EN 36C

What is EN 36C?

EN 36C is a three percent nickel chromium-molybdenum case-hardening steel. Its carbon content is 0.12 to 0.18 percent, which permits carburising. Its alloy content allows the core beneath the hardened case to retain toughness in heavy sections. The British designation system classifies it as a premium carburising grade. It is specified for gears, journals and other parts that combine surface contact stress with shock loading in the body.

The BS 970 designation is 832M13. The 800 band identifies the nickel-chromium-molybdenum group. The letter M indicates supply against a mechanical property specification. The number 13 represents the mean carbon content multiplied by one hundred. Other designations include 14NiCrMo13-4, material number 1.6657, and the French equivalent 16NCD13.

A case-hardening steel develops its specified component properties through heat treatment. Bar is supplied soft and low in carbon. The properties stated on a drawing apply to the carburised, hardened and tempered component. The drawing therefore requires the case depth and core hardness in addition to the grade designation.

Laxcon Steels lists EN 36C in its grades reference as an alloy steel grade. The same steel is written EN36C and 832M13.

What is the chemical composition of EN 36C?

Composition limits in weight percent are carbon 0.12 to 0.18, manganese 0.3 to 0.6, sulphur 0.05 maximum, phosphorus 0.05 maximum, silicon 0.1 to 0.35, chromium 0.6 to 1.1, nickel 3.0 to 3.75 and molybdenum 0.1 to 0.25.

Mechanical properties of EN 36C

The specified strength of EN 36C refers to the treated core rather than the bar as supplied. Core strengths from 850 to 1230 N/mm squared are attainable after carburising and hardening. The resulting value depends on the section, carburising cycle and tempering temperature. In the soft annealed condition used for machining, hardness is held to about 217 Brinell.

The case requires a separate specification. It is defined by its depth and surface hardness. Case depth is usually stated as the effective depth to a specified hardness at a specified distance below the surface. These properties result from the carburising cycle rather than from the grade alone. Components made from the same EN 36C bar with different case depths can therefore have different service lives.

Heat treatment of EN 36C

Heat treatment produces separate case and core structures. The bar is first machined in the soft annealed condition. This condition is produced by heating to about 825 degrees Celsius and cooling slowly. Carburising then diffuses carbon into the surface layer to a controlled depth and carbon potential. The surface becomes high in carbon while the core remains at 0.12 to 0.18 percent carbon. Hardening follows at 825 to 850 degrees Celsius with an oil quench. This treatment converts the carbon-rich case to hard martensite and develops the core strength enabled by nickel, chromium and molybdenum. Tempering at 140 to 200 degrees Celsius relieves quenching stress while retaining case hardness.

Nickel supports hardening in heavy sections. The core must support the case against collapse and absorb shock that the case cannot accommodate. It must develop strength and toughness at cooling rates attainable in large parts. Three percent nickel increases hardening depth and core toughness. EN 36C is therefore specified where a smaller carburising steel could leave a soft and weak core at the centre.

EN 36C within the EN 36 group

The EN 36 group includes EN 36A, EN 36B and EN 36C. Each grade contains 3.0 to 3.75 percent nickel and 0.6 to 1.1 percent chromium. EN 36A contains a maximum of 0.15 percent carbon. EN 36B contains 0.12 to 0.18 percent carbon. EN 36C has the composition of EN 36B with an additional 0.1 to 0.25 percent molybdenum.

Molybdenum gives EN 36C greater hardenability than the molybdenum-free grades. It allows the core to develop strength at greater depth in a large part. It also improves core strength after heat treatment. Where either grade is permitted, selection between 655M13 and 832M13 depends on section size rather than general material quality.

Machining EN 36C

Machining takes place before carburising while the steel is soft and annealed. This permits complex geometry to be cut before the component develops its final hardness. Surfaces that must remain soft can be protected with copper plating or stop-off paint during carburising. They can also retain a machining allowance that is removed after treatment, thereby removing the carburised layer.

Running surfaces are normally ground after hardening. The grinding allowance must be related to the effective case depth rather than the total case depth. Otherwise, finishing can remove the part of the case intended to carry the load.

Applications of EN 36C

EN 36C is used for high-duty aircraft gears, heavy vehicle and automobile transmission components, gear shafts and pinions, steering worms, track rod pins, timing wheels, cams and camshafts, breech mechanisms and small arms components. These applications combine rolling or sliding surface contact with shock and bending in the body. Their sections are also too heavy for a leaner carburising grade to support.

Where EN 36C is not the right choice

EN 36C is unsuitable when a component requires uniform through-hardening. Its carbon content is deliberately too low to harden the core. A shaft that requires strength throughout its section, rather than surface hardness, belongs in EN 19 or EN 24. Without carburising, EN 36C remains a soft, low-carbon bar despite its alloy content and certification.

EN 36C is also unsuitable when the drawing omits case depth, surface hardness or core hardness. The grade designation alone specifies chemistry but does not define the properties developed by case hardening. Suppliers can therefore produce components with different service lives from the same incomplete drawing.

The grade is not appropriate for light or lightly loaded parts that do not use its nickel content. SAE 8620 contains roughly half a percent each of nickel and chromium. It is the standard carburising grade for ordinary duty and can develop a comparable case with a fraction of the alloy content. EN 36C also has no corrosion resistance. A carburised case provides wear resistance rather than rust resistance, so wet service still requires a coating.

Other alloy steel grades