---
title: "Case Hardening of Steel: Carburising, Nitriding, Induction"
source: "https://www.laxconsteels.com/case-hardening-of-steel/"
description: "Case hardening gives steel a hard surface over a tough core. Carburising, carbonitriding, nitriding, induction and flame hardening, grades and case depth."
---
# Case Hardening of Steel: Carburising, Carbonitriding, Nitriding, Induction and Flame Hardening

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Case hardening of steel is the group of surface treatments that give a part a hard, wear-resistant outer layer, the case, over a softer and tougher interior, the core. Two principles are used. Diffusion treatments change the composition of the surface: carburising adds carbon at about 900 to 950 degrees Celsius and is followed by a quench; carbonitriding adds carbon and nitrogen at a lower temperature; nitriding adds nitrogen at about 500 to 550 degrees Celsius and needs no quench. Selective hardening treatments, induction and flame hardening, leave the composition unchanged and harden only the surface of a medium-carbon or alloy steel by heating it rapidly and quenching it. A carburised gear tooth reaches around 60 HRC at the surface while its core stays tough enough to carry shock. The case is specified by its depth and hardness, and the effective case depth of a carburised part is the depth at which its hardness falls to 550 HV.

## Why a hard case over a tough core

Gear teeth, cam lobes, bearing journals, pins and splines are loaded in two ways at once. Their surfaces slide and roll under high contact stress, which calls for hardness, while the body of the part bends and takes impact, which calls for toughness. A steel hardened right through to the hardness a gear flank needs would be brittle in the tooth root. A steel soft enough to be tough throughout would wear and pit at the flank. Case hardening separates the two requirements, applying to the surface alone the hardening that the [heat treatment of steel](https://www.laxconsteels.com/heat-treatment-of-steel/) article describes for the whole section. It also leaves the case in residual compression, which raises the fatigue strength of the surface, where bending and contact fatigue cracks begin.

## Carburising

Carburising starts with a low-carbon steel, usually 0.1 to 0.25 percent carbon, which cannot reach a high hardness on its own. The part is held in a carbon-rich furnace atmosphere at about 900 to 950 degrees Celsius. At that temperature the steel is austenite, which dissolves carbon freely, and carbon diffuses inward from the surface. Diffusion sets the pace: the case depth grows with the square root of the time, so doubling the depth takes about four times as long. Effective case depths range from a few tenths of a millimetre on small parts to several millimetres on heavy gearing.

The carburised part is then quenched, either directly from the carburising temperature or after cooling and reheating to a lower hardening temperature, which refines the grain and reduces distortion. The high-carbon case transforms to martensite of around 60 HRC. The low-carbon core transforms to a tough, lower-hardness structure whose strength depends on the alloy content and the section size. A low temper at about 150 to 200 degrees Celsius relieves the quenching stress without lowering the case hardness.

Carburising is the usual treatment for transmission gears. Its drawback is distortion: a part held for hours at 900 degrees Celsius and then quenched changes size and shape, so running surfaces are ground after hardening. Areas that must stay soft, such as threads, are masked with copper plating or a stop-off paint, or left oversize and machined after carburising to remove the case.

## Carbonitriding

Carbonitriding adds ammonia to the carburising atmosphere, so that nitrogen diffuses into the surface along with carbon. It runs at a lower temperature, around 850 degrees Celsius. The nitrogen raises the hardenability of the case, which allows plain carbon steels to harden in oil rather than water, and the lower temperature and milder quench reduce distortion. The case is shallower than a carburised case, usually well under a millimetre, and it resists softening on tempering better.

## Nitriding and nitrocarburising

Nitriding diffuses nitrogen into the surface at about 500 to 550 degrees Celsius, below the temperature at which steel transforms to austenite. The nitrogen source is ammonia gas in gas nitriding, an ionised gas in plasma nitriding, or a molten salt. Because there is no transformation and no quench, dimensional change is small, and parts can be nitrided close to their finished size. The steel is hardened and tempered before nitriding, which fixes the core properties, and the tempering temperature is set above the nitriding temperature so that the core does not soften during the cycle.

The hardness comes from fine nitrides of the alloying elements, chiefly chromium, molybdenum and aluminium. Plain carbon steels form few of them and gain little. Chromium-molybdenum steels such as [SAE 4140](https://www.laxconsteels.com/grades/sae-4140/) and [EN 19](https://www.laxconsteels.com/grades/en-19/) nitride to a hard, tough case. Aluminium-bearing nitriding steels such as EN 41B, with 0.9 to 1.3 percent aluminium and 1.4 to 1.8 percent chromium, reach a harder case again, at the cost of a more brittle surface. Nitriding is slow. Cycles run from several hours to several days, and case depths are usually a few tenths of a millimetre. A nitrided case keeps its hardness at temperatures that would soften a carburised case.

Nitrocarburising adds some carbon with the nitrogen, commonly at about 570 degrees Celsius, in a short cycle. It forms a thin compound layer that resists wear and scuffing, and it is applied to plain carbon and low-alloy parts such as hydraulic piston rods and spindles.

## Induction and flame hardening

Induction and flame hardening change no chemistry. They heat the surface of a steel that already has enough carbon to harden, about 0.3 to 0.6 percent, above its transformation temperature and quench it before the heat reaches the core. In induction hardening an alternating current in a copper coil induces eddy currents in the surface of the part, which heats in seconds and is then sprayed with water or a polymer quench. The depth of the hardened layer is set by the frequency: the lower the frequency, the deeper the current penetrates. Depths run from under a millimetre to several millimetres. Flame hardening does the same with an oxy-fuel torch, and the depth is set by the speed of the burner. It suits large parts and short runs, with less precise control than induction.

The surface hardness follows the carbon content, about 55 HRC for a 0.4 percent carbon steel. The core keeps the properties it had before, so the bar is normalised or hardened and tempered first. [EN 8](https://www.laxconsteels.com/grades/en-8/) in the normalised condition, and EN 19 or 4140 hardened and tempered, are induction hardened along shaft journals, bearing seats and splines while the rest of the shaft stays as supplied.

## The processes compared

*Case hardening processes compared by surface change, temperature, quench, typical steels, case depth and distortion*

| Process | Surface change | Temperature | Quench | Typical steels | Case depth | Distortion |
| --- | --- | --- | --- | --- | --- | --- |
| Carburising | Carbon added | 900 to 950 degrees Celsius | Yes, then low temper | EN 36C, SAE 8620, 16MnCr5, EN 353 | A few tenths of a millimetre to several millimetres | High |
| Carbonitriding | Carbon and nitrogen added | About 850 degrees Celsius | Yes, often oil | Low-carbon plain and alloy steels | Usually under 1 mm | Moderate |
| Nitriding | Nitrogen added | 500 to 550 degrees Celsius | None | EN 41B, SAE 4140, EN 19 | A few tenths of a millimetre | Low |
| Nitrocarburising | Nitrogen and some carbon added | About 570 degrees Celsius | None | Plain carbon and low-alloy steels | Thin compound layer | Low |
| Induction hardening | None; surface transformed | Surface above transformation temperature for seconds | Yes, spray | EN 8, EN 19, SAE 4140 | Under 1 mm to several millimetres, set by frequency | Local |
| Flame hardening | None; surface transformed | Surface above transformation temperature | Yes, spray | EN 8, EN 19, SAE 4140 | Set by burner speed | Local |

## Case hardening steel grades

Carburising grades combine low carbon with enough alloy to harden the core in the section concerned. [EN 36C](https://www.laxconsteels.com/grades/en-36c/), BS 970 832M13, carries 0.12 to 0.18 percent carbon with 3.0 to 3.75 percent nickel, 0.6 to 1.1 percent chromium and 0.1 to 0.25 percent molybdenum; the nickel gives a tough core in heavy sections. [SAE 8620](https://www.laxconsteels.com/grades/sae-8620/), also written 20NiCrMo2-2 and EN 362, carries 0.18 to 0.23 percent carbon with about half a percent each of nickel and chromium and 0.15 to 0.25 percent molybdenum, and it is the usual carburising grade in American practice. 16MnCr5, material number 1.7131 in EN 10084, relies on 1.0 to 1.3 percent manganese and 0.8 to 1.1 percent chromium. EN 353 carries 0.14 to 0.20 percent carbon with 1.0 to 1.5 percent nickel, 0.75 to 1.25 percent chromium and a small molybdenum addition.

Medium-carbon steels are not carburised. EN 8 and 4140 already carry about 0.4 percent carbon, too much for a carburised case to add usefully, and a quench from the carburising temperature would harden them right through. They are case hardened by induction or flame, or nitrided.

## Case depth and how it is measured

A case is specified on the component drawing by its depth, its surface hardness and the core hardness, since the grade alone defines none of them. The effective case depth of a carburised and hardened part is the distance from the surface to the point where the hardness falls to 550 HV. It is measured on a sectioned sample with a row of Vickers micro-hardness impressions running inward from the surface. A nitrided case is measured the same way to a limit 50 HV above the core hardness, and an induction-hardened layer to a limit hardness stated on the drawing.

The grinding allowance is set against the effective depth, not the total depth of enrichment. Grinding past it removes the part of the case that carries the load, and such a part can still pass a surface hardness check.

## Case hardening stainless steel

Stainless steels are rarely case hardened. The austenitic grades cannot be hardened by quenching at all. Carburising or nitriding at the conventional temperatures ties the chromium at the surface into carbides or nitrides, and the surface loses the chromium it needs for its passive film, so the case gains hardness and loses corrosion resistance. Specialised low-temperature nitriding and carburising processes, run well below the conventional nitriding range, dissolve nitrogen or carbon into the austenite without forming chromium compounds. They give a thin hard layer that keeps the corrosion resistance of the grade. Martensitic grades such as 420 transform like an alloy steel and can be surface hardened by induction.

## Grades for case hardening at Laxcon Steels

Laxcon Steels supplies [hot rolled round bars](https://www.laxconsteels.com/products/hot-rolled-round-bars/) in EN 8, EN 19 and SAE 4140, the grades used for induction and flame hardening and for nitriding, and [bright bars](https://www.laxconsteels.com/products/bright-bars/) in EN 8. The carburising grades EN 36C and SAE 8620 are listed in the grade reference. Case hardening is done on the finished part after machining, so the bar is ordered in its supply condition and the case is specified on the component drawing.

## Frequently asked questions

### What is the difference between case hardening and through hardening?

Through hardening quenches the whole section of a medium-carbon or alloy steel and tempers it to one hardness. Case hardening makes only the surface hard and leaves the core softer and tougher, either by adding carbon or nitrogen to the surface or by heating and quenching the surface alone. Gear teeth and cam lobes are typically case hardened; bolts are through hardened.

### Which steels can be case hardened?

Carburising needs a low-carbon steel, about 0.1 to 0.25 percent carbon, such as EN 36C, SAE 8620, 16MnCr5 or EN 353. Nitriding needs nitride-forming elements, as in EN 41B, EN 19 and 4140. Induction and flame hardening need about 0.3 to 0.6 percent carbon, as in EN 8, EN 19 and 4140.

### Is nitriding better than carburising?

Neither is better in general. Nitriding runs at a lower temperature with no quench, so distortion is small and a part can be treated close to finished size, but the case is shallow and the cycle is long. Carburising gives a much deeper case for heavily loaded gears, at the cost of distortion and a grinding allowance.

### Can stainless steel be case hardened?

Austenitic stainless steel cannot be hardened by quenching, and conventional carburising or nitriding lowers its corrosion resistance. Specialised low-temperature processes harden a thin surface layer and keep the corrosion resistance. Martensitic grades such as 420 can be induction hardened.

## Sources

- ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes.
- ASM Handbook, Volume 4C, Induction Heating and Heat Treatment.
- DIN 50190-3, Hardness depth of heat-treated parts: determination of the effective depth of hardening after nitriding.
- EN 10084, Case hardening steels: technical delivery conditions.
- EN 10085, Nitriding steels: technical delivery conditions.
- BS 970, Wrought steels for mechanical and allied engineering purposes.
- SAE J404, Chemical compositions of SAE alloy steels.

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