annealed), commonly heat treated to 28-32 HRC for most applications, can reach 54 HRC
Machinability rating: 66%
What to expect: Excellent all-around alloy steel. Machines well in the annealed or pre-hard (28-32 HRC) condition. At 28-32 HRC, chip formation is good, surface finish is excellent, and tool life is reasonable. Above 40 HRC, tool life drops significantly -- use coated carbide or ceramic.
Common uses: Gears, shafts, axles, bolts, hydraulic components, tooling. The workhorse of the alloy steels.
Tip: Pre-hard 4140 (bought already heat treated to 28-32 HRC) eliminates the need for heat treatment and is the preferred starting point for many shops. It costs slightly more per pound but saves the heat treat cycle.

AISI 4340 (Nickel-Chrome-Moly Alloy Steel)

Composition: 0.40% carbon, 0.70-0.90% chromium, 1.65-2.00% nickel, 0.20-0.30% molybdenum
Hardness: 217 HB (annealed), commonly used at 36-42 HRC, can reach 57 HRC
Machinability rating: 57%
What to expect: Tougher than 4140 in every sense. Higher cutting forces, more tool wear, but excellent mechanical properties. When heat treated to 40+ HRC, it gets demanding. Use strong insert geometries, reduce speeds 15-20% from 4140 values, and maintain positive coolant flow.
Common uses: Landing gear, crankshafts, power transmission, high-strength structural components. The upgrade from 4140 when you need more toughness and fatigue resistance.

Tool Steels (ISO P/H depending on hardness)

Tool steels are designed to be heat treated to high hardness and hold an edge or resist wear. They are typically machined in the annealed condition (soft) and then heat treated.

D2 (Cold-Work Die Steel)

Composition: 1.50% carbon, 11.50% chromium
Hardness: ~230 HB annealed, 58-62 HRC hardened
What to expect: High chrome content makes it abrasive even when annealed. Carbide tools wear faster than in 4140. When hardened to 60 HRC, only CBN or ceramic inserts will cut it. It also moves during heat treatment, so leave 0.010-0.015 per side for finish grinding.
Common uses: Stamping dies, blanking dies, slitting knives, gauges.

A2 (Air-Hardening Die Steel)

Composition: 1.00% carbon, 5.00% chromium
Hardness: ~200 HB annealed, 57-62 HRC hardened
What to expect: Easier to machine than D2 in the annealed state. Less distortion in heat treatment than water- or oil-hardening steels (air-hardening is more dimensionally stable). Good balance of toughness and wear resistance.
Common uses: Blanking dies, forming dies, gauges, punches, injection mold components.

M2 (High-Speed Steel)

Composition: 0.85% carbon, 6.00% tungsten, 5.00% molybdenum
Hardness: ~230 HB annealed, 60-65 HRC hardened
What to expect: This is the stuff drill bits and taps are made from. Very abrasive when annealed due to tungsten and molybdenum carbides. Use coated carbide inserts and keep speeds moderate (200-250 SFM annealed).
Common uses: Cutting tools (drills, taps, reamers, end mills), broaches, cold-forming tools.

S7 (Shock-Resistant Tool Steel)

Composition: 0.50% carbon, 3.25% chromium, 1.40% molybdenum
Hardness: ~200 HB annealed, 54-58 HRC hardened
What to expect: Designed for impact resistance. Machines similarly to 4140 in the annealed state. Tougher than D2 or A2 at the same hardness.
Common uses: Chisels, punches, shear blades, hammer dies, injection mold components requiring impact resistance.

H13 (Hot-Work Die Steel)

Composition: 0.40% carbon, 5.00% chromium, 1.00% vanadium, 1.50% molybdenum
Hardness: ~192 HB annealed, 44-52 HRC hardened (typically used at 44-48 HRC)
What to expect: Designed to handle hot work (die casting, forging, extrusion). Machines well annealed. At working hardness (44-48 HRC), it is in the transition zone where carbide still works but tool life is limited. Coated carbide with strong edge prep is essential.
Common uses: Die casting dies, forging dies, extrusion dies, hot shear blades, injection mold inserts.

Stainless Steels (ISO M and P)

303 (Free-Machining Austenitic)

Composition: 18% chromium, 8% nickel, added sulfur for machinability
Hardness: 160 HB
Machinability rating: 78% -- the best of the austenitic stainless grades
What to expect: The sulfur inclusions act as chip breakers, giving much better chip control than 304. Still work hardens, but far less problematic than 304 or 316. If you have a choice and the part does not need welding or high corrosion resistance, specify 303.
Common uses: Shafts, fittings, fasteners, valve components. Anywhere you need stainless and the part is machined from bar.

304 (General-Purpose Austenitic)

Composition: 18% chromium, 8% nickel
Hardness: 150-170 HB (annealed)
What to expect: This is where stainless gets difficult. 304 work hardens aggressively. If your tool rubs instead of cuts (dull tool, too light a chip load, dwelling in a cut), the surface hardens and the next pass is cutting harder material. The fix: sharp tools, positive rake, adequate chip load (do not take light finish passes -- commit to the cut), and flood coolant.
Common uses: Food/beverage equipment, chemical processing, architectural, general corrosion-resistant applications.
Tip: Never use reground or chipped tools on 304. A dull edge generates heat and work hardening. Keep your inserts fresh and index at the first sign of wear.

316 (Marine-Grade Austenitic)

Composition: 16% chromium, 10% nickel, 2% molybdenum
Hardness: 150-170 HB
What to expect: Everything that makes 304 difficult, but slightly worse. The molybdenum improves corrosion resistance but increases cutting forces and work hardening. Reduce speeds 10-15% from 304 parameters.
Common uses: Marine hardware, chemical processing, pharmaceutical, medical devices.

17-4 PH (Precipitation-Hardening)

Composition: 17% chromium, 4% nickel, 4% copper
Hardness: 150 HB (Condition A, solution annealed), 35-44 HRC after aging (H900-H1150 conditions)
What to expect: In Condition A, it machines like a mild steel -- relatively easy. After precipitation hardening (aging), machinability drops significantly. Most shops rough machine in Condition A, then age, then finish machine. At H900 (44 HRC), use coated carbide with reduced speeds.
Common uses: Aerospace structural components, valve stems, gears, shafts requiring both strength and corrosion resistance.

440C (Martensitic, High-Carbon)

Composition: 17% chromium, 1.00% carbon
Hardness: ~230 HB annealed, 56-60 HRC hardened
What to expect: In the annealed state, it is abrasive due to chromium carbides. Machine it annealed, heat treat, then grind to finish dimensions. Machining after hardening requires CBN.
Common uses: Bearings, valve seats, surgical instruments, high-quality knives.

Cast Iron (ISO K)

Grey Cast Iron (ASTM A48, Class 30-40)

Composition: 2.5-4.0% carbon (present as graphite flakes)
Hardness: 160-210 HB
What to expect: Produces short, discontinuous chips (essentially dust). No coolant needed for most operations -- run dry with air blast or vacuum for chip removal. The graphite flakes act as natural lubricant. Very easy on tools but the abrasive nature of the casting skin (first cut) wears inserts quickly. Always take a heavy first cut to get below the casting skin.
Common uses: Engine blocks, machine bases, pump housings, pipe fittings.
Tip: The dust is abrasive and gets everywhere. Use way covers and vacuum systems. Cast iron dust will destroy linear guides.

Ductile Iron (ASTM A536, 65-45-12)

Composition: Similar to grey iron but with nodular (spheroidal) graphite from magnesium treatment
Hardness: 170-250 HB depending on grade
What to expect: Tougher than grey iron. Produces short chips but not as cleanly as grey. Higher cutting forces than grey iron. More similar to steel in cutting behavior. Some grades require coolant.
Common uses: Crankshafts, gears, suspension components, hydraulic cylinders, pipe fittings.

CGI (Compacted Graphite Iron)

Composition: Graphite in a vermicular (worm-like) form, between flake and nodular
Hardness: 200-260 HB
What to expect: 75% higher tensile strength than grey iron, which means higher cutting forces. More abrasive than ductile iron. Tool life can be 50% less than grey iron. Increasingly used in diesel engine blocks where the old grey iron was not strong enough.
Common uses: Diesel engine blocks, exhaust manifolds, brake components.

Aluminum Alloys (ISO N)

6061-T6 (General-Purpose Wrought)

Composition: 1.0% magnesium, 0.6% silicon
Hardness: 95 HB (T6 temper)
What to expect: The default aluminum for machining. Machines beautifully at high speeds (1000+ SFM with carbide, 300+ SFM with HSS). Chips break well, surface finish is excellent. Watch for built-up edge (BUE) at low speeds -- keep the speed up or use a polished/coated insert.
Common uses: Structural components, brackets, housings, fixtures, jigs.
Tip: Use 2- or 3-flute end mills to get chip clearance. Aluminum packs in flutes quickly at high feed rates. Uncoated polished carbide or ZrN-coated tools perform best.

7075-T6 (High-Strength Wrought)

Composition: 5.6% zinc, 2.5% magnesium, 1.6% copper
Hardness: 150 HB (T6 temper)
What to expect: Significantly harder than 6061. Still machines well at high speeds but tool life is shorter. Chips are more segmented. Cutting forces are 30-40% higher than 6061. Excellent for aerospace structural parts.
Common uses: Aircraft structural components, mold plates, high-performance sporting goods.

2024-T351 (Aircraft Alloy)

Composition: 4.4% copper, 1.5% magnesium
Hardness: 120 HB
What to expect: Harder and tougher than 6061 but not as strong as 7075. Good machinability with long, curly chips. The copper content makes it machines differently from 6061 -- slightly more abrasive.
Common uses: Aircraft structures (fuselage, wing skins), fittings.

Titanium (ISO S)

Ti-6Al-4V (Grade 5)

Composition: 6% aluminum, 4% vanadium, balance titanium
Hardness: 334 HB (annealed), 36-39 HRC (STA)
What to expect: Titanium is a demanding material. Key challenges:
Poor thermal conductivity -- heat does not dissipate into the chip or workpiece; it concentrates at the cutting edge. This destroys tools quickly
Work hardening -- like stainless but worse. Never let the tool rub. Maintain positive chip load at all times
Spring-back -- titanium has a low modulus of elasticity. It deflects under cutting pressure and springs back, causing rubbing on the flank face. Use sharp tools with relief
Chemical reactivity -- at high temperatures, titanium reacts with most tool materials. Use uncoated carbide (C2 grade) or PCD. TiAlN coatings can cause diffusion wear because of the titanium in the coating
Cutting parameters: 150-250 SFM with carbide (significantly lower than steel), heavy chip loads (0.004-0.008 IPT), high-pressure coolant (1000+ PSI through the tool is ideal), climb milling strongly preferred
Common uses: Aerospace structural components, medical implants, marine hardware, racing components.
Tip: If you are new to titanium, start conservative (150 SFM) and work up. A crashed tool in titanium often means a scrapped part because titanium work hardens the surface where the tool failed.

Nickel-Based Superalloys (ISO S)

Inconel 718

Composition: 52.5% nickel, 19% chromium, 5.1% niobium, 3% molybdenum, 18% iron
Hardness: 200-350 HB (solution annealed), 36-44 HRC (aged)
What to expect: Everything difficult about titanium, but worse. Inconel 718 is arguably the most challenging common material to machine:
Extreme work hardening -- the depth-of-cut line becomes a hardened layer that destroys subsequent passes if you vary your DOC
Very high cutting forces -- 50-100% higher than carbon steel
Heat generation -- worse thermal conductivity than titanium
Abrasive carbide particles -- niobium carbides in the microstructure act like sandpaper on the cutting edge
Notch wear -- the DOC line creates a notch on the insert that grows rapidly
Cutting parameters: 80-120 SFM with carbide (round or high-positive inserts), high-pressure coolant mandatory, ceramic inserts can run 600-1000 SFM but require rigid setups and specific geometries
Common uses: Jet engine components (turbine disks, blades, casings), nuclear reactors, high-temperature fasteners.
Tip: Program varying depth of cut (randomize between passes by 0.005-0.010) to spread the notch wear across more of the insert edge.

Copper, Brass, and Bronze (ISO N)

Free-Cutting Brass (C36000)

Hardness: 100-150 HB
What to expect: One of the easiest materials to machine. Excellent chip breakage, low cutting forces, beautiful surface finish. Runs at very high speeds (600-1000 SFM). No coolant required for most operations.
Common uses: Fittings, valve bodies, electrical connectors, decorative hardware.

Bronze (C93200 Bearing Bronze)

Hardness: 65-80 HB
What to expect: Soft and gummy. Long chips that want to wrap. Use sharp positive-rake tools and adequate chip load. Tin bronzes are abrasive and wear tools faster than you might expect for a "soft" material.
Common uses: Bushings, bearings, wear plates, gears.

Copper (C11000, ETP)

Hardness: 40-80 HB (varies with temper)
What to expect: Very soft, very gummy, very thermally conductive. The thermal conductivity means heat leaves the cutting zone quickly (good for tool life) but the softness means built-up edge is constant at low speeds. Run fast with sharp tools. PCD tooling gives excellent results.
Common uses: Electrical bus bars, heat sinks, electrodes for EDM.

Heat Treatment

Why Heat Treat?

Heat treatment changes the mechanical properties (hardness, toughness, strength, ductility) of steel by controlling its microstructure. As a machinist, you need to know when heat treatment happens relative to your machining operations and how much the part will move.

Annealing

Process: Heat to austenitizing temperature (1400-1600F for most steels), hold, then cool slowly (furnace cool). Purpose: Softens the steel for machining. Relieves internal stresses from rolling, forging, or prior machining. Machinist impact: Parts are at their softest and easiest to machine after annealing. This is when you do heavy roughing.

Normalizing

Process: Heat to austenitizing temperature, hold, then cool in still air. Purpose: Refines grain structure, relieves stress, produces more uniform hardness than annealing. Slightly harder than annealed. Machinist impact: Normalized stock is common for forged parts before machining. Predictable, uniform cutting behavior.

Hardening (Quench and Temper)

Process: Heat to austenitizing temperature, quench in oil/water/air (depending on steel grade), then temper at a lower temperature to achieve target hardness. Purpose: Produces the required hardness for service. Machinist impact: This is the big one. Hardened parts are harder to machine (obviously) and they DISTORT during quenching. A perfectly round shaft will come back oval. A flat plate will warp. Always leave finishing stock (0.010-0.020 per side minimum, more for long or thin parts) for post-heat-treat machining or grinding.

Tempering

Process: Reheating a hardened part to a temperature below the critical point (typically 300-1200F), holding, and cooling. Purpose: Reduces brittleness while retaining most of the hardness. Higher tempering temperatures reduce hardness but increase toughness. Machinist impact: The hardness specification on the print (e.g., "28-32 HRC") is achieved by selecting the tempering temperature. No dimensional change occurs during tempering.

Case Hardening (Carburizing)

Process: Heating low-carbon steel (e.g., 1018, 8620) in a carbon-rich atmosphere at 1600-1700F. Carbon diffuses into the surface to a specified depth (case depth, typically 0.020-0.060 inches). Then quench and temper. Purpose: Hard, wear-resistant surface (58-62 HRC) with a tough, ductile core. The best of both worlds. Machinist impact: Machine the part to finish dimensions BEFORE carburizing, leaving grinding stock only on critical surfaces. The case is thin and must not be machined through -- if you take a heavy cut after case hardening, you remove the hard surface and expose the soft core.

Nitriding

Process: Heating steel (typically 4140, Nitralloy 135M, or H13) in a nitrogen-rich atmosphere at 950-1050F. Nitrogen diffuses into the surface creating extremely hard nitrides. Purpose: Very hard surface (65-70 HRC equivalent) with minimal distortion because the temperature is below the transformation range. Machinist impact: Parts are machined to final dimensions BEFORE nitriding. Distortion is minimal (0.001 or less on most features). No grinding is needed after nitriding in most cases. This is a significant advantage over carburizing.

Rockwell Hardness Testing

Rockwell hardness is the most common hardness test in machine shops. It is fast, non-destructive (almost -- it leaves a small indent), and directly readable.

HRC (Rockwell C scale): Used for hardened steels. A diamond cone indenter with a 150 kg load. Range: 20-70 HRC (below 20, use HRB).

HRB (Rockwell B scale): Used for softer metals (annealed steel, aluminum, brass). A 1/16-inch steel ball with a 100 kg load. Range: 0-100 HRB.

Brinell hardness (HB): Older test using a 10mm ball. Common in material specs. Rough conversion: HRC = (HB - 100) / 4 for the range of 200-400 HB (very approximate -- use conversion charts for precision).

Practical hardness ranges:

18-22 HRC: Mild steel, easy machining
28-32 HRC: Pre-hard 4140, good combination of machinability and strength
38-42 HRC: Mold steel (P20), prehardened. Carbide inserts, moderate speeds
45-52 HRC: Die steel (H13), difficult with carbide, good for ceramic or CBN
55-62 HRC: Hard turning territory. CBN inserts only. Can replace grinding in many cases
62-67 HRC: Only for CBN or PCD. This is HSS drill bit and tap hardness

Machinability Ratings

The AISI machinability rating system compares materials to AISI 1212 free-machining steel (rated at 100%). A higher number means easier to machine.

Material	Rating	Notes
12L14 (leaded free-machining)	170%	The easiest steel to machine
1212 (resulfurized)	100%	The baseline
1018	78%	Gummy, poor chip control
4140 (annealed)	66%	Good all-around
4340 (annealed)	57%	Tougher, more tool wear
303 stainless	78%	Best austenitic stainless
304 stainless	45%	Work hardening is the enemy
316 stainless	36%	Worse than 304
Ti-6Al-4V	22%	Demanding
Inconel 718	12%	Extremely difficult

These ratings apply to annealed conditions. Heat-treated materials are significantly harder to machine.

Work Hardening: What It Is and How to Deal With It

Work hardening (strain hardening) occurs when plastic deformation increases the density of dislocations in the crystal structure, making the material harder. In machining, this happens when:

The tool rubs instead of cutting (dull tool, too light a chip load)
The tool passes over the same spot repeatedly at low DOC
The chip is compressed against the rake face without adequate shearing

Materials most susceptible to work hardening:

Austenitic stainless steels (304, 316, 321, 347)
Nickel-based alloys (Inconel, Hastelloy, Monel)
Titanium alloys
Hadfield manganese steel (austenitic manganese -- the worst offender)

How to avoid work hardening problems:

Maintain adequate chip load. The minimum chip thickness must exceed the work-hardened layer from the previous pass. If your finish pass takes 0.001 per side with a radiused insert, you may be cutting entirely in the hardened layer
Use sharp tools. Index inserts at the first sign of flank wear. Do not push worn tools
Do not dwell in the cut. Program smooth tool paths without stops. On a lathe, do not let the tool sit at the bottom of a groove
Climb mill whenever possible. In climb milling, the chip starts thick and gets thin. In conventional milling, the chip starts at zero thickness (rubbing through the hardened layer)
Use coolant. Reducing temperature slows the work-hardening rate
Avoid spring passes. Taking "one more pass at the same dimension" on stainless is counterproductive -- you are rubbing through the hardened layer and making it worse

Practical Material Selection Guide

When a customer asks "what material should I use?" consider:

Strength requirements -- what loads will the part see?
Corrosion resistance -- indoor, outdoor, chemical exposure, food contact?
Weight -- does it need to be light (aluminum, titanium)?
Wear resistance -- sliding surfaces, abrasive environment?
Temperature -- will it see high heat in service?
Cost -- material + machining cost. A material that costs more per pound but machines faster can be cheaper overall
Availability -- can you get the stock size you need? Exotic materials have long lead times

Quick decision tree for steel:

General purpose, no special requirements: 1018 or A36
Need some strength, easy to machine: 12L14
Moderate strength, heat treatable: 1045
High strength, good toughness: 4140 pre-hard
Maximum toughness, fatigue resistance: 4340
Corrosion resistance, non-magnetic: 304 or 316 stainless
Corrosion resistance, must machine well: 303 stainless
Very hard surface, tough core: 8620 carburized