Steel to which chromium, molybdenum, nickel, manganese, vanadium or boron have been added in controlled amounts so that it can be hardened through a section and reach strengths plain carbon steel cannot. Alloy engineering steels are the material of shafts, gears, bolts, axles and oil-tool parts.
Also called
Engineering steel
Low-alloy steel
Quenched and tempered steel
Case-hardening steel
Chromoly
4140
4340
42CrMo4
EN19
EN24
AR
الصلب السبائكي
ZH
合金结构钢
DE
Legierter Stahl
FR
Acier allié
ES
Acero aleado
IT
Acciaio legato
TR
Alaşımlı çelik
RU
Легированная конструкционная сталь
Main formsHot-rolled and forged bars, peeled and bright bars, wire rod, forgings, seamless tubes, plates
Typical sizesRolled bars 10–300 mm; forged rounds and blocks to 1,000 mm and over; plates 6–200 mm
Main standardsEN ISO 683-2, EN ISO 683-3, ASTM A29 / A322 / A434, SAE J404, JIS G4053, GB/T 3077
Typical usesShafts, gears, crankshafts, high-strength bolts, axles, drill collars, oil-tool and hydraulic parts
Overview
An alloy steel in the trade sense is a low-alloy engineering steel: an iron–carbon steel with up to about 5% of alloying elements in total. Chromium and manganese raise hardenability, molybdenum adds strength at temperature and resists temper embrittlement, nickel adds toughness at low temperature, and vanadium, niobium and boron refine the grain or add hardenability for a few parts per thousand. Stainless, tool and heat-resistant steels are also alloyed, but they are sold and specified as families of their own.
Two groups cover most of the tonnage. Quenched-and-tempered (through-hardening) steels such as 42CrMo4 (AISI 4140) and 34CrNiMo6 (4340) are hardened and tempered to a strength level — often given as a condition such as QT 900–1100 MPa — and machined into shafts, bolts and heavy parts. Case-hardening steels such as 16MnCr5, 20MnCr5, 18CrNiMo7-6 and 8620 have low carbon and are carburised after machining, so a gear or pinion gets a hard, wear-resistant surface over a tough core.
Alloy steel is made in electric arc or oxygen furnaces, refined in the ladle and usually vacuum degassed to bring hydrogen and oxygen down, then cast and rolled or forged. Clean steel matters: inclusions start fatigue cracks, so specifications for gears and bearings set limits on inclusion content and ultrasonic soundness as well as on chemistry.
In the Middle East the main users are the oil and gas industry — drill collars, subs, valve bodies, flanges and bolting to API and ASTM specifications — and machine shops, gearbox repairers and heavy-equipment workshops. Almost all of it is imported, from Europe, Turkey, India, China and Japan, through stockists who carry 4140, 4340 and EN-grade bars in the annealed or quenched-and-tempered condition and cut to length.
Forms & sizes
Alloy steel is bought as much by condition as by size: the same 42CrMo4 bar can be supplied as-rolled, annealed for machining, or already quenched and tempered to a strength range.
Round bars
Hot-rolled 10–300 mm, forged above that; black, peeled, turned or ground
Bright bars
Cold-drawn or peeled and polished, h9–h11 tolerance, for machining
Forgings
Open-die blocks, discs, rings and stepped shafts; also closed-die parts
Plates and flats
Cut plate 6–200 mm, mainly for wear parts, moulds and machine bases
Seamless tubes
Hollow bars for bushes, cylinders and drill pipe components
Heat-treatment condition
+AR as rolled, +A soft annealed, +N normalised, +QT quenched and tempered, +FP ferrite-pearlite for case-hardening grades
Grade and standard together — '42CrMo4 to EN ISO 683-2' or 'AISI 4140 to ASTM A29' — the two are close but not identical in chemistry.
Delivery condition: as-rolled, soft annealed, normalised or quenched and tempered, with the strength or hardness range for +QT.
Size and tolerance: black, peeled or turned, h9/h11 for bright bar; length, fixed or random.
Ultrasonic testing to a stated standard and acceptance class for forgings and large bars, and a grain size or cleanliness limit for gears and bearings.
Hardenability (H-band, Jominy) where the part's heat treatment depends on it.
An EN 10204 3.1 certificate with heat analysis and mechanical tests; 3.2 where the end user or API specification requires third-party witnessing.
Quality checks
Check the heat number stamped or painted on each bar against the certificate.
Spot-check chemistry with a portable spectrometer — 4140 and 1045 look identical on the rack.
Measure hardness on +QT and annealed bars against the range ordered.
Look for surface seams, laps and decarburisation on black bars, and check straightness.
For critical parts, ultrasonically test a sample before machining.
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