40Cr vs 42CrMo steel selection

Start with the ruling section and the failure mode

A procurement comparison should begin with the finished component, not the supplier's grade list. Record the maximum effective section that must respond to heat treatment, the smallest fatigue-critical diameter, transitions and keyways, thread or spline roots, unsupported length, torque or bending spectrum, impact temperature, wear interface, and whether the bar will be forged before final heat treatment. The largest purchased diameter is not always the ruling section: a stepped shaft may have a large heat-treatment section but fail at a smaller fillet, while a gear blank may be dominated by tooth-root fatigue and core support.

40Cr is often commercially attractive where a moderate section and an established quench-and-temper route deliver the drawing requirement without excessive process risk. That is not the same as saying 40Cr is suitable for every general shaft. 42CrMo is normally reviewed when the purchaser needs a stronger hardenability margin, higher standard reference-specimen strength, or more confidence in larger-section response. It is still possible to make a poor 42CrMo part through inadequate austenitizing, delayed quenching, excessive tempering variation, decarburization, grinding damage, or an unfavorable notch design.

Write a one-sentence decision criterion before requesting prices: for example, 'select the least complex approved route that meets the drawing properties at the stated sampling position after final heat treatment.' This prevents the quote comparison from drifting into alloy-content shorthand and makes an alternative offer auditable. If the design authority has not defined strength, toughness, hardness distribution, or inspection acceptance, procurement should return that gap rather than ask a mill to infer fitness for service.

Sources:[1]

Compare the GB/T 3077 ranges on their actual test basis

The table below is a buyer's comparison of the cast-analysis ranges and the standard reference heat-treated specimen values for GB/T 3077-2015. The mechanical figures are useful for understanding the different grade routes, but they are not an unconditional promise for a full-diameter bar. The standard test specimen, prescribed laboratory heat treatment, sampling provisions, product condition, and any order-specific agreement control how conformity is established. A mill certificate showing chemistry alone therefore does not demonstrate the mechanical condition of a quenched-and-tempered production bar.

The molybdenum range is the most obvious compositional difference, but procurement should not convert it into a fixed percentage improvement. Heat chemistry within the permitted ranges, prior microstructure, actual section, furnace loading, quench transfer, agitation, temper temperature and time all influence the result. If a supplier proposes 42CrMo4 or SAE 4140 instead of 42CrMo, their ISO or SAE chemistry documents are separate controlling routes; a familiar cross-reference does not silently replace GB/T 3077.

Values are condensed from the named standards and must be checked against the purchaser's licensed controlling edition, product form, sampling rules, and delivery condition. Only accepted order terms define the requirements for a specific supply.

Sources:[1][2][4]

Sources:[1][2][4]

Turn section-size risk into a heat-treatment evidence plan

Hardenability is the ability to develop hardness with depth under a stated cooling condition; it is not the surface hardness reading itself. The ISO 642 Jominy method measures response along a standardized end-quenched specimen, and the current standard also permits an agreed calculation route within a defined field of application. Neither route directly guarantees the core of the buyer's shaft. For a high-consequence or large-section part, use the design's required core and surface condition to decide whether the order needs a hardenability band, a sacrificial prolongation coupon, a separately heat-treated qualification piece, a production test ring, or tests taken from an agreed location in the supplied product.

Compare suppliers on the whole thermal route. Ask where the bars are heat treated, the maximum furnace load and bar spacing for the proposed lot, how time at temperature is controlled, expected transfer delay, quenchant type and temperature, agitation control, tempering route, straightening sequence, and whether machining allowance will remove decarburized material. These are process-review questions, not parameters to prescribe blindly. The supplier should confirm a capable route against the accepted result requirements and record any order-specific process qualification that the purchaser requires.

Section-sensitive evidence becomes more important as diameter increases or as the drawing narrows the permissible strength-toughness window. A single surface hardness can hide a soft core, and an axial tensile sample can miss circumferential variation. Conversely, requiring extensive testing on a low-consequence small shaft may add cost without changing the decision. Link every test to a failure risk and a disposition rule: what result passes, what triggers retest, and who can approve a deviation.

Sources:[1][3]

Price the processing and inspection consequences, not alloy alone

The cheapest quoted tonne can become the expensive route if it arrives in the wrong condition or consumes excessive machining and qualification time. Compare the offered bar size against the finished envelope and allowance, because extra stock raises material cost, quench mass, turning time, and scrap. Confirm whether the offer is hot rolled, forged, normalized, annealed, quenched and tempered, peeled, or rough machined. A QT quote should state where the required mechanical properties apply and whether straightening occurs before or after final tempering; a machining quote should state the inspection size and which surface defects the allowance is intended to remove.

40Cr may provide the better procurement outcome where the design accepts it and a stable process reaches the requirement with fewer alloy and heat-treatment burdens. 42CrMo may reduce technical risk where the specified section and property window exceed the demonstrated 40Cr route. Neither conclusion should be based on nominal grade price alone. Evaluate yield loss, minimum order quantity, heat-treatment batch size, test coupons, destructive-test loss, re-straightening, surface preparation, UT accessibility, certificate review, and the cost of a rejected or delayed qualification lot.

Inspection scope should follow consequence. Chemistry and heat identity are baseline controls. Add tensile, impact, hardness traverse, grain size, decarburization, macrostructure, or cleanliness only when the drawing, governing specification, prior failure, or qualification plan gives them a decision role. Ultrasonic testing needs its own method, coverage, calibration/reference basis, acceptance criteria, test stage, personnel qualification, and reporting requirement; 'UT passed' is not an acceptance specification. Preserve heat-to-bar traceability through cutting if test results must follow individual blanks.

Sources:[1][2]

Write one RFQ that makes both routes comparable

Ask the supplier to quote the required route and identify every deviation in a separate column. If alternatives are allowed, require two technically complete offers rather than an unlabeled substitution: one for the drawing grade and one for the proposed route, each with standard edition, production size, delivery condition, property and sampling basis, certificate, inspection, quantity, and commercial impact. The engineering authority—not the purchasing system or supplier cross-reference table—should approve the alternative before order acceptance.

The worked line below is intentionally specific about what is known and explicit about what remains purchaser-controlled. Replace the example diameter, length, quantity, test values, and UT criteria with the actual drawing and governing documents. Do not copy its numerical requirements into another component merely because the grade is similar. The phrase 'or equivalent' has been excluded because it gives no decision rule; a supplier may propose an alternative only through the stated deviation process.

At bid comparison, reconcile the offer to the RFQ line item by item. Record whether the mechanical values apply to a standard reference specimen, the delivered bar, a separately heat-treated coupon, or a test location in the finished section. Resolve contradictions before the purchase order, including any supplier standard terms that redefine tolerances or inspection. Only accepted order terms define the requirements for a specific supply.

Item: Alloy steel round bar for stepped drive shaft
Required grade: 42CrMo
Standard and edition: GB/T 3077-2015
Size and quantity: Ø120 mm × 6,000 mm, 18 bars; finished drawing and revision attached
Delivery condition: Quenched and tempered; straightening and final temper sequence to be declared
Mechanical requirements: Per drawing at the stated sampling location; supplier to distinguish production-bar results from reference-specimen data
Inspection: Heat analysis; agreed tensile/impact sampling; dimensions and surface; UT only to attached method, coverage, acceptance and report requirements
Certificate: EN 10204 type 3.1 or purchaser-approved equivalent document, with heat-to-bar traceability and actual results
Alternatives: 40Cr may be quoted as a separate deviation offer only; no substitution without written purchaser approval
Packing/marking: Grade, heat, bar number, size and PO item retained through shipment
Destination: [city, country and delivery term]

Sources:[1][3]