New Alternative Low-Lead Copper Alloy Shows Promise
C83470 is a viable option to produce castings that will be in compliance with the Safe Drinking Water Act or other standards requiring castings to be low in lead.
AFS Copper Alloy Division
(Click here to see the story as it appears in the February issue of Modern Casting.)
Many alternative low-lead brass and bronze alloys are available to metalcasting facilities for use in producing castings that will be in compliance with the Safe Drinking Water Act or other standards. Recently, a new no-lead alloy has entered the market that provides a combination of improved casting pressure tightness, mechanical strength, machinability (chip-breaking), alloy recyclability and pattern yield when compared to other low-lead alloy alternatives.
The Copper Development Association (CDA) lists a number of low-lead alloys identified by UNS number on its website (www.copper.org) and in print. Most of these alloys are also listed in applicable ASTM standards, specifying their required chemical composition and mechanical property requirements. Each of these alternative alloys have properties that make them unique from the others, and though they are all low in lead, each alloy presents differing challenges to metalcasters and machine shops when chosen to replace traditional lead-bearing alloys.
To date, no low-lead alloy alternative has proven to be an identical replacement for traditional lead-bearing alloys regarding casting, machining, pressure tightness and overall performance characteristics in typical waterworks casting applications.
A new sulfide-bearing tin bronze was developed in the 2000s by Shiga Valve Cooperative, Hikone, Japan, in conjunction with Kansai University, Suita, Japan. The new alloy has since been added to the CDA alloy database and given UNS number C83470.
With the introduction of this new alloy, copper-based casting facilities will have another tool to help meet the needs of the castings produced in no- or low-lead applications.
Learning by Trial
Beginning in 2011, the AFS Copper Alloy Division performed trials at Ford Meter Box Co., Inc., Wabash, Indiana, and A.Y. McDonald Mfg. Co. Inc., Dubuque, Iowa, to study alloy use in North American facilities. Green sand molds were produced with automatic molding machines utilizing warm box and shell cores.
The C83470 material was melted in a coreless induction furnace and a lift-swing furnace. Many different castings were manufactured in a size range consistent with standard casting sizes of these facilities. Casting weights were from 0.25-7.7 lbs., with wall thicknesses varying from 0.10-0.75 in. C83470 appears to be sensitive to high turbulence during pouring. While some existing gating and risering techniques designed for traditional leaded alloys, such as C83600, were successful without modification, other casting geometries required larger ingates. In some cases, due to the higher copper content of C83470, increased riser volumes were needed. In most cases, the thin ingates in a pressurized system resulted in poor casting quality. A non-pressurized gating system, along with generous parting-line venting, tended to result in more favorable casting quality.
Overabundance of superheat can cause problems, so degassing the metal is advised. Targeting the maximum of the zinc range also can be beneficial with degassing. Mixed results were found while purging the furnace with nitrogen gas, while phosphorous additions to the furnace appeared to help in some instances. Gas defects were the leading causes for scrap castings both before and after machining, and additional work with minimizing turbulence, gating design, pouring temperatures, pouring practices and other practices may be needed.
Pouring temperatures in the 2,100-2,275 F (1,149-1,246 C) range were needed to reduce misrun scrap, but may have contributed to higher amounts of centerline shrinkage porosity. These temperatures are 25-50 F higher than the current melting and pouring practices for bismuth alloy (C89833) castings. Different techniques of both horizontal and vertical venting were found to be effective in removing some of the gas.
As experience was gained during the trials, the use of proper mold venting addressed many of the gas issues. With proper venting techniques and minor gating changes, typical degassing methods were suitable.
The C83470 material was poured manually. No issues were found pouring this material, although some gating modifications may help reduce turbulence and gas issues. Some extra slag build-up in the ladles was noticed at times. Air monitoring did not pick up any significant levels of sulfur dioxide during melting or pouring.
Castings were found to have good surface finish when steps were taken to control gas absorption. In some instances, sawing and grinding the material was challenging due to material build-up on the wheels or blades. Additional investigation into cutting wheels and methods is needed. No major concerns were seen with grinding flash or trimming parting lines.
Castings made from the C83470 material (Fig. 1) were machined using both single-point tooling and gang tooling. The chips produced from both processes were larger and longer than those from other traditional plumbing alloys. The gang tooling could not evacuate the large chips easily, causing heat build-up and premature tooling failure. Elevated sulfur content can help break up the chips and reduce the size of the long, curly ribbons. Dye penetrant was used on selected machined castings, and no cracking was found in these castings.
Pressure tests of the C83470 material found it to be pressure tight. After successful pressure testing (up to 150 psi), many castings were fractured to examine the internal grain structure for potential defects. Defects found in these castings had minimal impact on pressure testing. Cold shuts and gas defects, where the defect did not penetrate the entire wall thickness, still allowed for positive pressure test results even with less than desirable metal structure (Fig. 2). The pressure tests resulted in almost no failures for leaking.
Testing of tensile bars poured during the trials resulted in mechanical properties that exceeded the published mechanical properties of C83470. Table 2 shows example tensile bar results from the trials. Castings made from the C83470 material were very ductile.
Leachate testing was conducted on multiple waterworks ball valve assemblies. The valve sizes ranged between 0.625-2 in., with all passing the NSF/ANSI Standard 61-G requirements.
Comprehensive recyclability of the current low-lead alternative family of alloys has been an industry-wide concern for recyclers, ingot makers and metalcasting facilities. This issue affects quality, cost and value. During the trials, C83470 provided advantages in this regard.
Current available low-lead alloys offer casting solutions, but vary in their ability to be safely and effectively recycled if they should become cross-contaminated with, or cause cross-contamination of, some of the other low lead alloys or the leaded brasses. Currently, the value of the resultant cross-contaminated material may be considerably reduced because this material must be refined or diluted when consumed back either by the facility or an ingot maker. Elements such as lead, bismuth, silicon or aluminum must be either removed by refining or diluted to bring the metal back into specification for reuse.
C83470 contains no lead, bismuth, silicon, aluminum or other element that causes cross-contamination and/or recycling concerns. C83470 will be easier for facilities to manage from this perspective and should potentially be more cost effective to recycle both in the facility and by an ingot maker. Should cross contamination of C83470 with another alloy occur in the facility, the sulfide present in C83470 is less likely to be a detriment to the casting quality, castability and value of returns, such as gates and risers. This is because alloys other than C83470 have significantly higher tolerance for sulfur than other alloying elements. Sulfur is relatively easy to remove by refining during the recycling process, negating the need for dilution with copper.
C83470 directly addresses the big picture of recyclability and the costs of finite natural resources.
C83470 most likely will be used in applications in the waterworks industry and castings that must meet the requirements of the Safe Drinking Water Act or other international standards restricting lead content. The alloy also can be used in applications that require pressure tightness, whether for air, water, gas or oil. Other viable applications include pump components, water impellors and housings, and small gears.
C83470 also has applicability for continuous cast rod, bar and shapes, as well as potential as a choice for bearings.
Based upon the trials conducted over the last three years, the authors believe this alloy is a viable option to produce castings that will be in compliance with the Safe Drinking Water Act or other standards requiring castings to be low in lead. Standard melting, deoxidation and pouring practices apply with C83470 as with other leaded and non-leaded waterworks and plumbing alloys.
Each facility should thoroughly investigate this alloy and implement the needed controls at each process step to ensure best results. Further research may be needed based on product applications. Results may vary.
This article is based on research conducted by members of the AFS Copper Alloy Division.