Why is the temper as important as the grade?
A grade such as DC04 fixes the chemistry and the broad family of properties, but it does not fix the state the strip is in when it reaches the press. That state is the temper, produced by the amount of cold reduction after annealing, and it is what decides how the material behaves in a draw. Two coils of the same grade, same thickness and same certificate can be supplied at different tempers, and the softer one will form a deep cup that the harder one tears. Temper shows up in the measurements that matter to a die: yield strength, elongation, the r-value that describes how the material resists thinning, and the yield-point elongation that produces stretcher strains. Buying by grade alone is how a part that ran for two years suddenly starts cracking on a new coil.
What the grade fixes, and what it does not
- Fixed by the grade: chemistry and the range of carbon, manganese and residual elements; whether the steel is aluminium-killed and suitable for drawing at all; the general level of formability.
- Not fixed by the grade: yield strength within the permitted band, elongation, r-value, n-value (the work-hardening exponent), grain size, and whether the surface is free from the Luders-band stretcher strains that appear as visible lines after forming.
- Fixed by the mill to the customer order: thickness tolerance, width, surface finish, oiling, and the temper itself. These are ordered parameters, not defaults.
The four numbers that actually predict the draw
- Yield strength. A higher yield needs more force to start the material moving and leaves less margin before the wall thins. On a small housing the difference between a soft and a quarter-hard temper can be the difference between a two-stage and a three-stage draw.
- Elongation. The total strain the material can take before it fractures. It is the headline ductility number, and a low-elongation coil is where tearing at the punch nose starts.
- r-value (plastic strain ratio). How much the material resists thinning when it is stretched in the plane of the sheet. A higher r-value means the wall stays thicker and more uniform, which is exactly what a pressure-carrying housing needs. It is a property of the crystallographic texture, influenced by the alloying and the annealing practice rather than by the temper alone.
- n-value (work-hardening exponent). How quickly the material hardens as it deforms. A higher n delays the onset of localised necking, which raises the draw ratio the material can survive in a single stage.
Key point
The practical test is not whether the certificate matches the grade - it is whether the coil carries the temper the die was set up for. When a part that has run for years starts cracking, ask for the incoming material test report before anyone talks about a new die.
Temper also decides whether the surface shows the forming
A temper that is too soft, or an annealed strip with the wrong yield-point elongation, forms visible stretcher strains: parallel bands across the part where the material yielded unevenly before work hardening caught up. They are cosmetic on a hidden bracket and a defect on a visible housing. The usual fixes are to skin-pass the strip, to specify a slightly harder temper, or to run the blank through a roller leveller before the press. The reverse problem is just as real: a temper hard enough to form beautifully on a shallow part can be too hard to form a deep one at all, so the temper has to be chosen against the deepest feature of the part, not against the average of it.
What to put on the drawing and the material order
- Name the grade and the standard - for example DC04 to EN 10130 - rather than leaving it as mild steel.
- Name the temper or the mechanical targets - the mill designation, or the yield and elongation range that works in the tool.
- State the r-value or n-value where the part is deep drawn and the wall has to stay uniform. If the designer does not know the number, asking the supplier for the forming data on the grade they propose is a reasonable substitute.
- Call out stretcher strain on visible parts, and say whether a roller-levelled or skin-passed surface is required.
- Say if the material is fixed by the customer. Where an OEM specifies the exact grade and mill, the freedom to adjust temper disappears and the die has to be designed around what is available.
What we do and do not claim
Balford reviews the drawing for material and process route together, and where a part is deep drawn the review includes the draw ratio, the wall-thickness distribution and whether the specified strip has the temper the route needs. We do not publish a rejection percentage or a guaranteed draw ratio for a grade, because both depend on the tooling and the thickness ratio as well as on the material. Quality management runs to ISO 9001:2015 with IATF 16949-aligned controls for the automotive programmes we serve. Material certificates are supplied with the shipment and are traceable to the coil or the heat.
Balford has built more than 4,000 single-hit drawing dies since 2000, runs presses from 25 t to 350 t, and deep draws to a maximum diameter of 250 mm. The plant covers 12,000 m2 in Zhuji, Zhejiang with 500+ machines and 120+ engineering and production staff, producing roughly 50 million parts a year. Drawings and material specifications are normally reviewed and quoted within 24 hours.
Frequently asked questions
Can I substitute a different temper of the same grade?
Not without checking the draw. The chemistry is the same, so the certificate looks right, but yield strength, elongation and r-value move with the temper and those are the numbers the die responds to. In practice a softer temper can often be substituted for a harder one on a deep part with little risk, while the reverse frequently cracks. If a substitution is unavoidable, ask for the incoming mechanical properties on the actual coil rather than the grade typical values, and run a trial before releasing production.
What are stretcher strains and how do I stop them?
Stretcher strains are the visible bands that appear when a soft, fully annealed strip yields unevenly at the start of forming. They come from the yield-point elongation of the material and are most visible on flat or lightly formed areas of a panel. The usual controls are a skin-passed (temper-rolled) strip, a slightly harder temper, or roller levelling the blank immediately before the press. On a visible housing it is worth specifying explicitly, because the part is otherwise structurally fine.
Does temper matter for stainless steel too?
Yes, and in stainless it is often the dominant variable. Austenitic grades such as 304 work-harden steeply, so the state of the incoming strip and the amount of cold work it has already seen change how many draws the material will accept. A 304 strip supplied at a higher cold-reduction level can need an extra interstage anneal that the same grade at a softer temper would not. In austenitic stainless the grade tells you very little about the draw behaviour.
How do I specify temper on a purchase order?
Use the mill own designation for the grade - for example a temper-rolling grade or a quarter-hard designation - and add the mechanical targets that matter for your part: a yield range, a minimum elongation, and where it applies the r-value. Then ask for the actual test values with the coil rather than accepting typical values from the grade datasheet. The certificate that comes with the steel and the values quoted in the datasheet are often not the same thing.