42CrMo4

Overview

Designation system

European steel name to EN 10027-1; material number to EN 10027-2

Product forms

Hot-formed semi-finished products, Bars, Wire rod, Finished flat products, Hammer and drop forgings, Bright steel products under the applicable product standard, Seamless tubes under the applicable tube standard

Condition

Untreated (+U), Treated to improve shearability (+S), Soft annealed (+A), Quenched and tempered (+QT), Specified hardenability (+H, +HH or +HL), Flame or induction hardened after suitable core heat treatment

Density

7.8 g/cm³ (Representative physical value, not a chemical or acceptance requirement of the grade.)

What the designation defines

42CrMo4 is a direct-hardening alloy steel rather than a structural steel strength class. The designation primarily identifies a controlled chemical composition: approximately 0.42% carbon with chromium and molybdenum additions. Chromium increases hardenability and wear response; molybdenum supports through-section hardening and reduces susceptibility to temper embrittlement. The grade is normally used with a tempered-martensitic or related quenched-and-tempered microstructure.

A certificate stating only “42CrMo4” does not establish a finished component strength. Delivery condition, dimensions, heat-treatment batch, test location and applicable product standard are essential parts of the material definition.

Standards and product-form boundaries

EN 10083-3 is the commonly cited predecessor for alloy steels for quenching and tempering. Existing drawings may legitimately reference a particular historical edition, but new procurement should identify the required current standard or explicitly retain the legacy specification when contractually necessary. Requirements from different product standards must not be combined into a single apparent property set.

Chemical composition

These are cast-analysis limits. Permitted product-analysis deviations are separately defined by the standard. Product analysis must be specified at enquiry and order if independent verification is required.

Delivery condition and heat treatment

Heat-treatment response is section dependent. The surface can transform rapidly while the core of a large component cools too slowly to form the same microstructure. This is why the standard bases mechanical properties on the ruling section rather than merely the overall nominal diameter or stock size. Manufacturer claims for through-hardening diameter are process- and chemistry-dependent and should not be treated as universal grade requirements.

Mechanical properties in the quenched-and-tempered condition

Values apply to standardized test pieces taken from prescribed locations. A is based on proportional gauge length. Z values are optional and must be agreed at enquiry and order. KV2 is the average of three longitudinal Charpy V-notch results using a 2 mm striker radius; no individual result may be below 70% of the specified minimum average.

The progressive reduction in specified strength with increasing ruling section reflects achievable cooling rate and microstructural uniformity. A small test coupon heat treated separately from a large component may substantially overstate the component's core properties. For critical parts, the heat-treatment qualification, test location and sacrificial prolongation or integral test material should be agreed before manufacture.

Hardenability and localized surface hardening

42CrMo4 is valued primarily for hardenability rather than corrosion resistance or inherent as-supplied strength. ISO 683-2 permits ordering with normal or narrowed Jominy bands. Specifying +H, +HH or +HL is particularly useful for serial heat-treated components where variations in core hardness, distortion or quench response must be controlled more closely than broad chemistry limits alone permit.

Flame or induction hardening can provide a wear-resistant martensitic surface over a quenched-and-tempered core. The standardized minimum surface hardness is 53 HRC for appropriately processed sections up to 250 mm diameter. Effective case depth, transition hardness, retained stress and distortion are process-design variables and must be specified separately. Surface decarburization can prevent the required hardness from being achieved.

42CrMo4 is not a conventional low-carbon carburizing grade. If a deep carburized case with a tough low-carbon core is required, a purpose-designed case-hardening steel should normally be evaluated instead.

Machining, forming and welding

Machinability is strongly condition dependent. The soft-annealed condition is appropriate for heavy stock removal; quenched-and-tempered stock requires tooling and parameters matched to its actual hardness. Where enhanced chip control is essential, 42CrMoS4 or a calcium-treated proprietary machinability variant may be considered, but sulfur modification and inclusion engineering can affect transverse ductility, polishability and fatigue performance.

Hot forging is practical, but the forging supplier must control finishing temperature, cooling and subsequent heat treatment to avoid coarse grain, excessive scale or decarburization. Final properties should be established after the last property-controlling heat treatment, not inferred from the starting billet certificate.

Welding requires specific procedure development because the carbon and alloy content create a hardenable heat-affected zone and significant hydrogen-cracking susceptibility. Low-hydrogen consumables, controlled preheat and interpass temperature, appropriate heat input and controlled cooling are normally required. A representative high-restraint procedure may use approximately 250 °C preheat, but this is not a universal set point: the WPS must account for actual composition, thickness, restraint, initial heat-treatment condition, filler strength and service requirements. Tempering or another qualified postweld heat treatment may be necessary where a brittle HAZ or loss of the original +QT property balance is unacceptable.

Welding can locally erase or alter the certified quenched-and-tempered condition. Matching weld-metal tensile strength alone does not demonstrate adequate HAZ toughness, fatigue performance or resistance to delayed cracking.

Applications and engineering limitations

Typical uses include shafts, axles, spindles, pins, highly loaded bolts, connecting rods, steering and drivetrain parts, gear blanks, tooling supports and general machine components exposed to combined bending, torsion, impact or fatigue. Suitability derives from the selected heat-treated condition and component design rather than from the designation alone.

42CrMo4 is not stainless and requires suitable corrosion protection where atmospheric, aqueous or chemical attack matters. The base grade also does not carry a general low-temperature toughness class, pressure-equipment approval, hydrogen-service qualification, fracture-toughness guarantee or elevated-temperature design strength. Such requirements must come from the applicable component specification, product standard and qualification program.

Ordering and acceptance checklist

For stock offered as “42CrMo4 QT,” compare the certified property range, test location and original ruling section with the intended component. A quoted hardness or tensile band from a distributor is not automatically the ISO 683-2 requirement for every size or product form.

Related designations

A related designation or cross-standard counterpart is not a declaration of equivalence; confirm the standard, product form and condition before substituting.