Die Casting Steel - why not? 2

[cf10]

We are unable to get a satisfactory answer to why steel/iron is not, or cannot be, pressure cast with permanent molds. It can be gravity cast, as I understood, into cooled copper molds. Does the high temperature required make an injection system too short lived, or are there properties of the steel that would be lost in die casting?
It seems to me that die casting steel would be a valuable foundry service for producing large quantities of strong parts-if the prices were even remotely near those for die casting the lighter metals, e.g. zinc, Al, Mg, wouldn't that make more sense than forming/machining/welding steel for certain products?

 

[swall]

Back in the late 1970's, a company was experimenting with die casting of steel. Because of the high temperatures, the dies had to be made of molybdenum. Resulted in expensive castings and was thus limited high value alloys, such as stainless. One of my former coworkers, now retired, has a couple of open end wrenches that were die cast from 17-4 stainless.

 

[arunmrao]

Carbon steel casting is poured at 1650 C . Stainless steel around 1550C. Now the die material has to withstand this high temperature and also the thermal shocks due to sudden filling of the mold cavity and associated thermal stresses during cooling of the castings. Also the die should be immediately available for pouring the next batch of liquid metal. Add to this the ease of fabrication of the die .

Some small parts weighing a few grams have been attempted in steel. But gray iron castings are processed by die casting process.

 

[cf10]

Thank you for your input. Sounds like mold stresses are the main factor in limiting die casting of steel.
The permanent molds currently used for gravity casting steel, which include refractory metals and some sort of water cooled copper(?), could they be cooled more, to allow a higher production rate? Which could eventually justify the use of pressure to fill the mold?
Is sand casting the main method for getting near net shapes in steel?
I appreciate your help with these questions; I am trying to get a better handle on why certain manufacturing processes work well and others don't.

 

[TVP]

If the molds/dies were to be more intensely cooled, problems with thermal stresses & distortion would be the limiting factor. As arunmrao stated, the pouring temperatures are very high, and if the ejection temperature was limited to ~ 250 C, then the die has to reduce the temperature of the metal 1300 C or more, and then repeat this every 10 seconds or so... If H13 tool steel dies used for high pressure die casting of aluminum develop fatigue cracks due to thermal cycling (heat checking), then imagine the problems with significantly temperatures.

 

[arunmrao]

water cooled copper mold that you are referring to is used in continuous casting of steel. Also to avoid sticking of the molten metal to the mold lineseed oil is applied onthe mold surface. This way you can cast billets and not shaped parts.

Conventional sand casting is the only method available now to produce steel casting.(Investment casting technology is there for making small and complex parts.)

 

[cf10]

That makes lots of sense, arunmrao, TVP, and swall.

The only question left then, which isnt precisely metallurgical: what to do when you want a strong, intricate part?
Is the answer: Just run costs on A) redesigning with the stronger Mg or Al alloys and the tooling (die cast) versus B) design for multi part welding/machining with steel parts but no significant tooling (not die cast).

From a prototype standpoint, it seems safer, and many orders cheaper, to make a steel prototype.
How big is the net-shape steel casting industry? Most local foundries just do bronze, al...

 

[arunmrao]

What part do you have in mind and what is the service condition it is subjected to. YOu can select suitable alloy and then heat treat the part to achieve your mechanical properties.

To develop a prototype sand castings are a lot cheaper and faster. Locate some small foundry in your area who may do it. Or else in India you can contact kasc@satyam.net.in. This foundry can easily handle your enquiry.

 

[arunmrao]

I just remembered that you could use graphite molds for casting steel. Also use a positive pressure on liquid metal in a covered ladle so that mold filling is quick and clean. This technique is patented and I have seen it used by a foundry manufacturing railway wheels. Hope you can extend your thinking along these lines too!!.

Good Luck in your search for a sound and cost effectve casting

 

[swall]

Don't forget about investment casting (lost wax). Lots of low alloy steel near net shape parts are made this way, in addition to the more exotic nickel and cobalt alloys.

 

[TVP]

cf10,

You should definitely investigate investment casting or countergravity casting (commonly called the Hitchiner process). Steel can be cast into components with wall thickness down to ~ 2 mm. Use the following link for more information:

http://www.hitchiner.com

 

[jsokal]

An interesting alternative we have found very sucessful is MIM. Metal Injection Molding. Metal powder (stainless or whatever)is combined with a plastic like material and injected into molds. The product is then baked to remove the plastic and HIPed (Hot Isostatic Pressed) to strengthen the product. Many stainless steel surgical tools are made this way and we have found it a cheaper way of producting a complicated near net shape.

 

[cf10]

Thank you for all the ideas- very helpful in getting an overview of the industry.
Investment/ceramic shell does look like it can produce the detail necessary, especially Hitchiner's process. Time is reduced by gang-molding the parts. How long does it take, does anyone know, to go from wax to pour?
What kind of pressure is used in regular die casting? Would the pressure just explode a typical refractory plaster/ceramic mold?

arunmarao: I have read that graphite crucibles introduce iron into ferrous castings; would the same apply to molds? Regardless, sounds like a valuable process.

 

[swall]

Time, or shall I say time delay, is a big factor with investment castings. From what I can remember, having been in three investment casting houses, about four layers of ceramic have to be built up on the wax pattern.The wax pattern has to be dipped, and each layer has to dry.Even with oven drying, you are looking at HOURS for each layer, if not days. So that is a major constraint if you are looking at high volume production.

 

[TVP]

swall is correct-- the time from wax to parts is hours. Hitchiner does have automated facilities (one in New Hampshire and one is Mexico) that reduce the moldmaking and casting time as well as massively increase the number of parts on the tree so that high volume production is possible.

Regarding pressure in high pressure die casting, during the intensification stage it can exceed 69 MPa (10 ksi), which would definitely destroy ceramic molds like those used in investment casting. Die casting machines require locking forces ranging from 450 kN to ~ 35 MN (50-4000 tonf) in order to resist the high pressures developed.