Surface hardening

Purpose of a hardened case

Through hardening a shaft to 58 HRC makes the core brittle and distorts the whole section in the quench. Surface hardening puts hardness at the load and keeps a core that takes bending and shock. A crack that starts in the case stops in the tough core.

Surface hardening processes fall into 3 groups:

Group	Processes	Mechanism	Result
Diffusion + quench	Carburizing, carbonitriding	Carbon (and nitrogen) added to a low-carbon steel, then quenched	Deep case, high hardness, high distortion
Local heating + quench	Induction, flame	Surface of a medium-carbon steel heated and quenched, no chemistry change	Deep case on selected zones only
Low-temperature layer	Nitriding, nitrocarburizing, PVD, hard chrome	Diffusion below the tempering temperature, or a deposited film	Thin layer, low distortion

The hero chart compares typical hardness profiles on a log depth scale. A PVD film is about 1000 times thinner than an induction case. Base steel families are introduced in the overview of hard steel families.

Carburizing and case hardening

Low-carbon steel (about 0.15-0.25% C) is held at about 900-950 °C in a carbon-rich atmosphere until carbon diffuses to the required depth. The part is then quenched and tempered at about 150-200 °C. Case hardness reaches 58-62 HRC and the core stays at about 30-45 HRC.

Item	Typical value
Steels	8620, 9310, 20MnCr5, 16MnCr5
Effective case depth	0.5-2.0 mm, gears and shafts
Depth vs time	Depth grows with the square root of time, so 2x depth needs about 4x cycle
Soft areas	Masked with copper plating or stop-off paint, or extra stock removed before the quench

Sequence for soft threads: machine with extra stock → carburize → remove the case at the threads → harden → temper → grind. The comparison with through hardening is in case hardening vs through hardening.

Carbonitriding

Carbonitriding runs at about 820-900 °C with ammonia added, so nitrogen enters with the carbon. The case is shallower, typically 0.1-0.75 mm, with less distortion than carburizing. It is used on small, high-volume parts in low-carbon and free-machining steels.

Induction and flame hardening

An induction coil heats a surface layer of medium-carbon steel above its hardening temperature in seconds. A water-polymer spray quenches it, and only the heated zone hardens. Surface hardness is about 50-58 HRC, set by the carbon content.

Item	Typical value
Steels	1045, 4140, 42CrMo4, 4340
Case depth	About 1-6 mm, set by frequency and heating time
Core	Often quenched and tempered beforehand to 28-32 HRC
Distortion	Low to moderate, local to the hardened zone

Flame hardening uses a gas torch on large parts such as machine ways and big gears. Control of depth and pattern is less precise than with induction.

Nitriding and plasma nitriding

Nitrogen diffuses into the surface at about 500-550 °C in ammonia gas or a plasma. Hard nitrides form with chromium, molybdenum, vanadium and aluminium, so the result depends on the alloy. There is no phase change and no quench.

Steel	Typical surface hardness after nitriding
H13	About 900-1200 HV
4140	About 550-700 HV
P20	About 550-700 HV
Nitriding steels (Al-bearing)	About 900-1100 HV

Case depth is typically 0.1-0.4 mm. Gas nitriding cycles often run 20-60 hours. The part grows by about 0.005-0.025 mm per surface.

The steel is hardened and tempered at least about 30 °C above the nitriding temperature. Otherwise the core softens. Low-tempered D2 is rarely nitrided for this reason.

A brittle compound layer (white layer), typically 5-15 µm, forms on top of the case. Plasma nitriding can suppress it where impact or thermal shock would flake it. Sequence and stock are covered in nitriding and PVD after hard machining.

Nitrocarburizing

Nitrocarburizing adds nitrogen and some carbon at about 560-590 °C for a few hours, in gas or salt bath. The main product is a compound layer of about 10-20 µm over a diffusion zone. It improves wear and galling resistance on most carbon and alloy steels, and corrosion resistance after post-oxidation.

The layer is thin and does not carry high contact stress. Under heavy load it collapses into the softer steel beneath.

PVD and CVD coatings

Physical vapour deposition (PVD) lays down a ceramic film 1-5 µm thick at about 200-500 °C. Method background: physical vapor deposition.

Coating	Typical hardness	Typical use
TiN	About 2300 HV	General wear, punches
TiAlN	About 3000-3500 HV	Cutting edges, higher temperature
CrN	About 1800-2200 HV	Forming, anti-galling
DLC	Varies by type, deposited below about 250 °C	Low friction, non-ferrous contact

The film copies the surface under it, so the sequence is polish → hone edges → coat. The substrate is hardened and tempered above the coating temperature.

Chemical vapour deposition (CVD) gives thicker, adherent films at about 900-1050 °C. A tool steel part is re-hardened after coating, with the distortion that follows. Use on tool steel is limited to forming and drawing tools where adhesion justifies the cost.

Hard chrome plating

Electroplated chromium is about 5-250 µm thick at about 800-1000 HV. Plating runs near 50-60 °C, so the part does not distort. Thick deposits are ground back to size.

Hardened steel is baked after plating, usually within a few hours, to remove hydrogen. Without the bake, the part can crack from hydrogen embrittlement.

Process comparison table

Process	Case or layer depth	Surface hardness	Distortion	Typical steels	Machining sequence
Carburizing	0.5-2.0 mm effective	58-62 HRC	High	8620, 9310, 20MnCr5	Machine soft with grind stock → mask → harden → grind
Carbonitriding	0.1-0.75 mm	58-63 HRC	Moderate	1018, 12L14, 8620	Machine near size → treat → grind critical fits
Induction	About 1-6 mm	50-58 HRC	Low to moderate, local	1045, 4140, 42CrMo4	Machine with stock on zones → harden → temper → grind zones
Flame	About 1.5-6 mm	50-58 HRC	Moderate	1045, 4140, cast iron	As induction, larger stock
Gas or plasma nitriding	0.1-0.6 mm	550-1200 HV (alloy dependent)	Low, growth 5-25 µm	H13, 4140, P20, nitriding steels	Harden, temper → stress relieve → finish to size minus growth → nitride → light lap
Nitrocarburizing	10-20 µm compound + diffusion zone	500-1000 HV (steel dependent)	Low	Most carbon and alloy steels	Finish machine → treat → polish
PVD	1-5 µm	1800-3500 HV	Negligible	Hardened tool steels tempered above coating temperature	Finish grind → polish → hone → coat
CVD	5-15 µm	About 3000 HV	High (re-hardening)	Selected tool steels	Machine → coat → re-harden in vacuum → minimal finishing
Hard chrome	5-250 µm	800-1000 HV	None	Most steels	Grind undersize → plate → bake → grind to size

Figures are typical production ranges. The heat treater or coater confirms values per steel, section and drawing.

Case depth, core support and grinding

• Case depth against the load. Under contact stress the peak shear stress sits below the surface. A case shallower than that depth is pushed into the core and spalls. Thin layers suit wear and galling, and deep cases suit gears, rolls and loaded journals.
• Core support. A thin case needs a hard core beneath it. Typical pairs: 4140 at 28-32 HRC under an induction case, H13 at 44-50 HRC under nitriding, 60 HRC tool steel under PVD.
• Grinding after treatment. Each 0.1 mm ground off a carburized or induction case is 0.1 mm less case. On nitrided parts, grinding more than a few microns removes the hardest layer. Typical stock figures are in the heat treat allowance table.

Process selection by part type

Requirement	Typical process
Whole surface hard, complex shape, high contact load (gears, cams, pins)	Carburizing
Round part, hardness on journals or seats only, low distortion	Induction on 4140 or 1045
Finished precision part, wear or galling, no size change allowed	Nitriding on H13, 4140 or P20; nitrocarburizing
Hardened tool steel edge, longer wear life	PVD, with nitriding first under high load
Rods, rolls, sliding surfaces, low friction and corrosion resistance	Hard chrome, or nitrocarburize + oxidize

Parts that need hardness through the whole section belong on the heat treatment page. Base steel choice is covered in toughness vs wear resistance.

Drawing requirements for surface-hardened parts

• Process and steel. “Carburize and harden, 8620” or “Gas nitride, H13 hardened and tempered 46-50 HRC before nitriding”.
• Surface hardness scale. HRC for carburized and induction cases. HV with a stated load for nitrided cases, because an HRC indent passes through a thin case.
• Effective case depth. With its limit hardness, e.g. “ECD 0.8-1.2 mm at 550 HV, after grinding”. For nitriding, state the depth and its definition.
• Core hardness. Where the core carries load.
• Hardened and soft zones. Induction zones dimensioned on the drawing, masked areas marked.
• Compound layer. A thickness limit, or “no compound layer” for nitrided parts under impact.
• Coating type and thickness. Coated surfaces marked, film thickness included in the tolerance of precision fits.
• Verification. A sectioned coupon from the same batch with a microhardness traverse.

Terms such as case depth and compound layer are defined in the glossary. Drawings with these notes can be sent through the RFQ form.