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Breaking cast iron's vicious cycle.

Gray-iron machinability ain't what it used to be, just ask any manufacturing engineer. Yet good machinability once was a primary reason for selecting this work material. What changed? Is it possible for cast gray iron to meet modern part-design criteria and not raise havoc in the machine shop?

I think so. And it can be done cost effectively. But, before discussing how, a brief disgression is necessary to understand why gray iron evolved to its present state.

The vicious cycle

Over the years, there's been an increasing demand for stronger cast irons. Circa 1950, when ductile iron came on the scene at twice the strength of cast gray iron, there suddenly was less general interest in making high-strength gray iron. The emphasis in gray iron changed to cost reduction.

Alloying a base iron with chromium (compared to copper, nickel, molybdenum, and vanadium) was a low-cost, expedient way for foundries to meet strength requirements. Chromium, however, causes chill iron carbide and intercellular iron carbide--two enemies of machinability.

Unfortunately, chill and intercellular carbides were thought to be the penalty for increasing gray iron's strength. Silicon and/or inoculants were added to offset (balance) these deterious by-products, which was fine except for a couple of "little details" that got lost in the shuffle.

Silicon, for example, can offset chill, but it also reduces strength, thereby compromising the motivation for adding chromium in the first place. Adding more chromium, however, restores strength. Over the years, chromium and silicon were successively added to the point that it's now common to have gray irons with as much as 2.4 percent silicon when no more than 2 percent should be necessary.

Too much silicon creates a condition where casting hardness becomes extremely sensitive to cooling rate, which varies within a workpiece (i.e., a thin section cools faster than a thick one), Figure 1. Hardness variations lead to machinability problems, and that is a story unto itself.

Moreover, even though silicon reduces chill carbides, it doesn't necessarily reduce intercellular iron carbides. Therefore, chill-free castings still can contain plenty of troublemakers, Figure 2.

Intercellular carbides damage cutting tools the same way stones ruin a farmer's plow. Tests indicate that even 3 to 5 percent intercellular carbides, a level often considered tolerable, can adversely impact tool life.

Eventually, the situation got out of the metallurgists' hands and into those of the accountants'. Low-cost melt became the game, and to the accountants' delight it turned out that the lowest-cost charge material also had a high dose of chromium (e.g., bumpers, die parts, etc).

There was no longer an option to add chromium for strengthening--more than enough was already in the scrap charge. The only way to preserve a semblance of machinability was to add silicon. As a consequence, your perishable tooling perishes faster.

What's the real cost?

The low-cost melt approach even is tainting the way end users do business. For example, many metalworking firms have embraced accounting practices that overmephasize up-front material cost. The real cost of high-strength gray-iron castings is the cost of the raw casting plus the cost of machining. And machining costs easily can exceed that of the casting. To drive this point home, consider the following example.

Cast gray iron was picked by a large metalworking manufacturer for a complex valve design because it was possible to minimize machining by using cores in the casting. The raw casting cost $45, plus $60 to machine it, or $105/part.

The problem was that variations in section thickness (and, consequently, cooling rates) associated with the cores produced hard spots and porosity. This led to machining problems and "leakers," resulting in a 60 percent scrap rate. The actual cost for a good part, then, was $62.50!

The manufacturer took extreme action and changed the material spec to cast-iron bar stock. Material cost was lower (only $20/part), but machining cost jumped to $150/part. But because scrap fell to about 1 percent, the real cost was $172/part. No bad, but not as good as it could have been.

What really was needed was an iron of both superior quality and machinability--one with properties not sensitive to section thickness and cooling rate. Such an iron would inherently have the low scrap rate associated with bar stock, while retaining the economies of a near-net-shape casting.

You may be beginning to realize that the metallurgy of producing the lowest-cost casting to meet a specified minimum strength is substantially different from the metallurgy of producing a high-strength casting with superior machinability. The trick is to minimize the carbides.

Walking the high-strength wire

Producing machinable high-strength iron at lowest total cost requires controlling carbon equivalent, graphite structure, chill and intercellular carbides, ferrite content, and pearlite refinement. Alloying elements play a major role here. Each has distinct advantages (some have disadvantages) that must be considered.

Chill is best avoided, for example, by keeping chromium and other strong iron-carbide formers as low as possible. Moreover, chill can be reduced by raising carbon equivalent, and by adding silicon (pointed out earlier), inoculants, and graphitizing elements.

The major strengthening effects of alloying elements result from forming pearlite and then refining it. To obtain maximum strength from pearlite refinement, though, it's essential to first have pearlite, i.e., eliminate free ferrite. The reason? A structure of 30 percent ferrite/70 percent pearlite receives only 70 percent of the potential benefit of strengthening by refinement.

Copper-molybdenum and tin-molybdenum combinations are effective for this purpose. Copper and tin promote pearlite (suppress ferrite), and molybdenum refines it for further strengthening. Copper and moly promote neither chill nor intercellular carbides.

Moly delays formation of both ferrite and pearlite. By delaying the pearlite reaction to a lower temperature, moly causes a finder pearlitic structure that increases both strength and hardness.

Silicon acts to coarsen pearlistic structure, by the way, which tends to weaken iron. Significant strengthening by pearlite formation and refinement is possible by simply lowering silicon content. The level, necessarily high in a chromium iron, can be much lower for cooper-molybdenum or tin-molybdenum iron.

Molly is an alloying element that's very compatible with lower silicon levels because it does not promote chill. Thus, the balancing act necessary with chromium isn't necessary--copper/molybdenum is an efficient metallurgical system.

Molybdenum is usually added in concentrations up to 0.5 percent, which can increase tensile strength by 5000 to 10,000 psi; however, because the element refines pearlite (but doesn't promote it), full benefit is obtained only when a fully pearlitic condition is ensured. This can be done by adding pearlite forming elements (such as tin or copper), or by reducing silicon.

Proof positive

Metcut Reserach Associates, Cincinnati, OH, recently corroborateed the machinability benefits of this approach to making high-strength gray irons. Ten Class 45C cast-iron test plates (ASTM A48; 45,000 psi minimum tensile strength), alloyed with chromium, nickel, and copper for strengthening, contained an average intercellular-iron-carbide content of 2.8 percent. A second group of 10 plates, alloyed with copper and molybdenum, contained less than 0.1 percent.

A Cincinnati Model Cinova 80 vertical mill, tooled up with a 6"-dia fly cutter using a Carboloy Grade 883 SNG-432 carbide insert, did the machining (feed was 0.010" per tooth; depth of cut, 0.050"). As expected, the relatively carbide-free iron machined much better. The extent of the improvement, however, was more dramatic than expected, Figure 3.

Based on the findings, a user can either increase productivity or redice costs by eliminating intercellular carbides. If tool life is 1 hr, for example, cutting speed for the carbide-free iron can be increased 41 percent. Or, at fixed cutting speeds, tool life can be extended up to 300 percent.

Metcut's tests prove that machinability doesn't have to be sacrificed for strength. High-strength gray iron can be produced using molybdenum in combination with other alloying elements that don't promote chill and intercellular iron carbides.

Many innovative foundries have been following this prescription for years. They're the ones supplying castings that provide the lowest real cost to end users.
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Author:Janowak, J.F.
Publication:Tooling & Production
Date:Dec 1, 1984
Words:1336
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