Specifying materials

In one lineA complete callout names the alloy and condition, the product-form standard, and the finish with its own spec.

Before thisSelecting materials · Designations

Why this matters

The material block is the part of a drawing that is easiest to write badly and most expensive to get wrong. “6082-T6” or “D6AC” tells a supplier the chemistry and nothing about what you will receive: which product form, what tolerance class, what melt practice, what strength, what certificate. The strength you designed to only arrives if the drawing asks for it in terms an inspector can check.

The three-part rule

Think of it like…

Ordering a coffee

Ask for “a coffee” and you will get a coffee. Ask for “a large flat white with oat milk and an extra shot” and you get what you meant — and if it arrives wrong, you can say exactly how. A material callout works the same way: the alloy is “coffee”; the form, standard, condition and finish are everything that makes the order checkable.

Every metallic callout needs three things, and most drawings carry only one or two:

1. The alloy and its condition — temper or heat treatment, stated so it can be tested (for example a minimum UTS), with the process specification that delivers it.
2. The product form and its standard — properties and tolerances differ between bar, plate, extrusion and forging, so the standard for that form governs. Many forms have a separate tolerance standard.
3. The finish, with its own specification — anodise, plating, passivation, primer, peening. Never “to suit”.

Anatomy of a complete material callout

1. D6AC STEEL (UNS K24728)AlloyChemistry — and nothing else. On its own it is a wish.
2. BARProduct formGrain direction, tolerances, how much you machine away.
3. TO AMS 6431Material standardProperties, test method, sampling and acceptance for that form.
4. VARMelt practiceCleanliness — fatigue and fracture depend on it.
5. HARDEN + DOUBLE TEMPER, 1520 MPa MIN UTS, AMS 2759/2ConditionThe strength you designed to, and the process that delivers it.
6. PRIMER AND PAINT TO <SPEC>FinishIts own specification, every time.

"D6AC" names a chemistry. Only the full block — form, standard, melt, condition and finish — names something an inspector can accept or reject.

Product form and grain direction

Product form sets the grain — and the weak direction

Wrought metal has a grain, like wood. Plate and bar are strongest along the rolling direction and weakest through the thickness. Forging lets the grain follow the part, which is why pressure-vessel closures and bosses are forged.

Rolled and extruded metal is anisotropic. In thick plate the short-transverse direction, through the thickness, typically has the lowest elongation and fracture toughness and the highest sensitivity to stress-corrosion cracking — the classic failure path for high-strength aluminium and steel fittings loaded through their thickness. Two consequences for the drawing:

• Choose the form for the load path. A closure, boss or Y-ring that carries pressure load around a contour is usually forged so the grain follows the shape, rather than machined from bar or plate.
• Say which way the grain runs when it matters. A grain-direction note on the drawing and on the forging drawing removes the guesswork.

Builder

Pick a sheet, then each of the three parts. The builder assembles the block from the standards on that sheet; the example underneath is how REF·DB v1 wrote a complete callout for the same material.

Material specification builderCopy callout

Material

1 · Product form

2 · Condition

3 · Finish

Extra lines

Inspection certificate (BS EN 10204 3.1) Batch marking and traceability Dimensions apply after finish

Your callout

Placeholders in <angle brackets> are yours to fill. Check every standard is the issue your programme invokes.

How v1 wrote it for this material

Practical tips

Worked example — a D6AC motor case component

The builder’s default is the D6AC sheet, one of the two case-steel sheets added in v2.

1. Alloy and form. Bar to AMS 6431, the material specification for D6AC bar, forgings and tubing, which also fixes vacuum-arc remelting (VAR). Sheet and plate for a rolled-and-welded cylinder come under AMS 6439.
2. Condition. The case strength — around 1520 MPa minimum UTS in v1’s example — comes from hardening and double tempering, invoked through the heat-treatment specification (AMS 2759/2 for low-alloy steels at this strength) with furnace pyrometry to AMS 2750. Write the strength you designed to on the drawing, not just the process.
3. Finish. Paint over a phosphate or primer base. Electroplating ultra-high-strength steel risks hydrogen embrittlement, so it is used only where justified and always followed by an embrittlement-relief bake.
4. Extras that matter for a pressure part: an inspection certificate (BS EN 10204 type 3.1 in the UK), batch or heat marking by a method that doesn’t create stress raisers, and NDT after heat treatment — the v1 example shows magnetic particle inspection to ASTM E1444.

Special processes get their own lines

Heat treatment, welding, NDT, coating and cleanliness are each separate notes with their own specification. If a requirement can’t be verified by measuring the finished part, it needs an approved process and a certificate.

Where this stops

Check the current issue

Standards are revised, superseded and sometimes withdrawn. The sheets here are a starting point carried over from REF·DB v1 plus two v2 additions; work to the issue your programme invokes, and use your organisation’s preferred specifications where they exist.

• Equivalent is not identical. Two specifications for “the same” alloy can differ in impurity limits, melt practice, testing and acceptance. See designations before substituting across systems.
• Fracture-critical parts add flaw-size and NDE requirements and controlled quality classes to the drawing.
• Composites and non-metallics are specified quite differently: the lay-up, process and cure are the material.

References and further reading

• BS 8888 — the UK framework for technical product specification, including how drawings invoke standards.
• SAE AMS specifications for aerospace materials and processes (AMS 6431, AMS 6439, AMS 2759 series, AMS 2750).
• BS EN 10204 — types of inspection documents for metallic products.
• ASM Handbook, Vol. 4 (Heat Treating) and Vol. 14 (Forming and Forging).

Note

Screening and educational material. Drawing callouts are approved under your organisation’s drawing and materials procedures.