Page 1 of 1
Posted: Thu Jun 29, 2017 7:15 am
Many of us just cast a "Witch's Brew" of scrap metal, but in the real world they have a lot of problems selecting alloys:
Re: Copper Alloy
Posted: Fri Jun 30, 2017 3:56 pm
Formerlly leaded yellow brass, or plumbing brass was a simple copper alloy that contained about 3 % lead to improve tightness and machine qualities where profit margins are razor thin for general products in use. It should be noted that much old pipe work in underground supplies of old cities were of lead. chemical additives and treatment ph levels are closely monitered to keep the oxide skin stable on these old lines, to prevent leaching into the water supply. This treatment then in turn keeps the lead in the brass. When the treatment fails in standard, then a few oz of water in the valve and faucet can pick up some lead. In Chicago where this phenomonon could occur, it was practice to turn on the faucet, and then place the catch container in the steram, the lost water having cleared the valve and expelled the exposed water. All the hoopla was started over bad sampling, using distilled 'clean' water, that did not stop the leaching, and having the wrong PH, and numerous other problems that really should not be in the equation. In response, industry started getting the lead out of solders, for near pure tin, and bismuth became the wetting agent of choice. It is expensive by comparison to tin, and prone to creep in high amounts. In the real world of casting revert scrap, machinings and failed castings returned to the furnace, are limited, and added in proportion to a virgin mix, thereby creating a predictable base metal, that follows the current recipe in use for that product. And of course commercial cast shops have the tools to assay the melt on the fly to make minor bumps in the melt if needed. Very much different than the home guy that has to guess about what is in the pot for scrap. Indeed a virgin melt of copper and tin, in the home shop can get substantial lead from older solder that is not pure. Buying tin off the net from some backyard shmo can get you bricks of unknown soft metal, as lead is cheap, tin is not. Most guys cannot accurately tell yellow plumbing brass from gunmetal bronze or navel bronze, thinking the rich copper color of bronze may be a patina, and not alloy. Good Naval bronze will maintain a polish, due to the fact it lacks lead.. Leaded bronze, more common for bearings, will tarnish because of the lead. The wise home caster will use a best guess scrap melt where the quality of the part is appropriate, and certified bar metal, where quality is paramount.
Engineers use the reference books and go by the numbers to determine the vagaries of tool life and shear strength, to head off production problems as these properties are well known with their behavior for investment, sand or lost wax casting.