Investment Casting Material Options

Taylor Topper:

Hi, everyone. I’m Taylor Topper. You have received quite a few emails from me in the last couple of weeks leading up to the webinar, so thanks for being patient with me. Welcome to our webinar series, Metal Solutions. Today we’ll be featuring investment casting materials. If you missed Investment Casting 101 where we kind of do an overview of the investment casting process and a little bit about Signicast you can actually view that on our website. If you go to signicast.com/webinars you can get a direct download of our video recording of our last seminar which I think would be really helpful in case you missed it or if you want to share with any colleagues.

Before we get started I’d like to go over a few things to help you participate in today’s event. We’re taking a screenshot of the attendee interface. You should see something that looks like this on your own screen in the upper right corner. For everyone’s sake we do have all attendee microphones muted. You’ll have the opportunity to submit text questions to today’s presenter by typing your questions into the question pane of the control panel which you see here. You may send in your questions at any time during the presentation. I’ll collect them and we’ll address during the live Q&A session at the end of the webinar.

Today we have Pat Morrison presenting for us today. Pat is our technical director and is responsible for process engineering, material science and R&D at Signicast. He is also responsible for plant and sales, technical support in the areas of metallurgy, alloy selection and heat treatment. Prior to Signicast, Pat was a process metallurgist and then plant metallurgist at a foundry owned by Manoir Industries in Ohio. Prior to that he worked at a heat treater in Cleveland, Ohio and in R&D related to powder metallurgy. Pat has a BS in material science from Case Western Reserve University in Cleveland, Ohio. Though he would not classify himself as a materials expert, I do and I thank you very much, Pat, for volunteering to present for all of us today. 

Pat Morrison:

Thanks, Taylor. Hello, everybody. I’m Pat Morrison. We’ve got quite a bit of material to cover so I’d like to get started. Today I just want to talk a little bit about Signicast, what we pour, some of the implications that might have for heat treatment, a quick summary and then we’ll answer some questions. So if we’re ready let’s get going.

We’ve been producing investment castings since 1959. We started with casting boat transoms out of stainless steel and from that we grew to pouring over a hundred plus alloy grades from nine alloy families. We have two locations in Wisconsin with six foundries total. So the alloy families we pour most often are plain and low alloy steels, plain carbon and low alloy steels, 300 and 400 series stainless steels, tool steels, nickel-based corrosion resistant. We don’t do very much in the heat resistant alloy class. Cobalt-based wear resistant alloys, some aluminum alloys and then we have some alloys for special applications.

So, plain carbon steels really are what the name implies. They contain carbon, manganese, silicon and iron. There’s a name in convention that allows us to easily understand the compositions of a particular alloy. It’s basically four digits. It’s 1-0, the first two denote the alloy family, and the second two denote the carbon content. So, for us we pour mainly the 1-0 family of plain carbon steels and that would be like 10/25, 10/45 are very common for us. They would contain .25% and .45% carbon respectably. We generally produce to ASTM A216 and A352. A216 WCB, that’s basically a 1025 alloy that’s good for high temperature meaning room temperature to moderately high temperature surface. And LCB and LCC those are very similar alloys to WCB but they’re produced with tighter chemistry controls so they can be used at lower temperatures.

With the plain carbon steels we get a wide range of strengths depending on the grade and the heat treatment. We can have ultimate tensile strengths from 50 to 200 ksi and that would be 200,000 psi, yield strengths from 40 to 150 ksi, quite wide range of ductility from really very respectful numbers of greater than 20% to around five, a very wide hardness range from HRB 60 to HRC 50. The heat treatment response depends greatly on carbon content and section thickness. The low carbon grades, which are generally considered to be less than .35% carbon, are very weldable and machinable.

So as a class these materials are very versatile and economical. They’re an excellent choice for low to medium strength structural parts and small, high hardness parts, and we’ll talk about why the size is important in a little bit. Since these are just fields they require painting or plating for protection from the environment. The casting you see in the picture is a structural component for a truck. It’s basically a joint. So, you can see the ends look like squares. Well they accept tubing that will then form the framework for a truck body. So on we go. 

Low alloy steels are plain carbon steels with additions to improve heat treatment response or grade size, you know, some property that we’re looking to enhance. They have a similar name and convention. The first two digits again denote a family and the second two denote the carbon content as before. So the 4100 family denotes chrome and moly additions. 4300, 8100, 8600, are all nickel-chrome-moly steels. So the 4300 would be a family. The only real difference is that the amounts of nickel, chrome and moly would change in the family. So a 4324 would have .25% carbon with nickel-chrome-moly addition that would be different from an 8125.

With these materials we get an even wider range of strengths depending on the grade and the heat treatment. UTS or ultimate tensile strengths from 50 to 300 ksi, yields from 50 to 260%, elongations from about 20 to around five, and again, a very wide hardness range of HRB 75 to HRC around 58. The other thing that’s worth noting is that we get this wide range of properties with a tighter carbon range than in the case with the plain carbon steels.

Heat treatment response depends greatly on carbon and alloy content and less on section thickness and that is the benefit of the alloy and elements that we’re adding. So we can get a predictable heat treatment response in very large sections of thicknesses. So again, these are versatile and economical. They’re an excellent choice for medium to high strength structural parts and surface hardened parts. They require painting and plating for protection from the environment just like the plain carbon steels do. 4140 and 8620 are produced in the highest volumes here at Signicast. So 4140 is a chrome-moly- steel with .4% carbon. 8620 is a chrome-moly-nickel-steel with .2% carbon.

So, we talked about surface hardening there or mentioned surface hardening, 8620 would be a grade alloy for a surface hardened application where you would quench and temper the part so that it has a very hard surface and a relatively soft, tough core. So it could take impact but still maintain a good wear resistance.

Moving on to stainless steels, we’ll start with the 300 series stainless steels. So again, these are iron-based alloys with additions of chrome and nickel to promote corrosion resistance. So when people think of 18-8 they’re really thinking of a 300 series stainless steel. The microstructure is primarily austenitic. So that’s a face-centered cubic crystal structure. Room temperature iron for instance or normal steels would be body-centered cubic. So I’m assuming some of you will know what that means and some of you won't but it’s important for heat treating and stability.

So, other elements are added to promote resistance in certain environments or for weldability. So, 316 is basically 304 or CF-8 with moly added. The molybdenum increases the general corrosion resistance but is especially effective at improving the pitting resistance in the presence of chloride, so like marine environments, that sort of thing. It also controls carbide precipitation when these materials are welded. 347 or CF- 8C has niobium added and that also stabilizes carbides. So there are several ways sometimes to produce the same general properties. Moly though is very good for the chloride resistance.

So, it’s always austenitic. It does not respond to heat treatment but we do solution annealed these materials to optimize the corrosion resistance. So, as cast these materials will have some carbides in their structure because of the solidification behavior so we heat treat these by heating to like 1950 F or above to redissolve all of those carbides and then we quench to keep them in solution. That way they don’t interfere with the corrosion resistance of the material. The room temperature properties are actually quite low at 70 ksi ultimate tensile strength, 30 ksi yield, but the ductility is quite high at 35%, and that depends on the grade but they’re all generally pretty ductile.

Other common grades for us are CK20, which is the cast version of 310. That would be a 25 chrome, 20 nickel alloy which has much improved hot strength over 304. We produce most of the grades listed in ASTM A743 and A351. Applications for these materials would include pump and meter bodies in generally oxidizing or mildly reducing environments. So that again depends greatly on the alloying elements that are added to the material.

Another wide range of applications is food processing, pots and pans are made out of these materials, pumps, pumps for food, pumps for chocolate, that sort of stuff. What you see in the pictures here on the left is a pump cover and on the right is a manifold, and for those of you in the food processing or sanitary industries you’ll notice that those fittings are compatible with a tri-clover style clamp. These parts both have been electropolished.

So 400 series stainless steels are iron-based alloys, again, with additions of chrome, 11 to 18% as a family, and carbon 1.2% max. Now, if you're looking at the chrome content it varies from relatively low relative to the 300 series stainless steels to about the same level as many of the 300 series stainless steels. Carbon is much higher. So this is important and we’ll talk about it in a little bit. So, depending on the composition the microstructure will consists of martensite and ferrite or just ferrite.

The martensitic grades respond to heat treatment so that means we can quench and temper them and achieve quite high strengths. Ferritic grades are solution annealed for maximum corrosion resistance. So martensitic grades will give us strengths that are close to 200 ksi and 150 ksi yield. Ferritic grades are similar to 300 series stainless steels in their strengths. Sometimes they’re a little bit weaker. The part you see in this picture is a steering arm for a marine application and that is cast out of CA15. So, it’s roughly 11% chrome. So applications for martensitic materials, 410, 420, 440C, are tools, cutters, structural parts in oxidizing environments.

So I keep talking about the environment, it’s the chrome in this particular case that gives stainless steels their resistance to corrodents. The chromium is really only good in oxidizing environments. Applications for ferritic steels such as 409, 409 with molybdenum and 442 are fittings for automative exhausts. The castings you see here on the left is part of a climbing rig. So someone who’s a recreational rock climber would wear this thing with a rope and if he slips the rope will grab and then it will hold him and keep him from falling any further, and on the right there’s a punch. That punch puts the ear tag into a cow’s ear for identification purposes.

So precipitation hardening alloys really are a separate class but they’re often included in the 400 series stainless category. So these materials sort of combine some of the best properties of the 300 series stainless and the 400 series stainless steels. We produce two grades, 17-4 PH. The cast version of this is CB7-Cu1, and 15-5 PH, which would be CB7-Cu2. So they contain 17% chromium, 4% nickel and 15-5 would contain roughly or nominally 15% chromium and 5% nickel. The copper addition is 3% and niobium is .3%.

So heat treatment consists of at a minimum of a solution annealed followed by rapid cooling to below 90 degrees F. Like if these parts are going to be used for aerospace applications there would be a homogeneization added to that. The castings are then reheated to a specified temperature and held for a specified time, that’s where it’s called aging, depending on the desired properties as defined in ASTM A747. The part you see above is part of the heater section in a residential furnace.

So when I said that it combines the best properties of 300 and 400 series stainless steels, this is where that starts to become apparent. The strengths approach 200 ksi UTS and 160 yield. The corrosion resistance however is pretty close to the 300 series stainless steels and far superior to that of the general class of 400 series stainless steels. So we have a combination of corrosion resistance and strength that’s kind of hard to beat. It’s used for structural marine components, construction equipment and boat props.

So this is a good material where you need strength, corrosion resistance in oxidizing environments, and you don’t want to like keep it immersed in salt water for long periods of time because it will corrode but in general if the oxygen content of the salt water is high it can stay there almost indefinitely. Again, it’s oxidizing environments is where the chrome provides protection. The parts you see on your right in this slide, that’s actually a large part. It probably weighs about 15 to 20 pounds. It’s eight inches across from ear to ear and it’s for a pump body. It’s about an inch thick. The casting on the left is a cage for I think a poppet valve.

So, I’ve included an application guide that I borrowed from...it’s an ASM desk reference, and basically it just says sort of a rough guide of where you can use these materials or what environments they’ve been designed to be used in. The materials on the left are the austenitic stainless steels and you can see that really only the alloys with moly additions in this family are designed for use in reducing environments or salt water. So, the salt water contains chloride and the molybdenum provides resistance to that. On the right is the 400 series stainless steels and you can see that they’re much less corrosion resistant.    

So we’ll move on to tool steels. Tool steels is an extremely broad classification used to describe materials that are basically designed to allow us to form other materials, specifically metals but as industry progressed they’ve been used for other things. Alloys are named for what they do, how they are heat treated or for the properties they might have in common.

So, A series are air hardening. That means that we would heat them up and cool them in air, at least in the good old days. The water series or W series are water hardening. So they require a quench to produce maximum...a water quench actually to produce maximum hardness. So what that means is they need to be cooled much faster than the A series to attain hardness. The S series are shock resistant so they might be used for punches, that sort of stuff. And the H series are hot work materials. So they would be used for like forging dies.

These alloys consist of iron again and contain .25% to 2.5% carbon with substantial additions of chrome, moly, tungsten, vanadium, cobalt and often some nickel to achieve the desired hardenability, hardness, toughness and wear resistance. It’s interesting, the total alloy content can really be quite high, almost 50%. Nickel isn’t used that often but cobalt and tungsten are.

So, Signicast produces a few grades from each of the series on the slide, the A series, the D series, the hot works family, the molybdenum-bearing high-speed steels, the O series which stands for oil quench, and the P series which was originally designed for injection dies, plastic injection molding dies. The applications for these materials include knives, sheers, chain links, high-strengthened wear resistant components. So with all that carbon and alloying elements we can produce an awful lot of carbide in these materials and they’re martensitic so they’re very strong, very hard and quite wear resistant.

The part on the left will look familiar to anyone who has a mixer with a grinder attachment on it, and the part on the right in this slide is a chain link. The heat treatment can be very tricky because if the section thicknesses are high then the stresses that occur during cooling can crack the parts. So it’s often a very complicated heat treatment procedure.

So nickel-based corrosion resistant alloys are used in some of the harshest environments consisting primarily of nickel, chromium, copper and moly with sometimes smaller additions of other alloying elements that are used to create alloys of very high corrosion resistance. They’re used in environments and or in contact with corrosives that exceed the capabilities of 300 series stainless steels. We produce most of the hastelloy C variance that are in ASTM A494 but the most popular alloys are CW-2M and CX2-MW. We also produce some of the nickel-copper alloys as well.

CW-2M, if we take those two alloys, CW-2M and CX-2MW, within that group CW-2M is by far more popular and the reason for that is that it’s very, very stable and very weldable, and it maintains its corrosion resistance in the welded state far better than the other alloys in that family. Actually, let me go back. The slides up here basically compare most of the materials in this family to 316L which would be cast CF-3M. And if you look the CF-3M is the black line and it it’s corroding very quickly. The other materials are actually quite resistant, and I’m looking at the left in hydrochloric acid. Hydrochloric acid is very, very corrosive. If you look at the plot on the right it’s the .1 millimeter per year line in sulfuric acid, the materials generally do a lot better.

Look at the temperature scale. These materials are being used almost to 200 degrees F in sulfuric acid whereas they’re used considerably at lower temperatures in hydrochloric acid. But in all cases here the stainless steel is not doing very well. So it’s very important to understand a little bit about the metallurgy of these materials. So it’s all again about the chrome in oxidizing environments but the nickel and the moly provide resistance for reducing environments or environments where there’s not that much oxygen. If there’s not sufficient oxygen to form a chromia or a chromium oxide scale the parts in general will corrode quickly.

We also produce a fair amount of cobalt-based wear-resistant alloys. They’re very corrosion and heat resistant due mostly to the unique properties of cobalt. Cobalt is very stable. It doesn’t tend to stick to much including itself. So it has very good galling resistance. For us, we produce alloys for heat resistance and generally wear resistance but not so much for hot strength although they do a pretty good job there as well. They contain very large amounts of carbon, chromium and carbide formers like tungsten. The microstructure will consist of large amounts of primary carbides in a cobalt chrome matrix.

A primary carbide is a carbide that forms during solidification. We call secondary carbides those that form generally later, say either during heat treatment or as the part cools in the solidified state. The alloys are very hard and maintain their hardness at very high temperatures. For instance, the reason cobalt is added to tool steels is to maintain their hot strength. So these alloys are very hard to machine and precision casting such as investment casting is often the most cost effective method for producing components. The part you see in the slide is a wear plate. We also make some sheer plates and some cutting knives.

 

Moving on to aluminum, this is like 180 degrees away from cobalt. Aluminum alloys possess a unique combination in mechanical properties and corrosion resistance. The corrosion resistance is in oxidizing environments. So for instance on the top of the Washington Monument there’s an aluminum cone, or point I should say, that has been there since the Washington Monument was completed that is there to keep the monument looking pointy. It’s the original one like I said and it’s got at least another hundred years left. The reason it’s so corrosion resistant is that if forms aluminum oxide, a very stable oxide film on the surface that protects it from further oxidation. So in oxidizing environments at relatively low temperatures aluminum alloys do very well.

 

The other important thing is that aluminum alloys are very light. They’re one-third as dense as steel. So with the proper alloy you can create materials that have high strength to weight ratios. Strength to weight ratios that are greater than steel. On the downside their elastic modulus is also about one-third that of steel. So we need to design castings carefully or other structures so they’re rigid but with careful design those structures can be lighter and more rigid than those made out of steel. So as I mentioned before in oxidizing environments aluminum alloys form a tight, protective layer of aluminum oxide which essentially prevents further oxidation.

That part happens to be a clamp. Part of that, if you look at the body it’ll wrap itself around a tube and get clamped in place and then it clamps another tube to it. So it’s part of a construction device. We also produce some parts for exhaust, or actually fuel delivery in some engine components. Aluminum alloys are almost always provided in the solution in the annealed and aged condition. Unlike steels they don’t have a fatigue limit and are therefore not usually selected for applications involving high cycle fatigue. That is actually quite important. Once the defects achieve a certain size failure will occur catastrophically, which really just means fast. So in aviation, aerospace applications they spend a fair amount of time measuring, you know, assessing defect size to keep things in service.

The most common grade poured at Signicast is A356.2 and we supply it in the T6 condition. So that’s solution annealed and aged to the T6 condition and that would be 34 ksi UTS, 24 ksi yield and 3.5% minimum elongation. If you know this part happens to be...I don’t know how I should say it but it’s a tapper. So it goes on...you’ll see it in bars if you frequent such establishments, if you look at the beers being served. This part is kind of neat in that it’s about a foot, a little over a foot tall, and if you look at the image on the left not only will you see my hand but you’ll see that it’s hollow. We did a pretty good job with this part.

So, I mentioned special applications and we’re always looking to work with our current customers and new customers on any applications they might have, and recently we had an opportunity to explore some low-expansion alloys. These are a class of materials that have a very low thermal expansion coefficient and some of you might know them as invars. So we successfully cast invar 36. We heat treated it. It’s actually a fairly involved heat treatment but the mechanical properties and the thermal expansion coefficient were phenomenally...we were phenomenally successful in achieving what we desired. So that’s a new area for us and we hope to grow into that area as we move along. But that’s the kind of thing that we can do here because we have a very good technical staff. We have very good people in the plant and we can do a lot of cool things.

So there are cost implications. As much fun as alloy development and pouring cobalt-based alloys is there is a cost, and generally speaking this little graphic is quite accurate. So plain carbon and alloy steels are the cheapest or most economical materials that we cast, both because they don’t cost too much, you know, iron is very plentiful, carbon is both plentiful and fairly cheap, and we also produce them in high volumes. These are very versatile materials like I had mentioned before and find wide application.

400 series stainless steels, they’re they least cost stainless steel because for the most part they only contain chrome and they only contain enough of it to make them stainless. So they’re not very stainless and in most environments they will rust eventually. 300 series stainless steels are fairly expensive materials as metals go and they’re highly engineered to provide service in the environments they’re designed for. And as you go up things become more expensive because in general their processing cost are higher or their alloying costs are higher. So in the case of 17-4 they’re less highly alloyed than 300 series stainless steels but they require more processing and that creates a little more expense. So, it’s basically a rough guide.

So, as an overview we provide a wide range of alloys and a wide range of chemistries. We can customize to meet specific customer requirements and we also heat treat to also produce specific customer requirements. I’d like to thank you for attending and I hope you found it beneficial.

Taylor Topper:

Thanks, Pat. That was great.

Pat Morrison:

Thank you. Cool.

Taylor Topper:

I think the pictures are really helpful. Everyone we do have a few questions so if anybody has any questions you can submit them in the questions pane as you can see here. So we’ll get into the live Q&A portion of the webinar. And before we do that I know that this is a ton of information to take in all at once. We are recording this so you will receive an email with the recording and the slides in the next day or so, and our engineers are really great with helping with alloy selection and alloy support during DFM. So that’s just another opportunity there for more information, definitely more specific. We only have so much time we can over. But I did want to share with you our website.  

Download the video recording to listen to the live Q&A portion of the seminar.