Wire breaks and strength reversal in strand-annealed MP35N fine wire (0.02–0.065 mm)

MP35N is a cobalt-nickel-chromium-molybdenum alloy with good resistance to body-fluid corrosion and fatigue, and a standard conductor material for cardiac pacing leads. As leads get thinner, conductor wire is drawn to a few tens of microns. At that scale two problems appear that are minor in coarser sizes: breakage during drawing, and the straightness and strength of the wire after annealing.

This article summarizes our production records for 0.02–0.065 mm MP35N fine wire, describes what we observed, and interprets it in terms of how the alloy strengthens.

Production conditions

The strength figures are approximate values noted during production, not results of a formal test program, and elongation after annealing was not recorded. The analysis below relies on the trend, not on the precision of individual values.

Observation 1: breaks at 0.02 mm concentrate in the final dies

Above 0.035 mm, breaks during drawing are occasional. At 0.02 mm they become the main problem, and they cluster in the last few dies.

The direct consequence is spools that never fill. Each break ends a spool early, so spool weight and length vary. Braiding and coiling need full spools of consistent length; more breaks mean more splices and changeovers downstream.

Defect size relative to wire diameter. A defect of a given size takes up a larger share of a thinner wire:

A defect that is negligible at 0.065 mm is large enough to start a fracture at 0.02 mm. This is why breaks cluster in the final dies: the closer the wire gets to finished size, the larger any defect becomes in relative terms.

Feedstock factors. Tracing broken lots back, three factors matter most:

1. Feedstock diameter tolerance. Uneven diameter means uneven reduction in each die; spots that take too much reduction fail first.
2. Surface defects. Scratches, laps and pressed-in scale move toward the center of the wire over successive passes and surface as break points in the final dies.
3. Melt practice. MP35N contains titanium, which forms hard nitride and carbide inclusions. These particles do not deform with the matrix and become fracture origins in fine wire; in finished leads they are also a common origin of fatigue cracks. Vacuum-melted, remelted material has fewer and smaller inclusions.

ASTM F562 allows up to 1.0% titanium and does not specify a melt practice. Different heats that all meet F562 can therefore behave quite differently when drawn to 0.02 mm.

Observation 2: strength reversal in the strand anneal

As-drawn MP35N fine wire has pronounced cast: released, it coils into loops and cannot go straight into braiding or tubing. Annealing relieves the residual drawing stress and straightens it.

Results for 0.035 mm (0.0014 in.) wire strand-annealed at different temperatures:

Below 600 °C the wire does not straighten effectively. As temperature rises, strength does not fall steadily as it would in most metals. It falls and then rises: lowest at 590 °C, above as-drawn by 700 °C.

Discussion: two competing processes

Heating cold-drawn MP35N sets off two processes that pull strength in opposite directions.

Stress relief (recovery). Residual drawing stress relaxes and dislocations rearrange. The wire straightens and loses a little strength. This is fast; seconds are enough.

Age strengthening. MP35N reaches its highest strengths through cold work plus aging: the cold-worked structure strengthens further when heated. The standard aging treatment is 540–645 °C (1000–1200 °F) for 4 hours, a time measured in hours.

A strand anneal lasts seconds. At lower temperatures aging has no time to develop and stress relief dominates. At higher temperatures aging speeds up enough to catch and overtake stress relief within the same few seconds:

• 590 °C: stress relief has begun and strength drops about 12%, but residual stress remains and the wire is not yet straight.
• 650 °C: residual stress is essentially gone and the wire is straight. Aging starts to act and strength recovers, though it stays below as-drawn.
• 700 °C: aging within a few seconds outweighs the loss from stress relief, and strength ends about 6% above as-drawn.

Under the strand-anneal conditions described here, 650 °C is therefore the tipping point: the wire is straight and strength has not been pushed up. Hotter, and strength can exceed the range a lead design calls for; cooler, and the wire does not straighten. The window is narrow, and furnace temperature has to be held steady.

These temperatures apply only to strand annealing on a scale of seconds. A batch anneal of whole coils holds for hours, aging proceeds fully at lower temperatures, and the temperature relationship is different.

Relation to the ASTM F562 fine-wire conditions

ASTM F562 is the usual specification for medical MP35N wire. It defines three conditions for fine wire: annealed, cold-worked hard and cold-worked spring. Our 0.02–0.035 mm sizes fall in its finest band (under 0.0508 mm / 0.002 in.); 0.06 and 0.065 mm fall in the band above.

Strength is specified as a range; wire above the upper limit fails as well.

The 0.035 mm wire stress-relieved at 650 °C, at about 1500 MPa, is above the annealed range and well below the cold-worked hard minimum. It belongs to none of the three conditions.

The 25–30% elongation in the annealed column applies to wire given a full high-temperature anneal that recrystallizes and softens it. Stress-relieved, straightened wire keeps most of its cold-work strengthening, and its elongation is far below that level.

For conditions outside the table, F562 leaves the mechanical properties to agreement between purchaser and supplier. Elongation in fine wire depends heavily on gauge length, so the gauge length should be agreed as well.

Conclusions

1. When MP35N is drawn to 0.02 mm, breaks concentrate in the final dies, mainly because defects become large relative to wire diameter. Feedstock diameter tolerance, surface quality and melt practice (inclusion level) decide whether the size can be produced consistently.
2. In a strand anneal lasting seconds, tensile strength of 0.035 mm wire first falls and then rises with temperature: down about 12% at 590 °C, recovering at 650 °C, about 6% above as-drawn at 700 °C. The cause is the different rates of stress relief and age strengthening.
3. 650 °C is the tipping point that balances straightness and strength. The resulting stress-relieved wire falls outside the annealed, hard and spring conditions of ASTM F562; its tensile range, elongation and gauge length need to be agreed separately.
4. The data are approximate production values, and the conclusions rest on the trend. Elongation after annealing is not included.