Annealed Stainless Profile Wire - Grain Size Control
Annealed Stainless Profile Wire
,Grain Size Control Profile Wire
,Stainless Steel Profile Wire
Annealed Stainless Profile Wire - Grain Size Control
Two coils of annealed stainless wire can share the same grade, the same diameter and the same tensile strength, and still behave differently on the same press. The difference is usually grain size. Grain size controls how the metal distributes strain during bending and cold forming, how much it springs back when the tool releases, and whether the surface stays smooth or develops an orange peel texture. This page explains what the ASTM E112 number actually describes, how grain size is set during annealing, and how to specify it so that forming behaviour repeats from coil to coil.
Product Specifications
| Item | Specification |
|---|---|
| Product Name | Annealed Stainless Steel Profile Wire |
| Material Grades | 304, 304L, 316, 316L, 201, 430 |
| Diameter Range | 0.15-12 mm |
| Diameter Tolerance | ±0.01 mm (fine wire), ±0.02 mm (0.5-3 mm), ±0.03 mm (above 3 mm) |
| Delivery Condition | Bright annealed, soft annealed |
| Grain Size | ASTM E112 No. 6-9, single phase, no duplex structure |
| Tensile Strength | 500-700 MPa (304 soft annealed) |
| Elongation | ≥ 40% (gauge length 100 mm) |
| Annealing | Bright annealing 1050-1120 °C under protective atmosphere |
| Coil Weight | 1-500 kg |
| Applicable Standards | ASTM A555, ASTM E112, EN 10270-1, GB/T 4240 |
Why Grain Size Decides Formability
What the ASTM E112 Number Means
The ASTM E112 grain size number is an inverse index: the higher the number, the finer the grain. No. 6 corresponds to an average grain diameter of roughly 0.045 mm, No. 8 to about 0.022 mm, and No. 10 to about 0.011 mm. For wire forming the useful working range is generally No. 6 to No. 9. Above that range the material is strong but the annealing window becomes narrow; below it, bendability drops and surface roughening appears at moderate strain.
Grain Size and Spring-back
Spring-back is elastic recovery after the tool withdraws, and it scales with yield strength, which in turn follows grain size through the Hall-Petch relationship. A change of two grain size numbers can shift spring-back by enough to move a formed angle outside tolerance, even though the tensile strength on the certificate still reads within the agreed band. Where a part is bent to a fixed angle and the angle is critical, grain size belongs in the purchase specification next to the tensile strength.
What Changes Grain Size During Production
Three variables set the final grain size. The annealing temperature and holding time decide how far grains grow. The amount of cold reduction before annealing decides how many nucleation sites exist. And the cooling rate decides whether the structure stays single phase. An undersized reduction before the final anneal leaves too few nucleation sites and produces coarse grains; a reduction in the critical range of roughly 5-15% can produce abnormal grain growth, where a few grains absorb the rest and the coil ends up with a duplex structure.
Checking Grain Size in Practice
Sample Preparation and Method
The test is taken on a transverse cross section, mounted, polished and etched electrolytically in oxalic acid for austenitic grades. Grain size is then rated by comparison with the standard charts or by the intercept method. The intercept method is preferable on wire because the field is small and the comparison charts were designed for wider sections.
Duplex Structure Is the Hidden Problem
A certificate reporting No. 8 does not say whether the structure is uniform. A duplex structure with a few coarse grains in a fine matrix averages out to No. 8 while behaving like a much coarser material at the bend. Ask for the rating to state that the structure is single phase, and where the part is critical, ask for the micrograph to be attached to the certificate.
Applications
Profile Wire for Custom Sections
Profile wire is rolled or drawn to a non-circular section, and the deformation during profiling is concentrated at the edges of the section. A uniform fine grain distributes that strain evenly and keeps the corner radii free of cracks, while a coarse or duplex structure tends to crack at the sharpest corner first.
Formed Parts with Tight Bend Radii
Parts bent to a radius below three times the wire diameter are sensitive to both grain size and surface condition. No. 7 to No. 9 annealed wire with a bright surface is the usual choice, and the bend radius should be stated on the order so the grain size can be matched to it.
Annealed Wire for Welded Assemblies
Welded wire assemblies need consistent fit-up before welding and predictable distortion after it. Uniform grain size keeps the spring-back of the cut lengths consistent, so the fixture closes without forcing, and a single phase structure avoids the local hardening that appears where coarse and fine grains meet.
Product Advantages
- Grain size held to ASTM E112 No. 6-9, stated on the certificate for each lot rather than quoted as a general capability.
- Single phase structure verified by micrograph on request, avoiding duplex grain conditions that average out on paper but crack at the bend.
- Bright annealing at 1050-1120 °C with controlled cooling, keeping the surface free of scale for subsequent forming or plating.
- Cold reduction before the final anneal controlled above the critical range, preventing abnormal grain growth.
- Diameter tolerance of ±0.01 mm on fine sizes, so the formed section matches the drawing without adjustment.
Quality Control
Grain size is checked on each annealing lot, not only on the first lot of an order. Diameter, tensile strength and elongation are tested on every lot, and the surface is inspected over the full coil length. When a grain size outside the agreed range is found, the lot is held rather than shipped with a note. Records are kept against the heat number and the annealing charge, so a forming problem in the field can be traced back to the specific furnace run.
Selection and Ordering Notes
- State the grain size range, for example ASTM E112 No. 7-9, in addition to grade and diameter. Tensile strength alone does not fix spring-back.
- Give the minimum bend radius or the profile corner radius the part requires, since both scale with grain size.
- Ask for a single phase structure to be stated on the certificate where the part is bent or profiled.
- For parts that will be welded after forming, mention the welding process, as grain size affects distortion and heat affected zone behaviour.
- Confirm whether grain size is to be rated by comparison or by intercept method, since the two do not always agree at the same decimal.
Grain size is the property that connects the annealing furnace to the press, and it is the one most often left off a purchase order. Specify it together with grade, temper and tolerance, and the forming behaviour of the wire becomes a number that can be checked rather than a result that has to be discovered.
