Zirconium Fabrication Guide

Zirconium is a hard, shiny, grayish white metal. With its superior corrosion resistance capabilities, it has increasingly become the material of choice in the fabrication of chemical processing equipment. For non-nuclear applications, zirconium has two main alloys: ASTM R60702 (Zr702), which is unalloyed, and ASTM R60705, which is alloyed with 2.0-3.0 percent niobium.

The ductility and workability of zirconium allows standard shop equipment to be used for machining and forming operations, with few modifications or new techniques required. There are, however, some special considerations that need to be made when working with zirconium. The information here will provide a starting point for less experienced fabricators, or those working with zirconium for the first time, listing guidelines, recommendations, and specific data when applicable. MACHINEABILITY There are three basic principles that should be followed when machining zirconium: slow speeds, high feed rates, and a flood coolant system using a water-soluble oil lubricant. Zirconium does tend to gall and workharden, which requires higher than normal clearance angles on tools to penetrate the work-hardened surface and cut a clean coarse chip. In most cases, both carbide and high-speed steel tools give satisfactory results when machining zirconium; however, carbide will usually allow higher productivity and give a better finish. Performance is greatly enhanced by keeping the tools sharp. It is also important to prevent the accumulation of fine chips, since zirconium can be pyrophoric; small machine chips or turnings with a high surface-area-to-mass ratio are easily ignited and burn at extremely high temperatures

Turning

There are no special requirements for turning zirconium. It can be done without difficulty using standard equipment, as long as sharp tools and a coolant lubricant are used.

Milling

Both vertical face and horizontal slab milling of zirconium give good results. Whenever possible, zirconium should be climb milled to penetrate the work at the maximum approach angle and depth of cut while emerging through the work-hardened area. It is important that the milling cutters be kept very sharp and the work area flooded or sprayed with coolant to completely wash away the chips from the tool. Specific operating set points for milling zirconium are given in Tables 4-9. Lead angles on tools should be between +15 and +30 degrees, with positive axial and positive radial rake angles.

Drilling

Good results for drilling zirconium can be achieved using the standard fresh ground (118º) thin webbed drill and a coolant lubricant. It is important to use a firm backing to prevent burrs at the exit. Also, ensuring a sufficient amount of stock material is left for reaming can minimize smearing. Tapping zirconium is done best using chip driver or gun type taps, they must be kept sharp.

Grinding

Grinding Both wheel grinding and belt grinding can be used for zirconium. For wheel grinding, conventional speeds and feeds are satisfactory with silicon carbide wheels giving better results than aluminum oxide. The effect of grinding fluid on zirconium is the same as on other metals; grinding oils alone produce higher grinding ratios than water miscible fluids, which means less wheel breakdown and finer finishes.

FORMABILITY

Although zirconium can be formed using standard shop equipment, it does have a tendency to react with gases in the air at elevated temperatures and to gall and seize under sliding contact with other metals. A thin oxide layer, acting as a lubricant, protects against galling in forming operations. If additional lubricant is needed, use any oil or grease that does not contain halogen or sulfur (with the exception of molydisulfide).

A.   Bending

Sheet or strip zirconium can be easily bent on conventional press brake or roll forming equipment to a 5T bend radius at room temperature and to 3T at approximately 200ºC. For cold forming zirconium tube, a minimum bend radius of 3 times the OD dimension is advisable. For smaller radius bends, hot forming at temperatures from 200ºC to 425ºC or the use of special bending techniques is required. When bending zirconium tube, spring back may be encountered due to its work hardening behavior. Also, both the inside and outside surfaces at the bend area must be in tension during any bending operation to prevent buckling and wall thinning.

B.   Punching

Zirconium requires high plate pressures or excessive side flow occurs during punching operations. Very close punch and die tolerances, 1 to 2 percent of the metal thickness, provide the best results. It is essential that the dies remain sharp when punching zirconium. There is a tendency for zirconium to build up on the punch sides thus making stripping difficult. A die lubricant will minimize galling and reduce die wear.

C.   Drawing and Spinning

Despite its work hardening characteristics, zirconium’s formability by hot and cold operations is good. Designs that eliminate severe or abrupt section changes, and allow generous radii are a must. Dies of non-galling material with tolerances and clearances comparable to those used for austenitic stainless steels should be employed. As in the case of tube bending, die designs should allow for the spring back tendency of the material.

SPECIAL PRECAUTIONS

There is one major precaution that must be considered when working with zirconium. It is non-toxic, but because of its very high heat-producing reaction with oxidizing elements such as oxygen, zirconium is pyrophoric. Large pieces of sheet, plate, bar, tube, and ingot can be heated to high temperatures without burning, however, small machine chips and filings are easily ignited and will burn at extremely high temperatures. Therefore, it is necessary to prevent large accumulations of chips or other small pieces of zirconium, and care should be taken to store this material in non-flammable containers and isolated areas. Zirconium chips and filings should be frequently cleared from machines and moved to a proper storage container. An effective method of storage for zirconium chips and filings is to keep the material covered with water in the containers, with a layer of oil on top of the water to prevent evaporation. It is important to keep any storage container covered to prevent stray sparks from igniting the zirconium. If a fire does start in zirconium, ordinary fire extinguishers or water must not be used. Only dry sand, powdered graphite or special Metal-X powder should be used.