35CrMo vs 42CrMo for fastener bars

Decide whether the comparison is answering the right question

First define the finished component: headed bolt, stud bolt, tie rod, anchor component, machined blank, or bar for another processor. Record the product standard and drawing, thread system and tolerance, stressed and unthreaded sections, preload route, steady and cyclic loads, impact or minimum temperature, corrosion environment, coating, expected life, and fracture consequence. This prevents procurement from using an alloy designation to answer questions that belong to joint design, fastener geometry, surface integrity, or finished-product validation.

Then map the manufacturing sequence. Show the ordered bar size, the blank size during quenching and tempering, the final thread and transitions, and whether threads are rolled or cut before or after heat treatment. The heat-treatment section controls cooling depth; the final geometry controls local stress and gauging. A small qualification coupon, a bar-surface tensile specimen, and a finished stud proof-load test do not measure the same thing. State who owns each operation and which evidence releases the bar, the treated blank, and the finished component.

ISO 898-1 can govern defined metric bolts, screws, and studs within its scope, but a 35CrMo or 42CrMo certificate does not automatically create one of its property classes. The standard also excludes universal claims about fatigue, corrosion, shear, torque-clamp performance, and other service behavior. Use the relevant finished fastener standard and drawing additions, and keep application-specific validation outside the grade comparison.

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Compare chemistry and mechanics inside their controlling standards

GB/T 3077 provides the relevant Chinese routes for 35CrMo and 42CrMo. The listed mechanical lines below are prescribed reference heat-treatment specimen values; they are not universal values at every radial position or in a finished thread. ISO 683-2 instead provides 42CrMo4 chemistry and size-banded quenched-and-tempered properties. SAE J404 provides SAE 4140 chemistry but no universal mechanical-property line. A purchaser can evaluate these routes together, but should not splice their most favorable limits into one unnamed requirement.

Within GB/T 3077, 42CrMo carries a higher carbon minimum and a higher listed reference yield/tensile line than 35CrMo. That can support a higher-strength route, subject to section and heat-treatment evidence. It can also increase manufacturing sensitivity depending on hardness, thread sequence, straightening, surface condition, plating, and service stress. 35CrMo can be the better controlled route when its approved performance is sufficient and its manufacturing/qualification path is established. Neither conclusion should be made from nominal alloy content alone.

Values are condensed from the named standards for buyer-side RFQ comparison. Confirm licensed editions, analysis type and deviations, product form, delivery condition, ruling section, specimen location/orientation, impact method and temperature, and all order options. Similar chemistry does not establish interchangeability, and reference-specimen or size-banded values are not universal finished-fastener guarantees. Only accepted order terms define the requirements for a specific supply.

Sources:[1][2][3]

Sources:[1][2][3]

Test the proposed grade against the real section and process

For each grade, ask how the proposed heat and production route will meet the required section. Hardenability evidence, prior qualification, representative blanks, and production tests can all contribute, but they answer different questions. A Jominy curve characterizes hardenability under a standardized end-quench route; it does not reproduce the quench severity, blank geometry, furnace load, tempering, or residual stress of a threaded component. If core response is acceptance-critical, define an agreed production-section qualification and show where specimens come from.

Compare the entire heat-treatment plan: starting and rough-machined size, austenitizing and quench process ownership, lot definition, tempering, straightening, stress relief, surface cleanup, reheat restrictions, and controlled process changes. The buyer does not need a proprietary furnace recipe, but it needs to know whether a small qualification piece follows the production load and whether a new subcontractor, larger section, changed quenchant, or altered process stage requires approval. Allocate responsibility clearly when bar is purchased annealed for later buyer-controlled treatment.

The thread route can reverse a simple grade ranking. Rolling and cutting produce different surface and residual-stress states; performing either before or after heat treatment changes scale, decarburization, distortion, cleanup, and final gauging. State the sequence, starting allowance, thread and tolerance standard, runout and end details, surface-discontinuity criteria, specified decarburization limits, and the point at which hardness and dimensions are accepted. A stronger reference tensile line is of little value if the final thread surface is uncontrolled.

Sources:[2][4]

Separate static strength, fatigue, coating, and release evidence

Define the tests at the stage where the characteristic is final. Bar chemistry and agreed bar properties belong to the material lot; blank properties belong to the heat-treatment lot; thread dimensions, proof load or finished tensile performance, surface condition, and coating belong to the finished manufacturing lot. Maintain identity across cutting and subcontract operations so each report can be traced to delivered fasteners. Do not accept a pre-thread bar certificate as the only release record when the order requires finished-product compliance.

Fatigue deserves its own evidence path. ISO 3800 specifies axial-load fatigue testing conditions for threaded fasteners, but the official scope also makes clear that test conditions affect results. A fatigue result from one thread geometry, mean stress, stress amplitude, surface state, size, and lot should not be transferred silently to another. If the joint design relies on fatigue performance, engineering should define the specimen/product, loading, runout, statistical treatment, acceptance, and relationship to production. Static Rm and hardness cannot be used as fatigue-life guarantees.

Coating can also change the decision. ISO 4042 addresses electroplated fastener coating systems and measures to minimize hydrogen-embrittlement risk. It does not make either grade immune. Require the coating system and thickness, cleaning restrictions, thread allowance and final gauging, the approved hydrogen-control and relief route, lot traceability, tests, and approval for alternatives. Review high-hardness and sustained-stress fasteners with the manufacturer and plater before selecting a material route, not after a delayed failure concern appears.

Sources:[4][5][6]

Use controlled RFQ wording and a bid-deviation matrix

The sample below is a format, not an approved engineering requirement. Replace all bracketed fields with the drawing and project standard. It selects 35CrMo and asks for a 42CrMo option only as a separate deviation so the buyer can compare a common scope. If 42CrMo is mandatory, reverse the wording and do not permit downgrade by inference. Do not write 42CrMo4 or SAE 4140 in the same material line unless their standards and approval status are separately defined.

Have bidders return the exact material designation and edition, chemistry route, source product form, bar and heat-treatment sizes, process ownership, property basis and specimen relationship, thread sequence, coating route, inspection document, traceability, deviations, quantity/yield, destructive-test allowance, lead time, and nonconformance disposition. Compare total manufacturing and qualification cost, not just alloy price per tonne. Established 35CrMo qualification can be lower risk than an unqualified 42CrMo route; the reverse can be true when section demand or approved history requires 42CrMo.

Before production, resolve the supplier acknowledgment against the drawing and purchase order. Freeze required and alternative grades, standards, blank size, heat treatment, thread route, properties and test locations, surface and coating controls, lot definitions, records, witness points, deviation authority, packing, and destination. Only accepted order terms define the requirements for a specific supply.

Item: Alloy steel bar for high-strength stud bolts
Material route: 35CrMo to GB/T 3077-2015; quote 42CrMo only as a separate technical deviation requiring written purchaser approval
Finished fastener: M42 × 4.5 stud to drawing SB-709 rev D and [finished fastener standard/property requirement]
Bar size: Ø[approved blank diameter] × [length], [quantity]; state heat-treatment section and finished-thread allowance
Heat treatment: Quench and temper as production blanks; heat-treatment owner, lot definition, representative specimen location and retest plan to be stated
Thread route: Roll after heat treatment; final thread tolerance, runout, surface-discontinuity and decarburization requirements per drawing
Inspection: Actual chemistry, agreed section-representative mechanical tests, hardness, finished dimensions/gauges, surface, heat/process-lot traceability and release report
Coating: [system/thickness]; ISO 4042 route and hydrogen-control plan where contractually applicable; final gauging after coating
Certificate: [required inspection document] with actual results, test-piece relationship and report references
Alternatives/deviations: 42CrMo4 or SAE 4140 not accepted as equivalents without a separate clause-by-clause deviation and written approval
Packing/marking/destination: Retain heat and lot identity; [packing]; [city, country and delivery term]

Sources:[1][4][5]