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Casting more light on gray-iron machinability.

The technology and alloying techniques for producing a readily machinable high-strength gray iron exist, and have for some time. That's the good news. The bad news is that the resulting castings often reflect increased foundry costs. The bone of contention is whether it's in your best interests to pay a premium for carbide-free high-strength gray irons and realize the cost saving from improved machinability.

Chromium is popular as an alloying agent for strengthening gray irons because of its relatively low cost. A side effect, however, is that it can cause both chill carbides and intercellular carbides to form in the iron--and carbides detract from machinability. To balance that effect, silicon-bearing materials usually are added to eliminate chill. But silicon reduces strength, so some foundries are caught in a cycle of increasing both chromium and silicon content. The resulting gray irons don't always machine well.

For years, Jay Janowak, manager of foundry development, AMAX Corp, Arlington Heights, IL, and others have advocated using molybdenum (moly) and copper as alloying elements, maintaining that their higher cost offsets losses from scrap castings and lost machining time. Some foundries have worked with machine shops and are able to use this technique. A great many do not.

Let's talk

"While there often has been poor communication and an adversarial relationship between machine shops and foundries," comments Iron Casting Research Institute (ICRI) Executive Director Bill Shaw, in a latter to his membership, "we are in the process of trying to remove these barriers." He points out that emphasis on fully meeting customer material specifications has tied to foundrymen's hands to an extent, further reducing the opportunity for needed dialogue.

Specs given to the foundries vary from one machine shop to the next, depending on the end products. Some specify only tensile strength and Brinell hardness, while others go into great detail concerning alloying techniques and microstructure control. Some shops dictate specs to the point where foundry metallurgists have no room to exercise sound judgment. Then comes the repeated cry for lower-cost castings.

One foundryman combating this trend is Bob Hachtman, president, Kulp Foundry, East Stroudsburg, PA. "Price must not be the only consideration of a castings buyer," he remarks. "We work with end users to make them aware of areas where a higher quality casting is in their best interest."

Hachtman has worked for years with such people as John Briggs of Foseco Internation, AMAX's Janowak, and Vern Patterson, a retired metallurgist from Miller & Co in Chicago. As a result, he has adopted alloying techniques that reflect an awareness of machinability, while maintaining required hardness and tensile strength. Recognizing that chromium-silicon gray irons can cause machining problems because of carbides and hard spots, Hachtman has opted since 1976 to use more copper and moly in place of chromium and silicon.

"It was a drastic change, but one that has worked out well," he says. "Before, we were modifying the iron's base chemistry at the furnace by chaning the carbon equivalent, thus producing different grades. This caused scheduling complications, and also made precise control of chemistry more difficult. Now we take a base Class 30 iron and add alloys at the cupola, which also permits producing sheaves for heavy-section castings without risers."

Kulp employees follow a posted mathematical program (developed by Hachtman) that indicates the amount of each alloy to be added to the batch iron. All alloys are preweighed and packaged. Kulp foundrymen merely toss in the prescribed alloys and inoculants to produce a specific iron grade. By using more moly and copper, casting cost goes up. Hacthman combats a "knee-jerk reaction" to higher costs by working closely with end users.

"Bob has been here several times to talk with out peopel," comments Rick Witte, Dover Elevator Systems, Cincinnati, OH. "His expertise and ability to explain how what goes on in the foundry impacts what we do in the machine shop helps tremendously.

"Originally we were following everyone else, specifying tensile strength and Brinell hardness range for our Class 35 gray irons. What started us looking at alternative approaches was the horrendous machining problems with many of our castings. Bill Shaw at ICRI helped by determining that the presence of free carbides was the root of the problem. He then suggested a major revision to our specifications.

"We reexamined the alloying techniques of our foundries and decided to reduce chromium and add copper and more moly. In addition, we opted to more closely control cooling rate of the iron in the mold. These changes improved casting machinability significantly."

At Dover, cutting-tool life on castings has tripled. Witte also is quick to point out the scrap savings. "We have greatly reduced the unmachinable scrap that is returned to our vendors. If you scrutinize scrap cost, it actually costs three times the apparent cost of the workpiece because you must setup again and return the part, thereby losing production time. Also, when you are pushing as hard as we are, there is no room for the additional inventory associated with running execessive scrap."

Lean and mean

Emphasis on reduced inventory and the need for good machinability is an integral part of Dover's commitment to embracing just-in-time inventory control. "This means that when we order a casting and set it up on a machine tool, we must know that it's going to machine right the first time," asserts Witte. "Any businessman will tell you that inventory is an expense that drags down profits. Not only have savings in tool life justified higher-cost castings, but they have contributed to inventory savings as well."

AMAX's Janowak has promoted this total approach to cost savings for years. "Many productivity improvements and cost-reduction opportunities begin with higher-value castings," he maintained in a recent presentation to the ASM International Conference on High Productivity Machining, Materials, and Processing. "Users will benefit significantly by being open to paying extra for castings that reduce machining and other manufacturing costs."

"Standards for microstructure testing have created confusion at the foundry level, which is another issue that must be addressed," says al Hochstein at Grede Foundries. He also is chairman of the American Foundry Society Gray Iron Research Committee. "AFS is sponsoring a research project as Southern Research Institute designed to redefine the impact of various alloying elements on gray iron's physical and mechanical properties. This work will give us a better understanding of how alloying agents affect the microstructure of castings and, ultimately, machinability."

Interpretations of ASTM and other specifications have encountered similar problems. Readings can vary depending on the persom administering the tests.

Brinell hardness provokes uniform criticism as an indicator of machinability. Hochstein describes the Brinell test as " of the few working tools we have, but it's certainly only part of the equation when determining machinability." Hachtman calls the Brinell test "inconsistent at best" as a reading for machinability. Janowak says that using only BHN as an indicator of machinability "can lead to erroneous, often costly, results." Witte is more specific, saying that "Brinell is truly no indicator of machinability."

Mike Saunders, a CNC programmer for Delco Products Div, General Motors, DAyton, OHM reflecting on the wide variations in machinability of castings with the same BHN, declares, "Brinell isn't a good test for machinability--period."

Hachtman has taken that controversy one step further in collaboration with Dover Elevator's Witte. "Bob sent me a test casting in the 260 to 280 BHN range and asked us to process it, without telling the machinist the hardness," says Witte. "The operator had such an easy time, he guessed it to be 240 BHN, when in fact he was machining a good, very-high-strength gray iron that was free of carbides."

Besides the consensus concerning the inability of BHN to perform as a sole indicator of machinability, there is one other factor that affects quality--length of time a casting remains in the mold. Several machine shops are adding a minimum mold-cooling time to their casting specs; foundrymen are acknowledging its role in reducing hard spots.

"Last year I suggested a review of our casting techniques and materials," says Delco's Saunders. "We were looking to improve consistency and predictability of castings because of machinability problems. As we altered specs on some of our gray-iron castings, we found that machinability, machining speed, and tool life decreased.

"One variable that we examined was the time the casting stayed in the mold. After talking with out foundries, we concluded this had a direct bearing on hard spots and as such should be included in the specifications. We've made the change."

Witte corroborates Saunders' experience, saying, "We attribute the improved quality of castings to reduced chrome in the charge material and increased control over cooling rates. We can tell if the foundry pops the castings out of the mold too quickly because we can't touch them with a cutting tool, regardless of BHN."

Hachtman agrees. "Early shakeout detracts from machinability because of an increase in the matrix hardness, which isn't related to the presence of carbides. The solution under these circumstances is to make adjustments to both base chemistry and mold-cooling time."

While specifics concerning production of machinable high-strength gray irons are yet to be agreed upon, several aspects can't be ignored. One is the problem of getting good data from working foundries and their customers. An aggressive businessman earns whatever advantages he develops, and may not be inclined to share them with potential competitors. Moreover, professional ssociations that compile information for research studies are purposely deliberate. They must document findings, and that slows down the process. Under this scenario, progressive foundries and end users will continue to hold an edge.

Which brings us to a final observation of the market in general. Foreign competition is making its presence felt more strongly than ever. The threat of losing market-share to foreign firms should prompt more US companies to reexamine their relationship with foundries. Superior service and favorable locations in conjunction with a better ability to fit in with JIT inventory procedures are advantages provided by domestic foundries that firms in this country will do well to consider.

For more information about machinable high-strength gray-iron castings use the following reader service numbers: AMAX circle 555. Grede Foundries cirlce 556. Kulp Foundry circle 557.
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Publication:Tooling & Production
Date:Dec 1, 1985
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