# Havar (alloy)

Havar, or UNS R30004, is an alloy of cobalt, possessing a very high mechanical strength. It can be heat-treated. It is highly resistant to corrosion and is non-magnetic. It is biocompatible. It has high fatigue resistance. It is a precipitation hardening superalloy.

## Chemical composition

The composition of Havar alloy is the following:
Composition of Havar alloy (wt. %) :
| Metal | Symbol | Average | Min | Max |
| (—) | (—) | (wt. %) | (wt. %) | (wt. %) |
| Cobalt | Co | 42.0 | 41.0 | 44.0 |
| Chromium | Cr | 19.5 | 19.0 | 21.0 |
| Nickel | Ni | 12.7 | 12.0 | 14.0 |
| Tungsten | W | 2.7 | 2.3 | 3.3 |
| Molybdenum | Mo | 2.2 | 2.0 | 2.8 |
| Manganese | Mn | 1.6 | 1.35 | 1.8 |
| Carbon | C | 0.2 | 0.17 | 0.23 |
| Beryllium | Be | 0.05 | 0.02 | 0.08 |
| Iron (balance) | Fe | 19.05 | 22.16 | 12.79 |

## Physical properties

Havar melting point is about 1480 °C. It will retain three quarters of its room temperature strengths up to 510 °C. Its density is 8.3 g/cm$^3$. Its thermal conductivity is 13.0 W/m·K. Its tensile strength is 960-970 MPa and its modulus of elasticity is 200 – 210 GPa. It can be joined by welding: Gas metal arc welding (GMAW), resistance welding, soldering, and brazing.

## Applications

Havar foils of various thickness are used as diaphragms for pressure sensing in process control equipment, biocompatible medical implants, as particle beam windows for beamlines of particle accelerators in nuclear physics, and various other high-temperature applications.
Havar foils are frequently used as window material for high-energy proton beams used in the production of fluorine-18 from oxygen-18 enriched water. $^{18}$F is a beta plus emitter commonly used in positron emission tomography (PET) scan. The high mechanical strength and the good corrosion resistance of Havar foils are both essential for the tightness and the reliability of the cyclotron targets in hospitals while also minimizing the contamination of H$_2$$^{18}$O in the targets by the activation products of the transition metals (Cr, Mn, Fe, Co, Ni, W, Mo) present in the alloy.

## History

Havar was originally developed in the late 1940s by Hamilton Watch Company as an alloy for the mainsprings used in watches, and named Dynavar. Later it was used as sensing diaphragms and other uses, under its current name Havar.

## Corrosion resistance

Its corrosion resistance allows use in stress corrosion resistant springs and diaphragms in oilfield equipment handling sour crude oil. Havar outperforms 316L stainless steel in resistance to pitting corrosion and crevice corrosion in medical implant environment. In cold-rolled and aged form, its yield and tensile strength are higher than of other cobalt-based implant alloys.
In Green death, a solution used to test the resistance to corrosion of metals, the Havar alloy does not corrode at all at room temperature, starts corroding rapidly (15 mm/year) at 70 °C, and reaches rate of 56 mm/year at boiling point (~103 °C).

## Machining

Havar is difficult to machine, as it undergoes rapid work hardening under the cutting tool. The tool should be as sharp as possible and the machine should be rigid, with minimal backlash. Higher power is required than to machine ordinary steels of similar hardness.