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Home/Grade library/Cr, Cr-Mo, boron and non-heat-treated steels

Cr, Cr-Mo, boron and non-heat-treated steels

Roles of the alloying elements, heat treatment and typical uses of chromium, chromium-molybdenum, boron and H steels, with an outline of non-heat-treated steels.

Family overview

SCr grades are chromium steels and SCM grades are chromium-molybdenum steels. Both are machine-structural alloy steels under JIS G 4053, in which chromium, or chromium and molybdenum, is added to carbon steel to raise hardenability, that is, how far into the section quenching hardens the steel. With the same heat treatment they reach the required strength and toughness in thicker sections more readily than carbon steel, so they are used for machine parts such as gears, shafts and bolts.

H steels carry an H after the grade number, and under JIS G 4052 their hardenability band is guaranteed in the Jominy test, an end-quench test in which only one end of the test bar is cooled with water. Even within one grade, a composition that varies inside the permitted range can change the quenching result. H steels are meant to narrow that spread, and they are used for parts such as gears and shafts, where hardness and distortion after heat treatment must not differ from part to part.

Boron steels owe their higher hardenability to a very small boron addition. Quenching hardens them effectively without large additions of elements such as chromium or molybdenum, so they are used for fasteners and similar parts. 10B38 and 51B20 are SAE/AISI designations; the B in the number indicates a boron addition.

Non-heat-treated steels are used for hot-forged parts without quenching and tempering; the required strength is obtained by controlling the cooling rate after forging. The method relies on fine carbonitrides formed by microalloying elements such as vanadium, niobium and titanium, which raise strength and refine the grain. It saves heat-treatment steps and reduces quench distortion, but the steel may not suit parts that need the toughness or the through-section uniformity of a quenched and tempered steel.

Role of the main elements

  • Carbon (C) and manganese (Mn): the basic elements of structural steel. Carbon sets the limit of hardness and strength reached after quenching, and manganese raises hardenability and strength. The last two digits of a grade number indicate the carbon level; a larger number means more carbon.
  • Chromium (Cr): raises hardenability and forms carbides that add strength and wear resistance. It is not added in amounts that give corrosion resistance, so these steels are distinct from stainless steel.
  • Molybdenum (Mo): raises hardenability, reduces softening during tempering and reduces temper embrittlement. Together with chromium it makes a quench-and-temper effect easier to obtain in thick sections.
  • Boron (B): a very small amount raises hardenability markedly. Boron that combines with nitrogen or oxygen loses its effect, so the composition has to fix nitrogen with titanium, aluminum or similar elements, and the effect is also sensitive to the heating temperature.
  • Vanadium (V), niobium (Nb) and titanium (Ti): microalloying elements added in very small amounts to non-heat-treated steels. They precipitate as fine carbonitrides while the part cools after forging, which raises strength, and some of them refine the grain to limit the loss of toughness.

Heat treatment and delivery condition

  • Case-hardening grades (SCr415, SCr420, SCM415, SCM420): a low-carbon steel is heated in a carburizing furnace so that carbon diffuses into the surface, then quenched and tempered at a low temperature. The surface is hard and wear resistant while the core keeps its toughness, which suits parts such as gears and cams that carry load in contact with a mating part. Surface hardness, effective case depth and core hardness are specified on the drawing.
  • Quench-and-temper grades (SCM435, SCM440): heated, quenched and tempered to balance strength and toughness. In a large section the core may harden less, so section size and hardenability are considered together with the tempering conditions. These grades are also used for induction hardening, which hardens only the surface.
  • H steels: the hardness range at set distances from the quenched end in the Jominy test is guaranteed. The composition range and the hardenability range are confirmed against the applicable standard, and whether the test result is reported on the mill certificate is set in the order specification.
  • Boron steels: fasteners are often cold headed or formed from spheroidize-annealed stock and then quenched and tempered to reach the strength class. Because the hardenability effect of boron varies with the composition balance and the heating temperature, the stock composition and the heat-treatment conditions should be confirmed together.
  • Delivery condition: bars and wire rod are generally divided into as-rolled (black), annealed, spheroidize-annealed, quenched and tempered, and drawn conditions. Machinability before heat treatment and performance after it differ with the condition, so it is chosen to suit the downstream machining and heat-treatment plan.
  • Non-heat-treated steels: the hot-forged part is cooled from the forging heat at a set rate, such as air cooling, to obtain the required structure. Strength varies with the cooling rate and the section size, so the specification is set together with the forging conditions.

Standards cross-reference

GroupGradeSAE/AISIEN
Chromium steelSCr415511517Cr3
SCr420——
Chromium-molybdenum steelSCM415——
SCM420——
SCM4354135—
SCM440——
H steels (guaranteed hardenability)SCr415H——
SCr420H5120H20Cr4
SCM415H——
SCM420H——
SCM435H——
SCM440H4140H42CrMo4
Boron steel (SAE/AISI designations)10B38——
51B20——

Composition range

Confirm the composition range against the applicable standard.

Typical applications

  • Case-hardening grades (SCr415, SCr420, SCM415, SCM420): parts that need a wear-resistant surface and a tough core, such as gears, pinions, cams, splined shafts and pins
  • Quench-and-temper grades (SCM435, SCM440): parts that need a balance of strength and toughness, such as drive shafts, shafts, bolts, studs and hydraulic equipment parts
  • H steels: parts in which hardness and distortion after heat treatment must vary as little as possible, such as transmission and reducer gears and shafts
  • Boron steels: cold-headed fasteners such as bolts, nuts and screws, and general machine parts that need hardenability with reduced alloy content
  • Non-heat-treated steels: hot-forged automotive and industrial machinery parts that omit quenching and tempering, for example connecting rods, crankshafts and steering parts

Products covering these grades

Grades not listed on a product page (10B38, 51B20): send us the standard, size and quantity, and we will confirm availability. Quote & technical inquiry

Notes

  • The grade lists explain standards; they do not indicate stock or availability, which we confirm on inquiry.
  • Cross-references are for reference only; the final grade is confirmed against your drawing and standard.
  • A dash (—) marks a pair not yet confirmed.
  • H steels differ from the regular grades in whether hardenability is guaranteed, and their composition ranges may also differ. Where the drawing shows an H, the grade is confirmed against the H steel standard, and any grade change follows the customer approval process.
  • Boron steels in SAE/AISI designations may not correspond one to one with a JIS grade, so the applicable standard and the hardenability requirement are confirmed against your drawing.
  • Non-heat-treated steels are explained as a concept only, without a grade list. Whether they can be sourced is confirmed separately against the requested specification, such as the required strength level, part shape and cooling conditions after forging.

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