Understanding Kovar and Invar 36: Low-Expansion Alloys
Kovar (ASTM F15, UNS K94610) and Invar 36 (ASTM F1684, UNS K93600) are iron-nickel alloys engineered for extremely low coefficients of thermal expansion (CTE). Kovar, containing approximately 29% nickel and 17% cobalt with the balance iron, achieves a CTE of 5.0–5.6 × 10⁻⁶/°C from 25–400°C — closely matching borosilicate and alumina glasses. Invar 36, with 36% nickel and the balance iron, exhibits an even lower CTE of 1.2 × 10⁻⁶/°C near room temperature, the lowest of any commercially available ferrous alloy.
These alloys serve critical roles in glass-to-metal seals, hermetic electronic packages, optical bench components, cryogenic structures, LNG membrane systems, and precision measurement instruments. Their machining characteristics differ significantly from standard carbon steels due to work hardening behavior, gumminess, and the need to preserve dimensional stability.
Tool Materials for Kovar and Invar
Carbide Grades (Primary Selection)
Coated carbide inserts in the ISO M15–M30 range (ANSI C5–C7) are the standard choice. These alloys machine similarly to austenitic stainless steels but with greater tendency to work-harden and produce long, tough chips. Recommended grades include:
- Turning (roughing): CVD-coated TiCN/Al₂O₃ grades, M20–M25 designation. Inserts: CNMG 120408, WNMG 080408, DNMG 150408.
- Turning (finishing): PVD-coated TiAlN grades, M10–M15. Inserts: CCMT 09T304, VBMT 160404, DCMT 11T304.
- Milling: PVD TiAlN-coated, positive-rake, APKT 1604PDER, LNKT 1506PNTR.
- Drilling: Solid carbide, TiAlN-coated, 140° point angle, 30–35° helix.
Cermet and Ceramics (Finishing Only)
TiCN-based cermets (ISO P05–P15) offer excellent surface finish on Kovar and Invar due to their chemical inertness with iron-nickel alloys. Use only for continuous light finishing cuts with minimal interrupted cutting.
Cutting Parameters: Turning Kovar
| Operation | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) | DOC (in) | DOC (mm) |
|---|---|---|---|---|---|---|
| Rough Turning | 200–300 | 61–91 | 0.008–0.012 | 0.20–0.30 | 0.060–0.100 | 1.5–2.5 |
| Semi-Finish | 300–450 | 91–137 | 0.005–0.008 | 0.13–0.20 | 0.020–0.060 | 0.5–1.5 |
| Finish Turning | 450–650 | 137–198 | 0.003–0.005 | 0.08–0.13 | 0.005–0.020 | 0.13–0.50 |
Cutting Parameters: Turning Invar 36
| Operation | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) | DOC (in) | DOC (mm) |
|---|---|---|---|---|---|---|
| Rough Turning | 180–280 | 55–85 | 0.008–0.012 | 0.20–0.30 | 0.060–0.100 | 1.5–2.5 |
| Semi-Finish | 280–420 | 85–128 | 0.005–0.008 | 0.13–0.20 | 0.020–0.060 | 0.5–1.5 |
| Finish Turning | 420–600 | 128–183 | 0.003–0.005 | 0.08–0.13 | 0.005–0.020 | 0.13–0.50 |
Invar 36 machines approximately 10–15% slower than Kovar due to its higher nickel content and more pronounced work-hardening behavior. Tool life: 25–50 minutes per edge roughing, 60–120 minutes finishing.
Cutting Parameters: Milling
| Operation | Speed (SFM) | Feed/Tooth (IPT) | Feed/Tooth (mm) | Axial DOC | Radial DOC |
|---|---|---|---|---|---|
| Face Milling – Kovar | 250–400 | 0.005–0.008 | 0.13–0.20 | 0.060–0.120 in | 60–75% of Ø |
| Face Milling – Invar | 220–350 | 0.005–0.008 | 0.13–0.20 | 0.060–0.120 in | 60–75% of Ø |
| End Milling – Kovar | 250–400 | 0.003–0.006 | 0.08–0.15 | 1.0× Ø | 10–20% of Ø |
| End Milling – Invar | 220–350 | 0.003–0.006 | 0.08–0.15 | 1.0× Ø | 10–20% of Ø |
| Slot Milling – Both | 180–300 | 0.002–0.004 | 0.05–0.10 | 0.5× Ø | Full width |
Cutting Parameters: Drilling and Tapping
| Operation | Speed (SFM) | Feed (IPR) | Notes |
|---|---|---|---|
| Drilling (≤ 1/4″) | 80–120 | 0.002–0.003 | 135° split point, peck 2× Ø |
| Drilling (1/4″ – 1/2″) | 100–150 | 0.003–0.005 | Peck cycle 3× Ø depth |
| Drilling (> 1/2″) | 120–180 | 0.005–0.007 | Pilot then step drill |
| Tapping Kovar | 15–25 SFM | — | Spiral-point, TiN-coated HSS, H3 limit |
| Tapping Invar | 12–20 SFM | — | Spiral-point, higher rake angle |
Work Hardening: The Primary Challenge
Both Kovar and Invar work-harden rapidly during machining. Surface hardness can increase from 130–160 HV (annealed) to 250–350 HV after a single machining pass with insufficient depth of cut. This hardened surface layer accelerates tool wear on subsequent passes and can induce residual stresses that compromise the alloy’s low-expansion properties.
Critical rules for managing work hardening:
- Maintain depth of cut above 0.005″ (0.13 mm). Shallower cuts rub rather than shear, creating a hardened glazed surface.
- Never dwell or spark out. Holding the tool stationary against the workpiece while the spindle rotates work-hardens a localized zone.
- Use sharp tools with positive rake. Dull tools increase plastic deformation ahead of the cutting edge.
- Progressive depth reduction. Rough at full DOC, then reduce in controlled steps — avoid “finishing” at 0.001–0.002″ DOC.
Dimensional Stability During Machining
Since Kovar and Invar are specified for dimensional stability, machining-induced stresses must be minimized and managed:
- Stress-relieve between operations. For precision components, intermediate stress relief at 850–950°C (Kovar) or 830–900°C (Invar) in a hydrogen or vacuum atmosphere prevents warpage after final machining.
- Symmetric stock removal. Remove equal material from opposing faces to balance residual stress distribution.
- Minimize clamping forces. Use soft jaws, spread clamping points, or vacuum fixturing for thin sections.
- Coolant temperature control. Maintain coolant at 20 ± 1°C for Invar parts. Thermal gradients from warm coolant can cause measurable expansion even in low-CTE alloys.
Coolant Strategy
Flood coolant at standard pressure (40–80 PSI) with semi-synthetic coolant at 8–10% concentration is adequate for most operations. Through-tool coolant at 150–300 PSI is recommended for drilling and deep boring. Coolant serves the dual purpose of heat extraction and chip flushing — long, stringy chips from these ductile alloys must be evacuated promptly to prevent recutting and surface damage.
Glass-to-Metal Seal Preparation
For Kovar components destined for glass sealing, the machined surface must be oxidized in a controlled wet-hydrogen atmosphere at 1,000–1,100°C to form a uniform FeO/Fe₂O₃ oxide layer. The machined surface finish directly affects seal quality: 32–63 μin Ra (0.8–1.6 μm) is optimal. Rougher surfaces create oxide pooling; smoother surfaces may not develop adequate oxide thickness for reliable bonding. After machining, components must be thoroughly degreased and free of any sulfur-containing cutting fluid residue, as sulfur contamination causes porosity in the glass seal.
Summary
Kovar and Invar 36 require the same disciplined approach used for austenitic stainless steels, with additional attention to work-hardening management and dimensional stability. Coated carbide in the M15–M30 range, cutting speeds of 180–650 SFM depending on operation, positive-rake sharp geometry, and controlled stress-relief heat treatment produce reliable results. The glass-sealing application demands particular care with surface finish and cleanliness to ensure hermetic seal integrity.
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