EN 31

What is EN 31?

EN 31 is a high-carbon chromium through-hardening bearing steel. Its nominal composition contains one percent carbon and about one and a half percent chromium. The grade is hardened and tempered to approximately 62 HRC. It is used for rolling contact fatigue life rather than tensile strength. Most ball and roller bearings use this steel under one of several designations.

The European designation is 100Cr6 with material number 1.3505. EN ISO 683-17 standardises it as a ball and roller bearing steel. Other designations include SAE 52100 in the United States, SUJ2 in Japan and 100C6 in the French system. Its carbon content permits the formation of a hard martensitic matrix. Its chromium content produces fine, evenly distributed chromium carbides and provides sufficient hardenability for bearing-sized sections.

Steel cleanliness governs the service life of EN 31 components. Rolling contact fatigue cracks initiate at inclusions. The oxide and sulphide content of the cast can therefore determine bearing life even when different suppliers meet the same composition.

Laxcon Steels lists EN 31 in its grades reference as an alloy steel grade. The same steel is written EN31, 535A99, 100Cr6, 52100 and 1.3505.

What is the chemical composition of EN 31?

Composition limits in weight percent are carbon 0.9 to 1.2, manganese 0.3 to 0.75, sulphur 0.05 maximum, phosphorus 0.05 maximum, silicon 0.1 to 0.35 and chromium 1.0 to 1.6.

What is EN 31 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Mechanical properties and hardness

EN 31 is specified by hardness rather than a tensile range. Hardened bearing components operate under compression and rolling contact and are not intended to yield. Bar is normally supplied for machining in the soft annealed condition, with a maximum hardness of about 207 Brinell. The surface reaches approximately 62 HRC after hardening and tempering. Published working ranges for finished bearing components vary slightly with the selected tempering treatment.

The principal properties are compressive strength, hardness retention and dimensional stability. Tensile strength is not a useful primary design basis at working hardness. A component at 62 HRC has almost no ductility. Design calculations instead address contact stress and fatigue cycles.

Heat treatment

Hot forming takes place between about 1100 and 850 degrees Celsius. Normalising uses a temperature of 870 to 900 degrees Celsius followed by air cooling. Soft annealing uses 780 to 800 degrees Celsius followed by furnace cooling. This treatment produces a spheroidised structure suitable for machining. Hardening uses 800 to 830 degrees Celsius with a water quench, or 830 to 870 degrees Celsius with an oil quench. Oil quenching is the usual industrial process because water quenching can crack high-carbon steel. Tempering follows immediately at 150 to 180 degrees Celsius. Mill practice requires tempering within two hours of quenching because residual stress can crack a fully hardened component.

The low tempering temperature retains hardness while relieving quenching stress and stabilising the structure. It also limits the service temperature. A component tempered at 160 degrees Celsius continues to temper if operated above that temperature. This causes hardness loss and dimensional change. Bearings for hot service use a higher stabilising treatment and accept lower hardness.

EN 31 has moderate hardenability. Mill data for 100Cr6 covers balls and rollers of any dimension, but limits rings and discs to about 30 mm effective thickness. The standard grade does not harden through above this thickness. Larger sections require a more heavily alloyed bearing steel or a carburising process.

Steel cleanliness and bearing life

Rolling contact produces maximum shear stress a fraction of a millimetre below the surface. Fatigue cracks initiate at nearby discontinuities. In clean and correctly hardened bearing steel, the relevant discontinuity is usually a non-metallic inclusion. Examples include alumina or silicate particles remaining from deoxidation and manganese sulphides. Two EN 31 casts with identical certificates can differ by an order of magnitude in bearing life if one is vacuum degassed and the other is not.

Bearing steel specifications therefore address more than composition. EN ISO 683-17 covers ball and roller bearing steels. The American specification family for high-carbon anti-friction bearing steel is ASTM A295. These specifications also address melting practice, inclusion ratings and macrostructure.

Machining, grinding and joining

EN 31 is machined in the spheroidised annealed condition. In this condition, its carbides are rounded rather than lamellar and the steel cuts predictably. Grinding provides the final finish after hardening because other economic finishing methods are unsuitable at 62 HRC. Stockholder machinability ratings place the grade well below free-cutting steel. Chromium carbides govern tool life during the machining of annealed bar more than the hardness value does.

Welding is not a normal process for EN 31, and stockholder guidance does not provide a welding procedure. At about 1.05 percent carbon, its International Institute of Welding carbon equivalent is roughly 1.4 when calculated from the midpoints of the published composition limits. This value is three times the level at which preheating and low-hydrogen practice become mandatory. Welding produces brittle untempered martensite beside the joint and disrupts the required temper.

Applications

Principal applications include bearing races, balls, rollers, needles and cages. Other applications include precision spindles, collets, gauges, gauge blocks, punches and dies for light forming, mandrels, cams, cam followers, rolls, taps, reamers, and hardened guide and location surfaces. These applications involve high contact stress in clean, lubricated environments.

Where EN 31 is not the right choice

EN 31 is unsuitable for structural components. At working hardness, the grade is brittle and has low notch toughness. Shock loading or bending stress concentrated at a corner can cause fracture instead of deformation. EN 19 and EN 24 are through-hardening engineering steels used where the primary duty is load rather than contact.

EN 31 is unsuitable for large rings and heavy sections. Its hardenability becomes insufficient above roughly 30 mm effective thickness. Large bearing rings use case-carburising bearing grades. These grades produce a hardened surface and retain a tough core, while a through-hardened EN 31 section would remain soft at its centre.

EN 31 is unsuitable for wet service because it has no corrosion resistance. Lubricants and seals protect bearings made from the grade. Applications in which water, condensation or washdown reaches the raceway require a hardenable stainless grade such as AISI 440C or a bearing based on a corrosion-resisting material. A coating on EN 31 does not provide the required solution.

EN 31 is unsuitable for service above its tempering temperature. A component tempered at 150 to 180 degrees Celsius continues to temper when operated at a higher temperature. It then loses hardness and dimensional stability. Hot-running applications require a stabilised heat treatment specified before service and accept a lower hardness.

Other alloy steel grades