AWS WHC-5.07:2015

Reactive, Refractory, and Precious Metals and Alloys
Scope : This chapter differs from other chapters in that it covers the weldability of reactive, refractory, and precious metals: 17 metals rather than a single metal system. The section of the periodic table of elements in Figure 7.1 shows the relative positions of these metals. Groups IVB through IB and Periods 5 and 6 (and beryllium and uranium, but not technetium) comprise the reactive, refractory, and precious metals. To make this chapter easier to use, the metals have been categorized into these three groups and identified using the Unified Numbering System (UNS).1 The associated metals in these groups and are the following: 1. Reactive metals: zirconium, hafnium, beryllium, and uranium; 2. Refractory metals: tantalum, niobium, molybdenum, tungsten, and rhenium; and 3. Precious metals: gold, silver, platinum, palladium, osmium, iridium, rhodium, and ruthenium. The generally accepted description of a reactive metal is that the metal reacts with oxygen and may absorb oxygen at elevated temperatures. A refractory metal is typically identified as a metal that has a melting point above about 2000°C (3632°F) and maintains some strength characteristics at elevated temperatures. Precious metals include a group of metals that are considered rare and have high economic value. The challenges of welding these metals are many, including the following: 1. The ability to absorb contaminants at elevated temperatures, which causes the metal to become hard and brittle; 2. Cleaning of the raw material may require complicated and hazardous operations, such as acid pickling; 3. Many of these metals undergo multiple phases, which can result in difficulty in forming and fabricating; 4. Unique processing and heat treatment procedures are often required to prevent damaging the metal; 5. These metals may be difficult to obtain in a desired product form and are costly; 6. Long lead times may be necessary to acquire sufficient quantity for a project; and 7. Replacement material is not nearly as available as more common metals.