201 Stainless Screw Wire - Cold Heading Crack Control
201 stainless screw wire
,cold heading crack control wire
,stainless steel spring wire
201 Stainless Screw Wire - Cold Heading Crack Control
Cold heading failures rarely start at the head surface. They start where the deformation is largest and the material has the least room to move: the fillet under the head, or the centre of a deeply upset section. For 201 stainless steel, which work hardens faster than 304 because of its manganese and nitrogen content, the margin between a clean upset and a split head is narrow. This page covers the three properties that decide heading performance, how to work out the heading ratio for a specific screw, and what to specify on the purchase order so that the wire behaves the same way lot after lot.
Product Specifications
| Item | Specification |
|---|---|
| Product Name | 201 Stainless Steel Screw Wire |
| Material | AISI 201: C ≤ 0.15%, Cr 16-18%, Mn 5.5-7.5%, Ni 3.5-5.5%, N ≤ 0.25% |
| 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 | Soft annealed, light drawn |
| Tensile Strength | 600-750 MPa, soft annealed |
| Reduction of Area | ≥ 60%, typical requirement for heading grades |
| Elongation | ≥ 40% (gauge length 100 mm) |
| Surface Treatment | Bright, oxalate coated, soap coated |
| Coil Weight | 1-500 kg |
| Applicable Standards | ASTM A555, EN 10263-5, GB/T 28904 |
What Determines Cold Heading Performance
Reduction of Area Comes Before Tensile Strength
A wire can show an acceptable tensile strength and still split at the head. Tensile strength describes resistance to being pulled apart; cold heading pulls and pushes material sideways at once, and what controls that is ductility in the transverse direction, reported as reduction of area. For heading grades the requirement is commonly set at 60% minimum, measured on a tensile specimen after fracture. Two lots both at 700 MPa can differ by ten points of reduction of area, and the difference appears as cracked heads rather than as a failed certificate. When a heading process is marginal, ask for reduction of area to be reported per lot instead of only tensile strength.
Centre Quality and Segregation
The centre of the wire is where casting segregation survives. Carbon, sulphur and phosphorus concentrate slightly in the middle of the original billet, and drawing reduces the diameter without redistributing them. In a shallow upset this is harmless; in a heading ratio above three, the segregated core is pushed outward into the head and acts as a plane of weakness, producing the classic centre burst. A macro etch test on a wire cross section shows the condition in a few minutes and is worth requesting for critical fasteners.
Surface Condition at the Upset Zone
Longitudinal seams, laps and deep die marks open up when the wire is compressed. A seam that is invisible on the straight wire becomes a crack running from the fillet into the head. Because 201 work hardens quickly, the surface layer already carries more strain than the core after drawing, and a defect in that layer has little reserve left. Surface inspection over the full coil, not only at the ends, is part of the specification rather than an optional extra.
Working Out the Heading Ratio for a Part
Reading the Ratio
The heading ratio is the cross-sectional area of the finished head divided by the cross-sectional area of the wire, which is the same as the square of the head diameter divided by the square of the wire diameter. A screw with a 4 mm head made from 2 mm wire runs at a ratio of four. For 201 in a single blow, ratios above four are commonly treated as a warning point, and above six the process usually needs a two step upset or an intermediate anneal. The same ratio that works in 304 may crack in 201 because of the faster work hardening rate.
Trial Heading Before Mass Production
Run twenty to fifty pieces at the intended machine setting and inspect the fillet under a low power magnifier. Cracks begin as short radial lines at the transition radius. If they appear, softening the wire is the usual remedy, together with a change in coating or die geometry. Recording the trial result against the wire lot number makes it possible to trace a later failure back to a specific delivery.
Applications
Screws, Bolts and Self-Tapping Fasteners
201 is widely used for dry-service fasteners where cost matters more than corrosion resistance in chloride environments. Diameters from 1 mm to 8 mm cover most screw production, with soft annealed wire and an oxalate plus soap coating to carry lubricant into the die.
Rivets and Cold Headed Pins
Rivets combine a large head with a small shank, so the heading ratio is high relative to the wire diameter. Wire with reduction of area above 60% and a clean surface upsets reliably at these ratios, while cold drawn temper without annealing typically does not.
Inserts, Nuts and Small Pressed Parts
Parts formed by combined forward and backward extrusion place the same demands on the wire as heading, with the addition of high tool pressure on the surface. Consistent diameter keeps the extruded wall thickness uniform, and a well bonded coating reduces pickup on the punch.
Product Advantages
- Soft annealed 201 with tensile strength held in a 600-750 MPa band, giving predictable metal flow at the die.
- Reduction of area reported per lot on request, so heading behaviour can be checked before the coil reaches the machine.
- Oxalate plus soap coating applied as a bonded carrier, reducing friction and punch pickup during upsetting.
- Diameter tolerance of ±0.01 mm on fine sizes, keeping the headed head concentric to the shank.
- Macro etch inspection available for fastener grades where centre quality is critical.
Quality Control
Each lot is tested for tensile strength, elongation and reduction of area, with results recorded against the heat number. Diameter is checked at the coil ends and at two intermediate points. Surface is inspected for seams, laps and die marks over the full length. Where a customer specifies a heading ratio, a trial upset is run at the agreed ratio before dispatch and the result is documented with the certificate.
Selection and Ordering Notes
- Give the finished head diameter and the wire diameter, or the heading ratio directly, so the temper can be matched to the deformation.
- State the required reduction of area if the part is a critical fastener; a tensile strength figure alone does not describe heading ability.
- Name the coating system required by your heading process: bright, oxalate plus soap, or a copper flash where galling is a problem.
- For 201 specifically, mention the environment. If the fastener will see chlorides or persistent moisture, a 304 or 316 grade should be evaluated instead.
- Agree the surface acceptance limit in writing, since a seam that passes a visual check on the coil can still open at the fillet.
Heading performance is decided by reduction of area, centre quality and surface, in that order, with grade and temper setting the limits. Send the part drawing with the head and shank diameters, and the wire specification can be matched to the actual deformation rather than to a general purpose grade.
