QT500-7 Ductile Iron: Properties, Heat Treatment, Inspection and Drawing Requirements

QT500-7 is a Chinese spheroidal-graphite cast-iron designation commonly requested for components that need a defined balance of strength and elongation. The designation should not be reduced to a chemical-composition table. Acceptance depends on the governing material standard, test-sample method, casting section, microstructure, heat treatment where required, dimensional drawing and inspection plan.

This guide is for engineers and buyers preparing an RFQ for QT500-7 castings or comparing it with an international spheroidal-graphite iron designation. It explains what the designation means, why test-piece results may differ from a casting, how matrix and graphite affect behavior, when heat treatment is discussed, and what quality records to request. Final suitability remains the purchaser’s engineering responsibility.

What the QT500-7 designation communicates

Under the applicable Chinese material system, the designation communicates a mechanical-property class. Buyers must state the exact standard and edition in the purchase documents rather than treating “QT500-7” as a complete specification. The test location, separately cast or cast-on sample, wall thickness and heat-treatment condition can affect how results relate to the finished component.

International projects may refer to ISO 1083:2018, which classifies spheroidal graphite cast irons using mechanical properties measured on machined test pieces prepared from cast samples. ISO confirmed this edition in 2023. A cross-reference must be reviewed against the complete standard; similar strength/elongation labels do not automatically make grades interchangeable.

Why chemistry alone is not an acceptance basis

Ductile iron properties arise from graphite nodularity, matrix structure, casting soundness, section size and processing. Chemistry controls the metallurgical process but is not normally a complete substitute for specified mechanical properties and microstructure. A fixed “universal composition” copied from a website may conflict with a foundry’s control practice or the governing standard.

Control area	What it influences	RFQ requirement
Base iron and treatment	Graphite formation and matrix potential	Specify material standard and required evidence.
Nodularization/inoculation	Graphite form, count and distribution	Define microstructure evaluation if required.
Cooling rate/section	Ferrite-pearlite balance and local properties	Provide critical wall thicknesses and test locations.
Heat treatment	Matrix adjustment and stress condition	Require only when technically justified and approved.
Defects and geometry	Local load capacity and fatigue behavior	Define inspection zones and acceptance criteria.

Graphite form and matrix structure

Spheroidal graphite reduces the severe stress concentration associated with flake graphite, but nodularity is not the only variable. Graphite size, distribution, degeneracy and matrix structure need context. ISO 945-1:2019 provides visual graphite-classification methods and explicitly does not determine suitability for a particular application.

QT500-7 castings often rely on a ferritic-pearlitic or pearlitic contribution to reach the requested strength/elongation balance, but the final matrix requirement should come from the governing standard or purchaser specification. Do not infer full mechanical compliance from one photomicrograph.

Mechanical properties and test-piece location

When tensile strength, proof strength and elongation are required, state the sample type and acceptance standard. Results from a separately cast coupon may not reproduce every local property in a heavy or highly variable section. The purchaser should identify critical zones and determine whether cast-on samples, separately cast samples or additional casting tests are appropriate.

Question	Why it matters	Purchase-document action
Which standard/edition?	Property values and sampling rules depend on it.	State it explicitly.
Which sample type?	Cooling history can differ from the component.	Define sample method and location.
Which wall thickness?	Section size changes solidification and matrix.	Provide drawing and critical section map.
Which temperature?	Low-temperature impact or service limits may matter.	Add only justified requirements.

Hardness: useful, but not a replacement for tensile testing

ASTM E10 defines Brinell hardness testing and notes that a result at one location may not represent the whole part. Hardness can support process control or local mapping, but it does not independently verify tensile strength, elongation, nodularity, soundness or fatigue performance.

If hardness is specified, identify the method, surface preparation, test zone, minimum section around the indentation, number of readings and acceptance range. Avoid converting hardness to tensile strength as the sole acceptance method unless the governing specification permits it.

When heat treatment may be considered

Heat treatment can be used to adjust matrix structure, relieve stress or recover a property balance when the process specification allows it. The route must be developed for the actual casting section and required properties. A generic temperature/time recipe should not be copied into an RFQ without foundry and purchaser review.

Objective	Possible discussion	Engineering boundary
Increase ferritic content	Annealing strategy and cooling control	Must still meet strength and dimensional requirements.
Increase pearlitic contribution	Alloy/process control or approved heat treatment	May reduce ductility; verify required properties.
Stress control	Thermal cycle compatible with geometry	Does not repair shrinkage or poor nodularity.
Dimensional stability	Sequence of rough machining, heat treatment and finish machining	Requires drawing-based process planning.

Casting design and section sensitivity

Heavy bosses connected to thin walls, abrupt transitions, isolated hot spots and constrained cores can affect feeding, shrinkage, graphite condition and matrix. Share the complete 2D drawing and, when possible, a 3D model. Identify machined surfaces, pressure or sealing boundaries, critical loads, datums and areas where repair is prohibited.

Machining and dimensional requirements

Define casting tolerances, machining allowance, datums, final dimensions, surface finish and geometric controls. Do not apply machined tolerances to an as-cast surface unless intended. For pressure-containing or sealing parts, provide the applicable test and acceptance requirement rather than assuming the material grade alone establishes leak tightness.

Inspection and traceability

Requirement	Define before ordering	Possible evidence
Mechanical properties	Standard, sample type and acceptance values	Tensile test report linked to batch.
Microstructure	Nodularity/graphite method, matrix and sampling	Microstructure report or images where agreed.
Hardness	Method, zones and range	Mapped readings.
Dimensions	Critical features, datums and sampling	Dimensional inspection report.
NDT/leak test	Method, area, coverage and acceptance criteria	Project-specific report.
Traceability	Part marking, heat/batch and certificate linkage	Material and inspection dossier.

Common specification mistakes

• Ordering only “QT500-7” without a standard, drawing or sample method.
• Using a chemistry range copied from one producer as a universal guarantee.
• Assuming a coupon represents every heavy and thin section.
• Using hardness alone to accept tensile and elongation requirements.
• Requesting microstructure without method, location or acceptance criteria.
• Ignoring machining allowance, datums and pressure/sealing boundaries.
• Approving a “similar” international grade without a formal equivalence review.

RFQ checklist for QT500-7 castings

1. Approved drawing, revision and 3D model when shareable.
2. Governing material standard and edition.
3. Critical sections, service loads, temperature and environment.
4. Mechanical-property values and test-sample method.
5. Microstructure, hardness, NDT or leak-test requirements where justified.
6. Heat-treatment condition and repair policy.
7. Machining, tolerances, datums and surface requirements.
8. Marking, traceability, certificate and packing requirements.
9. Prototype/production quantity and delivery destination.

How to assess a proposed equivalent grade

An equivalence review should compare more than the nominal strength number. Check the standard edition, tensile and proof-strength requirements, elongation, impact requirements where applicable, sample type, section limits, hardness classification, graphite/matrix requirements, delivery condition and supplementary tests. Differences must be documented and approved by the purchaser’s materials engineer.

Comparison area	Question	Decision
Property class	Are minimum values and test temperatures equivalent?	Document gaps.
Sampling	Separately cast, cast-on or cut from casting?	Confirm applicability.
Section size	Are thickness-dependent requirements aligned?	Map critical sections.
Microstructure	Are nodularity and matrix requirements comparable?	Define inspection.
Supplementary tests	Hardness, impact, NDT or pressure testing?	Add project criteria.

Do not state that QT500-7 “equals” another designation solely because both names include 500 and 7. The number format may suggest a similar nominal property class, but standards use different sampling and acceptance rules. The approved drawing and purchase specification must identify the accepted designation.

Process-control questions for the foundry

A buyer does not need confidential process recipes, but the quotation should explain how material identity, treatment batch, test coupons, heat treatment, inspection and nonconforming product are controlled. Ask how coupon identity stays linked to the castings and how results are reviewed before release. For critical parts, agree hold points before production rather than requesting unplanned evidence after shipment.

Where machining exposes internal material or controls a sealing interface, state whether rough machining precedes final inspection. If a repair is permitted, define approval, procedure qualification, affected zones and post-repair inspection. A material certificate alone cannot demonstrate dimensional conformity or casting soundness.

How to compare QT500-7 quotations

Quotation line	Required clarity	Why
Material	Standard, edition, grade and delivery condition	Prevents name-only comparison.
Testing	Coupon type, properties, microstructure and hardness	Defines acceptance evidence.
Manufacturing	Casting, heat treatment, machining and repair scope	Separates included operations.
Quality	Dimensions, NDT/leak test, marking and dossier	Aligns consequence and records.
Commercial	Tooling, sample quantity, production quantity and packing	Makes offers comparable.

Frequently asked questions

Is QT500-7 defined by chemistry?

It is primarily ordered as a mechanical-property grade under the applicable standard. Chemistry is process-dependent and cannot replace the required properties, microstructure and inspection evidence.

Is QT500-7 the same as EN-GJS-500-7?

Do not assume automatic equivalence. Compare complete standards, sampling, section rules and supplementary requirements, then obtain purchaser approval.

Does higher pearlite always improve the casting?

More pearlite can increase strength or hardness but may reduce ductility. The acceptable balance depends on the specified properties and service duty.

Can heat treatment repair poor nodularity or shrinkage?

No. Heat treatment can modify matrix or stress condition, but it cannot correct all metallurgical or casting defects. Prevention and appropriate inspection remain necessary.

Match the grade to the component duty

QT500-7 can be considered for many machinery castings, but the grade name does not approve a pressure boundary, safety-critical bracket, fatigue-loaded hub, low-temperature part or wear component. Define static and cyclic loads, temperature, corrosion, impact, required stiffness, fatigue consequence, sealing duty and regulatory requirements. The design engineer must confirm section stresses and allowable properties.

For fatigue-sensitive geometry, surface condition, fillet radius, machining marks, casting discontinuities and local microstructure may matter more than the nominal tensile class. For sealing or pressure duty, a separate leak or pressure test may be required. For wear duty, contact, lubrication, counterpart material and surface treatment must be considered. Do not apply general material advantages as a universal performance claim.

Prototype and first-article planning

A new or transferred casting may justify a first-article plan. Identify which dimensions receive a full report, which material tests are linked to the batch, whether microstructure is checked at a representative location, and whether machining or assembly trials are needed. If destructive sectioning is required, plan an additional sample rather than sacrificing a production part without agreement.

Review the first article before releasing recurring production. Record approved deviations and update the drawing or specification instead of allowing an undocumented concession to become the new standard. The order should state whether tooling, gauges and inspection fixtures belong to the purchaser or supplier and how revisions are controlled.

Preserve traceability through machining and shipment

Part identity can be lost when gates are removed, surfaces are blasted or machining separates components from a batch. Agree a marking location and method compatible with the drawing. Inspection reports, test coupons and certificates should reference the same heat, batch or order identity. Packing lists should preserve part number, revision and quantity so receiving inspection can match the shipment to the approved records.

Receiving inspection should confirm markings and documents before components enter stores or machining. If traceability is broken, quarantine the affected parts and resolve the discrepancy through the purchaser’s quality process rather than recreating an identity from appearance or packaging alone.

For recurring orders, compare new certificates and inspection results with the approved first article. Trend changes in hardness, dimensions or microstructure where the control plan requires it, but investigate context before treating ordinary variation as a failure.

Related products and drawing-based quotation

For relevant component examples, review QT500 cylinder liners, ductile iron cylinder sleeves, quality assurance and the factory overview. Use the contact page to send drawings for quote.

Final material, test sampling, matrix, dimensions, machining, inspection and acceptance must follow purchaser-approved documents. This guide does not guarantee equivalence between standards, component performance or service life.