EN 9

What is EN 9?

EN 9 is an unalloyed high-carbon engineering steel containing 0.5 to 0.6 percent carbon. It is used for parts that require wear resistance and retention of an edge or contact surface rather than resistance to shock. Supply conditions include normalised and hardened and tempered. The grade is also used for flame and induction hardening.

EN 9 corresponds to 070M55 in BS 970. In this designation, 070 represents the mean manganese content multiplied by one hundred. The letter M indicates that the bar is specified by mechanical properties rather than chemical analysis. The number 55 represents the mean carbon content multiplied by one hundred. EN 9 is the older 1955 designation for the same steel. C55, with material number 1.0535, is the closest continental designation.

EN 9 contains 0.15 percent more carbon than EN 8. This difference increases attainable hardness and wear resistance. It reduces ductility, toughness and weldability. It also reduces the margin against cracking during quenching.

Laxcon Steels lists EN 9 in its grades reference as a carbon steel grade. The same steel is written EN9, 070M55 and C55.

What is the chemical composition of EN 9?

Composition limits in weight percent are carbon 0.5 to 0.6, manganese 0.5 to 0.8, sulphur 0.06 maximum, phosphorus 0.06 maximum and silicon 0.05 to 0.35.

Mechanical properties and supply condition

EN 9 is a direct-hardening steel specified in BS 970 by supply condition rather than by a single strength value. The order therefore states the required condition. Normalised bar is the usual stock condition and is commonly used for machining. Hardened and tempered bar is supplied within a tensile band, with a corresponding hardness range and limiting ruling section. The limiting ruling section is the largest diameter for which the specified property is guaranteed at the centre of the bar.

The tempered tensile strength of EN 9 overlaps that of medium-carbon grades. Its distinguishing property is its higher hardness limit. The maximum quenched hardness of plain steel depends mainly on carbon content. At 0.5 to 0.6 percent carbon, EN 9 can reach substantially greater hardness than EN 8. Its principal functional properties are hardness and wear resistance rather than tensile strength.

Heat treatment and hardenability

EN 9 is hardened at about 820 to 850 degrees Celsius. The section is soaked until heated through and then quenched in oil or water. Tempering takes place at 550 to 660 degrees Celsius. The holding time is one hour per 25 mm of thickness, with a minimum of two hours, followed by air cooling. Annealing takes place at 680 to 710 degrees Celsius, followed by slow furnace cooling. Customer-side forging is carried out at temperatures up to about 1100 degrees Celsius and stops before the steel cools below 850 degrees Celsius.

The quench medium affects both hardening depth and cracking risk. Water removes heat faster and hardens more deeply. At 0.55 percent carbon, however, the transformation stresses can crack parts that contain a sharp section change, keyway or unblended fillet. Oil reduces this risk where the geometry contains stress raisers, but produces a shallower hardened region.

EN 9 has hardenability comparable to that of EN 8 because, apart from manganese, its analysis contains no element that substantially delays transformation. Carbon increases the hardness of the martensite that forms, but does not increase the depth at which martensite forms. Flame or induction hardening therefore provides an alternative to through hardening. Local heating and quenching create a hard, wear-resistant surface at the contact area while leaving the core in the normalised condition. This treatment avoids the through-hardening limit.

Machining and welding

EN 9 is machined in the normalised or annealed condition. Where dimensional tolerances permit, machining is completed before hardening because hardened surfaces require grinding rather than cutting. At the same hardness, EN 9 causes more tool wear than EN 8 because its structure contains more carbide.

EN 9 has limited weldability. Using the midpoints of the published composition limits, its International Institute of Welding carbon equivalent is about 0.66. Trade practice uses 0.45 as the level above which preheating, low-hydrogen consumables and interpass temperature control become necessary. The value for EN 9 is half as high again. Welding therefore requires a controlled procedure. A weld also alters the hardened or tempered condition locally, so welded joints must form part of the component design rather than a repair procedure.

Applications

EN 9 is used for components with sliding, rolling or cutting contact. Applications include moderate-duty gears and sprockets, cams and cam followers, worms, guides and slides, rollers and bushes, machine tool components, hammers, knives and blades, woodworking drills, axes and sickles. It is also used for shafts and crankshafts where journal wear is more important than notch toughness. Flame-hardened tracks and ways use the grade when a hard surface is required along a line while the main body remains unchanged.

Comparison with adjacent carbon steel grades

EN 9 occupies a higher carbon range than EN 8 but does not provide the properties required for spring service. EN 8 contains 0.36 to 0.44 percent carbon and is used for general shafting and machined components. It is more suitable where shock loading or welding occurs. EN 9 contains 0.5 to 0.6 percent carbon and exchanges some toughness for increased hardness and wear resistance. SAE 9260 is a spring steel containing 0.56 to 0.64 percent carbon and 1.8 to 2.2 percent silicon. The silicon increases the elastic limit, which is a principal spring property and is not provided by carbon alone.

Where EN 9 is not the right choice

EN 9 is unsuitable for components governed by shock or impact loading. Its carbon content increases hardness but reduces notch toughness. A hardened EN 9 component subjected to sudden loading at a stress raiser can fail by brittle fracture instead of bending. EN 8 is more appropriate for components that must withstand a blow. EN 19 or EN 24 provides combined strength and toughness in heavy sections.

EN 9 is unsuitable for fabricated components. At a carbon equivalent of about 0.66, the heat-affected zone can transform into untempered martensite during cooling and crack without preheating and post-weld tempering. Welding also reverses the specified heat-treated condition in the region of the joint.

EN 9 is not a spring steel despite its carbon content. Springs require fatigue resistance and a high elastic limit. These properties are associated with silicon-bearing grades such as SAE 9260 rather than plain carbon steel. EN 9 also provides no corrosion resistance. In wet service, its wear surface corrodes as plain steel. A martensitic stainless grade such as AISI 420 provides a hard surface where corrosion resistance is also required.

Other carbon steel grades