Stainless vs. mild steel, the throw down.

Monday, November 15th, 2010

Where to start, where to start.  There are so many different aspects of these two metals that we could talk about but so we don’t bore anyone by making the article 20 pages we will (try) to focus on the aspects of the metals that will be of interest to the coffee reader.  This article is specifically meant to talk about these two metals as they would be of interest to building and designing a roast chamber weather it is a fluid-bed or a drum roaster.  Although you could easily apply this to brewers as well, that is something that we will be discussing in another entry.

Before we get too far into it, it is important to define what we are talking about when we talk about carbon steel and stainless steel.  Stainless steel is defined as a metal that has a minimum of 10.5% chromium added.  Carbon steel is a little more difficult to define because there are a broad range of metals that fall under this category.  But if you walked into a sheet metal shop and asked for something to be made out of regular steel you will get what is called mild steel which has a carbon content that is less than 0.15%.  Mild steel can also contain small amounts of other metal such as nickel, chromium, aluminum and others but it is the carbon content that defines it as a mild steel.

Mild steel has a reasonable strength and hardness; it is easier to weld than stainless, and it is cheaper.  However; stainless steel offers several advantages in terms of strength and hardness. It also doesn’t rust, corrode or react as easily as mild steel.  This is important for any food product but it becomes especially crucial when you are dealing with coffee, where any variation in the processing or any foreign material introduced into the coffee may significantly affect the end result.

An important property of mild steels that you lose with stainless steel is ductility, the ability of a material to withstand plastic deformation without breaking. Plastic deformation is when a material gets to the point of bending or stretching when the material will no longer go back to its original shape, it has been permanently deformed.  For reference because I know you are probably writing this all down to share with your friends and you just know they are going to say, “wait, wait a minute, then what is it called when the material does go back to it’s original shape?”  And just so you don’t look like a total fool, elastic deformation is the range of bending or stretching where the material will return to its original shape.

I am super happy right now because I am just having a complete nerd-out, but all of this is just a fancy way of saying that mild steels can have relatively large forces applied to them without being permanently deformed or broken.  So why do we care about the elasticity of these two metals?  To be honest the forces that are required to actually plastically deform either of these two metals is pretty high and not something that you have to worry too much about when building a roast chamber unless the metals get really thin.  But we care because it is a large part of why mild steel is cheaper to work with, is easier to form, shape and weld than stainless.

At this point I feel we are pretty even between the two types of steel, we are gaining strength and hardness with less rust and corrosion with stainless. But the material is less ductile and we have lost some workability plus stainless is more expensive. Now we come to the two factors that will have a much higher impact on which metal you will use: thermal conductivity and specific heat.

Thermal conductivity is a material’s ability to conduct heat through itself over a temperature gradient (temperature difference). The specific heat is the amount of energy required to change the temperature of a material a specific amount.

Many of mild steel’s structural properties can be contributed to its processing rather than any additives; pure metals typically transfer heat better than alloys (metals with other additives) because they don’t have those extra atoms that disrupt the movement of energy through them.  Stainless steel, on the other hand, has by definition a minimum of 10.5% chromium added to it which gives it it’s non-corrosive properties. But chromium atoms that are mixed in with the iron have the added effect of making the material worse at conducting heat than mild steel.

In a fluid-bed roaster where you are heating your beans with the hot inlet air and not the walls of the roast chamber a low thermal conductivity is good because you want the roast chamber to insulate and keep the heat in.  If you are using a drum roaster on the other hand the drum is being used as a heat source. One side of the drum is heating the beans and the other is being heated by natural gas. So for a drum you want a number that is somewhere in the middle so that the heat from the gas is transferred easily from the outside of the drum to the beans on the inside.  At the same time you don’t want it to lose all its heat to the beans at once, you want it to retain some of it and you want it to retain it in as even a way as possible.

The specific heat of the metal that you chose is going to be similar for both drums and fluid-bed roasters.  It is simply a case of not having to use a bunch of energy to heat your roast chamber.  Again since in a fluid-bed roaster you are not using the chamber to heat the beans it will only take away some of the heat from your beans after they are heated by the air, until it reaches the ambient temperature of it’s surroundings.  For a drum since you have a relatively large mass of metal that you have to heat before you heat the beans it will take longer to heat up and it will be a waste of energy if you have a metal that takes a lot of energy to increase it’s temperature.

Let’s recap: Relative to stainless steel, mild steels have lower strength and hardness but are more ductile. In a roasting system, the strength, hardness, and ductility differences between mild steel and stainless steel are not going to play a significant role because we are not using these materials for structural purposes.  But it will cut the cost of manufacturing, as mild steel is easier to work with from a manufacturing standpoint.  So there are really only three significant factors left to consider: the smaller amount of corrosion in stainless steels, the thermal conductivity and the specific heat of each.

So I hate to be the bearer of bad news because I know how sad you are going to be when this article is over but this is the end.  Hopefully this has been informative and interesting and if there is anything that you think that we might have missed we would love to know.  We realize that there are several factors that one needs to consider when designing a roast chamber but between these two metals this will cover most of your bases.

We are going to make our blog into a regular feature on our web site.  We will be blogging about technical aspects of the coffee industry, anything from motors to burners to metals to questions submitted by our readers.  So if you have any questions or suggestions just send us an e-mail.

1 Comment so far

  • Carrie
    January 29, 2011

    I love it when engineers nerd out over such fun things! Fun to read and a nice little metals review. keep it up!

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